Promoting columnar-to-equiaxed transition via TiB2 nano-nucleants to achieve fully equiaxed microstructure in WAAM AlMg alloys

The structural application of Al Mg alloys fabricated via Wire Arc Additive Manufacturing (WAAM) is often hindered by mechanical anisotropy, which arises from coarse columnar grains formed under the large temperature gradients induced by rapid cooling. To overcome this, we introduced varying concentrations of in-situ TiB₂ particles into Al Mg feed wires and fabricated components using the Cold Metal Transfer (CMT) process. Coupled with finite element (FE) simulations of the thermal and solidification dynamics within the molten pool, this study reveals that TiB₂ particles successfully promote a Columnar-to-Equiaxed Transition (CET). A dual mechanism of explosive nucleation and grain boundary pinning synergistically promotes this transition. Furthermore, the effective nucleation potency of TiB₂ exhibits a saturation-like behavior, whereby excessive addition leads to particle overlap and agglomeration and, in turn, reintroduces anisotropy. This work elucidates the intrinsic mechanisms governing TiB₂-mediated microstructural evolution during WAAM and provides important guidance for the additive manufacturing of high-performance, nearly isotropic aluminum alloy components.