laser thesis.unlocked.pdf

8/19/2019 laser thesis.unlocked.pdf

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DOCTORAL T H ES I S

Luleå University of Technology

Department of Applied Physics and Mechanical Engineering,

Division of Manufacturing Systems Engineering

:|: -|: - -- ⁄ --

:

Laser Cladding: An Experimental

and Theoretical Investigation

Hans Gedda


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Preface

Since April 1999 I have been conducting experimental and theoretical research in the field oflaser 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 beendone at Duroc AB in Umeå, Luleå and at Nottingham University.

Several people have been important in completition of this work. I sincerely thank mysupervisor 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 fordiscussions, suggestions through this work. I would also like to thank all my friends andcolleagues 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


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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 overviewof the two common laser cladding methods and some applications for the processes. Thischapter 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 lasercladding. Experimental absorption measurements by calorimetry were carried out to analysehow 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 whencladding into pre machined groves including; groove geometry, powder application methodand laser type.

Chapter four presents preliminary experimental results from two new processes; Laser castingand Laser clad-casting. Laser casting is a process similar to blown powder laser cladding butwithout the final product joined to the substrate. The substrate acts as a mould and the castingretains the topological features of the substrate. Laser clad-casting involves the production of aclad layer between machined copper blocks. Clad tracks can therefore be achieved with largedepth to width ratios and pre determined cross sections.

Chapter five describes a new technique for the production of solid wire or rods from powderby laser melting. Three techniques have been developed to ensure that the molten powdersolidifies as a rod or wire rather than a series of droplets. The techniques can be used toproduce welding rods, tensile test samples and other solid pieces from a wide range of powdermixes.

Chapter six presents experimental data in conjunction with mathematical models are used toexplain 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 lasercastings have microscopically uneven surfaces.


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Contents

Page

Preface i

Abstract ii

Contents iii

Chapter I: Introduction to laser cladding 1

Chapter II: Energy Redistribution in Laser Cladding: A comparison ofNd:YAG and CO2 lasers which combines information from twopublished papers; 25

1. Gedda, H., Powell, J., Wahlström, G., Li, W-B., Engström, H.,Magnusson, C.: Energy Redistribution During CO 2 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 Comparisonof Nd:YAG and CO 2 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 PacificInternational Conference on Applications of Lasers and Optics(PICALO)April 19-21, 2004 Melbourne, Australia. Submitted forpublication in Journal of Laser Applications.

Chapter IV: Laser Casting and Laser Clad-Casting: New processesfor rapid prototyping and production 53

Gedda, H., Powell, J., Kaplan, A.: Conference proceedingsInternational 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 inMetallurgical and Material Transactions B.


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H.Gedda: Chapter I-Introduction to Laser Cladding

1

Chapter I

Introduction to laser cladding


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1 Introduction to laser cladding

Industrial applications require parts with special surface properties such as good corrosionresistance, 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 acheaper substrate. Laser surface treatment includes several different surfacing techniques usingthe heat of the laser beam to modify the structure and physical characteristics of the surface of amaterial [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 highbond strength to the substrate [3]. Surface coating by laser is a method that has been developedover the last two decades. The lasers minimal and easily controllable energy delivery makes itpossible to alloy, impregnate, clad, and harden components that are exposed to wear and

corrosion. The method offers great advantages compared with traditional hardening andalloying methods. The method is used commercially in the aircraft engine industry and in thecar 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 thetwo-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 achosen alloy in powder form or by a blend of powders with the required composition. Lasercladding with powder offers the possibility of the development of new material combinationsfor the future.

(a)

(b)

Cladding material

Figure 1. Schematic diagrams of laser cladding process.

a) Preplaced powde,r b) blown powder cladding.


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Relative motion between the laser/powder supply and the substrate can be used tocontinuously apply a surface coating. To cover larger surfaces, overlapping tracks are made (seefigure 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 fromRolls 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 theinterface and the molten particles are trapped in the melt pool. The energy must be highenough 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 sideblown powder there is a directional effect on the clad bead shape [6] and the powderutilisation 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

WorkpieceFocal point

Figure 3. Cross section of a coaxial nozzle.


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Figure 5. Cladding on turbine blade.

Duroc AB in Sweden Umeå has developed the technology for cladding material on valves etcfor 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 anuntreated tool.

Figure 6. Laser clad chopping tool for the wood industry.


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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 toinvestigate how much of the laser power is used in the cladding process and how much isreflected etc. In this investigation an Nd:YAG and a CO2 laser have been compared as energysources for the process. Every aspect of the energy redistribution during cladding has beenanalysed. The main energy loss to the process for both lasers is by reflection from the melt pooland 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 P A, P B etc).

(Power lost byconvection

PFPF

PA PA

PBPB

PE PE

P PD

P Laser beam

Substrate

Powder stream

Substrate

P Laser beam

(Power radiated)

(Power reflected offthe surface of the clad)

(Power reflected offthe powder particles)

(Power lost byconduction)

PD

PA


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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 laserpower 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 (5kW) range covered in our earlier work [2]) i.e. given the same laser power, Nd:YAGlasers 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 lasercladding. (* 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 andreflection of the energy by the powder cloud.

Laser Beam

PowderParticles

InsulatedCalorimeter


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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(CO 2 laser).

4 mm

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

A B C D 4 mm


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3 mm

Machiningline

Figure 12 shows an example of laser casting.

Figure 12. Successful laser clad-casting of cross hatched grooves. a) substrate (mould), b) substrate andcasting, c) casting. Process parameters: laser power 3 kW (Nd:YAG), beam diameter 5 mm, process speed0.8 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 cladcross 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)


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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, claddingspeed 0.5 m/min, laser power 3.5 kW (CO 2 ), beam diameter 4 mm.

Conclusions

It has been demonstrated that two new laser cladding techniques are possible and that they mayprovide novel answers to future production requirements.

Laser casting can be used to produce surface castings in high strength alloys to generate toolbits or stamping dies etc.

Laser clad-casting can be employed to make clad tracks with large depth to width ratios andpre determined cross sections.

Figure 14. Cross section of the clad cast

mould.

Clamping

SubstrateMachinedcopper blocks

3 mm


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Laser power 3kWSpeed 0.4 m/minMould separation

3 mm


5 mm


≥ 6 mm

Cross section Cross section Cross section

General view General view General view

5 mm 6 mm3 mm

5 cm

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


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

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

Figure 18. The use of a net shape mould toorm 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 crosssection 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 thepowder 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 anexplanation of the large range of process parameters over which low dilution clad deposits canbe produced. Also the interaction of the melt pool with the powder bed is analysed to identifywhy laser castings have microscopically uneven surfaces.


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Figure 20. Cross sections of clad tracks made under identical conditions (laser power 3500 W, powder beddepth 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 substratemelting over a wide range of process speeds. This phenomenon was first discussed by Powellwho 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 interfacedoes not move into the body of the substrate.

3. If the laser energy continues to irradiate the top surface of the melt, the energywill eventually move the melt/solid interface back down through the clad layerand 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 timesteps [9].

Figure 23. Calculated maximum melting depth through the powder (1 mm thick) and substrate (>>

1mm) as a function of the processing speed.


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0

10

20

30

40

50

0 50 100 150 200

Particle Diameter [um]

E n e r g y ,

P a r t i c l e D i s t r i b u t i o n

[ 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 particlesof different sizes in this batch.


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The calculated surface shape and motion is shown in figure 26 for four different grain sizes as afunction 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 surfaceshown is that which was in contact with the substrate. This photograph supports the modelresults presented in Figure 10 as it demonstrates that the liquid surface was covered in partiallymelted 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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H.Gedda: Chapter II-Energy Redistribution During in Laser Cladding; A comparison ofNd:YAG and CO2 lasers

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Chapter II

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


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Energy Redistribution in Laser Cladding; A

comparison of Nd:YAG and CO2 lasersThe 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 NG72TR, 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*.



AbstractBlown powder laser cladding is a cost effective way of producing a surface layer to withstandwear and corrosion. However, the cladding process is slow. Therefore is it of great interest toinvestigate how much of the laser power is used in the cladding process and how much isreflected etc. In this investigation an Nd:YAG and a CO2 laser have been compared as energysources for the process. Every aspect of the energy redistribution during cladding has beenanalysed. The main energy loss to the process for both lasers is by reflection from the melt pooland the powder cloud. It was found that the Nd:YAG laser cladding process is approximatelytwice as energy efficient as the CO2 laser cladding process.

Keywords: Laser cladding; Laser processing, Energy redistribution, Surface treatment.


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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 canbe overlapped to cover areas of the substrate with a harder and/or more corrosion resistantsurface. The process is not energy efficient as a large proportion of the incoming laser power isreflected or reradiated from the cladding zone as shown in figure 2. Figure 2 demonstrates allthe different ways in which the incident laser energy is redistributed during the claddingprocess.

Figure 1. Blown powder laser cladding.

Powder particles

Laser beam

Clad layer


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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 P A,P B etc).

(Power lost byconvection

PFPF

PA PA

PBPB

PE PE

P PD

P Laser beam

Substrate

Powder stream

PD

PA

Substrate

P Laser beam

(Power radiated)

(Power reflected offthe surface of the clad)

(Power reflected off thepowder particles)

(Power lost byconduction)


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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 someextent necessary to the cladding process; It is not possible to heat a metal to well above itsmelting point without having radiant or convective thermal losses, a liquid sitting on acomparatively cool solid will always lose heat by conduction etc. For the purpose of thisdiscussion however, it will be taken that any influence which could minimise PA, PB, PD, PE, PFor PG would increase the efficiency of the cladding process. This reduction in any of the factorsof equation 3 would, of course, increase the proportion of the power available to the claddingprocess.

The aim of commercial cladding is to cover the surface of one metal with another at the lowestcost. 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 ratewith a high quality interfacial bond.

Returning to equation 1 it is clear that the process can be speeded up if there is an increase inthe proportion laser power available producing the clad layer PC. The requirement here wouldbe to melt enough powder to achieve the correct clad thickness at a faster linear speed. Such anincrease in PC must not be employed to melt the substrate to a greater depth. The process mustbe 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 lossesin equation 3. This would lead to an increase in PC and the process could beaccelerated 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 and3 for CO2 laser cladding. The results of that work concluded that the laser power wasredistributed 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%


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Of the power absorbed by the process (40%) three quarters of it was employed in simplyheating the substrate and only the remaining 10% of the original laser power was used to meltmaterial 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 laserfor this process. These experimental trials were carried out at a laser power of approximately 3kW for both types of laser. This allowed a direct comparison of the lasers and also aconfirmation of the previous published results [1] at a different power level (the earlier workwas 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 aRofin Sinar RS 6000 CO2 laser with a maximum output power of 6 kW and the Nd:YAGlaser was a Haas Laser HL 3006 D 4 kW. The powder feeder was a TECFLO TM 5102. Theshielding/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 toreach the cladding zone (see figure 2). A proportion of the laser energy is reflected off thepowder cloud and is lost to the cladding process. Another portion of the incident energy isabsorbed by the particles but some of this energy is also lost to the process because not all theheated particles join the cladding melt pool.

A simple experiment was set up to discover what proportion of the original laser power wouldpenetrate the powder cloud (see figure3 below). A commercially available “power probe” wasused to measure the laser power with and without the powder stream turned on. The powderflow rates were typical of the cladding process as were all the other process parameters. Theaverage results from several such tests are presented in table 3. The energy absorbed by thepowder cloud was directly measured by measuring the average temperature rise of the powder


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after it had passed through the beam (see figure 3). The power reflected off the powder cloudcould 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

Lasertype

LaseroutputPower *

(Watts)

Powderflowrate

(g/min)

Postpowdercloudpower

(Watts)

Totalpowerreflectedandabsorbed bythe powder

cloud ** (Watts)

Powerabsorbedby powdercloud **

(Watts)

PowerReflectedoff powdercloud (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 offthe 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 sameparameters were used for actual cladding. This parameter is PG, the level of power absorbed byparticles which do not enter the cladding melt pool. A number of cladding trials were carriedout and these showed that, over this range of parameters, the proportion of particles whichformed 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 thecladding process.

i.e. PG

= 0.4 % Ptot

for both types of laser (5)

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

Laser Beam

Powder Particles

InsulatedCalorimeter


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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 onethen 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 surfacearea 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 (P E).

The cladding zone is a molten alloy with a surface temperature of ≈ 2300 Kand a known surface area. This melt is exposed to a stream of argon which carries the powderto the clad zone. The argon flow was measured and found to have an average flow velocity of4.3 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 100W/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)


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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 thismeasurement it is possible to measure the power reflected off the cladding zone (PA) in thefollowing way:

PA = Ptot – ( PB+PD+PE+PG+Pin) (13)

Table 4 shows the average results from the calorimetric measurements over a range of processparameters.

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 anappreciable difference. In summary it can be said that, for the CO2 laser, approximately half ofthe laser power is reflected from the cladding zone. For the Nd:YAG laser this value is reducedto approximately 40%. These generally high reflectivity values confirm the work of otherauthors in the field [3] who suggest that the onset of melting is associated with a rise in materialreflectivity. This is because a molten surface in an inert atmosphere (in this case argon) issmooth 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 radiationof CO2 lasers and this is confirmed by the above results. As we will see later in this paper, thisreduction in reflectivity for the Nd:YAG laser results in a marked increase in process efficiencywhen cladding as compared to a CO2 laser.

Lasertype

Laser outputPower (Ptot)

(Watts)

Power input toclad sample (Pin)

(Watts)

Power % inputto sample

Nd:YAG 2743 1367 49%

CO2 2695 1044 39%


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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 infigures 4 and 5.

Figure 4. The cross sectional geometry of a blown powder laser clad layer. Note: the melted substrate andcladding material are mixed together during the process.

As figure 4 demonstrates, the production of a clad layer usually involves melting the surfacelayers of the substrate. The amount of substrate melting can range from minimal to levelswhere 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

Meltedsubstrate

Clad layer

Meltedsubstrate

Heat affected zone


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In order to evaluate Pc accurately for both the CO2 and the Nd:YAG laser a set of claddingtrials were carried out. Laser power and focusing conditions were kept identical for the twolasers (2700 W, 5 mm beam diameter). Powder flow rates of 30, 40 and 50 g/min wereemployed 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 showscross sections of clad traces produced by both types of laser at a powder flow rate of 40 g/minand 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 meltwas 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 propertiesnecessary for equation 18 were taken as being for a 50:50 mixture of cladding material andsubstrate.

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


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From these values and a melt temperature of 2300 K [1], the values for Pc for the two types oflaser 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 twiceas 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 laserpower was used to melt metal during cladding. This result is confirmed here for a different setof process parameters. For the Nd:YAG laser however, the proportion of the laser powerinvolved in melting is almost double the CO2 value. Section 2.5 revealed that 50% of the CO2laser light was reflected from the clad zone as compared 40% for the Nd:YAG laser. It seemsthen, that the difference of 10% is almost exclusively given over to material melting and thisrepresents 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 theworkpiece (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 theprocess when an Nd:YAG laser is used is involved in the melting process , (see previoussection).


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3 Discussion

Figure 7 presents schematics of the redistribution of energy during the laser cladding processfor 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 offigure 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%


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The major mechanism of energy loss to the process is that of reflection from the melt pool andthe powder cloud. Reflection off the melt pool is a function of the condition of the meltsurface. As this melt is produced in an inert atmosphere it experiences no surface oxidation andthus has a high reflectivity. Dilution of the inert shroud gas with an oxidising agent woulddecrease 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 alsoprove problematic.

A reduction of reflective losses from the powder cloud on the other hand would becomparatively easy. All that is necessary is an increase in the average particle diameter. Duringits passage through the laser beam the particle interacts with and reflects light over an area

equal to its cross sectional area (πr 2) rather than half its surface area (2πr 2). This because the

shadow cast by any particle has an area of πr 2 where r is the particle radius. A particle of twicethe original radius would cast a shadow four times as big but would have eight times the mass

(mass ∝ r 3). 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 claddingprocess will break down if the particles are too large.

One very important feature of figure 7 is that, although the power absorbed by the processincreases only from 40% to 50% when CO2 and Nd:YAG lasers are compared, the poweremployed in melting material increases by a factor of 2 from 10% to 20 %. This doubling ofthe energy efficiency of the process is clearly demonstrated in figure 8 which compares a lowspeed (0.7 m/min) CO2 laser clad sample with an Nd:YAG laser clad sample carried out attwice 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 ratherthan CO 2 laser.(The powder feed rate was increased from 30 g/min for the CO 2 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 absorbedby the process. PF remains steady (in this case at 30%) because it is determined by the amountof power the substrate needs to absorb before surface melting is initiated. This is a thresholdvalue, which will not charge with increasing absorptivity. This being the case, any increase inabsorbed power will be entirely available to the melting process.

3 mm


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

4 Conclusions

1. Ignoring the trivial contributions of convective and radiative cooling etc, the laserpower 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 inthe range of parameters covered in this paper ( and by implication, the higher power (5kW) range covered in our earlier work [2]) i.e. given the same laser power, Nd:YAGlasers 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 islikely that substrate pre heating by a cheaper power source* would improve theprofitability 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 LaserApplications, Vol. 14, pp. 78-82

2. Porier, D.R., Geiger, G.H. (1994). Transport Phenomena in MaterialsProcessing. The Minerals, Metals & Materials Society, ISBN 0-87339-272-8,pp. 219-236

3. Bloehs, W., Grünenwald, B., Dausinger, F., Hügel. (1996). Recent progress inlaser surface treatment. Part 1: Implications of laser wavelength. Journal of LaserApplications, 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%


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Chapter III

Laser Cladding into pre machined grooves

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


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Laser Cladding into pre machined grooves.

J.Powell+, H.Gedda*, 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]

AbstractWhen laser cladding is used to improve the wear characteristics of a substrate it is not alwaysnecessary to clad the whole surface. Wear resistant individual tracks can be clad directly ontothe substrate or into pre machined grooves. This paper investigates the process parameterswhich 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 metalsubstrate. 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 andsubsequently melted by the laser (see figure 1a).

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

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

a)

b)

Cladding material

Laser beam


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Laser cladding can be used to provide a protective coating of hard or corrosion resistant metalon a weaker substrate. Tracks of the harder, powdered material are laid down next to eachother 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 powdercharacteristics etc. Typical individual clad tracks produced by the preplaced and blown powdermethods are presented in cross section in figure 3.

It is clear from figure 3 that individual tracks of laser melted powder would not generally beuseful in an engineering context as the harder material forms a ridge on the substrate. This isdifferent situation from that experienced in the field of laser surface hardening. Surfacehardening [1-3] (which does not affect the substrate surface flatness) has been successfully usedto 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 thethermal input to the component.

HAZSignificant substratemelting

Substrate

4 mm

SubstrateHeat affected zone(HAZ)

Minimal clad layer/substrateinterfacial 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


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

In order to replicate the advantages of the single track approach for cladding it is necessary todeposit the cladding material into pre machined grooves. This paper investigates the effect ofthe 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 sectionalgeometry of the groove. For this experiment “V” shaped grooves were produced with includedangles 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 andthe 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 fivesteps from 22 to 46 grams per minute.

b) For pre placed powder a wedge of powder was prepared over the groove as shown infigure 4. The depth of the powder increased from zero at one end of the groove to 2mm 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 ofpowder is useful in demonstrating the progressive effect of an increase in powder depth. Allpowder 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 ) = 4mm

Substrate, SS 2172 Mild steel (0.16% C)

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

Powder feeder, Sulzer Metco Single 10 C


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Figure 6. Cross sections of grooves showing that even when there is sufficientmelt 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 thesample where there was enough melt to fill the top of the groove. It is clear that for all thesesamples the melt has not assumed a flat top surface. In all these cases the melt has retained itscircular 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 mmGroove

Powder depth 2mm

Wedge of powderPowderdepth 0 mm

Figure 5. A grooved powder wedge sample (see

figure 4) after cladding.

Figure 4. Schematic preplaced powder.

4 mm


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Figure 7 demonstrates that the addition of more powder to the melt results in a clad tracewhich over fills the groove. This sample also demonstrates the very low amount of substratemelting 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 apowder 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 doesnot project sufficient material into the bottom of the groove. The 45º and 60º groove aresuccessfully filled with almost flat top surface although there are small linear pores at the bottomof 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 becompletely 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/minCO2 laser .

A B C D 4 mm


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

C. Nd:YAG laser; Preplaced Powder

Figure 10 demonstrates that a change of laser type from CO2 to Nd:YAG does not produce aflat surface when preplaced powder is employed. The results are very similar to those given infigure 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 themaximum flow rate. Once again the 45º groove produces an almost flat top surface and becauseof its smaller cross section the 90º groove is overfilled. It was noticed however that the 90ºsamples 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 sinkcapacity 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 processspeed of 0.5m/min (Nd:YAG laser).


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4 Discussion

Before we analyse the cross section of the clad groove samples it is important to make a fewremarks about cladding onto flat surfaces. Figure 12 shows the cross sections of two clad tracksproduced 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 (andthose 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 asfar 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 determinedby the various surface tensions associated with the liquid, the solid and the surrounding air (seefigure 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).

lg

GasLiquid

Solid

θ sl

γ sg

γ lg = Surface tension; Solid liquid

γ sl = Surface tension; Liquid gas γ sg = Surface tension; Solid gas

4 mma b

Figure 12. A pair of preplaced powder clad tracks produced under identical conditions except forthe depth of the powder used. (3.5 kW CO 2 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.


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In the case of laser cladding the contact angle is not determined by the surface tension of theliquid. This is clearly demonstrated in figure 12 where the contact angle is close to 45º in onecase and close to than 90º in the other for the same liquid on the same substrate. Although it istrue that the two melts may have achieved different temperatures (and therefore surfacetensions) during their melting cycle this effect is unimportant compared with the influence ofthe 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 meltcannot spread laterally if more powder is added to the cladding process, the clad cross sectionchanges as shown in figure 15.

It is clear from figure 15 that the upper surface of the melt will always assume a shape which ispart of a circle but this circle is intersected by a cord of (approximately) fixed length andrepresents the melt- substrate interface. The cross section shape of an individual clad track istherefore 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 thisreason it is not possible to match the interaction of a groove angle to the contact angle of themelt in order to achieve a flat surface. On the other hand the grooves investigated here do havean 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 toolarge then the melt will not be able to fill it adequately. These figures also demonstrate thepoint 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 radiusachievable by the melt. The larger included angle of 90º tended to encourage completepenetration of the melt into the groove.

θ θ

Smaller droplet Larger droplet

Laser beam

x xxx

Figure 14. θ remains the same if the droplet sizechanges 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).


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

If a flat topped clad groove is required these results imply that preplaced powder will never givethe 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 somesingle track hardening studies that the track is accompanied on either side by softened areaswhich 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 asthey allowed the flow of lubricant to the hard, load bearing area and the removal of wearparticles [6].

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

Substrate

Removed excessclad material

Clad layer

Lubricant supplyand debrisremoval conduits

Figure 16. A Schematic cross section of a laser hardened

track.

Substrate

Laser hardened track Softened eroded area

Figure17. Schematic cross section.


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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 theprocess may be used to produce a clad track with shallow grooves on either side whichcould aid lubrication (Once the central protruding part of the clad layer has beenmachined 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 thelaser beam diameter and the amount of powder supplied to the melt.

6 Acknowledgements

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

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 themicrostructure and wear behaviour of grey iron. Materials and Design, ISSN0261-3069, Vol. 17, no. 5-6, pp. 267-273

3. Ion, J C. (2002). Review - laser transformation hardening. Surface Engineering, ISSN0267-0844, Vol. 18, no. 1, pp. 14-31

4. Eckersley, J, S. (1984) Laser Applications in Metal Surface Hardening. Advances inSurface Treatments, Technology Applications Effects, Vol. 1, pp. 211-231

5.

Powell. J. (1988) Laser Cladding With Preplaced Powder; Analysis of thermal cyclingand 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


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

53

Chapter IV

Laser Casting and Laser Clad-Casting: New

processes for rapid prototyping and

production.


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Laser Casting and Laser Clad-Casting: New

processes for rapid prototyping andproduction.

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 finalproduct is not joined to the substrate. The substrate surface therefore acts as a mould ina laser casting process and the eventual casting retains the topological features of thesubstrate.

2. Laser clad-casting involves the production of clad tracks which are welded as usual to asubstrate but which are laid down between machined copper blocks. The eventual cladtrack therefore has its cross sectional profile determined by the blocks which areremoved after completion of the cladding process. In this way clad tracks with largedepth to width ratios can be achieved with pre determinated cross sections.

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


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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 blownpowder laser cladding but the aim in this case is to produce a “clad” layer which is not fused tothe substrate. The resulting “clad “ layer retains the topological features of the surface of thesubstrate 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 sectionalprofile 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 (CO 2 ), 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


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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 twotechniques;

1. The powder mass flow is higher for clad-casting, typically 2 or 3 times the flow neededfor 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 normallyused 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 theprocess 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 irradiatesboth the surface of the molten cladding material and the substrate. The result is theestablishment of a fusion line beneath the original surface of the substrate. This ensures goodadhesion 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 infront of the molten cladding zone. This has two effects:

45°

10-15 mm 5 mm

45°

3a. Laser Cladding 3b. Laser Casting

Interfacial melting betweenthe clad layer and substrate

No interfacialmelting

Unmelted layer ofowder articles

Laser beam Powder feed


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1. The powder layer shields the substrate from direct laser heating and thereby inhibitssubstrate 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 shadowingeffect of the powder cloud. The powder cloud absorbs energy from the incident laser beamand 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]. Theparticle cloud therefore tends to transport energy from the laser beam towards the top part ofthe melt (where the hotter upper particles land) and away from the lower part (where thecooler, 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 wereemployed:

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

Cr C Si Mo Ni Fe Co

wt % 27.2 0.27 1.0 5.5 2.3 0.3 63.4

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


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3 mm

Table3. Fe based powder composition

Cr C V Mo Fe Cowt % 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 beampower and diameter, process speed, powder type, substrate type, powder mass feed rate andparticle speed etc.

This introductory paper will not therefore, attempt to map out the whole process. Our aimhere 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 groovedsubstrates. 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 processparameters, much poorer results are achieved if certain guidelines are not followed. Theseguidelines are presented in the following notes.

a b c Figure 4. Sucessful laser clad-casting of cross hatched grooves. a) substrate (mould), b) substrate andcasting, c) casting. Process parameters: laser power 3 kW (Nd:YAG), beam diameter 5 mm, process speed0.8 m/min., Ni based powder, powder flow 80 g/min (in argon), inter-track distance 3mm.


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Powder mass flow rate

If the powder mass flow rate is excessive the melt will rest on a bed of powder rather than thesubstrate. The resulting “cast” will therefore not take on the features of the mould. Anexample 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 thesubstrate (either directly or by conduction through the melt pool). In this case the clad layerwill 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 afunction 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 asP/VD is increased there is a tendency for increased welding of the clad layer to the substrate.

Figure 5. A polished andunetched cross section of thesample shown in figure 1showing the 95 % density ofthe casting.

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

Figure 7. An example of a casting which failed dueto excessive powder mass flow rate. (process

parameters as fig 1 except powder mass flow rateincreased to 110 g/min).


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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 byadding a clad layer to the outer diameter. This clad layer was to be of a wear resistant materialand, 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 isachieved when the track is semicircular in cross section as shown in figure 9b. This customerhowever, required a better aspect ratio than the 2:1 limit of a semicircle. They needed a postmachined aspect ratio of approximately 1:1 as shown in figure 9c. From figure 9c it is also clearthat 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 cladcross 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 toeither side of the substrate as shown in figure 10.

Clad layer


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Figure 10. Cross section of the clad castmould.

Clamping

SubstrateMachined

co er blocks

Cladding was now carried out with the laser and the powder stream aiming into the valleybetween the copper blocks. The beam diameter was 4 mm on the substrate surface and thusirradiated the copper blocks on either side. However, the high reflectivity of the copperprevented it from melting by direct laser irradiation and its high thermal conductivityprevented melting by contact with the molten cladding metal. As a result the copper blockscould be easily removed after the cladding was complete. The clad profile produced by thismethod 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, claddingspeed 0.5 m/min, laser power 3.5 kW (CO 2 ), beam diameter 4 mm.

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

3 mm

Machined

copper blocks


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

a)

The process is more energy efficient than standard laser cladding. In this case 24% of thelaser energy was utilised in the melting process as compared to 20% for standard lasercladding with an Nd:YAG laser [2] (This value is only 10% for standard CO2 lasercladding [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 claddingvalue ∼ 60%) [3]. This improves deposition rates and minimises substrate meltingbecause a greater proportion of the laser energy is involved in melting the incomingpowder. This improvement in powder catchment efficiency is clearly a function of thevalley-like geometry of the clad-cast melt zone. A geometry of this type tends tochannel powder into the weld pool rather than allowing it to spray all over the substratesurface. (Which happens in standard laser cladding).

5 Conclusions

It has been demonstrated that two new laser cladding techniques are possible and that they mayprovide novel answers to future production requirements.

Laser casting can be used to produce surface castings in high strength alloys to generate toolbits or stamping dies etc.

Laser clad-casting can be employed to make clad tracks with large depth to width ratios andpre determined cross sections.

6 References

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

2. Gedda, H., Powell, J., Kaplan, A. (2002). A Process Efficiency Comparison ofNd:YAG and CO2 Laser Cladding. Welding in the World, Vol. 46, SpecialIssue, 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 LaserApplications, Vol. 14, no. 2, pp. 78-82


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H.Gedda: Chapter V-Laser Wire Casting

65

Chapter V

Laser Wire Casting


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Laser Wire Casting

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



#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 bylaser melting. Three techniques have been developed to ensure that the molten powdersolidifies as a rod or wire rather than a series of droplets. The straight rods or wires produced inthis way have an almost circular cross section, are several millimetres in diameter and can bepore free. The techniques can be used to produce welding rods, tensile test samples and othersolid pieces from a wide range of powder mixes. The rapid thermal cycle involved means thathitherto difficult to produce mixtures and alloys can now be produced in the solid form inseconds.

1 Introduction

Previous work by the present authors [1] investigated novel applications of blown powder lasercladding 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 placedpowder beds. Simply traversing a defocused laser over the surface of a powder bed was found togive unsatisfactory results because the melt has a natural tendency to form a series of largedroplets 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


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Three techniques have been developed at the Sirius laboratory, Luleå University whichsuppress 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 wereemployed:

Laser model: Rofin Sinar RS 6000 CO2. Laser power 3500 W. Laser beam defocused to Ø 3mm.

Powder: Stellite 8 (Cobalt base). The powder chemical composition is presented in table 1below.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

Cr C Si Mo Ni Fe Co

wt % 27.2 0.27 1.0 5.5 2.3 0.3 63.4

Ni Cr Mo C Fe Wwt % 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


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2.2 Casting with moulds

The aim of this

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