Powder Bed Fusion – Laser Beam of a non-weldable Ni-base superalloy CM247LC: Microstructure control, crack mitigation, heat treatment and creep performance

Doktorsavhandling, 2025

Powder Bed Fusion – Laser Beam (PBF–LB) of Ni-base superalloys is highly attractive for components in high temperatures applications, particularly within the aerospace and industrial gas turbine sectors. This interest is due to the high degree of design freedom which allows complex internal cooling channels that significantly improve the component lifetime or enable higher operating temperatures, thereby boosting gas turbine efficiency. However, superalloys containing high volume fraction of γ′, like CM247LC with 60 to 70 vol.%, exhibit poor PBF–LB processibility, suffering from both micro- and macro-cracking, and have poor and anisotropic creep performance. The aim of the thesis is to understand the impact of the intricate relationships between PBF–LB process parameters, cracking, residual stresses, heat treatment, microstructure and creep performance. Using operando synchrotron X-ray radiography, the observed micro-cracking is confirmed to occur during solidification. The micro-cracks are observed at the high angle grain boundaries exhibiting a distinct solidification structure consistent with the solidification cracking mechanism. Solidification cracking is mitigated either through alloy/powder modification or through process parameter optimization with any remnant solidification cracks eliminated through hot isostatic pressing (HIP). However, the presence of macro-cracks, particularly around stress concentrators, is observed after HIP. This macro-cracking is identified as strain age cracking (SAC) due to the γ′ precipitation. SAC is minimized by a tailored HIP using higher pressure before reaching solution treatment temperature. Minimizing residual stress by varying PBF–LB process parameters, including scan strategies, is effective in minimizing SAC. The solution heat treatment developed for cast CM247LC when applied to CM247LC produced via PBF–LB indicated limited grain growth and poor creep performance. A re-designed HIP combined with a solution heat treatment at higher temperature of 1280 °C demonstrates 23% increase in creep rupture life compared to the standard 1250 °C, yielding slightly coarser grains, improved grain boundary decoration, and finer uniform cuboidal γ′ precipitates. Despite this optimization, the creep performance remained inferior to that of the cast alloy. Subsequent tailoring of the PBF–LB process to produce a highly columnar and anisotropic microstructure significantly increased the creep life along the build direction. In summary, this investigation into the PBF–LB of CM247LC demonstrates that the effective mitigation of cracking and improvement of creep performance requires tailored PBF–LB processing and post-processing heat treatment strategies.