Analysis and development of the laser cladding process

UNIVERSITY OF TECHNOLOGY

Analysis and Developn1ent of the Laser Cladding Process

Hans Gedda

Department of Applied Physics and Mechanical Engineering

This work has been carried out since the spring of 1999 at the Division of Manufacturing Systems at Luleå University of Technology. The work has been in the area of laser cladding and the experimental work was mostly performed in our laser laboratory. Some work has been done at Duroc AB in Umeå and Luleå.

I would like to thank all my friends and colleagues at the division for all their help and fruitful discussions.

I would like to express my gratitude to Professor Claes Magnusson, Luleå, Doctor John Powell, Nottingham and Professor Alexander Kaplan, Luleå for discussions, suggestions and guidance through this work.

I would finally thank my family, Birgitta, Petrus and Emilia for their love, support and patience during the work.

Luleå, May 2002

Hans Gedda

This thesis presents an investigation into the laser cladding process using CO, and Nd:YAG lasers. The work is divided into four chapters:

Chapter one is a literature review of the subject of laser cladding. This presents a general overview of the subject from a practical and theoretical point of view.

Chapter two is an investigation about the energy redistribution during CO, laser cladding.

Experimental absorption measurements by calorimetry were carried out to separate and analyse how much of the energy is lost by reflection etc. It was found that approximately 60% of the original laser power is lost by reflection.

Chapter three extends the work of paper two and compares the process efficiency of Nd:YAG and CO, laser cladding. The energy redistribution during cladding has been analysed for both processes. The Nd:YAG laser cladding process was found to be considerably more energy efficient than CO, laser cladding process.

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 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.

Preface

Abstract ii

Contents iii

Chapter I: An introduction to laser cladding 1

Chapter II: Energy Redistribution During CO, Laser Cladding 18 Gedda, H., Powell, J., Wahlström, G., W-B, Li., Engström, H., Magnusson,C. Accepted for publication in Journal of Laser Applications March 2002.

Chapter III: A process Efficiency Comparison of Nd:YAG

and CO, Laser Cladding 32

Gedda, H., Powell, J., Kaplan, A. Conference proceedings

International Conference on Advanced Processes and Technologies in Welding and Allied Processes Copenhagen,

Denmark 24-25 June 2002.

Chapter IV: Laser Casting and Laser Clad-Casting: New processes

for rapid prototyping and production 46

Powell, J., Gedda, H., Kaplan, A. Conference proceedings International Congress on Applications of Lasers & Electro-Optics (ICALEO)Scottsdale, Arizona, 14-17 October 2002.

Chapter I

An Introduction to laser cladding

An Introduction to Laser Cladding

H. Gedda

(An edited version of; Laser Surface Cladding- A literature survey issn1402-1536; LTU- TR- 2000/7—SE)

1 Background

Some industrial applications require parts with special surface properties such as good corrosion resistance, wear resistance and hardness. Alloys with those properties are usually very expensive and it is of great interest to reduce the cost of parts of this type [1]. This requirement has stimulated the growth of surface treatment technologies which produce the correct surface properties on cheap substrates. 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. Laser surface treatment can be divided into; alloying, cladding, dispersing, impregnation, and hardening [2]. The subject of this report is laser cladding.

Laser cladding involves the fusion of an alloying material layer to a substrate surface, with a low level of melting of the substrate. The surface alloy composition must be well controlled with a high bond strength to the substrate [3].

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 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.

H.Gedda: Chapter I-An Introduction to Laser Cladding

2 Techniques of laser cladding

2.1 Introduction

Laser cladding is a cost-effective way of reducing material loss by corrosion, wear or other forms of surface degradation. The aim of most cladding operations is to overlay one metal with another, with a sound interfacial bound. Laser cladding enables a low dilution of the clad layer with the bulk material combined with a metallurgical bonding between the layer and the substrate [4,5]. According to Steen [6], the two most common methods of supplying cladding material are:

• Preplacement of cladding material powder on the substrate (see figure la).

• Inert gas propulsion of material powder into a laser generated molten pool (see figure lb):

11

Cladding powder

a, Preplaced powder b, blown powder cladding.

Figure 1. Schematic diagrams of laser cladding processes [1] .

Relative motion between the laser/powder supply and the substrate can be used to

continuously apply a surface coating. Tracks are overlapped to cover larger surfaces (see figure 2). Clad coatings are usually of a thickness between 0.1 and 5 mm.

Figure 2. Schematic of the overlapping cladding process [7].

The effect of the laser radiation on the material depends on the energy density (P/VD).

Where:

P = Laser power (W)

V = Laser/material velocity (m/sec) D = Diameter of the laser spot (m)

Common power densities for laser cladding are 102-104 W/mm2 together with an interaction time of around 10-3- 1.0 s. Rapid solidification starts after the laser beam has scanned the surface. The solidification starts at the bottom in the melt pool and progresses towards the surface. The microstructure of the laser clad zone consists mainly of an as-cast region of dendrites and HAZ adjacent to the fusion line. The microstructure of the clad layer can be affected by the degree of dilution of the substrate [3, 8].

2.2 Laser cladding with preplaced powder

Cladding with preplaced powder is a very simple 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 to melt it and weld it to the substrate.

2.3 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 [9]. 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 and the molten particles are trapped in the melt pool. The energy must be high enough to melt the powder but low enough too avoid too much substrate melting [3]. The powder striking the substrate outside the melt pool 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 NOZZLE

ETERING UNIT LASER BEAM

(REFERENCE)

CONTROL

powder utilisation efficiency is low compared with coaxial powder nozzle feed [10]. The coaxial system shown in figure 3 can avoid this problem to some extent.

Figure 3. A schematic of the transverse section of a coaxial nozzle [le There are also other powder delivery systems (see figure 4). A detailed description is not necessary here because each process and application may require special engineering considerations [11].

Figure 4. Powder delivering system [11].

The blown powder cladding process offers a numbers of advantages compared to other laser surface treatments:

• Well-defined treated region.

• Low dilution.

• Adaptable to automatic processing.

• Good fusion bond.

H.Gedda: Chapter I-An Introduction to Laser Cladding

• The process window is large.

• High reproducibility.

2.4 Cladding with paste bound powders

In the cladding process with paste bound materials the clad material is fed directly into the interaction zone between the laser and substrate. The laser melts the paste and produces a layer on the substrate. The main disadvantage of this technique are problems with the binder materials, such as contamination and gas porosity in the layer. This is because the binder must be able to dry quickly, evaporate and, at same time keep the cladding material in compact form.

2.5 Laser cladding by wire feeding

Laser cladding with powder filler material gives a poor utilisation of the powder and dust can be emitted directly into the laser equipment and the working environment. One way to avoid the disadvantage with powder feeding is to use wire as a cladding material. This wire feeding process seems advantageous because the material efficiency is high in this one-step process together with an improved working environment. But the absorption of energy by the wire is inefficient, and to avoid this drawback it is necessary to raise the laser power or preheat the wire [12,13]. The principle of laser cladding by wire feeding is shown in figure 5 below:

Laser beam

E E

I

Substrate Wire Nozzle

Figure 5. Laser cladding by wire feeding.

Investigations with preheated wire,> 1000 °C, show improved efficiency of the cladding process [14]. The material usage efficiency is about 100% and melt rates as high as 3 kg/h are obtained, but with unwanted dilution as a result. Wiklund et al. [12], has shown that when laser cladding wire is combined with a MIG power supply, the amount of additive material can be increased over 4 times in clad welds up to 5 mm thick with low dilution between the substrate and cladding material. The clad welding rate can be three times higher if the

3 Theoretical modelling of the laser cladding process

3.1 Pre placed powder cladding

Preplaced powder cladding generally results in a low dilution clad layer over a wide range of process parameters. This phenomenon was investigated by Powell [15] who produced a model which divided up the laser-material interaction into 6 stages:

1. A layer of solid metal powder rests on a substrate with no laser irradiation. The powder (of known depth, density, and composition) and the substrate (of known composition, etc) are both at ambient temperature.

2. Laser irradiation begins and the surface powder particles heat up but are assumed not to conduct heat to the particles below them because of the low level of interparticle contact ( i.e. the bulk of the powder is assumed to be an insulator).

3. Under this irradiation the insulated surface particles melt and in the molten state can conduct heat, but only to neighbouring particles experiencing wet contact ( i.e. only the powder layer <2Ri, distance from the molten front; where RI, is the particle radius).

4. The molten front progresses through the insulating powder layer described by an energy balance on the melt pool:

Laser energy in =[temperature rise + phase change (solid liquid) in the melt pool] + (radiated loss from melt surface) + (convective loss from melt surface), i.e.

H = (C pi/ + QAV p) + ilp-411+[hA(Ts (1) Where H is the heat input (J) to an element of surface area, A, and volume V, C specific heat, Q the latent heat of melting, T, surface temperature, T ambient temperature, h heat transfer coefficient, a the Stefan-Boltzmann constant and p density.

5. Contact is established between the molten front and the substrate. This changes the energy balance to:

Energy in = (change in sensible heat of the melt pool) + (radiative loss from melt surface) + (convective loss from melt surface) + (conduction loss to the substrate), i.e.

H = pV AT + Q,AV p) + Uo-A(T5.41+[hA(T, — Ta kcIt + KA 11, ^{dt (2)}

Where K is the thermal conductivity.

This chilling effect of the substrate leads to a reversal in the direction of movement of the melt front. This now approaches the laser irradiated surface which is still being heated.

a E

2

3

6A. The laser is no longer heating due to being stopped or having moved away. The clad layer solidifies on the substrate surface and a bond exists between the two which involves minimal substrate melting.

6B. The laser is of insufficient power to cause remelting but continues to heat. This has the same result as found for 6A.

6C. The laser is sufficiently powerful to cause remelting but not to alter the single reversal in solidification direction. The result is the same as for 6A.

6D. The laser is sufficiently powerful to cause a reversal of the resolidification process but not powerful to enough to melt back to the interface. The result is a double change in the solidification direction but the interface is still formed from a solid/liquid contact.

6E. The laser is sufficiently powerful to remelt back to the interface. The result is a double change in solidification direction but an interfacial bond which could be a fusion bond if the remelting process is allowed to proceed back to the substrate surface otherwise it is once more a solid/liquid bond.

In practice the interactions described as 6C, 6D and 6E rarely have sufficient time to proceed to an equilibrium melt depth.

A computer model of this theory was developed which generated process maps of the type shown in fig 6.

or inal ,to tier surface

Position of the melt front

2000W incident absorbed power.

s

8.

•Ke •

e..

Poston bond

inc...-re t..,-.tug dizuttutl ,

0.1 0.2 0.3 0.4

Figure 6. Theoretical calculation of the position of the melt front during prep/aced powder cladding [6].

This figure demonstrates that the melt front needs to contact the substrate only once to produce a clad layer but needs to contact it twice to dilute the clad layer with molten substrate material. This explains the low level of dilution associated with the process.

3.2 Blown powder cladding

According to Steen [6], there is a reasonably large operating window for successful low dilution fusion bonded cladding as illustrated in figure 7 below This process window is limited by three boundaries namely:

1. The dilution limit (I), which corresponds to the laser beam power (P) needed to melt the powder and a thin layer of the substrate with a fixed powder flow rate (m). This can be expressed by J P/Dm where D is the beam diameter. For cobalt alloys the authors estimated that J 2500 J/g.mm.

2. The aspect ratio limit corresponds to the powder feed rate at fixed absorbed laser beam power, that leads to clad tracks which can be overlapped without defects. To avoid porosity in overlapped tracks the aspect ratio should be w/h >5, where iv is the width and h is the height of a single track

3. The lower power limit corresponds to the absorbed laser beam which just melts the substrate. This limit is given by the equation / = P (1 — r —s)/DV, where r is the reflectivity and s is the shadow coefficient. For example, more than 22 J/mm2 is required for

continuous cladding.

It has been suggested that the extensive low dilution region is due to the solidification front rising swiftly with the growth of the clad. The explanation of this is that the interface freezes almost as soon as it forms even if there is melt above.

Power /spot diameter

1000

500

no clad

0 0.1 0.2 0.3 0.4 0.5 Powder feed rate

Figure 7. Operating window for blown powder laser cladding [6].

In the early nineties Picasso and Hoadley presented a two dimensional Finite Element model for the cladding process [16]. They used a very simple gas-powder flow model, the heat and Navier-Stokes equations are solved in the material with the corresponding boundary conditions. Later Li et al [17] developed a mathematical model of the cladding process. The

cladding process. To do this, certain simplifying assumptions had to be made. They are summarised as follows:

1. The laser beam and the injecting powder stream are both assumed to be cylindrical. The two "cylinders" converge in the cladding zone (see fig 8)

2. The cladding particles are spheres of uniform size.

3. The temperature dependence of physical properties such as density, specific heat and the absorption coefficient of the particle material are neglected.

4. Energy reflected between particles or from the workpiece to the particles is ignored.

Figure 8. The geometry of the laser cladding process [17]

Li et al. [17] stated that the basic consideration for calculating the temperature of any particle is the law of energy conservation, i.e.

E,= Ea+ER+E,+E, ₍₃₎

Where

E,

is energy incident on the particle,

E,

is the energy absorbed,

E,

is energy reflected from the particle surface, Er,„ the radiant and convected losses and

E,

is the energy transmitted through the particle.

If the radiation and convected losses together with the energy-transmitted through a particle are neglected, we have

(4) The energy transmitted through a particle can be neglected because the penetration depth of the CO,- laser light is about 10 m. The powder particles are usually tens of microns in diameter and therefore the particles can be assumed to be opaque.

Energy Input to Any Particle (Ea)

The aim of the model was to calculate the temperature of a particle which impacts the melt pool at any point P* defined by (a*,b*) in X*0*Y* co-ordinate plane or by P(a,b) in the co- ordinates of XOY (see figure 8). From figure 8 it can be seen that x* = x / cos a and y* = y.

Considering a particle at a location B on its path of injection, the energy input to the particle is dependent on its depth in the powder stream, because the shielding effects of the particles above. Therefore, ignoring radiative, convective and transmitted losses the energy absorbed by the particle for a small time interval A t , E, can be expressed as:

E = (1— D) I (x, y) rt- r 2p e -EL A t (5)

Where D is the reflection coefficient of the particle, rp the particle radius, B the absorption coefficient of the particle material, L, the volume fraction of the particles in the powder cloud, I(x,y) the distribution function of the intensity of the laser beam and I the penetration depth of the laser light through the powder cloud to reach the particle concerned. In this work, described in more detail elsewhere [26,28], the equation above is rewritten and substituted to result in the final equation below.

3(1— D) 477

T—T, — I (x,b)e t

4rp pCvCos0 ⁽⁶⁾

This is the equation for calculating the temperature of the particle impacting on the melt pool at the point (a*,b*) in the X*0*Y* plane or at (a,b) in the XOY plane. The equation relates the temperature to a number of variables which include the beam parameters (I, rb, Q), geometrical parameters (0, a, 1) and the particle parameters (P,v,rp ,B, f,). The effect of these variables can be predicted separately by calculation.

Model results:

Power:

Results from the model showed what was expected i.e. there exists a linear relationship between laser power and particle heating.

Laser Beam Radius:

The relationship between the preheat temperature and beam diameter also showed a linear relationship.

, Aeee V 2

-0 0010 -0 0005 0 0000 0 0005 00(110

Particle Size:

Figure 9, shows that the amount

of pre heating experienced by a particle is inversely proportional to particle size. This is because the mass of the particle increases in proportion to r2 (r = radius) and the absorbing surface increases only in proportion to r2.

Or/ 0 00001 Rp =0 00002

=0 00003

00.

a (m)

Figure 9. Pre-heating temperature of powder particles of different sizes [14

Powder Flow Rate:

The powder flow rate is another important parameter in the cladding process represented by the volume fraction of powder Ç. Low flow rates often give high dilution whilst high flow rates create porosity and defects in the binding zone. The model showed that the shadowing effect of the topmost particles increased dramatically with increased powder fraction.

Particle speed:

Figure 10 demonstrates that the preheating is inversely proportional

to the powder velocity. This result depends on the reduced laser particle interaction time at higher velocities.

The particles are impacting along the line defined by b=0 for different powder particle speed [m/s].

a (m)

Figure 10. Pre-heating of powder particles for different speeds [181.

Distance From the Laser Beam Centre:

The particle preheat temperature decreases if the particle path is moved away from the centre line (i.e. as b increases). Particles travelling along the centre line of the beam are exposed to laser light for a longer time than those which are off centre. Figure 11 below shows the preheating temperature of powder particles impacting along lines defined by b=0, 0,2, 0,4, 0,6, and 0,8 mm for Gaussian beam intensity distribution. If the beam has a Gaussian energy distribution the particle travelling closer to the beam centre line are also exposed to higher beam intensities.

Maximum expoaure

and heaung Reduced exPc.ure and heaUng

Laser beam Cr0555CCUOn 00000 b=0.0000

aeleao b=0.0002 ecneent b-0.0004

»eft b0.0006 tree*0 b=0.0008

a (m)

xi

b=0 b>0

Figure 11. Pre-heating temperature of particles for Gaussian beam intensity distribution [18].

Various models have been published relating to numerical simulations of the physical processes involved in laser cladding. Collier et al [19] presents a two dimensional model for deterrning the temperature and the distribution of velocities in the workpiece, taking due account of the results obtained in experiments. Oilier states that the interaction between hydrodynamics, thermal, conduction and powder gas-flow determine the physical processes involved in cladding operations and the processing results.

4 Influence of process parameters on the cladding process

Powder feed rates and scanning speed influence the geometry of the clad layer. Cladding thickness depends on the amount of powder injected to the melt pool. It is usual to combine the powder feed rate m and the scanning speedy to a powder feed rate m /v [g/m], to calculate the amount of powder transported onto the sample per unit length [20,21]. A linear

relationship exists between the powder feed rate and the energy required, if sufficient energy is to be available to melt all arriving powder. An excessively high laser energy input leads to dilution with undesirable mechanical properties in the material e.g. decrease in wear resistance.

The surface of the substrate should be melted slightly so that a good bond betwen the substrate and cladding material is produced. A slightly larger laser beam diameter than the powder supply nozzle improves the coating efficiency in terms of reduced powder loss during the coating process.

Lugscheider [20] states that when coating larger areas by laying single tracks next to each other, the overlap of tracks is added as another process parameter. If the overlap of the tracks is too small, poor cladding is produced. With increasing overlap the surface profile becomes smoother, but every track produces a heat treatment in the preceding track. Between two overlapping tracks a heat-affected zone (HAZ) develops in the clad layer. This HAZ shows a coarsened microstructure with inferior properties of hardness and corrosion resistance.

Therefore, for coating larger areas, a broad track-width is advantageous, with fewer overlaps as a result.

The cladding microstructure is partly influenced by the structure and size of the powder particles but is mainly affected by the solidification rate, which depends on process parameters such as power density, absorption of laser energy and scanning speed [21].

By changing these parameters different solidification rates are achieved and faster solidification rates lead to a fine structure. Powders used in the cladding process contain mainly Co, Ni, Fe, and Cr. Their thermophysical properties are similar and average values are as follows: p=

8g/cm3, c= 0,5 J/g, AH„, = 300J/g and Tm =1500 °C. For a mass of 1g, the energy required for melting is E =1kj, and for a feed rate of 30 g/min (0,5 g/s), the absorbed power needed is 500W [33]. The power necessary for melting depends also on the absorption coefficient (A) of the powder particles. It is known that that this coefficient is small for metallic surfaces at = 10,6 gm (CO, laser wavelength)but the surface of the powder particles is often covered in oxides [22]which increases absorptivity. The absorption of laser radiation is also a function of angle of incidence of the beam with respect to the material surface [7]. This is clearly a complex variable when small powder particles are considered. The polarisation of the beam is another factor, which affects absorptivity. Experimental work [7] has shown that the best process efficiency can be achieved when using a linearly polarised CO,- laser at an angle of 75° to the direction of material surface.

The laser cladding process can therefore generally be divided into process and powder parameters:

Powder parameters are:

• Size and size distribution of the powder.

• Powder feed rate.

• Type and amount of protection and transport gas.

• Melting temperature.

• The point of injection.

• Absorption coefficient.

Process parameters are:

• Laser power and power density.

• Angle and position of the powder nozzle relatively the substrate

• Scanning speed.

5 Conclusion

Industrial applications require parts with good wear, corrosion, and hardness properties. Laser surface treatment is a process which can compete with traditional surface treatment methods.

Laser surface treatments are divided in hardening, dispersing, impregnation, alloying, and cladding. In this literature survey laser cladding technology has been the main object of study and the general conclusions are summarised as follows.

• Single stage (i.e. material injection) laser cladding technology offers some advantages compared with the preplaced powder cladding process as any substrate geometry can be clad.

• The technique based on direct powder injection into the melt pool has the advantage of possible deposition of a wide range of a alloys either by using a chosen alloy as powder or by a blend of powders with the required composition. A slightly larger laser beam diameter than the powder nozzle improves the powder utilisation.

• For coating larger areas, a broad track-width is advantageous, with fewer overlaps.

• Absorbing coating layers or sand blasting of the substrate increases the cladding rate.

• Laser cladding with wire, band or paste is a material efficient process compared to blown powder cladding. If wire cladding is combined with a MIG power supply, the cladding rate increases more then four times compared with the laser alone.

• A coaxial nozzle powder feeder showed some advantage compared with side blown powder technology concerning the powder utilisation and directional effects.

• Laser cladding is very complex because the processing parameters are numerous.

• Models of the laser cladding process are a useful tool to avoid undesired dilution and an understanding of the phenomena in the interaction zone.

6 References

Riabkina-Fishman, M., Zahavi, J. Laser alloying and cladding for improving surface properties, Applied Surface Science (Netherlands), vol. 106, no. 1-4, pp.

263-267, 1 Oct. 1996

2. König, W., Rozsnoki, Kimer, p. Laser Treatment of Materials 1992 Conference paper (ECLAT 92) ISBN 3-88355-185-6, pp. 217-221

3. Yellup, JM,. Laser Cladding using the powder blowing technique, Surface Coating Technology. (Switzerland)., vol. 71, no 2, pp. 121-128, Mar.1995

4. Bergman, HW., Muller, D., Endress, T., Damascheck, R., Domes, J., Brandsen, AS. Industrial applications of surface treatments with high power lasers, Materials science forum, vol.163-165. Part 1. 1994. pp 377-404.

Powell, J., Henry, P.S., Steen, W.M. Laser cladding with preplaced powder.

Analysis of thermal cycling and dilution effects. Surface engineering 1988 Vol 4.

No. 2.

6. Steen, W.M. Laser Material Processing. Laser surface treatment. Second eddition, 1998 ISBN 3-540-76174-8.

7. Frenk, A., Vandyoussefi, M., Wagnnre, J.-D., Zryd, A., Kurz,W. Analysis of the laser-cladding [laser surfacing] process for stellite on steel. Metallurgical and Material Transactions B, vol.28B, June 1997, Pp501-508.

8. Pelletier, JM., Fouqet, F., Dezert, D., Robbin, M., Vannes, AB. Improvement of mechanical properties of steels by addition of tungsten carbides. DGM

Informationsgesellschaft mbh (Germany), pp.211-216, 1992

9. Hoadley, A, Rappaz, M., A thermal model of laser cladding by powder injection.

Metallurgical Treansactions B, vol . 23B, no.5. Oct 1992. Pp631-641.

10. Hu, U.P., Chen, C.W., Mukherjee, K., An analysis of powder feeding systems on the quality of laser cladding, Metal Powder Industries Federation (USA).

Pp21.17-21.31, 1997

11. Webber,T., Laser material processing, Laser Weld Overlay ISBN 0-8247-9714-0 pp 231-255 USA 1996.

12. Wiklund, G., Flinkfeldt, J. Laser Cladding Combined with MIG-Equipment.

Luleå University of Technology, Division of materials processing. 1997:09 ISSN:

1402-1536, ISRN: LTU —TR — 1997/9—SE

13. Bouaffi, B., Bartzschj., Surface protection by laser beam deposition with hot wire addition, Schweissen und Schneiden, nr 4, vol.45, 1993 April, pp. E70-E72.

14. Hinse-Stem, A., Burchards, D., Mordike, B. L., Laser cladding with preheated wires, Conference; Laser treatment of materials, ECLAT 1992, Gottingen, Germany, pp. 223-228.

15. Powell, J., Laser Cladding. Phd Theses. Imperial College of Science and Technology, Dept. Of metallurgy, London, UK, (1993).

16. Picasso, M, Hoadley, A. F. A., The Influence Of Convection In The Laser Cladding Process. Proceeding of the 7th International Conference on Numerical Methods in Thermal Problems jul 8-12. 1991 Publ by pineridge Press Ltd Swansea pp.199-209.

17. Li, W.B, Engström, H., A Model of Laser Cladding by Powder Injection. Pre heating of the blown powders and power redistribution of the laser beam. Conf.

Proc. Of MSMM '96. June 11-13, 1996, Beijing, China.

18. Li, W.B, Engström, H, Powell, J, Tan, Z, Magnusson, C., Modelling of the pre- heating of the blown powder material in the laser cladding process. Conf. Proc.

Of 5" Conf. On Laser Materials Processing in the Nordic Countries, Nolamp 4, Sep 6-8, 1995, Oslo, Norway

19. Oilier, B, Pirch, N, Kreutz, E.W, Schluter , Gasser, H, Wissenbach, K., Cladding With Lasers Radiation: Properties and Analysis. DGM Informationsgesllschft mbH (Germany), pp. 687-692, 1992.

20. Lugscheider, E, Oberländer, BC, Leising, S.E., Influnce of Laser Cladding Parameters on the Microstructure and Properties of Claddings. Surface Modification Technologies V Edited by T.S Sudarshan and J.F Braza. The Institute of Matreials. 1992 Aachen University of Technology, Templergrabbe 55, D-5100 Aachen, Germany. pp 880-888.

21. Vilar, S, Neto,D., Interaction Between the laser Beam and the Powder Laser Alloying and Cladding. Conference proceedings 'CALE() 98 November 16-19, 1998 Orlando,FL USA. D180-D187.

22. Sahour, M.C, Vannes, A.B, Pelletier, J,M., Laser Cladding By Powder Injection:

Optimazation Of The Processing Conditions. Journal De Physique IV, Vol 1.

Dec 1991. Pp C7-51-C7-54.

Chapter II Energy Redistribution During CO2 Laser

Cladding

Energy Redistribution During CO2 Laser Cladding

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: Hans.Gedda@mb.luth.se

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

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

Abstract

This paper examines the factors that effect the efficiency of the CO, laser powder cladding process. By theoretical calculation and experimental work it has been possible to identify how much of the original laser energy contributes to the cladding process and how much is lost to the surrounding environment by reflection, radiation, convection etc. Every aspect of energy redistribution has been analysed and quantified and this has lead to a deeper understanding of the process. The paper concludes with a number of suggestions for improving the efficiency of blown powder laser cladding.

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

Wig

1 Introduction

Laser cladding is a method of welding one metal onto the surface of another. There are two basic techniques:

1. Single stage or blown powder cladding: This involves blowing the cladding metal powder into a laser generated melt pool on the surface of the substrate. (see figure 1).

This process is easily automated and has widespread commercial applications [1-3].

2. Two stage or pre-placed powder cladding: In this case a bed of powder is layed on the surface on the substrate and the laser subsequently passes over it (see figure 1). This process is difficult to automate and has limited commercial applications [4].

Blown powder particles

A

F

11

1— Substrate

Pre—placed cladding material

Figure 1. Schematic diagrams of laser cladding processes [5].

This paper considers the efficiency of the blown powder cladding process by analysing how much of the laser energy contributes to the cladding process and how much is lost to the surrounding environment by reflection etc.

Clad layer

2 Theoretical Discussion

An energy balance for laser cladding can be expressed as follows:

P tot PC+PL (1)

the output power of the laser

he power utilised in melting the cladding material and welding it to the surface of the substrate.

the power lost by reflection, radiation, convection etc.

Where:

Prot = Pc PL =

Pc in equation 1 can be expanded as follows:

Pc= Pp+Ps (2)

-Where:

P,

-= the power utilised in melting the cladding powder.

Ps =- the power utilised in melting the surface of the substrate in order to achieve a clad/substrate weld.

P,

in equation 1 can be similarly expanded:

PL PA±PB+PD±PE±PF±PG (3) Where:

PA = Power reflected off the surface of the clad zone.

P„

= Power reflected off the powder particles as they approach the weld pool.

P,=

Power lost by radiation from the cladding zone.

P,

= Power lost by convection from the cladding zone.

P,

= 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 must be taken that any influence which could minimise PA, PE, PD, PE,

P,

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.

P Laser beam

Powder stream

PD

Figure 2. The redistribution of laser power during 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 2 it is clear that the process can be speeded up if there is an increase in the laser power available for melting the cladding material Pi,. The requirement here would be to melt enough powder to achieve the correct clad thickness at a faster linear speed. Also an increase in

P,

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

P,

and the process could be accelerated to produce the same clad depth with a minimal depth of substrate melting.

The following experimental section details an investigation to identify the relative magnitude of the components of Pc and

P,

3 Experimental procedure

3.1 General

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

Table1. Steel composition (substrate)

C Si Mn P 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.

Table2. 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 laser used was a Trumpf CO. laser with a maximum output power of 12 kW. The shielding/carrier gas employed to propel the powder was argon. The substrate specimens were grit blasted before cladding was carried out.

A number of different experimental and theoretical analyses were carried out to estimate the relative importance of the components of equation 2 and 3. These are described separately below.

3.2 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 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.

H.Gedda: Chapter II-Energy Redistribution During CO, Laser Cladding

Laser Brans

Powder Particles

Insulated .4— calorimeter

Power reading

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

Table3. Power absorbed and reflected by the powder cloud 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)

5300 35 4800 500 106 (2.0%) 394 (7.4%)

5300 45 4660 640 119 (2.2%) 521 (9.8%)

5000 35 4510 490 102 (2.0%) 388 (7.8%)

5000 45 4370 630 117 (2.3%) 513 (10.2%)

4700 35 4200 500 101 (2.1%) 399 (8.5%)

4700 45 4150 550 116 (2.5%) 434 (9.2%)

Average 2.2%

(approx 2%)

8.8%

(approx 9%)

* Measured by the powder probe but with zero powder flow.

** Percentages are approximate

It is clear from table 3 that we now have an approximate value for

P,

for the commercial parameter range covered here:

PB= 9"01.r (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 130, 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%). Then it can be concluded that 40% of the heat collected by the powder cloud (2%

see table 3) does not contribute to the cladding process.

i.e. PG = 0.8 % (5)

3.3 The power lost by radiation from the cladding zone (Pp).

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 ^{= Cr} T4A (6)

Where

a is the Stefan-Boltzman constant (5.7*10-8 Wm-2K-4) 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 and its surface area was 32 mm2. The value of 2300 K has been chosen because it is half way between the melting point (1700 K) and the boiling point (3000 K) of the material

This gives a maximum value for P, of

Pd= 5.7 *10-8 *(2300)4 * 32 *10-6 = 51 Watts (7)

1% of P50 (8)

3.4 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 surface area of 32 mm2. 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 4.3m/sec.

The rate of convective cooling of a hot body exposed to a cooler gas is given by:

Q=11.AAt Watts (9)

Where:

h = heat transfer coefficient A = surface area of the hot body

At = The difference in temperature between the body and the cooling gas.

Evaluation of h from a standard text on the subject [6] gives us a value of approximately 100

Q= 100 *(,T* 0.0032 )* 2000 (10) PE = 5.6 Watts (11)

Or PE = 0.1%P 0 (12)

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

Calorimetry was employed to measure the heat input to each clad sample (Pm). From this measurement it is possible to measure the power reflected off the cladding zone in the following way:

PA = Pa+PD±PE±PG+P.) (13)

Table 4 shows the results from the calorimetric measurements taken at different laser powers and powder feed rates

Table4. Calorimetric measurements Laser output

Power (Watts)

Powder flow rate (g/min)

Power input to clad sample (Ph)

(Watts)

Power % input to sample

5300 35 1990 37.5 %

5300 45 1886 35.5 %

5000 35 2043 41.0%

5000 45 2147 43.0%

4700 35 1938 41.0%

4700 45 1730 37.0 %

Average 40 1955 39.0%

From our earlier results:

P A = 100— (9+1+0.1+0.8+39) (14)

PA = 50.1% (15)

In summary it can be said that the clad zone reflects approximately half of the laser energy.

This high reflectivity value confirms the work of other authors in the field [7] who suggest that the onset of melting is associated with a rise in material reflectivity. It is assumed that 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.

Heat affected zone

Melted substrate

6.2 mm Melted

substrate

3.6 The power utilised in melting the clad layer to the substrate (Pa)

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

Figure4. 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. For this reason cladding which involves substantial substrate melting is called laser surface alloying.

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

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

P,= Avp(CpAt + AHm) (16)

Where:

A = The melt cross sectional area (m2)

= The Cladding speed (m/s)

= The Density of the material.

Cp -= Specific heat capacity of the material.

At -= The difference between the average melt temperature and ambient AH„, = Specific heat of melting of the clad melt.

Taking approximate average values of these variables in the case of our experiments gives following:

A = 3.4*10-6m2

= 0.015 (m/sec) p = 8020 kg/m3 Cp = 500 J/kg K At = 2000K AHm = 300 kJ/Kg

P,= 3.4*10*0.015*8020*(500*2000+300*103) P,= 530 Watts

These values give us a typical value of P, of r-lt 500 W or 10 % of Pro,

3.7 Power lost by conduction from the clad zone to the substrate

This value is easily established by subtracting (Pa) from the total power absorbed by the workpiece (P1.). Taking average values:

PF= Pc •"'-' 30%1350, ⁽¹⁷⁾

Raw Power of beam (5 kW) (Pt,„)

% of power 100%

Power Reflected off the workpiece (PA) 50%

Power Re radiated from the workpiece

(P,)

1%

Power Reflected off particles

(P,)

9%

Power absorbed by the Process (P,+13,) 40%

-100%

Power absorbed by the cladding process 40%

Power used to melt the Clad layer

(P,)

10%

Power absorbed in heating the substrate (P r) ³⁰%

4 Discussion

Figure 6 presents schematics of the redistribution of energy during the laser cladding process (approximate percentages).

Figure 6. Schematic of the redistribution of energy during the cladding process

Our calculations show that two of the variables included in equation 3; PE and

P,

are negligible as they account for less than 1 % of

P„„

(see equation 5 and 12). 13, also accounts for only 1% of Pto, and can therefore be ignored.

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.

The reduction of reflective losses from the powder cloud is 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 (nr2) rather than half its surface area (27cr2). This because the shadow cast by any particle has an area of itr2 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 oc rs). 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.

5 Conclusions

Of the powder absorbed by the cladding process (40%) three quarters is used in heating the substrate and only the remaining quarter (10% of the original laser power) is employed in melting the cladding material. From this it is clear that a pre heat involving a cheaper energy source than a laser would improve the economic efficiency of the process. Such a preheating method might employ flame, plasma, oven or induction techniques.

1) In commercial blown powder cladding the laser power is eventually re distributed as follows:

a) 50 % is reflected off the cladding melt b) 10% is reflected off the powder cloud c) 30% is used to heat the substrate d) 10% is used to melt the clad layer

2) Reflections off the powder cloud could be reduced for a given mass flow rate if the powder particle size was increased (within certain limits above which the process would break down)

3) The proportion of laser energy required to heat the substrate could be reduced by exploring in pre heat method which uses a cheaper energy source (flame, plasma, induction etc)

4) Acting upon 2 or 3 above could improve the cost effectiveness of laser cladding.

6. References

1. Oilier, B., N, Kreutz, E.W, Schluter, Gasser, H,. Wissenbach, K.

(1992)Cladding With Laser Radiation: Properties and Analysis. DGM Informationsgesllesheft mbH (Germany), pp. 687-692.

1. Li, W.B., Engström, H., A Model Of Laser Cladding By Powder Injection.

(1996) Pre heating of the blown powders and powder redistribution of the laser beam. Conf. Proc. Of MSMIVI '96. June 11-13, Beijing, China.

3. Sahour, M.C, Vannes, AB, Pelletier, J,M., (1991) Laser Cladding By Powder Injection: Optimisation Of The Processing Conditions. Journal De Physique IV, Vol 1. Dec. pp C7-51-C7-54.

4. Powell, J., Laser Cladding. PhD Theses. Imperial College of Science and Technology, Dept. Of metallurgy, London, UK, (1983).

5. Riabkina-Fishman, M., Zahavi,J. Laser alloying and cladding for improving surface properties, Applied Surface Science (Netherlands), vol. 106, no. 1-4, 1 Oct. 1996

6. Porier, D.R., Geiger, G.H. Transport Phenomena in Materials Processing. Heat Transfer and The Energy Equation. The Minerals, Metals & Materials Society (1994). ISBN 0-87339-272-8.

7. Bloehs, W., Grünenwald, B., Dausinger, F., Hügel. Recent progress in laser surface treatment: I. Implications of laser wavelength. Journal of laser applications 8 (1996) pp 15-23.

Chapter III A Process Efficiency Comparison of Nd:YAG

and CO2 Laser Cladding

A Process Efficiency Comparison of Nd:YAG and CO2 Laser Cladding

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: Hans.Gedda@mb.luth.se

+ 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 CO3-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 CO, laser cladding process.

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

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 fig 1).

Powder particles

Figure 1. Blown powder laser cladding [1].

Powder stream

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. The redistribution of laser power during he cladding process (see text for definition of PA

P,

^etc).

A power balance for laser cladding can be expressed as follows:

Pt.= Pc+Pt (1)

Where:

P,0,= the output power of the laser

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

P,

= the power lost by reflection, radiation, convection etc.

P,

in equation 1 can be expanded as follows:

Pc= P,,+Ps (2)

Where:

= the power utilised in melting the cladding powder.

Ps = the power utilised in melting the surface of the substrate in order to achieve a clad/substrate weld.

P,

in equation 1 can be similarly expanded:

PL = PA+PB+131)+PE+PF+PG (3)

Where:

PA = Power reflected off the surface of the clad zone.

P„, = Power reflected off the powder particles as they approach the weld pool.

P, = Power lost by radiation from the cladding zone.

PE = Power lost by convection from the cladding zone.

P, = 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, P„, PE, P, or P„ 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.

Earlier work by the present authors [2] quantified the individual elements of equations 1,2 and 3 for CO, laser cladding. The results of that work concluded that the laser power was redistributed in the following proportions:

Power reflected off the workpiece (P„) = 50%

Power reradiated from the workpiece (P0) = 1%

Power reflected off the particles (PB) = 9%

Power absorbed by the process (Pc+13,) = 40%

100%

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 CO, 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 [2] 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 I. Steel composit'on (substrate)

C Si Mn P 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 CO3-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 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.

4— Laser Beam Powder Particles

4—

^Insulated Calorimeter

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

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

Laser Laser Powder Post Total power Power Power

type output flow powder reflected absorbed Reflected

Power * rate cloud and by powder off powder

power absorbed by the powder cloud **

cloud **

(Watts)

cloud (P,)

**

(Watts) (g/min) (Watts) (Watts) (Watts)

Nd:YAG 2743 30 2506 237 18 224

(100%) (91%) (9%) (1%) (8%)

CO, 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 P, (the power reflected of the powder cloud) for the parameter range covered here:

P, = 8 % Ro, for the Nd:YAG laser and the CO, 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%). Then it can be concluded that 40% of the heat collected by the powder cloud (1% P,o, see table 3) does not contribute to the cladding process.

i.e. PG = 0.4 % Pto, for both types of laser (5)

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:

P0 = cr (6)

Where:

cr is the Stefan-Boltzman constant (5.7*10-8Wm-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 [2] and its surface area was 19 mm2.

This gives a maximum value for P, of:

Pp= 5.7*10-8 *(2300)4 *19*10-6 = 30 Watts (7) 13,-= 1% of P,o, for both the Nd:YAG and the CO,-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 4.3m/sec.

The rate of convective cooling of a hot body exposed to a cooler gas is given by:

Q=hAAt Watts Where:

h = Heat transfer coefficient A = Surface area of the hot body

At = The difference in temperature between the body and the cooling gas.

(9)

Evaluation of h from a standard text on the subject [3] gives us a value of approximately 100 W/m2K.

Q = 100 * (7r * 0.00252 )* 2000 (10)

PE = 3.9 Watts

Or PE = 0.1% P,0, for both Nd:YAG and 032-laser (12)

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

Calorimetry was employed to measure the heat input to each clad sample

(P.).

From this measurement it is possible to measure the power reflected off the cladding zone (PA) in the following way:

P, = 1ror ( PB+P„+13E+PG+P.) (13)

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

Table 4. Calorimetric measurements (average values) Laser

type

Laser output Power

(P.,)

(Watts)

Power input to clad sample

(P.)

(Watts)

Power % input to sample

Nd:YAG 2743 1367 49%

CO, 2695 1044 39%

From our earlier results: PA = 100 — (8+1+0.1+0.8+49) (Nd:YAG) PA = 41.1 % (Nd:YAG)

PA = 100— (9+1+0.1+0.8+39) (CO2) PA = 51.1 % (CO,)

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 CO3-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 [4] 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 [5] that metals have a lower reflectivity for the 1.06pm radiation of Nd:YAG lasers that for the 10.61..tm radiation of CO, 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 CO, laser.

H.Gedda: Chapter Ill-A process Efficiency Comparison of Nd:YAG and CO, Laser Cladding

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

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

Heat affected zone

Clad layer

"7 7/v 2> Melted

substrate

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.

Melted substrate 6.2 mm

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

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

P,= Avp (CpAt + Where:

A = The melt cross sectional area (m2) v = The Cladding speed (m/s)

p = The Density of the material melted.

(18)

Cp Specific heat capacity of the material melted.

L'l.t The difference between the melt temperature and ambient L\H_m == Specific heat of melting of the clad melt.

In order to evaluate P, accurately for both the C0₂ 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 of30, 40 and 50 g/min were employed at cladding speeds of 0. 7, 0.8, 0. 9, 1.0, 1.2 andl.4 m/min.

From all these tests an average for P, was calculated for both types oflaser. Figure 6 shows cross sections of clad traces produced by both types oflasers at a powder flow rate of 40 g/min and speeds of0.7, 1.0 and 1.4 m/min.

a) C0₂laser

0.7 m/min b) Nd:YAG laser

0.7 m/min

, ' _<v'<' _v

-�

_{; '}

'N,,..,., >.-,,tA

LO m/min

1.0 m/min

1.4 m/min

1.4 m/min

Fig11re 6. Clad cross sections at increasing process speed for bot/, types oj laser. �aser power 2800 W, laser spot diameter 5 1111n, powder J/0111 rate 40 g/111in