Effects of the solution and first aging treatment applied to as-built and post-HIP CM247 produced via laser powder bed fusion (LPBF)

Abstract

In this work CM247LC, a low weldable Ni-Based alloy, was produced using selective laser melting (SLM). Despite the initial process parameter optimization, the low defect volume fraction was still uncompliant with manufacturing standards. This condition is principally caused by the high γ’ volume fraction which strongly affects the alloy weldability. Nonetheless, a crack free condition was eventually achieved applying a γ’-sub-solvus Hot Isostatic Pressing Cycle (HIP) which lowered the defects fraction down to 0.04%. The HIP cycle also demonstrated to play an important role in the stabilization of the microstructure, considerably limiting the carbides coarsening during the following heat treatment. Apart from the effectiveness of the healing process brought by HIP, the material microstructure still needs an optimization process which will be described along this paper. In fact, the Initial microstructure obtained after the printing process (the as-built condition) as well as the one obtained after HIP (post-HIP) won’t meet the desired requirements. Namely, the dendritic and γ’ free microstructure of the as-built material or the one with coarse and disordered particles obtained right after HIP, still need a tailored homogenization process. This paper will show how the combined effect of the solution and first aging treatment will profoundly alter the γ’ precipitation. More specifically, here, a new heat treatment recipe was developed to promote the precipitation of ordered cuboidal primary γ’ so as to improve creep and high temperature fatigue resistance. Moreover, the use of a γ’ super-solvus temperature allowed to achieve a γ’ volume fraction as high as 73% reducing its average size to 520 nm. At the same time, such heat treatment caused a profound alteration of the crystalline structures of the material promoting a general grain coarsening and the formation of equiaxial grains.