Stress relaxation in a nickel-base superalloy at elevated temperatures with in situ neutron diffraction characterization: Application to additive manufacturing

The complex thermal histories in additive manufacturing (AM) of metals result in the presence of residual stresses in the fabricated components. The amount of residual stress accumulated during AM depends on the high temperature constitutive behavior of the material. The rapid solidification and repeated thermal cycles with each laser pass result in material contraction, and subject the surrounding, constrained material to both elevated temperatures and internal stresses, providing driving forces for stress relaxation. In this study, the stress relaxation behavior and mechanisms of conventionally processed and additively manufactured Inconel 625 (CP-IN625 and AM-IN625) at 600 °C and 700 °C were investigated via compression tests up to an engineering strain of 9% with in situ neutron diffraction characterization. The stress decayed to a plateau stress equivalent to 18% of the peak stress in CP-IN625 and 16% in AM-IN625 at 600 °C, and 39% in CP-IN625 and 44% in AM-IN625 at 700 °C. At the same temperature, the stress relaxation rate in AM-IN625 was twice as high as that in CP-IN625, and the magnitude of the plateau stress in AM-IN625 was slightly lower than that in CP-IN625, as the textured AM-IN625 had much larger grains than the texture-free CP-IN625. The stress relaxation in CP- and AM-IN625 was deduced to be controlled by dislocation glide and climb, where dislocations interact with grain boundaries, solute atoms, and secondary phases. The stress relaxation constitutive behavior reported here is a necessary input for the development of accurate thermomechanical models used to predict and minimize residual stresses and distortion in AM, as well as to predict the stress relaxation behavior of Inconel 625 in high temperature structural applications.