Microstructure and mechanical properties of pure titanium models fabricated by selective laser melting

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1 711 Microstructure and mechanical properties of pure titanium models fabricated by selective laser melting E C Santos 1 *, K Osakada 1, M Shiomi 1, Y Kitamura 1 and F Abe 2 1 Division of Mechanical Science, Graduate School of Engineering Science, Osaka University, Osaka, Japan 2 Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, Nagoya, Japan Abstract: The pore structure, the hardness and the mechanical properties of three-dimensional titanium models formed by the selective laser melting method with a neodymium-doped yttrium aluminium garnet (Nd:YAG) pulsed laser are investigated. The optical and scanning electron micrographs show that pore structure depends on the peak power, the scan speed and the hatching pitch. The Vickers hardness of the laser formed specimens is around 240 HV (0.2 kgf), higher than that of the wrought material ( HV). Depth profiling by X-ray photoelectron spectroscopy (XPS) indicates that oxygen pick-up occurs during laser forming of the titanium model processed in a closed chamber filled with argon. The fatigue strength of the titanium models formed by changing the hatching pitch and the laser power were measured. It is possible to improve the fatigue strength of the as-formed models by decreasing the hatching pitch or by hot isostatic pressing (HIP). The specimens after HIP have a fatigue strength comparable to the wrought material. Keywords: selective laser melting, titanium, porosity, microstructure, mechanical properties 1 INTRODUCTION The MS was received on 2 December 2003 and was accepted after revision for publication on 3 March * Corresponding author: Division of Mechanical Science, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka , Japan. Rapid manufacturing (RM) of metal parts seems to be suitable for small-volume production. The strength, the surface roughness and the dimensional accuracy of the formed models are still the main problems faced for the widespread use of rapid prototyping (RP) techniques or layer manufacturing techniques (LMT) for RM [1 3]. The medical area is a good candidate for LMT. Implants and prostheses are highly individual components of high aggregate price. Three-dimensional scanning techniques such as computerized axial tomography (CAT) and magnetic resonance imaging (MRI) can be used for acquiring the geometric information necessary for production of the three-dimensional solid model, and RP techniques such as selective laser melting (SLM), selective laser sintering (SLS) and laser engineered net shaping (LENS) can be used for the production of the final models. Some post-processing of the laser formed parts is necessary to reach the strict final accuracy of the medical components. One advantage of LMT is the capability of fabricating parts with controlled porosity by varying processing parameters. Selective laser melting is one of the RM processes that use metal powder directly. It is an automated near-netshape technique for fabricating complex metal parts directly from computer aided design (CAD) data by fusing metal powders in the focal zone of a high-energy laser beam. It does not need to remove any binder and, theoretically, any castable materials can be used in the SLM process, although careful selection of forming conditions is essential for successful operation. The high thermal gradient induced by rapid solidification after laser melting may cause residual stresses or cracks. Heat treatment such as annealing and hot isostatic pressing can improve the final properties of the parts by stress relieving and full densification respectively [4]. Titanium has excellent corrosion resistance, biocompatibility and a high strength weight ratio. These characteristics make titanium a suitable material for the fabrication of implants and prostheses [5, 6]. The high reactivity of the metal to interstitial elements such as oxygen, carbon, nitrogen and hydrogen in the molten state is the main obstacle for laser processing of titanium. Titanium models made by SLM have shown that the density of the specimens is higher than 92 per cent and is

2 712 E C SANTOS, K OSAKADA, M SHIOMI, Y KITAMURA AND F ABE slightly affected by the peak power and scan speed; also tensile strength is high but impact strength and fatigue strength are low [7 9]. In the present work the influence of the peak power, the scan speed and the hatching pitch on the pore structure and on the fatigue strength of pure grade 1 titanium powder processed by a pulsed neodymium-doped yttrium aluminium (Nd:YAG) laser are investigated. The pore structure was investigated by optical and scanning electron microscopes. A torsional fatigue test was carried out in square bars with different surface conditions. Heat treatments such as annealing and hot isostatic pressing (HIP) were used to improve the fatigue strength of the specimens. 2 EXPERIMENTAL PROCEDURES Figure 1 shows a sketch of the selective laser melting equipment. An Nd:YAG laser of maximum peak power of 3 kw and maximum average power of 50 W was used. The pulsed laser can deliver high peak power in a short pulse, thus using the high irradiance (W/cm 2 ) of the pulsed laser; it is possible to have effective melting of the powder with a small heat-affected zone (HAZ) in the solidified part using proper parameters. Also, because of the better absorptivity of metals to short wavelength, Nd:YAG lasers seem to be more suitable than CO 2 lasers for material processing when melting is involved [10, 11]. The laser light is guided through the optical fibre and the laser beam diameter is 0.8 mm focused on to the powder bed. The laser head is attached to an x y table controlled by a computer. The laser beam moves on the titanium powder bed deposited in a steel substrate forming solidified layers. The substrate is attached to a piston which goes downward by one layer thickness of 0.1 mm (z direction). The process is carried out in a closed chamber and argon is flushed continuously in order to minimize oxygen and nitrogen pick-up. The material used was spherical grade 1 titanium powder (TILOP 45) supplied by Sumitomo Sitix Inc. The chemical analysis of the powder can be seen in Table 1. The powder had a very low amount of interstitial elements. The particle size was under 45 mm and the average particle size was 25 mm. Figure 2 shows the x and y scanning strategy for building the three-dimensional models. At first, the outline of the cross-section is scanned. The odd layers (layers 1, 3, 5,...) are also scanned in the x and y directions alternatively. The even layers (layers 2, 4, 6,...) are scanned only on its outline. Therefore a scanning cycle consists of a scanning outline and x direction (layer 1), scanning-only outline (layer 2), scanning outline and y direction (layer 3) and scanning-only outline (layer 4). This bidirectional scan strategy is repeated until the three-dimensional part is built. This scanning method decreases the interconnected porosity between the layers and reduces the anisotropy of the mechanical properties of the models (compared with one-directional scanning); the building time is also reduced because the even layers are not scanned in the x or y directions, only the outline. Cubes of length of 10 mm, blocks of 5 mm6 15 mm615 mm and blocks of 2 mm610 mm6 10 mm were built for investigation of pore structure and hardness measurements. The fatigue strength tests were done in square bars of 53 mm of height and Fig. 1 Selective laser melting process Table 1 Chemical composition of commercial pure titanium powder grade 1 TILOP 45 wt % Oxygen Nitrogen Carbon Iron Hydrogen Titanium Balance Source: Inspection Certificate, Sumitomo Sitix, Inc., Amagasaki, Japan. Proc. Instn Mech. Engrs Vol. 218 Part C: J. Mechanical Engineering Science C23503 # IMechE 2004

3 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF PURE TITANIUM MODELS 713 Fig. 2 Scanning strategy cross-section of 7 mm67 mm and 6 mm66 mm. The scan speed ðvþ and the peak power ðpþ were changed from 2 to 16 mm/s and from 0.5 to 1 kw respectively. The average power ðp av Þ and the frequency ð f Þ were kept at 50 W and 50 Hz. The hatching pitch ðh p Þ was varied from 0.2 to 0.75 mm. 3 PORE STRUCTURE AND HARDNESS Figure 3 shows the pore shape at the centre of the formed cubes at peak powers of 0.5, 0.75 and 1 kw. The pore shape changes from crack-like to round as the peak power increases. The crack-like pores are stress concentrators and are detrimental to the mechanical properties of the models [12]. A block with 5 mm615 mm of cross-section and 15 mm in height was built using different scan speeds for each 2 3 mm in the building direction ðzþ; an optical micrograph of the specimen can be seen in Fig. 4. The cross-section of the specimen in the building direction formed at scan speeds of 4, 6 and 8 mm/s shows high density. Some powder particles are not melted completely when the scan speed is high and the porosity increases. Vaporization of the powder at a very low speed (2 mm/s) was observed by a high-speed camera [7], so porosity is also high. Figure 5 shows the scanning electron micrographs of the samples at the centre of the layer (XY plane) at scan speeds of 6 and 16 mm/s and hatching pitches of 0.4 and 0.2 mm. As the scan speed increases the hatching pitch for producing high-density parts decreases. When the peak power is 1 kw, for example, hatching pitches of 0.4 and 0.2 mm should be used for scan speeds of 6 and 16 mm/s respectively [13]. The density can reach up to 98 per cent with proper parameters (e.g. h p ¼ 0:2 mm and v ¼ 16 mm=s). Figure 6 shows the influence of the scan speed and the peak power on the hardness. The maximum average hardness is around 255 HV at a scan speed of 4 mm/s and a peak power of 1 kw; the minimum is around 218 HV at a scan speed of 16 mm/s and a peak power of 1 kw. The hardness of wrought titanium grade 1 varies from 125 to 160 HV and depends on interstitial elements and thermomechanical processing [14]. The increase in hardness may be caused by oxygen pick-up. In order to determine the increase of the oxygen content during laser processing, depth profiling by X-ray photoelectron spectroscopy (XPS) is carried out. XPS spectra were measured with MgKa radiation as an excitation source and argon ion sputtering was achieved (with a 30 s interval) until 4.5 min under 4 kev of applied voltage. At a constant scan speed of 6 mm/s, the hardness increases with increasing peak power from 235 HV at a peak power of 0.25 kw to 265 HV at a peak power of 2 kw. Figure 7 shows the XPS profiles of the specimens built at peak powers of 1 kw (245 HV, 0.2 kgf) and 2 kw. Fig. 3 Influence of the peak power on the pore shape: (a) P ¼ 1 kw, (b) P ¼ 0.75 kw and (c) P ¼ 0.5 kw (P av ¼ 50 W, f ¼ 50 Hz, v ¼ 6 mm/s and h p ¼ 0.75 mm)

4 714 E C SANTOS, K OSAKADA, M SHIOMI, Y KITAMURA AND F ABE Fig. 4 Fig. 5 Influence of the scan speed on the porosity (Pav ¼ 50 W, P ¼ 1 kw, f ¼ 50 Hz and hp ¼ 0.75 mm) Influence of the hatching pitch on the porosity: (a) v ¼ 6 mm/s, hp ¼ 0.75 mm; (b) v ¼ 6 mm/s, hp ¼ 0.4 mm; (c) v ¼ 16 mm/s, hp ¼ 0.75 mm; (d) v ¼ 16 mm/s, hp ¼ 0.2 mm (Pav ¼ 50 W, P ¼ 1 kw and f ¼ 50 Hz) The peak areas give good qualitative information on the oxygen, carbon and titanium atomic concentration. Comparing the peak areas, it is possible to see that the specimen formed at a peak power of 2 kw has a higher Proc. Instn Mech. Engrs Vol. 218 Part C: J. Mechanical Engineering Science amount of oxygen. At a high peak power, the power density increases and the HAZ becomes large, which decreases the cooling rate. Therefore the amount of oxygen pick-up increases as the peak power increases. C23503 # IMechE 2004

5 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF PURE TITANIUM MODELS 715 Fig. 6 Hardness of laser-formed specimens: (a) influence of scan speed, P ¼ 1 kw; (b) influence of peak power, v ¼ 6 mm/s (P av ¼ 50 W and f ¼ 50 Hz) Fig. 7 Comparison between the XPS profiles until 4.5 min sputtering with Arþ ion at 4 kev of specimens processed at a peak power of (a) 1 kw and (b) 2 kw (P av ¼ 50 W, f ¼ 50 Hz, v ¼ 6 mm/s and h p ¼ 0.75 mm)

6 716 E C SANTOS, K OSAKADA, M SHIOMI, Y KITAMURA AND F ABE 4 FATIGUE STRENGTH The influence of surface condition is better examined by torsional or bending testing than by axial testing because the surface layers of the metal are most stressed during alternate bending or alternate twisting. Besides that, problems with a high variation of fatigue strength because of misalignment occur less in torsional testing [15]. The specimens used are square bars with maximum cross-section of 7 mm and length of 53 mm. The samples have two different surface conditions: the as-formed (just after laser processing) and machined (machining of 0.2 mm each side, grinding by 600-grit SiC paper and polishing by alumina 1 mm). Figure 8 shows the stress field during torsion and the samples. The scanning electron micrographs in Fig. 9 show the surface condition at the middle of the specimens. The sintered powder attached to the sides of the specimen in Figs 8b and 9a can be considered as a porous coating. During torsion of square bars the original plane crosssections are deformed or warped out of their own planes. The mathematical formulation for the case of circular cross-sections cannot be used. The problem was solved by Saint Venant and the stresses can be Fig. 8 calculated using the following equations [16]: t zx ¼ Gy Torsional fatigue test: (a) stress field during torsion test, (b) as-formed specimen (each division is 1 mm) and (c) machined specimen (P av ¼ 50 W, P ¼ 1 kw, f ¼ 50 Hz and v ¼ 6 mm/s) t zy ¼ Gy 16a p 2 2x 16a p 2 X ð 1Þ ðn 1Þ=2 n n 2 X ð 1Þ ðn 1Þ=2 n n 2! sin a n x cosh a n y ð1þ cosh a n b cos a n x sinh a n y cosh a n b ð2þ Here y ¼ T=ð16a 3 bbgþ, a n ¼ np=ð2aþ ðn ¼ 1, 3, 5,...Þ Fig. 9 Scanning electron micrographs of the surface of the fatigue specimens: (a) as-formed specimen (6 100), (b) neck formation between powder particles on the surface of the as-formed samples (6 2000) and (c) machined specimen (6 100) (P av ¼ 50 W, P ¼ 1 kw, f ¼ 50 Hz and v ¼ 6 mm/s) Proc. Instn Mech. Engrs Vol. 218 Part C: J. Mechanical Engineering Science C23503 # IMechE 2004

7 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF PURE TITANIUM MODELS 717 and the shape parameter b ¼ 0:141 for a square bar ða ¼ bþ. The stress is higher in the middle of each side of the bar and is zero at the corners. The fatigue cycles are carried out at a fully reversed mode (stress ratio R ¼ 1) and the frequency of the machine is 33 Hz. The specimens were formed at peak powers of 0.5, 0.75 and 1 kw. The scan speed was 6 mm/s and hatching pitches of 0.75 and 0.4 mm were used. The machined specimens built at a hatching pitch of 0.75 mm were submitted to heat treatment of annealing for stress relieving and HIP for densification. Annealing was done at a temperature of 750 8C, HIP was carried out at a temperature of 850 8C and pressure of 101 MPa in argon atmosphere and the specimens were kept for 1 h at the treatment temperatures. 5 RESULTS AND DISCUSSION Figure 10 shows the number of cycles before fracturing for stress amplitude varying from 35 to 100 MPa. The specimens formed at a hatching pitch of 0.75 mm and peak power lower than 1 kw (indicated by & and ~) are fractured before 10 5 cycles. The density of the specimens at peak powers of 1, 0.75 and 0.5 kw are 96, 94 and 94 per cent respectively. The small number of cycles before fracturing when the peak power is lower than 1 kw suggests a high influence of the crack-like pore shape and of the weak metallurgical bonding between the layers on the fatigue strength. It was shown in Fig. 3 that the pore shape changed from round to crack-like pores when peak power became low. In addition, the laser power has to be high enough to melt the powder and a portion of the previous layer for a good metallurgical bonding between the layers. Therefore the metallurgical bonding between the layers becomes weak as the peak power decreases. The use of roughened surfaces or porous coating to increase osseo-integration between the implant and the bone are under investigation [5]. The porous coating can improve osseo-integration by cement interlocks with the porous structure or better fixation via in-growth of bone tissue [5, 6]. Porous coatings are usually created by postprocessing of implants by plasma spraying. Because of the higher temperatures during conventional porous coating application, the grain size can undergo significant grain growth and notch effects, reducing the fatigue strength. Conventional porous-coated titanium implants are reported to have a fatigue strength onethird that of the uncoated implant [6]. The powder particles sticking to the as-formed specimens can be considered as a porous coating. Figure 9b shows a higher magnification of the sintered powder on the surface of the sample. This porous coating resulted in a slight decrease in the fatigue strength. The machined specimens have lower roughness and fewer defects (voids) on the surface and therefore were expected to stand a higher number of cycles before fracturing compared to the as-formed ones. However, the decrease in the number of cycles before fracturing for the as-formed specimens ðdþ was not high compared with the standard machined specimens ðsþ for the stress of 35 MPa because in the SLM process the coating is developed during the building up of the specimens. The fatigue strength of the specimens formed at a hatching pitch of 0.4 mm ð&þ is much higher than that of the ones formed at a hatching pitch of 0.75 mm because of the better metallurgical bonding between the scan tracks and higher density when the hatching pitch is smaller. Figure 11 shows the improvement of the fatigue strength after heat treatment. Annealing slightly Fig. 10 Fatigue strength results of the laser-formed specimens (P av ¼ 50 W, f ¼ 50 Hz and v ¼ 6 mm/s) Fig. 11 Fatigue strength results of the heat-treated specimens (P av ¼ 50 W, f ¼ 50 Hz, P ¼ 1 kw, v ¼ 6 mm/s and h p ¼ 0.75 mm)

8 718 E C SANTOS, K OSAKADA, M SHIOMI, Y KITAMURA AND F ABE Fig. 12 Microstructure of the specimens: (a) acicular martensitic before HIP, HV ¼ 245; (b) equiaxed structure after HIP, HV ¼ 220 increases the fatigue strength because the residual stresses of the laser-formed models may be reduced. After HIP, the density increases to near 100 per cent and the fatigue strength is very much improved. Figure 12 shows the microstructure section of the fatigue samples before and after HIP. Because of the high thermal gradient, the microstructure of the asformed specimens is of acicular martensitic. The microstructure of the specimens after HIP consists of alpha equiaxed grains. The torsional fatigue strength of the specimens ( MPa to 10 6 cycles) after HIP is comparable to that of the wrought material ( MPa for 10 6 cycles) and is adequate for dental implants. 6 CONCLUSIONS The influence of the laser parameters and forming conditions on the pore structure and fatigue strength of the parts was studied and the fatigue strength of the laser-formed components was improved by using different processing parameters and heat treatment. The main conclusions are: 1. The density after laser processing is higher than 92 per cent for all conditions. Density can be increased to up to 98 per cent by changing the forming conditions (scan pitch or layer thickness) or by hot isostatic pressing. 2. The Vickers hardness after laser processing is higher than the hardness of the wrought titanium. The increase in hardness is caused by oxygen pick-up. 3. The machined specimens withstand a higher number of cycles before fracturing. More tests need to be done to evaluate the decrease in fatigue strength of the as-formed specimens compared with the standard machined ones. The as-formed condition can be considered as a coated part with the advantage that the coating is developed during the forming process. 4. A high fatigue strength can be achieved by using a large hatching pitch (e.g. h p ¼ 0.75 mm for v ¼ 6 mm/s) followed by hot isostatic pressing or by using a small hatching pitch (e.g. h p ¼ 0.4 mm for v ¼ 6 mm/s). ACKNOWLEDGEMENTS The authors would like to thank Dr K. Kida, Research Associate at the Graduate School of Engineering Science at the Osaka University, for helping with the fatigue strength tests in the laboratory of Professor K. Ogura. They would like also to thank Dr Masanari Takahashi from the Osaka Municipal Technical Research Institute (OMTRI) for XPS measurements. REFERENCES 1 Lu, L., Fuh, J. Y. H. and Wong, Y. S. Laser Induced Materials and Processes for Rapid Prototyping, 2001 (Kluwer, Massachusetts). 2 Ramos, J. A., Murphy, J., Wood, K., Bourell, D. L. and Beaman, J. J. Surface roughness enhancement of indirect SLS metal parts by laser surface polishing. In Proceedings of the 12th Solid Freeform Fabrication Symposium, University of Texas at Austin, Texas, August 2001, pp Wohlers, T. Wholers Report, Rapid prototyping and tooling, state of the industry, Annual Worldwide Progress Report, 2000 (Wholers Associates Inc., Colorado). 4 Das, S., Wohlert, M., Beaman, J. J. and Bourell, D. L. Producing metal parts with selective laser sintering/hot isostatic pressing. J. Metals, 1998, 50(12), Brunette, D. M., Tengval, P., Textor, M. and Thomsen, P. Titanium in medicine. In Material Science, Surface Science, Engineering Biological Responses and Medical Applications, 2001 (Springer, Berlin). 6 Brown, S. A. and Lemons, J. E. Medical applications of titanium and its alloys. In The Material and Biological Issues, ASTM PCN: , STP 1272, May 1996 Proc. Instn Mech. Engrs Vol. 218 Part C: J. Mechanical Engineering Science C23503 # IMechE 2004

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