Effects of Alloy Composition and Solid-State Diffusion Kinetics on Powder Bed Fusion Cracking Susceptibility

Laser powder bed fusion (LPBF) has demonstrated its unique ability to produce customized, complex engineering components. However, processing of many commercial Al-alloys by LPBF remains challenging due to the formation of solidification cracking, although they are labelled castable or weldable. In order to elucidate this divergence, solidification cracking susceptibility from the steepness of the solidification curves, specifically |dT/dfS(1/2)|, as the fraction solidified nears 1 towards complete solidification, was calculated via Scheil-Gulliver model as a function of solute concentration in simple binary Al-Si, Al-Mg, and Al-Cu systems. Introduction of "diffusion in solid" into Scheil-Gulliver model resulted in a drastic reduction in the cracking susceptibility (i.e., reduction in the magnitude of |dT/dfS(1/2)|) and a shift in the maximum |dT/dfS(1/2)| to higher concentrations of solute. Overall, the calculated solidification cracking susceptibility correlated well with experimental observation made using LPBF AA5083 (e.g., Al-Mg) and Al-Si binary alloys with varying Si concentration. Cracking susceptibility was found to be highly sensitive to the composition of the alloy, which governs the variation of |dT/dfS(1/2)|. Furthermore, experimental observation suggests that the contribution of "diffusion in solids" to reduce the cracking susceptibility can be more significant than what is expected from an instinctive assumption of negligible diffusion and rapid cooling typically associated with LPBF.