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A comparative study of wire feeding and powder feeding in direct diode laser deposition for rapid prototyping
Applied Surface Science 247 (2005) 268–276 www.elsevier.com/locate/apsusc
A comparative study of wire feeding and powder feeding in direct diode laser deposition for rapid prototyping Waheed Ul Haq Syed *, Andrew J. Pinkerton, Lin Li Laser Processing Research Centre, School of Mechanical Aerospace and Civil Engineering, Sackville Street Building, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK Available online 26 February 2005
Abstract Metal powder feeding has been used widely in various rapid prototyping and tooling processes such as direct laser deposition (DLD) and layered engineered net shaping (LENS) to achieve near net shape accuracy. Although powder recycling has been practiced, the material usage efficiency has been very low (normally below 30%). This study compares the process characteristics, advantages and disadvantages of wire- and powder-feed DLD. A 1.5 kW diode laser is used to build multiple layer parts, which are compared and analysed in terms of surface finish, microstructure and deposition efficiency. Scanning electron microscopy (SEM), X-ray diffraction and optical microscopy are used for the material characterisation. The microstructure of samples from both the methods is similar, with some porosity found in powder-feed components, but the surface finish and material usage efficiency is better for wire-feed samples. The deposition angle is found to be critical in the case of wire feeding and the characteristics of different feed angles are explored. Possible reasons for the different characteristics of the two deposition techniques are discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Wire feeding; Powder feeding; Rapid prototyping; Laser deposition
1. Introduction Newer processes are being developed in rapid prototyping and manufacturing each year [1]. The shift is taking place from prototype plastic parts to * Corresponding author. Tel.: +44 161 306 3828; fax: +44 161 306 3803. E-mail address: [email protected] (W.U.H. Syed).
fully functional metallic parts by direct deposition of metallic powders, which give parts with good dimensional accuracy and surface finish [2,3]. In direct laser deposition (DLD), powder is delivered to the laser generated melt pool, melts and, after movement of the laser, solidifies. Manufacture of three-dimensional prototype parts is possible by overlapping multiple individual tracks [4]. So far, the widespread use of metallic wire as a deposition medium has been limited; about 80% of the
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.138
W.U.H. Syed et al. / Applied Surface Science 247 (2005) 268–276
application is for CO2 laser welding and there have been only a few studies of the basic phenomena of laser cladding with wire feeding [5]. Little effort has been made to investigate the advantages and disadvantages of powder- and wire-feed DLD, the differences between powder and wire made components and other related process characteristics. This contribution performs such an investigation. Parts are built by both methods, keeping all related processing parameters identical. The parts are analysed using various methods and the reasons for the discovered phenomena are discussed.
2. Experimental The laser used was a Laserline LDL 160–1500 diode laser with a maximum power of 1500 W, a focal length of 300 mm, mixed wavelengths of 808 and 940 nm and a rectangular beam shape. The beam size was experimentally measured by exposing an infrared detector card with a sensitivity of 1.75 10 9 W/ mm2, to the pilot beam (20 mW) of the diode laser and found to be 2.5 mm (slow axis) 3.5 mm (fast axis). The work piece was transversed along the slow axis of the beam. The wire feeder was a F4 replacement arc wire feed unit manufactured by Technical Arc with a modification to the potentiometer to reduce the wire feed rate. A FST PF-2/2 disk type powder feeder manufactured by Flame Spray Technologies Ltd. was used in the powder-feed experiment. Clad tracks were obtained by depositing 316L stainless steel (0.03% C, 2.0% Mn, 1.0% Si, 16.0–18.0% Cr, 10.0–14.0% Ni, 2.0–3.0% Mo) [6] wire of 0.8 mm diameter or powder with a particle size of 53–150 mm diameter. The substrate was mild steel blocks of EN43A mild steel with dimensions of 50 mm 50 mm 5 mm, which were first sand blasted in a Guyson sand blaster and then degreased using ethanol. The surface roughness (Ra) so obtained was 4–5 mm, measured using a Surtronic 3+ contact probe. The substrate was mounted onto a continuous flow water cooler, which was then mounted on a table that was manually adjustable in the z-axis (vertical). A CNC table controlled the movement in the x- and y-axis (horizontal plane). The whole setup was kept in an inert atmosphere (argon gas). The wire and powder were each supplied via a lateral feed nozzle along with argon gas.
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Different types of samples were prepared depending on the parameter to be measured. To measure the difference in surface roughness between the two deposition methods, samples were prepared using powder or wire (front feeding) by cladding five tracks side by side (x-axis deposition direction, y-axis displacement between tracks) with an overlap of 30%. Four layers (z-axis displacement) were deposited. Areas of 5 mm 10 mm on the top face and sidewalls of the built samples were scanned using an in-house laser scanning/profiling system. To measure the effect of feeding direction and location, experiments were performed with different wire/powder feeding directions. Wire or powder was fed into the melt pool from the ‘front’ and ‘rear’. In front feeding the movement of the table was away from wire-feeding nozzle and in the rear feeding the table movement was towards the wire-feeding nozzle, as shown in Fig. 1. Experiments were performed by feeding the wire into the centre, the trailing edge and
Fig. 1. Schematic diagrams for wire/powder feeding, (a) rear feeding and (b) front feeding.
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the leading point of the melt pool for front and rear feeding. To find the difference between the microstructure of wire fed and powder fed parts, samples were prepared using powder or wire (front feeding) by cladding three layers, each composed of three tracks 35 mm in length, side by side with an overlap of 30%. These were transversely sectioned at 18 mm from the start, polished to 4000 grit (Ra was 1–3 mm) and electrolytically etched using 6 V DC in 10% Oxalic acid. Composition and microstructure were measured using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction. To measure the deposition efficiency, single tracks were deposited at various power densities and powder or wire feed rates. Tracks were traversely sectioned and examined using optical microscopy. The crosssectional area and traverse speed during deposition was used to calculate the actual deposition rate, which was compared to the mass feed rate during the experiment.
with the spacing approximately proportional to the traverse speed. Positioning of the wire at or near the melt pool was also found to be critical. A good clad was obtained when the wire was positioned at the leading point of the melt pool for both front and rear feeding of the wire. A poor clad was obtained if the wire was positioned at the centre or the trailing point of the melt pool for both feed directions. The results obtained for powder feeding showed noticeable differences. High catchment efficiency was observed when the powder was fed from the front; there was an increase of 20–45% in efficiency compared to the rear powder feeding for the same operating conditions. However, rear powder feeding resulted in less oxidation and a 20% decrease in the surface roughness in agreement with the observations of Fabbro et al. [7]. No appreciable difference in the surface finish or roughness of the clad was visible, whether powder was fed at the centre, leading edge or trailing edge of the melt pool. 3.3. Effects of feeding angle
3. Results 3.1. Surface roughness The average surface roughness of the wire fed samples was between 40 and 60 mm (Ra) whereas the average surface roughness of the powder-feed samples was between Ra 70 and 90 mm, which is 20–30% higher. Physical examination of the powder-feed samples, surfaces revealed that the powder particles adhered to the surface of the clad in three different forms; loose particles, embedded particles and semi molten particles. Loose particles were easy to remove, embedded particles required some effort to remove and semi molten particles were difficult or impossible to remove. 3.2. Effects of feeding direction and location The results showed that the best feeding direction for the wire was from the front, as shown in Fig. 2(a). More severe undulations in the track surface developed in the rear feeding clads, as shown in Fig. 2(b). Serrations resembling periodic cuts across the track also occurred. These serrations were equally spaced,
Feeding angles were critical in case of wire feeding. The range of feeding angles for a continuous clad was found to be limited to a = 10–758 (front feeding) and a = 105–1708 (rear feeding). Feeding at low angles to the substrate, the wire was lifted up after the melt pool solidified, became misaligned and continuation of the clad ceased, as shown in Fig. 3(a). More rippling effects were seen on the surface of the clad for angles exceeding 608 and eventually resulted in a broken and very uneven clad, as shown in Fig. 3(b). The results showed that the quality clad was obtained with a = 20–608 (front feeding). This is a greater range than found by Breinan et al. [8]. For powder feeding, satisfactory clads were obtained from preplaced powders and for the full range of delivery angles, front and rear (a = 0–1808). 3.4. Micro structure and morphological characteristics There was no significant difference between the microscopic characteristics of the wire- and powderfeed samples. X-ray diffraction analysis of the raw powder, the wire and the samples produced by cladding did not show any appreciable difference in
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Fig. 2. Tracks produced by wire feeding. (a) Front fed and (b) rear fed, showing serration or furrows.
Fig. 3. Tracks produced by wire feeding at different angles to the substrate.
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Fig. 4. SEM of wire and powder feed samples, (a) powder fed part top layer and (b) wire fed part top layer.
elemental composition and the samples remained austenitic in phase. The microstructure was mainly dendritic and cellular with a cell size between 2 and 15 mm, as shown in Fig. 4. Various proportions of dendritic, cellular and columnar structures were visible at different locations in the samples as shown in Fig. 5; a finer structure was found in the first layer near to the substrate and then the grain size became coarser on moving up the sample. The top layer
showed a mainly dendritic structure and these structures was also seen to prevail at layer boundaries. No cracks were found but some porosity was visible in the cellular and dendritic structure of powder fed made components. Cavities between adjacent overlapped layers for both powder- and wire-feed samples were also present. These interlayer cavities were present at laser power below 900 W and above 1200 W for both powder- and wire-feed.
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Fig. 5. Microstructure of a powder feed sample.
3.5. Deposition efficiency Plotting the deposition rate, obtained by multiplying clad height, width, scanning speed and density of the material, against the material delivery rate allowed a comparison to be made as shown in Fig. 6. The figure shows that deposition efficiency is far better in the case of wire feed than powder feed, even for lower delivery rates. The graph also shows that by keeping all the parameters, including the powder delivery rate the same, the deposition rate was increased significantly by increasing the laser power. The deposition efficiency increased from 8% at 700 W to 40% at 1300 W for the same feed rate.
4. Discussion 4.1. Surface roughness The final shape of the clad largely depends upon the melt pool geometry and solidification process, which also depend upon many parameters. The higher
surface roughness of the powder-made components can firstly be attributed to some of the powder particles sticking to the solidified clad surface. The presence of three distinct types, indicates slightly different formation methods. Particles that heated up during the flight and struck the solidified surface of the clad loosely adhered to the clad surface. Embedding of particles developed when the solid powder particles fell into the trailing part of the melt pool and the temperature was not enough to melt these particles. Semi-molten particles developed when powder particles fell into the leading or centre part of the melt pool and the temperature of the melt pool was high enough to melt the embedded portion of the particle. All the above cases occurred simultaneously. However, adherent particles do not fully explain the differences in surface undulations that occurred. This can be attributed to the instability of the molten pool. The shape of the melt pool continuously changes during cladding and this change becomes more apparent with powder feeding. The number, size and shape of the powder particles entering the melt pool at any point are not constant whereas with wire
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Fig. 6. Deposition rate vs. material delivery rate at different laser powers for wire and powder feed deposition.
feeding the mass feed rate remains constant at any point. This change in mass flow distribution may cause some change to the melt pool shape and therefore the final track shape. A generally stronger gas flow in the centre of the pool due to the carrier gas when powder feeding, may also distort the pool. Peng et al. [9] observed that Marangoni flow enhances the heat flux at the pool rim and it is this region that eventually gives shape to the melt pool. Varying thermal gradients in the weld pool affect the stability of that thermocapillary flow. The thermal gradients in a melt pool produced by powder feeding undergo more changes because of the range of temperatures of the powder particles impacting at any point on the melt pool, as compared to the wire-feed melt pool in which the wire is fed at a constant temperature. Mills et al. [10] highlighted the direction of the thermocapillary flow caused by sulphur content as the cause of increased rippling in welding. However, since the X-ray diffraction did not give any substantial difference between the corresponding elemental compositions, the possibility of a difference in the surface roughness of powder and wire fed parts due to different
surface activant concentrations in the melt pool is ruled out in this case. 4.2. Effects of feeding direction and location Serrations were seen only when feeding wire from the rear. This has also been observed by Meinert et al. [11] who suggested it was due to misalignment of wire with the weld pool. There is a ‘make and break’ phenomenon in the production of serrations; the wire ploughs and pushes the semi molten melt pool to a distance where the melt pool melts the wire again and breaks it, but due to the movement of the table, the wire again pushes and makes way into the solidifying pool. For this reason, Kim and peng [12] advocated the front feeding of wire as it has a larger tolerance of wire feeding speed and position. The schematic diagram for front and rear powder feeding has been shown in Fig. 1. It is clear from the figure that in theory the inert gas shields the melt pool before, during and after the solidification, thus preventing formation of oxides. However, in front feeding, contamination of the melt pool and formation
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of oxides during and after solidification was observed. Because of the formation of these oxides, the melt pool would have absorbed more laser energy, enlarging the melt pool. Additionally, in front feeding, the melt pool is not shielded from incoming powder by the solidifying track. Together these factors account for the increased deposition efficiency seen with frontfeed powder deposition. Coaxially feeding powder into the melt pool would eliminate the effects associated with rear and front feeding. 4.3. Effects of feeding angle Cladding has been achieved with pre-placed powders, with lateral feed nozzles at a large range of angles to the substrate, both in front of and behind the melt pool, and with coaxial nozzles. Thus, for powders the feasible cladding range to obtain a good clad extends nearly the full range from a = 0 to 1808 (see Fig. 1). However, the results showed that wire feeding does not have this flexibility of feeding angles. For feed angles outside the observed process limits, the wire entered the laser beam and reflected some of the laser energy. This interruption of the laser energy to the substrate would produce rapid shrinkage of the melt pool, explaining the discontinuous track. Melting of the wire outside the melt pool may also have contributed to the poor track quality. 4.4. Micro structure and morphological characteristics The fine structure at the first layer above the substrate can be associated with the high initial cooling rate, because of three-dimensional heat transfer at the initial stage. Moving up the wall, the cooling becomes two-dimensional, which gives rise to a slower cooling rate and coarser grain size. This difference in structure remains even after the multiple reheat cycles caused by subsequent layer deposition. The dendritic structure at the top layers may suggest a very high cooling rate and reflects the fact that there has been no reheat cycle. The porosity visible between the dendrites in the powder made parts and absent in the wire made parts may suggest the presence of pores in the powder particles. Also, the thermocapillary flow determines the presence of porosity in the clad. Smooth fluid flow will give rise
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to less porosity and disturbed flow will give rise to high porosity. This reinforces the theory that there is more disturbed thermocapillary flow in powder-feed melt pools. The presence of the interlayer cavities that were found depends on the degree of overlap, laser mode structure and the track aspect ratio [13]. Selecting proper parameters can eliminate these cavities. 4.5. Deposition efficiency The increase in deposition efficiency only applied up to a certain limit and thereafter increasing the laser power gave no appreciable increase in the deposition rate. The results suggest the presence of an optimum process window. Above a certain power level there is no appreciable gain in deposition efficiency and a greater risk of interlayer cavities. 5. Conclusion In this paper, the process characteristics of wire and powder feed DLD were compared and multiple layer parts build by the two methods were analysed. The surface finish on the top surface and sidewalls was found to be better for wire-feed samples. The microstructures for both methods were similar, however some porosity was found and the dendrite structure was less refined in the powder-feed samples. Powder feeding allowed deposition angles from 0 to 1808, whereas the deposition angles for wire were limited to 10–758 to the substrate. Powder feeding from the rear gave less oxidation and a better surface finish, while front feeding gave 20–45% higher catchment efficiency. The deposition efficiency for wire feeding was found to be much higher than powder feeding. It was found that wire feeding from the front gave better-clad quality than rear feeding, which caused serrations in the clad. Results are consistent with a greater degree of instability in a powder fed melt pool than a wire fed one. References [1] D.T. Pham, S.S. Dimov, Rapid Manufacturing The Technologies and Applications of Rapid prototyping and Rapid Tooling, Springer-Verlag London Limited, London, 2001.
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[2] A.J. Pinkerton, L. Li, Time Compression Technologies Conference (TCT), Manchester, 2002, Steam B, CD. [3] A.J. Pinkerton, L. Li, Appl. Surf. Sci. 208–209 (2003) 411. [4] P.A. Kobryn, E.H. Moore, S.L. Semiatin, Scripta Mater. 43 (2000) 299. [5] A. Salminen, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics (ICALEO), Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 238 (section D). [6] Metals Handbook, Properties and Selection: Stainless Steel, Tool Materials and Special Purpose Metals, vol. 3, ninth ed., American Society for Metals, Metals Park, Ohio, USA, 1980. [7] R. Fabbro, M. Hamadou, L. Sabatier, F. Coste, K. Raissi, M. Zanelli, C. Delalondre, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics
[8] [9] [10] [11]
[12] [13]
(ICALEO), Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 166 (section C). E.M. Breinan, B.H. Kear, Laser Materials Processing, NorthHolland, Publishing Co., Amsterdam, 1983, p. 235. X.F. Peng, X.P. Lin, D.J. Lee, Y. Yan, B.X. Wang, Int. J. Heat Mass Transfer 44 (2001) 457. K.C. Mills, B.J. Keene, R.F. Brooks, A. Shirali, Phil. Trans. R. Soc. Lond. A 356 (1998) 911. K.C. Meinert Jr., E.W. Reutzel, R.P. Martukanitz, J.F. Tressler, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics (ICALEO), vol. 89, Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 107 (section C). J.D. Kim, Y. Peng, Opt. Lasers Eng. 33 (2000) 299. W.M. Steen, V.M. Weerasinghe, P. Monson, SPIE High Power Lasers and Their Industrial Applications, vol. 650, 1986, p. 226.
A comparative study of wire feeding and powder feeding in direct diode laser deposition for rapid prototyping Waheed Ul Haq Syed *, Andrew J. Pinkerton, Lin Li Laser Processing Research Centre, School of Mechanical Aerospace and Civil Engineering, Sackville Street Building, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK Available online 26 February 2005
Abstract Metal powder feeding has been used widely in various rapid prototyping and tooling processes such as direct laser deposition (DLD) and layered engineered net shaping (LENS) to achieve near net shape accuracy. Although powder recycling has been practiced, the material usage efficiency has been very low (normally below 30%). This study compares the process characteristics, advantages and disadvantages of wire- and powder-feed DLD. A 1.5 kW diode laser is used to build multiple layer parts, which are compared and analysed in terms of surface finish, microstructure and deposition efficiency. Scanning electron microscopy (SEM), X-ray diffraction and optical microscopy are used for the material characterisation. The microstructure of samples from both the methods is similar, with some porosity found in powder-feed components, but the surface finish and material usage efficiency is better for wire-feed samples. The deposition angle is found to be critical in the case of wire feeding and the characteristics of different feed angles are explored. Possible reasons for the different characteristics of the two deposition techniques are discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Wire feeding; Powder feeding; Rapid prototyping; Laser deposition
1. Introduction Newer processes are being developed in rapid prototyping and manufacturing each year [1]. The shift is taking place from prototype plastic parts to * Corresponding author. Tel.: +44 161 306 3828; fax: +44 161 306 3803. E-mail address: [email protected] (W.U.H. Syed).
fully functional metallic parts by direct deposition of metallic powders, which give parts with good dimensional accuracy and surface finish [2,3]. In direct laser deposition (DLD), powder is delivered to the laser generated melt pool, melts and, after movement of the laser, solidifies. Manufacture of three-dimensional prototype parts is possible by overlapping multiple individual tracks [4]. So far, the widespread use of metallic wire as a deposition medium has been limited; about 80% of the
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.138
W.U.H. Syed et al. / Applied Surface Science 247 (2005) 268–276
application is for CO2 laser welding and there have been only a few studies of the basic phenomena of laser cladding with wire feeding [5]. Little effort has been made to investigate the advantages and disadvantages of powder- and wire-feed DLD, the differences between powder and wire made components and other related process characteristics. This contribution performs such an investigation. Parts are built by both methods, keeping all related processing parameters identical. The parts are analysed using various methods and the reasons for the discovered phenomena are discussed.
2. Experimental The laser used was a Laserline LDL 160–1500 diode laser with a maximum power of 1500 W, a focal length of 300 mm, mixed wavelengths of 808 and 940 nm and a rectangular beam shape. The beam size was experimentally measured by exposing an infrared detector card with a sensitivity of 1.75 10 9 W/ mm2, to the pilot beam (20 mW) of the diode laser and found to be 2.5 mm (slow axis) 3.5 mm (fast axis). The work piece was transversed along the slow axis of the beam. The wire feeder was a F4 replacement arc wire feed unit manufactured by Technical Arc with a modification to the potentiometer to reduce the wire feed rate. A FST PF-2/2 disk type powder feeder manufactured by Flame Spray Technologies Ltd. was used in the powder-feed experiment. Clad tracks were obtained by depositing 316L stainless steel (0.03% C, 2.0% Mn, 1.0% Si, 16.0–18.0% Cr, 10.0–14.0% Ni, 2.0–3.0% Mo) [6] wire of 0.8 mm diameter or powder with a particle size of 53–150 mm diameter. The substrate was mild steel blocks of EN43A mild steel with dimensions of 50 mm 50 mm 5 mm, which were first sand blasted in a Guyson sand blaster and then degreased using ethanol. The surface roughness (Ra) so obtained was 4–5 mm, measured using a Surtronic 3+ contact probe. The substrate was mounted onto a continuous flow water cooler, which was then mounted on a table that was manually adjustable in the z-axis (vertical). A CNC table controlled the movement in the x- and y-axis (horizontal plane). The whole setup was kept in an inert atmosphere (argon gas). The wire and powder were each supplied via a lateral feed nozzle along with argon gas.
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Different types of samples were prepared depending on the parameter to be measured. To measure the difference in surface roughness between the two deposition methods, samples were prepared using powder or wire (front feeding) by cladding five tracks side by side (x-axis deposition direction, y-axis displacement between tracks) with an overlap of 30%. Four layers (z-axis displacement) were deposited. Areas of 5 mm 10 mm on the top face and sidewalls of the built samples were scanned using an in-house laser scanning/profiling system. To measure the effect of feeding direction and location, experiments were performed with different wire/powder feeding directions. Wire or powder was fed into the melt pool from the ‘front’ and ‘rear’. In front feeding the movement of the table was away from wire-feeding nozzle and in the rear feeding the table movement was towards the wire-feeding nozzle, as shown in Fig. 1. Experiments were performed by feeding the wire into the centre, the trailing edge and
Fig. 1. Schematic diagrams for wire/powder feeding, (a) rear feeding and (b) front feeding.
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the leading point of the melt pool for front and rear feeding. To find the difference between the microstructure of wire fed and powder fed parts, samples were prepared using powder or wire (front feeding) by cladding three layers, each composed of three tracks 35 mm in length, side by side with an overlap of 30%. These were transversely sectioned at 18 mm from the start, polished to 4000 grit (Ra was 1–3 mm) and electrolytically etched using 6 V DC in 10% Oxalic acid. Composition and microstructure were measured using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction. To measure the deposition efficiency, single tracks were deposited at various power densities and powder or wire feed rates. Tracks were traversely sectioned and examined using optical microscopy. The crosssectional area and traverse speed during deposition was used to calculate the actual deposition rate, which was compared to the mass feed rate during the experiment.
with the spacing approximately proportional to the traverse speed. Positioning of the wire at or near the melt pool was also found to be critical. A good clad was obtained when the wire was positioned at the leading point of the melt pool for both front and rear feeding of the wire. A poor clad was obtained if the wire was positioned at the centre or the trailing point of the melt pool for both feed directions. The results obtained for powder feeding showed noticeable differences. High catchment efficiency was observed when the powder was fed from the front; there was an increase of 20–45% in efficiency compared to the rear powder feeding for the same operating conditions. However, rear powder feeding resulted in less oxidation and a 20% decrease in the surface roughness in agreement with the observations of Fabbro et al. [7]. No appreciable difference in the surface finish or roughness of the clad was visible, whether powder was fed at the centre, leading edge or trailing edge of the melt pool. 3.3. Effects of feeding angle
3. Results 3.1. Surface roughness The average surface roughness of the wire fed samples was between 40 and 60 mm (Ra) whereas the average surface roughness of the powder-feed samples was between Ra 70 and 90 mm, which is 20–30% higher. Physical examination of the powder-feed samples, surfaces revealed that the powder particles adhered to the surface of the clad in three different forms; loose particles, embedded particles and semi molten particles. Loose particles were easy to remove, embedded particles required some effort to remove and semi molten particles were difficult or impossible to remove. 3.2. Effects of feeding direction and location The results showed that the best feeding direction for the wire was from the front, as shown in Fig. 2(a). More severe undulations in the track surface developed in the rear feeding clads, as shown in Fig. 2(b). Serrations resembling periodic cuts across the track also occurred. These serrations were equally spaced,
Feeding angles were critical in case of wire feeding. The range of feeding angles for a continuous clad was found to be limited to a = 10–758 (front feeding) and a = 105–1708 (rear feeding). Feeding at low angles to the substrate, the wire was lifted up after the melt pool solidified, became misaligned and continuation of the clad ceased, as shown in Fig. 3(a). More rippling effects were seen on the surface of the clad for angles exceeding 608 and eventually resulted in a broken and very uneven clad, as shown in Fig. 3(b). The results showed that the quality clad was obtained with a = 20–608 (front feeding). This is a greater range than found by Breinan et al. [8]. For powder feeding, satisfactory clads were obtained from preplaced powders and for the full range of delivery angles, front and rear (a = 0–1808). 3.4. Micro structure and morphological characteristics There was no significant difference between the microscopic characteristics of the wire- and powderfeed samples. X-ray diffraction analysis of the raw powder, the wire and the samples produced by cladding did not show any appreciable difference in
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Fig. 2. Tracks produced by wire feeding. (a) Front fed and (b) rear fed, showing serration or furrows.
Fig. 3. Tracks produced by wire feeding at different angles to the substrate.
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Fig. 4. SEM of wire and powder feed samples, (a) powder fed part top layer and (b) wire fed part top layer.
elemental composition and the samples remained austenitic in phase. The microstructure was mainly dendritic and cellular with a cell size between 2 and 15 mm, as shown in Fig. 4. Various proportions of dendritic, cellular and columnar structures were visible at different locations in the samples as shown in Fig. 5; a finer structure was found in the first layer near to the substrate and then the grain size became coarser on moving up the sample. The top layer
showed a mainly dendritic structure and these structures was also seen to prevail at layer boundaries. No cracks were found but some porosity was visible in the cellular and dendritic structure of powder fed made components. Cavities between adjacent overlapped layers for both powder- and wire-feed samples were also present. These interlayer cavities were present at laser power below 900 W and above 1200 W for both powder- and wire-feed.
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Fig. 5. Microstructure of a powder feed sample.
3.5. Deposition efficiency Plotting the deposition rate, obtained by multiplying clad height, width, scanning speed and density of the material, against the material delivery rate allowed a comparison to be made as shown in Fig. 6. The figure shows that deposition efficiency is far better in the case of wire feed than powder feed, even for lower delivery rates. The graph also shows that by keeping all the parameters, including the powder delivery rate the same, the deposition rate was increased significantly by increasing the laser power. The deposition efficiency increased from 8% at 700 W to 40% at 1300 W for the same feed rate.
4. Discussion 4.1. Surface roughness The final shape of the clad largely depends upon the melt pool geometry and solidification process, which also depend upon many parameters. The higher
surface roughness of the powder-made components can firstly be attributed to some of the powder particles sticking to the solidified clad surface. The presence of three distinct types, indicates slightly different formation methods. Particles that heated up during the flight and struck the solidified surface of the clad loosely adhered to the clad surface. Embedding of particles developed when the solid powder particles fell into the trailing part of the melt pool and the temperature was not enough to melt these particles. Semi-molten particles developed when powder particles fell into the leading or centre part of the melt pool and the temperature of the melt pool was high enough to melt the embedded portion of the particle. All the above cases occurred simultaneously. However, adherent particles do not fully explain the differences in surface undulations that occurred. This can be attributed to the instability of the molten pool. The shape of the melt pool continuously changes during cladding and this change becomes more apparent with powder feeding. The number, size and shape of the powder particles entering the melt pool at any point are not constant whereas with wire
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Fig. 6. Deposition rate vs. material delivery rate at different laser powers for wire and powder feed deposition.
feeding the mass feed rate remains constant at any point. This change in mass flow distribution may cause some change to the melt pool shape and therefore the final track shape. A generally stronger gas flow in the centre of the pool due to the carrier gas when powder feeding, may also distort the pool. Peng et al. [9] observed that Marangoni flow enhances the heat flux at the pool rim and it is this region that eventually gives shape to the melt pool. Varying thermal gradients in the weld pool affect the stability of that thermocapillary flow. The thermal gradients in a melt pool produced by powder feeding undergo more changes because of the range of temperatures of the powder particles impacting at any point on the melt pool, as compared to the wire-feed melt pool in which the wire is fed at a constant temperature. Mills et al. [10] highlighted the direction of the thermocapillary flow caused by sulphur content as the cause of increased rippling in welding. However, since the X-ray diffraction did not give any substantial difference between the corresponding elemental compositions, the possibility of a difference in the surface roughness of powder and wire fed parts due to different
surface activant concentrations in the melt pool is ruled out in this case. 4.2. Effects of feeding direction and location Serrations were seen only when feeding wire from the rear. This has also been observed by Meinert et al. [11] who suggested it was due to misalignment of wire with the weld pool. There is a ‘make and break’ phenomenon in the production of serrations; the wire ploughs and pushes the semi molten melt pool to a distance where the melt pool melts the wire again and breaks it, but due to the movement of the table, the wire again pushes and makes way into the solidifying pool. For this reason, Kim and peng [12] advocated the front feeding of wire as it has a larger tolerance of wire feeding speed and position. The schematic diagram for front and rear powder feeding has been shown in Fig. 1. It is clear from the figure that in theory the inert gas shields the melt pool before, during and after the solidification, thus preventing formation of oxides. However, in front feeding, contamination of the melt pool and formation
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of oxides during and after solidification was observed. Because of the formation of these oxides, the melt pool would have absorbed more laser energy, enlarging the melt pool. Additionally, in front feeding, the melt pool is not shielded from incoming powder by the solidifying track. Together these factors account for the increased deposition efficiency seen with frontfeed powder deposition. Coaxially feeding powder into the melt pool would eliminate the effects associated with rear and front feeding. 4.3. Effects of feeding angle Cladding has been achieved with pre-placed powders, with lateral feed nozzles at a large range of angles to the substrate, both in front of and behind the melt pool, and with coaxial nozzles. Thus, for powders the feasible cladding range to obtain a good clad extends nearly the full range from a = 0 to 1808 (see Fig. 1). However, the results showed that wire feeding does not have this flexibility of feeding angles. For feed angles outside the observed process limits, the wire entered the laser beam and reflected some of the laser energy. This interruption of the laser energy to the substrate would produce rapid shrinkage of the melt pool, explaining the discontinuous track. Melting of the wire outside the melt pool may also have contributed to the poor track quality. 4.4. Micro structure and morphological characteristics The fine structure at the first layer above the substrate can be associated with the high initial cooling rate, because of three-dimensional heat transfer at the initial stage. Moving up the wall, the cooling becomes two-dimensional, which gives rise to a slower cooling rate and coarser grain size. This difference in structure remains even after the multiple reheat cycles caused by subsequent layer deposition. The dendritic structure at the top layers may suggest a very high cooling rate and reflects the fact that there has been no reheat cycle. The porosity visible between the dendrites in the powder made parts and absent in the wire made parts may suggest the presence of pores in the powder particles. Also, the thermocapillary flow determines the presence of porosity in the clad. Smooth fluid flow will give rise
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to less porosity and disturbed flow will give rise to high porosity. This reinforces the theory that there is more disturbed thermocapillary flow in powder-feed melt pools. The presence of the interlayer cavities that were found depends on the degree of overlap, laser mode structure and the track aspect ratio [13]. Selecting proper parameters can eliminate these cavities. 4.5. Deposition efficiency The increase in deposition efficiency only applied up to a certain limit and thereafter increasing the laser power gave no appreciable increase in the deposition rate. The results suggest the presence of an optimum process window. Above a certain power level there is no appreciable gain in deposition efficiency and a greater risk of interlayer cavities. 5. Conclusion In this paper, the process characteristics of wire and powder feed DLD were compared and multiple layer parts build by the two methods were analysed. The surface finish on the top surface and sidewalls was found to be better for wire-feed samples. The microstructures for both methods were similar, however some porosity was found and the dendrite structure was less refined in the powder-feed samples. Powder feeding allowed deposition angles from 0 to 1808, whereas the deposition angles for wire were limited to 10–758 to the substrate. Powder feeding from the rear gave less oxidation and a better surface finish, while front feeding gave 20–45% higher catchment efficiency. The deposition efficiency for wire feeding was found to be much higher than powder feeding. It was found that wire feeding from the front gave better-clad quality than rear feeding, which caused serrations in the clad. Results are consistent with a greater degree of instability in a powder fed melt pool than a wire fed one. References [1] D.T. Pham, S.S. Dimov, Rapid Manufacturing The Technologies and Applications of Rapid prototyping and Rapid Tooling, Springer-Verlag London Limited, London, 2001.
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[2] A.J. Pinkerton, L. Li, Time Compression Technologies Conference (TCT), Manchester, 2002, Steam B, CD. [3] A.J. Pinkerton, L. Li, Appl. Surf. Sci. 208–209 (2003) 411. [4] P.A. Kobryn, E.H. Moore, S.L. Semiatin, Scripta Mater. 43 (2000) 299. [5] A. Salminen, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics (ICALEO), Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 238 (section D). [6] Metals Handbook, Properties and Selection: Stainless Steel, Tool Materials and Special Purpose Metals, vol. 3, ninth ed., American Society for Metals, Metals Park, Ohio, USA, 1980. [7] R. Fabbro, M. Hamadou, L. Sabatier, F. Coste, K. Raissi, M. Zanelli, C. Delalondre, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics
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(ICALEO), Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 166 (section C). E.M. Breinan, B.H. Kear, Laser Materials Processing, NorthHolland, Publishing Co., Amsterdam, 1983, p. 235. X.F. Peng, X.P. Lin, D.J. Lee, Y. Yan, B.X. Wang, Int. J. Heat Mass Transfer 44 (2001) 457. K.C. Mills, B.J. Keene, R.F. Brooks, A. Shirali, Phil. Trans. R. Soc. Lond. A 356 (1998) 911. K.C. Meinert Jr., E.W. Reutzel, R.P. Martukanitz, J.F. Tressler, in: Proceedings of the 19th International Congress on Applications of Lasers and Electro-optics (ICALEO), vol. 89, Laser Institute of America (LIA), Dearborn, MI, USA, 2000, p. 107 (section C). J.D. Kim, Y. Peng, Opt. Lasers Eng. 33 (2000) 299. W.M. Steen, V.M. Weerasinghe, P. Monson, SPIE High Power Lasers and Their Industrial Applications, vol. 650, 1986, p. 226.