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Basic Powder Metallurgical Aspects in Selective Metal Powder Sintering
Basic Powder Metallurgical Aspects in Selective Metal Powder Sintering J.-P. Kruth (1). B. Van der Schueren, J. E. Bonse. B. Mcrren, Faculty of Engineering, Department of Mechanical Engineering. Divisiori PMA, Heverlee, Belgium Received on January 9,1996
A recent evolution in Rapid Prototyping is the direct production of metal parts. The main bottle necks are part accuracy and appropriate material properties. This paper describes the approach of selective metal powder sintering (SMS) where liquid phase sintering actions as the basic binding mechanism between individual metal particles. The merits and drawbacks of this approach are highlighted from metallographic point of view. Next, a few material combinations, which have been experimentally verified, will be described from point of view of performance in the SMS process. Simultaneously, solutions for problems related to this performance are proposed and discussed.
Kevwords: Rapid Prototyping, Sintering, Laser
1. Introduction
Selective Metal Powder Sintering (SMS) is a Material Accretion Manufacturing technology to produce metallic parts. The main representative of Material Accretion Manufacturing is Stereolithography [4], introduced in 1987. Material Accretion Manufacturing can be distinguished from Material Removal and Material Forming techniques [S]in the way they make products: they build products by creating solid (bound) material where it is needed, without the use of special tools. Like most Material Accretion Manufacturing techniques, SMS produces parts in a layer by layer fashion. This allows a direct coupling with the CAD-model of the product in which successive cross sections are calculated. Figure l outlines the fundamentals of the SMS set-up. The basic material is an unbound metal powder mixture which is spread as a thin layer on top of a container by means of a deposition system [lo]. Layer thickness is in the order of magnitude of 0.4 mm. The powders are bound together using a focused energy source. During a feasibility study, an electron beam as well a Nd:YAG laser have been examined [l 11. Since the electron beam treatment of the powders showed important technological limitations, this energy source is not further investigated. The Selective Metal Powder Sintering set-up is based on a CW Nd:YAG laser, having a maximum power of 500 Watt. A deflection system scans the beam over the powder surface according to the product’s cross section [12]. By carefully controlling the energy input, the newly created 2D pattern is linked to the underlying one. Repeating deposition and pattern creation gives rise to a three dimensional part, called green product. This green part has a large degree of porosity and hence poor mechanical properties. The reason for this is the initial porosity in the loose powder Annals of rhe ClRP Vol. 45/1i’1996
(50 to 70%). So, a post-treatment is necessary to increase the product’s density. This process might consist in a thermal treatment or an impregnation with a liquid metal. This paper aims to describe the basic binding mechanism between adjacent powder particles as it is invoked by the laser beam. The theory is verified using a few powder mixtures in the experimental set-up depicted in figure 1. These experiments show the large number of parameters determining the process behaviour and the resulting part quality. But, by setting !he right parameters, a broad variety of different materials and related mechanical properties become available. 2. Basic Bindina Mechanism in SMS
To create the two dimensional patterns, an appropriate binding mechanism must be used for binding adjacent powder particles to form the green part. Two different approaches are powder melting and sintering (61. Melting implies a continuous feed of powders in the melt pool in order to compensate for shrinkage [3]. This may require complex powder feed and machine control systems. As all particles must be molten, the process is relatively. slow but the resulting parts have a high density, although containing important residual stresses [7].If no continuous powder feed is applied, shrinkage in the powder bed during melting results in trapped pores. These pores may lead to unpredictable mechanical properties of the parts, since neither size neither location of the pores is predictable. Figure 2 shows an example of a copper powder treated by a Nd:YAG laser. The micrograph reveals a homogeneous metallographic structure with pores trapped in the centre of the track. The second approach to consolidate powders consists in sintering. Distinction has to be made between solid state
183
Figure 1: Overview of the experimental SMS set-up with indication of fhe main components sintering (SSS)and liquid phase sintering (LPS). SSS is a thermal process that occurs at temperatures T:, %Tn c T, c Tn where T, is the melt point of the powder material. The driving force for binding is a physical diffusion of metal atoms from one particle to another. Since this is a slow phenomenon it makes out the main drawback of SSS for SMS. Experiments show that a laser beam particle interaction time of less than 1 ms is far too short to initiate sintering. Liquid Phase Sintering (LPS) is a much faster sinleddensification mechanism [Z]. The basic material here consists of a mixture of two metal powders: a high melting point metal, called the structural metal, and a low melting point metal, called the binder. Applying heat to the system causes the binder to melt and to flow into the pores formed by the non-molten particles. The main advantage of LPS is the very fast initial binding. This binding is based on capillary forces which might be very high: the reaction speed in this stage is determined by the kinetics of the solid-melt transformation. This transformation is orders of magnitudes faster than physical diffusion [a]. Figure 3 gives a qualitative view of the amount of sintering reaction as a function of time. This curve shows three different stages: the rearrangement (melt and capillary penetration), solution precipitation (grain growth by migration of atoms of the structural metal through the melt) and a solid state sintering (final densification). The SMS process only invokes the rearrangement mechanism. Once the binder metal is molten and spread out into the solid lattice. the system cools down (because the moving laser beam no longer feeds energy into the sys-
4
conventional LPS cor
'
.-
stage 2
liqud phase surlenng
tem) and the situation is frozen. This results in green parts with enough mechanical strength for further postprocessing. In case a thermal post process is applied, the product will be heated up again and the densification will continue following the dotted curve of figure 3. A general remark concerns the post-treatments, which require an interconnected porosity in the green part. This means that the pores are connected by means of micro channels allowing the evacuation of air to the outside or the flow of liquid metal to the inside. A complete melt may shut off these micro channels making the densification of remaining porosities impossible (figure 2). 3. ExamDles of Workina Powder Systems This paragraph gives evidence that Nd:YAG laser beam initiated LPS is possible, that the extent of the reactions can be controlled and that conventionally used powder combinations are also applicable in the SMS process. These examples also show the broad range of various parameters that must be appropriately set in order to get the required binding. 3.1 Fe-Cu Dowder mixture Fe-Cu is a well-described powder mixture for LPS [2,6] with attractive mechanical properties: the hard Fe lattice is dispersed in a ductile Cu matrix. New is the laser initiation of the binding reaction. Previous studies have shown that a mixture ratio of 20 to 30 wt% Cu is necessary [l 11. This arises from the relative high reflectivity of the Cu for laser light compared to Fe, meaning that, if no
LPS in SMS
bulk liquid phase ri~itenngof green SMS pan (stages 2 6 1)
sdidificafionof i n h l bindinp !hat ~ s u l l rm g m n SMS part
Figure 3: Liquid Phase Sintering shows three stages. from which the first is invoked during laser treatment in SMS. 184
Figure 2: Cross section of a melt track realised by scanning a Nd:YAG laser beam over a Cu powder bec. Note the pores in the centre of the track.
Figure 4: Nd:YAG laser initiated LPS in Fe-Cu mixture: white spots are Fe-particles, gray phase is Cu-binder and black area's are pores.
Line Width Lasar
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Scan speed ( m d s )
Figure 5: Experimentally determinded relationship between laser parameters (laser power and scan speed) and line width (I) and layer thickness (r)
special precautions are taken. Fe tends to melt before the Cu particles do what results in a complete melt. The same study revealed a second constraint to the powder particle size distribution: relative smaller Cu-particles enable a better Cu melt distribution. Both constraints result in following powder mixture: Fe (60-90 pm), 30 wt% Cu (25-40 um). The reaction type occurred during laser interaction is determined by light microscopy of sections along the tracks. Setting the right laser parameters (combination of laser power and scan speed) results in laser initiated LPS as represented by figure 4. This picture reveals spherical Fe-particles distributed in the Cu-matrix. but simultaneously a large degree of porosity. This means that no densification has taken place. This observation has also been verified by density measurements, showing a density of 50 to 55 wt%, what corresponds to the loose powder density. The green samples are typically brittle. This brittleness arises from oxides that do exist in the powders or that are formed during laser heating. To reduce the degree of oxidation, an inert atmosphere is applied in the reaction chamber by evacuating the chamber to 1 mbar and tlushing it with Argon. This allows the creation of useful green parts with limited mechanical properties. A thermal post treatment of the green samples results in a consolidation of the metallographic bindings with related improved mechanical strength. The post treatment consists in heating the samples in ti-atmosphere to 1100°C for 30 minutes. This treatment does not significantly decrease the porosity, what is reflected by a linear shrinkage of just 1 to 2 %. Two other sample properties are highlighted: the track width, being the width of a single line, and the track thickness. Both properties are related to the laser power (&) and the scan speed ( Y I limited to 120 mm/s by the setup), which define the energy density according to:
where 2.W=0.8 mm is the beam diameter on the powder surface. Figure 5 shows the experimentally determined relationships. From these results, it follows that the line width decreases with increasing scan speed until a width of 0.6 mm is reached. This corresponds to the zone in the laser spot where enough energy exists to melt the Cu
binder. At lower scan speeds. the applied heat is transferred to powder particles that are not directly irradiated by the laser beam. So, in order to accurately control the width of the laser tracks, it suffices to select a scan speed for which no heat transfer occurs in the XY-plane. At these scan speeds, the width is in a limited way dependent on the laser power: if not enough power is applied, no binding reaction occurs. In the other case, the laser power may vary up to 10 % without significant influence on the line width. Similar tendencies are valid for the layer thickness, that decreases with increasing speed and decreasing power level. However, the dependence on power is more pronounced, meaning that heat penetration in the Z-direction is several powder particles thick. It allows to control the layer thickness by varying the laser power for a given scan speed in order to get binding between successive layers. Notice that only ihe measurement points with the lowest energy density give rise to pure LPS while the others show a more or less significant degree of melt. This melting explains the improved heat penetration in the 2-direction compared to the XY-plane. 3.2 Cu-coated Fe oowder The Fe-Cu powder blend has some important disadvantages. Special care must be taken to avoid the Feparticles to melt, a homogeneous distribution of the Cu particles in the blend is difficult due to gravitational segregation and oxidation of Fe worsens capillary action (wetting). The most appropriate way to overcome these problems consists in coating the Fe-particles by Cu. The coating has been applied through liquid phase electroless deposition [l]. Figure 6-a shows an etched cross section of a particle revealing a homogeneous layer of Cu over the Fe-core. Note also some impurities in the interface between both metals originating from the chemical process. This powder givfs rise to a more uniform binding, compared to the blend, what is expected since no capillary action is necessary for the distribution of Cu-melt (figure 6-b). These samples also show a higher degree of Fe dispersed into the Cu-matrix what should result in improved mechanical properties.
185
Figure 6: A wide range of material combinations can be treated by SMS: (a) shows a cross section of a Cu-coated Fe powder while (b) is a laser sintered sample of this powder. (c) represents a WC-Co smtered sample. Track width and thickness vary with laser power and scan speed in the same way as the blend, but the settings are more critical. If the energy density is to high, heavy reactions occur by which material is erupted out of the powder surface and no bindings are formed. These reactions are caused by sublimation of the impurities in the interface between coating and core material. 3.3 WC-Co Dowder A third example is a combination of a ceramic powder WC (prismatic, 225 pm) and a metal binder (Co. irregular. 30 wt%, 20 um). The high melt point of WC and its chemical stability make that this material is not affected by the laser. Figure 6-c shows a light microscopy of a cross section in a sample. It reveals the nice wetting action of the Co melt, with just few pores (centre of photograph): the WC particles are well embedded in the matrix what gives the samples typical hard-metal properties. The Co matrix is characterised by a high degree of oxidation, resulting in brittle samples. A better control of the reaction atmosphere may overcome this problem. Once again, this powder blend does perform in the same way as the Cu-based powder systems with respect to scan speed and laser power, although the average energy density is shifted to a higher level. 4. Conclusions This paper has explained two different approaches to selectively bind metal powders. In case no continuous powder feed is applied, sintering is preferred over melting. Liquid phase sintering is most attractive due to the fast initial binding realised by capillary action. The examples, covered by the paper, prove that laser initiation of LPS is feasible, and that a broad range of material combinations is possible. However, special care must be paid to the correct blending of the powder systems and to the problems of laser-powder interaction and oxidation during heating. Coating of the structural powders with the binder material is proposed to overcome these problems. A successful example of Cu-coated Fe-powder is proposed. Also other powder combinations like WC-Co prove to be usefull in the SMS process. Finally, two parameters appear to allow dimensional control of the tracks: scan speed determines the width of the tracks, while laser power determines the thickness of the tracks for a given scan speed.
186
5. References Gabe. D.R.(1978) Principles of Metal Surface Treatment and Protection. 2M ed., Pergamon Press, Oxford. German, A.M. (1985) Liquid Phase Sinlering. Plenum Press, New York. Konig, W., Celiker, T., Herfurth, H.-J. (1993) Approaches to Prototyping of Metallic Parts. Proc. p d Int. Eur. Conf. on Rapid Prototyping, pp 303-316. Kruth. J.P. (1991) Material lncress Manufacturing by Rapid Prototyping Techniques. ClRP Annals Vol 40, 2 pp 603-614. Kruth. J.P. (1994) Advances in Physical and Chemical Machining. Proc. f hInt. Conf. on Production/ Precision Engineering, Chiba, pp K62-K76. Lenel, F.V. (1978) Powder Metallurgy, Principles and Applications. Metal Powder Industries Federation, New Jersey. Murphy, M.L.; Steen, W.M.; Lee, C. (1994) The Rapid Manufacturing of Metallic Components by Laser Surface Cladding. Proc. of the 2ghClRP Seminar on Mfg. Systems LANE'94, Erlangen, pp. 803-814. Porter, D.A., Easterling, K.E. (1992) Phase Transformations in Metals and Alloys. Chapman & Hall, 2nded., London. Sachs, E.; et aL(1993) 3D Printing, an Additive Process. Cirp Annals V0142l1, pp 257-260. [ l o ] Van der Schueren, 8.; Kruth, J.P. (1995) Powder Deposition in Selective Metal Powder Sintering. Proc. 4n Eur. Conf. on Rapid Prototyping and Manufacturing, Nottingham. pp. 197-211. [ l l ] Van der Schueren, B.; Kruth, J.P. (1994) Laser Based Selective Metal Powder Sintering: a Feasibility Study. Proc. of the 2dh ClRP Seminar on Mfg. Systems LANE'94, Erlangen, pp. 793-802. [12]Van der Schueren, B.; Kruth, J.P. (1995) Design Aspects of a "Selective Metal Powder Sintering" Apparatus. Proc. ISEM-XI International Symposium for Electro-Machining. Lausanne, pp 665-672. [ 131 Van der Schueren, B.( 1994) Selective Metal Powder Sintering: a Metal Accretion Manufacturing System. EARP Newletter, No 4, August, pp. 6-7.
A recent evolution in Rapid Prototyping is the direct production of metal parts. The main bottle necks are part accuracy and appropriate material properties. This paper describes the approach of selective metal powder sintering (SMS) where liquid phase sintering actions as the basic binding mechanism between individual metal particles. The merits and drawbacks of this approach are highlighted from metallographic point of view. Next, a few material combinations, which have been experimentally verified, will be described from point of view of performance in the SMS process. Simultaneously, solutions for problems related to this performance are proposed and discussed.
Kevwords: Rapid Prototyping, Sintering, Laser
1. Introduction
Selective Metal Powder Sintering (SMS) is a Material Accretion Manufacturing technology to produce metallic parts. The main representative of Material Accretion Manufacturing is Stereolithography [4], introduced in 1987. Material Accretion Manufacturing can be distinguished from Material Removal and Material Forming techniques [S]in the way they make products: they build products by creating solid (bound) material where it is needed, without the use of special tools. Like most Material Accretion Manufacturing techniques, SMS produces parts in a layer by layer fashion. This allows a direct coupling with the CAD-model of the product in which successive cross sections are calculated. Figure l outlines the fundamentals of the SMS set-up. The basic material is an unbound metal powder mixture which is spread as a thin layer on top of a container by means of a deposition system [lo]. Layer thickness is in the order of magnitude of 0.4 mm. The powders are bound together using a focused energy source. During a feasibility study, an electron beam as well a Nd:YAG laser have been examined [l 11. Since the electron beam treatment of the powders showed important technological limitations, this energy source is not further investigated. The Selective Metal Powder Sintering set-up is based on a CW Nd:YAG laser, having a maximum power of 500 Watt. A deflection system scans the beam over the powder surface according to the product’s cross section [12]. By carefully controlling the energy input, the newly created 2D pattern is linked to the underlying one. Repeating deposition and pattern creation gives rise to a three dimensional part, called green product. This green part has a large degree of porosity and hence poor mechanical properties. The reason for this is the initial porosity in the loose powder Annals of rhe ClRP Vol. 45/1i’1996
(50 to 70%). So, a post-treatment is necessary to increase the product’s density. This process might consist in a thermal treatment or an impregnation with a liquid metal. This paper aims to describe the basic binding mechanism between adjacent powder particles as it is invoked by the laser beam. The theory is verified using a few powder mixtures in the experimental set-up depicted in figure 1. These experiments show the large number of parameters determining the process behaviour and the resulting part quality. But, by setting !he right parameters, a broad variety of different materials and related mechanical properties become available. 2. Basic Bindina Mechanism in SMS
To create the two dimensional patterns, an appropriate binding mechanism must be used for binding adjacent powder particles to form the green part. Two different approaches are powder melting and sintering (61. Melting implies a continuous feed of powders in the melt pool in order to compensate for shrinkage [3]. This may require complex powder feed and machine control systems. As all particles must be molten, the process is relatively. slow but the resulting parts have a high density, although containing important residual stresses [7].If no continuous powder feed is applied, shrinkage in the powder bed during melting results in trapped pores. These pores may lead to unpredictable mechanical properties of the parts, since neither size neither location of the pores is predictable. Figure 2 shows an example of a copper powder treated by a Nd:YAG laser. The micrograph reveals a homogeneous metallographic structure with pores trapped in the centre of the track. The second approach to consolidate powders consists in sintering. Distinction has to be made between solid state
183
Figure 1: Overview of the experimental SMS set-up with indication of fhe main components sintering (SSS)and liquid phase sintering (LPS). SSS is a thermal process that occurs at temperatures T:, %Tn c T, c Tn where T, is the melt point of the powder material. The driving force for binding is a physical diffusion of metal atoms from one particle to another. Since this is a slow phenomenon it makes out the main drawback of SSS for SMS. Experiments show that a laser beam particle interaction time of less than 1 ms is far too short to initiate sintering. Liquid Phase Sintering (LPS) is a much faster sinleddensification mechanism [Z]. The basic material here consists of a mixture of two metal powders: a high melting point metal, called the structural metal, and a low melting point metal, called the binder. Applying heat to the system causes the binder to melt and to flow into the pores formed by the non-molten particles. The main advantage of LPS is the very fast initial binding. This binding is based on capillary forces which might be very high: the reaction speed in this stage is determined by the kinetics of the solid-melt transformation. This transformation is orders of magnitudes faster than physical diffusion [a]. Figure 3 gives a qualitative view of the amount of sintering reaction as a function of time. This curve shows three different stages: the rearrangement (melt and capillary penetration), solution precipitation (grain growth by migration of atoms of the structural metal through the melt) and a solid state sintering (final densification). The SMS process only invokes the rearrangement mechanism. Once the binder metal is molten and spread out into the solid lattice. the system cools down (because the moving laser beam no longer feeds energy into the sys-
4
conventional LPS cor
'
.-
stage 2
liqud phase surlenng
tem) and the situation is frozen. This results in green parts with enough mechanical strength for further postprocessing. In case a thermal post process is applied, the product will be heated up again and the densification will continue following the dotted curve of figure 3. A general remark concerns the post-treatments, which require an interconnected porosity in the green part. This means that the pores are connected by means of micro channels allowing the evacuation of air to the outside or the flow of liquid metal to the inside. A complete melt may shut off these micro channels making the densification of remaining porosities impossible (figure 2). 3. ExamDles of Workina Powder Systems This paragraph gives evidence that Nd:YAG laser beam initiated LPS is possible, that the extent of the reactions can be controlled and that conventionally used powder combinations are also applicable in the SMS process. These examples also show the broad range of various parameters that must be appropriately set in order to get the required binding. 3.1 Fe-Cu Dowder mixture Fe-Cu is a well-described powder mixture for LPS [2,6] with attractive mechanical properties: the hard Fe lattice is dispersed in a ductile Cu matrix. New is the laser initiation of the binding reaction. Previous studies have shown that a mixture ratio of 20 to 30 wt% Cu is necessary [l 11. This arises from the relative high reflectivity of the Cu for laser light compared to Fe, meaning that, if no
LPS in SMS
bulk liquid phase ri~itenngof green SMS pan (stages 2 6 1)
sdidificafionof i n h l bindinp !hat ~ s u l l rm g m n SMS part
Figure 3: Liquid Phase Sintering shows three stages. from which the first is invoked during laser treatment in SMS. 184
Figure 2: Cross section of a melt track realised by scanning a Nd:YAG laser beam over a Cu powder bec. Note the pores in the centre of the track.
Figure 4: Nd:YAG laser initiated LPS in Fe-Cu mixture: white spots are Fe-particles, gray phase is Cu-binder and black area's are pores.
Line Width Lasar
. 1 Powsr(W)
. . . . ,
.
,
.
,
-
.
.. . . . .. . .. . . . . . . . . . . . . . . . I
.
~
-40
., . , . .. . . . . . . . . . . .
-4-50
60
......
.- . .
. . . . . . . .I
0.4
. . . . . . . . . . . . . . . . . .
0.2
. . . . . . . . . . '.
'+30
.
\ y Layer Thickness
. .. .
...
. . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
. .
.
.
. . . .
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. . . . . .
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I
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1.4 ,
m
~
N
o
m
o l
O
c
b
Q
o
)
o
Scan speed ( m d s )
Figure 5: Experimentally determinded relationship between laser parameters (laser power and scan speed) and line width (I) and layer thickness (r)
special precautions are taken. Fe tends to melt before the Cu particles do what results in a complete melt. The same study revealed a second constraint to the powder particle size distribution: relative smaller Cu-particles enable a better Cu melt distribution. Both constraints result in following powder mixture: Fe (60-90 pm), 30 wt% Cu (25-40 um). The reaction type occurred during laser interaction is determined by light microscopy of sections along the tracks. Setting the right laser parameters (combination of laser power and scan speed) results in laser initiated LPS as represented by figure 4. This picture reveals spherical Fe-particles distributed in the Cu-matrix. but simultaneously a large degree of porosity. This means that no densification has taken place. This observation has also been verified by density measurements, showing a density of 50 to 55 wt%, what corresponds to the loose powder density. The green samples are typically brittle. This brittleness arises from oxides that do exist in the powders or that are formed during laser heating. To reduce the degree of oxidation, an inert atmosphere is applied in the reaction chamber by evacuating the chamber to 1 mbar and tlushing it with Argon. This allows the creation of useful green parts with limited mechanical properties. A thermal post treatment of the green samples results in a consolidation of the metallographic bindings with related improved mechanical strength. The post treatment consists in heating the samples in ti-atmosphere to 1100°C for 30 minutes. This treatment does not significantly decrease the porosity, what is reflected by a linear shrinkage of just 1 to 2 %. Two other sample properties are highlighted: the track width, being the width of a single line, and the track thickness. Both properties are related to the laser power (&) and the scan speed ( Y I limited to 120 mm/s by the setup), which define the energy density according to:
where 2.W=0.8 mm is the beam diameter on the powder surface. Figure 5 shows the experimentally determined relationships. From these results, it follows that the line width decreases with increasing scan speed until a width of 0.6 mm is reached. This corresponds to the zone in the laser spot where enough energy exists to melt the Cu
binder. At lower scan speeds. the applied heat is transferred to powder particles that are not directly irradiated by the laser beam. So, in order to accurately control the width of the laser tracks, it suffices to select a scan speed for which no heat transfer occurs in the XY-plane. At these scan speeds, the width is in a limited way dependent on the laser power: if not enough power is applied, no binding reaction occurs. In the other case, the laser power may vary up to 10 % without significant influence on the line width. Similar tendencies are valid for the layer thickness, that decreases with increasing speed and decreasing power level. However, the dependence on power is more pronounced, meaning that heat penetration in the Z-direction is several powder particles thick. It allows to control the layer thickness by varying the laser power for a given scan speed in order to get binding between successive layers. Notice that only ihe measurement points with the lowest energy density give rise to pure LPS while the others show a more or less significant degree of melt. This melting explains the improved heat penetration in the 2-direction compared to the XY-plane. 3.2 Cu-coated Fe oowder The Fe-Cu powder blend has some important disadvantages. Special care must be taken to avoid the Feparticles to melt, a homogeneous distribution of the Cu particles in the blend is difficult due to gravitational segregation and oxidation of Fe worsens capillary action (wetting). The most appropriate way to overcome these problems consists in coating the Fe-particles by Cu. The coating has been applied through liquid phase electroless deposition [l]. Figure 6-a shows an etched cross section of a particle revealing a homogeneous layer of Cu over the Fe-core. Note also some impurities in the interface between both metals originating from the chemical process. This powder givfs rise to a more uniform binding, compared to the blend, what is expected since no capillary action is necessary for the distribution of Cu-melt (figure 6-b). These samples also show a higher degree of Fe dispersed into the Cu-matrix what should result in improved mechanical properties.
185
Figure 6: A wide range of material combinations can be treated by SMS: (a) shows a cross section of a Cu-coated Fe powder while (b) is a laser sintered sample of this powder. (c) represents a WC-Co smtered sample. Track width and thickness vary with laser power and scan speed in the same way as the blend, but the settings are more critical. If the energy density is to high, heavy reactions occur by which material is erupted out of the powder surface and no bindings are formed. These reactions are caused by sublimation of the impurities in the interface between coating and core material. 3.3 WC-Co Dowder A third example is a combination of a ceramic powder WC (prismatic, 225 pm) and a metal binder (Co. irregular. 30 wt%, 20 um). The high melt point of WC and its chemical stability make that this material is not affected by the laser. Figure 6-c shows a light microscopy of a cross section in a sample. It reveals the nice wetting action of the Co melt, with just few pores (centre of photograph): the WC particles are well embedded in the matrix what gives the samples typical hard-metal properties. The Co matrix is characterised by a high degree of oxidation, resulting in brittle samples. A better control of the reaction atmosphere may overcome this problem. Once again, this powder blend does perform in the same way as the Cu-based powder systems with respect to scan speed and laser power, although the average energy density is shifted to a higher level. 4. Conclusions This paper has explained two different approaches to selectively bind metal powders. In case no continuous powder feed is applied, sintering is preferred over melting. Liquid phase sintering is most attractive due to the fast initial binding realised by capillary action. The examples, covered by the paper, prove that laser initiation of LPS is feasible, and that a broad range of material combinations is possible. However, special care must be paid to the correct blending of the powder systems and to the problems of laser-powder interaction and oxidation during heating. Coating of the structural powders with the binder material is proposed to overcome these problems. A successful example of Cu-coated Fe-powder is proposed. Also other powder combinations like WC-Co prove to be usefull in the SMS process. Finally, two parameters appear to allow dimensional control of the tracks: scan speed determines the width of the tracks, while laser power determines the thickness of the tracks for a given scan speed.
186
5. References Gabe. D.R.(1978) Principles of Metal Surface Treatment and Protection. 2M ed., Pergamon Press, Oxford. German, A.M. (1985) Liquid Phase Sinlering. Plenum Press, New York. Konig, W., Celiker, T., Herfurth, H.-J. (1993) Approaches to Prototyping of Metallic Parts. Proc. p d Int. Eur. Conf. on Rapid Prototyping, pp 303-316. Kruth. J.P. (1991) Material lncress Manufacturing by Rapid Prototyping Techniques. ClRP Annals Vol 40, 2 pp 603-614. Kruth. J.P. (1994) Advances in Physical and Chemical Machining. Proc. f hInt. Conf. on Production/ Precision Engineering, Chiba, pp K62-K76. Lenel, F.V. (1978) Powder Metallurgy, Principles and Applications. Metal Powder Industries Federation, New Jersey. Murphy, M.L.; Steen, W.M.; Lee, C. (1994) The Rapid Manufacturing of Metallic Components by Laser Surface Cladding. Proc. of the 2ghClRP Seminar on Mfg. Systems LANE'94, Erlangen, pp. 803-814. Porter, D.A., Easterling, K.E. (1992) Phase Transformations in Metals and Alloys. Chapman & Hall, 2nded., London. Sachs, E.; et aL(1993) 3D Printing, an Additive Process. Cirp Annals V0142l1, pp 257-260. [ l o ] Van der Schueren, 8.; Kruth, J.P. (1995) Powder Deposition in Selective Metal Powder Sintering. Proc. 4n Eur. Conf. on Rapid Prototyping and Manufacturing, Nottingham. pp. 197-211. [ l l ] Van der Schueren, B.; Kruth, J.P. (1994) Laser Based Selective Metal Powder Sintering: a Feasibility Study. Proc. of the 2dh ClRP Seminar on Mfg. Systems LANE'94, Erlangen, pp. 793-802. [12]Van der Schueren, B.; Kruth, J.P. (1995) Design Aspects of a "Selective Metal Powder Sintering" Apparatus. Proc. ISEM-XI International Symposium for Electro-Machining. Lausanne, pp 665-672. [ 131 Van der Schueren, B.( 1994) Selective Metal Powder Sintering: a Metal Accretion Manufacturing System. EARP Newletter, No 4, August, pp. 6-7.