A customised novel hybrid post-treatment process achieved excellent mechanical properties in additively manufactured Haynes 230 alloy

It is challenging to maintain decent plasticity while increasing strength in laser powder bed fusion (LPBF) manufactured Haynes 230 alloy. The fundamental issue of both inadequate affected depth and deteriorated plasticity exists for laser shock peening (LSP). To address this problem, the treatment method of heat treatment plus laser shock peening (HT-LSP) was proposed. After heat treatment, dispersive M23C6 carbides were precipitated, and dislocation was predominately decreased inside grains but increased at grain boundaries. The junction of three imperfect dislocations was identified at M23C6 after HT-LSP treatment, indicating the novel phenomenon of twin formation via the polar-axis mechanism of dislocation proliferation, given the inhibition of intergranular transfer of dislocations and adequate clean rooms without dislocation clusters inside grains. Grain rotation occurred after HT-LSP derived from dynamic recrystallisation and twin distortion. The hardness along depth direction of the sample demonstrated the increased effective affected depth of LSP from 900 to 1400 μm after heat treatment. The shear strength of the HT-LSP sample was increased to 662 MPa. The elongation of the sample reaching 13.4% was also higher than that with LSP. The mechanical performance improvement was mainly due to the gradient twin layer, carbides’ precipitation and high-density dislocation.