- Email: [email protected]
Direct laser sintering of a copper-based alloy for creating three-dimensional metal parts
Journal of Materials Processing Technology 140 (2003) 368–372
Direct laser sintering of a copper-based alloy for creating three-dimensional metal parts Y. Tang a , H.T. Loh a , Y.S. Wong b , J.Y.H. Fuh b,∗ , L. Lu b , X. Wang b b
a Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
Abstract Direct laser sintering of metal, as one of the important developments in rapid prototyping technologies, is discussed in this paper. A special copper-based alloy is used for this rapid prototyping process. Experiments on the sintering conditions of this material had been conducted in a self-developed high temperature metal sintering machine. The mechanism of laser sintering for this kind of material was disclosed by SEM analysis of microstructures of sintered parts. The density, surface roughness and mechanical properties of the laser sintering parts due to variation of process parameters were measured and analysed. The effect of process parameters to the accuracy of sintered parts was also investigated. Thus, optimum parameters were obtained for direct laser sintering of three-dimensional metal parts. © 2003 Elsevier B.V. All rights reserved. Keywords: Rapid prototyping (RP); Selective laser sintering (SLS); Direct laser sintering; Copper-based alloy; 3D metal part
1. Introduction Selective laser sintering (SLS) produces parts by fusing or sintering together successive layers of powder material. One of the major advantages is that it is able to process a very wide range of materials (standard polymers, metals, ceramics, foundry sand, etc.) in a direct way (i.e. sacrificial binder not mandatory), while yielding excellent material properties (i.e. close to those obtained with other manufacturing methods) [1]. For building metal parts, the SLS has two approaches. One is called indirect laser sintering, in which the metal powder is coated with low melting-point materials; and in fact it is only the coating material that has been sintered by the laser beam, and the metal particles are bound by the coating materials. The other one uses the direct process, in which the metal powder is directly sintered by the laser beam, hence the process is called direct laser sintering. In recent years, direct laser sintering of metals has been intensively studied by many researchers all over the world. For example, Simchi et al. [2] explored the mechanisms of particle bonding for direct metal laser sintering, Meiners et al. [3] developed the process of direct generation of metal parts and tools by selective laser powder re-melting (SLPR), Hauser et al. [4] worked on the direct SLS for stainless steel 314S. Das et al. [5] used direct laser sintering for fabricat-
∗ Corresponding author. Fax: +65-6779-1459. E-mail address: [email protected] (J.Y.H. Fuh).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00766-0
ing high performance metal components, Nyrhila et al. [6] discussed the industrial use of direct metal laser sintering. Durr et al. [7] proposed rapid tooling of EDM electrodes by means of SLS. It has already been understood that the metals are much more difficult to be laser-sintered than the polymer materials. Common problems such as oxidation, balling and shrinkage may result in low density, weak strength and rough surface of the sintering parts. And only a few materials and processes have been commercialised so far. This paper reports copper-based alloys for direct laser sintering based on our previous work [8]. The purpose of the research is to investigate the material, process and equipment developed for rapid tooling based on the direct laser sintering process. A special copper-based alloy has been developed and tested in an experimental metal sintering machine. Experiments were done to investigate the effect of process parameters to the properties and qualities for the final metal parts. Some good results have been obtained.
2. Experimental 2.1. Experimental equipment For the purpose of direct laser sintering of metals, experimental equipment called the high-temperature metal laser sintering (HTMLS) machine has been developed. In this machine (shown in Fig. 1), a 200 W CO2 laser is used as the energy source; it delivers out the laser beam at a certain
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
369
Fig. 1. Experimental equipment.
power. The laser beam is then transferred into the laser scanner to realize a 2D scan pattern on the surface of layered metal powder. A mechanism of two cylinders is used for feeding the powder layer by layer, and the thickness of the powder layer can be controlled from 0.05 mm. A sealed gas chamber can be filled with a protection gas (N2 or Ar), preventing the metals from oxidation at a high temperature in the laser sintering process. All the experiments were carried out in this machine. 2.2. Material The material system mainly consists of Cu and Cu3 P. The pre-alloyed SCuP metal powder acted as the binder, while the Cu powder acted as the structure during laser sintering. The phosphorus in the SCuP acts as a flux. It can dissolve and remove oxides from the powder surfaces, thus protecting the Cu from oxidisation and improve the wetting characteristics in the system. Two powders (Cu and SCuP) were then mixed together in a certain ratio. 2.3. Experimental procedures In order to determine the suitable ratio of the two components, mixed powder of Cu and SCuP at variable ratios was tested. The successful samples were analysed by SEM. The density, as the most important quality of the sintered parts was investigated using variable parameters, especially the original density caused by the shape of Cu particle. The influence of parameters to the strength, accuracy and surface finish of the sintered parts was also studied. Finally, actual 3D metal parts was built using the optimum parameters.
3. Results and discussion 3.1. Microstructures The SEM images of sintered samples due to variable amount of binder (SCuP) are shown in Fig. 2. All samples were sintered using a fixed laser power of 200 W, scan speed of 240 mm/s, scan line spacing of 0.2 mm and layer thickness of 75 m in air atmosphere at room temperature. From the images, it can be seen that the binder was molten and
Fig. 2. SEM images of sintered samples using different amount of binder: (a) 25 vol.% SCuP; (b) 40 vol.% SCuP; (c) 55 vol.% SCuP.
form the liquid phase during the laser sintering, then the liquid phase flew and filled the pores between the Cu particles and solidified to form a continuous solid phase. Therefore, the fraction of liquid phase influences the rearrangement and spreading. These influences are reflected by the microstructures. It can clearly be seen that the microstructural features, such as porosity, pore size and shape, and the agglomeration size and shape are associated with the variation of the amount of SCuP. At a low SCuP level (25 vol.%), there exists a large amount of pores. The agglomeration size is small and shows ball-like shape with sizes of about 200–300 m only (Fig. 2(a)). As the amount of SCuP increased to 40 vol.% (Fig. 2(b)), more molten binder flew and infiltrated into the pores between Cu particles, forming big agglomerates and a denser microstructure, but the pore sizes became larger. When 55 vol.% SCuP was used, several special features can be found. Firstly, ball-like agglomerates changed to long bar ones with directions along the laser scan direction. Secondly, the pores became narrower and longer. Lastly, a much denser
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
Table 1 Influence of parameters to the final density of sintered parts Loose powder density, ρ (%) Laser power (W) (scan speed 160 mm/s, scan spacing 0.15 mm)
Scan speed (mm/s) (laser power 100 W, scan spacing 0.1 mm)
Scan spacing (mm) (laser power 100 W, scan speed 160 mm/s)
27.2
76.8
100
69.0
80.1
150 200
73.8 74.3
81.9 82.2
160
72.5
80.1
200 240
66.5 64.7
76.7 75.0
0.1
72.5
80.1
0.15 0.2
69.0 65.7
80.1 77.0
microstructure was formed. At a low fraction of SCuP, the liquid phase is only able to bind those Cu particles near the SCuP particles, forming ball-like agglomerates. Since the existence of large amount of Cu powder particles (structural powder) and small amount of SCuP (binder) result in the low rearrangement force, it is impossible to rearrange Cu
Tensile strength (MPa)
21 19 17 15 13
Spacing=0.15mm
11
Spacing=0.20mm
9
Spacing=0.24mm
particles, hence leaving many small pores after laser sintering. At high fraction of SCuP, the rearrangement force is higher and viscosity of the mixture is lower, leading to faster rearrangement of Cu particles and spreading of binder. The molten SCuP spreads and wets with Cu particles well. The Cu particles are pulled together by the liquid towards the center of a laser scan path, thus forming long bar shaped tracks. However, high porosity is still left in the microstructure due to the short transient interaction duration (0.1–1 s) of laser and metal powder. 3.2. Density Table 1 gives the relative densities of the parts sintered by using variable process parameters. All the sintered parts used same layer thickness of 0.075 mm. The variable parameters include loose powder density, laser power, scan speed and scan spacing. From Table 1, it can be found that the final density of the sintered parts using higher loose powder density is much higher than that of sintered parts
Tensile strength (MPa)
370
7 70
(a)
120
170
Power (w)
Thickness=0.09mm
130
180
230
280
Scan speed (mm/s) 0.45 0.4
0.13 0.11 0.09
Spacing=0.15mm
0.07
Spacing=0.2mm Spacing=0.24mm
Error (mm)
Error (mm)
Thickness=0.07mm
(a)
0.15
120
170
220
Spacing=0.2mm Spacing=0.24mm
120 170 Power (W)
Thickness=0.09mm
0.2 0.15
(b)
Spacing=0.15mm
70
Thickness=0.07mm
0.25
80
Pow e r (W)
21 20 19 18 17 16 15 14 13 12
Thickness=0.05mm
0.3
0.05
Surface roughness (um)
70
(b)
0.35
0.1
0.05
Surface roughness (um)
Thickness=0.05mm
80
220
0.17
(c)
30 28 26 24 22 20 18 16 14 12
220
Fig. 3. Influence of laser power and scan spacing (a) tensile strength, (b) accuracy and (c) surface roughness.
180
23 22.5 22 21.5 21 20.5 20 19.5 19 18.5
230
280
Thickness=0.05mm Thickness=0.07mm Thickness=0.09mm
80
(c)
130
Scan speed (mm/s)
130
180
230
280
Scan speed (mm/s)
Fig. 4. Influence of scan speed and layer thickness (a) tensile strength, (b) accuracy and (c) surface roughness.
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372 Table 2 Optimum parameters for building 3D metal parts Laser power (W) Scan speed (mm/s) Scan spacing (mm) Layer thickness (mm)
200 190 0.2 0.075
371
The error decreases (accuracy becomes better) when the scan speed increases, and the layer thickness has a very little influence to the accuracy (Fig. 4(b)). The surface roughness decreases (surface finish becomes better) when the laser power increases and scan spacing decreases (Fig. 4(c)). 3.4. 3D metal parts
using lower loose powder density. At least 7.6% difference exists between these relative densities even using the same process parameters. The largest difference between these densities is as high as 11.1%. The final density also increases when the laser power increases, and the scan speed and scan spacing decrease.
Based on above results, optimum parameters were selected for building actual 3D metal parts using this direct laser sintering process. The parameters used in the process are shown in Table 2. The 3D metal parts built are shown in Fig. 5. It was measured that the parts had a relative density of 76%, surface roughness Ra of 16–21 m.
3.3. Strength, accuracy and surface roughness The influence of laser power and scan spacing to the tensile strength, accuracy and surface roughness of sintered parts is shown in Fig. 3. The tensile strength increases when the laser power increases and scan spacing decreases (Fig. 3(a)). The dimension error increases (accuracy becomes worse) when the laser power increases and scan spacing decreases (Fig. 3(b)). The surface roughness increases (surface finish becomes worse) when the laser power and scan spacing increase (Fig. 3(c)). Fig. 4 shows the influence of scan speed and the layer thickness to the tensile strength, accuracy and surface roughness of the sintered parts. The tensile strength decreases when the scan speed and layer thickness increase (Fig. 4(a)).
4. Conclusions (1) The direct laser sintering of the copper-based alloy is based on the mechanism of liquid phase sintering. The binder (SCuP) was molten and form the liquid phase during the laser sintering, then the liquid phase flew and filled the pores between the Cu particles (structure material) and solidified to form a continuous solid phase. The fraction of the binder in the mixed powder has an important influence to the final microstructure of the sintered parts. (2) The shape of the Cu particle is an important factor in the original density of loose powder, thus influence the final density of the sintered parts. The final density also increases when the laser power increases, and the scan speed and scan spacing decreases. (3) With the laser power increase, the strength of the sintered parts becomes better, but the accuracy and the surface finish become worse. The scan speed has a reverse influence to the strength, accuracy and surface finish. The scan spacing has an inverse relation to the strength and surface finish, but has a positive relation to the accuracy. The layer thickness however has an inverse relation to the strength and surface finish, but has a little influence to the accuracy. (4) Good quality 3D metal were successfully built by the direct laser metal sintering process using optimum parameters. References
Fig. 5. Direct laser sintered 3D metal parts: (a) mould inserts; (b) oil pump.
[1] J.P. Kruth, M.C. Leu, T. Nakagawa, Progress in additive manufacturing and rapid prototyping. Keynote papers, CIRP Ann. 98 (1998) 525–540. [2] A. Simchi, F. Petzoldt, H. Pohl, Direct metal laser sintering: material considerations and mechanisms of particle bonding, Int. J. Powder Metall. 37 (2) (2001) 49–61. [3] W. Meiners, C. Over, K. Wissenbach, R. Poprawe, Direct generation of metal parts and tools by selective laser powder re-melting (SLPR), in: Proceedings of the SFF, Austin, TX, August 9–11, 1999. [4] C. Hauser, T.H.C. Childs, K.W. Dalgarno, R.B. Eane, Atmospheric control during direct selective laser sintering of stainless steel 314S
372
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
powder, in: Proceedings of the SFF, Austin, TX, August 9–11, 1999. [5] S. Das, J.J. Beaman, M. Wohlert, D.L. Bourell, Direct laser freeform fabrication of high performance metal components, Rapid Prototyp. J. 4 (3) (1998) 112–117. [6] O. Nyrhila, J. Kotila, J.-E. Lind, T. Syvanen, Industrial use of direct metal laser sintering, in: Proceedings of the SFF, Austin, TX, August 10–12, 1998.
[7] H. Durr, R. Pilz, N.S. Eleser, Rapid tooling of EDM electrodes by means of selective laser sintering, Comput. Ind. 39 (1999) 35– 45. [8] C.C. Leong, L. Lu, J.Y.H. Fuh, et al., Formation of copper-based alloys for rapid tooling, in: Proceedings of the Eighth European Conference on Rapid Prototyping and Manufacturing, Nottingham, UK, July 6–8, 1999.
Direct laser sintering of a copper-based alloy for creating three-dimensional metal parts Y. Tang a , H.T. Loh a , Y.S. Wong b , J.Y.H. Fuh b,∗ , L. Lu b , X. Wang b b
a Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
Abstract Direct laser sintering of metal, as one of the important developments in rapid prototyping technologies, is discussed in this paper. A special copper-based alloy is used for this rapid prototyping process. Experiments on the sintering conditions of this material had been conducted in a self-developed high temperature metal sintering machine. The mechanism of laser sintering for this kind of material was disclosed by SEM analysis of microstructures of sintered parts. The density, surface roughness and mechanical properties of the laser sintering parts due to variation of process parameters were measured and analysed. The effect of process parameters to the accuracy of sintered parts was also investigated. Thus, optimum parameters were obtained for direct laser sintering of three-dimensional metal parts. © 2003 Elsevier B.V. All rights reserved. Keywords: Rapid prototyping (RP); Selective laser sintering (SLS); Direct laser sintering; Copper-based alloy; 3D metal part
1. Introduction Selective laser sintering (SLS) produces parts by fusing or sintering together successive layers of powder material. One of the major advantages is that it is able to process a very wide range of materials (standard polymers, metals, ceramics, foundry sand, etc.) in a direct way (i.e. sacrificial binder not mandatory), while yielding excellent material properties (i.e. close to those obtained with other manufacturing methods) [1]. For building metal parts, the SLS has two approaches. One is called indirect laser sintering, in which the metal powder is coated with low melting-point materials; and in fact it is only the coating material that has been sintered by the laser beam, and the metal particles are bound by the coating materials. The other one uses the direct process, in which the metal powder is directly sintered by the laser beam, hence the process is called direct laser sintering. In recent years, direct laser sintering of metals has been intensively studied by many researchers all over the world. For example, Simchi et al. [2] explored the mechanisms of particle bonding for direct metal laser sintering, Meiners et al. [3] developed the process of direct generation of metal parts and tools by selective laser powder re-melting (SLPR), Hauser et al. [4] worked on the direct SLS for stainless steel 314S. Das et al. [5] used direct laser sintering for fabricat-
∗ Corresponding author. Fax: +65-6779-1459. E-mail address: [email protected] (J.Y.H. Fuh).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00766-0
ing high performance metal components, Nyrhila et al. [6] discussed the industrial use of direct metal laser sintering. Durr et al. [7] proposed rapid tooling of EDM electrodes by means of SLS. It has already been understood that the metals are much more difficult to be laser-sintered than the polymer materials. Common problems such as oxidation, balling and shrinkage may result in low density, weak strength and rough surface of the sintering parts. And only a few materials and processes have been commercialised so far. This paper reports copper-based alloys for direct laser sintering based on our previous work [8]. The purpose of the research is to investigate the material, process and equipment developed for rapid tooling based on the direct laser sintering process. A special copper-based alloy has been developed and tested in an experimental metal sintering machine. Experiments were done to investigate the effect of process parameters to the properties and qualities for the final metal parts. Some good results have been obtained.
2. Experimental 2.1. Experimental equipment For the purpose of direct laser sintering of metals, experimental equipment called the high-temperature metal laser sintering (HTMLS) machine has been developed. In this machine (shown in Fig. 1), a 200 W CO2 laser is used as the energy source; it delivers out the laser beam at a certain
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
369
Fig. 1. Experimental equipment.
power. The laser beam is then transferred into the laser scanner to realize a 2D scan pattern on the surface of layered metal powder. A mechanism of two cylinders is used for feeding the powder layer by layer, and the thickness of the powder layer can be controlled from 0.05 mm. A sealed gas chamber can be filled with a protection gas (N2 or Ar), preventing the metals from oxidation at a high temperature in the laser sintering process. All the experiments were carried out in this machine. 2.2. Material The material system mainly consists of Cu and Cu3 P. The pre-alloyed SCuP metal powder acted as the binder, while the Cu powder acted as the structure during laser sintering. The phosphorus in the SCuP acts as a flux. It can dissolve and remove oxides from the powder surfaces, thus protecting the Cu from oxidisation and improve the wetting characteristics in the system. Two powders (Cu and SCuP) were then mixed together in a certain ratio. 2.3. Experimental procedures In order to determine the suitable ratio of the two components, mixed powder of Cu and SCuP at variable ratios was tested. The successful samples were analysed by SEM. The density, as the most important quality of the sintered parts was investigated using variable parameters, especially the original density caused by the shape of Cu particle. The influence of parameters to the strength, accuracy and surface finish of the sintered parts was also studied. Finally, actual 3D metal parts was built using the optimum parameters.
3. Results and discussion 3.1. Microstructures The SEM images of sintered samples due to variable amount of binder (SCuP) are shown in Fig. 2. All samples were sintered using a fixed laser power of 200 W, scan speed of 240 mm/s, scan line spacing of 0.2 mm and layer thickness of 75 m in air atmosphere at room temperature. From the images, it can be seen that the binder was molten and
Fig. 2. SEM images of sintered samples using different amount of binder: (a) 25 vol.% SCuP; (b) 40 vol.% SCuP; (c) 55 vol.% SCuP.
form the liquid phase during the laser sintering, then the liquid phase flew and filled the pores between the Cu particles and solidified to form a continuous solid phase. Therefore, the fraction of liquid phase influences the rearrangement and spreading. These influences are reflected by the microstructures. It can clearly be seen that the microstructural features, such as porosity, pore size and shape, and the agglomeration size and shape are associated with the variation of the amount of SCuP. At a low SCuP level (25 vol.%), there exists a large amount of pores. The agglomeration size is small and shows ball-like shape with sizes of about 200–300 m only (Fig. 2(a)). As the amount of SCuP increased to 40 vol.% (Fig. 2(b)), more molten binder flew and infiltrated into the pores between Cu particles, forming big agglomerates and a denser microstructure, but the pore sizes became larger. When 55 vol.% SCuP was used, several special features can be found. Firstly, ball-like agglomerates changed to long bar ones with directions along the laser scan direction. Secondly, the pores became narrower and longer. Lastly, a much denser
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
Table 1 Influence of parameters to the final density of sintered parts Loose powder density, ρ (%) Laser power (W) (scan speed 160 mm/s, scan spacing 0.15 mm)
Scan speed (mm/s) (laser power 100 W, scan spacing 0.1 mm)
Scan spacing (mm) (laser power 100 W, scan speed 160 mm/s)
27.2
76.8
100
69.0
80.1
150 200
73.8 74.3
81.9 82.2
160
72.5
80.1
200 240
66.5 64.7
76.7 75.0
0.1
72.5
80.1
0.15 0.2
69.0 65.7
80.1 77.0
microstructure was formed. At a low fraction of SCuP, the liquid phase is only able to bind those Cu particles near the SCuP particles, forming ball-like agglomerates. Since the existence of large amount of Cu powder particles (structural powder) and small amount of SCuP (binder) result in the low rearrangement force, it is impossible to rearrange Cu
Tensile strength (MPa)
21 19 17 15 13
Spacing=0.15mm
11
Spacing=0.20mm
9
Spacing=0.24mm
particles, hence leaving many small pores after laser sintering. At high fraction of SCuP, the rearrangement force is higher and viscosity of the mixture is lower, leading to faster rearrangement of Cu particles and spreading of binder. The molten SCuP spreads and wets with Cu particles well. The Cu particles are pulled together by the liquid towards the center of a laser scan path, thus forming long bar shaped tracks. However, high porosity is still left in the microstructure due to the short transient interaction duration (0.1–1 s) of laser and metal powder. 3.2. Density Table 1 gives the relative densities of the parts sintered by using variable process parameters. All the sintered parts used same layer thickness of 0.075 mm. The variable parameters include loose powder density, laser power, scan speed and scan spacing. From Table 1, it can be found that the final density of the sintered parts using higher loose powder density is much higher than that of sintered parts
Tensile strength (MPa)
370
7 70
(a)
120
170
Power (w)
Thickness=0.09mm
130
180
230
280
Scan speed (mm/s) 0.45 0.4
0.13 0.11 0.09
Spacing=0.15mm
0.07
Spacing=0.2mm Spacing=0.24mm
Error (mm)
Error (mm)
Thickness=0.07mm
(a)
0.15
120
170
220
Spacing=0.2mm Spacing=0.24mm
120 170 Power (W)
Thickness=0.09mm
0.2 0.15
(b)
Spacing=0.15mm
70
Thickness=0.07mm
0.25
80
Pow e r (W)
21 20 19 18 17 16 15 14 13 12
Thickness=0.05mm
0.3
0.05
Surface roughness (um)
70
(b)
0.35
0.1
0.05
Surface roughness (um)
Thickness=0.05mm
80
220
0.17
(c)
30 28 26 24 22 20 18 16 14 12
220
Fig. 3. Influence of laser power and scan spacing (a) tensile strength, (b) accuracy and (c) surface roughness.
180
23 22.5 22 21.5 21 20.5 20 19.5 19 18.5
230
280
Thickness=0.05mm Thickness=0.07mm Thickness=0.09mm
80
(c)
130
Scan speed (mm/s)
130
180
230
280
Scan speed (mm/s)
Fig. 4. Influence of scan speed and layer thickness (a) tensile strength, (b) accuracy and (c) surface roughness.
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372 Table 2 Optimum parameters for building 3D metal parts Laser power (W) Scan speed (mm/s) Scan spacing (mm) Layer thickness (mm)
200 190 0.2 0.075
371
The error decreases (accuracy becomes better) when the scan speed increases, and the layer thickness has a very little influence to the accuracy (Fig. 4(b)). The surface roughness decreases (surface finish becomes better) when the laser power increases and scan spacing decreases (Fig. 4(c)). 3.4. 3D metal parts
using lower loose powder density. At least 7.6% difference exists between these relative densities even using the same process parameters. The largest difference between these densities is as high as 11.1%. The final density also increases when the laser power increases, and the scan speed and scan spacing decrease.
Based on above results, optimum parameters were selected for building actual 3D metal parts using this direct laser sintering process. The parameters used in the process are shown in Table 2. The 3D metal parts built are shown in Fig. 5. It was measured that the parts had a relative density of 76%, surface roughness Ra of 16–21 m.
3.3. Strength, accuracy and surface roughness The influence of laser power and scan spacing to the tensile strength, accuracy and surface roughness of sintered parts is shown in Fig. 3. The tensile strength increases when the laser power increases and scan spacing decreases (Fig. 3(a)). The dimension error increases (accuracy becomes worse) when the laser power increases and scan spacing decreases (Fig. 3(b)). The surface roughness increases (surface finish becomes worse) when the laser power and scan spacing increase (Fig. 3(c)). Fig. 4 shows the influence of scan speed and the layer thickness to the tensile strength, accuracy and surface roughness of the sintered parts. The tensile strength decreases when the scan speed and layer thickness increase (Fig. 4(a)).
4. Conclusions (1) The direct laser sintering of the copper-based alloy is based on the mechanism of liquid phase sintering. The binder (SCuP) was molten and form the liquid phase during the laser sintering, then the liquid phase flew and filled the pores between the Cu particles (structure material) and solidified to form a continuous solid phase. The fraction of the binder in the mixed powder has an important influence to the final microstructure of the sintered parts. (2) The shape of the Cu particle is an important factor in the original density of loose powder, thus influence the final density of the sintered parts. The final density also increases when the laser power increases, and the scan speed and scan spacing decreases. (3) With the laser power increase, the strength of the sintered parts becomes better, but the accuracy and the surface finish become worse. The scan speed has a reverse influence to the strength, accuracy and surface finish. The scan spacing has an inverse relation to the strength and surface finish, but has a positive relation to the accuracy. The layer thickness however has an inverse relation to the strength and surface finish, but has a little influence to the accuracy. (4) Good quality 3D metal were successfully built by the direct laser metal sintering process using optimum parameters. References
Fig. 5. Direct laser sintered 3D metal parts: (a) mould inserts; (b) oil pump.
[1] J.P. Kruth, M.C. Leu, T. Nakagawa, Progress in additive manufacturing and rapid prototyping. Keynote papers, CIRP Ann. 98 (1998) 525–540. [2] A. Simchi, F. Petzoldt, H. Pohl, Direct metal laser sintering: material considerations and mechanisms of particle bonding, Int. J. Powder Metall. 37 (2) (2001) 49–61. [3] W. Meiners, C. Over, K. Wissenbach, R. Poprawe, Direct generation of metal parts and tools by selective laser powder re-melting (SLPR), in: Proceedings of the SFF, Austin, TX, August 9–11, 1999. [4] C. Hauser, T.H.C. Childs, K.W. Dalgarno, R.B. Eane, Atmospheric control during direct selective laser sintering of stainless steel 314S
372
Y. Tang et al. / Journal of Materials Processing Technology 140 (2003) 368–372
powder, in: Proceedings of the SFF, Austin, TX, August 9–11, 1999. [5] S. Das, J.J. Beaman, M. Wohlert, D.L. Bourell, Direct laser freeform fabrication of high performance metal components, Rapid Prototyp. J. 4 (3) (1998) 112–117. [6] O. Nyrhila, J. Kotila, J.-E. Lind, T. Syvanen, Industrial use of direct metal laser sintering, in: Proceedings of the SFF, Austin, TX, August 10–12, 1998.
[7] H. Durr, R. Pilz, N.S. Eleser, Rapid tooling of EDM electrodes by means of selective laser sintering, Comput. Ind. 39 (1999) 35– 45. [8] C.C. Leong, L. Lu, J.Y.H. Fuh, et al., Formation of copper-based alloys for rapid tooling, in: Proceedings of the Eighth European Conference on Rapid Prototyping and Manufacturing, Nottingham, UK, July 6–8, 1999.