Laser powder bed fusion of Ni-based Hastelloy X superalloy: Microstructure, anisotropic mechanical properties and strengthening mechanisms

Ni-based Hastelloy X (HX) superalloy is comprehensively employed in the aerospace industry because of its excellent oxidation resistance and high-temperature strength. Nevertheless, due to the insufficient overlapping feature of the process, extremely high porosity and surface roughness are formed between layers, which are primarily affected by processing parameters. In this study, statistical models regarding the effect of process parameters on density and surface roughness for laser powder bed fusion (LPBF)-fabricated HX were established via a response surface method, and interactions among density, surface roughness and processing parameters were systematically investigated. A multiple quantitative analysis of anisotropic strengthening mechanisms was established in terms of the grain boundary, solid solution and dislocation strengthening mechanisms. Experimental results demonstrated a maximum density (8.294 +/- 0.003 g/cm(3)) and minimum surface roughness (6.81 +/- 0.12 mu m) within a set of optimised parameters (laser power = 165 W, scanning speed = 1000 mm/s and scan spacing = 0.103 mm) using a multi-response optimisation approach. For the LPBF-fabricated HX, the XOY plane (perpendicular to build direction) exhibited higher strength (yield strength = 696 MPa; ultimate tensile strength = 892 MPa) than compared with the YOZ plane (parallel to the build direction) along the build direction, which was primarily attributed to high dislocation density (1.28 x 1014/m2) and refined grain size (13.9 mu m). Multiple quantitative analyses indicated that the difference in dislocation strengthening dominated the anisotropy in the mechanical properties between the two planes of the LPBF-fabricated HX, whereas solid solution and grain boundary strengthening contributions dominated in the wrought counterparts. This work offers new insights for meeting the requirements for a multi-degree freedom of design within Ni-based parts with customised performance in additive manufacturing.