Laser Cladding: An Experimental and Theoretical …989990/...(PICALO)April 19-21, 2004 Melbourne,...

DOCTORA L T H E S I S

Luleå University of TechnologyDepartment of Applied Physics and Mechanical Engineering,

Division of Manufacturing Systems Engineering

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:

Laser Cladding: An Experimental and Theoretical Investigation

Hans Gedda

This thesis is dedicated to my family

Birgitta, Petrus and Emilia.

i

Preface

Since April 1999 I have been conducting experimental and theoretical research in the field of laser cladding at the Division of Manufacturing Systems at Luleå University of Technology. The experimental work was mostly performed in our laser laboratory. Some work has been done at Duroc AB in Umeå, Luleå and at Nottingham University. Several people have been important in completition of this work. I sincerely thank my supervisor John Powell who has guided and supported me throughout this research. I would like to express my gratitude to Professors Alexander Kaplan and Claes Magnusson for discussions, suggestions through this work. I would also like to thank all my friends and colleagues at the division for all their help and fruitful discussions. I would finally thank my family, Birgitta, Petrus and Emilia for their love, support and patience during the work. Luleå, October 2004 Hans Gedda

ii

Abstract

This thesis presents an investigation into the laser cladding process using CO2 and Nd:YAG lasers. The work is divided into six chapters: Chapter one is an introduction the subject of laser cladding. This presents a general overview of the two common laser cladding methods and some applications for the processes. This chapter concludes with abstracts, main figures and conclusions from all chapters in the thesis. Chapter two is an investigation into the energy redistribution during CO2 and Nd:YAG laser cladding. Experimental absorption measurements by calorimetry were carried out to analyse how much of the energy is lost by reflection etc. It was found that the Nd:YAG laser cladding process is approximately twice as energy efficient as the CO2 laser cladding process. Chapter three investigates the process parameters which affect the finished product when cladding into pre machined groves including; groove geometry, powder application method and laser type. Chapter four presents preliminary experimental results from two new processes; Laser casting and Laser clad-casting. Laser casting is a process similar to blown powder laser cladding but without the final product joined to the substrate. The substrate acts as a mould and the casting retains the topological features of the substrate. Laser clad-casting involves the production of a clad layer between machined copper blocks. Clad tracks can therefore be achieved with large depth to width ratios and pre determined cross sections. Chapter five describes a new technique for the production of solid wire or rods from powder by laser melting. Three techniques have been developed to ensure that the molten powder solidifies as a rod or wire rather than a series of droplets. The techniques can be used to produce welding rods, tensile test samples and other solid pieces from a wide range of powder mixes. Chapter six presents experimental data in conjunction with mathematical models are used to explain various aspects of laser casting and laser cladding by the preplaced powder method. Also the interaction of the melt pool with the powder bed is analysed to identify why laser castings have microscopically uneven surfaces.

iii

Contents Page Preface i Abstract ii Contents iii Chapter I: Introduction to laser cladding 1 Chapter II: Energy Redistribution in Laser Cladding: A comparison of Nd:YAG and CO2 lasers which combines information from two published papers; 25 1. Gedda, H., Powell, J., Wahlström, G., Li, W-B., Engström, H., Magnusson, C.: Energy Redistribution During CO2 Laser Cladding (Published in Journal of Laser Applications. vol. 14, no. 2, pp. 78-82. May 2002) 2. Gedda, H., Powell, J., Kaplan, A.: A Process Efficiency Comparison of Nd:YAG and CO2 Laser Cladding (Published in Welding in the World, vol. 46, Special Issue. pp.75-86. July 2002) Chapter III: Laser Cladding into pre machined grooves 41 Powell, J., Gedda, H., Kaplan, A.: Proceedings of the 1st Pacific International Conference on Applications of Lasers and Optics (PICALO)April 19-21, 2004 Melbourne, Australia. Submitted for publication in Journal of Laser Applications. Chapter IV: Laser Casting and Laser Clad-Casting: New processes for rapid prototyping and production 53 Gedda, H., Powell, J., Kaplan, A.: Conference proceedings International Congress on Applications of Lasers & Electro-Optics (ICALEO) Scottsdale, AR, 14-17 October 2002. Chapter V: Laser Wire Casting 65 Gedda, H., Powell, J., Kaplan, A.: Conference proceedings International Congress on Applications of Lasers & Electro-Optics (ICALEO) Jacksonville, FL, 13-16 October 2003. Chapter VI: Melt-Solid Interactions in laser cladding and laser casting 75 Gedda, H., Powell, J., Kaplan, A.: Submitted for publication in

Metallurgical and Material Transactions B.

H.Gedda: Chapter I-Introduction to Laser Cladding

1

Chapter I

Introduction to laser cladding


3

1 Introduction to laser cladding

Industrial applications require parts with special surface properties such as good corrosion resistance, wear resistance and hardness. Alloys with those surface properties are usually very expensive and it is of great interest to reduce the cost of parts with these surface properties [1]. This cost reduction can be achieved by applying a hard or corrosion resistant surface layer to a cheaper substrate. Laser surface treatment includes several different surfacing techniques using the heat of the laser beam to modify the structure and physical characteristics of the surface of a material [2]. Laser cladding is the fusion of a different metal to a substrate surface, with a minimum of melting of the substrate. The surface alloy composition must be well controlled with a high bond strength to the substrate [3]. Surface coating by laser is a method that has been developed over the last two decades. The lasers minimal and easily controllable energy delivery makes it possible to alloy, impregnate, clad, and harden components that are exposed to wear and corrosion. The method offers great advantages compared with traditional hardening and alloying methods. The method is used commercially in the aircraft engine industry and in the car industry (G.M, etc.). Laser cladding can be carried out in a single or a two-stage process. In the single stage process, the powder is blown into the interaction zone between the laser beam and workpiece. In the two-stage process the cladding material is pre deposited on the substrate. Both techniques (see figure 1) have the advantage of the possible deposition of a wide range of alloys either using a chosen alloy in powder form or by a blend of powders with the required composition. Laser cladding with powder offers the possibility of the development of new material combinations for the future. (a)

(b)

Cladding material

V

V

Figure 1. Schematic diagrams of laser cladding process. a) Preplaced powde,r b) blown powder cladding.


4

Relative motion between the laser/powder supply and the substrate can be used to continuously apply a surface coating. To cover larger surfaces, overlapping tracks are made (see figure 2).

Figure 2. Schematic of the overlapping cladding process [4].

1.1 Blown powder laser cladding

The first reference that describes the laser cladding process by blown powder is a patent from Rolls Royce Ltd in the early eighties [5]. Blown powder laser cladding can produce a high quality cladding layer with low dilution. The powder is transported into the melt pool by a carrier gas and directed at an angle in the range 38-45° towards the substrate (see figure 2 ). The powder particles are heated when they pass through the laser beam. Melting starts at the interface and the molten particles are trapped in the melt pool. The energy must be high enough to melt the powder without too much substrate melting [3]. The powder striking the substrate ricochets but the powder striking the melt pool is completely melted. With side blown powder there is a directional effect on the clad bead shape [6] and the powder utilisation efficiency is low compared with coaxial powder nozzle feed [7]. The coaxial system in figure 3 can avoid this problem in some extent.

Inner nozzle Powder stream in Workpiece

Focal point

Figure 3. Cross section of a coaxial nozzle.


5

1.2 Laser cladding with preplaced powder

Cladding with preplaced powder is the simplest method provided the powder can be made to remain in place until melted, while the area is being shrouded by an inert gas. Some form of binder is usually used, often this is an alcohol [6]. The preplaced powder method involves scanning the laser beam over the powder bed. The general theory for cladding of pre-placed powder may be understood on the basis of the work of Powell et al. [8].

2 Commercial examples

Industrial applications require parts with good wear, corrosion and hardness properties and laser cladding is a process which can fulfill all these requirements. Laser cladding can be used to good effect in processes which require a high productivity combined with flexibility without compromising on quality. A high and uniform quality with a low heat input makes this process suitable for a wide range of applications in which minimum distortion is desired. Examples of industrial laser cladding applications are:

• Improved wear resistance of bearings, valves, axles, cutting tools and other parts where the working conditions are very severe

• Improved corrosion resistance

• Repairing turbine parts, moulds, tools etc

• Building up complex geometries

Figure 4. Laser clad parts.

Typical commercial applications of laser cladding are carried out by SIFCO in Ireland who are involved in the remanufacture of turbine engine components. In recent years they have devoted a large amount of resources to the research and development of new repair technologies for the gas turbine industry. The deposited layer can have different composition, and subsequently properties, to the underlying material. This potentially has a range of applications in a number of areas in particulary the aerospace and the automotive industries.


6

Figure 5. Cladding on turbine blade.

Duroc AB in Sweden Umeå has developed the technology for cladding material on valves etc for the nuclear power plant industry and the wood industry. Figure 6 below shows a part from a laser clad chopping tool of which the service life has increased 5-6 times compared to an untreated tool. Figure 6. Laser clad chopping tool for the wood industry.


7

3. Summary of the chapters

3.1 Chapter 1. Energy redistribution in laser cladding; A comparison of Nd:YAG and CO2 lasers Abstract Blown powder laser cladding is a cost effective way of producing a surface layer to withstand wear and corrosion. However, the cladding process is slow. Therefore is it of great interest to investigate how much of the laser power is used in the cladding process and how much is reflected etc. In this investigation an Nd:YAG and a CO2 laser have been compared as energy sources for the process. Every aspect of the energy redistribution during cladding has been analysed. The main energy loss to the process for both lasers is by reflection from the melt pool and the powder cloud. It was found that the Nd:YAG laser cladding process is approximately twice as energy efficient as the CO2 laser cladding process.

Figure 1. The redistribution of laser power during the cladding process (see text for definition of PA, PB etc).

(Power lost by convection)

PFPF

PA PA

PB PB

PE PE

PD PD

P Laser beam

Substrate

Powder stream

Substrate

P Laser beam

(Power radiated)

(Power reflected off the surface of the clad)

(Power reflected off the powder particles)

(Power lost by conduction)

PD

PA


8

Where: PA = Power reflected off the surface of the clad zone. PB = Power reflected off the powder particles as they approach the weld pool. PD = Power lost by radiation from the cladding zone. PE = Power lost by convection from the cladding zone. PF = Power lost by conduction from the clad zone to the substrate. PG = Power absorbed by the powder particles which do not enter the cladding melt pool.

Conclusions

1. Ignoring the trivial contributions of convective and radiative cooling etc, the laser power applied to the cladding process is redistributed in the following ways:

*This value includes powder and substrate melting.

2. Nd:YAG lasers are approximately twice as energy efficient as CO2 lasers for cladding in the range of parameters covered in this paper ( and by implication, the higher power (5 kW) range covered in our earlier work [2]) i.e. given the same laser power, Nd:YAG lasers are capable of approximately double the cladding rates of CO2 lasers.

As a large proportion (30%) of the laser power is consumed in heating the substrate it is likely that substrate pre heating by a cheaper power source* would improve the profitability of laser cladding. (* flame, plasma, induction etc).

Laser type CO2 Nd:YAG Power reflected off the cladding melt 50% 40% Power reflected off the powder cloud 10% 10% Power used to heat the substrate 30% 30% Power used to melt the clad layer* 10% 20%

Figure 2. The experimental arrangement for the analysis of the absorption and reflection of the energy by the powder cloud.

Laser Beam

Powder Particles

Insulated Calorimeter


9

Figure 4. Cross sections of grooves showing that even when there is sufficient melt to produce a flat surface the clad layer does not do so when preplaced

powder is used.

3.2. Chapter 3. Laser cladding into pre machined grooves

Abstract When laser cladding is used to improve the wear characteristics of a substrate it is not always necessary to clad the whole surface. Wear resistant individual tracks can be clad directly onto the substrate or into pre machined grooves. This paper investigates the process parameters which affect the finished product when cladding into groves including; groove geometry, powder application method and laser type.

Substrate

200 mm Groove

Powder depth 2 mm

Wedge of powder Powder depth 0 mm

Figure 3. Schematic preplaced powder.

4 mm


10

Figure 5. a) A cross section of the type of clad profile achieved for preplaced powder cladding if the groove must be completely filled. 5b) A micrograph showing the clad – substrate interface weld.

0.1 mm

4 mm

a)

b)

Figure 6. Blown powder cladding results for 46 g/min powder flow at a process speed of 0.5 m/min (CO2 laser).

4 mm

Figure 7. The concave top profile of an under filled groove clad by the blown powder method.

A B C D 4 mm


11

4 mma b

Laser beam

x xxx

Figure 8. A pair of preplaced powder clad tracks produced under identical conditions except for the depth of the powder used. (3.5 kW CO2 laser, spot size 4 mm, cladding speed 0.5 m/min)

powder deep a = 0.75 mm powder deep, b = 1.75 mm of powder.

Figure 9. The change in cross section of a clad track as more powder is added (“x” remains approximately constant as its width is determined by the laser beam diameter on the melt

pool).


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Conclusions 1. It is possible to produce almost flat topped filled grooves by either CO2 and Nd:YAG

laser if blown powder cladding is employed.

2. Pre placed powder cladding does not give flat typed clad filled grooves. However the process may be used to produce a clad track with shallow grooves on either side which could aid lubrication (Once the central protruding part of the clad layer has been machined away).

3. Grooves with too large an aspect ratio cannot be effectively filled with melt.

4. The contact angle of a clad melt on a substrate can be varied and is determined by the laser beam diameter and the amount of powder supplied to the melt.

Substrate

Removed excess clad material

Clad layer

Lubricant supply and debris removal conduits

Figure 10. Schematic cross section.


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3.3. Chapter 4. Laser casting and laser clad casting: New process for rapid prototyping and production

Abstract This paper presents preliminary experimental results from two new processes:

1. Laser casting involves a process similar to blown powder laser cladding but the final product is not joined to the substrate. The substrate surface therefore acts as a mould in a laser casting process and the eventual casting retains the topological features of the substrate.

2. Laser clad-casting involves the production of clad tracks which are welded as usual to a

substrate but which are laid down between machined copper blocks. The eventual clad track therefore has its cross sectional profile determined by the blocks which are removed after completion of the cladding process. In this way clad tracks with large depth to width ratios can be achieved with pre determinated cross sections.

Figure 13 shows the difference between laser cladding and laser casting.

Figure 11. Comparison of laser cladding and casting.

45°

10-15 mm 5 mm

45°

Interfacial melting between the clad layer and substrate

No interfacial melting

Unmelted layer of powder particles


14

3 mm

Machining line

Figure 12 shows an example of laser casting.

Figure 12. Successful laser clad-casting of cross hatched grooves. a) substrate (mould), b) substrate and casting, c) casting. Process parameters: laser power 3 kW (Nd:YAG), beam diameter 5 mm, process speed 0.8 m/min., Ni based powder, powder flow 80 g/min (in Ar), inter-track distance 3mm. Figure 13 shows the difference between standard laser cladding (a+b) and clad-casting (c). a) Standard clad b) Maximum height c) Required clad cross section clad track (semi circular cross section cross section)

Figure 13. Standard clad track cross section (a, b) and the required cross section (c ).

Clad layer

a) b) c)


15

Figure 14 shows the use of moulds in clad casting. Figure 15 shows a successful laser clad cast.

Figure 15. A cross section of the clad-cast track deposited between copper blocks. (substrate width: 3mm,clad track height: 3.5 mm). Process parameters: powder feed (Nickel alloy) 40 g/min, cladding speed 0.5 m/min, laser power 3.5 kW (CO2), beam diameter 4 mm.

Conclusions It has been demonstrated that two new laser cladding techniques are possible and that they may provide novel answers to future production requirements. Laser casting can be used to produce surface castings in high strength alloys to generate tool bits or stamping dies etc. Laser clad-casting can be employed to make clad tracks with large depth to width ratios and pre determined cross sections.

Figure 14. Cross section of the clad cast mould.

Clamping

Substrate Machined copper blocks

3 mm


16

3.4. Chapter 5. Laser wire casting

Abstract This paper describes a new technique for the production of solid wire or rods from powder by laser melting. Three techniques have been developed to ensure that the molten powder solidifies as a rod or wire rather than a series of droplets. The straight rods or wires produced in this way have an almost circular cross section, are several millimetres in diameter and can be pore free. The techniques can be used to produce welding rods, tensile test samples and other solid pieces from a wide range of powder mixes. The rapid thermal cycle involved means that hitherto difficult to produce mixtures and alloys can now be produced in the solid form in seconds.

Wire Powder

Mould Mould

Substrate

Laser beamPowder

a) Cross section of mould and powder before laser irradiation

c) The cross section after laser irradiation

b) During laser processing

Figure 16. Laser wire casting.


17

Laser power 3kW Speed 0.4 m/min Mould separation 3 mm


Laser power 3kW Speed 0.4 m/min Mould separation ≥ 6 mm

Cross section

Cross section

Cross section

General view

General view

General view

5 mm 6 mm 3 mm

5 cm

Figure 17. A selection of results of the side contact mould laser casting process.


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18 a) The powder filled mould 18b) After successful prior to laser melting production of a rod

19a) Before laser melting 19b) After laser melting

Figure 18. The use of a net shape mould to form a rod.

Figure 19. Casting with wires imbedded in powder beds.


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Conclusions

• Wires or rods can be cast from metal powder using a high power laser as a heat source.

• Metal powders which have been laser melted do not readily solidify as uniform cross

section rods unless the tendency to form strings of droplets is inhibited.

• The presence of side wall or net shape moulds can result in rods which are ovoid or circular in cross section and approximately 100% dense. Wires incorporated into the powder bed can have the same effect in the absence of moulds.

• The casting techniques discussed in this paper could be used to produce wires or rods

of a very wide range of alloys and alloy-ceramic mixtures. 3.5 Chapter 6. Melt-Solid Interactions in laser cladding and laser casting

Abstract

Experimental data in conjunction with mathematical models are used to explain various aspects of laser casting and laser cladding by the preplaced powder method. Results include an explanation of the large range of process parameters over which low dilution clad deposits can be produced. Also the interaction of the melt pool with the powder bed is analysed to identify why laser castings have microscopically uneven surfaces.


20

Figure 20. Cross sections of clad tracks made under identical conditions (laser power 3500 W, powder bed depth 1 mm) at different speeds.

a) 0,1 m/min b) 0,2 m/min

c) 0,9 m/min d) 2,1 m/min

e) 3,3 m/min f) 3,8 m/min

1 mm

a b c d e f (0,1 m/min) (0,2 m/min)(0,9 m/min)(2,1 m/min)(3,3 m/min)(3,8m/min) Figure 21. The top views of the clad tracks shown in figure 20.


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The main contra- intuitive feature of figure 20 is the surprisingly low amount of substrate melting over a wide range of process speeds. This phenomenon was first discussed by Powell who postulated a three stage melting process for preplaced powder laser cladding;

1. The laser rapidly melts the powder before the melt touches the substrate because,

prior to substrate contact the melt is surrounded by low conductivity powder. 2. Once the melt touches the substrate it looses a great deal of heat by conduction.

This leads to partial solidification of the melt. As a result the melt-liquid interface does not move into the body of the substrate.

3. If the laser energy continues to irradiate the top surface of the melt, the energy

will eventually move the melt/solid interface back down through the clad layer and across into the body of the substrate.

Figure 22 presents a graphical description of the three stage process derived from a one dimensional mathematical model.

Figure 22. Vertical temperature distribution through the preplaced powder and substrate for different time

steps [9]. Figure 23. Calculated maximum melting depth through the powder (1 mm thick) and substrate ( >> 1 mm) as a function of the processing speed.


22

0

10

20

30

40

50

0 50 100 150 200

Particle Diameter [um]

En

erg

y, P

arti

cle

Dis

trib

uti

on

[a.

u.]

N

E

N*E

Figure 24. Melt-substrate contact history in cross section. (Black = liquid, Grey = Powder , Shaded = Solid).

Figure 25. The particle size distribution and proportion of the incident energy needed to melt the particles of different sizes in this batch.


23

The calculated surface shape and motion is shown in figure 26 for four different grain sizes as a function of time.

Figure 26. Calculated heating and melting of powder grains of different diameter touched by the melting front and subsequent smoothing of the droplets. Figure 27 is a magnified photograph of the surface of a laser casting. The part of the surface shown is that which was in contact with the substrate. This photograph supports the model results presented in Figure 10 as it demonstrates that the liquid surface was covered in partially melted particles.

Figure 27. The surface of a laser cast specimen (This surface was in contact with the substrate).

0,1 mm


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4 Conclusions

This analysis of melt solid interactions has helped to explain the following points about the laser cladding and casting processes;

a) There is a wide parameter range over which dilution free cladding can be achieved by the preplaced powder process. This is primarily due to the difference in thermal conductivity of the powder bed and substrate.

b) If the process parameters are set outside the range mentioned above the result will be

either a dilute clad layer (see figure 3a) or a casting process (see figure 2) depending on whether or not the power input to the process is increased or decreased.

c) The physics of powder particle melting by contact with a liquid pool makes it different

to achieve laser casting with a smooth surface.

5 References

1. Riabkina-Fishman, M., Zahavi, J. (1996). Laser alloying and cladding for

improving surface properties. Applied Surface Science, Vol. 106, no. 1-4, pp. 263-267

2. König, W., Rozsnoki, V., Kirner, P. (1992). Laser Treatment of Materials.

Conference proceedings (ECLAT 92) ISBN 3-88355-185-6, pp. 217-221 3. Yellup, JM. (1995). Laser Cladding using the powder blowing technique. Surface

Coating Technology, Vol. 71, no. 2, pp. 121-128 4. Frenk, A., Vandyoussefi, M., Wagnière, J. D., Zryd, A., Kurz, W. (1997).

Analysis of the laser-cladding [laser surfacing] process for stellite on steel. Metallurgical and Material Transactions B, Vol. 28B, pp. 501-508

5. Hoadley, A, Rappaz, M. (1992). A thermal model of laser cladding by powder

injection. Metallurgical Transactions B, Vol. 23B, pp. 631-641 6. Steen, W.M. Laser Material Processing. (1998). Second edition. Springer-Verlag

London. ISBN 3-540-76174-8, pp. 199-202 7. Hu, U.P., Chen, C.W., Mukherjee, K. (1997). An analysis of powder feeding

systems on the quality of laser cladding. Metal Powder Industries Federation USA, pp. 21.17-21.31

8. Powell, J., Henry, P.S., Steen, W.M. (1988) Laser cladding with preplaced

powder. Analysis of thermal cycling and dilution effects. Surface engineering, Vol 4. no. 2, pp. 141-149

9. Powell, J. (1983). “Laser Cladding”, PhD-thesis, Imperial College of Science and

Technology

H.Gedda: Chapter II-Energy Redistribution During in Laser Cladding; A comparison of Nd:YAG and CO2 lasers

25

Chapter II

Energy Redistribution in Laser Cladding; A comparison of Nd:YAG and CO2 lasers


27

Energy Redistribution in Laser Cladding; A comparison of Nd:YAG and CO2 lasers The following chapter combines information from two published papers;

1) Energy Redistribution During CO2 Laser Cladding

(Published in Journal of Laser Applications. Vol. 14, no. 2, pp. 78-82. May 2002)

H.Gedda*, J.Powell+, G.Wahlström**, W-B. Li*, H.Engström*, C.Magnusson*.

* Luleå University of Technology, Division of System and Manufacturing Engineering,

S-971 87 Luleå, Sweden Phone: +46 920 91169, E-mail: [email protected]

+ Laser Expertise Ltd., Acorn Park Industrial Estate, Harrimans Lane, Nottingham NG7 2TR, U.K.

** Duroc AB, Industrivägen 8, S-90130 Umeå Sweden

2) A Process Efficiency Comparison of Nd:YAG and CO2 Laser Cladding (Published in Welding in the World, vol.46, Special Issue. pp.75-86. July 2002)

H.Gedda*, J.Powell+, A.Kaplan*.

* Luleå University of Technology, Division of System and Manufacturing Engineering, S-971 87 Luleå, Sweden Phone: +46 920 91169, E-mail: [email protected]

+ Laser Expertise Ltd., Acorn Park Industrial Estate, Harrimans Lane, Nottingham NG7

2TR, U.K.

Abstract

Blown powder laser cladding is a cost effective way of producing a surface layer to withstand wear and corrosion. However, the cladding process is slow. Therefore is it of great interest to investigate how much of the laser power is used in the cladding process and how much is reflected etc. In this investigation an Nd:YAG and a CO2 laser have been compared as energy sources for the process. Every aspect of the energy redistribution during cladding has been analysed. The main energy loss to the process for both lasers is by reflection from the melt pool and the powder cloud. It was found that the Nd:YAG laser cladding process is approximately twice as energy efficient as the CO2 laser cladding process. Keywords: Laser cladding; Laser processing, Energy redistribution, Surface treatment.


28

1 Introduction

Blown powder laser cladding involves projecting a stream of metal powder (in an inert gas jet) into a laser generated melt pool on the surface of a metal substrate (see figure 1). The result of this process is a clad track of the cladding metal on the substrate. Such tracks can be overlapped to cover areas of the substrate with a harder and/or more corrosion resistant surface. The process is not energy efficient as a large proportion of the incoming laser power is reflected or reradiated from the cladding zone as shown in figure 2. Figure 2 demonstrates all the different ways in which the incident laser energy is redistributed during the cladding process.

Figure 1. Blown powder laser cladding.

Powder particles

Laser beam

Clad layer


29

A power balance for laser cladding can be expressed as follows: Ptot = PC+PL (1) Where: Ptot = The output power of the laser.

PC = The power utilised in melting the cladding material and welding it to the surface of the substrate.

PL = The power lost by reflection, radiation, convection etc. Pc in equation 1 can be expanded as follows: PC= PP+PS (2) Where: PP = The power utilised in melting the cladding powder. PS = The power utilised in melting the surface of the substrate in order to achieve aclad/substrate weld. PL in equation 1 can be similarly expanded: PL = PA+PB+PD+PE+PF+PG (3)

Figure 2. The redistribution of laser power during the cladding process (see text for definition of PA,PB etc).

(Power lost by convection)

PFPF

PA PA

PB PB

PE PE

PD PD

P Laser beam

Substrate

Powder stream

PD

PA

Substrate

P Laser beam

(Power radiated)

(Power reflected off the surface of the clad)

(Power reflected off the powder particles)

(Power lost by conduction)


30

Where: PA = Power reflected off the surface of the clad zone. PB = Power reflected off the powder particles as they approach the weld pool. PD = Power lost by radiation from the cladding zone. PE = Power lost by convection from the cladding zone. PF = Power lost by conduction from the clad zone to the substrate. PG = Power absorbed by the powder particles which do not enter the cladding melt pool. Figure 2 gives a visual representation of equation 3. Of course these “losses” are to some extent necessary to the cladding process; It is not possible to heat a metal to well above its melting point without having radiant or convective thermal losses, a liquid sitting on a comparatively cool solid will always lose heat by conduction etc. For the purpose of this discussion however, it will be taken that any influence which could minimise PA, PB, PD, PE, PF

or PG would increase the efficiency of the cladding process. This reduction in any of the factors of equation 3 would, of course, increase the proportion of the power available to the cladding process. The aim of commercial cladding is to cover the surface of one metal with another at the lowest cost. Clad depths are usually stipulated and the biggest cost element of the process is laser time. Therefore the simple aim of commercial cladding can be expressed as follows:

• To cover metal A with a known thickness of metal B at the fastest possible rate with a high quality interfacial bond.

Returning to equation 1 it is clear that the process can be speeded up if there is an increase in the proportion laser power available producing the clad layer PC. The requirement here would be to melt enough powder to achieve the correct clad thickness at a faster linear speed. Such an increase in PC must not be employed to melt the substrate to a greater depth. The process must be accelerated to achieve the same (minimum) substrate melt depth at a higher process speed. To summarise:

• The efficiency of laser cladding could be improved by minimising any of the losses in equation 3. This would lead to an increase in PC and the process could be accelerated to produce the same clad depth with a minimal depth of substrate melting.

Earlier work by the present authors [1] quantified the individual elements of equations 1,2 and 3 for CO2 laser cladding. The results of that work concluded that the laser power was redistributed in the following proportions: Power reflected off the workpiece (PA) Power reradiated from the workpiece (PD) Power reflected off the particles (PB) Power absorbed by the process (PC+PF)

= 50% = 1% = 9% = 40% 100%


31

Of the power absorbed by the process (40%) three quarters of it was employed in simply heating the substrate and only the remaining 10% of the original laser power was used to melt material to produce a clad layer. This present work involves repeating this quantification of the power redistribution for Nd:YAG and CO2 laser cladding in order to compare the efficiency of the two types of laser for this process. These experimental trials were carried out at a laser power of approximately 3 kW for both types of laser. This allowed a direct comparison of the lasers and also a confirmation of the previous published results [1] at a different power level (the earlier work was carried out at a power level of 5 kW).

2 Experimental work

2.1 General

The substrate material used in this study was (SS 2172) steel with the following composition:

Table 1. Steel composition (substrate)

C Si Mn P S V N Fe wt % 0.16 0.22 0.94 0.014 0.022 0.06 0.009 98.6

The cladding material was cobalt based with the following composition:

Table 2. Cladding powder composition

Cr C Si Mo Ni Fe Co wt % 27.2 0.27 1.0 5.5 2.3 0.3 63.4

The substrate specimens were grit blasted before cladding was carried out. The laser used was a Rofin Sinar RS 6000 CO2 laser with a maximum output power of 6 kW and the Nd:YAG laser was a Haas Laser HL 3006 D 4 kW. The powder feeder was a TECFLO TM 5102. The shielding/carrier gas employed to propel the powder was argon.

2.2 The power absorbed by or reflected off the powder cloud above the clad zone

During the cladding process the laser beam must travel through the powder cloud in order to reach the cladding zone (see figure 2). A proportion of the laser energy is reflected off the powder cloud and is lost to the cladding process. Another portion of the incident energy is absorbed by the particles but some of this energy is also lost to the process because not all the heated particles join the cladding melt pool. A simple experiment was set up to discover what proportion of the original laser power would penetrate the powder cloud (see figure3 below). A commercially available “power probe” was used to measure the laser power with and without the powder stream turned on. The powder flow rates were typical of the cladding process as were all the other process parameters. The average results from several such tests are presented in table 3. The energy absorbed by the powder cloud was directly measured by measuring the average temperature rise of the powder


32

after it had passed through the beam (see figure 3). The power reflected off the powder cloud could then be easily calculated as shown in table 3.

Table 3. Power absorbed and reflected by the powder cloud irradiated by the two types of laser

Laser type

Laser output Power *

(Watts)

Powder flow rate (g/min)

Post powder cloud power (Watts)

Total power reflected and absorbed by the powder cloud ** (Watts)

Power absorbed by powder cloud ** (Watts)

Power Reflected off powder cloud (PB) ** (Watts)

Nd:YAG

2743 (100%)

30

2506 (91%)

237 (9%)

18 (1%)

224 (8%)

CO2 2695 (100%)

30 2457 (91%)

238 (9%)

22 (1%)

218 (8%)

* Measured by the power probe but with zero powder flow. ** Percentages are approximate It is clear from table 3 that we now have an approximate value for PB (the power reflected off the powder cloud) for the parameter range covered here: PB = 8 % Ptot for the Nd:YAG laser and the CO2 laser (4) One other component of equation 3 can also be identified from table 3 after the same parameters were used for actual cladding. This parameter is PG, the level of power absorbed by particles which do not enter the cladding melt pool. A number of cladding trials were carried out and these showed that, over this range of parameters, the proportion of particles which formed the clad track was 60% (The range was 57%-63%). It can then be concluded that 40% of the heat collected by the powder cloud (1% Ptot see table 3) does not contribute to the cladding process. i.e. PG = 0.4 % Ptot for both types of laser (5)

Figure 3. The experimental arrangement for the analysis of the absorption and reflection of the energy by the powder cloud.

Laser BeamPowder Particles

Insulated Calorimeter


33

2.3 The power lost by radiation from the cladding zone (PD).

The total energy radiated from the clad pool can be calculated from the pool temperature, surface area and emissivity. If the emissivity of the liquid metal pool is taken as equal to one then the calculation is simplified and the maximum possible radiation power can be estimated: PD = σ T4A (6) Where:

σ is the Stefan-Boltzman constant (5.7*10-8 Wm-2K-4) T is the surface temperature of the melt (K) A is the area of the melt surface (m2)

In this case the surface temperature of the melt was approximately 2300 K [1] and its surface area was 19 mm2. This gives a maximum value for PD of: PD= 648 10*19*)2300(*10*7.5 −− = 30 Watts (7) PD ≈ 1% of Ptot for both the Nd:YAG and the CO2 laser (8)

2.4 Power lost by convection from the clad zone (PE).

The cladding zone is a molten alloy with a surface temperature of ≈ 2300 K and a known surface area. This melt is exposed to a stream of argon which carries the powder to the clad zone. The argon flow was measured and found to have an average flow velocity of 4.3 m/sec. The rate of convective cooling of a hot body exposed to a cooler gas is given by: Q=hA∆t Watts (9) Where: h = Heat transfer coefficient. A = Surface area of the hot body. ∆t = The difference in temperature between the body and the cooling gas. Evaluation of h from a standard text on the subject [2] gives us a value of approximately 100 W/m2K. Q = ( ) 2000*0025.0**100 2π (10) PE = 3.9 Watts (11) Or PE = 0.1% Ptot for both Nd:YAG and CO2 laser (12)


34

2.5 Power reflected off the surface of the clad zone (PA).

Calorimetry was employed to measure the heat input to each clad sample (Pin). From this measurement it is possible to measure the power reflected off the cladding zone (PA) in the following way: PA = Ptot – ( PB+PD+PE+PG+Pin) (13) Table 4 shows the average results from the calorimetric measurements over a range of process parameters.

Table 4. Calorimetric measurements (average values)

From our earlier results: PA = 100 – (8+1+0.1+0.8+49) (Nd:YAG) (14)

PA = 41.1 % (Nd:YAG) (15)

PA = 100 – (9+1+0.1+0.8+39) (CO2) (16)

PA = 51.1 % (CO2) (17)

So far this is the first time that the measurements from the two types of laser have shown an appreciable difference. In summary it can be said that, for the CO2 laser, approximately half of the laser power is reflected from the cladding zone. For the Nd:YAG laser this value is reduced to approximately 40%. These generally high reflectivity values confirm the work of other authors in the field [3] who suggest that the onset of melting is associated with a rise in material reflectivity. This is because a molten surface in an inert atmosphere (in this case argon) is smooth and oxide free. This smooth, oxide free surface acts as a better reflector than the solid, rough, oxidised surface which exists before melting. It is well known [4] that metals have a lower reflectivity for the 1.06 µm radiation of Nd:YAG lasers than for the 10.6 µm radiation of CO2 lasers and this is confirmed by the above results. As we will see later in this paper, this reduction in reflectivity for the Nd:YAG laser results in a marked increase in process efficiency when cladding as compared to a CO2 laser.

Laser type

Laser output Power (Ptot) (Watts)

Power input to clad sample (Pin) (Watts)

Power % input to sample

Nd:YAG 2743 1367 49% CO2 2695 1044 39%


35

2.6 The power utilised in melting the clad layer to the substrate (PC)

Blown powder laser clad layers usually have a cross sectional geometry similar to that shown in figures 4 and 5.

Figure 4. The cross sectional geometry of a blown powder laser clad layer. Note: the melted substrate and cladding material are mixed together during the process. As figure 4 demonstrates, the production of a clad layer usually involves melting the surface layers of the substrate. The amount of substrate melting can range from minimal to levels where the clad layer is really a dilute alloy of the substrate and cladding material.

Figure 5. Macrographs of a typical laser clad sample in cross section (see also figure 4).

PC, the power utilised in melting the cladding material and welding it to the surface of the substrate can be calculated as follows:

PC = Avρ(Cp∆t + ∆Hm) (18) Where: A = The melt cross sectional area (m2). v = The Cladding speed (m/s). ρ = The Density of the material melted. Cp = Specific heat capacity of the material melted. ∆t = The difference between the melt temperature and ambient. ∆Hm = Specific heat of melting of the clad melt.

6.2 mm

Clad layer

HAZ

Melted substrate

Clad layer

Melted substrate

Heat affected zone


36

In order to evaluate Pc accurately for both the CO2 and the Nd:YAG laser a set of cladding trials were carried out. Laser power and focusing conditions were kept identical for the two lasers (2700 W, 5 mm beam diameter). Powder flow rates of 30, 40 and 50 g/min were employed at cladding speeds of 0.7, 0.8, 0.9, 1.0, 1.2 and 1.4 m/min. From all these tests an average for Pc was calculated for both types of laser. Figure 6 shows cross sections of clad traces produced by both types of laser at a powder flow rate of 40 g/min and speeds of 0.7, 1.0 and 1.4 m/min. a) CO2 laser b) Nd:YAG laser Figure 6. Clad cross sections at increasing process speed for both types of laser. (laser power 2700 W, laser spot diameter 5 mm, powder flow rate 40 g/min. It is clear that a substantial amount of substrate was melted in each case. On average the melt was found to consist of a 40% substrate; 60% clad material mix for the CO2 laser and 55% substrate and 45% metal mix for the Nd:YAG laser. As a simplification, the material properties necessary for equation 18 were taken as being for a 50:50 mixture of cladding material and substrate. ρ = 8020 kg/m3 Cp = 500 J/kg K ∆Hm = 300 kJ/Kg

0.7 m/min 1.0 m/min 1.4 m/min

3 mm

0.7 m/min 1.0 m/min 1.4 m/min


37

From these values and a melt temperature of 2300 K [1], the values for Pc for the two types of laser were: PC (Nd:YAG) = 506 W (19) Which represents 18% of Ptot PC (CO2) = 266 W (20) Which represents 9.5 % of Ptot This is a remarkable result. Here we can see that the Nd:YAG laser melts approximately twice as much material its CO2 counterpart. In our earlier work [1] we found that for a CO2 laser at a power of 5 kW only 10% of the laser power was used to melt metal during cladding. This result is confirmed here for a different set of process parameters. For the Nd:YAG laser however, the proportion of the laser power involved in melting is almost double the CO2 value. Section 2.5 revealed that 50% of the CO2

laser light was reflected from the clad zone as compared 40% for the Nd:YAG laser. It seems then, that the difference of 10% is almost exclusively given over to material melting and this represents a doubling of the energy available for melting.

2.7 Power lost by conduction from the clad zone to the substrate (PF)

This value is easily established by subtracting (PC) from the total power absorbed by the workpiece (Pin). Taking average values:

(Nd:YAG) PF = PIn- PC ≈ 30%Ptot (21)

(CO2) PF = PIn- PC ≈ 28%Ptot (22)

This is the result which would be expected given that all extra laser power which joins the process when an Nd:YAG laser is used is involved in the melting process , (see previous section).


38

3 Discussion

Figure 7 presents schematics of the redistribution of energy during the laser cladding process for both types of laser.

Figure7. Schematic of the redistributions of energy during the laser cladding process (percentages are approximate).

For the sake of clarity PE (convective losses) and PG (lost powder losses) have been left out of figure 7 as their contribution to the energy balance is negligible.

Power absorbed by the cladding process 40% 50% Power used to melt the Clad layer (PC) 10% 20% Power absorbed in heating the substrate (PF) 30% 30%

CO2 Nd:YAG % of power % of power Raw Power of beam (∼3 kW) (Ptot) 100% 100% Power Reflected off the workpiece (PA) 50% 40% Power Re radiated from the workpiece (PD) 1% 1% Power Reflected off particles (PB) 8% 8% Power absorbed by the Process (PC+PF) 40% 50%

∼100%


39

The major mechanism of energy loss to the process is that of reflection from the melt pool and the powder cloud. Reflection off the melt pool is a function of the condition of the melt surface. As this melt is produced in an inert atmosphere it experiences no surface oxidation and thus has a high reflectivity. Dilution of the inert shroud gas with an oxidising agent would decrease this reflectivity but may have disruptive consequences on the stability of the process and the metallurgy of the clad track. Overlapping such deliberately oxidised tracks could also prove problematic. A reduction of reflective losses from the powder cloud on the other hand would be comparatively easy. All that is necessary is an increase in the average particle diameter. During its passage through the laser beam the particle interacts with and reflects light over an area equal to its cross sectional area (πr2) rather than half its surface area (2πr2). This because the shadow cast by any particle has an area of πr2 where r is the particle radius. A particle of twice the original radius would cast a shadow four times as big but would have eight times the mass (mass ∝ r3). Thus it is clear that, for a set mass flow rate, larger particles interact with (and reflect) less of the beam. This is of course only useful within certain limits as the cladding process will break down if the particles are too large. One very important feature of figure 7 is that, although the power absorbed by the process increases only from 40% to 50% when CO2 and Nd:YAG lasers are compared, the power employed in melting material increases by a factor of 2 from 10% to 20 %. This doubling of the energy efficiency of the process is clearly demonstrated in figure 8 which compares a low speed (0.7 m/min) CO2 laser clad sample with an Nd:YAG laser clad sample carried out at twice that speed (1.4 m/min).

a) CO2 laser 0,7 m/min b) Nd:YAG laser 1.4 m/min

Figure 8. A demonstration of the doubling of the process speed possible when using an Nd:YAG rather than CO2 laser.(The powder feed rate was increased from 30 g/min for the CO2 laser to 50 g/min for the Nd:YAG laser but the laser power (∼ 3kW) and spot size (5 mm) were kept constant.) The doubling of the process efficiency shown in figures 7 and 8 would not be possible if the “powder absorbed in heating the substrate” (PF see fig 7) changed as more power was absorbed by the process. PF remains steady (in this case at 30%) because it is determined by the amount of power the substrate needs to absorb before surface melting is initiated. This is a threshold value, which will not charge with increasing absorptivity. This being the case, any increase in absorbed power will be entirely available to the melting process.

3 mm


40

The CO2 laser results given in figure 7 are almost identical to the earlier published figures from experiments carried out at 5 kW on different equipment [1]. It is therefore possible to say that these results are typical of multi kilowatt laser cladding.

4 Conclusions

1. Ignoring the trivial contributions of convective and radiative cooling etc, the laser power applied to the cladding process is redistributed in the following ways:

*This value includes powder and substrate melting.

2. Nd:YAG lasers are approximately twice as energy efficient as CO2 lasers for cladding in the range of parameters covered in this paper ( and by implication, the higher power (5 kW) range covered in our earlier work [2]) i.e. given the same laser power, Nd:YAG lasers are capable of approximately double the cladding rates of CO2 lasers.

3. As a large proportion (30%) of the laser power is consumed in heating the substrate it is

likely that substrate pre heating by a cheaper power source* would improve the profitability of laser cladding. (* flame, plasma, induction etc).

5 References

1. Gedda, H., Powell, J., Wahlström, G., W-B, Li., Engström, H., Magnusson, C. (2002). Energy Redistribution During CO2 Laser Cladding. Journal of Laser Applications, Vol. 14, pp. 78-82

2. Porier, D.R., Geiger, G.H. (1994). Transport Phenomena in Materials

Processing. The Minerals, Metals & Materials Society, ISBN 0-87339-272-8, pp. 219-236

3. Bloehs, W., Grünenwald, B., Dausinger, F., Hügel. (1996). Recent progress in

laser surface treatment. Part 1: Implications of laser wavelength. Journal of Laser Applications, Vol. 8, pp. 15-23

4. Steen, W.M. Laser Material Processing. (1998). Laser surface treatment.

Springer-Verlag London. Second edition, ISBN 3-540-76174-8, pp. 199-202

Laser type CO2 Nd:YAG Power reflected off the cladding melt 50% 40% Power reflected off the powder cloud 10% 10% Power used to heat the substrate 30% 30% Power used to melt the clad layer* 10% 20%

41

Chapter III

Laser Cladding into pre machined grooves

H.Gedda: Chapter III-Laser Cladding into pre machined Grooves


43

Laser Cladding into pre machined grooves.

J.Powell+, H.Gedda*, A.Kaplan*.




Abstract

When laser cladding is used to improve the wear characteristics of a substrate it is not always necessary to clad the whole surface. Wear resistant individual tracks can be clad directly onto the substrate or into pre machined grooves. This paper investigates the process parameters which affect the finished product when cladding into groves including; groove geometry, powder application method and laser type.

1 Introduction

Laser cladding is a process by which a metal powder is melted onto the surface of a metal substrate. There are two common methods of providing powder for this process;

a) Pre placed powder; where a layer of powder is applied to the surface of the substrate and subsequently melted by the laser (see figure 1a).

b) Blown powder; where powder is propelled into the cladding melt pool by means of a non oxidising gas stream (see figure 1b).

Figure1. (a) Preplaced and (b) blown powder

laser cladding.

a)

b)

Cladding materialLaser beam


44

Laser cladding can be used to provide a protective coating of hard or corrosion resistant metal on a weaker substrate. Tracks of the harder, powdered material are laid down next to each other to form a new surface as shown in figure 2.

The individual clad tracks which go to make up a clad layer have their cross sectional shape determined by a number of factors including laser power, laser beam width and powder characteristics etc. Typical individual clad tracks produced by the preplaced and blown powder methods are presented in cross section in figure 3.

It is clear from figure 3 that individual tracks of laser melted powder would not generally be useful in an engineering context as the harder material forms a ridge on the substrate. This is different situation from that experienced in the field of laser surface hardening. Surface hardening [1-3] (which does not affect the substrate surface flatness) has been successfully used to extend the wear life of components by applying single tracks rather than covering an entire surface with a hardened layer. This use of single tracks reduces laser processing costs and the thermal input to the component.

HAZ Significant substrate melting

Substrate

4 mm

Substrate Heat affected zone (HAZ)

Minimal clad layer/substrate interfacial melting

1 mm

Figure 2. A cross section of a laser clad layer of Ni-based material on a low carbon (SS 2172) steel.

Figure 3 a. A typical cross section of a single clad track produced by the preplaced powder method.

Figure 3b. A typical cross section of a clad track produced by the blown powder method.

1 mm


45

One early application of single track hardening was employed by the automobile industry to improve the wear characteristics of a piston and cylinder [4]. In this case a spiral track was produced on the piston and this interacted with three or four straight hardened lines down the length of the internal face of the cylinder.

In order to replicate the advantages of the single track approach for cladding it is necessary to deposit the cladding material into pre machined grooves. This paper investigates the effect of the cross sectional shape of the grooves on the eventual clad track.

2 Experimental work

If grooves are to be filled with cladding material it is important to optimise the cross sectional geometry of the groove. For this experiment “V” shaped grooves were produced with included angles of 30º, 45º, 60º, and 90º.

The gap at the top of the grooves was kept constant at 4 mm. These grooves were clad using CO2 and Nd:YAG lasers both operating at a power of 3 kW. The substrate was mild steel and the cladding material was Nickel based super alloy (see table 1).

Both the pre-placed and blown powder techniques were investigated as follows;

a) For blown powder the mass flow of the powder stream (in Argon) was increased in five steps from 22 to 46 grams per minute.

b) For pre placed powder a wedge of powder was prepared over the groove as shown in figure 4. The depth of the powder increased from zero at one end of the groove to 2 mm at the other end (200 mm away).

A photograph of a groove clad in this way is presented in figure 5. This use of a wedge of powder is useful in demonstrating the progressive effect of an increase in powder depth. All powder wedge samples were produced at a process speed of 0.5 m/min.

Table I. Equipment and Parameters

CO2 laser, Rofin Sinar RS 6000 (6 kW)

Nd:YAG, Haas Laser HL 3006 D (4 kW)

Spot size at top of groove (both laser ) = 4 mm

Substrate, SS 2172 Mild steel (0.16% C)

Cladding Material, Nickel based (80% Ni, 20% Cr)

Powder feeder, Sulzer Metco Single 10 C


46

Figure 6. Cross sections of grooves showing that even when there is sufficient melt to produce a flat surface the clad layer does not do so when preplaced

powder is used.

3 Results

A. CO2 laser; Preplaced Powder

Figure 6 shows cross sections of the preplaced powder cladding trials at the section in the sample where there was enough melt to fill the top of the groove. It is clear that for all these samples the melt has not assumed a flat top surface. In all these cases the melt has retained its circular curvature towards the top of its cross section. It is also noticeable that there is a pore along the bottom of the clad groove for angles less than 90º.

Substrate

200 mm Groove

Powder depth 2 mm

Wedge of powder Powder depth 0 mm

Figure 5. A grooved powder wedge sample (see figure 4) after cladding.

Figure 4. Schematic preplaced powder.

4 mm


47

Figure 7 demonstrates that the addition of more powder to the melt results in a clad trace which over fills the groove. This sample also demonstrates the very low amount of substrate melting which is often typical of pre placed powder cladding [5].

B. CO2 laser; Blown Powder

Figure 8 shows the cross sections of blown powder cladding for the four types of groove at a powder flow rate of 46 g/min and a process speed of 0.5 m/min.

Figure 8 reveals that the 30º groove is unsuitable to the process because the powder stream does not project sufficient material into the bottom of the groove. The 45º and 60º groove are successfully filled with almost flat top surface although there are small linear pores at the bottom of the grooves. The 90º grooved sample has become overfilled with melt at this powder flow rate as its cross sectional area is considerably smaller than those for the other angles.

0.1 mm

4 mm

7a)

7b)

Figure 7. A cross section of the type of clad profile achieved for preplaced powder cladding if the groove must be completely filled. 7b A micrograph showing the clad – substrate interface weld.

Figure 8. Blown powder cladding results for 46 g/min powder flow at a process speed of 0.5 m/min (CO2 laser).

A B C D 4 mm


48

As far as producing an overall flat surface is concerned, samples b and c in figure 8 are much more successful than the preplaced powder samples shown in figure 6. The reason why these cross section are flatter must be attributed to the action of the powder jet gas flow on the solidification dynamics of the melt. This point is supported by figure 9 which shows that under filled grooves produced by the blown powder method had concave rather than convex top profiles. Figure 9 also demonstrates the increased substrate melting common to blown powder cladding.

C. Nd:YAG laser; Preplaced Powder

Figure 10 demonstrates that a change of laser type from CO2 to Nd:YAG does not produce a flat surface when preplaced powder is employed. The results are very similar to those given in figure 6 for the CO2 laser.

D. Nd:YAG laser; Blown Powder

Figure 11 shows the results of blown powder cladding with the Nd:YAG laser and the maximum flow rate. Once again the 45º groove produces an almost flat top surface and because of its smaller cross section the 90º groove is overfilled. It was noticed however that the 90º samples for both types of laser did not produce flat clad surfaces even at lower powder flows. This retention of a curved upper melt surface is possibly related to the superior heat sink capacity of the 90º grooves.

Figure 9. The concave top profile of an under filled groove clad by the blown powder method.

4 mm

4 mm

Figure 10. Tracks made by Nd:YAG laser and preplaced powder when there is enough melt to fill the groove. Once more the melt retain its curved upper surface.

4 mm

Figure 11. Blown powder cladding results for 46 g/min powder flow at a process speed of 0.5m/min (Nd:YAG laser).


49

4 Discussion

Before we analyse the cross section of the clad groove samples it is important to make a few remarks about cladding onto flat surfaces. Figure 12 shows the cross sections of two clad tracks produced under identical conditions except for the depth of the preplaced powder involved.

At first glance the two cross sections in figure 12 look similar to the types of cross section (and those shown in figure 3) we would expect of droplets of any liquid on a substrate.

It is tempting therefore to apply the same type of physical analysis to the clad cross sections as far as contact angle and surface tension are concerned. However, the situation for laser cladding is not that simple. For a droplet of a liquid on a solid surface, the contact angle θ is determined by the various surface tensions associated with the liquid, the solid and the surrounding air (see figure 13). If more liquid is added to the droplet the contact angle does not change but the contact area between the droplet and the solid increases (see figure 14).

γlg

Gas Liquid

Solid

θ γsl γsg

γlg = Surface tension; Solid liquid γsl = Surface tension; Liquid gas γsg = Surface tension; Solid gas

4 mm a b

Figure 12. A pair of preplaced powder clad tracks produced under identical conditions except for the depth of the powder used. (3.5 kW CO2 laser, spot size 4 mm, cladding speed 0.5 m/min)

powder deep a = 0.75 mm powder deep, b = 1.75 mm of powder.

Figure 13. The surface tension forces which determine the contact angle θ for a droplet of liquid on a solid.


50

In the case of laser cladding the contact angle is not determined by the surface tension of the liquid. This is clearly demonstrated in figure 12 where the contact angle is close to 45º in one case and close to than 90º in the other for the same liquid on the same substrate. Although it is true that the two melts may have achieved different temperatures (and therefore surface tensions) during their melting cycle this effect is unimportant compared with the influence of the laser beam diameter on the cladding zone. The effect of the laser beam diameter on the cladding process is to (approximately) fix the width of the melt-substrate contact. As the melt cannot spread laterally if more powder is added to the cladding process, the clad cross section changes as shown in figure 15.

It is clear from figure 15 that the upper surface of the melt will always assume a shape which is part of a circle but this circle is intersected by a cord of (approximately) fixed length and represents the melt- substrate interface. The cross section shape of an individual clad track is therefore largely determined by two parameters; the volume of the clad track per unit length (which gives us the size of the part circle in figure 15) and the diameter of the laser beam on the cladding melt pool (which gives us the width of the melt-substrate interface). For this reason it is not possible to match the interaction of a groove angle to the contact angle of the melt in order to achieve a flat surface. On the other hand the grooves investigated here do have an effect on the clad finished product.

It is clear from the results given in figures 6 and 8 that if the aspect ratio of the groove is too large then the melt will not be able to fill it adequately. These figures also demonstrate the point that grooves with an acute internal angle will tend to have a cavity at their base.

This cavity is probably the result of melt surface tension which would limit minimum radius achievable by the melt. The larger included angle of 90º tended to encourage complete penetration of the melt into the groove.

θ θ

Smaller droplet Larger droplet

Laser beam

x x x x

Figure 14. θ remains the same if the droplet size changes for a normal liquid droplet.

Figure 15. The change in cross section of a clad track as more powder is added (“x” remains approximately constant as its width is determined by the laser beam diameter on the melt

pool).


51

The result given here also reveal that there is no fundamental effect on the process by changing from CO2 to Nd:YAG laser.

If a flat topped clad groove is required these results imply that preplaced powder will never give the desired product. On the other hand it seems clear that the downward thrust of the gas/powder feed in blown powder cladding can help to flatten the top surface of the melt. However, a flat clad surface may not give optimum performance. It has been noted in some single track hardening studies that the track is accompanied on either side by softened areas which are prone to accelerated wear. This wear results in erosion as shown in figure 16.

This eroded channels were found to be beneficial to the wear behaviour of the components as they allowed the flow of lubricant to the hard, load bearing area and the removal of wear particles [6].

This principle could be extended to laser cladding of grooves in certain cases. If a clad groove of the type shown in figure 6 was produced and the protecting clad material was ground away the remaining shallow grooves next to the clad track could supply lubricant and debris removal conduits as shown in figure 17.

Substrate

Removed excess clad material

Clad layer

Lubricant supply and debris removal conduits

Figure 16. A Schematic cross section of a laser hardened track.

Substrate

Laser hardened track Softened eroded area

Figure17. Schematic cross section.


52

5 Conclusions

1. It is possible to produce almost flat topped filled grooves by either CO2 and Nd:YAG laser if blown powder cladding is employed.

2. Pre placed powder cladding does not give flat typed clad filled grooves. However the process may be used to produce a clad track with shallow grooves on either side which could aid lubrication (Once the central protruding part of the clad layer has been machined away).

3. Grooves with too large an aspect ratio cannot be effectively filled with melt.

4. The contact angle of a clad melt on a substrate can be varied and is determined by the laser beam diameter and the amount of powder supplied to the melt.

6 Acknowledgements

We gratefully acknowledge the financial support from VINNOVA, SSF and Kempe Foundation.

7 References

1. Migliore, L. (1996). Laser material processing. Marcel Dekker Inc New-York. ISBN 0-

8247-9714-0, pp. 209-237 2. Ruiz, J., Lopez, V., Fernandez, B J. (1996). Effect of the surface laser treatment on the

microstructure and wear behaviour of grey iron. Materials and Design, ISSN 0261-3069, Vol. 17, no. 5-6, pp. 267-273

3. Ion, J C. (2002). Review - laser transformation hardening. Surface Engineering, ISSN

0267-0844, Vol. 18, no. 1, pp. 14-31 4. Eckersley, J, S. (1984) Laser Applications in Metal Surface Hardening. Advances in

Surface Treatments, Technology Applications Effects, Vol. 1, pp. 211-231 5. Powell. J. (1988) Laser Cladding With Preplaced Powder; Analysis of thermal cycling

and dilutions effects. Surface Engineering, Vol. 4, no. 2, pp. 141-149 6. Steen W, M., Powell, J. (1981). Laser Surface Treatment Materials in Engineering, Vol.

2, no. 3, pp. 157-162

H.Gedda: Chapter IV-Laser Casting and Laser Clad-Casting: New process for rapid prototyping and production

53

Chapter IV Laser Casting and Laser Clad-Casting: New

processes for rapid prototyping and production.


54


55

Laser Casting and Laser Clad-Casting: New processes for rapid prototyping and

production.

J.Powell+, H.Gedda*, A.Kaplan*.




Abstract

This paper presents preliminary experimental results from two new processes:

1. Laser casting involves a process similar to blown powder laser cladding but the final product is not joined to the substrate. The substrate surface therefore acts as a mould in a laser casting process and the eventual casting retains the topological features of the substrate.

2. Laser clad-casting involves the production of clad tracks which are welded as usual to a

substrate but which are laid down between machined copper blocks. The eventual clad track therefore has its cross sectional profile determined by the blocks which are removed after completion of the cladding process. In this way clad tracks with large depth to width ratios can be achieved with pre determinated cross sections.

Keywords: Laser cladding, Laser processing, Laser casting, Laser clad-casting


56

1 Introduction

This paper presents the preliminary results of an experimental program investigating two new processes: Laser casting and laser clad-casting. As a technique, laser casting is similar to blown powder laser cladding but the aim in this case is to produce a “clad” layer which is not fused to the substrate. The resulting “clad “ layer retains the topological features of the surface of the substrate which effectively acts as the mould in a casting process. An example of the detached “casting” and its mould is presented in figure1.

During laser clad-casting the clad track is welded to the substrate as usual but the cross sectional profile of the track is determined by copper blocks which act as moulds and are later removed. An example of such a clad-cast track is shown in figure 2.

Figure 2. A clad-cast track on the edge of a sample.

(Process parameters: laser 3,5 kW (CO2), cladding speed 0.7 m/min, powder feed 45 g/min)

The reminder of this paper will discuss these two processes separately.

Figure 1. A Laser casting and the mould it was produced with. (Process parameters: laser 3 kW (Nd:YAG), cladding speed 0.8 m/min, powder feed (cobalt alloy)

80g/min).

3 mm


57

2 Laser casting

The experimental set up for laser casting is similar to that for blown powder laser cladding. Schematics of both processes are presented in figure 3.

Figure 3. Comparison of laser cladding and casting.

From a process parameter point of view there are only three differences between the two techniques;

1. The powder mass flow is higher for clad-casting, typically 2 or 3 times the flow needed for cladding under the same conditions.

2. The powder feed nozzle is much nearer the melt pool than it is for cladding.

3. The laser beam is defocused to approximately twice the original diameter normally

used for cladding (in this case from 4 to 8 mm diameter). This reorganisation of the powder delivery and power density has a fundamental effect on the process which prevents the substrate from melting. This effect is demonstrated in figure 3. Figure 3a shows that, during blown powder laser cladding, the laser beam directly irradiates both the surface of the molten cladding material and the substrate. The result is the establishment of a fusion line beneath the original surface of the substrate. This ensures good adhesion of the clad layer as it is welded to the substrate. In the case of laser casting (figure3b) the powder flow conditions are such that a layer of unmelted powder builds up immediately in front of the molten cladding zone. This has two effects:

45°

10-15 mm 5 mm

45°

3a. Laser Cladding 3b. Laser Casting

Interfacial melting between the clad layer and substrate

No interfacial melting

Unmelted layer of powder particles

Laser beam Powder feed


58

1. The powder layer shields the substrate from direct laser heating and thereby inhibits substrate melting.

2. The powder layer cools the lower part of the melt by becoming melted into it.

Another important influence on the temperature distribution in the melt is the shadowing effect of the powder cloud. The powder cloud absorbs energy from the incident laser beam and casts an increasingly dense shadow over the melt pool as the mass flow rate is increased. Also, the upper particles in the powder cloud cast a shadow over the lower particles [1]. The particle cloud therefore tends to transport energy from the laser beam towards the top part of the melt (where the hotter upper particles land) and away from the lower part (where the cooler, shadowed particles land). All of these effects reduce the ability of the cladding melt-laser combination to melt the substrate. The result is a “clad” layer which is not welded to the surface of the substrate.

3 Experimental procedure

3.1 General

For the purposes of the experimental runs the following equipment and materials were employed: Laser model: Haas Laser HL 3006 D (4 kW) Nd:YAG. Laser power 3 kW. Powders: Stellite 8 (Cobalt base), Deloro Alloy NO 35 S (Nickel base), ASP 60 (Iron base). The powder chemical compositions are presented in table 2-4 below. Powder feeder: TECFLO TM 5102.

Powder feeding: 80-110 g/min. Powder feed gas: Argon. Process speed: 0.6-1.0 m/min. Substrate (mould): single and cross-hatched “V” shaped groves with an internal angle of 90° with depths of 2,4 and 6 mm. The mould chemical composition is presented in table 1 below. Chemical Composition

Table1. Steel composition (mould)

C Si Mn P S V N Fe wt % 0.16 0.22 0.94 0.014 0.022 0.06 0.009 98.6

Melting point (Tm) = 1773 °K

Table2. Co based powder composition


Powder size 45-150µm (Tm) = 1459-1656 °K


59

3 mm

Table3. Fe based powder composition

Cr C V Mo Fe Co wt % 4.2 2.3 6.5 7.0 69.5 10.5


Table4. Ni based powder composition

Cr C Si B Ni Fe wt % 3.7 0.4 3.5 1.6 86.5 2.0


3.2 Laser casting: results and discussion

Laser casting involves a large number of inter dependant process variables such as; laser beam power and diameter, process speed, powder type, substrate type, powder mass feed rate and particle speed etc. This introductory paper will not therefore, attempt to map out the whole process. Our aim here is to demonstrate that which is easily achievable and to point out areas of difficulty. Figures 1 and 4 show successful examples of the process for single and cross hatched grooved substrates. Cross sections of these two samples are shown in figures 5 and 6 and these clearly show that the castings are close to 100% dense. (The actual figures are ∼95% for these two cross sections). Although these two samples involved a correct balancing of the process parameters, much poorer results are achieved if certain guidelines are not followed. These guidelines are presented in the following notes.

a b c

Figure 4. Sucessful laser clad-casting of cross hatched grooves. a) substrate (mould), b) substrate and casting, c) casting. Process parameters: laser power 3 kW (Nd:YAG), beam diameter 5 mm, process speed 0.8 m/min., Ni based powder, powder flow 80 g/min (in argon), inter-track distance 3mm.


60

Powder mass flow rate If the powder mass flow rate is excessive the melt will rest on a bed of powder rather than the substrate. The resulting “cast” will therefore not take on the features of the mould. An example of this is presented in figure 7.

If the powder mass flow rate is too low then the excess laser energy will melt the surface of the substrate (either directly or by conduction through the melt pool). In this case the clad layer will be welded to the substrate and it will not be possible to separate the two later.

Process speed / laser power / laser spot diameter. These three parameters are inter related in their effect on the process and can be described as a function of the energy density:

Energy density =VD

P (1)

Where P is the laser power, V the process speed and D the laser spot diameter. Generally as P/VD is increased there is a tendency for increased welding of the clad layer to the substrate.

Figure 5. A polished and unetched cross section of the sample shown in figure 1 showing the 95 % density of the casting.

Figure 6. A polished and unetched cross section of the sample shown in figure 4 showing the 95% density of the casting.

Figure 7. An example of a casting which failed due to excessive powder mass flow rate. (process parameters as fig 1 except powder mass flow rate increased to 110 g/min).


61

As P/VD is decreased there is a tendency for the melt not to contact the substrate. In this case, as for excessive powder, the casting bears no relationship to the mould.

Powder and substrate type In our investigations we found that the easiest and best quality results were achieved with the nickel based alloy. Tables 1-4 reveal that, of the three powder alloys, the nickel based alloy has a melting point (≈ 1300 °K) which is furthest removed from that of the substrate (≈ 1800 °K). This means that the operating window for molten/solid contact between the two materials without welding would be greater than for the other alloys. Wet contact without welding is necessary for an accurate casting and this is made easier by a large difference in casting – mould melting point or by melt substrate incompatibility. Other types of incompatibility could be chemical (i.e ceramic substrate moulds could be used) or thermal (e.g. water cooled substrates or high conductivity alloys could be used). The difference in performance of the three alloys used here is clear from figure 8. The melting point, although important, is not the only powder characteristic governing the castability of the material. Melt viscosity and surface tension also play a significant role in the casting process. Both these material properties should be minimised for high quality casting and this would an interesting topic for future work.

3.3 Possible future development

These early results have demonstrated that it is possible to use the laser as a power source to produce shallow surface casts in high strength alloys. Although the castings take on the macroscopic surface features of the moulds they are not microscopically smooth. This is because the outer surface is covered in powder particles which melted immediately before solidification began. This is an area for future investigation which may involve the use of high melting point moulds and cladding material with suitable liquid flow characteristics. At this early stage it is not possible to forecast future applications of this technique although these may include the production of abrasive surfaces or hard cutting tools. Refinement of the process could result in the production of stamping dies.

a ) Cobalt based b) Iron based c) Nickel based Figure 8. A comparison of the casting capabilities of the three alloys tested. Process parameters: 3 kW (Nd:YAG), beam diameter 5 mm, process speed 0.8 m/min, Ni based powder, powder flow 90 g/min.


62

Machining line

4 Laser Clad-Casting

Laser clad-casting is a simple development of standard laser cladding which was stimulated by an industrial inquiry. The company involved wanted to extend the life of piston rings by adding a clad layer to the outer diameter. This clad layer was to be of a wear resistant material and, to prolong life even further, was to be gradually increasing in thickness towards the edge. The first concern from the laser cladding point of view was the aspect ratio of the clad track. Generally, blown powder laser cladding gives a single clad track cross section which is a truncated semicircle as shown in figure 9a. The maximum height of a single clad track is achieved when the track is semicircular in cross section as shown in figure 9b. This customer however, required a better aspect ratio than the 2:1 limit of a semicircle. They needed a post machined aspect ratio of approximately 1:1 as shown in figure 9c. From figure 9c it is also clear that they required the clad layer deposit to have sides which were diverging from the line of the substrate at an angle of 10°.

a) Standard clad b) Maximum height c) Required clad cross section clad track (semi circular cross section cross section)

Figure 9. Standard clad track cross section (a, b) and the required cross section (c ).

In order to achieve the clad profile required, copper blocks were machined and clamped to either side of the substrate as shown in figure 10.

Clad layer


63

Figure 10. Cross section of the clad cast mould.

Clamping

Substrate Machined copper blocks

Cladding was now carried out with the laser and the powder stream aiming into the valley between the copper blocks. The beam diameter was 4 mm on the substrate surface and thus irradiated the copper blocks on either side. However, the high reflectivity of the copper prevented it from melting by direct laser irradiation and its high thermal conductivity prevented melting by contact with the molten cladding metal. As a result the copper blocks could be easily removed after the cladding was complete. The clad profile produced by this method is shown in profile in figure 11. Figure 11. A cross section of the clad-cast track deposited between copper blocks. (substrate width: 3mm,clad track height: 3.5 mm). Process parameters: powder feed (Nickel alloy) 40 g/min, cladding speed 0.5 m/min, laser power 3.5 kW (CO2), beam diameter 4 mm.

Figure 11 clearly shows that the required clad profile has been achieved. The integrity and low dilution levels of the clad layer are typical of the standard laser cladding process. In this case a single, high aspect ratio, track has been produced on the edge of a narrow substrate. The depth of the deposit could of course be increased by overalying another track on this one. The process could also be extended to the laying of tall, narrow walls on flat substrates to produce enclosures or stamping tools.

3 mm

Machined copper blocks


64

Apart from the ability to produce deep clad layers laser clad-casting has two other advantages over the standard process:

a) The process is more energy efficient than standard laser cladding. In this case 24% of the laser energy was utilised in the melting process as compared to 20% for standard laser cladding with an Nd:YAG laser [2] (This value is only 10% for standard CO2 laser cladding [2]).

b) The powder catchment efficiency is higher for clad-casting than for cladding. i.e. in this

example of clad-casting the powder catchment efficiency was ∼96 %. (Standard cladding value ∼ 60%) [3]. This improves deposition rates and minimises substrate melting because a greater proportion of the laser energy is involved in melting the incoming powder. This improvement in powder catchment efficiency is clearly a function of the valley-like geometry of the clad-cast melt zone. A geometry of this type tends to channel powder into the weld pool rather than allowing it to spray all over the substrate surface. (Which happens in standard laser cladding).

5 Conclusions

It has been demonstrated that two new laser cladding techniques are possible and that they may provide novel answers to future production requirements. Laser casting can be used to produce surface castings in high strength alloys to generate tool bits or stamping dies etc. Laser clad-casting can be employed to make clad tracks with large depth to width ratios and pre determined cross sections.

6 References

1. Li, W.B, Engström, H, Powell, J, Tan, Z, Magnusson, C. (1995). Modelling of the laser cladding process; Pre-heating of the Blown Powder Material. Lasers in Engineering, Vol 4, pp. 329-341

2. Gedda, H., Powell, J., Kaplan, A. (2002). A Process Efficiency Comparison of Nd:YAG and CO2 Laser Cladding. Welding in the World, Vol. 46, Special Issue, pp.75-86 3. Gedda, H., Powell, J., Wahlström, G., W-B, Li., Engström, H., Magnusson, C. (2002). Energy Redistribution During CO2 Laser Cladding. Journal of Laser Applications, Vol. 14, no. 2, pp. 78-82

H.Gedda: Chapter V-Laser Wire Casting

65

Chapter V

Laser Wire Casting


66


67

Laser Wire Casting

J.Powell+, H.Gedda*, A.Kaplan*, Katja Rüstig# .



S-971 87 Luleå, Sweden Phone: +46 920 491169, E-mail: [email protected] #Material Science and Materials Technolog, Technische Universität Bergakademie Freiberg, D

Abstract This paper describes a new technique for the production of solid wire or rods from powder by laser melting. Three techniques have been developed to ensure that the molten powder solidifies as a rod or wire rather than a series of droplets. The straight rods or wires produced in this way have an almost circular cross section, are several millimetres in diameter and can be pore free. The techniques can be used to produce welding rods, tensile test samples and other solid pieces from a wide range of powder mixes. The rapid thermal cycle involved means that hitherto difficult to produce mixtures and alloys can now be produced in the solid form in seconds.

1 Introduction Previous work by the present authors [1] investigated novel applications of blown powder laser cladding techniques to produce castings or castings which were simultaneously clad to substrates [2]. This paper extends this work to the production of cast wires or rods from pre placed powder beds. Simply traversing a defocused laser over the surface of a powder bed was found to give unsatisfactory results because the melt has a natural tendency to form a series of large droplets which may or may not be connected to each other [3] as shown in figure 1.

Figure1. A series of droplets formed by the interaction of a moving, defocused laser and a bed of metal powder.

15 mm


68

Three techniques have been developed at the Sirius laboratory, Luleå University which suppress the formation of droplets and allow the production of wires or rods. These methods involve the employment of moulds or the use of preliminary wire within the powder bed.

2 Experimental work

2.1 Equipment and materials used For the purposes of the experimental runs the following equipment and materials were employed: Laser model: Rofin Sinar RS 6000 CO2. Laser power 3500 W. Laser beam defocused to Ø 3 mm. Powder: Stellite 8 (Cobalt base). The powder chemical composition is presented in table 1 below. Process speed: 0.4 m/min. Wire: (Ni-based) The chemical compositions is presented in table 2 below. Substrate: The substrate (mould bottom) chemical composition is presented in table 3 below. Mould: Cu-blocks. Chemical Composition

Table 1. Co based powder composition

Powder size 45-150µm (Tm) = 1459-1656 K

Table2. Ni-based wire composition

Ø 1.3 mm (Tm) 1600 K

Table 3. Steel composition (substrate)

Melting point (Tm) =1773 K


Ni Cr Mo C Fe W wt % 59.3 21.2 13.2 0.2 3.3 2.7

C Si Mn P S V N Fe wt % 0.16 0.22 0.94 0.014 0.022 0.06 0.009 98.6


69

2.2 Casting with moulds The aim of this work was to cast approximately circular cross section wires or rods. Two types of mould were employed to attempt this; side contact moulds and net shape moulds.

2.2.1 Side contact moulds Figure 2 shows the very simple principle of the use of side contact moulds. A channel between two metal blocks is filled with metal powder which is subsequently melted by the laser beam.

Figure 2. The principle of application of side contact moulds.

The side walls of the mould constrain the melt and prevent the formation of droplets (see figure1). The walls also help to cool the melt by conduction. It is a primary requirement that the melt does not become clad to the substrate and so the process is designed to leave a residual layer of powder at the bottom of the channel as shown in figure 2c. This Lack of contact with the substrate also helps to encourage an approximately circular cross section. A number of wires were produced in this way and a selection of results is presented in figure 3.

Powder Mould

Substrate

Laser beam Powder

a) Cross section of mould and powder before laser irradiation

c) The cross section after laser irradiation

b) During laser processing

Wire


70

Figure 3. A selection of results of the side contact mould laser casting process.

It is clear from the results presented in figures 3a-b that the wires or rods produced by this method are of ovoid cross section and are close to 100% dense. They are also generally straight along most of their length. Figure 3c demonstrates that, if the mould walls are too far apart, the melt reverts to its droplet forming behaviour. It is the suppression of this behaviour which is the main advantage of the use of side walls. The mechanism of suppression will be discussed later.



Laser power 3kW Speed 0.4 m/min Mould separation ≥ 6 mm

Cross section

Cross section

Cross section

a) General view

b) General view

c) General view

5 mm 6 mm 3 mm

5 cm


71

4 mm

2.2.2 Net shape moulds A direct extension of the use of side contact moulds is the employment of moulds which are in complete contact with the melt and thus control its final shape. Such a mould is shown in figure 4 in its powder filled state (4a) and after rod production (4b).

Figure 4. The use of a net shape mould to form a rod.

This process produces very straight almost 100% dense, circular cross section rods as shown in figure 5. In this case the adhesion of the melt to the mould has been avoided by making the mould from high conductivity copper.

4a) The powder filled mould 4b) After successful prior to laser melting production of a rod

200 µm

5a) Cross section 5b) Polished and unetched cross section

5c) Macroscopic view Figure 5. An example of the high quality wire created when using net shape moulds.


72

2.3 Casting with wires imbedded in powder beds The aim in this case was to dispense with moulds but to control the casting process by using a wire imbedded in the powder prior to melting as shown in figure 6.

Figure 6. Casting with wires imbedded in powder beds. Once again this proved to be a successful technique for producing rods or wires as the results of figure 7 show.

Figure 7. Examples of rods cast by the wire and powder route.

6a) Before laser melting 6b) After laser melting

a) Cross sections b) General view

6 mm 50 mm

Ø 1.3 mm


73

In this case the wire acts to inhibit droplet formation and this will be discussed in the following section. 3 Discussion Figure1 demonstrates that there is a natural tendency for liquid metal to gather together in droplets. (This is not true if the mass of liquid is large because, in that case, gravity overcomes the surface tension effects which create droplets). Droplets are formed because the liquid attempts to achieve its lowest energy state and this is satisfied when the liquid has the lowest surface to volume ratio. The shape with the lowest surface to volume ratio is the sphere and therefore small volumes of liquid tend to form spheroids. If a laser melts a track across a bed of metal powder and does not simultaneously weld the track to the substrate (as in laser cladding) [4], then the solidifying liquid will attempt to form spheroidal droplets in order to minimise its surface energy. Figure 1 shows that these spheroids are extended in the direction of movement of the laser and are often joined together. This sort of morphology is a compromise between the sphere forming influence of the surface tension of the melt and the line forming influence of the movement of the melt front (which, obviously, follows the laser movement). The presence of a side wall or net shape mould prevents the bulging of the melt bead necessary to the production of spheroids. This is clearly demonstrated by figure 3 which shows that spheroids are created if the gap between the side wall moulds is too great. At narrower gaps the side walls only allow the melt to solidify with a linear morphology which creates a wire with a consistent cross section. The use of a wire as a melt guidance device also produces rectilinear solidification without droplet formation. In this case the wire and powder material were Ni-based (Tm ≈ 1400º K) and Co-based alloy (Tm ≈ 1600º K) respectively. This proximity in melting point resulted in the original wire being entirely melted during the process but this doesn’t necessarily have to be the case. Whether or not the original wire is melted it acts as a solidification nucleation site and inhibits the formation of droplets. Of the three processes discussed in this paper the one which produces the highest quality product is, of course, the use of net shape moulds. Using this technique it should certainly be possible to produce wires or rods over a wide range of diameters and lengths. The compositional range of rods formed in this way is extremely wide and could include metal matrices with hard, abrasive ceramic particles as well as exotic alloy combinations.


74

3 Conclusions

• Wires or rods can be cast from metal powder using a high power laser as a heat source.

• Metal powders which have been laser melted do not readily solidify as uniform cross section rods unless the tendency to form strings of droplets is inhibited.

• The presence of side wall or net shape moulds can result in rods which are ovoid or

circular in cross section and approximately 100% dense. Wires incorporated into the powder bed can have the same effect in the absence of moulds.

• The casting techniques discussed in this paper could be used to produce wires or rods

of a very wide range of alloys and alloy-ceramic mixtures. 4 References 1. Powell, J., Gedda, H., Kaplan. A. (2002). Laser Casting and Laser Clad-Casting: New processes for rapid prototyping and production. Conf. Proc. (ICALEO) Scottsdale, AR, 14-17, October 2002 2. Stimper, B. (2000). Using Laser Powder Cladding To Build Up Worn Compressor Blade Tips. Conf. Proc. Advanced Materials in Aerospace Industry Berlin 2000 3. Powell, J. (1983). Laser Cladding. PhD Theses Imperial College of Science and Technology, Dept. Of Metallurgy, London UK. 4. Powell. J. (1988). Laser Cladding With Preplaced Powder; Analysis of thermal cycling and dilutions effects. Surface Engineering, Vol. 4, no. 2. pp. 141-149

H.Gedda: Chapter VI-Melt-Solid Interactions in laser cladding and laser casting

75

Chapter VI

Melt-Solid Interactions in laser cladding and

laser casting


76


77

Melt-Solid Interactions in laser cladding and

laser casting

H.Gedda*, A.Kaplan*, J.Powell*+,

* Luleå University of Technology, Division of Manufacturing Systems Engineering, S-971 87 Luleå, Sweden Phone: +46 920 49 1169, E-mail: [email protected] + Laser Expertise Ltd., Acorn Park Industrial Estate, Harrimans Lane, Nottingham NG7

2TR, U.K.

Abstract

Experimental data in conjunction with mathematical models are used to explain various aspects of laser casting and laser cladding by the preplaced powder method. Results include an explanation of the large range of process parameters over which low dilution clad deposits can be produced. Also the interaction of the melt pool with the powder bed is analysed to identify why laser castings have microscopically uneven surfaces. Kewords: laser, cladding, casting


78

3 mm

1 Introduction

The process of laser cladding has been the subject of scientific research and commercial applications since the late 1970s [1-5]. There are two basic methods of using a laser to clad one metal with another; the preplaced powder method and the blown powder method. These two techniques are illustrated in figure 1.

This paper investigates the thermo and fluid dynamics of the preplaced powder cladding method and of a more recently developed technique called laser casting. Laser casting was developed at Luleå university of technology [6] and involves the deposition of a laser melted metal into a mould (see figure 2). The process parameters are deliberately chosen to prevent the “clad” deposit forming a bond with the substrate and thus cast objects can be produced.

Figure 1. The preplaced and blown powder cladding methods a, preplaced powder b, blown powder cladding.

Cladding material

V

V

a

b

Figure 2. Laser casting.

V

Laser beam


79

Laser casting can be described as laser cladding which has no substrate – clad layer bond. This paper is an investigation into the process by which such substrate clad layer bonds are created or avoided. This work uses experimental results to support two theoretical models. The first, which is a extension of earlier work by Powell et al. [1, 2], investigates macroscopic aspects of melt-solid interactions during preplaced powder laser cladding. The second model, which builds on earlier work by Kaplan et al. [7, 8] looks at the microscopic effects of the interaction of the melt with individual powder particles.

2 Experimental procedure

For the purpose of this study a number of clad tracks were produced over a wide range of process speeds. The experimental details are provided in Table 1.

Table 1. Equipment and Parameters

CO2-laser, Rofin Sinar RS 6000 (6 kW) Laser power range 100 - 4000 W Laser spot diameter = 4 mm Substrate, SS 2172 Mild steel (0.16% C) Cladding Material, Cobalt based (Stellite 21) Process speed range 0,1 m/min- 4 m/min

3 Macroscopic Melt-Solid interaction

Figure 3 shows the cross sections of six clad tracks laid down under identical conditions except for the process speed which is indicated by each photograph. The photographs are all printed at the same scale and it is clear that as the speed is increased from 0.1 m/min to 2.1 m/min the amount of material melted decreases. However, figures 3 e and f show a larger cross section of melt compared to the lower speeds. This is easily understood by reference to figure 4, which shows the top views of the clad tracks and reveals that the cross section is very unstable for the highest speeds.


80

Figure 3. Cross sections of clad tracks made under identical conditions (laser power 3500 W, powder bed

depth 1 mm) at different speeds.

a b c d e f (0,1 m/min) (0,2 m/min)(0,9 m/min)(2,1 m/min)(3,3 m/min)(3,8m/min) Figure 4. The top views of the clad tracks shown in figure 3.

a) 0,1 m/min b) 0,2 m/min

c) 0,9 m/min d) 2,1 m/min

e) 3,3 m/min f) 3,8 m/min

1 mm


81

Returning to figure 3, a number of the observations are clear:

• Substantial substrate melting is only a feature of the lowest speed even though sound cladding is possible at speeds over twenty times this value.

• There are a broad range of process speeds over which the amount of substrate melting is trivial (0,2 m/min-2,1 m/min).

• Although the melt cross sectional area decreases with process speed the rate of decrease is surprisingly small. For example the cross section of b is 5.1 mm2 and this decreases to 3.5 mm2 (69 %) for d which was produced at approximately ten times the speed of b.

• The contact angles of the melt to the substrate show good wetting characteristics from a to d but poor wetting in cases e and f. The clad layer in the case of the highest speeds (e and f) is not in full contact with the substrate.

The main contra- intuitive feature of figure 3 is the surprisingly low amount of substrate melting over a wide range of process speeds. This phenomenon was first discussed by Powell [1, 2] who postulated a three stage melting process for preplaced powder laser cladding;

1. The laser rapidly melts the powder before the melt touches the substrate because, prior to substrate contact the melt is surrounded by low conductivity powder.

2. Once the melt touches the substrate it looses a great deal of heat by conduction.

This leads to partial solidification of the melt. As a result the melt-liquid interface does not move into the body of the substrate.

3. If the laser energy continues to irradiate the top surface of the melt, the energy

will eventually move the melt/solid interface back down through the clad layer and across into the body of the substrate.

Figure 5 presents a graphical description of the three stage process derived from a one dimensional mathematical model.

Figure 5 Vertical temperature distribution through the preplaced powder and substrate for different time

steps [1,2].


82

The first-stage, the melting of the unmelted powder is shown in the 15 ms and 37 ms curves as the temperature of the powder rises rapidly above the melting point Tm. At 37 ms the melt makes contact with the substrate and looses heat by conduction. This second stage is described by the 46, 66 and 142 ms lines which shows that the Tm point is now back above the powder-substrate interface. It is only after 252 ms that the melt front returns to the powder-substrate interface and subsequent laser irradiation will result in substrate melting. This model therefore suggests that over a wide range of interaction times (or process speeds), the clad layer will have experienced wet contact with the substrate without melting it. The mathematical model which given us the results presented in figure 5 is highly simplified in some respects in order to present the main point clearly. (The main point being the halting or reversal of the movement of the melt front is the vertical direction as a result of contact with the substrate) As a continuation of the one-dimensional calculations which gave rise to the results given in figure 5 a combined two and three dimensional model has been developed as follows; Before melt-substrate contact the vertical position of the melt above the substrate can be described by an extension of a standard equation for a moving point source of heat [9]:

( ) ( )

−++−

+++=

κπ 2exp

2,,

222

222

xzyxv

zyxK

APTzyxT L

a (1)

Where : x,y and z = A Cartesian coordinate system T(x,y,z) = The temperature field at any position x, y, z.

Ta = Ambient temperature

PL = Laser beam power

A = Absorptance of the surface to the incident laser light v = The speed traveled by the laser beam in the x direction K = Thermal conductivity (assumed constant)

κ = Thermal diffusivity (assumed constant) The extension of Equation 1 involves the addition of an infinite number of mirror sources [9] arranged along the vertical axis. This mathematical method replicates the insulating effect of the powder bed. Thus expanded, Equation 1 becomes;

( )( )

( )

−+++

−+++

+= ∑∞

−∞= κπ 2

2exp

2

1

2,,

222

222

xidzyxv

idzyxK

APTzyxT

i

La (2)

where i = Mirror source index d = The powder depth After melt substrate contact the heat flow reverts to that described in Equation 1. This constitutes a change from two dimensional heat flow (Equation 2) to three dimensional (Equation 1).


83

These equations have been used to produce figure 6 which shows the maximum depth of melting for different processing speeds.

Figure 6. Calculated maximum melting depth through the powder (1 mm thick) and substrate ( >> 1

mm) as a function of the processing speed.

Figure 6 qualitatively supports the results shown in figure 3; only the lowest speeds involve substrate melting, corresponding to 3-dimensional heat conduction, (Equation 1). Then there is a range of speeds over which no substrate melting takes place but the powder is melted completely. Throughout this range the calculation continuously flips from three dimensional to two dimensional heat-flow and this effectively freezes the movement of the melt front at the substrate surface. At the highest speeds (here 1.6 m/min or higher) the powder is not completely melted throughout its depth and no melt substrate contact is achieved. The calculation in this region is governed entirely by Equation 2

Figure 7. Melt-substrate contact history in cross section.

(Black = liquid, Grey = Powder , Shaded = Solid).

This phenomenological model is summed up by figure 7 which indicates the position of the melt front and the geometry of the melt as time progresses for any point along the cladding line. (The roughness of the melt-solid contact line on initial contact is one of the topics which will be covered in the next section).


84

4 Microscopic melt-solid interaction

This section of the paper considers the interaction of the melt with the powder particles which surround it. During the cladding process the melt moves through the powder layer melting and collecting these particles.

Figure 8. The geometry of preplaced powder cladding.

4.1 Particle Heating by the Melt Front

The process of particle melting by the melt pool is sensitive to the size of the particles involved. Commercially available powders have a range of particle sizes in each batch and the size of a particle determines how rapidly it will melt. The energy needed to heat and melt the particle is proportional to rp

3;

( )[ ]mampp HTTc

rP ∆+−= ρ

πτ

3

4 3

(3)

Where: P = Thermal power flowing into the particle

τ = Interaction duration until melting rp = Powder particle radius

ρ = Specific mass density of the particle cp = Specific heat capacity Tm = Melting temperature

∆Hm = Latent heat of melting Figure 9 shows the distribution of the particle diameters in the powder used in this investigation. The dotted line on this figure shows the level of energy needed to melt the particles as a function of radius and the broken line demonstrates the proportion of the incident energy needed to melt the various particle sizes in the batch.

Clad layer

Melt

Powder bed

Substrate


85

0

10

20

30

40

50

0 50 100 150 200

Particle Diameter [um]

En

erg

y, P

arti

cle

Dis

trib

uti

on

[a.

u.]

N

E

N*E

Figure 9. The particle size distribution and proportion of the incident energy needed to melt the particles of

different sizes in this batch.

As particle heating by the advancing melt front is a highly complex mechanism, the simplified model of Eq. (3) is applied here in order to estimate the duration for heating and melting a particle for given heat flux.

According to figure 9 the energy required for heating and melting a particle is proportional to its volume. Table 2 gives the melting times for a number different sized particles and it can be seen that doubling the particle diameter multiplies the melting time by a factor of eight. It must also be born in mind that a change in incident power (or heat flux) leads only to a linear change in melting time.

Diameter 2rp [µm] 60 µm 90 µm 120 µm 150 µm

Melting time τ [ms] 0.8 ms 2.7 ms 6.4 ms 12.5 ms

Table 2: Calculated melting time of a single particle depending on its diameter (steel,

prescribed power per particle 0,885 W)

4.2 Surface Tension Driven Droplet-Melt Front Interactions

As the particle comes in contact with the hot melt it melts and becomes incorporated into the main body of the liquid. Surface tension forces tend to minimize the surface to volume ratio of

any melt. The surface tension forces acting on liquid bodies of radii Rϕ and Rθ which are in contact with each other can be expressed at each point of the surfaces follows in the Young – Laplace Equation :

+=

θϕ

σRR

pS

11 (4)


86

Where:

ps = Pressure from surface tension

σ = Surface tension

Rϕ , Rθ = Local radii of curvature of the two liquids

Earlier work by Kaplan [7] modelled the collapse of gaseous cavities (bubbles and keyholes) in liquid weld pools and the force acting upon them. This approach, based on the theory of water bubbles and drops [12-15] can be used to model the reverse situation i.e. liquid droplets surrounded by a gas. Potential flow theory is applied via the time dependent Bernoulli Equation which considers the curvature affected surface tension force and the inertia of the melt close to the surface:

2

2ns,

l

ssup

t−=

∂Φ∂

ρ (5)

sΦ = Velocity potential at the surface

lρ = Liquid density

2ns,u = Surface speed normal component

Locally the flow can be split into two cylindrical components ϕ,θ perpendicular to each other, which can be superimposed. Continuity liquid mass flow in a cylindrical coordinate system with the radial coordinate r is determined by the flow potential field [12, 14]:

( ) ( )r

tMt

1

4 lπρ&

−=Φ (6)

Spatial derivation yields the velocity field:

( ) ( )2

l

1

4 r

tMtu

πρ&

= (7)

Note that the mass flow rate M& implicitely results from the calculation. As for the heat conduction equation, superposition of the surface solutions of two

perpendicular cylinders of radius Rϕ, Rθ, respectively, at their coinciding surface gives:

( )

+−=Φ

θϕπρ RR

Mt

11

4 ls

&

(8)

( ) ( )( ) ( )

+=

tRtR

tMtu

22l

ns,

11

4 θϕπρ&

(9)

Again, this solution is valid only at the surface and its vicinity, but is important to take into account the local surface curvature in the radial and vertical direction governing the consolidation situation studied here. The above solution can be locally applied to the surface position r

s as a function of (z,t).


87

( ) ( ) ( ) ( )∫ ′′′+=t

tdtztzuzrtzr0

ns,ss ,cos,0,, β (10)

Where:

β = Inclination of the surface

From the above equation the advancement of the front can be calculated point by point in space and iteratively in time, resulting in a model of the surface tension driven melt front motion. Particles touching the melting front are first heated and melted, e.g. according to Table 2, followed by consolidation which is driven by surface tension forces that are in turn governed by the surface curvature, according to Eq. (4). The calculated surface shape and motion is shown in figure. 10 for four different grain sizes as a function of time.

Figure 10. Calculated heating and melting of powder grains of different diameter touched by the melting

front and subsequent smoothing of the droplets.

The black spheres correspond to the solid state and the percentage of the enthalpy consumed related to the enthalpy required for melting is shown in each solid sphere, (refer also to Table 1). In the calculation the particles are assumed to have penetrated the melt by 20 % of their radius, however, the sensitivity of this parameter is low. Note that wetting and liquid motion is


88

neglected in this calculation, based on the assumption that the speed of the process is governed by inertia, i.e. by the acceleration of the melt. It is probable that the melt front in contact with the substrate is covered in a disproportionate number of larger particles because the smaller ones are much more rapidly absorbed into the melt. As the melt moves through the powder bed it effectively coats itself in larger particles as they have a longer residence time on the liquid surface. This type of melt surface, covered in semi molten protrusions will not wet the substrate surface sufficiently to allow a strong bond or weld to form. This sort of surface is typical of laser casting (see figure 2) and is equivalent to the cladding process being interrupted at stage c of figure 7. In the case of laser cladding there is sufficient time and heat available to produce a sound band (see figure 3a-d and figure 7d-f) Figure 11 is a magnified photograph of the surface of a laser casting. The part of the surface shown is that which was in contact with the substrate. This photograph supports the model results presented in figure 10 as it demonstrates that the liquid surface was covered in partially melted particles.

Figure 11. The surface of a laser cast specimen (This surface was in contact with the substrate).

5 Conlusions

This analysis of melt solid interactions has helped to explain the following points about the laser cladding and casting processes;

a) There is a wide parameter range over which dilution free cladding can be achieved by the preplaced powder process. This is primarily due to the difference in thermal conductivity of the powder bed and substrate.

b) If the process parameters are set outside the range mentioned above the result will be

either a dilute clad layer (see figure 3a) or a casting process (see figure 2) depending on whether or not the power input to the process is increased or decreased.

c) The physics of powder particle melting by contact with a liquid pool makes it difficult

to achieve laser casting with a smooth surface.

0,1 mm


89

6 Acknowledgements

The authors gratefully acknowledge the support from the Swedish funding bodies VINNOVA and SSF within the VINST programme.

7 References

1. Powell, J. (1983). “Laser Cladding”, PhD-thesis, Imperial College of Science and Technology

2. Powell, J., Henry, P.S, Steen, W. M. (1988). Laser cladding with preplaced

powder. Analysis of thermal cycling and dilution effects. Surface Engineering, Vol.4 no.2, pp. 141-149

3. Riabkina- Fishman, M., Zahavi, J. (1996). Laser alloying and cladding for

improving surface properties. Applied Surface Science, Vol. 106, no. 1-4, pp. 263-267

4. Yellup, JM. (1995). Laser Cladding using the powder blowing technique.

Surface Coating Technology, Vol 71. no. 2, pp 121-128 5. Steen, W. M. Laser Material Processing. (1998). Laser surface treatment.

Springer-Verlag, London. Second edition, ISBN 3-540-76174-8, pp. 199-202 6. Powell, J., Gedda, H., Kaplan, A. (2002). Laser Casting and Laser Clad

Casting: New processes for rapid prototyping and productionn. Conf. Proc. (ICALEO) Scottsdale, AR, 14-17 October 2002

7. Kaplan, A. F. H. and G. Groboth. (2001) Process analysis of laser beam

cladding, Transactions of the ASME: Journal of Manufacturing Science and Engineering, Vol. 123, pp. 609-61

8. Kaplan, A. F. H., G. Liedl, J. Zimmermann and D. Schuöcker. (1998). Laser

dispersing of TiC-powder into Al-substrates, Lasers in Engineering, Vol. 7, no. 3-4, pp. 165-178

9. Carslaw, H. S., Jaeger, J. C. (1959). Conduction of Heat in Solids (Oxford,

Clarendon), pp. 258-259 and pp. 338-339 10. Kaplan, A. F. H. (1997). Surface processing by non-Gaussian beams, Applied

Physics Letters, Vol. 70, no 2, pp 264-266 11. Kaplan, A. F. H., Mizutani, M., Katayama, S., Matsunawa, A. (2002).

Unbounded keyhole collapse and bubble formation during pulsed laser interaction with liquid zinc, Journal of Physics D: Applied Physics, Vol. 35, no. 11, pp. 1218-1228

12. Rayleigh, L. (1917). Philos. Mag. Vol. 34, pp. 94-98


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13. Plesset, M. S., Zwick, A. (1954). Journal of Applied Physics, Vol. 25, pp. 493-500

14. Florschuetz, L. W., Chao, B. T. (1965). Transactions of the ASME, Journal of

Heat Transformation, Vol. 87, pp. 209-220 15. Legendre, D., Borée, J., Magnaudet, J. (1998). Physics of Fluids, Vol. 10, pp.

1256-1272 16. Picasso, M., Marsden, C. F., Wagnière, J. D., Frenk, A. Rappaz, M. (1994). A

simple but realistic model for laser cladding, Metallurgical Transactions B, Vol. 25B, no. 2, pp. 281-291.

17. Kaplan, A. F. H., Groboth, G. (2001). Process analysis of laser beam cladding.

Transactions of the ASME, Journal of Manufacturing Science and Engineering, Vol. 123, pp. 609-614

Laser Cladding: An Experimental and Theoretical …989990/...(PICALO)April 19-21, 2004 Melbourne,...

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