RESUMO
Laser powder bed fusion (LPBF) provides a rapid and versatile approach for producing parts with complex geometries. However, many parts with intricate geometries have overhang structures, which are not easily fabricated by using LPBF and are often downgraded by staircase effects, warpage, cracks, and dross formation. Thus, the present study proposes a combined numerical and experimental approach for determining the optimal settings of the laser power and scanning speed that minimize the surface roughness and maximize the density of Inconel 718 LPBF overhang structures. In the proposed approach, the heat transfer simulations are employed to determine the melt pool depth, the melt pool length, and the solid cooling rate within the feasible input space of laser power and scanning speed combinations. Notably, the simulations take account of both the difference in the material properties of the solid and powder materials, respectively, and the variation of the laser absorptivity in the depth direction of the powder layer. The simulation results are then used to train artificial neural networks for predicting the melt pool depth for 3600 combinations of the laser power and scanning speed within the input space. The resulting processing maps are screened in accordance with three quality criteria (namely the melt pool depth, the melt pool length, and the solid cooling rate) to determine the optimal processing region, which improves the surface roughness. The feasibility of the proposed approach is demonstrated by fabricating 10 × 10 and 20 × 20 mm2 horizontal overhang structures using parameter settings chosen from the optimal processing map. It shows that the optimal processing conditions result in a low surface roughness and a maximum density of 99.78%.
RESUMO
Laser powder bed fusion (L-PBF) has attracted great interest in recent years due to its ability to produce intricate parts beyond the capabilities of traditional manufacturing processes. L-PBF processed biomedical implants are usually made of commercial pure titanium (CP-Ti) or its alloys. However, both alloys are naturally bio-inert, and thus reduce the formation of apatite as implants are put into the human body. Accordingly, in an attempt to improve the bioactivity of the materials used for making orthopedic implants, the present study decomposed fluorapatite material (FA, (Ca10(PO4)6F2)) into the form of nano-powder and mixed this powder with CP-Ti powder in two different ratios (99%Ti + 1%FA (Ti-1%FA) and 98%Ti + 2%FA (Ti-2%FA)) to form powder material for the L-PBF process. Experimental trials were conducted to establish the optimal processing conditions (i.e., laser power, scanning speed and hatching space) of the L-PBF process for the two powder mixtures and the original CP-Ti powder with no FA addition. The optimal parameters were then used to produce tensile test specimens in order to evaluate the mechanical properties of the different samples. The hardness of the various samples was also examined by micro-Vickers hardness tests. The tensile strength of the Ti-1%FA sample (850 MPa) was found to be far higher than that of the CP-Ti sample (513 MPa). Furthermore, the yield strength of the Ti-1%FA sample (785 MPa) was also much higher than that of the CP-Ti sample (472 MPa). However, the elongation of the Ti-1%FA sample (6.27 %) was significantly lower than that of the CP-Ti sample (16.17%). Finally, the hardness values of the Ti-1%FA and Ti-2%FA samples were around 63.8% and 109.4%, respectively, higher than that of the CP-Ti sample.
