Crack mitigation in Laser Powder Bed Fusion processed Hastelloy X using a combined numerical-experimental approach

Laser Powder Bed Fusion (LPBF) is an additive manufacturing (AM) technique associated with large thermal gradients and thermal strains resulting from the inherent nature of the process. Due to the non-equilibrium solidification conditions, certain alloys suffer from thermally induced micro-cracking, which hinders their exploitation. In this paper, we propose a novel approach towards the processing of a defect-free nickel-based superalloy, Hastelloy X, by means of Laser Powder Bed Fusion (LPBF). A combined experimental and numerical approach was used in order to evaluate the influence of the laser beam intensity distribution and melt track overlap on the temperature evolution, solidification behavior and crack formation. The use of a laser beam with narrow diameter and Gaussian beam intensity distribution leads to keyhole mode melting and a steep cooling rate, whereas a large diameter beam with top-hat intensity distribution leads to the formation of wide and shallow conduction mode melt pools, with a lower cooling rate and consequently a coarser sub-grain microstructure. The decrease of hatch spacing did not show a significant change in the local thermal history within a melt pool when a Gaussian beam was used. On the other hand, with the laser with the top-hat beam profile was used, an overall increase in the temperature of the layer and significantly lower cooling rates were observed. As a result, by decreasing the hatch spacing to 125 µm, micro-crack free Hastelloy X material was produced by LPBF for the first time, using a commercially available powder, simply by optimizing the scanning strategy and without modification of the chemical composition of the alloy. The material in the as-build condition exhibited a higher yield tensile strength (500 ± 17 MPa) than conventionally produced Hastelloy X (340 MPa), with elongation of 31.5 ± 3.0%.