THE EFFECT OF THE LASER PROCESS PARAMETERS IN THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TI6AL4V PRODUCED BY SELECTIVE LASER SINTERING/MELTING

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1 THE EFFECT OF THE LASER PROCESS PARAMETERS IN THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TI6AL4V PRODUCED BY SELECTIVE LASER SINTERING/MELTING João Batista FOGAGNOLO 1, Edwin SALLICA-LEVA 1, Eder LOPES 1, André Luiz JARDINI 2, Rubens. CARAM 1 1 School of Mechanical Engineering, State University of Campinas, Campinas, Brazil. 2 School of Chemical Engineering, State University of Campinas, Campinas, Brazil. Abstract Direct metal laser sintering or selective laser sintering is a rapid prototyping technique that consolidates, layer by layer, parts with complex shape from powders of a wide range of metallic materials. It has been used for fabrication of patient specific net-shaped parts of dense and porous titanium and its alloys for use as implants. But depending on the laser parameters, the powder can be fully melted during its consolidation. In this case, selective laser melting seems to be a more appropriate process denomination than selective laser sintering. In this work we use selective laser sintering/melting to consolidate Ti6Al4V porous parts from prealloyed powders in order to investigate the laser process parameters on the microstructure and mechanical properties of the parts. The consolidated samples were characterized by optical and scanning electron microscopy and compressive tests. The results showed a straight relationship between laser energy and mechanical properties. Keywords: Rapid prototyping, titanium alloys, porous materials, mechanical properties. 1. INTRODUCTION Rapid prototyping is a fabrication process based on the successive addition of flat layers of material. From a three-dimensional geometric model produced by a CAD (Computer Aided Design), rapid prototyping allows to obtain both fully-densified materials and materials with controlled porosity. The model is virtually cut, generating a sequence of layers, which defines where the material will be added. The piece is then generated by processing a sequential stacking of layers [2,3]. Direct Metal Laser Sintering (DMLS), one of the rapid prototyping techniques, produces parts form metal powders, using the energy of a laser beam to promote sintering, in an inert and thermally controlled environment within a chamber [1].The incidence of the laser can promote the sintering of powder particles or its complete fusion, bonding them to the previous layer. After obtaining each layer, an additional layer of powder is superimposed on the part, and the laser beam once again scans the desired areas, according to the virtual model, thereby obtaining a new layer, and subsequently, to complete construction of the part. Custom orthopedic implants with controlled porosity can be fabricated by sintering DMLS, due the geometric freedom inherent to this technique. Such porous parts have attract interest due to their inherent lower modulus of elasticity, which is closer to the tissue stiffness, and the osseointegration possibility. In this work, we obtained Ti-6Al-4V porous parts with cubic geometry by DMLS, using models with two different porosity levels. Two different energy input were tested, by varying the DMLS process parameters. The parts presented the designed porosity, according to the model. Depending on the energy input, inherent porosity was also obtained with different intensities. Their influence on the mechanical properties was determined.

2 2. EXPERIMENTAL PROCEDURE Cubic models with 15 mm diameter were designed using a CAD program. The first model has 49 internal connected porous with 1.23 mm per 1.23 mm and struts of 0.80 mm. This configuration produces 61% of porosity. The second model has 36 internal connected porous with 1.57 mm per 1.57 mm and struts of 0.80 mm, producing 69% of porosity. Figure 1 shows these models, which was taken from the work reported by Parthasarathy et. al. [3]. Our intent was to compare the results of the mechanical properties of Ti-6Al-4V porous parts obtained by DMLS with the mechanical properties of the same alloy with the same geometry obtained by electron beam melting, reported by Parthasarathy et. al. [3]. Fig. 1 Cubic models used to produce the DMLS parts. The Ti-6Al-4V porous parts were obtained by DMLS, using a EOSINT M 270, a device specially developed to sinter metal powders. To sets of parameters of process were chosen, as displayed in Table 1. Table 1 Process parameters used to obtain the porous parts. Set 1 2 Laser powder (W) Scanning speed (mm/s) Distance between laser scanning lines (um) Thickness of layer (um) Energy density (J/mm 2 ) The energy density (E) was calculated from the laser power (P), distance between laser scanning lines or laser hatching (LS) scanning speed (V), using Equation 1 [4]. Hereafter the set 1 will be referred as low energy input and the set 2 as high energy input. P( W ) E( J 2) (1) mm HS( mm) V ( mm ) s During laser sintering, argon was used as protective atmosphere, to avoid metal oxidation. The parts were oriented at 45 o respect to the base. The direction of scanning was always altered in 45 o after completing a whole layer. After sintering, the parts were photographed using a Canon Power Shot S31S digital camera. The sintered parts was characterized by optical microscopy (OM), using a Olympus BX60M. Kroll reagent was used to revel the microstructure. The Young modulus, the yield strength and the ultimate compressive

3 strength were determined using a universal machine testing EMIC DL 2000, 30 kn maximum, equipped with extensometer. 3. RESULTS AND DISCUSSION Figure 2 shows the parts obtained by DMLS. Parts almost free of defects, with interconnected porous could be obtained for both 61 and 69 %of designed porosity. Fig. 2 Porous parts obtained by DMLS with 61 (upper) and 69 (bottom) % of porosity. However, OM analysis (see Figure 3) exposed a substantial porosity inside the struts when using high energy input parameters, here denominated as no-controlled porosity, to differentiate it from the designed one. Struts with very low porosity were obtained when using low energy input parameters. The no-controlled porosity may be due to the excessive energy input, which could promote boiling of metal. The spherical feature of the porous is an indication of boiling of metal. Regardless of the process parameters and the designed porosity, the sintered parts presented martensitic structure, as shown in the Figure 4. The small thickness of the layers in the DMLS causes a high cooling rate, which results in the martensitic transformation. The mechanical properties of the sintered parts were determined and the results are shown in the Figure 5. For comparison, the results of Young modulus and ultimate compressive strength, published by Parthasarathy et. al. [3], using electron beam melting to obtain parts with the same geometry and alloy, are plotted together. As expected, the porosity decreases the mechanical properties of the parts, including the Young modulus, the yield strength and the ultimate compressive strength, and the decrease is directly proportional to the fraction of porosity. Comparing parts with the same designed porosity, obtained by DMLS, one can find that the use of high energy input produced parts with lower mechanical properties. This result is due to the additional porous introduced inside the struts. Comparing the mechanical properties obtained by DMLS with those obtained by EBM and reported by Parthasarathy et. al. [3], on can find that the ultimate compressive strength of the EBM parts was higher than those of the parts obtained by DMLS using high energy input and lower than those of the parts obtained by

4 DMLS using low energy input. However, the Yong modulus of the EBM parts was significantly lower than those of the DMLS parts. Although the work of Parthasarathy et. al. [3] did not show the microstructure of the EBM parts and consequently we cannot presume their internal, no-controlled porous, we believe that the differences are more related with the internal porosity than the structure formed, as both process should produce martensitic transformations. Fig. 3 Struts of the porous parts with 61 (a and b) and 69 (c and d) % of designed porosity, obtained using low (a and c) and high (b and d) energy input, observed by OM. Fig. 4 Microstructure of the sintered parts with 61 (a and b) and 69 (c and d) % of designed porosity, obtained using low (a and c) and high (b and d) energy input, observed by OM.

5 Fig. 5 Mechanical properties of the sintered parts, compared with the results of the same-geometry parts obtained by electron beam melting and published by Parthasarathy et. al. [3].

6 4. CONCLUSIONS Ti-6Al-4V porous parts were consolidated from Ti-6Al-4V powder alloy by DMLS. The structure of the parts after sintering was martensitic. As expected, the mechanical properties are strongly related with the porous volume fraction. The use of high energy density parameters produced a non-controlled porosity inside the struts, which resulted in decrease of the mechanical properties. ACKNOWLEDGEMENTS The authors would like to thank the the National Council for Scientific and Technological Development (CNPq), Brazil, for financial support. LITERATURE [1] ENGEL, B., BOURELL, D.L. Titanium alloy powder preparation for selective laser sintering, Rapid prototyping Journal, 2000, 6, [2] GIBSON, I., CHEUNG, L.K., CHOW, S.P., CHEUNG, W.L., BEH,S.L., SAVALANI, M. LEE, S.H. The use of rapid prototyping to assist medical applications, Rapid Prototyping Journal, 2006, 12, page 53. [3] PARTHASARATHY, J., STARLY, B., SHIVAKUMAR, R., CHRISTENSEN, A. Mechanical evaluation of porous titanium (Ti6Al4V) structures with electron beam melting (EBM), Journal of the Mechanical Behavior of Biomedial Materials, 2010, 3, page 249. [4] GIBSON, I., SHI, D. Materials properties and fabrication parameters in selective laser sintering process, Rapid Prototiping Journal, 1997, 3, page 129.