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2 high speed steel by selective laser sintering
Scripta Materialia, Vol. 39, No. 1, pp. 67–72, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00
Pergamon PII S1359-6462(98)00126-2
LIQUID PHASE SINTERING OF M3/2 HIGH SPEED STEEL BY SELECTIVE LASER SINTERING H.J. Niu and I.T.H. Chang School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (Received March 13, 1998) (Accepted March 20, 1998) Introduction Selective laser sintering (SLS) is a rapid prototyping process which offers unique advantages over conventional thermomechanical processes (1,2). In this process, a laser is scanned across the surface of a loose powder bed, sintering the powders into the shape of the required cross section. The part is built up layer by layer from the bottom to the top. Thus, it is a sequential layered approach to manufacture any desired three-dimensional part that may have a simple or complex shapes. High speed steels (HSSs) are conventional cutting tool materials with a desired combination of hot hardness, wear resistance, and toughness. However, as HSSs are of inherently high alloy content and complex carbide structure, they need a complicated, expensive and carefully controlled processing, such as hot working, heat treating and machining, in order to produce the finished product. Powder processing of HSSs can overcome these problems and offers a cost effective method for producing near net shape components. Therefore, extensive studies have been carried out in sintering HSSs and HSS metal matrix composites using conventional high temperature furnaces (3–5). This present work focuses on understanding the sinterability of M3/2 HSS powder using a CW CO2 laser beam. This paper reports the effect of SLS processing parameters on the surface morphologies and microstructure, and describes the mechanism of liquid phase sintering of M3/2 HSS by SLS. Experimental Procedure The starting material was water atomized M3 class 2 (M3/2) HSS powders supplied by Powdrex Limited, with the composition (wt.%) 1.10C-5.81W-5.05Mo-4.08Cr-2.95V-0.23Co-0.12Ni-0.12Cu0.44Si-0.20Mn-1030 (ppm)O-196 (ppm)N. The laser used in this experiment was a 25 W, CW, CO2 laser with a beam size of 0.5 mm. The laser was operated at nominal powers (P) of 6.25, 12.5 and 25.0 W with a scan rate (V) in the range from 1 to 30 mm/s and scan line spacing (t) of 0.15 mm. The loose M3/2 HSS powders were placed in an Al2O3 tray and they were levelled so as to obtain a flat powder surface. The samples with dimension 10 3 10 mm were made by scanning a laser beam over the surface in air. After sintering, the samples were air-cooled to room temperature. The surfaces of the as-sintered samples were observed in a JEOL 5410 scanning electron microscope (SEM). Samples for metallographic examination were prepared using standard techniques and etched in 2% nital. Both unetched and etched samples were examined using a JEOL 6300 SEM. 67
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Figure 1. SEM images of laser sintered M3/2 HSS surfaces using different scan rates and laser powers, and a scan line spacing of 0.15 mm: (a) 6.25 W, 1.0 mm/s; (b) 12.5 W, 1.0 mm/s; (c) 6.25 W, 3.0 mm/s; (d) 12.50 W, 3.0 mm/s; (e) 6.25 W, 8.0 mm/s; (f) 12.50 W, 8.0 mm/s.
Results Fig. 1 shows the surface morphologies of laser sintered samples using various scan rates (V 5 1– 8 mm/s), laser powers (P 5 6.25–12.50 W), and a fixed scan line spacing (t 5 0.15 mm). For the lower scan rate of 1 mm/s, the SLS sample surface under a laser power of 6.25 W consisted of agglomerates which were connected by a continuous network of solid. As laser power increased to 12.5 W, a
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Figure 2. Variation of the agglomerate diameter with scan rate for various laser power from 6.25 W to 25.00 W.
relatively smooth and highly dense surface was obtained. However, at a higher scan rate above 3 mm/s, the SLS sample surface consisted of the agglomerates and interconnected pores. The size of the agglomerates, the size of interconnected pores and the surface roughness increased with increasing laser power and decreasing scan rate, as shown in Fig. 1c, d, e and f). However, the agglomerate connectivity increased with increasing laser power and decreasing scan rate. Fig. 2 shows the effect of scan rate and laser power on the agglomerate diameter. The diameter of the agglomerates increased slowly with decreasing scan rate until a critical scan rate had been reached and a rapid growth of the agglomerates was followed with further decreasing scan rate, but the value of the critical scan rate and the corresponding diameter of the agglomerates decreased as laser power decreased. The diameter of the agglomerates increased linearly with increasing laser power, as shown in Fig. 3. Fig. 4 shows the microstructure of the as-supplied M3/2 powders and the SLS samples sintered using different laser powers (P 5 6.25–25.00 W) for a given scan rate (V 5 1 mm/s). In the as-supplied M3/2 powders, the M6C and MC carbides randomly dispersed in a ferrite matrix, as shown in Fig. 4a. However, at the heating stage, the carbide particles have begun to redistribute regularly along the grain boundaries with solid state diffusion, as shown in Fig. 4b.
Figure 3. Variation of the agglomerate diameter with laser power for scan rate of 5.0 and 8.0 mm/s.
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Figure 4. Back scattered SEM images of the as-supplied M3/2 powders and the laser sintered M3/2 HSS samples using different laser powers for scan rate of 1.0 mm/s: (a) as-supplied powder; (b) heat affected zone; (c) 6.25 W; (d) 12.50 W; (e) 25.00 W; (f) 25.00 W (etched).
Fig. 4c shows that the agglomerates (light grey area), forming at low laser power of 6.25 W and scan rate of 1 mm/s, were connected by the liquid phase (the network dark regions). The agglomerates consisted of thick and angular carbides embedded mainly at the three grain junctions. SLS at higher laser power of 12.50 W led to formation of continuous matrix and a decrease in the quantity of carbides, which were still dispersed at the three grain junctions, as shown in Fig. 4d. However, at a laser power of 25.00 W and scan rate of 1 mm/s (Fig. 4e), the sintering density increased
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further and the carbides formed thin layers along the grain boundaries. The carbide films consisted of many isolated lens-shaped particles and formed necklace microstructure. The etched sample exhibited that there existed the thick ring microstructure reprecipited around the grain boundaries (Fig. 4f). Discussion SLS of M3/2 HSS results in both (a) liquid phase sintering, i.e. the laser melts the HSS powders on the sample surface and melting alloy penetrates into underlying solid particles and (b) supersolidus liquid phase sintering, i.e. M3/2 particles are heated to a temperature at which the austenite 1 carbide 1 liquid phases are in equilibrium, and the liquid phase forms along the austenitic grain boundary within the particles. The liquid phase produced during the SLS process flows around and wets the solid particles, or the grain boundaries. The presence of the liquid phase around the solid particles and the grain boundaries will lead to rapid densification by the rearrangement of the solid particles and subsequent solution-reprecipitation mechanism (Fig. 4e and f). However, as SLS is carried out line by line, laser scan causes melting along a row of powder particles, thereby forming a track of molten region of cylindrical shape. This liquid cylinder is likely to break up into a row of spheres so as to reduce the surface area, leading to the formation of the agglomerates, i. e. “balling” phenomenon as observed in the SLS processing of tin and stainless steel (1, 6). The instability of the liquid cylinder was originally described by Rayleigh (7). The analyses show that the liquid cylinder will be stable against any perturbation if a wavelength l , pD (where D is the initial diameter of an unperturbed cylinder). Hence, the time required for breaking up the cylinder increases with D (8). Considering that the breakup of M3/2 liquid cylinder into the agglomerates is a process of a constant volume change, the diameter of M3/2 liquid cylinder is then proportional to the diameter of the agglomerates, which is increased with increasing laser power, or decreasing scan rate (Fig. 2 and 3). Therefore, increasing laser power, or decreasing scan rate makes it difficult for the liquid cylinder to break up into the agglomerates so as to obtain a continuous surface. However, it is difficult to obtain the continuous surface only by increasing laser power. An increasing scan rate causes a rapid decrease of the D value, together with an increase of l, This tends to overshadow the effect of large D value produced by increasing laser power. Hence, the liquid cylinder is likely to break up more readily. Conversely, low scan rate tends to keep the instantaneous liquid cylinder of l , pD, and a continuous and smooth surface will be obtained. The experiments show that scan rate of 1 mm/s under laser power of 6.25 and 25.00 W produces a continuous and highly dense surface as compared with higher scan rate (Fig. 1). The effect of laser power on the microstructure of laser sintering of M3/2 HSS is similar to that of temperature for supersolidus liquid phase sintering in vacuum furnace (4, 5). The resulting morphologies of the carbide are related to the morphologies of the liquid phase forming at the grain boundaries. When the samples are sintered at low laser power of 6.25 and 12.50 W, the liquid phase forms angular morphology at the three grain junctions because of higher dihedral angle (9). As the increase of laser power results in a decrease of dihedral angle, the liquid phase spreads and forms a continuous film along the grain boundaries. Due to capillary effect, the continuous film breaks up into a network of closed spaced lens-shaped island of liquid along the grain boundaries giving the necklace microstructure (10). Summary SLS of M3/2 HSS results in both a liquid phase sintering and supersolidus liquid phase sintering. The formation of the agglomerates on the surface and the necklace carbide along the grain boundaries can
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be explained by the model of Rayleigh instability. The agglomerate size increases with increasing laser power, or decreasing scan rate. With a lower scan rate of 1 mm/s, the smooth and dense surface can be obtained. With laser power increasing from 6.25 to 25.00 W for a given scan rate of 1 mm/s, the carbide changes from the angular carbide at the three grain junction to a necklace microstructure along the grain boundaries. Acknowledgments The author would like to thank Powdrex Limited and Quantum Laser Engineering Ltd. for the provision of the powders and laser facility, respectively. In addition, we would like to thank Professor I. R. Harris for the provision of laboratory facilities and ORS awards for the financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
D. L. Bourell, H. L. Marcus, J. W. Barlow, and J. J. Beaman, Int. J. Powder Metall. 28, 369 (1992). A. Manthiram, D. L. Bourell, and H. L. Marcus, JOM. 45, 66 (1993). F. A. Kirk, Powder Metall. 24, 70 (1981). C. S. Wright and B. Ogel, Powder Metall. 36, 213 (1993). J. D. Bolton and A. J. Gant, Powder Metall. 40, 143 (1997). B. V. Schueren and J. P. Kruth, in Laser Assisted Net Shape Engineering Proceedings of the LANE’ 94, ed. M. Geiger and F. Vollertsen, vol. 11, pp. 793– 801, Meisenbach Bamberg, Erlangen (1994). L. Rayleigh, Proc. London Math. Soc. 10, 4 (1878). F. A. Nichols and W. W. Mullins, Trans. Met. Soc. AIME. 233, 1840 (1965). P. J. Wray, Acta Metall. 24, 125 (1976). R. M. German, Int. J. Powder Metall. 22, 31 (1986).
Pergamon PII S1359-6462(98)00126-2
LIQUID PHASE SINTERING OF M3/2 HIGH SPEED STEEL BY SELECTIVE LASER SINTERING H.J. Niu and I.T.H. Chang School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (Received March 13, 1998) (Accepted March 20, 1998) Introduction Selective laser sintering (SLS) is a rapid prototyping process which offers unique advantages over conventional thermomechanical processes (1,2). In this process, a laser is scanned across the surface of a loose powder bed, sintering the powders into the shape of the required cross section. The part is built up layer by layer from the bottom to the top. Thus, it is a sequential layered approach to manufacture any desired three-dimensional part that may have a simple or complex shapes. High speed steels (HSSs) are conventional cutting tool materials with a desired combination of hot hardness, wear resistance, and toughness. However, as HSSs are of inherently high alloy content and complex carbide structure, they need a complicated, expensive and carefully controlled processing, such as hot working, heat treating and machining, in order to produce the finished product. Powder processing of HSSs can overcome these problems and offers a cost effective method for producing near net shape components. Therefore, extensive studies have been carried out in sintering HSSs and HSS metal matrix composites using conventional high temperature furnaces (3–5). This present work focuses on understanding the sinterability of M3/2 HSS powder using a CW CO2 laser beam. This paper reports the effect of SLS processing parameters on the surface morphologies and microstructure, and describes the mechanism of liquid phase sintering of M3/2 HSS by SLS. Experimental Procedure The starting material was water atomized M3 class 2 (M3/2) HSS powders supplied by Powdrex Limited, with the composition (wt.%) 1.10C-5.81W-5.05Mo-4.08Cr-2.95V-0.23Co-0.12Ni-0.12Cu0.44Si-0.20Mn-1030 (ppm)O-196 (ppm)N. The laser used in this experiment was a 25 W, CW, CO2 laser with a beam size of 0.5 mm. The laser was operated at nominal powers (P) of 6.25, 12.5 and 25.0 W with a scan rate (V) in the range from 1 to 30 mm/s and scan line spacing (t) of 0.15 mm. The loose M3/2 HSS powders were placed in an Al2O3 tray and they were levelled so as to obtain a flat powder surface. The samples with dimension 10 3 10 mm were made by scanning a laser beam over the surface in air. After sintering, the samples were air-cooled to room temperature. The surfaces of the as-sintered samples were observed in a JEOL 5410 scanning electron microscope (SEM). Samples for metallographic examination were prepared using standard techniques and etched in 2% nital. Both unetched and etched samples were examined using a JEOL 6300 SEM. 67
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Figure 1. SEM images of laser sintered M3/2 HSS surfaces using different scan rates and laser powers, and a scan line spacing of 0.15 mm: (a) 6.25 W, 1.0 mm/s; (b) 12.5 W, 1.0 mm/s; (c) 6.25 W, 3.0 mm/s; (d) 12.50 W, 3.0 mm/s; (e) 6.25 W, 8.0 mm/s; (f) 12.50 W, 8.0 mm/s.
Results Fig. 1 shows the surface morphologies of laser sintered samples using various scan rates (V 5 1– 8 mm/s), laser powers (P 5 6.25–12.50 W), and a fixed scan line spacing (t 5 0.15 mm). For the lower scan rate of 1 mm/s, the SLS sample surface under a laser power of 6.25 W consisted of agglomerates which were connected by a continuous network of solid. As laser power increased to 12.5 W, a
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Figure 2. Variation of the agglomerate diameter with scan rate for various laser power from 6.25 W to 25.00 W.
relatively smooth and highly dense surface was obtained. However, at a higher scan rate above 3 mm/s, the SLS sample surface consisted of the agglomerates and interconnected pores. The size of the agglomerates, the size of interconnected pores and the surface roughness increased with increasing laser power and decreasing scan rate, as shown in Fig. 1c, d, e and f). However, the agglomerate connectivity increased with increasing laser power and decreasing scan rate. Fig. 2 shows the effect of scan rate and laser power on the agglomerate diameter. The diameter of the agglomerates increased slowly with decreasing scan rate until a critical scan rate had been reached and a rapid growth of the agglomerates was followed with further decreasing scan rate, but the value of the critical scan rate and the corresponding diameter of the agglomerates decreased as laser power decreased. The diameter of the agglomerates increased linearly with increasing laser power, as shown in Fig. 3. Fig. 4 shows the microstructure of the as-supplied M3/2 powders and the SLS samples sintered using different laser powers (P 5 6.25–25.00 W) for a given scan rate (V 5 1 mm/s). In the as-supplied M3/2 powders, the M6C and MC carbides randomly dispersed in a ferrite matrix, as shown in Fig. 4a. However, at the heating stage, the carbide particles have begun to redistribute regularly along the grain boundaries with solid state diffusion, as shown in Fig. 4b.
Figure 3. Variation of the agglomerate diameter with laser power for scan rate of 5.0 and 8.0 mm/s.
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Figure 4. Back scattered SEM images of the as-supplied M3/2 powders and the laser sintered M3/2 HSS samples using different laser powers for scan rate of 1.0 mm/s: (a) as-supplied powder; (b) heat affected zone; (c) 6.25 W; (d) 12.50 W; (e) 25.00 W; (f) 25.00 W (etched).
Fig. 4c shows that the agglomerates (light grey area), forming at low laser power of 6.25 W and scan rate of 1 mm/s, were connected by the liquid phase (the network dark regions). The agglomerates consisted of thick and angular carbides embedded mainly at the three grain junctions. SLS at higher laser power of 12.50 W led to formation of continuous matrix and a decrease in the quantity of carbides, which were still dispersed at the three grain junctions, as shown in Fig. 4d. However, at a laser power of 25.00 W and scan rate of 1 mm/s (Fig. 4e), the sintering density increased
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further and the carbides formed thin layers along the grain boundaries. The carbide films consisted of many isolated lens-shaped particles and formed necklace microstructure. The etched sample exhibited that there existed the thick ring microstructure reprecipited around the grain boundaries (Fig. 4f). Discussion SLS of M3/2 HSS results in both (a) liquid phase sintering, i.e. the laser melts the HSS powders on the sample surface and melting alloy penetrates into underlying solid particles and (b) supersolidus liquid phase sintering, i.e. M3/2 particles are heated to a temperature at which the austenite 1 carbide 1 liquid phases are in equilibrium, and the liquid phase forms along the austenitic grain boundary within the particles. The liquid phase produced during the SLS process flows around and wets the solid particles, or the grain boundaries. The presence of the liquid phase around the solid particles and the grain boundaries will lead to rapid densification by the rearrangement of the solid particles and subsequent solution-reprecipitation mechanism (Fig. 4e and f). However, as SLS is carried out line by line, laser scan causes melting along a row of powder particles, thereby forming a track of molten region of cylindrical shape. This liquid cylinder is likely to break up into a row of spheres so as to reduce the surface area, leading to the formation of the agglomerates, i. e. “balling” phenomenon as observed in the SLS processing of tin and stainless steel (1, 6). The instability of the liquid cylinder was originally described by Rayleigh (7). The analyses show that the liquid cylinder will be stable against any perturbation if a wavelength l , pD (where D is the initial diameter of an unperturbed cylinder). Hence, the time required for breaking up the cylinder increases with D (8). Considering that the breakup of M3/2 liquid cylinder into the agglomerates is a process of a constant volume change, the diameter of M3/2 liquid cylinder is then proportional to the diameter of the agglomerates, which is increased with increasing laser power, or decreasing scan rate (Fig. 2 and 3). Therefore, increasing laser power, or decreasing scan rate makes it difficult for the liquid cylinder to break up into the agglomerates so as to obtain a continuous surface. However, it is difficult to obtain the continuous surface only by increasing laser power. An increasing scan rate causes a rapid decrease of the D value, together with an increase of l, This tends to overshadow the effect of large D value produced by increasing laser power. Hence, the liquid cylinder is likely to break up more readily. Conversely, low scan rate tends to keep the instantaneous liquid cylinder of l , pD, and a continuous and smooth surface will be obtained. The experiments show that scan rate of 1 mm/s under laser power of 6.25 and 25.00 W produces a continuous and highly dense surface as compared with higher scan rate (Fig. 1). The effect of laser power on the microstructure of laser sintering of M3/2 HSS is similar to that of temperature for supersolidus liquid phase sintering in vacuum furnace (4, 5). The resulting morphologies of the carbide are related to the morphologies of the liquid phase forming at the grain boundaries. When the samples are sintered at low laser power of 6.25 and 12.50 W, the liquid phase forms angular morphology at the three grain junctions because of higher dihedral angle (9). As the increase of laser power results in a decrease of dihedral angle, the liquid phase spreads and forms a continuous film along the grain boundaries. Due to capillary effect, the continuous film breaks up into a network of closed spaced lens-shaped island of liquid along the grain boundaries giving the necklace microstructure (10). Summary SLS of M3/2 HSS results in both a liquid phase sintering and supersolidus liquid phase sintering. The formation of the agglomerates on the surface and the necklace carbide along the grain boundaries can
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be explained by the model of Rayleigh instability. The agglomerate size increases with increasing laser power, or decreasing scan rate. With a lower scan rate of 1 mm/s, the smooth and dense surface can be obtained. With laser power increasing from 6.25 to 25.00 W for a given scan rate of 1 mm/s, the carbide changes from the angular carbide at the three grain junction to a necklace microstructure along the grain boundaries. Acknowledgments The author would like to thank Powdrex Limited and Quantum Laser Engineering Ltd. for the provision of the powders and laser facility, respectively. In addition, we would like to thank Professor I. R. Harris for the provision of laboratory facilities and ORS awards for the financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
D. L. Bourell, H. L. Marcus, J. W. Barlow, and J. J. Beaman, Int. J. Powder Metall. 28, 369 (1992). A. Manthiram, D. L. Bourell, and H. L. Marcus, JOM. 45, 66 (1993). F. A. Kirk, Powder Metall. 24, 70 (1981). C. S. Wright and B. Ogel, Powder Metall. 36, 213 (1993). J. D. Bolton and A. J. Gant, Powder Metall. 40, 143 (1997). B. V. Schueren and J. P. Kruth, in Laser Assisted Net Shape Engineering Proceedings of the LANE’ 94, ed. M. Geiger and F. Vollertsen, vol. 11, pp. 793– 801, Meisenbach Bamberg, Erlangen (1994). L. Rayleigh, Proc. London Math. Soc. 10, 4 (1878). F. A. Nichols and W. W. Mullins, Trans. Met. Soc. AIME. 233, 1840 (1965). P. J. Wray, Acta Metall. 24, 125 (1976). R. M. German, Int. J. Powder Metall. 22, 31 (1986).