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Investigations on the Influence of Operating Parameters on the Lubricated Wear of SLS Materials
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 11319–11325
www.materialstoday.com/proceedings
ICMMM - 2017
Investigations on the Influence of Operating Parameters on the Lubricated Wear of SLS Materials C. D. Naijua, P. M. Anila, Arjun Mahadevana and Joseph Kurianb a
b
School of Mechanical Engineering, VIT University, Vellore 14, Tamil Nadu, India Department of Management Engineering, Technical University of Denmark, Lyngby, Denmark
Abstract Selective laser sintering is a layered manufacturing method that fuses the powder material with the help of a laser into a physical part directly from a 3D model without intermediate tooling. Iron powder of grain size 20 μm with a resin coating of 3μm was sintered by selective laser sintering method, which was later infiltrated with bronze particles. To find out the behavior of the sintered ferrous-bronze in functional applications, it was necessary to find out the wear rate of this material under real world operating conditions. Tests were conducted using a linearly reciprocating ball-on-flat sliding wear testing machine at different temperatures in a lubricated environment. Wear rates were determined under different operating conditions. Micro structural studies of the samples were conducted using Field Emission Scanning Electron Microscope (FESEM). EDAX analysis was done to study the chemical changes occurred during tests. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: SLS; Inclusion; Pores; Sintering; Wear.
1. Introduction Rapid Prototyping (RP) is widely used to fabricate components without intermediate tooling directly from computer aided three dimensional data. The main advantage of using this technology is the speed at which these modeling systems can generate complicated three-dimensional shapes. Ferrous-bronze material has been in use for decades for making collars and various types of bushes for automotive and industrial applications. The research objective was to study the material behavior under various temperatures in a lubricated environment.
2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
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C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
Selective Laser Sintering (SLS) Selective laser sintering (SLS) is one of the additive manufacturing technique to fabricate physical part using different kinds of materials such as metal, polymers, ceramics etc by fusing these by the help of a laser source using CAD data. The powder is usually sintered on a powder bed which can be lowered by a layer thickness. The powder get fused by the heat source on each layer as per the 2D cross sectional data. This process continues till the final cross section is sintered. In order to make layer manufacturing in to regular production process, alternative to conventional manufacturing process. Several attributes of manufacturing parts are to be taken into consideration such important attributes of wear rate are vital from application point of view. Therefore an investigation have devoted at considerable attention of these aspects. Conventionally processed material was evaluated for SLS/HIP processed material for mechanical properties such as hardness and tensile by microstructional studies [1]. Laser sintering of metallic powders with different melting points(low and high) has shown that for one-component solid state sintering is less than that of two-metal sintering by liquid phasing, which was analyzed by beam technology for 3D micro structuring [2]. When the components are built when the opening faces downwards, distortion of the part occurs due to the formation of trapped material inside RP parts. The component opening can be reoriented such that the opening faces upwards so as to avoid the trapped volume for an SLS part [3]. The hardness and density of components diminishes by increasing layer thickness and hatching distance [4]. The temperature of the infiltration chamber should be higher so as to get a better infiltration by maintaining the melting point of the infiltrating liquid slightly above [5]. The friction coefficient and wear resistance of laser treated surfaces are good and the wear rate is significantly less [6]. Comparing the wear resistance of materials used for sintering using SLS and SLM, SLS has better wear resistance and surface properties [7]. Scan spacing has more influence on wear compared to slice thickness and infiltration for a study carried for sliding wear for metallic SLS components [8]. While comparing the reciprocating wear behavior, Laser power has more influence compared to other process parameters of test specimens manufactured by selective laser sintering (SLS) [9]. Another study on reciprocating wear behavior shows that sintering speed had more influence on wear as compared to other process parameters for direct metal laser sintering (DMLS) components. Better bonding is observed for lower sintering speed and in turn gives better wear rate [10]. 2. Analysis Roughness and Wear tests A contact type of surface roughness tester was used to measure the roughness of the flat samples. The roughness values of the samples used are presented in Table 1. Wear testing was carried out on these samples using a reciprocating wear testing machine The machine consists of reciprocating arm to which the ball is fixed and a bottom holder for holding the lower flat specimen. The upper ball reciprocates over the lower specimen. Thus wear either occurs to the ball or to the specimen or for both. A data acquisition system that interfaces the machine with a computer collects the real time data. The test parameters like load to be applied, stroke length, frequency and duration of test are presented in Table 2. Table1. Roughness values of the samples
Component No.
Slice thickness (mm)
Scan spacing (mm)
Laser Power (W)
Surface roughness (Ra) (microns)
1
0.08
0.08
55
0.4820
2
0.08
0.08
55
0.4543
3
0.08
0.08
55
0.5264
4
0.08
0.08
55
0.5226
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
11321
Table 2. Test Parameters
Parameter
Load (N)
Frequency (Hz)
Stroke Length (mm)
Duration (min.)
Value
40
10
15
120
Initial mass of the ball and flat samples were found out using a high precision analytical balance. Samples were fixed on the machine and the test parameters were set. Pure SAE 30 base oil was used as the lubricant. The lubricant was applied at the contact zone and test was commenced. Wear depth, coefficient of friction and temperature were noted. After completion of the test the samples were removed and the mass loss was found out. Wear Calculation
(
)=
( )=
− (
=
)
( )×
=
×
( ) ℎ(
) × 0.002
The calculated values are shown in Table 3. Table 3. Wear rate of the components
No.
Oil Temp (°C)
Initial Mass (gm)
Final Mass (gm)
Mass loss (gm)
Wear volume (mm3)
Wear rate (mm3/ m)
1
32
20.8786
20.8775
0.0011
0.11765
5.447E-05
2
80
21.1139
21.1137
0.0002
0.02353
1.089E-05
3
130
20.3977
20.3954
0.0023
0.27059
1.253E-04
4
180
21.4212
21.4194
0.0018
0.21176
9.804E-05
Microstructural Analysis The component was polished using emery paper of grades 200, 400,600, 800, 1000 and 1200. Then the component was finally polished in diamond polishing machine using diamond paste with acetone. The component was kept in scanning electron microscope chamber for viewing microstructures of the components. The microstructure was obtained with different levels of magnification. Image Analyzer was used to take photographs of microstructure. As shown in Fig. 1, scanning electron microscope images (SEM) was taken on three different points on the specimen. Images were taken at three points on the specimen as shown in the Fig. 1.
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C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
Fig.1. Schematic Diagram of the points on the Component where the SEM images were obtained
3. Results and Discussions The coefficient of friction was found to be increasing with lubricant temperature. The explanation for this is the decrease in viscosity if the lubricant with increasing in temperature. Wear depth was found to be less for the sample tested at 80°C. For higher temperatures, the wear rate was found to be high. These variations are shown in Fig. 2 and Fig. 3.
Fig. 2. Variation of coefficient of friction with respect to time
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
11323
Fig. 3. Variation of wear depth with respect to time
Microstructural Analysis Two types of pores (i) rounded pores and (ii) irregular shaped cavities were generally observed in the sintered component. Neck formation on the Fe particle, regions of bronze infiltration and irregular shaped interconnected cavities was observed which are due to insufficient infiltration. Micro-structural features observed on sample 1 are shown in Fig. 4. In addition, coarse oxide inclusions were also observed. The images show oxide inclusions at an increasing pattern from component 2 having the least and component 3 have the highest oxide inclusion pattern. Component 3 had the maximum wear as the sintering was irregular, non uniform and contained large number of pores whereas component 2 had the least wear as the sintering was regular comparatively and had lesser pores. Irregular shaped pores in Fe-bronze are observed in SLS parts which are regions of weaknesses in the microstructure and are considered as defects. The presence of oxides inclusions in the structure can considerably degrade the mechanical properties of the material.
Fig.4. Micro-structural features from Sample 1
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Fig.5. Interconnected pores on Sample 3
The un-etched polished specimens reduced under all processing conditions clearly reveal the presence of oxide inclusions under the scanning electron microscope. Their presence was confirmed by the EDAX analyses that clearly revealed the presence of a high concentration of oxygen in those areas. Though these inclusions were found both in the interior as well as near the edges, volume fraction was found to be different in all the specimens. It was found to be highest in specimen no.3. The oxide inclusions observed in the structure have been shown in Fig.5. It is observed that inclusions often occurred as a cluster and at times formed a network. At higher magnifications they were found to have micro cracks that would have resulted due to thermal expansion. It must understood that inclusions in SLS parts can arise due to (a) their presence in the starting powder itself, (b) due to the presence of the occluded gas in the powder, (c) impurities levels the protecting gas cover or leakage occurring in it during laser sintering. EDAX Analysis Fig. 6. shows the EDS graph of component 2 which gave the lowest wear rate. The EDS data shows multiple peaks of Fe confirming its presence both in elemental and compound state.
Fig. 6. EDAX data of Sample 2
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
11325
This validates the slight oxide formation observed in the SEM images of component 2. The result of the EDAX analysis confirmed the oxide patterns from the SEM images of component 3; hence explaining the high wear rate and oxide formation. 4. Conclusions Reciprocating wear tests were conducted on components made by SLS manufacturing technique. Coefficient of friction was found to be directly proportional to temperature in this case under lubricated conditions. Material defects like cavities, segregation and thermal cracks were found to affect wear rate. Components having networks of inter-connected cavities gives higher wear rate. Sintering quality affects wear rate. Components having improper sintering had higher wear rate. Oxide formation was proportional to temperature during lubricated wear contact. Oxides of more elements formed at higher lubricant temperature. The wear data obtained was generally satisfactory showing SLS is an advantageous manufacturing technique. Temperature had a direct influence on wear rate. Individual pores were found in the SEM images. These pores increase the wear rate of the material. Infiltration was not uniform in some of the areas. Improper infiltration affects sintering quality and hence the wear performance of the component. However, there is huge scope of further studies on the effect of operating parameters on lubricated reciprocating wear testing of parts produced by Selective Laser Sintering. The values of the process parameters should be varied more to get good results. Lubricants of different grades should be used during wear tests for finding the influence of lubricant grade on wear. The process parameters like layer thickness, laser power, grain size, which were kept constant, should be varied while conducting experiments since these parameters can also influence the wear rate. Xray diffraction should be done on the wear track to identify the oxides formed during wear test. Load, which was kept constant, should be varied along with temperature to mimic real life conditions more closely. Acknowledgements The authors wish to acknowledge the support of school of mechanical engineering, VIT University, Vellore, India. We also thank PSG Tech, Coimbatore, India for their valuable support to manufacture the components. References [1] Suman Das, Martin Wohlert, Joseph J. Beaman, David L. Bourell, Producing Metal Parts with Selective Laser Sintering/Hot Isostatic Pressing, Journal of Materials.50 (1998) 17-20. [2] Y. P. Kathuria, Microstructuring by selective laser sintering of metallic powder, Surface and Coatings Technology. 116–119 (1999) 643–647. [3] B Y Ang, C. K Chua, Z. H Du, Study of Trapped Material in Rapid Prototyping Parts. The International Journal of Advanced Manufacturing Technology. 16(2000)120-130. [4] A. N Chatterjee, Sanjay Kumar, P Saha, P. K Mishra, A Roy Choudhury, An experimental design approach to selective laser sintering of low carbon steel, Journal of Materials Processing Technology. 136 (2003) 151–157. [5] J. DuckJ, F. Niebling, T. Neeße, A. Otto, Infiltration as post-processing of laser sintered metal parts, Powder Technology. 145(2004) 62–68. [6] J. Takacs, L. Toth, F. Franek, Pauschitz T. Sebestyen, Friction and wear measurements of laser sintered and coated parts, Wear. 256(2004). [7] S. Kumar, J.P Kruth, Wear Performance of SLS/SLM Materials, Advanced Engineering Materials. 10 (2008) 750–753. [8] C. D.Naiju, K. Annamalai, Joseph Kurian, Tarun Thomas George, Study on the Effect of Process Parameters on Sliding Wear Behavior of Components Produced by Selective Laser Sintering (SLS), Advanced Materials Research. 488-4892012)1419-1423. [9] C. D.Naiju, P. K Manoj, Tarun Thomas George, Joseph Kurian, Study on the Effect of Process Parameters on Reciprocating Wear Behavior of Components Produced by Selective Laser Sintering (SLS), Advanced Materials Research.. 488-489 (2012) 1424-1428. [10] C. D.Naiju, M. Adithan, P. Radhakrishnan, Study on the Effect of Process Parameters on the Reciprocating Wear Behavior of Components Produced by Direct Metal Laser Sintering (DMLS), Advanced Materials Research. 488 – 489(2012) 1424-1428.
ScienceDirect Materials Today: Proceedings 5 (2018) 11319–11325
www.materialstoday.com/proceedings
ICMMM - 2017
Investigations on the Influence of Operating Parameters on the Lubricated Wear of SLS Materials C. D. Naijua, P. M. Anila, Arjun Mahadevana and Joseph Kurianb a
b
School of Mechanical Engineering, VIT University, Vellore 14, Tamil Nadu, India Department of Management Engineering, Technical University of Denmark, Lyngby, Denmark
Abstract Selective laser sintering is a layered manufacturing method that fuses the powder material with the help of a laser into a physical part directly from a 3D model without intermediate tooling. Iron powder of grain size 20 μm with a resin coating of 3μm was sintered by selective laser sintering method, which was later infiltrated with bronze particles. To find out the behavior of the sintered ferrous-bronze in functional applications, it was necessary to find out the wear rate of this material under real world operating conditions. Tests were conducted using a linearly reciprocating ball-on-flat sliding wear testing machine at different temperatures in a lubricated environment. Wear rates were determined under different operating conditions. Micro structural studies of the samples were conducted using Field Emission Scanning Electron Microscope (FESEM). EDAX analysis was done to study the chemical changes occurred during tests. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: SLS; Inclusion; Pores; Sintering; Wear.
1. Introduction Rapid Prototyping (RP) is widely used to fabricate components without intermediate tooling directly from computer aided three dimensional data. The main advantage of using this technology is the speed at which these modeling systems can generate complicated three-dimensional shapes. Ferrous-bronze material has been in use for decades for making collars and various types of bushes for automotive and industrial applications. The research objective was to study the material behavior under various temperatures in a lubricated environment.
2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
11320
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
Selective Laser Sintering (SLS) Selective laser sintering (SLS) is one of the additive manufacturing technique to fabricate physical part using different kinds of materials such as metal, polymers, ceramics etc by fusing these by the help of a laser source using CAD data. The powder is usually sintered on a powder bed which can be lowered by a layer thickness. The powder get fused by the heat source on each layer as per the 2D cross sectional data. This process continues till the final cross section is sintered. In order to make layer manufacturing in to regular production process, alternative to conventional manufacturing process. Several attributes of manufacturing parts are to be taken into consideration such important attributes of wear rate are vital from application point of view. Therefore an investigation have devoted at considerable attention of these aspects. Conventionally processed material was evaluated for SLS/HIP processed material for mechanical properties such as hardness and tensile by microstructional studies [1]. Laser sintering of metallic powders with different melting points(low and high) has shown that for one-component solid state sintering is less than that of two-metal sintering by liquid phasing, which was analyzed by beam technology for 3D micro structuring [2]. When the components are built when the opening faces downwards, distortion of the part occurs due to the formation of trapped material inside RP parts. The component opening can be reoriented such that the opening faces upwards so as to avoid the trapped volume for an SLS part [3]. The hardness and density of components diminishes by increasing layer thickness and hatching distance [4]. The temperature of the infiltration chamber should be higher so as to get a better infiltration by maintaining the melting point of the infiltrating liquid slightly above [5]. The friction coefficient and wear resistance of laser treated surfaces are good and the wear rate is significantly less [6]. Comparing the wear resistance of materials used for sintering using SLS and SLM, SLS has better wear resistance and surface properties [7]. Scan spacing has more influence on wear compared to slice thickness and infiltration for a study carried for sliding wear for metallic SLS components [8]. While comparing the reciprocating wear behavior, Laser power has more influence compared to other process parameters of test specimens manufactured by selective laser sintering (SLS) [9]. Another study on reciprocating wear behavior shows that sintering speed had more influence on wear as compared to other process parameters for direct metal laser sintering (DMLS) components. Better bonding is observed for lower sintering speed and in turn gives better wear rate [10]. 2. Analysis Roughness and Wear tests A contact type of surface roughness tester was used to measure the roughness of the flat samples. The roughness values of the samples used are presented in Table 1. Wear testing was carried out on these samples using a reciprocating wear testing machine The machine consists of reciprocating arm to which the ball is fixed and a bottom holder for holding the lower flat specimen. The upper ball reciprocates over the lower specimen. Thus wear either occurs to the ball or to the specimen or for both. A data acquisition system that interfaces the machine with a computer collects the real time data. The test parameters like load to be applied, stroke length, frequency and duration of test are presented in Table 2. Table1. Roughness values of the samples
Component No.
Slice thickness (mm)
Scan spacing (mm)
Laser Power (W)
Surface roughness (Ra) (microns)
1
0.08
0.08
55
0.4820
2
0.08
0.08
55
0.4543
3
0.08
0.08
55
0.5264
4
0.08
0.08
55
0.5226
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
11321
Table 2. Test Parameters
Parameter
Load (N)
Frequency (Hz)
Stroke Length (mm)
Duration (min.)
Value
40
10
15
120
Initial mass of the ball and flat samples were found out using a high precision analytical balance. Samples were fixed on the machine and the test parameters were set. Pure SAE 30 base oil was used as the lubricant. The lubricant was applied at the contact zone and test was commenced. Wear depth, coefficient of friction and temperature were noted. After completion of the test the samples were removed and the mass loss was found out. Wear Calculation
(
)=
( )=
− (
=
)
( )×
=
×
( ) ℎ(
) × 0.002
The calculated values are shown in Table 3. Table 3. Wear rate of the components
No.
Oil Temp (°C)
Initial Mass (gm)
Final Mass (gm)
Mass loss (gm)
Wear volume (mm3)
Wear rate (mm3/ m)
1
32
20.8786
20.8775
0.0011
0.11765
5.447E-05
2
80
21.1139
21.1137
0.0002
0.02353
1.089E-05
3
130
20.3977
20.3954
0.0023
0.27059
1.253E-04
4
180
21.4212
21.4194
0.0018
0.21176
9.804E-05
Microstructural Analysis The component was polished using emery paper of grades 200, 400,600, 800, 1000 and 1200. Then the component was finally polished in diamond polishing machine using diamond paste with acetone. The component was kept in scanning electron microscope chamber for viewing microstructures of the components. The microstructure was obtained with different levels of magnification. Image Analyzer was used to take photographs of microstructure. As shown in Fig. 1, scanning electron microscope images (SEM) was taken on three different points on the specimen. Images were taken at three points on the specimen as shown in the Fig. 1.
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C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
Fig.1. Schematic Diagram of the points on the Component where the SEM images were obtained
3. Results and Discussions The coefficient of friction was found to be increasing with lubricant temperature. The explanation for this is the decrease in viscosity if the lubricant with increasing in temperature. Wear depth was found to be less for the sample tested at 80°C. For higher temperatures, the wear rate was found to be high. These variations are shown in Fig. 2 and Fig. 3.
Fig. 2. Variation of coefficient of friction with respect to time
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
11323
Fig. 3. Variation of wear depth with respect to time
Microstructural Analysis Two types of pores (i) rounded pores and (ii) irregular shaped cavities were generally observed in the sintered component. Neck formation on the Fe particle, regions of bronze infiltration and irregular shaped interconnected cavities was observed which are due to insufficient infiltration. Micro-structural features observed on sample 1 are shown in Fig. 4. In addition, coarse oxide inclusions were also observed. The images show oxide inclusions at an increasing pattern from component 2 having the least and component 3 have the highest oxide inclusion pattern. Component 3 had the maximum wear as the sintering was irregular, non uniform and contained large number of pores whereas component 2 had the least wear as the sintering was regular comparatively and had lesser pores. Irregular shaped pores in Fe-bronze are observed in SLS parts which are regions of weaknesses in the microstructure and are considered as defects. The presence of oxides inclusions in the structure can considerably degrade the mechanical properties of the material.
Fig.4. Micro-structural features from Sample 1
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C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
Fig.5. Interconnected pores on Sample 3
The un-etched polished specimens reduced under all processing conditions clearly reveal the presence of oxide inclusions under the scanning electron microscope. Their presence was confirmed by the EDAX analyses that clearly revealed the presence of a high concentration of oxygen in those areas. Though these inclusions were found both in the interior as well as near the edges, volume fraction was found to be different in all the specimens. It was found to be highest in specimen no.3. The oxide inclusions observed in the structure have been shown in Fig.5. It is observed that inclusions often occurred as a cluster and at times formed a network. At higher magnifications they were found to have micro cracks that would have resulted due to thermal expansion. It must understood that inclusions in SLS parts can arise due to (a) their presence in the starting powder itself, (b) due to the presence of the occluded gas in the powder, (c) impurities levels the protecting gas cover or leakage occurring in it during laser sintering. EDAX Analysis Fig. 6. shows the EDS graph of component 2 which gave the lowest wear rate. The EDS data shows multiple peaks of Fe confirming its presence both in elemental and compound state.
Fig. 6. EDAX data of Sample 2
C D Naiju et. al/ Materials Today: Proceedings 5 (2018) 11319–11325
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This validates the slight oxide formation observed in the SEM images of component 2. The result of the EDAX analysis confirmed the oxide patterns from the SEM images of component 3; hence explaining the high wear rate and oxide formation. 4. Conclusions Reciprocating wear tests were conducted on components made by SLS manufacturing technique. Coefficient of friction was found to be directly proportional to temperature in this case under lubricated conditions. Material defects like cavities, segregation and thermal cracks were found to affect wear rate. Components having networks of inter-connected cavities gives higher wear rate. Sintering quality affects wear rate. Components having improper sintering had higher wear rate. Oxide formation was proportional to temperature during lubricated wear contact. Oxides of more elements formed at higher lubricant temperature. The wear data obtained was generally satisfactory showing SLS is an advantageous manufacturing technique. Temperature had a direct influence on wear rate. Individual pores were found in the SEM images. These pores increase the wear rate of the material. Infiltration was not uniform in some of the areas. Improper infiltration affects sintering quality and hence the wear performance of the component. However, there is huge scope of further studies on the effect of operating parameters on lubricated reciprocating wear testing of parts produced by Selective Laser Sintering. The values of the process parameters should be varied more to get good results. Lubricants of different grades should be used during wear tests for finding the influence of lubricant grade on wear. The process parameters like layer thickness, laser power, grain size, which were kept constant, should be varied while conducting experiments since these parameters can also influence the wear rate. Xray diffraction should be done on the wear track to identify the oxides formed during wear test. Load, which was kept constant, should be varied along with temperature to mimic real life conditions more closely. Acknowledgements The authors wish to acknowledge the support of school of mechanical engineering, VIT University, Vellore, India. We also thank PSG Tech, Coimbatore, India for their valuable support to manufacture the components. References [1] Suman Das, Martin Wohlert, Joseph J. Beaman, David L. Bourell, Producing Metal Parts with Selective Laser Sintering/Hot Isostatic Pressing, Journal of Materials.50 (1998) 17-20. [2] Y. P. Kathuria, Microstructuring by selective laser sintering of metallic powder, Surface and Coatings Technology. 116–119 (1999) 643–647. [3] B Y Ang, C. K Chua, Z. H Du, Study of Trapped Material in Rapid Prototyping Parts. The International Journal of Advanced Manufacturing Technology. 16(2000)120-130. [4] A. N Chatterjee, Sanjay Kumar, P Saha, P. K Mishra, A Roy Choudhury, An experimental design approach to selective laser sintering of low carbon steel, Journal of Materials Processing Technology. 136 (2003) 151–157. [5] J. DuckJ, F. Niebling, T. Neeße, A. Otto, Infiltration as post-processing of laser sintered metal parts, Powder Technology. 145(2004) 62–68. [6] J. Takacs, L. Toth, F. Franek, Pauschitz T. Sebestyen, Friction and wear measurements of laser sintered and coated parts, Wear. 256(2004). [7] S. Kumar, J.P Kruth, Wear Performance of SLS/SLM Materials, Advanced Engineering Materials. 10 (2008) 750–753. [8] C. D.Naiju, K. Annamalai, Joseph Kurian, Tarun Thomas George, Study on the Effect of Process Parameters on Sliding Wear Behavior of Components Produced by Selective Laser Sintering (SLS), Advanced Materials Research. 488-4892012)1419-1423. [9] C. D.Naiju, P. K Manoj, Tarun Thomas George, Joseph Kurian, Study on the Effect of Process Parameters on Reciprocating Wear Behavior of Components Produced by Selective Laser Sintering (SLS), Advanced Materials Research.. 488-489 (2012) 1424-1428. [10] C. D.Naiju, M. Adithan, P. Radhakrishnan, Study on the Effect of Process Parameters on the Reciprocating Wear Behavior of Components Produced by Direct Metal Laser Sintering (DMLS), Advanced Materials Research. 488 – 489(2012) 1424-1428.