Engineering graded microstructures in a Ti alloy through high-cycle laser rescanning in powder bed fusion – Laser beam

Powder Bed Fusion – Laser Beam (PBF – LB) is an additive manufacturing (AM) technique enables flexibility in material design. Ti6Al4V, a proven AM alloy, is widely used in the biomedical industry as an implant material. However, its typically poor wear behavior hampers its use. Often, increasing surface hardness can improve wear resistance, but orthopedic implants require low stiffness to minimize stress-shielding. Although pushing the stiffness of a metallic implant material close to the bone is a difficult task, PBF – LB offers potential solutions. This study aims to develop a Ti-alloy using PBF – LB with a lower stiffness while having a harder surface compared to common PBF – LB Ti6Al4V. A two-step strategy was applied, combining powder mixing and laser rescanning. We demonstrate that by employing laser rescanning to a mixed powder of Ti6Al4V and 316L stainless steel, we can print a Ti alloy with a graded microstructure. The resulting Ti alloy possesses harder (> 5 GPa) α’ microstructure on the surface while having a relatively low bulk stiffness (90 GPa) from a β-matrix microstructure. These findings show the combination of powder mixing and laser rescanning is effective in tailoring the mechanical properties of Ti6Al4V alloy.