Improving the Usability

of Waste Ti-6Al-4V

Powder

by

Michael Tack

Supervisor: Dr DC Blaine

24 October 2014

Improving the Usability of

Waste Plasma Rotated

Electrode Process (PREP)

Powder

Final Report for Mechanical Project 478

MF Tack

Student Number: 16544897

Supervisor: Dr DC Blaine

24 October 2014

i

EXECUTIVE SUMMARY

Title of Project

Utilising Boeing’s Plasma Rotating Electrode Powder (PREP) Waste

Objectives

Identify the most effective method for improving the compressibility of coarse PREP Ti6Al4V powder whilst maintaining the overall alloy stoichiometry

What aspects of the project are new/unique?

Determining new manufacturing procedures for utilising an otherwise wasted powder product

What are the expected findings?

A suitable powder metallurgy process for using coarse PREP Ti-6Al-4V powder exists that can produce good quality press-and-sintered material

What value do the results have?

To determine whether or not a feasible solution can be produced in terms of utilising the wasted powder batches

What contributions have/will other students made/make?

Previous students have investigated the press-and-sinter processing of HDH titanium powder which will assist this project

Which aspects of the project will carry on after completion?

Determining whether the manufacturing procedures generated are economically viable on a long term scale. Exploring possibilities of even more innovative manufacturing

procedures.

What are the expected advantages of continuation?

To ensure that South Africa is at the forefront of titanium processing, when the global demand increases, as it is naturally abundant here and develop a collaborative research

partnership with Boeing as they are a primary titanium consumer

What arrangements have been made to ensure the project continuation?

As per the project topic outline, the project results will be shared with Boeing’s global materials research team who will decide on further development of the project

ii

iii

ACKNOWLEDGEMENTS

The author would like to thank Dr Deborah Blaine for her time, effort and

guidance in supervising this project and helping make it possible to complete the

project in the limited time available.

For their assistance with the lab equipment used in the completion of the project,

the reader would like to thank Mr. Hendrik Bosman, Miss Melody van Rooyen

and Mr. Brendon Boulle.

iv

ECSA OUTCOMES

ECSA Outcome Assessed in this Module

Outcome Addressed in sections:

1. Problem solving:

Demonstrate competence to identify, assess, formulate

and solve convergent and divergent engineering

problems creatively and innovatively.

1, 2, 3

2. Application of scientific and engineering

knowledge:

Demonstrate competence to apply knowledge of

mathematics, basic science and engineering sciences

from first principles to solve engineering problems.

3, 4, Appendix A,

Appendix C, Appendix

E, Appendix G

3. Engineering Design: Demonstrates competence to perform creative,

procedural and no procedural design and synthesis of

components, systems, engineering works, products or

processes

3, 4, Appendix A,

Appendix F, Appendix G

5. Engineering methods, skills and tools, including

Information Technology:

Demonstrate competence to use appropriate

engineering methods, skills and tools, including those

based on Information technology.

4, Appendix A,

Appendix D, Appendix

E, Appendix G

6. Professional and technical communication: Demonstrate competence to communicate effectively,

both orally and in writing, with engineering audiences

and the Community at large.

Project proposal,

Progress report, Oral

presentation, Final

Report, Final oral

presentation, Project

poster

8. Individual, team and multi-disciplinary working:

Demonstrate competence to work effectively as an

individual, in teams and in multi-disciplinary

environments

1, 2, 5, Appendix G

9. Independent learning ability:

Demonstrates competence to engage in independent

learning through well-developed learning skills

2, 3, 5, Appendix A

v

TABLE OF CONTENTS

Page

Executive Summary .................................................................................................. i

Plagarism Declartaion ............................................ Error! Bookmark not defined.

Acknowledgements ................................................................................................. iii

ECSA Outcomes ..................................................................................................... iv

Table of contents ...................................................................................................... v

List of figures ......................................................................................................... vii

List of tables ............................................................................................................ ix

1. Introduction .................................................................................................... 1 1.1 Project Introduction ................................................................................ 1 1.2 Objectives ............................................................................................... 2

1.3 Motivation ............................................................................................... 2

2. Literature Review ........................................................................................... 4 2.1 Titanium and Titanium Alloys Overview ............................................... 4 2.2 Powder Metallurgy ................................................................................. 7

2.3 Production of Titanium Powders ............................................................ 8 2.3.1 Overview of Production Processes ................................................... 8

2.3.2 Plasma Rotating Electrode Process (PREP) ..................................... 9 2.4 Methods for Producing Ti-6Al-4V ....................................................... 11

2.4.1 Pre-alloyed Approach ..................................................................... 11 2.4.2 Blended Elemental Approach ........................................................ 12

2.5 Review of Previous Projects ................................................................. 14

3. Experminetnal Procedure ............................................................................. 16 3.1 Powder Characterisation ....................................................................... 16

3.2 PREP Ti-6Al-4V Powder Compaction ................................................. 19 3.3 Using the Blended Elemental Approach to Mix Powders .................... 20 3.4 Compact Final Ti-6Al-4V powder mixture using Cylindrical Die Set . 21 3.5 Compact Final Ti-6Al-4V powder mixture using TRB Die Set ........... 22

3.6 Sintering of the TRB Specimens .......................................................... 23 3.7 Strength Testing .................................................................................... 25

4. Results and Discussion ................................................................................. 27 4.1 Powder Characterisation ....................................................................... 27 4.2 Powder Compaction .............................................................................. 30

4.2.1 Cylindrical Die-Set Compaction .................................................... 30 4.2.2 TRB Die-Set Compaction .............................................................. 32

4.3 TRB Specimen Sintering ...................................................................... 36 4.4 Strength Testing .................................................................................... 38

4.4.1 TRB Green Strength ....................................................................... 38 4.4.2 TRB Sintered Strength ................................................................... 40

5. Risk Assessment ........................................................................................... 43

vi

6. Conclusion .................................................................................................... 44

7. Recommendations ........................................................................................ 46

8. References .................................................................................................... 47

Appendix A: Experimental Calculations ............................................................... 49 A.1 Ti-6Al-4V Characterisation: Apparent Density, Flow Rate and Sieve

Analysis ................................................................................................ 49 A.2 Theoretical Analysis of Powder Mixture .............................................. 51 A.3 Ti6Al4V Characterization: Laser Diffraction ....................................... 53 A.4 TRB Green Density: Archimedes Principle .......................................... 58

Appendix B: Quote received for PA Ti-6Al-4V Powder ...................................... 59

Appendix C: Conversion tables ............................................................................. 60

Appendix D: Green Density Results ...................................................................... 61

D.1 Cylindrical Specimen Green Densities ................................................. 61 D.2 TRB Green Densities ............................................................................ 63

Appendix E: Strength Test Results ........................................................................ 64 E.1 TRB Green Specimens Failure Force ................................................... 64

Appendix F: Specifications of Re-Designed Spacer .............................................. 66

Appendix G: Techno-Economic Analysis ............................................................. 67

vii

LIST OF FIGURES

Page

Figure 1: Ultimate Tensile Strength vs. temperature comparison between different

alloys (Goso & Kale, 2010) ..................................................................................... 5

Figure 2: Metal price listing for titanium alloy and competitors

(www.metalprices.com/charts) ................................................................................ 6 Figure 3: Simple pressing of a green compact (Clinning, 2012) ............................. 7 Figure 4: SEM images of typical titanium alloy powders (ASM, 2009) ................. 9 Figure 5: Plasma rotating electrode process (ASM, 2011) .................................... 10

Figure 6: (a) Micrometrics Saturn DigiSizer (b) Olympus SZX7 stereomicroscope

system .................................................................................................................... 16

Figure 7: Flow chart of experimental procedure ................................................... 17 Figure 8: (a) Hall flow meter (b) A&D FX-1200i scale ........................................ 18 Figure 9: (a) Layout of sieve analysis equipment (b) Largest size mesh (c)

Smallest size mesh ................................................................................................. 19 Figure 10: (a) Cylindrical die set (b) TRB die set ................................................. 19

Figure 11: Carver® 12 ton manual press ............................................................... 20 Figure 12: Mechanical mixer ................................................................................. 21 Figure 13: Data acquisition system ........................................................................ 22 Figure 14: Amsler 25 ton automatic press ............................................................. 23

Figure 15: Vacuum furnace system ....................................................................... 24

Figure 16: Furnace end-seal and argon inlet valve ................................................ 24

Figure 17: MTS tensile testing machine ................................................................ 25 Figure 18: TRS tooling .......................................................................................... 26

Figure 19: Stereomicroscope images of PREP Ti-6Al-4V powder ....................... 27 Figure 20: Cumulative particle size distribution of PREP Ti-6Al-4V powder ...... 27 Figure 21: Sieve analysis graph ............................................................................. 28

Figure 22: Ejection of compacted Ti6Al4V BE 75:25 powder mixture using fine

Ti powder at (a) 500 MPa (b) 600 MPa ................................................................. 30

Figure 23: Green density of cylindrical specimens ............................................... 31 Figure 24: (a) Buckled spacer (b) Re-designed spacer .......................................... 33 Figure 25: TRB specimen green density ................................................................ 33

Figure 26: TRB and cylindrical green density comparison ................................... 34 Figure 27: TRB sintered densities ......................................................................... 36 Figure 28: Fractured green specimen ..................................................................... 38

Figure 29: TRB specimen green strength .............................................................. 39 Figure 30: Sintered strength of the TRB specimens .............................................. 41 Figure 31: Laser diffraction result page 1 of 6 ...................................................... 53 Figure 32: Laser diffraction result page 2 of 6 ...................................................... 54 Figure 33: Laser diffraction result page 3 of 6 ...................................................... 54

Figure 34: Laser diffraction result page 4 of 6 ...................................................... 55 Figure 35: Laser diffraction result page 5 of 6 ...................................................... 56 Figure 36: Laser diffraction result page 6 of 6 ...................................................... 57

Figure 37: Quote received for fine PREP Ti-6Al-4V powder ............................... 59 Figure 38: Green density with 200 mesh Ti at 500MPa compaction .................... 61

viii

Figure 39: Green density with 200 mesh Ti at 600MPa compaction .................... 61 Figure 40: Green density with 100 mesh Ti at 500MPa compaction .................... 62 Figure 41: Green density with 100 mesh Ti at 600MPa compaction .................... 62 Figure 42: TRB compressibility chart with 200 mesh Ti ...................................... 63 Figure 43: TRB compressibility chart with 100 mesh Ti ...................................... 63

Figure 44: Force required to rupture the 40:60 green specimens .......................... 64 Figure 45: Force required to rupture the 25:75 green specimens .......................... 64 Figure 46: Force required to rupture the 10:90 green specimens .......................... 65 Figure 47: Gantt chart: project schedule ............................................................... 67

ix

LIST OF TABLES

Page

Table 1: Properties of titanium ................................................................................ 4 Table 2: Compositions and mechanical properties of selected alloys (Groover,

2011) ........................................................................................................................ 5 Table 3: Production stage cost comparison between steel, aluminium and titanium

................................................................................................................................. 6 Table 4: Typical titanium alloy powders (ASM, 2011) ........................................... 8 Table 5: Quote received for PA PREP Ti-6Al-4V powder ................................... 11

Table 6: Price of elemental titanium and master alloy powders supplied by

Stellenbosch University ......................................................................................... 13

Table 7: Summary of results from study completed by Kirchener (2009) ............ 14 Table 8: Summary of results from study completed by Laubscher (2012) ........... 14 Table 9: Mixing ratios used to create powder mixtures ......................................... 20 Table 10: Comparison between current study and previous study powders .......... 29 Table 11: Average green densities for TRB specimens ......................................... 35

Table 12: Average TRB sintered densities ............................................................ 37 Table 13: Average TRB specimen green strength and breaking force .................. 40 Table 14: Average TRB sintered strength ............................................................. 41 Table 15: PREP powder apparent density test result ............................................. 49

Table 16: PREP powder flow rate test results ....................................................... 49

Table 17: Sieve analysis 1st attempt ....................................................................... 50

Table 18: Sieve analysis 2nd attempt .................................................................... 50 Table 19: Mass of powders required for 100g final mix ....................................... 52

Table 20: Conversion table for Carver press and dia.10mm cylindrical die ......... 60 Table 21: Conversion table for Amsler press and TRB die ................................... 60 Table 22: Budgeted and actual cost of the project ................................................. 68

1

1. INTRODUCTION

Discussed in this section of the report is the project introduction as well as the

projects stated objectives. It will also provide a motivation as to why this project

is applicable and of use to Boeing.

1.1 Project Introduction

Powder metallurgy (PM) is a continually and rapidly advancing technology used

to fabricate a variety of products. The fact that it can produce net to near net shape

components underpins its importance in manufacturing as it produces little to no

waste. High precision forming allows manufacturers to produce products across a

wide range of applications with more consistent and predictable behaviours

(Boyer, 2010). More often than not, parts generated from powder metallurgy are

finished with minimal need for further machining and tooling. The more common

powder metallurgy techniques are powder injection moulding and the press-and-

sinter process. For the purpose of this project, the press and sinter process will be

the main focus as it is more cost effective than powder injection moulding

(Campbell, 2013).

Titanium, although relatively expensive, is far superior to many of its competitors

in terms of its strength to weight ratio and its resistance to corrosion (Campbell,

2013). It is therefore highly desirable in industry, such as aerospace, where its

mechanical properties can be exploited. Due to titanium’s desirable properties,

two principle areas of application have evolved: (1) in the commercially pure

state, Ti, for corrosive resistant components; and (2) titanium alloys for high-

strength-minimal-weight components where high temperature fluctuations exist

(Groover, 2011).

The University of Stellenbosch has been provided, by Boeing, with a batch of Ti-

6Al-4V PREP (plasma rotating electrode process) powder for analyses and

testing. PREP produces spherical powder particles which are typically used in

additive manufacturing procedures. Boeing uses fine Ti6Al4V PREP powder

particle, 10-100µm, for its additive manufacturing. These spherical powders

possess good flow ability, do not agglomerate and sinter easily. The remaining

larger Ti6Al4V powder particles (>100µm) are sieved out and subsequently

discarded as they are not suitable for the targeted manufacturing process.

This project aims to investigate the viability of making the larger, waste PREP Ti-

6Al-4V powder usable through two proposed solutions: (1) by using the pre-

alloyed approach in combining the wasted, coarse Ti6Al4V powder with a finer

pre-alloyed Ti6Al4V powder; or (2) by using the blended elemental approach to

combine the coarse Ti6Al4V powder with a compressible titanium powder,

blended with a 60Al:40V master alloy powder. These two methods will be

compared on a conceptual level to decide which is most feasible. The preferred of

2

the two methods will be used to improve the compressibility of the PREP powder

so that it can be used in a press-and-sinter process. This project is a proof of

concept as PREP powders are typically difficult to compact due to their particle

shape. The ultimate goal of this study is to compact, sinter and strength test the

improved PREP powder blends in order to determine whether or not the

methodology discussed in this study, is a feasible means of using Boeing’s waste

powder.

1.2 Objectives

The aim of this project is to develop manufacturing procedures to better utilise

waste Ti-6Al-4V PREP powder. The main goals for the project can be

summarized as follows:

Characterize the supplied PREP and evaluate its compressibility.

Compare two methods of improving the PREP’s compressibility on a

conceptual level: Pre-alloyed Ti6Al4V powder and blended elemental

Ti6Al4V powder mixtures

Maintain the alloy stoichiometry in the improved powder blends

Determine the green and sintered strengths of the compacted powder

blends

1.3 Motivation

South Africa has the second largest mineral reserve of titanium ore (titanium

oxide, ilmenite and rutile) in the world (Campbell, 2013). Unfortunately very little

of this titanium ore is processed locally as it is almost entirely exported. The vast

majority of South Africa’s titanium ore resources are found in black sand deposits

in the upper regions of KwaZulu Natal. Because of its natural abundance in this

country, it makes economic sense to enhance the countries processing capabilities

of this raw titanium ore.

Currently the global production of titanium is low, in comparison to other metals,

with a value of approximately 180 000 tons per annum (van Buuren, 2009). The

reason for titanium’s low production is due to its high price, which is a result of

the energy intensive, outdated technology used to extract the metal from its raw

material (van Buuren, 2009). Therefore, to enhance the market for titanium, either

advances have to be made into the processing and extraction of the metal or

innovative manufacturing procedures involving titanium need to be developed,

which gives rise to projects such as this one.

Titanium is known for its material properties such as high strength to density

ratio, good ductility, high melting point (which ultimately makes it difficult to

cast) and favourable corrosive properties, making it desirable form many

applications (Boyer, 2010). Currently the chemical and aerospace industries are

3

the largest consumers of titanium because of its excellent corrosive resistance and

its abilities to save weight and handle extreme temperatures (Boyer, 2010).

Titanium and its alloys have the potential to replace more common metals, such as

steel and aluminium, in the manufacturing sector because of their superior

properties. At present, this is not set to happen but as technology advances and

interests into titanium deepen, it is only a matter of time before the full potential

of titanium can be harnessed at a fraction of the cost.

To date, the powder supplied to Stellenbosch University by Boeing has been

considered nothing more than waste. The useable/smaller size particles of the

powder are sieved out and the larger particles are discarded as they are not

favoured in additive manufacturing techniques. As this powder is produced using

PM techniques, regarding a portion of the powder as waste undermines the core

advantage of PM: its minimal waste level (Boyer, 2010). Therefore enhancing the

compressibility of the waste PREP Ti-6Al-4V powder is essential in increasing

the efficiency of the PM process used by Boeing in the production of their parts.

This may lead to a decrease in losses incurred by discarding the powder as waste.

Therefore it is in Boeing’s interest to find a feasible method to improve the

usability of their wasted powder.

Countries who have titanium reserves in abundance, such as South Africa, need to

be at the forefront of producing innovative manufacturing procedures to process

titanium and its alloys. By 2020 South Africa is hopeful to have a titanium

industry that could amount to about R5 billion in revenue per annum and up to

10% of the international markets share (Clark, 2012). With innovative

technologies and manufacturing procedures, South Africa will gain the necessary

edge over its competitors to take advantage of a potentially emerging market.

4

2. LITERATURE REVIEW

The following sections will discuss the literature examined in preparation for

completing the project and its stated objectives.

2.1 Titanium and Titanium Alloys Overview

Titanium is a fairly abundant element, constituting approximately 1% of the

Earth’s crust compared to the most abundant metal, aluminium, which constitutes

approximately 8% (Groover, 2011). Due to titanium’s unique properties, its

importance in a vast array of industries has grown in recent decades. Titanium is

lightweight and possesses a very good strength-to-weight ratio which has led to its

use in the aerospace industry. General properties of titanium can be found in

Table 1 below (Groover, 2011). The principle ores from which titanium is

extracted are rutile and ilmenite. Rutile (TiO2) is preferred as an ore because it

contains a higher concentration of titanium than ilmenite (FeTiO3). Both ilmenite

and rutile are naturally abundant in South Africa with ilmenite accounting 90% of

South African production and rutile the other 10% (Clark, 2012). To recover pure

titanium from both of these ores, additional processing is required. Rutile (or

ilmenite) is reacted with chlorine gas to produce titanium tetrachloride (TiCl4)

which is subsequently distilled to remove impurities and form a highly

concentrated TiCl4. This highly concentrated compound is then reduced to

metallic titanium through a reaction with either magnesium or sodium; these are

known as the Kroll Process and Hunter Process, respectively (Groover, 2011).

Table 1: Properties of titanium

Symbol Ti

Atomic Number 22

Specific Gravity 4.51

Crystal Structure HCP (Hexagonal close-packed)

Melting Temperature (K) 1941

Elastic Modulus (GPa) 117

Alloying Elements Aluminium, tin, vanadium, copper, magnesium

Yield Strength (MPa) 170-485

Elongation % 12-25

Titanium is difficult to extract and process which has a direct correlation to its

high expense. The extraction processes are not only energy intensive/expensive

but they are also highly toxic as well. Even though the properties titanium

possesses are highly sought after, its high expense is the primary fact limiting its

more extensive use.

From Table 1 it can be seen that titanium has many alloying elements. These

alloys are used in a wide range of applications and although the Ti6Al4V alloy is

5

considered to be the most popular, there are a vast majority that are currently used

in industry today.

Table 2: Compositions and mechanical properties of selected alloys (Groover,

2011)

Typical Composition, % Tensile

Strength,

MPa

Elongation,

% Code

a

Ti Al Cu Fe V Other

R50520 99.8 0.2 240 24

R56400 89.6 6.0 0.3 4.0 1000 12

R54810 90.0 8.0 1.0 1Mo 985 15

R56620 84.3 6.0 0.8 0.8 6.0 2Sn 1030 14 aUnited numbering system

Table 2 is a comparison of selected commonly used titanium alloys along with

their mechanical properties, namely tensile strength and elongation. It can be seen

from this table that although the Ti6Al4V alloy has only the second largest tensile

strength, it has the lowest elongation percentage. Along with this data and that

presented in Figure 1, the decision to use titanium alloys in place of other more

commonly known alloys can be justified.

Figure 1: Ultimate Tensile Strength vs. temperature comparison between

different alloys (Goso & Kale, 2010)

The need to improve technologies involved with each production stage of a

typical titanium ingot can be further justified when looking at Table 3 below. The

information contained in Table 3 was adapted from van Tonder’s (2010) report.

6

Table 3: Production stage cost comparison between steel, aluminium and

titanium

In each production stage, titanium is considerably more expensive to process than

its competitors in industry such as conventional steel and aluminium. It can also

be noted that for titanium, the increase in cost in the ingot formation and sheet

formation production stages are more significant than costs involved with ore

extraction and metal refining. With advances in titanium processing technologies,

these prices will most certainly decrease.

Figure 2 below is a comparison in metal price listing for the Ti-6Al-4V metal

alloy and its competitors over the last three years. Cold rolled steel has been used

for this comparison as it is one of the more expensive ways to produce steel

products. Although the price for the Ti-6Al-4V ingot has decreased dramatically

of the last three years, it still has a significantly higher cost per lb weight than

both aluminium and steel. This is a direct result of the higher production stage

process costs seen in Table 3. In order for titanium and titanium alloys to be more

competitive in terms of their consumption, the processing cost at each production

stage needs to be decreased.

Figure 2: Metal price listing for titanium alloy and competitors

(www.metalprices.com/charts)

Production

Stage Units Steel Aluminium Titanium

Factor

to steel

Factor

to steel

Factor to

aluminium

Ore Extraction $/tonne 9.07 45.36 5 136.08 15 3

Metal Refining $/tonne 45.36 308.44 7 907.18 20 3

$/m3 1708.67 4027.58 2 20137.19 12 5

Ingot

Formation

$/tonne 68.04 317.51 5 2041.17 30 6

$/m3 2624.03 4149.63 2 44547.51 17 11

Sheet

Formation

$/tonne 204.12 1360.78 7 14741.75 72 11

$/m3 7780.56 17941.0

5 2 322816.87 42 18

7

2.2 Powder Metallurgy

PM is the manufacturing of commercial components from powdered metals and

alloys. Many different PM processes exist such as additive manufacturing, powder

injection moulding and press-and-sinter. The press-and–sinter process is the

simplest application of PM and it can be described as the compilation of four

different stages (Clinning, 2012): Powder manufacture, blending of powders,

compacting and sintering. The press-and-sinter technique can be further described

as follows:

Subsequent to the manufacturing of the powders, they are blended together to

form the required alloys to be compacted into a certain shape. The resulting

compact is termed the “green compact” and it will stay together due to mechanical

bonding from the compacting process as opposed to chemical bonds forming

between the particles during sintering (German, 2005). The green compact is then

heated to a temperature below that of the main element present in the compact.

This is known as sintering. The high temperature allows the individual powder

particles to chemically bond with one another which improves the mechanical

properties of the material. Figure 3 is a simple representation of the pressing

process of a green compact.

Figure 3: Simple pressing of a green compact (Clinning, 2012)

Die Filling Start of

Compaction

Specimen

Compacted

Part Ejection

8

2.3 Production of Titanium Powders

Titanium and titanium alloy powders are produced by a number of different

processes. The resulting powder characteristics from each process are different as

well as their end applications. Section 2.3.1 gives a brief overview of the different

processes used in the production of titanium powder whilst section 2.3.2 further

discusses the PREP process.

2.3.1 Overview of Production Processes

Chemical reduction, hydride/dehydride process (HDH), gas atomization and

plasma rotating electrode process (PREP) are the most practiced methods to

produce metal alloy powders. Table 4 lists the useful powder particle size, typical

powder particle shape and typical powder application for each of these processes.

Table 4: Typical titanium alloy powders (ASM, 2011)

Production

Technique

Typical Useful

Powder Size

Typical Powder

Shape

Typical

Application

Chemical Reduction <150µm Irregular Die compaction

Hydride/Dehydride

Process (HDH) <150µm Angular Die compaction

Gas Atomisation <100µm Spherical

Powder injection

moulding/ additive

manufacturing

Plasma Rotating

Electrode Process

(PREP)

<100µm Spherical

Powder injection

moulding/ additive

manufacturing

Both the angular and irregular shaped powders are better suited to die compaction

whereas the spherical powder particles are preferably used in powder injection

moulding and additive manufacturing. As can be seen from the SEM images of

the different powders, Figure 4, the irregular and angular powder particles are

clearly noticeable. It is this angular and irregular shape that allows the particles to

interlock with one another during die compaction. This interlocking leads to the

powders ability to be tightly packed and hold its shape once compacted. On the

other hand, spherical particle powders cannot be compacted as the compact

specimen is ultimately too porous due to the lack of interlocking between the

particles. Thus, spherical powders can only be used in the application mention in

Table 4: powder injection moulding and additive manufacturing.

9

Figure 4: SEM images of typical titanium alloy powders (ASM, 2009)

2.3.2 Plasma Rotating Electrode Process (PREP)

PREP is the most relevant production process to this project and as a result will be

discussed in more detail. PREP is a method of producing metal powders whereby

the end of a metal bar is melted while it rotates about a longitudinal axis as can be

seen in Figure 5. As the end of the bar melts, the molten metal is centrifugally

ejected and forms droplets which solidify into spherical powder particles, see

Figure 4. As this process relies on a plasma or electric arc to melt the

electrode/metal bar, the process is termed plasma rotating electrode process

“Sponge Fines” as a result of

chemical reduction

Hydride/Dehydride powder particles

Gas atomisation powder particles

PREP powder particles

10

Figure 5: Plasma rotating electrode process (ASM, 2011)

Figure 5 shows one of two machines, long bar machine, used to produce PREP

powders. The other type is known as a short bar machine and it is characterised by

its much shorter consumable anodes (ASM, 1998). It is reported that an estimated

80% of the length of the short bar is converted into powder and that removal and

introduction of new electrodes into the machine is carried out manually. In the

long bar machine, increased productivity and conversion efficiency are realized

(ASM, 1998). The ability for the machine to continually push successive long bars

through the seal housing, allows it to achieve almost 100% conversion rate from

bar to powder. In both of these machines, the rotational speed is used to determine

the particle size of the end powder.

There are many characteristics which make the PREP process highly suitable for

the fabrication of alloy powders. Firstly, it is a means of contactless melting and

atomization which results in powders with possibly the highest capable

cleanliness (ASM, 1998). This is a particularly important feature when it comes to

high-melting-temperature alloys, which in their molten state, are highly corrosive

and attack conventional ceramic crucibles. Titanium is one such alloy of which

others are zirconium, molybdenum and vanadium. Secondly, as the atomised

droplets are radially dispersed, there is little chance of collisions between the

particles. If the particles and droplets did collide they would merge and form

irregularly shaped clusters (Roberts, 1987). However, as they do no collide with

each other on a regular basis, the resulting PREP powder is almost perfectly

spherical and virtually satellite free (Roberts, 1987).

11

In general, the individual spherical nature of the powder particles results in PREP

powder being very free flowing and having a high packing density. Comparatively

speaking, gas atomisation and PREP are the two methods which produce spherical

particle shapes. The advantages of PREP over the gas atomisation process are that

it has a less dispersed particle size distribution and a larger median size particle

can be produced (ASM, 1998). The other main advantage that the PREP holds is

that because it is produced by centrifugal forces as oppose to aerodynamic drag,

the powder is essentially porosity free when compared to gas atomized particles

(Roberts, 1987).

2.4 Methods for Producing Ti-6Al-4V

There are two approaches used to manufacture parts from titanium alloy powders:

1) pre-alloyed approach and 2) the blended elemental approach. Each has their

own unique characteristics and this section will compare them on a conceptual

level to determine the best method to take forward into testing.

2.4.1 Pre-alloyed Approach

The pre-alloyed approach (PA) uses ready-mixed alloys prior to subsequent

alloying. In this approach, a coarse grain powder can be mixed with a finer

grained powder of the same composition. As these powders are both pre-alloyed,

their chemical compositions are known prior to mixing. Therefore powders of the

same chemical composition, irrespective of their particle size and shape, can be

combined and the overall final powder will have the same composition as its

alloying elements.

In order to improve the compressibility of the supplied PREP Ti-6Al-4V powder,

a finer grain Ti-6Al-4V powder will be blended with it. As there are no local

producers of the powder in South Africa, the powder will have to be sourced from

elsewhere. As such, with a struggling economy, sourcing the powder is expensive

considering the weak Rand/Dollar and Rand/Euro exchange rates. Table 5 is an

example of amount that can be paid for PREP Ti6Al4V powder. The relevant

quote can be found in Appendix B.

Table 5: Quote received for PA PREP Ti-6Al-4V powder

Supplier Particle Size (µm) Batch Size (kg) Cost

AP&C (Advanced

Powder & Coating)

0-25

5 1440 USD

(±R 15869)

10 2380 USD

(± R 26228)

12

The PA approach does however hold certain advantages over the blended

elemental approach. For one, there is no need to worry about the chemical

composition of the powders in the mixing process itself. This is because the PA

approach is carried out using powder created from an ingot of the desired alloy

produced by conventional techniques. Essentially, the PA powders have the same

chemical composition. The PA process, as previously mentioned, is expensive but

its ease of manufacturing near-net-shaped components justifies its use over the

cheaper conventional PM options. The PA process itself is expensive but costs

can be saved as little to no post machining is required on the near-net-shaped

components, which in turn, decreases the amount of wasted material (Clinning,

2012).

2.4.2 Blended Elemental Approach

The blended elemental approach (BE) requires elemental titanium powder to be

prepared (by one of the methods discussed in section 3) and then combined with

individual alloying elements or a 60Al:40V master alloy (MA). This is the

cheapest method to produce titanium alloy powders as the method can make use

of direct reduction powder (Froes et al, 2004). Direct reduction is a process of

using gas as a reducing agent and reducing an ore into a solid form. The PA

process, on the other hand, uses powder which has gone through an energy

intensive melting and casting operation which is of much higher expense than the

direct reduction method. Also, the BE approach is known for its ability to produce

alloys whose mechanical abilities surpass even those created through conventional

metallurgy (German, 2005).

The BE approach does however have its own limitations. It is known to produce

components with high impurities which hinder the achievement of high density

powders which ultimately decreases its mechanical properties (Froes et al, 2004).

Specifically for this project, where the powder at the end of the mixing process

must have the same chemical composition as the powder supplied by Boeing,

careful attention must be given to the mixing process. The commercially pure

(CP) elemental titanium powder and MA powder need to be blended with the Ti-

6Al-4V powder in such a way that the overall stoichiometry of the end powder

remains the same. In comparison to the PA approach, this is a certain

disadvantage. The MA powder will need to have a 60:40 Al:V composition ratio

to be able to mix with the CP titanium powder and form a Ti-6Al-4V alloy.

Conceptually when these two powders are then mixed together with the waste

PREP Ti-6Al-4V powder, the overall stoichiometry of the end powder will be

maintained. Practically however, it will have to be carefully monitored and

controlled to ensure this is indeed the case. The master alloy powder needs to be

pre-alloyed 6Al:4V for the BE approach to be effective. If aluminium and

vanadium were to be added to the overall mixture as elemental powders, the

13

aluminium would melt before the other metal powders and destroy the desired end

stoichiometry.

The University of Stellenbosch had already purchased the CP titanium powders

and the 6Al:4V MA powder needed for the BE approach prior to the

commencement of this project. Therefore if this method were to be used over the

PA approach, no extra powder would have to be imported at the high cost seen in

Table 5. The prices for the various BE approach powders evaluated were

gathered from invoices for the powders purchased in 2013 by Dr DC Blaine.

These prices can be found in Table 6. Unfortunately the 60Al:40V master alloy

powder was purchased by the University of Cape Town so there is no invoice

available for it.

Table 6: Price of elemental titanium and master alloy powders supplied by

Stellenbosch University

Powder Preparation

Method Batch Size (kg)

Cost (R) (incl. shipping

and customs)

-200 mesh titanium

powder HDH sponge fines 5.4kg 8550

-100 mesh titanium

powder HDH sponge fines 5.4kg 5700

60Al:40V master

alloy Crushed and milled NPA NPA

Considering the extra cost that will be incurred to the University of Stellenbosch

if the PA approach were to be used, it has been decided that the BE approach will

be the method of choice for this project. Not only will it minimise the cost of this

project to the University of Stellenbosch but the necessary powders for the BE

approach are far more accessible in terms of their availability. The compressibility

of BE powder mixtures are better than the PA powder mixtures. This is because

the CP titanium and 4Al:6V MA powders are more compressible than the PA

Ti6Al4V powder. Therefore there is a greater chance that the BE approach will

yield better results which further justifies its use.

14

2.5 Review of Previous Projects

Although this project does not specifically build on any previous projects, it may

still be of some use to compare the results obtained in this project to results

obtained from more conventional and wider used powders.

Table 7: Summary of results from study completed by Kirchener (2009)

Powder Characterisation

Supplier and Grade Alfa Aesar -200 mesh Ti powder

Flow Rate Powder did not flow

Apparent Density (g/cm3) 1.07

Particle Size - range (µm) 2 - 80

Particle Size – mean (µm) 32.27

Packing Density (%) 24

Powder Compaction

Cylindrical Specimens – Average Green

density (405 MPa) (g/cm3)

3.13

TRB Specimens – Average Green Density

(compaction pressure 380MPa) (g/cm3)

3.14

Green Strength (Mpa) 22.00

Sintering

Sintered Density (g/cm3) 3.99

Sintered Strength (MPa) 661.21

Table 8: Summary of results from study completed by Laubscher (2012)

Powder Characterisation

Supplier and Grade Global Titanium +325-100 mesh

Ti powder

Flow Rate Powder did not flow

Apparent Density (g/cm3) 1.38

Particle Size - range (µm) 5 – 140

Particle Size – mean (µm) 77.57

Packing Density (%) 30.6

Powder Compaction

Cylindrical Specimens – Average Green

density (500 MPa) (g/cm3)

3.38

15

TRB Specimens – Average Green Density

(compaction pressure 300-500 MPa) (g/cm3)

3.51

Sintering

Sintered [@ 1300 oC] Density (g/cm

3) 4.05

Sintered [@ 1300 oC] Strength (MPa) 1260

Table 7 and Table 8 summarise the results obtained by Kirchener (2009) and

Laubscher (2012), respectively, in their final year projects. Both Kirchener and

Laubscher used varied sizes of elemental titanium powder in their studies.

Although titanium is not an extensively used material, these results will be an

appropriate benchmark against which the current studies PREP Ti-6Al-4V powder

performance can be compared.

As previously mentioned, this project does not pick up from a previous students

work. However, this project does use previous student’s results and testing

procedures/parameters as a point of reference for the tests that will be conducted.

Also, these projects made use of equipment which is particularly applicable to the

current study such as the Amsler and Carver presses, vacuum furnace and MTS

load frame.

A problem which occurred in Kirchener’s study is that of delamination in some

of his samples after compaction. In particular, this problem occurred in his TRB

samples. The delamination was caused by inadequate lubrication of the TRB die

walls prior to compaction which caused significant friction between the die wall

and powder compact during ejection. The significant frictional forces caused the

compacts to crack during ejection.

The use of an industry proven die-wall lubricant is a suggestion made by

Kirschener to solve the issue of delamination. This may be a viable solution

however; the die-wall lubricant has the potential to contaminate the microstructure

of the compact samples. It is the hope that because of the larger size powder

particles used in this project, the issue of delamination will be less persistent. If it

is found that delamination is severe in the execution of this project then the die-

wall lubricant will be considered.

16

3. EXPERMINETNAL PROCEDURE

The procedure followed in executing the project experiment is represented

visually by the flow chart in Figure 7 below. Each step from the flow chart is

elaborated as follows:

3.1 Powder Characterisation

Step 1 in Figure 7: The PREP Ti-6Al-4V powder was characterised in order to

determine the particle size and distribution, flow rate, apparent density and

composition. These characteristics are important as they help to provide a better

understanding of the powder and its behaviour (German, 2012).

The particle size distribution of the PREP powder was measured using laser

diffraction. This technique measures the angular variation in intensity of light

scattered as a beam passes through a dispersed particle sample. The principle on

which this process is based is the theory that large particles scatter light at small

angles relative to the laser beams whilst small particles scatter light at large

angles. Therefore based on the angular scattering intensity data measured, the

particle size responsible for creating the scatter pattern can be determined (ASM,

1998).

The laser diffraction size distribution of the PREP powder was measured by Mrs

H Botha from the Process Engineering Department of Stellenbosch University.

The equipment needed to conduct the laser diffraction size distribution can be

seen in Figure 6 (a) [Make: Micromeritics®, Model: Saturn Digitizer]. The laser

diffraction results were then visually confirmed using a stereomicroscope system,

see Figure 6 (b). [Make: Olympus Model: SZX7 unit, KL 1600 LED light source,

ACH1X objective/camera, SC30 observation tube].

Figure 6: (a) Micrometrics Saturn DigiSizer (b) Olympus SZX7

stereomicroscope system

(a) (b)

17

2. Test Compressibility of PREP powder

3. Compare results (i.e. green density) to previous

studies performed on -200 mesh/-100 mesh titanium powders

4. Create Ti-6Al-4V PremixBlend the - 200 mesh titanium

powder with the master alloy powder

5. Blend Premix and PREP powderMix the two powders in the following

weight percent ratios (premix : PREP)

25:7550:5060:4075:2510:90

6. Test the compressibility of new Ti6Al4V mixture

Mixture is made up of the premix and original PREP powder

7. Analyze Results

8. Compact all three powder mixtures

(i.e. the five ratios) using a TRB die set

Results Acceptable

Use -100 mesh titanium powder in place of -200

mesh powder

Re-run test

Carver Manual Press Φ10mm cylindrical

die Compact at 500MPa

and 600MPa

9. Sinter TRB specimens

10. Strength test sintered specimensTransverse

Rupture Test

Vacuum Furnace

Amsler Automatic Press

1. Powder Characterisation

Figure 7: Flow chart of experimental procedure

18

The flow rate and apparent density were measured according to ASTM standards

B212 and B213. The flow rate of the powder is a good indication of the inter-

particle friction. Inter-particle friction refers to the resistance to movement of

particles in contact with one another. The apparent density is the density of the

powder when it is in its loose state without any agitation (ASTM, 2014). Both the

apparent density and the flow rate were determined using a Hall flowmeter and an

A&D FX-1200i precision scale which can be seen in Figure 8 a) and b)

respectively.

Figure 8: (a) Hall flow meter (b) A&D FX-1200i scale

The particle size distribution was also measured by sieving analysis acccording to

ASTM standard B214. The results from the sieving analysis were used to confrm

the laser diffraction results. The sieve analysis was conducted using Endecotts test

sieves.The size and order of the sieves were as follows (top to bottom) 710, 260,

425, 250, 180, 150, 106, 75, 45. Each of the formentioned sizes are in

micrometers (µm). The setup of the test can be seen in Figure 9 (a). The top mesh

has the greatest mesh opening size (710 µm) whilst the mesh right at the bottom

has the smallest mesh opening size (45 µm) which can be seen in Figure 9 (b) and

(c) respectively. The sieves are stacked in decending order in terms of their mesh

opening sizes.

(b) (a)

19

Figure 9: (a) Layout of sieve analysis equipment (b) Largest size mesh (c)

Smallest size mesh

3.2 PREP Ti-6Al-4V Powder Compaction

Step 2 of Figure 7: The original supplied PREP Ti-6Al-4V powder was

compacted at 500 MPa and 600 MPa as a baseline against which further powder

samples could be compared. By determining the current compressibility of the

PREP powder, any notable improvements through the addition of other powders

can be easily assessed. The powder was compacted using a Carver® 12 ton

manual press, see Figure 11, and a ∅10 mm cylindrical die set, see Figure 10 (a),

which was designed according to ASTM standard B 312-96. Table 20 of

Appendix C was required to determine the internal compaction pressure from the

gauge pressure reading of the hydraulic oil in the manual press.

Figure 10: (a) Cylindrical die set (b) TRB die set

(a) (b)

(a) (b)

(c)

20

Figure 11: Carver® 12 ton manual press

3.3 Using the Blended Elemental Approach to Mix Powders

Step 4 and step 5 of Figure 7: As previously discussed, the BE approach was used

to try and improve the compressibility of the PREP Ti-6Al-4V powder. The PREP

powder, elemental titanium powder and the master alloy powder were combined

to establish the ratios seen in Table 9.

Table 9: Mixing ratios used to create powder mixtures

PREP powder (weight %) Ti powder + MA powder (weight %)

75 25

50 50

40 60

25 75

10 90

Both -100 mesh titanium powder and -200 mesh titanium powder would be used

to create the mixture ratios seen above with the aim to later compare their results

against one another as well as against previous studies on just pure titanium

powder. From this point on, -100 mesh titanium will be referred to as the coarse

titanium powder and the -200 mesh titanium will be referred to as fine titanium

powder.

In order to ensure that the overall stoichiometry of the Ti-6Al-4V was maintained

during mixing, a theoretical analysis for the required mass of each powder at a

given ratio was conducted. This can be found in Appendix A.2. This calculation

was necessary as the atomic weight percent of each element in the powder needed

21

to be converted to a weight percent. Using this information, the amount of powder

necessary to satisfy the above ratios could be determined.

For the cylindrical die set, the powders were individually weighed using the A&D

scale, see Figure 8b, placed in a small container and then mixed. Due to the size

of the container, it was shaken by hand to mix the powders within it. In total, 2

grams of powder was used for each cylindrical die set compaction test. With the 2

grams comprising of the PREP powder, titanium powder and the master alloy

powder in the ratios mentioned in Table 9.

For the TRB die set, the total amount of powder needed (for each powder ratio) to

compact the required amount of specimens was first determined. The 75:25 and

50:50 powder ratios were excluded from the TRB compaction process for reasons

that will be discussed in Section 4.2.2. As a result, four batches of final power

mixture were made, one batch for each powder ratio. Each batch contained

enough powder to compact the required amount of TRB specimens. As each batch

of powder was now in a larger container than what was needed for the cylindrical

die set tests, the powder could be mixed not by hand, but rather by a mechanical

mixer seen in Figure 12.

Figure 12: Mechanical mixer

3.4 Compact Final Ti-6Al-4V powder mixture using Cylindrical Die Set

Step 6 of Figure 7: The Carver 12 ton manual press, see Figure 11, and the

cylindrical die set, see Figure 10 (a), were used to compact the different powder

mixtures into cylindrical specimens. Die wax was first applied to the inside of the

die to ensure ease of specimen ejection once compacted. To avoid excessive

density gradients within the die-set during compaction, it was advised that 2 g of

powder should be compacted at a time. First, 2 g of powder from the 75:25 final

mixture of powder was poured into the die and compacted at a compaction

pressure of 500 MPa. Another 2 g of the same powder was then subsequently

22

compacted at 600 MPa. This process was repeated for each powder ratio for both

the coarse and finer titanium powder mixtures. As in section 3.2, Table 20 was

used to convert from gauge pressure to compaction pressure found in Appendix C

had to be used.

The green density of each compacted specimen was then determined by dividing

the mass of the specimen by its volume. Rough edges that formed on the

specimens post compaction needed to be sanded flat with very fine grit sandpaper.

By removing the rough edges it allowed for the most accurate measurement of the

specimen’s volume. Measurements were carried out using a Mitutogo Absolute

Digimatic digital Vernier which was accurate to the nearest 0.01mm. In total 48

cylindrical specimens were compacted, 3 specimens for each powder ratio at each

compaction pressure (3 x 4 x 4 = 48). The green density of each cylindrical

specimen was determined.

3.5 Compact Final Ti-6Al-4V powder mixture using TRB Die Set

Step 8 of Figure 7: Once the green densities of the cylindrical specimens were

determined, the tests (i.e. compaction pressure and powder ratios) which produced

the best results were reproduced using the Amsler press, Figure 14 , and the

rectangular die set, Figure 10 (b). This step in the experiment was necessary so as

to produce rectangular specimens which could later be sintered and strength

tested. In a similar fashion to the cylindrical die-set, die wax was applied to the

inner die walls to ensure ease of ejection and avoid delamination.

A Spider 8 data acquisition system (600 Hz model), Figure 13, was needed to

operate the Amsler Press. The data acquisition system allowed for the forces

exerted by the 300 kN load cell to be visually seen as well as recorded. Table 21

in Appendix C had to be used in order to convert the force measured by the load

cell into powder compaction pressure. Appendix C also contains the calculation

on how to perform this conversion. It was determined that a force of 200 kN and

240 kN would produce compaction pressures of 500 MPa and 600 MPa

respectively.

Figure 13: Data acquisition system

23

There were a total of 24 TRB specimens compacted. For reasons explained in

section 4.2.2, the TRB specimens were only compacted at 500 MPa. The

specimen green densities were determined in a similar manner to the cylindrical

specimen green densities. All the rough edges were removed using the fine grit

sandpaper and then the specimen was measured and weighed from which, the

green density could be calculated.

3.6 Sintering of the TRB Specimens

Step 9 of Figure 7: Only halve of the compacted TRB specimens were sintered so

that a comparison could be made between their green strength and sintered

strength in the next phase of testing. The sintering system that was used

comprised of a vacuum system combined with a sintering oven. The vacuum

system is made up of an Adixen rotary vane pump [Model: Pascal 2012SD],

Varian turbo pump [Model: Turbo-V 81-M] and an Adixen vacuum gauge

[Model: ACS 2000] (Laubscher, 2012). The sintering oven used was an Elite 1500 oC Horizontal tube furnace [Model: TSH 15-50-180]. Figure 15 below shows the

vacuum furnace system in its entirety.

Figure 14: Amsler 25 ton automatic press

24

Figure 15: Vacuum furnace system

The oven crucible allowed for three specimens to be sintered at a time which

meant that four separate sintering attempts would have to be made to sinter all

twelve specimens. Once the specimens were placed in the crucible, the ends of the

crucible were sealed, as seen in Figure 16, to ensure that an effective vacuum

could be drawn. The rotary pump was then turned on and a vacuum was drawn to

below 0.133 mbar in order to remove any major contaminants. This vacuum was

sufficient to begin flushing the system with argon. The argon supply cylinder

pressure regulator was first set at 50 kPa and then the inlet valve to the crucible

was slowly opened to allow the argon to flow through the system. The inlet valve

to the crucible can also be seen in Figure 16. The inlet valve was turned until the

vacuum gauge measured a pressure of 6.26 mbar in the system. After 30 minutes

of flushing at this pressure, the inlet valve was closed to stop the flow of argon.

The argon flushing phase was now completed.

Figure 16: Furnace end-seal and argon inlet valve

The turbo pump could only be turned on once the rotary pump had removed the

remaining argon from the system. This was to protect the sensitive turbo pump

blades which would otherwise be damaged by the argon. The turbo pump was

necessary as it helped draw a larger vacuum which allowed for high sintering

temperatures to be achieved. The furnace was then programmed to reach 1300 oC

and sinter at this temperature for 2 hrs.

25

Once the specimen had been sintered at the desired temperature for the required

time, the furnace cooled to the point where the turbo pump could be switched off.

After allowing sufficient time for the turbo pump blades to stop rotating, the

system was backfilled with argon using the rotary pump. The backfill method is

essentially the same as the flushing method mentioned above. Once the system

was backfilled and the system/specimens had sufficiently cooled to be handled,

the specimens were removed and the process was repeated.

Subsequent to all the specimens being sintered, their sintered densities were

determined using the simple method of dividing the mass by the reduced volume

of the specimen which was measured once again using the Mitutogo Absolute

Digimatic digital Vernier calliper. The Archimedes principle was used to verify

the densities determined from these measurements. The calculation methodology

can be found in Appendix A.4.

3.7 Strength Testing

Step 10 of Figure 7: The last phase of the experimental procedure was to

determine the strength of the TRB specimens. The green and sintered strength of

all the TRB specimens was measured according to ASTM standard B312-96. For

the green (un-sintered) TRB specimens, this test was done using a MTS Criterion

model 44 load frame, MTS LPS 304 force transducer and MTS Testworks 4

software.

Figure 17 shows the setup of the entire system for the green strength tests will all

of the mentioned components. The transverse rupture strength (TRS) tooling used

to conduct the three point bend test can be seen in Figure 18 and the design

specifications of the tooling can be found in Laubscher’s (2012) report.

Figure 17: MTS tensile testing machine

26

Green specimens typically break at lower than 500 N so therefore the 1 kN load

cell, as appose to the 30 kN load cell, was used for strength testing the green

specimens. The reason for this is that the 1kN load cell would provide better

resolution if specimens broke at for instance, 100 N. To ensure that the force

applied by the load cell was evenly distributed across the specimen, the specimen

was checked for any surface defects or rough edges which may have developed

during handling and moving. If any were found then they would be sanded flat

using the fine grit sand paper.

Figure 18: TRS tooling

The 30 kN load cell did not have sufficient load capacity to fracture the sintered

TRB specimens. The TRS tooling would therefore be taken off the MTS load

frame and subsequently attached to the Amsler Press, Figure 14, as the 300 kN

load cell on the Amsler press would be more than capable of fracturing the

sintered specimens. Once again the rough edges resulting from compaction and

sintering were sanded away to improve the accuracy of the achieved results.

The applied load in the MTS load frame and Amsler press were the controlled

variable in the experiment. The applied load increased to the point where the

specimen failed. At this point of failure, the force measured by the respective load

cells was used to determine the rupture strength of the specimen: ultimately, the

sintered strength of the TRB specimen was determined.

27

4. RESULTS AND DISCUSSION

The following sections contain the results and discussions pertaining to the

different experiments mentioned. It is reiterated that in this section, -100 mesh and

-200 mesh Ti powder are referred to as coarse and fine Ti powder, respectively.

4.1 Powder Characterisation

The stereomicroscope images seen in Figure 19 show two different samples of the

PREP powder; the powder particles can be seen to be spherical in shape. This was

expected due to the general nature of powder particles produced weir the PREP

method. It can also be observed from Figure 19 that the particles range anywhere

from 89 µm to 235 µm.

Figure 19: Stereomicroscope images of PREP Ti-6Al-4V powder

The size distribution estimate of the powder particles can be verified from the

laser diffraction test results. As seen from Figure 20 the PREP powder particles

range from 50 µm to 400 µm with a mean particle diameter of 180 µm. The

complete set of results from the laser diffraction tests can be found in Appendix

A.3.

Figure 20: Cumulative particle size distribution of PREP Ti-6Al-4V powder

28

0

10

20

30

40

50

60

<45 45 75 106 150 180 250 425 560 710

We

igh

t %

re

tain

ed

Sieve mesh opening size (µm)

Sieve Analysis on PREP Ti6Al4V

First sieveanalysis

Second sieveanalysis

From the results obtained from Figure 19 and Figure 20, it can be seen that the

Stereomicroscope images and the laser diffraction results correlate with one

another. Supplied with the PREP powder was a metal powder certification which

gave an indication that the powder is +149 / -500 µm which once again correlated

with the achieved laser diffraction results and stereomicroscope images.

From the sieve analysis conducted on the PREP powder, the results of which can

be seen in Appendix A.1, the particle size distribution could once again be

evaluated. Figure 21 shows the particle size distribution obtained from the sieve

analysis.

Figure 21: Sieve analysis graph

For both sieve analysis tests, the 150 µm mesh retained the largest percentage of

powder. Therefore the powder particles were small enough to pass through the

180 µm screen but too large to pass through the 150 µm screen. This correlates

with the estimated mean particle size of 180 µm from the laser diffraction results.

The results from both sieve analyses show that the particles were greater than 106

µm and smaller than 425 µm. This particle size distribution again correlates with

the laser diffraction test as well as the reported size distribution form the metal

powder certificate.

The apparent density of the PREP powder was calculated to be 2.59 g/cm3, see

Appendix A.1. The exact density of the Ti-6Al-4V alloy varies according to

chemical composition but it is typically 4.42 g/cm3

(ASM, 2014). This

information can be used to determine the packing density of the powder which is

calculated as the ratio of the apparent density to the alloy’s density. The packing

density for the PREP powder is 58.6%.

29

The flow rate test indicated that the powder flowed at a rate of 39 s/50g powder,

see Appendix A.1. This reiterates the properties that PREP powders flow freely

and the powders particles do not agglomerate. The fact that the powder does flow

indicates weak inter-particle frictional forces between particles (i.e. the powder

displays low resistance to particles slipping past one another). This is an important

result in terms of the powders compressibility as in general, powders which flow

freely, allow for high production rates for compaction.

Table 10 is a summary of the results obtained from the characterisation of the

current studies PREP Ti-6Al-4V powder as well as the results obtained from

previous studies on purely titanium powder.

Table 10: Comparison between current study and previous study powders

PREP Ti-6Al-4V

powder

Laubscher

(2012)

Kirchener

(2009)

Powder Type PREP Ti-6Al-4V HDH Ti HDH Ti

Apparent density

(g/cm3)

2.59 1.38 1.07

Flow Rate (s/50g) 39 Did not flow Did not flow

Packing Density (%) 58.6 30.6 24

Mean Particle

Diameter (µm) from

laser diffraction

180 77.57 32.27

It can be seen that due to the smaller mean particle size for both of the previous

studies, the apaprent densities of these powders are much lower than the apparent

density for the current study. The reason for this is that a larger number of pores

form between smaller particles. This is also evident in the packing density values.

The current study’s powder possess a packing density of 58.6% which translates

to the powder having 41.4% porosity between its particles when freely packed.

The powders from Laubscher (2012) and Kirchener (2009) both possess 69.4%

and 76% porosity, respectively, between their particles.

30

4.2 Powder Compaction

As previously mentioned, the aim of the powder compaction was to determine the

green densities of the different powder ratio combinations at different compaction

pressures. Compaction was first conducted using the cylindrical die-set and then

using the TRB die set.

4.2.1 Cylindrical Die-Set Compaction

As mentioned in section 3.2 of the experimental procedure, an attempt was made

to compact the PREP powder in its existing state. The powder was first compacted

at 500 MPa and then again at 600 MPa. At both compaction pressures the powder

failed to compact into the cylindrical shape of the die; the powder compact

disintegrated upon ejection. This was expected due to the nature of the particle

size and shape, as well as, the result from the flow rate test conducted on the

powder. Therefore it was concluded that the current powder possessed no

compressibility.

The next compaction attempt was the 75:25 powder ratio of PREP : Ti + MA

powder. The fine titanium form of this powder ratio was compacted at 500 MPa

and 600 MPa, Figure 22 (a) and (b) respectively.

Figure 22: Ejection of compacted Ti6Al4V BE 75:25 powder mixture using

fine Ti powder at (a) 500 MPa (b) 600 MPa

It can be seen that in the case of the 500 MPa compaction pressure, as soon as the

specimen was ejected from the die it disintegrated. At 600 MPa compaction

pressure, the specimen stayed intact but then began flaking and subsequently fell

apart shortly after it was ejected. As a result, it was decided that the 75:25 powder

ratio would not contribute any meaningful results to this study and was

consequently excluded from any further testing. Only the 50:50, 40:60, 25:75 and

10:90 ratio powders will be investigated and discussed for the remainder of this

study.

(a) (b)

31

3.300

3.350

3.400

3.450

3.500

3.550

3.600

50:50 40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

Compressibility Chart at 500MPa and 600MPa

500MPawith fine Ti

600MPawith fine Ti

500MPawith coarseTi600MPawith coarseTi

The remainder of these powder ratios were compacted at 500 MPa and 600 MPa

using both the coarse and finer titanium powder. The volume and weight of each

green specimen was recorded and then used to determine the specimen green

density. The green density results can be found in Appendix D.1. The average

green density of the three samples for each powder ratio at each compaction

pressure was determined and represented in the form of a compressibility chart as

seen in Figure 23.

Figure 23: Green density of cylindrical specimens

The error bars on Figure 23 show the largest and smallest deviation of the

measured densities from the average density. It can therefore be interpreted from

these error bars that compaction was done accurately as there is very little

deviation around the mean. Also from Figure 23, the general trend of the green

densities for both the coarse and fine Ti powder mixtures increases as the ratio of

premix to PREP powder increases. This being said, for each of the tests

conducted, the 10:90 powder ratio showed a decrease in green density. This

decrease can be seen for each compressibility chart, Figure 38 to Figure 41, in

Appendix D.1.

A reason for this decrease in green density, at a powder ratio of 10:90, is that the

Ti-6Al-4V powder mixture used for compaction is almost entirely comprised of

the elemental titanium powder (i.e. 82.96% of the mixture is either fine or coarse

Ti powder). Both the fine and coarse titanium powders possess packing densities

and mean particle diameters which are far less than those for the PREP powder

(Table 10). Thus, the smallest amount of pores between powder particles exists in

32

the 10:90 powder mixes. This results in the drop in green density, as seen in the

compressibility charts, because there are fewer vacancies in the loose powder

which can be filled by the powder particles during compaction.

From Figure 38 to Figure 41, it can be seen that there is a slight variation in the

results achieved for each of the attempted tests, as was evident from the error bars

of Figure 23. This could be due to the inaccuracy of the Carver press pressure

gauge which gives an indication of the oil pressure within the press itself. Due to

the ratio of oil pressure to compaction pressure, 1:25, a reading error of 1MPa on

the gauge could result in a compaction pressure error of 25 MPa. This could have

an effect on the green densities achieved.

Ejection pressure was not considered as a die wax was used during compaction of

the cylindrical specimens. Specimen delamination was occurring as a result of the

high ejection pressures caused by friction between the die-wall and specimen. As

high ejection pressures contribute to density gradients and tool wear it was

decided that the use of die-wax was justified. The ejection pressure peaked at

about 7 MPa gauge pressure when die wax and a compaction pressure of 600 MPa

was used. As this ejection pressures was significantly lower than the compaction

pressure it was not included in the results. After each specimen was ejected, the

inside of the die-set was cleaned to ensure cross contamination of powder

mixtures did not occur.

Figure 23 clearly shows that the optimal powder ratio is 25:75 for both the fine

and coarse Ti mixtures. However, this is not the most economically viable powder

ratio as it uses the second least amount of waste PREP Ti-6Al-4V powder for a

given amount of mixture. The most ideal powder ratio to make use of would be

the 50:50 powder ratios as they would be the cheapest to produce (due to the

reduced amount of titanium and master alloy powder required). Unfortunately, as

this ratio produced the lowest green densities, the 50:50 powder ratio mixtures

were not taken forward into the next phase of testing.

4.2.2 TRB Die-Set Compaction

As mentioned in section 4.2.1, the 40:60, 25:75 and 10:90 powder ratios were

used to create a total of 24 TRB compact specimens. Four specimens of each

powder ratio were compacted for both the fine and coarse Ti powder mixtures.

The original plan was to compact the specimens at a compaction pressure of 500

MPa and 600 MPa as mentioned in the experimental procedure, section 3.5.

However, a problem arose in that the spacer used with the TRB die-set buckled

during the first compaction test; see Figure 24 (a). Buckling occurred before the

compaction pressure of 600 MPa was achieved and therefore a new spacer had to

be designed. The specification of the old spacer can be found in Laubscher’s

(2012) report. The new spacer was made according to the technical drawing found

33

3.390

3.400

3.410

3.420

3.430

3.440

3.450

3.460

3.470

40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

TRB Compressibility Chart at 500MPa

Fine Timixture

Coarse Timixture

in Appendix F. This spacer was 5 mm smaller in height and also made of 1mm

thicker steel, see in Figure 24 (b).

Figure 24: (a) Buckled spacer (b) Re-designed spacer

The spacer was used during compaction in order to keep the test procedure

consistent with Laubscher (2012) so that a comparison could be made between the

values achieved in his project and the values achieved in this one. Refer to section

7 of this report for the recommendation on the use of the spacer.

The new spacer was placed between the die and the lower punch and an attempt

was made to compact a TRB specimen at 600 MPa. Once again the spacer was

showing signs of yielding, see Figure 24 (b), before the 600 MPa was achieved.

The thickest steel available in the engineering workshop at the University of

Stellenbosch was used to make the new spacer. Thus, it was decided that a

compaction pressure of only 500 MPa would be used for the remainder of the

tests. Figure 25 shows the average green density achieved for the different powder

ratios, using both the fine and coarse titanium powders, at a compaction pressure

of 500 MPa.

Figure 25: TRB specimen green density

(a) (b)

34

3.34

3.36

3.38

3.4

3.42

3.44

3.46

3.48

3.5

40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

Green Density Comparison (TRB and Cylindrical) at 500MPa

Fine Tiaverage - TRB

Coarse Tiaverage - TRB

Fine Tiaverage -cylindrical

Coarse Tiaverage -cylindrical

The individual results for each specimen can be seen in Figure 42 and Figure 43

in Appendix D.2. The TRB green specimens followed the same general trend as

the cylindrical green specimens in that the optimal powder ratio was determined

to be the 25:75, as can be seen in Figure 25. This trend is also visible in Figure 42

and Figure 43. From the error bars in Figure 25, it can be seen that the deviation

of the measured results from the mean increases as the amount of PREP powder

decreases. More samples should be compacted to determine whether this is a

recurring feature or just a slight error in the compaction pressures applied in these

tests.

The fine titanium powder mixture achieved higher green density values for both

the 40:60 and 25:75 powder ratios and lower green density values for the 10:90

powder ratio when compared to the coarse Ti mixtures. This outcome was also

evident in Figure 23 when the cylindrical specimens were compacted at 500MPa.

Although the coarse Ti powder mixture produced lower green density values on

average, its optimal powder ratio was also 25:75. There is however a noticeable

variation in the green density values, at 500M Pa, when comparing the cylindrical

and TRB specimens. This variation can be seen in Figure 26.

Figure 26: TRB and cylindrical green density comparison

When looking at the fine titanium mixtures, the cylindrical specimens possess a

higher green density at the 40:60 and 25:75. At a powder ratio of 10:90, the green

density value for the cylindrical specimens drops off sharply to 3.37 g/cm3. This is

lower than the green density value of 3.414 g/cm3 achieved with the TRB

specimen.

35

For the coarse titanium mixtures, the TRB specimen green densities are all higher

than the cylindrical specimen green densities for the given compaction pressure. It

can be noted that the trends for the coarse titanium TRB and cylindrical

specimens are almost parallel to one another as the green densities vary with the

different powder ratios.

Specimen geometry does have a definite effect on green density as can be seen in

Figure 26. It is interesting to note that there is an opposing relationship between

specimen geometry and achievable green densities when looking at the fine and

coarse Ti mixtures. For the coarse Ti mixtures, the TRB green densities were

higher than the cylindrical green densities for each powder ratio. This could be

due to the fact that the compacting surface area is larger for the TRB specimens

than it is for the cylindrical specimens which lead towards a greater green density

being achieved. The fine Ti mixtures should in theory produce a similar pattern to

what was achieved with the coarse Ti mixture. However, the cylindrical green

densities were higher than the TRB green densities for each powder ratio except

the 10:90 mixtures. Thus, it can be said that powder type and specimen geometry

have an effect on achievable green density.

Table 11 shows a comparison of the TRB green densities achieved in the current

and past projects.

Table 11: Average green densities for TRB specimens

Average Green Densities at 500 MPa Compaction Pressure (g/cm3)

Current Study Laubscher Kirchener

Ti-6Al-4V (100

mesh Ti mixture)

Powder

ratio

Green

density

3.68 ±3.395

40:60 3.436

25:75 3.456

10:90 3.433

Ti-6Al-4V (200

mesh Ti mixture)

40:60 3.441

25:75 3.460

10:90 3.414

When comparing the compressibility of the current studies PREP Ti6Al4V

mixtures to previous studies conducted on HDH titanium powder, it can be seen

that the powder of the current study performs relatively well. The Ti-6Al-4V

powders produce green density values which range from 3.414 g/cm3 to 3.460

g/cm3. These values lie in between the 3.68 g/cm

3 and 3.395 g/cm

3 achieved by

Laubscher and Kirchener respectively. As this study focusses on making a waste

PREP powder usable, the compressibility results are an indication that this is

indeed possible.

There were a series of issues involved with the TRB specimen compaction which

may have led to discrepancies in the results. For instance, to achieve a compaction

36

3.700

3.800

3.900

4.000

4.100

4.200

4.300

4.400

4.500

4.600

40:60 25:75 10:90

Sin

tere

d D

en

sity

(g/

cm3 )

Powder Ratio (PREP : Ti + MA)

TRB Specimen Sintered Density

Fine TiSpecimens

Fine Ti (averageline)

Coarse TiSpecimens

Coarse Ti(average line)

pressure of 500 MPa for the TRB die-set, a force of 201.61 kN was required as

previously mentioned. Stopping the press when it has exerted a force of 201.61

kN repeatedly is near impossible, so there was a compaction error involved as

exactly 500 MPa was not achieved for each compaction attempt. It was also noted

that when the Amsler press was stopped to end each compaction attempt, the

vibration induced by the motor switching off caused a large spike in the force

measured by the data acquisition system. Whether or not this spike in the force

affects the compaction pressure is not for certain, but for the purpose of this report

it is worth mentioning.

4.3 TRB Specimen Sintering

The TRB specimens were sintered at 1300oC. The vacuum furnace system was set

to increase the temperature at 10 oC/min until it reached the 1300

oC target. As

three specimens could be sintered at a time, four lots of sintering had to be done in

order to sinter all twelve TRB specimens. Completing the sintering process for

one set of specimens took 9.5 hours which in turn meant it took 38 hours to

completely sinter all of the specimens.

Once all of the specimens were sintered their sintered densities were determined

according to section 3.6 above. Figure 27 shows the sintered densities achieved

for both the fine and coarse Ti mixtures at the different powder ratios.

Figure 27: TRB sintered densities

37

As can be seen in Figure 27 and Table 12, the fine Ti powder mixtures yielded the

best results post sintering. The fine Ti specimens produced, on average, larger

sintered densities for each powder ratio when compared to the coarse Ti mixtures.

The most surprising result after sintering the specimens was that the 10:90

mixtures consistently produced the highest sintered densities. After the green

density measurements revealed that the 25:75 powder ratio was the optimal

mixture, it was expected that this would also be the case with the sintered

densities. However, when compared to the TRB green densities, the sintered

density values for the 10:90 mixtures increased by the greatest amount. The

reason for this is that the 10:90 mixtures contain the largest amount of titanium

powder which, due to its larger powder particle surface area, possesses the best

sinterability.

Table 12 shows a comparison between the PREP Ti-6Al-4V powders’ sintered

densities and the sintered densities achieved by Laubscher using the same press-

and-sinter process.

Table 12: Average TRB sintered densities

Average Green Densities at 500 MPa Compaction Pressure (g/cm3)

Current Study Laubscher

Ti-6Al-4V (100

mesh Ti mixture)

Powder ratio Green density Green density

40:60 3.86

4.15

25:75 4.00

10:90 4.25

Ti-6Al-4V (200

mesh Ti mixture)

40:60 4.05

25:75 4.22

10:90 4.44

The PREP powder sintered densities fluctuate around the average sintered density

achieved by Laubscher (2012). The expectation was that the Ti-6Al-4V mixtures

should have produced sintered densities lower than those achieved by Laubscher.

This is because Laubscher used pure titanium powder whereas the highest

percentage of titanium used in the current study was 82.96% for the 10:90 powder

mixtures. An influencing factor which could account for these results is the fact

that more powder was used for each TRB specimen in the current study, 14 g, as

opposed to the 9 g used by Laubscher. The slightly larger PREP Ti6Al4V

specimens could produce the higher than expected densities which were found

here. More samples should be compacted at 500 MPa and sintered at 1300 oC to

determine the accuracy of the results and ultimately if the trend of Figure 27 is

valid.

38

4.4 Strength Testing

The last phase of the experimental procedure, step 10 of Figure 7, was to

determine the transverse rupture strength (TRS) of the TRB specimens. There

were twenty four strength tests conducted in total, twelve green specimen tests

and twelve sintered specimen tests. This was done in order to investigate the

effect that the sintering process has on the strength of a specimen.

4.4.1 TRB Green Strength

Green specimen strength testing was done using the MTS load frame, Figure 17,

along with the TRS tooling, Figure 18. The specimens were placed in the TRS jig

and a force applied to the specimen mid-section until it fractured as in Figure 28.

Figure 28: Fractured green specimen

The specimen in Figure 28 is larger than the specimens tested by Laubscher

because to reproduce his tests, more powder had to be used to achieve an

effectively compacted specimen. In Appendix E.1, the applied load is plotted

against the MTS load frame crosshead displacement. It can be seen that the green

strength dramatically increases as the amount of PREP powder in the mixtures

decreases (i.e. from 40:60 to 10:90). For each powder ratio, the fine Ti mixtures

outperformed the coarse Ti mixtures. This was due to the higher force required to

fracture each of the fine Ti specimens.

It is interesting to note that for the 40:60 powder mixtures, Figure 44, the coarse

Ti mixtures displayed some ductility prior to failing. The force peaked at around

27 N and 22 N for the two tests conducted but then each of them decreased by

about 9 N and 5 N respectively before they truly failed. This phenomenon only

occurred for these tests as the rest of the specimens ruptured abruptly at a certain

applied load.

39

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

40:60 25:75 10:90

Gre

en

Str

en

gth

(M

pa)

Powder Ratio (PREP : Ti + MA)

TRB Specimen Green Strength

Fine Tispecimens

Fine Ti(average line)

Coarse Tispecimens

Coarse Ti(average line)

To calculate the green strength from the applied load, the following relationship

had to be used:

𝐺𝑆 = 3×𝐹𝑏×𝐿

2×𝑇2×𝑊 (4.1)

where GS is the green strength in MPa, Fb is the breaking force required to rupture

the specimen, L is the length between the supports on the bottom half of the TRS

tool, T is the thickness of the specimen and W is the width of the test specimen.

The results from the green strength tests can be seen in Figure 29 and Table 13.

Figure 29: TRB specimen green strength

The fine Ti mixtures produced higher green strengths than the coarse Ti mixtures

for each of the mixture ratios. Essentially the fine Ti specimens were more

resistant to rupturing under an increasing force. However, there is large concern

that the green strengths achieved are lower than what is generally desirable

(German, 2005).

Green strength is important in powder metallurgy processing, such as the press-

and-sinter process, for a number of reasons. Lower green density values generally

indicate specimen vulnerability which is of particular concern to parts

manufactures (German, 2005). This vulnerability typically manifests itself in the

form of specimen delamination and specimen cracking which can lead to

handling, and more importantly, automated handling problems (King et al, 2005).

It is this automated handling problem which creates the concern from the

40

manufacturer’s point of view as parts could be damaged at any stage of their

production process.

Table 13: Average TRB specimen green strength and breaking force

Average Green Strength at 500 MPa Compaction Pressure (g/cm3)

Current Study Laubscher

Ti-6Al-4V (100

mesh Ti mixture)

Powder ratio Green strength

(MPa) Green strength (MPa)

40:60 0.933

43.06

25:75 3.431

10:90 6.609

Ti-6Al-4V (200

mesh Ti mixture)

40:60 0.702

25:75 2.345

10:90 5.358

Typically, it is desirable for parts used in industry to have green strengths of over

10 MPa (German, 2005). All of the specimens tested in this project achieved

green strength values of lower than 10 MPa. It is expected that the low green

strengths were due to issues surrounding the use of the spacer during TRB

specimen compaction. Recommendations will be given in section 7 on how to

improve the testing process so that perhaps more accurate results can be achieved

in any future development of this topic.

4.4.2 TRB Sintered Strength

The Amsler press, Figure 14, was used to strength test the sintered TRB

specimens. Unfortunately by using the Amsler press, crosshead displacement

could not be measured as was the case with the MTS load frame. To determine the

TRS of the sintered TRB specimens, equation 4.1 was used as was done when

calculating the green specimen TRS’s.

According to Figure 30, the strength of the sintered TRB specimens increases

with decreasing amount of PREP Ti-6Al-4V powder. In terms of the specimen

strength performance, the fine Ti powder mixtures produced higher rupture

strengths for each powder ratio when compared to the coarse Ti mixtures. If one

compares the general trend lines of Figure 27 and Figure 30, it is expected that the

fine Ti specimens perform better than the coarse Ti specimens. This is because

density and strength are directly related to one another and seen as though the fine

Ti mixtures produce higher TRB specimen sintered densities, they should in

theory also have higher strengths. This is reflected in the results obtained. There is

a very clear relationship which has been developed between the amount of PREP

powder in the overall mixture and the specimen transverse rupture strength: the

higher the amount of PREP powder, the lower the transverse rupture strength.

41

0

200

400

600

800

1000

1200

1400

1600

40:60 25:75 10:90

Sin

tere

d S

tre

ngt

h (

MP

a)

Powder Ratio (PREP : Ti + MA)

TRB Sintered Strength

Fine Tispecimens

Fine Ti(average)

Coarse Tispecimens

Coarse Ti(average)

Figure 30: Sintered strength of the TRB specimens

The increase in strength is more gradual between the 40:60 and 25:75 powder

ratios when compared to the increase in strength between the 25:75 and 10:90

powder ratios. This is also marginally evident in Figure 27 when looking at the

sintered densities of the TRB specimens. This correlation between the sintered

strength and sintered densities indicates that the strength test results are accurate.

Table 14 gives a comparison between the transverse rupture strengths achieved in

this project, compared to the results obtained by Laubscher (2012).

Table 14: Average TRB sintered strength

Average Sintered Strength at 500 MPa Compaction Pressure

Current Study Laubscher

Ti-6Al-4V (100

mesh Ti mixture)

Powder

ratio

Sintered strength

(MPa) Sintered strength (MPa)

40:60 773.97

~1510

25:75 943.69

10:90 1397.41

Ti-6Al-4V (200

mesh Ti mixture)

40:60 512.99

25:75 739.86

10:90 1274.64

It can be seen that the Ti-6Al-4V powder mixtures all produced lower TRS values

than what was achieved by Laubscher. It valuable to note however, that the 10:90

fine Ti mixture produced strength values very close to what Laubscher achieved,

with only a 7.46% difference between the two. One needs to remember that the

42

Ti6Al4V specimens were slightly larger than the specimens tested by Laubscher.

Therefore the results need to be viewed from the point of view that if the same

size specimens were used, lower sintered strengths would most probably be

achieved. This being said, the results obtained still prove that waste PREP powder

can be incorporated in a press-and-sinter process and produce high strength

specimens.

43

5. RISK ASSESSMENT

As with any project involving the use of machinery, there are inherent dangers

that can cause harm to the operator if the right safety procedures are not followed.

In the case of this project, areas of potential risk were the use of the Amsler press,

MTS load frame and vacuum furnace system. Safety documents for each of these

pieces of equipment have been submitted separate to this report as they are

themselves quite detailed. The safety reports were compiled before using the

respective piece of equipment to demonstrate that the health and safety

regulations regarding the use of the equipment were known and that the right

precautions would be taken in the event of an emergency. Safety Reports are also

essential as they create awareness of potential areas of danger concerning the

machinery as well as the testing environment.

The full details concerning safe operating procedures and general housekeeping of

the test areas are discussed in the safety documents themselves. Below is a

summary of the more general risks and procedures associated with using the test

equipment.

There should always be more than one person present when using any of the

equipment to offer advice and ensure that the equipment is handled properly as

well as safely operated. They can also provide help in the case of an emergency.

The location of the fire escapes as well as the nearest fire extinguishers should

also be known when using the vacuum furnace as it is a definite fire hazard.

Proper ventilation through the testing area should also be ensured when using the

vacuum furnace. The reason for this is that when the system is flushed or

backfilled with argon, the backing pump expels this argon gas into the air which

could make occupants of the test area uncomfortable if it is not properly

ventilated.

Hands and loose clothing should be kept away from the Amsler press and MTS

load frame as both of these machines have the ability to crush anything caught in

their clamps/fixtures. In terms of protective clothing, the necessary protective

shoes and protective eye wear should be worn where applicable. Protective eye

wear is particularly applicable when strength testing the sintered TRB specimens.

This is due to the fact that the rupture forces are so large that when failure occurs,

the two halves of the specimen are flung into the air.

44

6. CONCLUSION

The purpose of this project was to assess whether or not waste PREP Ti-6Al-4V

powder could be made usable through the blended elemental approach and tested

using a conventional powder metallurgy technique: the press-and-sinter process.

In this report, the objectives and motivation for the mentioned project was given

and the general properties and preparation methods for titanium powders was

discussed. Powder metallurgy and more specifically the press-and-sinter process

were also discussed in this report. Lastly, the experimental procedure along with

a detailed description of the necessary equipment was also given.

There were 48 cylindrical specimens compacted in total and from the green

densities calculated for each cylindrical specimen, the optimal powder ratio was

determined to be 25:75. The results obtained for the cylindrical specimen green

densities also allowed for the 75:25 and 50:50 powder ratios to be excluded from

the further testing as they yielded unfavourable densities.

Full scale testing successfully commenced with the compaction of 24 TRB

specimens at a compaction pressure of 500 MPa. The green densities calculated

for each TRB specimen revealed that the optimal powder ratio remained at 25:75.

It can be concluded from the compaction results that the type of titanium powder

used, either fine or coarse, has an influence on achievable green density value.

Specimen geometry is also determined to be an influencing factor when looking at

achievable green densities but it has the opposite effect on the fine Ti and coarse

Ti mixtures.

Halve of the TRB specimens were successfully sintered at a temperature of

1300oC for duration of 2 hours. This allowed for a comparison to be made

between green strength and sintered strength. The resulting sintered densities

revealed that the optimal powder ratio for both the fine Ti and coarse Ti mixtures

was no longer 25:75 but rather 10:90.

Finally, Strength testing was conducted on all of the TRB specimens. The results

revealed that the sintered specimens are significantly stronger than the green

specimens when tested for transverse rupture strength. The strongest sintered

specimen showed a 7.54% difference in strength when compared to a pure

titanium specimen tested under the same parameters.

The results obtained from this project show that waste PREP powder can be made

usable through the master alloy approach and implemented in the press-and-sinter

process. The effect of sintering time and compaction pressure on transverse

rupture strength should be investigated in further projects so that the most

commercially viable production option can be determined. This project has

determined that the optimal powder ratio is 10:90 (PREP : -200 mesh Ti + MA)

for the given sintering and compaction parameters. This is the least economical

45

powder ratio as it uses the least amount of waste powder, but it is certainly more

cost effective solution than completely discarding the waste powder. As a proof of

concept, this project can be deemed successful. Not only this, but the results

obtained in this project can be used and built upon in any future development of

this topic.

46

7. RECOMMENDATIONS

The following recommendations can be made after the completion of this project:

The die-wall lubricant did not seem to have an effect of the compressibility of the

powder mixtures and should therefore be considered in future projects for its ease

of specimen ejection from both the cylindrical and TRB die-set.

The largest area of concern for this project was the TRB specimen compaction.

The original spacer buckled prematurely and the re-designed spacer began

buckling at a compaction pressure of over 540 MPa. Also, the use of both the

original and re-designed spacer created too large a gap between the top punch and

the bottom punch which resulted in the powders not being completely compacted

to the desired 500 MPa compaction pressure. More powder could have been used

for each compaction test, but this could have inadvertently led to density gradient

issues within the die during compaction. So it is therefore recommended that the

spacer not be used for any future TRB compaction tests.

When the TRB specimens are sintered, each specimen is placed in steel cylinders

which have one end open and one end sealed. It is recommended that these

housing cylinders not be used and rather a cylinder with both ends open be used

instead. If this is not possible then one should ensure that the open end of the

cylinder is facing the direction of the turbo/backing pump so that the air can be

drawn out of the housing when the vacuum is created.

The housing cylinders should also be checked prior to use as some of them

showed signs of surface delamination which could cause specimen contamination.

The vacuum furnace itself should also be checked and cleaned if necessary to

prevent further specimen contamination.

There should be more TRB specimens compacted and sintered to improve the

accuracy of the results as well as ensure that the tests are repeatable.

Higher compaction pressures as well as other variables such as sintering

temperature and sintering time should be investigated to determine what effect

they have in green and sintered density as well as sintered strength of the

compacted specimens.

TRS testing on the sintered TRB specimens should be conducted on the

University of Cape Town’s Zwick 100 kN testing rig, as opposed to the Amsler

press, if possible as the load cell range is closer to the test results which would

provide better resolution. Crosshead displacement can also be monitored on this

machine.

47

8. REFERENCES

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its Alloys”, [Online] Available From:

http://www.tms.org/pubs/journals/JOM/1005/boyer-1005.html

Campbell, K. 2013,”SA moves to use titanium-ores platform to build new high-

tech industry” [Online] Available From:

http://www.engineeringnews.co.za/article/sa-moves-to-use-titanium-ores-

platform-to-build-new-high-tech-industry-2013-08-30

Clark, J, 2012, “R5bn titanium Industry Planned for South Africa by 2020”,

[Online] Available From: http://www.moneyweb.co.za/moneyweb-south-

africa/r5bn-titanium-industry-planned-for-sa-by-2020?sn=2009+Detail

Van Vuuren, D.S, 2009 “Titanium—an opportunity and challenge for South

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Pohlman N.A, Roberts J.A, Gonser M.J, 2012, “Characterization of titanium

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ASM International, 1998 “ASM Handbook. Vol 7, Powder metal technologies

and Application”, Handbook Committee

ASM International, 2011 “ASM Handbook. Vol 7, Powder metal technologies

and Application”, Handbook Committee

German R M, 2005.“Powder Metallurgy and Particulate Processing”. Metal

Powder Industries Federation

H.H. Laubscher. 2012. “Press and Sinter Processing of HDH Ti Powder”,

Mechanical Project 478 Final Report, Department of Mechanical Engineering.

Stellenbosch

Groover, M P, 2007, “Fundamentals of Modern Manufacturing”. John Wiley &

sons inc.

Callister WD, R. D. (2011). Material Science and Engineering. Asia: John Wiley

& Sons Pte Ltd.

Goso, X and Kale, A. 2010. “Production of Titanium Powder by the HDH

Process”, Advanced Metals Initiative, Light Metals Conference

48

Froes, F H and Eylon, D, 1990, “Powder Metallurgy of Titanium Alloys”, Int.

Mater Rev, Vol 35

Moxson, V, Senkov, O N, Froes, F H, 1997, “Production and Charaterization of

Titanium Powder Products for the Environtal, Medical and Other Applications”,

Advanced Particulate Materials and Processes, F.H. Froes and J.C. Hebeisen, Ed.,

American Powder Metallurgy Institute

Froes, F H and Aeolian, D, 1984, “Production of titanium Powder”, Metals

Handbook 9th

ed. , American Society for Metals ,Colorado

Robertson, I M and Schaffer, G B, 2010, “Comparison of Sintering of Titanium

and Titanium Hydride Powders”, Powder Metallurgy, 53(1), pp. 12-19

Froes, F H, Suryanarayana, C, 1993, “Powder Processing of Titanium Alloys”,

Vol 1, pg. 223-275

Clinning, N, 2012, “Thermomechanical Processing of Blended Elemental Powder

Ti-6Al-4V Alloy”, Masters Dissertation, University of Cape Town

FH Froes, SJ Mashl, JC Hebeisen, VS Moxson, and VA Duz,2004, “The

technologies of titanium powder metallurgy”.JOM Journal of the Minerals, Metals

and Materials Society, 56(11):46–48

ASM, 2014, “ASM Aerospace Specification Metals inc.” [Online] Available

From: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MTP641

P King, G Poszmik, R Causton , 2005, “Higher Green Strength Materials for

Green Handling”, Hoeganaes Corporation. Presented at PM2 Tec 2005, Montreal,

Canada

49

APPENDIX A: EXPERIMENTAL CALCULATIONS

A.1 Ti-6Al-4V Characterisation: Apparent Density, Flow Rate and Sieve

Analysis

1. Apparent Density:

The apparent density of the powder was determined according to the ASTM

standard B212-12. The test was conducted four times in order to gain the most

accurate result. First the volume of the density cup was determined to be 28.53

cm3. The powder mass was determined and the resulting apparent density

determined by the equation:

𝐴𝐷𝐻 =𝑀

𝑉 (A.1)

where M is the mass of the powder, V is the volume of the density cup and ADH is

the apparent density. The results were tabulated as follows:

Table 15: PREP powder apparent density test result

Test Number Powder Mass (g) Apparent Density

(g/cm3)

1 73.96 2.592

2 73.63 2.581

3 74.1 2.597

4 73.88 2.589

Average 73.89 2.59

2. Flow Rate:

The flow rate was determined according to the ATSM standard B213-11. Two

different methods were used in determining the flow rate: Static Flow method and

Dynamic Flow Method. For each method there were three tests conducted. The

results obtained were as follows:

Table 16: PREP powder flow rate test results

Time (s)

Test Number Method 1: Static Flow Method 2: Dynamic Flow

1 38.87 36.42

2 39.04 37.92

3 38.94 37.08

Average 38.95 37.14

50

3. Sieve Analysis:

The sieve analysis done on the power was in accordance with ASTM standard

B214-07(2011). As the apparent density of the powder was determined to be

greater than 1.5 g/cm3, a test specimen of between 90g and 110g was used. Both

the first and second sieve analysis tests were done with 100.00g of PREP Ti-6Al-

4V powder. Table 17 and Table 18 are summaries of the results obtained from the

two sieve analyses conducted:

Table 17: Sieve analysis 1st attempt

Sieve:

New U.S.

Series 40 60 80 100 140 200 325 Pan

Opening

(μm) 710 560 425 250 180 150 106 75 45 <45

Mass retained on

screen 0 0.04 0.23 8.41 33.09 49.22 8.94 0.06 0.01 -

%Retained on

screen 0 0.04 0.23 8.41 33.09 49.22 8.94 0.06 0.01

% Finer than this

size 100 99.96 99.73 91.32 58.23 9.01 0.07 0.01 0

Table 18: Sieve analysis 2nd attempt

Sieve:

New

U.S.

Series

40 60 80 100 140 200 325 Pan

Opening

(μm) 710 560 425 250 180 150 106 75 45 <45

Mass retained on

screen 0 0.04 0.12 4.77 27.46 56.25 11.24 0.11 0.01 -

%Retained on

screen 0 0.04 0.12 4.77 27.46 56.25 11.24 0.11 0.01 -

% Finer than this

size 100 99.96 99.84 95.07 67.61 11.36 0.12 0.01 0

51

A.2 Theoretical Analysis of Powder Mixture

Using the master alloy approach, elemental titanium powder (Ti) and master alloy

powder (MA) are mixed together to form a premix Ti-6Al-4V powder. This

premix is then blended with the existing PREP Ti-6Al-4V powder. Theoretically

this process can be shown as follows:

Assuming 100g of Ti-6Al-4V final mixture:

This mixture will contain:

𝑥 𝑔𝑟𝑎𝑚𝑠 PREP powder

(100 − 𝑥) 𝑔𝑟𝑎𝑚𝑠 Ti and MA powder

To work out the theoretical mass of Ti and MA powder necessary for the (100 −𝑥)g mixture:

𝑇𝑖 + 𝑀𝐴 𝑔𝑖𝑣𝑒𝑠 𝑇𝑖 − 6𝐴𝑙 − 4𝑉

∴ 90𝑎𝑡% 𝑇𝑖

6𝑎𝑡%𝐴𝑙

4𝑎𝑡%𝑉

The percentage of each element present in the (100-x) g mixture is currently in

atom percent. This can be converted to weight percent using the equation

described in (Callister WD, 2011):

𝐶1 =𝐶1

′𝐴1

𝐶1′𝐴1+𝐶2

′𝐴2+𝐶3′𝐴3

(A.2)

Where Cx denotes the weight percent of the element you are looking at. Cx’

denotes the atom percent of the element and Ax denotes the atomic weight of an

element. The atomic weights used for titanium, aluminium and vanadium were

47.87 g/mol, 26.982 g/mol and 50.942 g/mol (Callister WD, 2011). Using the

above equation the weight percent of each element can be represented as follows:

𝐶𝑇𝑖 =90 (47.87

𝑔𝑚𝑜𝑙

)

90 (47.87𝑔

𝑚𝑜𝑙) + 6 (26.982

𝑔𝑚𝑜𝑙

) + 4 (50.942𝑔

𝑚𝑜𝑙)

× 100

𝐶𝑇𝑖 = 92.18 𝑤𝑡%

𝐶𝐴𝑙 =6 (26.982

𝑔𝑚𝑜𝑙

)

90 (47.87𝑔

𝑚𝑜𝑙) + 6 (26.982

𝑔𝑚𝑜𝑙

) + 4 (50.942𝑔

𝑚𝑜𝑙)

× 100

𝐶𝐴𝑙 = 3.46 𝑤𝑡%

52

𝐶𝑉 =4 (50.942

𝑔𝑚𝑜𝑙

)

90 (47.87𝑔

𝑚𝑜𝑙) + 6 (26.982

𝑔𝑚𝑜𝑙

) + 4 (50.942𝑔

𝑚𝑜𝑙)

× 100

𝐶𝑉 = 4.36 𝑤𝑡%

Now that the weight percent of each element is known, the following mass

requirements can be determined for the titanium powder and the master alloy

powder.

𝑇𝑖 𝑝𝑜𝑤𝑑𝑒𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 0.9218 × (100 − 𝑥)𝑔 (A.3)

𝑀𝐴 𝑝𝑜𝑤𝑑𝑒𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 0.0782 × (100 − 𝑥)𝑔 (A.4)

Table 19: Mass of powders required for 100g final mix

For 100g of Ti-6Al-4V final mixture

Powder Ratio

(PREP powder:

Ti + MA premix)

Mass of PREP Ti-

6Al-4V

Required (g)

Mass of 200 mesh

Ti powder

required (g)

Mass of Master

Alloy powder

required (g)

75:25 75 23.05 1.95

50:50 50 46.09 3.91

40:60 40 55.31 4.69

25:75 25 69.14 5.86

10:90 10 82.96 7.04

53

A.3 Ti6Al4V Characterization: Laser Diffraction

Particle size analysis was done on the batch of PREP Ti-6Al-4V powder supplied

by Boeing. Laser diffraction was conducted by Mrs H Botha from the Process

Engineering Department at the University of Stellenbosch.

Figure 31: Laser diffraction result page 1 of 6

54

Figure 32: Laser diffraction result page 2 of 6

Figure 33: Laser diffraction result page 3 of 6

55

Figure 34: Laser diffraction result page 4 of 6

56

Figure 35: Laser diffraction result page 5 of 6

57

Figure 36: Laser diffraction result page 6 of 6

58

A.4 TRB Green Density: Archimedes Principle

In order to determine the sintered density of the TRB specimens the Archimedes

principle was used in conjunction with physical measurements of the specimen.

The Archimedes principle tests conducted to determine the sintered densities were

done in accordance to ASTM standard B962-13.

The Archimedes principle works on the basis that when an object is partially or

fully submerged in a fluid, the fluid exerts a force on the object which is equal in

magnitude to the weight of fluid displaced. A Mitutogo Absolute Digimatic digital

Vernier was used to determine the amount of fluid displaced when the basket

containing the TRB specimen was lowered into the water container. The forces

acting on the submerged specimen were as follows:

𝑊𝑎 + 𝐵 − 𝑊𝑠 = 0 (A.5)

where Wa is the apparent weight of the object, Ws is the weight of the specimen

and B is the buoyancy force. The buoyancy force B is determined as follows:

𝐵 = 𝜌𝑤𝑎𝑡𝑒𝑟 ∗ 𝑉 ∗ 𝑔 (A.6)

where V is the volume of the object, g is the gravitational acceleration constant

and ρwater is the density of the water. Combining equations A.5 and A.6 the density

of the specimen can be determined as follows:

𝜌𝑠 =𝑀𝑠

𝑀𝑠−𝑀𝑎∗ 𝜌𝑤𝑎𝑡𝑒𝑟 (A.7)

where Ms is the mass of the specimen and Ma is the apparent mass of the object

(Ma=Wa/g).

59

APPENDIX B: QUOTE RECEIVED FOR PA TI-6AL-4V POWDER

Figure 37: Quote received for fine PREP Ti-6Al-4V powder

60

APPENDIX C: CONVERSION TABLES

Table 20 and Table 21 are the conversion tables for both the Carver and Amsler

presses. The information provided by Table 20 is adapted from a conversion table

provided with the machine. The Amsler press reads compaction as a force exerted

on the die. In order to determine the force which needs to be exerted on the TRB

die to achieve the required compaction pressures, the following relation needs to

be used:

𝐹 = 𝑃 × 𝐴

F is the force exerted on the die-set, P is the required compaction pressure and A

is the area of the die where the force is being exerted. Using a value of A =

403.23mm2, the force necessary to achieve the compaction pressures can be

determined.

Table 20: Conversion table for Carver press and dia.10mm cylindrical die

Compaction Pressure (MPa) Gauge Pressure - 10mm cylindrical die

(MPa)

400 15.02

450 16.90

500 18.78

550 20.24

600 22.18

Table 21: Conversion table for Amsler press and TRB die

Compaction Pressure (MPa) Force Required (kN) Force Required (ton)

400 161.29 16.44

450 181.45 18.50

500 201.61 20.55

550 221.77 22.61

600 241.94 24.66

61

APPENDIX D: GREEN DENSITY RESULTS

D.1 Cylindrical Specimen Green Densities

The figures below contain the results for green density of the cylindrical

specimens. Figure 38 and Figure 39 show the compressibility charts for the

powder ratios when the fine Ti powder was used. Figure 40 and Figure 41 show

the compressibility charts for the considered powder ratios when the coarse Ti

powder was used.

Figure 38: Green density with 200 mesh Ti at 500MPa compaction

Figure 39: Green density with 200 mesh Ti at 600MPa compaction

3.25

3.3

3.35

3.4

3.45

3.5

3.55

50:50 40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3)

Powder Ratios (PREP : Ti + MA)

Compressibilty Chart at 500MPa with 200 mesh Ti

1st Test Data

2nd Test Data

3rd Test Data

3.45

3.46

3.47

3.48

3.49

3.5

3.51

3.52

50:50 40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3)

Powder ratio (PREP : Ti + MA)

Compressibility Chart at 600MPa with 200 mesh Ti

1st Test Data

2nd Test Data

3rd Test Data

62

Figure 40: Green density with 100 mesh Ti at 500MPa compaction

Figure 41: Green density with 100 mesh Ti at 600MPa compaction

3.32

3.34

3.36

3.38

3.4

3.42

3.44

50:50 40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

Compressibility Chart at 500MPa with 100 mesh Ti

1st Test Data

2nd Test Data

3rd Test Data

3.44

3.46

3.48

3.5

3.52

3.54

3.56

3.58

50:50 40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3)

Powder Ratio (PREP : Ti + MA)

Compressibility Chart at 600 Mpa with 100 mesh Ti

1st Test Data

2nd Test Data

3rd Test Data

63

D.2 TRB Green Densities

Figure 42 and Figure 43 show the results achieved for the green densities of the

fine and coarse Ti mixtures respectively.

Figure 42: TRB compressibility chart with 200 mesh Ti

Figure 43: TRB compressibility chart with 100 mesh Ti

3.370

3.380

3.390

3.400

3.410

3.420

3.430

3.440

3.450

3.460

3.470

40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

TRB Compressibility Chart for 200 mesh Ti at 500MPa

1st Test Data

2nd Test Data

3rd Test Data

4th Test Data

3.400

3.410

3.420

3.430

3.440

3.450

3.460

3.470

40:60 25:75 10:90

Gre

en

De

nsi

ty (

g/cm

3 )

Powder Ratio (PREP : Ti + MA)

TRB Compressibility Chart for 100 mesh Ti at 500MPa

1st Test Data

2nd Test Data

2nd Test Data

4th Test Data

64

APPENDIX E: STRENGTH TEST RESULTS

E.1 TRB Green Specimens Failure Force

The MTS load frame, along with the TestWorks4 software, allowed for the

applied force to be plotted against the crosshead displacement until the specimen

fractured/ruptured. The rupture force for the different powder mixtures can be

seen from Figure 44 to Figure 46.

Figure 44: Force required to rupture the 40:60 green specimens

Figure 45: Force required to rupture the 25:75 green specimens

0

5

10

15

20

25

30

35

40

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Forc

e (

N)

Crosshead Displacement (mm)

Rupture Force for 40:60 (PREP : Ti + MA) Mixtures

200 mesh (1)

200 mesh (2)

100 mesh (1)

100 mesh (2)

0

20

40

60

80

100

120

140

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Forc

e (

N)

Crosshead Displacement (mm)

Rupture Force for 25:75 (PREP : Ti + MA) Mixtures

200 mesh (1)

200 mesh (2)

100 mesh (1)

100 mesh (2)

65

Figure 46: Force required to rupture the 10:90 green specimens

0

50

100

150

200

250

300

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25

Forc

e (

N)

Crosshead Displacement (mm)

Rupture Force for 10:90 (PREP : Ti + MA) Mixtures

200 mesh (1)

200 mesh (2)

100 mesh (1)

100 mesh (2)

66

APPENDIX F: SPECIFICATIONS OF RE-DESIGNED SPACER

67

APPENDIX G: TECHNO-ECONOMIC ANALYSIS

Figure 47 shows the intended schedule of the project, grey bars, and the actual

progress of the project, blue bars. It can be seen that the actual progress and

scheduled progress differ considerably. The reason for this is that the project

schedule was completed prior to any knowledge of workload that would be

received by the author during the year. As a result it was impossible to stick to the

schedule which caused every task to start later than originally intended. It is worth

noting that although tasks started a considerable time later than planned; each task

required only a fraction of the time to complete than was scheduled. This allowed

for the project to still be completed by the due date as can be seen. Unfortunately

the delayed start to each task did have an impact in terms of what could be handed

in for the progress report and final draft deliverables.

Figure 47: Gantt chart: project schedule

Table 22 reflects the budgeted and actual cost of the project. It takes into account

the author’s time to complete the task, the running cost of the project as well as

the cost of using the equipment to perform the experimental procedure. The

running costs are the costs incurred when consuming “goods”, which in the case

of this project is the different powders. The facility costs were determined on an

hourly rate with the amount charged per hour depending on the cost of the

equipment used. As the majority of the tasks took shorter than was originally

scheduled, see Figure 47, the actual cost of the project decreased when compared

to the budgeted cost. This can be seen in Table 22.

68

Table 22: Budgeted and actual cost of the project

The cost of this project is justified due to the importance of the results. This

project has proved that waste PREP powder can be made usable through the BE

approach and implemented in a press-and-sinter-process. This project succeeded

in being a proof of concept, further justifying the cost.

Activity

Planned Cost Actual Cost

Engineering Time Facility

Use

Running

Cost Engineering Time

Facility

Use

Running

Cost

Time

(hr) Cost (R) Cost (R) Cost (R)

Time

(hr) Cost (R) Cost (R) Cost (R)

Literature Review 25 8750 25 8750

PREP Powder

Characterisation 30 10500 1250 20 7000 900 1250

Compact PREP

Ti6Al4V Powder

as a Baseline

15 5250 1500 15 5250 1500

Mix Powders

Using the BE

Approach

15 5250 1000 10 3500 500

Compaction with

Cylindrical Die-

set

30 10500

8000 5000

25 8750

6000 3390

Compaction with

TRB Die-set 40 14000 30 10500

Specimen

Sintering 30 10500 2500 4000 40 14000 3300 5000

Strength Testing 45 15750 2500 5000 10 3500 800 1000

Consolidation of

Final Report 50 17500 60 21000

Total 280 98000 15750 15000 235 82250 12500 11140

Grand Total

128750 105890