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CHAPTER 1

INTRODUCTION

1.1 GENERAL INTRODUCTION

Titanium and its alloys find numerous applications in aerospace

and nuclear industries due to their high strength to weight ratio and good

corrosion resistance. Joining of various components is inevitable for the

manufacture of machine, plant and equipment. Titanium alloys are joined

with nickel alloys for aerospace engines to withstand high temperatures.

Titanium - stainless steel joints are made in nuclear applications for better

corrosion resistance. Conventional fusion welding is not a feasible technique

to join these kinds of dissimilar joints due to the formation of chemical,

mechanical and structural heterogeneities. Solid state joining is a suitable

alternate to overcome the difficulties. No melting is involved in solid state

welding; hence melting related defects are avoided. The joining of immiscible

or partially miscible alloy systems which is cumbersome in conventional

fusion welding is also possible by solid state joining. Numerous solid state

joining processes have been in use on an industrial scale now-a-days.

1.2 SOLID STATE JOINING PROCESSES

Among the modern joining techniques used in engineering

industries today, solid state joining processes probably have the longest

history. Solid state joining is a group of welding processes that are carried

out in the solid state of the components. A wide range of processes are

grouped under solid state joining category and they differ in the method of

application of key parameters namely temperature, time and stress.

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Solid state joining processes include cold pressure welding,

diffusion bonding, explosive welding, forge welding, friction welding, hot

pressure welding, roll bonding and ultrasonic welding. In all these processes,

joints are produced by coalescence of the base metals without significant

melting of the base metals. Solid state joining has many advantages. No

melting is involved so the metals being joined retain their original properties

without the heat affected zone problems which are common in fusion welding

processes. When dissimilar metals are joined, their thermal expansion and

conductivity are of much less importance with solid state welding than with

conventional fusion welding. The joining of immiscible or partially miscible

alloy systems is also possible by solid state joining.

In solid state joining, the joint quality is based on the process

parameters like time, temperature and stress maintained during the process.

In some processes, the time of welding is extremely short, up to a few

seconds like in friction welding. In some cases, the time of welding is

extended to several hours like in diffusion bonding. As temperature increases,

time is usually reduced. The solid state joining processes have been in use for

quite some time on an industrial scale in the field of aerospace and nuclear

engineering.

1.3 DIFFUSION BONDING PROCESS

Diffusion bonding is a solid state joining process in which two

metals with clean surfaces are brought into contact at elevated temperature

and stress for a predetermined time. International Institute of Welding (IIW)

defines diffusion bonding as “a solid state bonding process for making a

monolithic joint through the formation of bonds at atomic level, as a result of

closure of the mating surface due to the local plastic deformation at elevated

temperatures which aids inter-diffusion at the surface layers of the materials

being joined”.

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Some of the solid state joining processes like cold pressure welding

needs no significant heating. But they require extensive deformation. Hence

the processes are applicable only for ductile materials. Processes like friction

welding, magnetically impelled arc butt welding (MIAB) and explosive

welding result in significant distortion due to plastic deformation at high

temperatures. But diffusion bonding gives a sound bond without any

macroscopic deformation because it involves only inter-atomic diffusion with

microscopic deformation at the interfaces.

In diffusion bonding, two clean metal surfaces are brought into

contact at elevated temperatures under stress for a specified period of time.

During that time atoms diffuse across the interface between the two metals.

Energy required for the movement of atoms depends on the applied stress and

temperature. This bonding process can be applied for joining metals and

alloys of similar as well as dissimilar material combinations. This process can

also be extended to non-metals, ceramics and intermetallics. The conventional

fusion welding processes have limitations like the formation of weak heat

affected zone, distortion of the base metals etc. These limitations are

overcome in diffusion bonding process. One important advantage of diffusion

bonding is that the individual characteristics of the base materials are retained

after bonding, except probably at the interface.

Temperature required for the diffusion bonding is usually in the

range of 0.5 to 0.8 times the absolute melting point of the lower melting point

metal. Stress is typically a small fraction of yield strength to avoid macro

plastic deformation while the time period can vary from a few minutes to

several hours.

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1.3.1 Mechanism of Diffusion Bonding

The mechanism of diffusion bonding is schematically illustrated in

Figure 1.1. During diffusion bonding, the surfaces, which are smooth and free

from contaminants, are brought into intimate contact by applying sufficient

stress. First, the applied stress causes yielding of the asperities at the

interface, which establishes an intimate contact between the surfaces to be

bonded. Subsequently continuous creep deformation and atomic diffusion

take place leading to closure of interfacial voids followed by bonding of

materials. The stress applied will cause only deformation of asperities and not

bulk deformation.

Figure 1.1 The mechanism of diffusion bonding

1.3.2 Process Parameters for Diffusion Bonding

Diffusion is a thermally activated process. An incremental change

in temperature will cause large change in process kinetics. Temperature must

be controlled to promote or avoid certain metallurgical transformations like

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allotropic transformation, recrystallisation and solution of precipitates or

formation of intermetallics. Generally the temperature used for diffusion

bonding is 0.5 to 0.8 times Tm, where Tm is the absolute melting point of the

lower melting point material of the two materials to be bonded. Diffusion rate

is expressed as a function of temperature as:

D = Do e (-Q / RT)

where, D = Diffusion rate (m2/s)

D0 = Diffusivity (m2/s)

Q = Activation energy for diffusion (J/mol)

T = Temperature (K)

R = Gas constant (8.314 J/mol/K)

Bonding at lower temperature will lead to insufficient atoms

diffusion and weak bond. On the other hand, higher bonding temperature can

speed up the atoms diffusion and shorten the bonding time, but will induce

intermetallic formation at the interface, grain coarsening and oxidation

(He et al 1999).

The applied stress contributes to improve the joint quality by

crushing the asperities on the surface and increasing the diffusion path length.

When the applied stress is increased to values that produce substantial plastic

flow of sub-surface layers, full contact is made. Further increase in stress does

not contribute to the rate of diffusion across the interface.

The process time enhances the bonding quality significantly as

diffusion is time - dependent phenomenon. Surface roughness is also an

important factor. Diffusion bonding is generally carried out in inert

atmospheres or in vacuum to prevent contamination and oxidation of the base

metals.

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1.3.3 Advantages and Limitations of Diffusion Bonding

In diffusion bonded joints, strength of the interface layer is equal or

nearly equal to that of the parent metal. Distortion is minimum in the diffusion

bonding process. Diffusion bonding can yield joints with more joint efficiency

even for large area bonds. No melting takes place and consequently the joints

are free from melting defects. Thin sections to very thick sections can be

joined by diffusion bonding. All types of materials such as cast, wrought or

sintered powder products can be joined by diffusion bonding with the same

materials or dissimilar materials. Diffusion bonding produces no burrs or

splatter, so machining cost is reduced. Diffusion bonding can be done in

combination with superplastic forming to take advantage of both the processes.

However, diffusion bonding has some drawbacks. The process

duration is more when compared to that of the conventional joining processes.

The preparation of the surface is difficult since very smooth finish is required

for diffusion bonding to occur.

1.4 FRICTION WELDING

Friction welding is a solid-state welding process that produces a

weld under a compressive contact stress between the work pieces while

rotating or moving relative to one another to produce heat and plastically

displace material from the faying surfaces. When a suitable high temperature

has been reached, rotational motion ceases, additional stress is applied and

coalescence occurs.

1.4.1 Mechanism of Friction Welding

Friction welding is a solid state welding process in which the heat

for welding is produced by the relative motion of the two interfaces being

joined. This method relies on the direct conversion of mechanical energy to

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thermal energy for the weld, without the application of heat from any other

source. Under normal conditions, no melting occurs at the interface.

There are three variants of friction welding process, namely, rotary

process, linear process and orbital process. Rotary friction welding can be

used only for circular cross section and in this process one component is

rotated about its axis while the other remains stationary with simultaneous

application of stress for joining. Based on the manner by which rotational

energy is transferred to make the weld, rotary friction welding is further

divided into two classes: direct drive (continuous drive) and inertia drive

(stored energy). Direct drive friction welding uses a motor running at constant

speed while inertia drive uses the kinetic energy stored in a rotating flywheel

to input energy to the joint during friction stage.

In the case of linear friction welding, one part is stationary and

another part moves in a reciprocating fashion under friction stress through a

small linear displacement. Linear friction welding has become a key

manufacturing and repair technology for aero-engine blocks in the

aeronautical engine manufacturing. Orbital friction welding is a combination

of linear and rotational friction welding where the two parts to be joined are

rotated around their longitudinal axes in the same constant angular speed.

Here the two longitudinal axes are parallel but with a small linear distance

offset. When motion of the components stops, the parts need to be correctly

aligned to form a weld. Linear and orbital friction welding can weld

noncircular parts and the heat generation is almost uniform throughout the

joint cross section unlike rotary friction welding. However, the rotary friction

welding which is the oldest is still the most popular method.

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Figure 1.2 The mechanism of typical friction weld

Figure 1.2 shows the mechanism of typical rotary friction welding

(direct - drive friction welding) in which a non rotating work piece is held in

contact with a rotating work piece under constant or gradually increasing

stress until the interface reaches the welding temperature. The rotation speed,

the axial stress, and the welding time are the principal variables that are

controlled in order to provide the necessary combination of heat and stress to

form weld. These parameters are adjusted so that the interface is heated into a

plastic temperature range where welding take place. Once the interface is

heated, axial stress is used to bring the weld interface into intimate contact.

During the last stage of the welding process, atomic diffusion occurs while

the interfaces are in contact, allowing a metallurgical bond to form between

the two materials. Friction welding involves heat generation through friction

abrasion, heat dissipation, plastic deformation, and chemical inter-diffusion.

1.4.2 Friction Welding Parameters

There are three important factors involved in producing friction

welded joints: (1) Rotational speed: Selection of this parameter depends on

the materials to be friction welded and the diameter of the weld at the

interface. Normally the speed ranges between 700 rpm to 2000 rpm. (2) Stress

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between parts to be welded: Stress changes during the weld sequence. It is

very low at the start, but is increased to create the frictional heat. When the

rotation is stopped, stress is rapidly increased. (3) Welding time: It is related

to the shape and the type of metal and the surface area. It is normally a matter

of few seconds. It is generally represented in terms of Burn-off-Length

(BOL). The actual operation of the friction welding machine is automatic and

is controlled by a sequence controller which can be set according to the weld

schedule established for the parts to be joined.

1.4.3 Advantages and Limitations of Friction Welding

Dissimilar metals, some of which are considered incompatible or

unweldable, can be easily joined by friction welding method. The friction

welding process is at least twice - and up to 100 times - as fast as other

welding techniques. For friction welding, joint preparation is not critical.

Machined, saw cut, and even sheared surfaces are weldable. Self cleaning

action reduces or eliminates surface preparation time and cost. Since there is

no melting during welding, solidification defects like gas porosity,

segregation and slag inclusions do not occur. During welding, there are only a

few sparks without any weld spatter.

However, the friction welding has some drawbacks. At least one of

the matting components must be rotationally symmetrical, for rotary friction

welding so that the component can rotate around the axis of the weld plane.

Typical part geometries that can be friction welded are bar to bar, bar to tube,

bar to plane, tube to tube and tube to plane. If tubing is welded, flash removal

from inside the joint will become difficult. This process is normally limited to

make flat and angular (or conical) butt joints. The material of at least one

component must be plastically deformable under the given welding

conditions. For example, alumina can be joined to aluminum, but cannot be

joined to alumina.

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1.5 APPLICATIONS OF DIFFUSION BONDING

It is possible to manufacture a near net shape of heavy, solid part

with diffusion bonding (Lee et al 2008). By diffusion bonding, parts have

been manufactured from pre-sized multi-sheets of Ti-6Al-4V alloy. Using this

method, the complex shape of components can be manufactured in bulk at solid

state with metallurgically homogeneous microstructure. Diffusion bonding of

copper with stainless steel 304L using nickel as interlayer has been carried out to

manufacture matrix heat exchanger with good strength, leak tightness and free

flow of the heat-exchanging fluids (Krishnan et al 1997).

Diffusion bonding combined with superplastic forming is a well

established method of manufacturing honeycomb structures using Ti-6Al-4V

sheet material as given in Figure 1.3 (Wenbo et al 2007). Manufacture of

four-sheet hollow aircraft engine blades made of Ti-6Al-4V sandwich

structure as shown in Figure 1.4, is a practical example of application of

technology based on superplastic forming combined with diffusion bonding

(Xun et al 2000). The microstructures at the interfaces indicate that a bonding

ratio of 100% can be reached in most bonding areas. However, low bonding

ratios in some regions such as cell corners and triple angles were unavoidable

in this application.

Figure 1.3 Honeycomb structure using Ti-6Al-4V sheet material

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Figure 1.4 Schematic of the cross section of a hollow aircraft engine blade

1.6 Ti-6Al-4V ALLOY

Since the introduction of titanium and titanium alloys in the early

1950s, these materials have in a relatively short time become backbone

materials for aerospace, energy and chemical industries. The combination of

high strength to weight ratio, excellent mechanical properties and corrosion

resistance makes titanium alloys excellent material choices for many critical

applications. Today, titanium alloys are being used for several demanding

applications such as static and rotating components in gas turbine engines.

Some of the most critical and highly stressed air frame parts in civilian and

military aircrafts are made out of these alloys.

Titanium exists in two crystallographic forms. At room

temperature, titanium has a hexagonal close packed (HCP) crystal structure

referred to as alpha ( ) phase. At 883OC, pure titanium transforms to body

centered cubic (BCC) structure known as beta ( ) phase. Development of a wide

range of titanium alloys is based on the manipulation of the nature, amount and

distribution of crystallographic forms through alloying additions and thermal and

thermo mechanical processing. Based on the phases present, titanium alloys can

be classified as alpha alloys, beta alloys and alpha - beta alloys. Alpha - beta

alloys have compositions that support a mixture of alpha and beta phases. They

contain between 10 and 50% beta at room temperature.

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The most common alpha - beta alloy is Ti-6Al-4V. Alpha - beta

alloys generally have good formability. The properties of these alloys can be

controlled through heat treatment, which adjusts the amount and the types of

phases present. Solution treatment followed by ageing at 480 to 650OC

precipitates alpha, resulting in a fine mixture of retained or transformed beta

along with alpha phase. Ti-6Al-4V is the workhorse alloy of the titanium

industry. The alloy is fully heat treatable in section sizes up to 15 mm and is

used up to approximately 400OC. Ti-6Al-4V is used in aircraft gas turbine

compressor blades and disk, forged airframe fittings, sheet metal airframe

parts and also in major non-aerospace applications in the marine, offshore and

power generation industries. The properties of Ti-6Al-4V at room

temperature are given in Table 1.1 (Balram Gupta 1996).

Table 1.1 Properties of Ti-6Al-4V

Physical Properties

Melting point (°C) 1550 -1650

Beta transition (°C) 980 - 1010

Density (kg/m3) 4420

Specific heat (J/kg/K) 620

Thermal conductivity (W/m-K) 5.8

Mechanical Properties

Hardness (HV) 305 - 340

Elastic modulus (GPa) 106

Ultimate tensile strength (MPa) 985 - 1090

Yield strength (MPa) 830 - 930

Elongation (%) 14

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1.7 INCONEL 718 ALLOY

The ease and economy with which Inconel 718 alloy can befabricated, combined with good tensile, fatigue, creep, and rupture propertieshave resulted in its use in a wide range of applications. This alloy is used formaking components for liquid fueled rockets, rings, casings and variousformed sheet metal parts for aircraft and land-based gas turbine engines, andcryogenic tank. It is also used for fasteners and instrumentation parts.

Inconel 718 retains strength over a wide range of temperatures.This makes it particularly attractive in high temperature applications wherealuminum and steel would “soften”. High temperature strength of Inconel 718is due to the solid solution strengthening or precipitation strengthening(depending on the alloy). In precipitation strengthening varieties, smallamounts of niobium combine with nickel to form the intermetallic compoundNi3Nb or gamma prime ( '). Gamma prime forms small cubic crystals thatinhibit slip and creep effectively at elevated temperatures. The properties ofInconel 718 at room temperature (Balram Gupta, 1996) are given inTable 1.2.

Table 1.2 Properties of Inconel 718

Physical Properties

Melting Point (°C) 1260 - 1335

Density (kg/m3) 8190

Specific Heat (J/kg/K) 435

Thermal Conductivity (W/m/K) 11.4

Mechanical Properties

Hardness (HV) 410 - 420

Elastic Modulus (GPa) 211

Ultimate tensile strength (MPa) 1275

Yield Strength (MPa) 1030

Elongation (%) 12

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1.8 SS 304L ALLOY

Austenitic stainless steels have excellent corrosion and heat

resistance with good mechanical properties over a wide range of temperature.

They are non-magnetic and non heat-treatable steels that are usually annealed

and cold worked. Out of the different classes of austenitic stainless steels, 304

grade, a chromium - nickel type, is the most widely used steel and is also

known as 18 - 8 (Cr-Ni) steels. In the case of SS 304L, carbon content is

reduced to improve intergranular corrosion resistance. It has an excellent

weldability. SS 304L is used in heat exchangers, nuclear plants etc. The

properties of SS 304L at room temperature are given in Table 1.3

(ASM 1990).

Table 1.3 Properties of SS 304L

Physical Properties

Melting Point (°C) 1400 - 1450

Density (g/cm3) 8000

Specific Heat (J/kg-K) 500

Thermal Conductivity (W/m-K) 16.2

Mechanical Properties

Hardness (HV) 125 - 140

Elastic Modulus (GPa) 190

Ultimate Tensile Strength (MPa) 490

Yield Strength (MPa) 170

Elongation (%) 40

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1.9 OBJECTIVES AND SCOPE OF THE STUDY

The main purpose of the present study is to investigate and

compare the characteristics of dissimilar joining of Ti-6Al-4V with Inconel

718 and Ti-6Al-4V with SS 304L using diffusion bonding and friction

welding processes.

The objectives of this investigation are:

1. a) To produce diffusion bonded joints of Ti-6Al-4V with

Inconel 718 and Ti-6Al-4V with SS 304L without and

with interlayer with varying process parameters namely,

temperature, stress and time.

b) To study the effect of diffusion bonding process

parameters on the bond characteristics of the joints and

consequently to optimise the bonding conditions to get a

good bond.

2. a) To produce friction welded joints of Ti-6Al-4V with

Inconel 718 and Ti-6Al-4V with SS 304L without and

with interlayer with varying process parameters namely,

rotational speed, upset load, friction load and burn-off-

length.

b) To study the effect of friction welding process parameters

on the bond characteristics of the joints and consequently

to optimise the welding conditions to get a good weld.

3. To evaluate the mechanisms of joining of the two dissimilar

material combinations using the two solid state processes and

to compare them.

4. To examine the suitability of ultrasonic C-scan as non-

destructive testing method for diffusion bonded and friction

welded industrial components.

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The scope of the work includes making of dissimilar joints with

each of the two chosen solid state processes by varying the key process

parameters, characterising the joints by various methods and relating the

parameters with bond characteristics. The characterization tests include

ultrasonic C-scan analysis, optical microscopy, SEM-EDS tension test and

hardness survey. Since formation of intermetallic compounds is usually an

important cause detoriating the bond quality in dissimilar joints, an attempt is

made to find out presence of various intermetallics formed. An attempt is also

made to avoid these intermetallics by using appropriate interlayer.