Influence of process parameters in selective laser melted Ti6Al7Nb alloy for biomedical applications

Single track investigations are commonly used to understand defect formation and alloy printability in selective laser melting (SLM). While the approach is practical, it does not account for the effects of thermal accumulation and potential issues that may arise from successive layer printing after single track optimizations. This, in turn, highlights a need for evaluation of melt pool characteristics through integrated single and multi-layer analysis. The present study investigated the printability of Ti6Al7Nb alloy for biomedical applications by analysing the outcomes based on the alloy's laser-matter interactions. This was achieved by varying the laser power and scan speed between 300 and 400 W and between 1900 and 2300 mm/s, respectively. The adopted framework investigates the interdependence between the laser power and scan speed and their effects on the microstructure and defect formation across single tracks, single layers, and cubic samples. It is shown that the linear energy densities (LED) of 0.184 J/mm & 0.181 J/mm produce the best single tracks. During layer formation, 0.184 J/mm LED yielded the lowest single layer surface roughness (R_a) of 7.49 ± 0.56 μm and relative density of 99.3 %, comparable to those obtained from 0.181 J/mm of 7.64 ± 0.22 μm and 99.1 %, respectively. X-ray diffraction (XRD) patterns and microscopy investigations also indicated that the microstructures formed at an LED of 0.184 J/mm were characterized by few defects and a coarse grain structure. This was attributed to the improved melting, reduced presence of spatter, and prolonged solidification times at this process parameter. The tensile strength tests revealed that stress-relief treatment at 650 °C led to an increase in ultimate tensile strength (UTS) (1195 ± 5.62 MPa) and a decreased elongation to an average of 4.60 % owing to the fine β dispersions. Post subsequent annealing, the UTS decreased to 1081 ± 41.5 MPa while the elongation increased to 5.45 %, which is considered unfavourable for biomedical applications. This observed behaviour was probably influenced by factors such as the presence of grain boundary α (GB _α) and microstructural size. The findings from this study demonstrate that the employed methodology enables a thorough investigation of the optimized tracks as the number of layers is increased. Furthermore, it provides insights into the microstructural manipulation of the alloy during heat treatment, which will greatly assist in improving the mechanical properties.