Significant improvement in the strength-toughness and isotropy of laser powder bed fused Ti6Al4V alloy by combining heat treatment with subsequent laser shock peening

In this study, the influences of heat treatment (HT) in combination with subsequent laser shock peening (LSP) on the strength-toughness and isotropy of laser powder bed fused (LPBFed) Ti6Al4V alloy were investigated. Results suggested that the ultimate tensile strength (UTS) of the 0°, 45°, and 90° HT1-LSPed specimens were 1283 MPa, 1276 MPa, and 1273 MPa, respectively, with corresponding elongation (EL) of 20.5%, 19.6%, and 19.7%, respectively. Compared to these three LPBFed specimens, the UTS values increased by 14.3%, 20.0%, and 24.8% while the EL values increased by 133.0%, 117.8%, and 72.8%, respectively. Meantime, the differences in UTS and EL were significantly reduced in different build directions (BDs). The fracture modes of the three HT1-LSPed specimens changed from mixed fracture (brittle and ductile) to ductile fracture. Microstructural characterization indicated that HT transformed the originally needle-like α′ grains into stable α and β grains in the LPBFed Ti6Al4V alloy in different BDs, significantly reducing the anisotropy of the microstructures. The subsequent LSP induced deeper plastic deformation layers, generating favorable gradient CRS, gradient microstructures (high-density dislocation structures, nanocrystallines, and nanotwins), and gradient microhardness. HT1-LSP-induced CRS strengthening, microstructural strengthening (deformation strengthening, fine grain strengthening, and twinning strengthening), and microhardness strengthening contributed decisively to the significant improvement in strength-toughness and isotropy of the Ti6Al4V alloy fabricated by LPBF in different BDs.