Introduction. Keywords Lasers, Sintering, Coatings technology, Metals. Paper type Research paper

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1 Preparation and selective laser sintering of nylon-coated metal powders for the indirect SLS process State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan, China Abstract Purpose The purpose of this paper is to report a new method, the dissolution-precipitation process, to prepare nylon-coated metal powders for the indirect selective laser sintering (SLS) process. Design/methodology/approach The nylon-12 coated carbon steel powders were prepared by the dissolution-precipitation process. The powder characteristics are examined by scanning electron microscope (SEM) and laser diffraction particle size analysis. The effect of the applied laser energy density on the three-point bend strength and dimensional accuracy of the SLS specimens are studied. The influence of nylon-12 content on the bend strength are also investigated. Findings The SEM and laser diffraction particle size analysis results indicate that the steel particles are well coated by nylon-12 resin. The bend strength of the SLS specimens increases with increasing the applied energy density until it reaches a maximum value, and then further increasing energy density will cause the decrease in the bend strength. The bend strength of the SLS specimens increases with increasing the nylon-12 content over the investigated range. The dimensional errors in the X-Y-and Z-directions are all increased with the increase in energy density. Research limitations/implications This paper only concerns the preparation and SLS of the coated powders. Further investigations are planned into post-processing, such as binder decomposition and high-temperature sintering, of the green parts made from the coated powders. Originality/value This paper provides a useful method for preparing nylon-coated metal powders for making metal parts by the indirect SLS process. Keywords Lasers, Sintering, Coatings technology, Metals Paper type Research paper Introduction Selective laser sintering (SLS) is a powder-based rapid prototyping process, which directly forms solid components according to a three-dimensional Computer-aided design (CAD) model by selective sintering of successive layers of powdered raw materials (Kumar, 2003; Kruth et al., 2003). While the capability of SLS to produce functional objects directly from metals is still under development, indirect methods of producing functional objects from metals have been widely used. In an indirect method of SLS process, green parts can be formed through selectively fusing a polymer binder to bond metal particles with a small power laser. The polymer binder is subsequently removed, usually by thermal processes, and the polymer-free part is then sintered or further processed to form a finished part with geometric precision that is comparable to that of the green part (Dalgarno and Stewart, 2001; Stewart et al., 1999; Uzunsoy et al., 2003; Shishkovskii and Kupriyanov, 1997; Goode, 2003). At present, two methods have been used to produce polymer binder/metal composite powders for the indirect SLS process. The current issue and full text archive of this journal is available at 15/5 (2009) q Emerald Group Publishing Limited [ISSN ] [DOI / ] The simple method is to mechanically mix metal powders with polymer binder powders. In general, the mixed composite powders are not commercially viable due to problems associated with powder segregation during shipping and with poor binder efficiency (Beaman et al., 1997). The other method is that polymer binders uniformly coat metal powders. The coated composite powders are widely used for the relatively lower binder content and higher binder efficiency. Nowadays, the polymer binders for the indirect SLS process are mainly amorphous polymers such as poly(methyl methacrylate) (PMMA), PMMA-co-n-butyl methacrylate (BMA)) and epoxy. A class of polymer binders, based on PMMA or copolymers of methyl methacrylate and BMA monomers have been developed at the University of Texas at Austin to permit the formation of metal parts by the indirect SLS process (Beaman et al., 1997). PMMA binder emulsions in water are coated on to the inorganic particulate by spray drying a slurry of the particulate in the polymer emulsion. Agglomerated, porous particles are formed by this coating process, which typically contain 20 vol.% polymer. The paper was invited from ICRPM conference This paper is financially supported by Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China ( ) and Postdoctoral Foundation in China ( ). The authors greatly thank the Analytical and Testing Center of HUST for the measurements. Received: 4 October 2008 Revised: 13 November 2008 Accepted: 17 December

2 Preparation and SLS of nylon-coated metal powders The PMMA-based polymer binder has been successfully used to prepare ceramic and metal shapes by the indirect SLS process form a variety of materials including glass, alumina, and silica/zircon mixture, copper, silicon carbide, calcium phosphate and so on (Vail et al., 1996; Subramanian et al., 1995; Evans et al., 2005; Dalgarno and Stewart, 2001; Stewart et al., 1999; Uzunsoy et al., 2003; Goode, 2003). Liu et al. (2006) used an admixture of metal and epoxy resin powders to make metal parts via the indirect SLS process. Epoxy resin content in this admixture is about 4 Wt% (24 vol.%). It is important that the green parts show sufficient mechanical properties to retain the desired shape and dimensions during handling and post-processing. One way to increase green part strength would be simply to increase the amounts of polymer binders. However, as the binders are removed by thermal processes, void spaces are left behind. High contents of polymer binders result in relatively larger amounts of void spaces upon high-temperature sintering, which can lead to unacceptable amounts of shrinkage in the finished part. Another problem with incorporating high content of polymer binders requires longer annealing times to remove the binder, which obviously reduces efficiency and add costs. Therefore, it is desirable to optimize binder contents to achieve enough strength of the green parts (at least 1.7 MPa) for the post-processing processes with a minimum of binder (Beaman et al., 1997). At the same polymer binder content, polymer-coated metal composites should be preferred because the part strength of the coated composites is much higher than that of the mixed composites (Beaman et al., 1997). In addition, strengths of green parts can be improved by selecting appropriate polymer binders which have good interfacial adhesion with meal powders and high strengths of their sintered parts. In this study, nylon-12 resin was used as a polymer binder, and the dissolution-precipitation process was developed to prepare nylon-12 coated carbon steel powders for making metal parts via the indirect SLS process. Experimental Materials The nylon-12 pellets were obtained from Degussa Co in Germany. The carbon steel powder was purchased from Central South University, China and treated with dilute hydrochloric acid before using. The mixed solvent was comprised of 95 wt% ethanol, 4.5 wt% butanone and 0.5 wt% distilled water. Powder preparation The procedure of the dissolution-precipitation process for the nylon-12 coated carbon steel powder (coated powder) was as follows: add the nylon-12 pellets, mixed solvent (1:7 wt/wt) and carbon steel powder into a 10 l reactor; vacuum the reactor and add N 2 gas to protect the reactants from being oxidized; elevate the temperature to 145 o C to resolve the nylon-12 pellets thoroughly; stirring intensively, cool at a speed of 10 o C/h-105 o C or so at which nylon-12 began to precipitate, keep the temperature until precipitation was complete and distill out the mixed solvent. The obtained precipitation materials underwent milling to obtain the finished coated powder product. A mechanically mixed nylon-12/carbon steel powder (mixed powder) was also prepared. The preparing procedure was as following: prepare a neat nylon-12 powder by the dissolution-precipitation process; mix the carbon steel powder and neat nylon-12 powder, and ball-mill the admixture for 2 h. Selective laser sintering process The sintering experiments were carried out in an atmosphere of nitrogen gas using an HRPS-III SLS system made by Huazhong University of Science and Technology (HUST), People s Republic of China. The SLS system was equipped with a continuous wave CO 2 laser (wavelength ¼ 10.6 mm), the power of which could be continuously adjusted from 0 to 50 W. Energy density (v), defined as the relative applied laser energy per unit area, can be calculated by equation (1) (Ho et al., 1999): v ¼ P H v where P is the fill laser power, H is the scan spacing, and v is the laser beam traversing speed. In this work, v was 2,000 mm/s; H was 0.1 mm; P was in the range from 8 to 24 W. Therefore, v was in the range from 0.04 to 0.12 J/mm 2. The part bed temperature was 165 o C and the powder layer thickness was 0.1 mm. All test specimens were fabricated using the scanning method that scans each successive layer in alternate, perpendicular directions. Therefore, a three-point bend specimen ( mm 3 ) will have alternate layers of shortscan vectors, lv ¼ 10 mm, and long-scan vectors, lv ¼ 80 mm. Measurements The microscopic morphologies of the coated powder (1.0 wt% nylon-12), carbon steel powder and bend fractured surface of the green specimen were examined by FEI Quanta 200 environmental scanning electron microscope (SEM). All the samples are sputter coated with gold-palladium to avoid charging. The average particle sizes and particle size distributions of the coated powder (1.0 wt% nylon-12) and carbon steel powder were determined by Master Min laser particle size analyzer, Malvern Instruments Ltd The threepoint bend strengths of the green specimens were tested using a Z010 universal testing machine, Zwick/Roell Corporation in Germany. The three-point bend strengths were tested at the speed of 2 mm/min. The nominal dimension of the threepoint bend specimen was mm 3. For each data point, five specimens were tested and the average value was taken. Dimensional accuracy of the green specimens is represented with the dimensional error S, which is defined as: ð1þ S ¼ A 1 2 A 0 A 0 100% ð2þ where A 0 is the design size given by computer, A 1 is the actual size measured by a propeller micrometer. As shown in Figure 1, the dimension of the specimen for dimensional accuracy test was mm 3. The building orientation was along the Z-direction. Results and discussions Powder characteristics The SEM micrographs of the carbon steel powder and coated powder (1.0 wt% nylon-12) are shown in Figure 2 (a) and (b), respectively. As shown in Figure 2(a), 356

3 Figure 1 Scheme of the specimen for dimensional accuracy test 15 mm Preparation and SLS of nylon-coated metal powders 30 mm 30 mm Figure 2 SEM micrographs of (a) the carbon steel powder; (b) coated powder (1.0 wt% nylon-12) Z Y X it is observed that most particles of the carbon steel powder have a spherical shape and rough surfaces. By comparing Figure 2(a) and (b), it can be found most of the particles of the coated powder become less regular and have rougher surfaces than those of the carbon steel powder. This result indicates that the carbon steel particles are coated by nylon-12 resin in the dissolution-precipitation process. At present, the dissolutionprecipitation process is widely used to prepare neat nylon powders, such as nylon-11 and nylon-12 powders. In this study, this process is used to prepare nylon-12 coated metal powders for making metal parts by the indirect SLS process. By means of organic solvents, nylon-12 resin is uniformly coated on the surfaces of the metal particles. Although organic solvents are used, it does no harm to environment because the whole process is carried out in a closed vessel and solvents are all recovered. Therefore, this study provides a useful method for preparing nylon-coated metal powders. The particle size distributions of the carbon steel powder and coated powder (1.0 wt% nylon-12) obtained from the laser diffraction particle size analysis are shown in Figure 3(a) Figure 3 Particle size distributions of (a) the carbon steel powder; (b) coated powder (1.0 wt% nylon-12) Volume percentage/% (a) Particle size/µm (a) 20 Volume percentage/% (b) Particle size/µm (b) 357

4 Preparation and SLS of nylon-coated metal powders and (b), respectively. From Figure 3, it can be seen that both of the powders have a particle size distribution of 2-29 mm, but the particles with the sizes of 2-5 and 5-8 mm decrease and the particles with the sizes of mm obviously increases in the coated powder as compared with the metal powder. The average particle sizes of the coated powder and carbon steel powder were also obtained from the laser diffraction particle size analysis. The result shows that the average particle size of the coated powder is mm, which is greater than that of the carbon steel powder, mm. All these results indicate that the particle sizes of the coated powder increase as compared with those of the metal powder, which further proves that the metal particles are coated with nylon-12 resin. Effect of energy density on the bend strength Figure 4 shows the three-point bend strengths of the green specimens made from the coated powder and mixed powder at different energy densities. The two powders have the same nylon-12 content of 0.9 wt%. From Figure 4, it is seen that the bend strength of the green specimens increases with increasing energy density until it reaches a maximum value, and then further increasing energy density will result in the decrease in the bend strength. Similar results have been reported previously in epoxy/iron system (Liu et al., 2006) and PMMA/alumina system (Subramanian et al., 1995). In general, increasing energy density can elevate the temperature of PA12 resin; hence the viscosity of the polymer melt is decreased. According to the Frenkel model for viscous sintering, the sintering rates of polymers are inversely proportional to the melt viscosity at low-shear rates (Beaman et al., 1997; Nelson, 1993). Therefore, the sintering rate increases, and then the bend strength increases. However, when energy density increases to a relatively high value, the polymer binder decomposes seriously, and the binder contents in the green specimens decrease greatly. Therefore, the bend strength begins to drop. From Figure 4, it is additionally observed that the bend strength of the green specimens made from the coated powder is much higher than that of the green specimens made from Figure 4 Effect of energy density on the bend strengths of the specimens made from the coated powder (0.9 wt% nylon-12) and mixed powder (0.9 wt% nylon-12) the mixed powder at the same energy density. At the energy density of 0.08 J/mm 2, for instance, the green specimens of the coated powder have the bend strength of 2.24 MPa, which is about 1.95 times of the bend strength of the green specimens made from the mixed powder, 1.15 MPa. This result may be mainly attributed to the following two reasons: 1 Since there is a big difference in density between the neat nylon-12 and carbon steel powders, it is very difficult to uniformly disperse the binder particles in the metal matrix by the mechanically mixing method. Therefore, there are some weakly bonded regions where there is little or no polymer binder in the green parts of the mixed powder, which makes strengths of the green parts decrease greatly. On the contrary, because the polymer binder well coats the metal particles in the coated power, the polymer binder can be uniformly dispersed in the green parts, resulting in virtually on weakly bonded regions. 2 For the coated powder, the interfacial bonding of the polymer binder to the metal particles has been finished during its preparing process before the SLS process. In the dissolution-precipitation process, there is a sufficient time for the two materials to form a good interfacial adhesion. However, in the case of the mixed powder, the bonding of the polymer binder to the metal particles has to be accomplished in the SLS process. But, because the time of laser scanning is extremely short, the formed interfacial adhesion between the polymer binder and metal particles is limited. To meet the required green strengths for post-processing, the coated powder require a smaller amount of the polymer binder than the mixed powder. Therefore, the coated powder is preferred. Effect of binder contents on the bend strength The influence of binder contents on the bend strengths of the coated powder SLS specimens is shown in Figure 5. Figure 5 shows that the higher the binder content, the greater the bend strength of the SLS specimens at the same energy density. The green strengths at 0.6 wt% binder are less than 1.4 MPa which cannot meet the need of post-processing processes, indicating the binder content should be greater than 0.6 wt%. Figure 5 Effect of binder contents on the bend strengths of the specimens made from the coated powders Bend strength (MPa) Coated powder Mixed powder Bend strength (MPa) Nylon-12 content 1.0 wt% 0.8 wt% 0.6 wt% Energy density (J/mm 2 ) Energy density (J/mm 2 )

5 Preparation and SLS of nylon-coated metal powders The green strengths at 1.0 wt% binder are in the range of 1.87, 3.12 MPa, when energy density changes from 0.06 to 0.12 J/mm 2. These strengths are sufficient for parts to be handled in post-processing processes as well as for features as small as 1.0 mm to be built and cleaned. Therefore, the binder content is determined as 1.0 wt%. Dimensional accuracy Figure 6 shows the relationship between the dimensional errors of the green specimens made from the coated powder (1.0 wt% nylon-12) in the X-Y-and Z-directions and the applied energy density. From Figure 6, it is observed that the dimensional errors in the X-, Y- and Z-directions are all increased with increasing energy density. This may be due to a phenomenon referred to as part growth in the SLS process, which is schematically shown in Figure 7. In the SLS process, the sintered part has a higher temperature relative with the surrounding loose powder. Therefore, thermal energy stored in the sintered part propagates outward into the surrounding loose powder and raise local temperatures (as shown in Figure 7(a)). When the temperature of the surrounding powder is elevated beyond the caking temperature, which is T m for semi-crystalline polymers, an uncontrolled or secondary sintering layer that resides adjacent to the surface of the desired part geometry is generated (as shown in Figure 7(b)), and thus part growth happens. Part growth causes the increase in the dimensions of SLS parts. The larger energy density causes more serious part growth, because increasing energy density can enlarge the temperature gradient between the sintered part and the surrounding powder. Since part growth increases when greater energy density is applied, the dimensional errors increase with the higher energy density. It is well-known that accuracy is very important for SLS parts. Consequently, part accuracy is preferentially considered when energy density is selected. At the energy density of 0.06 J/mm 2, the green specimens made from the coated powder (1.0 wt% nylon-12) have relatively highdimensional accuracy, with the dimensional errors in the X, Y and Z-directions being 0.207, and per cent, respectively. The bend strength of the green specimens is 1.87 MPa, which is sufficient for parts to be handled during post-processing. Therefore, the energy density is confirmed as 0.06 J/mm 2 in this study. Figure 8 is the SEM micrograph of the fractured surface of a three-point bend specimen. As shown in Figure 8, the steel particles are bonded together by the polymer necks between particles. Figure 9 shows two SLS green parts made from the coated powder (1.0 wt% nylon-12) with the energy density of 0.06 J/mm 2. Figure 8 SEM micrograph of the bend fractured surface of the green specimen made from the coated powder (1.0 wt% nylon-12) with the energy density of 0.06 J/mm 2 Figure 6 Variations of the dimensional errors of the green specimens made from the coated powder (1.0 wt% nylon-12) in the X, Y and Z directions with the applied energy density Dimensional error (%) X Y Z Energy density (J/mm 2 ) 0.12 Figure 9 SLS green parts made from the coated powder (1.0 wt% nylon-12) with the energy density of 0.06 J/mm 2 Figure 7 Schematic illustration of part growth in the SLS process Loose powder Sintered part (a) (b) Secondary sintering layer 359

6 Summary The nylon-12 coated carbon steel powders for the indirect SLS process were prepared by the dissolution-precipitation process. The SEM and laser diffraction particle size analysis results indicate that the steel particles are well coated by nylon-12 resin. Therefore, this study provides a useful method for preparing nylon-coated metal powders. The bend strength of the SLS specimens increases with increasing the applied energy density until it reaches a maximum value, and then further increasing energy density will cause the decrease in the bend strength. At the same energy density, the bend strength of the SLS specimens made from the coated powder is much higher that that of the mixed powder SLS specimen. The bend strength of the SLS specimens increases with increasing the nylon-12 content over the investigated range. The dimensional errors in the X-, Y- and Z-directions are all increased with increasing energy density. In this study, the nylon-12 content and energy density are determined as 1.0 wt% and 0.06 J/mm 2, respectively. The green specimens have relatively high-dimensional accuracy, with the dimensional errors in the X-, Y- and Z-directions being 0.207, and per cent, respectively. The bend strength of the green specimens is 1.87 MPa, which is sufficient for parts to be handled in post-processing processes. References Preparation and SLS of nylon-coated metal powders Beaman, J.J., Barlow, J.W., Bourell, D.L., Crawford, R.H., Marcus, H.L. and McAlea, K.P. (1997), Solid Freeform Fabrication: A New Direction in Manufacturing, Kluwer Academic, Boston, MA. Dalgarno, K.W. and Stewart, T.D. (2001), Manufacture of production injection mould tooling incorporating conformal cooling channels via indirect selective laser sintering, P.I. Mech. Eng. B-J. Eng, Vol. 215 No. 10, pp Evans, R.S., Bourell, D.L., Beaman, J.J. and Campbell, M.I. (2005), Rapid manufacturing of silicon carbide composites,, Vol. 11 No. 1, pp Goode, E. (2003), Selective laser sintering system & materials, Advanced Materials & Processes, Vol. 1 No. 161, pp Ho, H.C.H., Gibson, I. and Cheung, W.L. (1999), Effects of energy density on morphology and properties of selective laser sintered polycarbonate, J. Mater. Process. Tech, Vol , pp Kruth, J.P., Wang, X., Laoui, T. and Froyen, L. (2003), Lasers and materials in selective laser sintering, Rapid Prototyping Journal, Vol. 23 No. 4, pp Kumar, S. (2003), Selective laser sintering: a qualitative and objective approach, JOM, Vol. 55 No. 3, pp Liu, J.H., Shi, Y.S., Lu, Z.L., Xu, Y. and Huang, S.H. (2006), Rapid manufacturing metal parts by laser sintering admixture of epoxy resin/iron powders, Advanced Engineering Materials, Vol. 8 No. 10, pp Nelson, J.C. (1993), Selective laser sintering: a definition of the process and an empirical sintering model, PhD thesis, The University of Texas at Austin, Austin, TX, May. Shishkovskii, L.V. and Kupriyanov, N.L. (1997), Thermal fields in metal-polymer powder compositions during laser treatment, High Temperature, Vol. 5 No. 35, pp Stewart, T.D., Dalgarno, K.W. and Childs, T.H.C. (1999), Strength of the DTM RapidSteel TM 1.0 material, Material and Design, Vol. 20, pp Subramanian, K., Vail, N., Barlow, J. and Marcus, H. (1995), Selective laser sintering of alumina with polymer binders,, Vol. 1 No. 2, pp Uzunsoy, D., Chang, I.T.H. and Bowen, P. (2003), Fracture behaviour of selective laser sintered RapidSteel2.0 under static and dynamic loading, Materials Science and Technology, Vol. 7 No. 19, pp Vail, N.K., Balasubramanian, B., Barlow, J.W. and Marcus, H.L. (1996), A thermal model of polymer degradation during selective laser sintering of polymer-coated ceramic powders,, Vol. 2 No. 3, pp Further reading Volpato, N., Childs, T.H.C., Stewart, T.D. and Watson, P. (2001), Indirect selective laser sintering of metal parts with overhung features, P.I. Mech. Eng. B-J. Eng, Vol. 215 No. 6, pp About the authors C.Z. Yan has received his PhD from the State Key Laboratory of Material Processing and Die and Mould Technology in HUST, China, and is now working in China University of Geosciences. His research interests encompass the field of selective laser sintering of polymerbased composite materials, including studies on material preparation, forming via selective laser sintering and the properties of sintered parts. C.Z. Yan is the corresponding author and can be contacted at: yesoar@ gmail.com Y.S. Shi is a Professor of the State Key Laboratory of Material Processing and Die and Mould Technology in HUST, China. He has authored or co-authored more than 30 research papers that have appeared in internationally assessed journals. His current research interests are the fields of rapid prototyping and tooling technologies, including related materials, equipment and applications. J.S. Yang is a Postdoctoral Researcher in the State Key Laboratory of Material Processing and Die and Mould Technology in HUST, China. J.H. Liu is a Professor of Heilongjiang University of Science and Technology, China. To purchase reprints of this article please reprints@emeraldinsight.com Or visit our web site for further details: 360