Investigation of hot cracking in additive manufactured nickel-base superalloys

Abstract

Additive manufacturing (AM) offers an unprecedented freedom of design and enables production of complex geometries with competitive mechanical properties, such as components for aerospace engines and gas turbines. Some commonly used materials, such as γ’ precipitation strengthened Ni-base superalloys, are susceptible to cracking during both AM processing and subsequent densification and/or heat treatment. Therefore, to fully utilize the potential that AM offers, it is crucial to optimize the manufacturing process in order to minimize the amount of defects in the final product. The aim of this study is to reduce the amount of defects, primarily micro-cracks, in a laser powder bed fusion (LPBF) processed γ’ precipitation strengthened Ni-base superalloy. LPBF process parameter optimization, by using a design of experiment approach lead to reduction of defect density to very low levels. This was followed by a statistical data analysis to investigate how different parameters relate to defect formation. Furthermore, it is shown that post-AM hot isostatic pressing (HIP) can completely eliminate remaining micro-cracks. In addition, after HIP above a certain temperature and pressure (1210°C and 1000 bar), cracks did not re-open during subsequent high temperature heat treatment. SEM/EDX analysis showed that only small non-metallic inclusions remained after HIP+HT at these conditions. Finally, it is shown that by applying a tailored temperature-pressure profile, strain age cracking during HIP and heat treatment can be completely avoided.