Crack mitigating and microstructure engineering through in-process mechanical work in wire directed energy deposited Invar alloy

Abstract Wire-based directed energy deposition (WDED) of Invar alloys is prone to cracking due to high thermal stresses and microstructural anisotropy. In this study, in-process machine hammer peening (MHP) was integrated into WDED to mitigate cracking and control the microstructure of Invar alloy. The as-deposited material exhibited millimetre-scale columnar grains with strong build-directional alignment and severe solidification cracking along grain boundaries, indicating a dominant role of deposition-induced thermal stresses. Interlayer MHP promoted a transition from epitaxial growth to heterogeneous nucleation, producing a refined and more equiaxed grain structure with improved tolerance to thermal straining. Deformation-related FCC texture components and an increased fraction of low-energy Σ3 coincidence site lattice boundaries were observed in the MHP-treated material, contributing to enhanced grain-boundary-mediated plasticity and cracking resistance. Mechanical characterisation further revealed increased hardness and elastic modulus achieved by interlayer MHP, together with progressive build-height-dependent strengthening. In general, in-process MHP offers an effective and cost-efficient thermo-mechanical strategy for crack mitigation and is expected to contribute to microstructure-property optimisation in additively manufactured Invar alloys.