Decoupling the heat source and remelting depth for equiaxed transition in wire arc additive manufacturing of titanium alloy

Abstract

The columnar to equiaxed transition (CET) of grain structures presents significant challenges in titanium alloy additive manufacturing (AM), especially in wire arc additive manufacturing (WAAM) with highly localized heat input and large temperature gradient. In this work, the strategy of decoupling the relationship between heat source and remelting depth was proposed, which was achieved by altering the electrode connection type with arc discharge between the tungsten electrode and the welding wire (IPAW-Wire method). Compared to the conventional WAAM methods based on tungsten inert gas welding (Conventional-TIG method), the IPAW-Wire method reduces the average β grains width from 2 mm to around 200 μm and the maximum texture intensity by approximately three times. The decoupling strategy combined with thermal undercooling and periodic solidification effect of low-frequency pulse arc promotes CET results. The IPAW-Wire method increases tensile strength by 50–80 MPa without altering the alloy composition or making external equipment modifications, and significantly weaken the anisotropy of mechanical properties, both in terms of ultimate strength and plasticity. The strength enhancement and anisotropy reduction are attributed to the coupling of β grains refinement, weakened α crystallographic texture, fine needle-like α′ martensite, and high-density dislocation with multiple types of <a> dislocations, <c> dislocations and <c+a> dislocations. This innovative IPAW-Wire method effectively mitigates coarse columnar grains and anisotropy by decoupling the relationship between heat source and remelting depth. This control strategy can inspire other heat sources and material additive manufacturing process, addressing hotspot challenges such as programmable microstructure, metamaterials structure, multi-material, and bioinspired printing.