Abstract:

The strength and damage tolerance of additively manufactured (AM) alloys are significantly influenced by their heterogeneous microstructures. However, establishing quantitative relationships between these microstructural characteristics and the resulting mechanical properties remains a challenge. Here, a microstructure-based mechanical model is established based on the heterogeneous grain distribution within the melt pool, with particular emphasis on the strain gradient effect arising from the deformation incompatibility between distinct grain regions. The strengthening mechanisms and local deformation response of AM alloys are elucidated with the finite element method (FEM). The strain gradient effect generated by the deformation incompatibility between the columnar and equiaxed grain regions enhances the local stress near the equiaxed-columnar interface, which is an important reason for the overall work hardening. Concurrently, the local stress concentration makes it easier to reach the critical stress for microcrack nucleation at the interface, leading to failure and a lack of synergy between the strength and damage tolerance. The prediction of the crack initiation location based on the simulation results is consistent with the previous experiments. By further quantitatively predicting the comprehensive effects of melt pool size on strength, strain hardening, and damage rate, small-melt-pool structures produce high strength, but microcracks originate early, whereas large-melt-pool structures have weak strengthening effects but fast damage evolution in the later stages of deformation. This study provides a pathway to predict the optimal melt pool size for achieving superior combinations of strength and damage tolerance in AM alloys.

Key words: additive manufacturing, strain gradient, strain hardening, damage tolerance, microcrack nucleation