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Selective laser sintering of TiC–Al2O3 composite with self-propagating high-temperature synthesis
Journal of Materials Processing Technology 118 (2001) 173±178
Selective laser sintering of TiC±Al2O3 composite with self-propagating high-temperature synthesis A. Slocombe, L. Li* Manufacturing Division, Department of Mechanical Aerospace and Manufacturing Engineering, University of Manchester Institute of Science and Technology (UMIST), P.O. Box 88, Manchester M60 1QD, UK
Abstract Self-propagating high-temperature synthesis (SHS) processes have previously been used to manufacture various high strength/ temperature materials. A method for combining this process with the selective laser sintering (SLS) of rapid prototype parts has been investigated. TiC±Al2O3 composite has the potential for high wear resistance, high hardness and toughness. By controlling the SHS propagation wave it has been demonstrated possible to produce a complex part manufactured from TiO2, Al and C synthesis utilising the SLS process. This paper reports basic experimental investigation on TiC±Al2O3 composite synthesis and that by varying the amount of diluent within the initial reactants control over the SHS can be achieved through controlled solid state combustion during SLS. # 2001 Published by Elsevier Science B.V. Keywords: Laser; Sintering; Prototyping; High temperature; Self-propagating high temperature synthesis
1. Introduction The need for developing a means of using materials closely matching (if not the same as) those of the original product, e.g. metals, ceramics and composites for rapid prototyping, has led to the development of the selective laser sintering (SLS) machine and the current ongoing research within this ®eld [1±3]. Basically thin layers of heat-fusible powder are deposited and then laser scanned one on top of another from a sterolithography ®le. This process is repeated until a ®nal 3D part is produced. Current problems with the SLS has been the dif®culty for producing high-temperature functional parts from metals and ceramic materials. These require expensive preheating and normally the parts contain signi®cant amounts of porosity. However, work has been performed on compacting these powders which has led to a reduction in porosity and possible removal of the preheating phase [4,5]. Within the ®eld of engineering, the development of new materials which offer a combination of high mechanical strengths, low densities, high melting points and good oxidation resistance, has always been desirable. With the introduction of new materials such as Si±C [6,7], Si±Ti±C *
Corresponding author. Tel.: 44-161-200-3816; fax: 44-161-200-3803. E-mail address: [email protected] (L. Li). 0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 0 5 - 0
[8], Ti±C [9±13] and Ti±N [14], this aim would appear to have been reached to a certain extent. However, due to the signi®cantly high strengths and melting temperatures of these materials, their production can both be time and energy consuming, hence reducing their cost ef®ciency within industry. In order to reduce the amount of energy and time required to produce these materials a relatively new technique termed self-propagating high-temperature synthesis (SHS) has recently received much attention [15]. The SHS technique, also called solid state combustion, uses the formation of initial compounds from reactant substances to develop exothermic reactions which in turn generates enough energy to initiate the formation of initial compounds from the mixture of reactants. The reaction becomes selfpropagating and a combustion wave travels through the reactants completely converting them to the ®nal product. The advantages of the technique are that it is simple and energy ef®cient as once initiated no further externally applied energy is required [15]. The processing time is reduced to seconds rather than hours and the products are often of high purity, as impurities are expelled during the high-temperature reactions, leading to increased mechanical properties [15]. This reduction in processing time means that the production chain is signi®cantly reduced leading to higher operating ef®ciencies and economic gains. Although, the use of lasers have been considered for the ignition stage of the SHS process [15,16], their degree of
174
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
usage could be increased further and to more effect if the SHS process were to be combined with the selective SLS process which has the capabilities for free form fabrication of 3D parts. This would further increase the range of geometries produced through the SHS process and the applications for which the hard alloy and intermetallics could be employed. Within this current work a method for combining these two processes has been investigated for the generation of a TiC±Al2O3 composite material. TiC±Al2O3 composite has the potential for high wear resistance, high hardness and toughness. By controlling the SHS propagation wave it may be possible to produce a complex part manufactured from TiO2 Al and C utilising the SLS process. This paper reports basic experimental investigation on TiC±Al2O3 composite and that by varying the amount of diluent within the initial reactants control over the SHS can be achieved allowing for SLS to produce controlled intralayer and interlayer parts. 2. Experimental procedure The following exothermic reaction was investigated to produce a ceramic composites: 3TiO2 4Al 3C ! 3TiC 2Al2 O3 exothermic energy (1) The reactant powders were TiO2, Al and C all of <90 mm in size. Initially stoichiometric measures were prepared using a rotating mixing vessel. Blends of powder reactants were then processed using the Nd:YAG laser source, and examined for their chemical composition and physical characteristics. The blends consisted of non-compacted stoichmetric blend, 50% compacted stoichmetric blend. Layers of reactant powders were prepared by two mild steel spacer sheets of 100 mm 100 mm 0:18 mm, with a cut off hole of identical sizes (40 mm diameter in this experiment) in the middle, on the sintering platform. The spacer plates were placed one on top of the other and aligned each other. The cut off holes were ®lled with the powder blend and powders were levelled with the top surface of the spacer using a sharp edge of a ¯at plate and residual powders were removed from the surface of the spacer plate. Compaction was achieved by the removal of the top plat and a pressure of around 70±180 N/cm2 was applied to the top plate to compress the powder, so that the powders were levelled with the spacer below making about 50% powder volume reduction for this layer to about 0.18 mm thick. Finally, the top plate was removed and the powder bed was ready for sintering. Both a focused (spot size 0.2 mm) and defocused (spot size 0.6 mm) Q-switched Nd:YAG laser beam were used within the experiment. The initial investigation was to determine whether the SHS could be achieved and whether
Fig. 1. Scan beam path to create simple shape.
the propagation wave could be controlled by the compaction of the reactant by 50% of the apparent volume. The laser operated at 10±70 W with 18 ns pulse width. The Nd:YAG laser raster scanned the sample in a pattern shown in Fig. 1 under different powers, frequencies and speeds. A laser beam overlap of 0.1 mm was used to generate the scanned shape. All samples were sintered under atmospheric condition in air. Samples were then analysed using scanning electron microscope (SEM), energy dispersive analysis by X-ray (EDX) and X-ray diffraction analysis (XRD). Surface roughness measurements were also taken using Taylor± Hobson Surtronic 3. 3. Results and discussion It was found that the usual effect of using a focused Q-switch Nd:YAG beam resulted in the ablation of the powder material, hence samples could only be attained using a defocused beam. From viewing the XRD results (Figs. 2 and 3), it can be seen that below a certain threshold (20 kHz frequency, 2 mm s 1 speed and 60 W power) no SHS reaction takes place. It would appear that the only reaction to have occurred was the removal of C from the system, which may have been combined with O2 to become CO2. 3TiO2 4Al 3C 3O2 ! 3TiO2 4Al 3CO2
(2)
Above this threshold it would appear the reactants contained enough thermal energy to develop into the required products, TiC and Al2O3 (Figs. 4 and 5). However, viewing the optical photographs (Figs. 6 and 7), it is clear that although both compacted and non-compacted samples have reacted to form the required products, high levels of porosity are clearly visible within the non-compacted sample. The non-compacted sample (Fig. 7) also displays an extremely poor rectangular shape development. This is highlighted by the difference between the laser scan area, within the white rectangular boarders overlaid and the excess surrounding
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 2. XRD analysis of non-compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 10 mm s
material, while the 50% compacted sample (Fig. 6) displays less porosity and a more de®ned rectangular shape development. In fact the non-compacted sample would appear to have increased in area compared to the compacted sample, due to uncontrolled SHS propagation. The SEM photograph in Fig. 8, displays a dense sintered regions of TiC and Al2O3 which were generated along the
175
1
(SHS did not take place).
laser track of the compacted sample. Through XRD and EDX analyses which indicated that all the reactants were contained within this region, it could be concluded that this region represented the sintered composite material of TiC and Al2O3. From surface roughness measurements made on samples using the Taylor±Hobson Surtronic roughness tester. The
Fig. 3. XRD analysis of 50% compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 10 mm s
1
(SHS did not take place).
176
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 4. XRD analysis of non-compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 2 mm s 1.
surface roughness values (Ra) of these samples were 20 and 100 mm for the compacted and non-compacted samples, respectively. Hence, the compacted sample displayed a higher surface quality than the non-compacted sample. Numerous factors could effect the combustion waves progress through the reactants. However, the porosity and dilute within the system are the most in¯uential factors and can be used to develop the SHS reaction to determine how and whether the SHS process would proceed.
According to the Fourier heat balance equation for a single heat source [17] @T @2T @f n k 2 qr (3) @t @x @t where Cp is heat capacity (J kg 1 K), r the density (kg m 3), T the absolute temperature (K), x the direction along which wave propagation (m), @f(n)/@t the rate of the reaction (ms 1) and q the heat of the reaction (J).
Cp r
Fig. 5. XRD analysis of 50% compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 2 mm s 1.
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 6. Optical photograph of 50% compacted sample sintered under defocused beam at a power of 60 W, frequency 20 kHz and speed 2 mm s 1 (SHS has taken place). The white border encloses the laser scanned area.
Since, the density and the conductivity of the powders are a function of the porosity, by controlling the amount of porosity within the system the SHS propagation wave can be reduced. Hence, the propagation length and reaction rate are reduced. By compacting the reactants an increase in the heat ¯ux transfer ahead of the wave can be developed which lowers the temperature of the wave. Effectively this acts as a form of diluent and is manifested by a decrease in the wave velocity leading to the extinction of the wave [18]. Hence, by reducing the amount of porosity of the system more control over the formation of the ®nal product along the laser track is attained. However, from the experimental observation there is still some excessive wave propagation which leads to an
177
Fig. 8. SEM photograph of sintered region of TiC and Al2O3.
increase in the desired area of the initial shape (Figs. 6 and 7). Possible higher compaction levels may further increase the control over the SHS combustion wave and lead to less porosity within the ®nal product. From the XRD analysis it is clear that a reactant threshold needs to be attained in order for the desired reaction to occur. This needs further investigation to determine the operating window of the system. Also, it would appear that a defocused beam is favoured over a focused beam which causes ablation to occur. This is possibly due to the high peak powers generated by the pulsed Nd:YAG laser and may be overcome by using a continuous wave laser system. Also it is clear from the XRD results that a certain amount of reactant remained unaltered by the whole process. This may be due to the oxidation of the carbon with the surrounding air. By using a controlled atmosphere of argon, more reactant may develop into the desired product. The surface quality of the compacted sample is signi®cantly higher than that of the non-compacted sample. This may be due to the effect of reducing the amount of porosity which allows the product material to be sintered more effectively. 4. Conclusion
Fig. 7. Optical photograph of non-compacted sample sintered under defocused beam at a power of 60 W, frequency 20 kHz and speed 2 mm s 1 (SHS has taken place). The white border encloses the laser scanned area.
By controlling one of the major factors (porosity) which affects the combustion wave propagation of the SHS reaction, it would appear possible to develop a product reaction where the solid state combustion wave occurs over an extremely small region, allowing a simple geometry to be developed under the resulting action of laser sintering TiO2, Al and C with a Q-switched Nd:YAG laser source. From this work it can be concluded that a focused Qswitched Nd:YAG laser beam causes ablation of the reactant powders. Hence, a defocused beam of spot size 0.6 mm is required (for the Nd:YAG laser used within this experiment).
178
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
This may be due to the high peak power generated by this pulsed Nd:YAG laser source and a continuous wave laser may allow for smaller spot sizes to be utilised. A threshold for the reaction at power 60 W, frequency 20 kHz and scan speed 2 mm s 1 has been determined. However, further work is required to produce an operating window for the reaction. This would need to concentrate on the increasing of the scanning speed which would be an important parameter for rapid prototyping. Although some control over the SHS reaction has been achieved through 50% compaction of the reactant powders there is still further need for investigation into introducing other controlling factors, such as diluent material, e.g. TiC and Al2O3, which may allow for accurate and precise scanned geometry to be produced. Surface quality is greatly enhanced by compacting the reactants before sintering with the Nd:YAG laser sources. The advantage of the combined SLS and SHS is that it may be possible to produce high-temperature functional prototype parts at low cost. Further work is still required to determine the best methods for controlling the SHS combustion wave to produce a rapid prototype functional part. References [1] W. Meiners, K. Wissenbach, R. Poprawe, Direct selective laser sintering of steel powders, in: Proceedings of Second Laser Assisted Net Shape Engineering, LANE'97, 1997, pp. 615±621. [2] K.C. Chee, Three-dimensional rapid prototyping technologies and key development areas, Comput. Contr. Eng. J. (1994). [3] A. Kochan, Rapid developments in rapid prototyping, Assem. Autom. 15 (4) (1995) 18±19. [4] L. Li, K.L. Ng, A. Slocombe, Diode laser sintering of compacted metallic powders for desk top rapid prototyping, in: Proceedings of
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
the Seventh European Symposium on Rapid Prototyping and Manufacturing, Nottingham University, 1998, pp. 281±296. L. Li, A. Slocombe, K.L. Ng, Selective laser sintering of compacted Cu/Sn powders, in: Proc. ICALEO'98, Laser Institute of America, Laser Material Proc., Vol. 85, No. E, 1998, pp. 38±47. R. Raghunathan, R. Chowdhury, K. Jagannadham, J. Narayan, Synthesis of highly dense beta-SiC pellet by self-propagating high temperature, Mater. Manuf. Process. 10 (1995) 559±563. R. Pampuch, L. Stobierski, J. Lis, Synthesis of sinterable beta-SiC powders by a solid combustion method, J. Am. Ceram. Soc. 72 (1989) 14334±14335. R. Pampuch, J. Lis, L. Stobierski, M. Tymkiewicz, Solid combustion synthesis of Ti//3SiC//2, J. Eur. Ceram. Soc. 5 (1989) 283±287. J. Subrahmanyam, R. Rao, R. Mohan, Combustion synthesis of MoSi//2-TiC composites, J. Mater. Res. 10 (1995) 1226±1236. J.Y. Hung, L.L. Ye, Y.K. Wu, H.Q. Ye, Direct evidence for combustion reaction on TiNi/TiC composites formation by mechanical alloying, Metall. Mater. Trans. A 26 (1995) 2755±2759. A. Niiler, L.J. Kecskes, Impurities in the combustion synthesis of titanium carbide, J. Am. Ceram. Soc. 72 (1989) 655±661. R.V. Raman, S.V. Rele, S. Poland, J. Lasalvia, M.A. Meyers, A.R. Niiler, One-step synthesis of dense titanium carbide tiles, JOM 47 (1995) 23±25. M.E. Mullins, E. Riley, Effect of carbon morphology on the combustion synthesis of titanium carbide, J. Mater. Res. 4 (1989) 408±411. Z.A. Munir, S. Deevi, M. Eslamloo-Grami, Synthesis of titanium nitride by a self-sustaining combustion method, High Temp. High Press. 20 (1988) 19±24. McCauley, W. James, Historical and technical perspective on SHS, Ceram. Eng. Sci. Proc. 11 (1990) 1137±1181. M.I. Shilyeav, V.E. Borzukh, A.R. Dorokhov, On laser ignition of powder nickel aluminium systems, Fizika Goreniya I Vzryva 30 (1994) 14±18. Z.A. Muir, U. Anselmi-Tamburini, The kinetics of the selfpropagating high temperature synthesis process, Mater. Sci. Rep. 3 (1989) 277. N. Sata, Z.A. Munir, in: V.W.B. Johnson, R.A. (Eds.), Proceedings of the Symposium on High Temperature Materials Chemistry, The Electronchemical Society, Penninton, NJ, Vol. 90, 1990.
Selective laser sintering of TiC±Al2O3 composite with self-propagating high-temperature synthesis A. Slocombe, L. Li* Manufacturing Division, Department of Mechanical Aerospace and Manufacturing Engineering, University of Manchester Institute of Science and Technology (UMIST), P.O. Box 88, Manchester M60 1QD, UK
Abstract Self-propagating high-temperature synthesis (SHS) processes have previously been used to manufacture various high strength/ temperature materials. A method for combining this process with the selective laser sintering (SLS) of rapid prototype parts has been investigated. TiC±Al2O3 composite has the potential for high wear resistance, high hardness and toughness. By controlling the SHS propagation wave it has been demonstrated possible to produce a complex part manufactured from TiO2, Al and C synthesis utilising the SLS process. This paper reports basic experimental investigation on TiC±Al2O3 composite synthesis and that by varying the amount of diluent within the initial reactants control over the SHS can be achieved through controlled solid state combustion during SLS. # 2001 Published by Elsevier Science B.V. Keywords: Laser; Sintering; Prototyping; High temperature; Self-propagating high temperature synthesis
1. Introduction The need for developing a means of using materials closely matching (if not the same as) those of the original product, e.g. metals, ceramics and composites for rapid prototyping, has led to the development of the selective laser sintering (SLS) machine and the current ongoing research within this ®eld [1±3]. Basically thin layers of heat-fusible powder are deposited and then laser scanned one on top of another from a sterolithography ®le. This process is repeated until a ®nal 3D part is produced. Current problems with the SLS has been the dif®culty for producing high-temperature functional parts from metals and ceramic materials. These require expensive preheating and normally the parts contain signi®cant amounts of porosity. However, work has been performed on compacting these powders which has led to a reduction in porosity and possible removal of the preheating phase [4,5]. Within the ®eld of engineering, the development of new materials which offer a combination of high mechanical strengths, low densities, high melting points and good oxidation resistance, has always been desirable. With the introduction of new materials such as Si±C [6,7], Si±Ti±C *
Corresponding author. Tel.: 44-161-200-3816; fax: 44-161-200-3803. E-mail address: [email protected] (L. Li). 0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 0 5 - 0
[8], Ti±C [9±13] and Ti±N [14], this aim would appear to have been reached to a certain extent. However, due to the signi®cantly high strengths and melting temperatures of these materials, their production can both be time and energy consuming, hence reducing their cost ef®ciency within industry. In order to reduce the amount of energy and time required to produce these materials a relatively new technique termed self-propagating high-temperature synthesis (SHS) has recently received much attention [15]. The SHS technique, also called solid state combustion, uses the formation of initial compounds from reactant substances to develop exothermic reactions which in turn generates enough energy to initiate the formation of initial compounds from the mixture of reactants. The reaction becomes selfpropagating and a combustion wave travels through the reactants completely converting them to the ®nal product. The advantages of the technique are that it is simple and energy ef®cient as once initiated no further externally applied energy is required [15]. The processing time is reduced to seconds rather than hours and the products are often of high purity, as impurities are expelled during the high-temperature reactions, leading to increased mechanical properties [15]. This reduction in processing time means that the production chain is signi®cantly reduced leading to higher operating ef®ciencies and economic gains. Although, the use of lasers have been considered for the ignition stage of the SHS process [15,16], their degree of
174
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
usage could be increased further and to more effect if the SHS process were to be combined with the selective SLS process which has the capabilities for free form fabrication of 3D parts. This would further increase the range of geometries produced through the SHS process and the applications for which the hard alloy and intermetallics could be employed. Within this current work a method for combining these two processes has been investigated for the generation of a TiC±Al2O3 composite material. TiC±Al2O3 composite has the potential for high wear resistance, high hardness and toughness. By controlling the SHS propagation wave it may be possible to produce a complex part manufactured from TiO2 Al and C utilising the SLS process. This paper reports basic experimental investigation on TiC±Al2O3 composite and that by varying the amount of diluent within the initial reactants control over the SHS can be achieved allowing for SLS to produce controlled intralayer and interlayer parts. 2. Experimental procedure The following exothermic reaction was investigated to produce a ceramic composites: 3TiO2 4Al 3C ! 3TiC 2Al2 O3 exothermic energy (1) The reactant powders were TiO2, Al and C all of <90 mm in size. Initially stoichiometric measures were prepared using a rotating mixing vessel. Blends of powder reactants were then processed using the Nd:YAG laser source, and examined for their chemical composition and physical characteristics. The blends consisted of non-compacted stoichmetric blend, 50% compacted stoichmetric blend. Layers of reactant powders were prepared by two mild steel spacer sheets of 100 mm 100 mm 0:18 mm, with a cut off hole of identical sizes (40 mm diameter in this experiment) in the middle, on the sintering platform. The spacer plates were placed one on top of the other and aligned each other. The cut off holes were ®lled with the powder blend and powders were levelled with the top surface of the spacer using a sharp edge of a ¯at plate and residual powders were removed from the surface of the spacer plate. Compaction was achieved by the removal of the top plat and a pressure of around 70±180 N/cm2 was applied to the top plate to compress the powder, so that the powders were levelled with the spacer below making about 50% powder volume reduction for this layer to about 0.18 mm thick. Finally, the top plate was removed and the powder bed was ready for sintering. Both a focused (spot size 0.2 mm) and defocused (spot size 0.6 mm) Q-switched Nd:YAG laser beam were used within the experiment. The initial investigation was to determine whether the SHS could be achieved and whether
Fig. 1. Scan beam path to create simple shape.
the propagation wave could be controlled by the compaction of the reactant by 50% of the apparent volume. The laser operated at 10±70 W with 18 ns pulse width. The Nd:YAG laser raster scanned the sample in a pattern shown in Fig. 1 under different powers, frequencies and speeds. A laser beam overlap of 0.1 mm was used to generate the scanned shape. All samples were sintered under atmospheric condition in air. Samples were then analysed using scanning electron microscope (SEM), energy dispersive analysis by X-ray (EDX) and X-ray diffraction analysis (XRD). Surface roughness measurements were also taken using Taylor± Hobson Surtronic 3. 3. Results and discussion It was found that the usual effect of using a focused Q-switch Nd:YAG beam resulted in the ablation of the powder material, hence samples could only be attained using a defocused beam. From viewing the XRD results (Figs. 2 and 3), it can be seen that below a certain threshold (20 kHz frequency, 2 mm s 1 speed and 60 W power) no SHS reaction takes place. It would appear that the only reaction to have occurred was the removal of C from the system, which may have been combined with O2 to become CO2. 3TiO2 4Al 3C 3O2 ! 3TiO2 4Al 3CO2
(2)
Above this threshold it would appear the reactants contained enough thermal energy to develop into the required products, TiC and Al2O3 (Figs. 4 and 5). However, viewing the optical photographs (Figs. 6 and 7), it is clear that although both compacted and non-compacted samples have reacted to form the required products, high levels of porosity are clearly visible within the non-compacted sample. The non-compacted sample (Fig. 7) also displays an extremely poor rectangular shape development. This is highlighted by the difference between the laser scan area, within the white rectangular boarders overlaid and the excess surrounding
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 2. XRD analysis of non-compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 10 mm s
material, while the 50% compacted sample (Fig. 6) displays less porosity and a more de®ned rectangular shape development. In fact the non-compacted sample would appear to have increased in area compared to the compacted sample, due to uncontrolled SHS propagation. The SEM photograph in Fig. 8, displays a dense sintered regions of TiC and Al2O3 which were generated along the
175
1
(SHS did not take place).
laser track of the compacted sample. Through XRD and EDX analyses which indicated that all the reactants were contained within this region, it could be concluded that this region represented the sintered composite material of TiC and Al2O3. From surface roughness measurements made on samples using the Taylor±Hobson Surtronic roughness tester. The
Fig. 3. XRD analysis of 50% compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 10 mm s
1
(SHS did not take place).
176
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 4. XRD analysis of non-compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 2 mm s 1.
surface roughness values (Ra) of these samples were 20 and 100 mm for the compacted and non-compacted samples, respectively. Hence, the compacted sample displayed a higher surface quality than the non-compacted sample. Numerous factors could effect the combustion waves progress through the reactants. However, the porosity and dilute within the system are the most in¯uential factors and can be used to develop the SHS reaction to determine how and whether the SHS process would proceed.
According to the Fourier heat balance equation for a single heat source [17] @T @2T @f n k 2 qr (3) @t @x @t where Cp is heat capacity (J kg 1 K), r the density (kg m 3), T the absolute temperature (K), x the direction along which wave propagation (m), @f(n)/@t the rate of the reaction (ms 1) and q the heat of the reaction (J).
Cp r
Fig. 5. XRD analysis of 50% compacted sample sintered at a power of 60 W, frequency 20 kHz and speed 2 mm s 1.
A. Slocombe, L. Li / Journal of Materials Processing Technology 118 (2001) 173±178
Fig. 6. Optical photograph of 50% compacted sample sintered under defocused beam at a power of 60 W, frequency 20 kHz and speed 2 mm s 1 (SHS has taken place). The white border encloses the laser scanned area.
Since, the density and the conductivity of the powders are a function of the porosity, by controlling the amount of porosity within the system the SHS propagation wave can be reduced. Hence, the propagation length and reaction rate are reduced. By compacting the reactants an increase in the heat ¯ux transfer ahead of the wave can be developed which lowers the temperature of the wave. Effectively this acts as a form of diluent and is manifested by a decrease in the wave velocity leading to the extinction of the wave [18]. Hence, by reducing the amount of porosity of the system more control over the formation of the ®nal product along the laser track is attained. However, from the experimental observation there is still some excessive wave propagation which leads to an
177
Fig. 8. SEM photograph of sintered region of TiC and Al2O3.
increase in the desired area of the initial shape (Figs. 6 and 7). Possible higher compaction levels may further increase the control over the SHS combustion wave and lead to less porosity within the ®nal product. From the XRD analysis it is clear that a reactant threshold needs to be attained in order for the desired reaction to occur. This needs further investigation to determine the operating window of the system. Also, it would appear that a defocused beam is favoured over a focused beam which causes ablation to occur. This is possibly due to the high peak powers generated by the pulsed Nd:YAG laser and may be overcome by using a continuous wave laser system. Also it is clear from the XRD results that a certain amount of reactant remained unaltered by the whole process. This may be due to the oxidation of the carbon with the surrounding air. By using a controlled atmosphere of argon, more reactant may develop into the desired product. The surface quality of the compacted sample is signi®cantly higher than that of the non-compacted sample. This may be due to the effect of reducing the amount of porosity which allows the product material to be sintered more effectively. 4. Conclusion
Fig. 7. Optical photograph of non-compacted sample sintered under defocused beam at a power of 60 W, frequency 20 kHz and speed 2 mm s 1 (SHS has taken place). The white border encloses the laser scanned area.
By controlling one of the major factors (porosity) which affects the combustion wave propagation of the SHS reaction, it would appear possible to develop a product reaction where the solid state combustion wave occurs over an extremely small region, allowing a simple geometry to be developed under the resulting action of laser sintering TiO2, Al and C with a Q-switched Nd:YAG laser source. From this work it can be concluded that a focused Qswitched Nd:YAG laser beam causes ablation of the reactant powders. Hence, a defocused beam of spot size 0.6 mm is required (for the Nd:YAG laser used within this experiment).
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This may be due to the high peak power generated by this pulsed Nd:YAG laser source and a continuous wave laser may allow for smaller spot sizes to be utilised. A threshold for the reaction at power 60 W, frequency 20 kHz and scan speed 2 mm s 1 has been determined. However, further work is required to produce an operating window for the reaction. This would need to concentrate on the increasing of the scanning speed which would be an important parameter for rapid prototyping. Although some control over the SHS reaction has been achieved through 50% compaction of the reactant powders there is still further need for investigation into introducing other controlling factors, such as diluent material, e.g. TiC and Al2O3, which may allow for accurate and precise scanned geometry to be produced. Surface quality is greatly enhanced by compacting the reactants before sintering with the Nd:YAG laser sources. The advantage of the combined SLS and SHS is that it may be possible to produce high-temperature functional prototype parts at low cost. Further work is still required to determine the best methods for controlling the SHS combustion wave to produce a rapid prototype functional part. References [1] W. Meiners, K. Wissenbach, R. Poprawe, Direct selective laser sintering of steel powders, in: Proceedings of Second Laser Assisted Net Shape Engineering, LANE'97, 1997, pp. 615±621. [2] K.C. Chee, Three-dimensional rapid prototyping technologies and key development areas, Comput. Contr. Eng. J. (1994). [3] A. Kochan, Rapid developments in rapid prototyping, Assem. Autom. 15 (4) (1995) 18±19. [4] L. Li, K.L. Ng, A. Slocombe, Diode laser sintering of compacted metallic powders for desk top rapid prototyping, in: Proceedings of
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