Selective laser melting of nickel superalloys for aerospace applications: defect analysis and material property optimisation

Selective laser melting (SLM), as a main branch of additive manufacture (AM), is drawing increasing interest within both academia and industry, particularly in aerospace, owing to the ability to produce complex net-shape objects directly from designed files without expensive tooling. However, the knowledge on the influence of SLM processing parameters on defect and microstructure formation are limited. Therefore, this research investigates the SLM-fabricated nickel superalloys IN713C and LR8. IN713C is a commercially used γ’ precipitation hardenable Ni-based superalloy, which can be limited by the high weld-crack susceptibility during processing. While LR8 is a newly designed material specific for AM, and thus the knowledge of its properties is very limited.
This research aims to establish a viable SLM processing route for both IN713C and LR8. Parametric studies are carried out to understand the influence of process variables on defects and microstructure. With the help of a melting pool simulation and defect classification script, the mechanism for defects and microstructures can be understood. The concepts of “laser travel time” and “apparent power” are introduced to analyse the effect of volumetric parameters, including part geometry and scan strategy. The effect of powder geometry on part quality was revealed in LR8 experiments, which need to be studied further.
The viable heat treatment (HT) for SLM-fabrication of IN713C has been investigated to refine the γ/γ’ microstructure to determine if its mechanical properties can be improved. Scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), and X-ray powder diffraction (XRD) were used to reveal the microstructure evolution through processing stages.
Mechanical tests (i.e., tensile strength, indentation hardness) were applied to compare SLM-fabricated and casted materials. The SLM IN713C generally shows better mechanical properties in all conditions. In general, both SLM-fabricated IN713C and LR8 show the potential to be used as aerospace components with proper processing parameters.