Interplay of strain and phase evolution of laser powder bed fusion Ti-6Al-4V

While additive manufacturing (AM) provides a method of producing geometrically complex and highly detailed structures, the generation of residual strain in AM processes like laser powder bed fusion (L-PBF) can negatively impact performance-enabling properties. In applications such as orthopedic implants, specific performance windows require optimized microstructures in order to obtain desirable properties from multi-phase alloys like Ti-6Al-4V. This research aims to quantify the microscale origins of strain in L-PBF manufactured Ti-6Al-4V by understanding how strain is distributed at the grain and sub-grain scale, the interplay between phase evolution and strain, and examining post-processing strain relief strategies to control these features. Model spinal cage implants were manufactured from Ti-6Al-4V powder via L-PBF and then subjected to strain relieving heat treatment cycles above and below the Ti-6Al-4V beta transus as a function of time and cooling rate. Residual strain was then studied via high resolution electron backscatter diffraction (HR-EBSD), and 2D strain maps with sub-micron resolution were generated for each post-processing state. It was found that macroscale thermal strains decreased with heat treatment time, but additional contributions from phase stabilizing residual strains retained primarily in the alpha & PRIME; grains as lattice distortive strain remained. Additionally, the retention of beta phase significantly changed the strain and dislocation distribution while reducing overall residual strain. These results were vali-dated and reinforced with 3D mesoscopic micromechanical modeling of strain behavior across simulated mi-crostructures, confirming that the local lattice dilation of alpha' martensite is a primary contributor of microscale strain generation and retention in L-PBF Ti-6Al-4V.