Realizing Crack Inhibition of Haynes 230 Alloy during Laser Powder Bed Fusion by Grain Refinement under Decreased Energy Density

Nickel-based superalloys (Haynes 230) fabricated by laser powder bed fusion suffer from high cracking susceptibility, leading to a decrease in mechanical performance. In this study, the cracking mechanism of Haynes 230 was investigated based on microstructural and thermodynamic calculations. It was found that C and carbide-forming elements (such as Mo and Cr) were segregated at the grain boundaries, which increased the solidification range and impeded liquid film backfalling by forming nano-carbides. Additionally, the coalescence of high-angle grain boundaries (>15°) requires a higher undercooling ΔTbthan that of low-angle grain boundaries (2-15°), which increases the susceptibility to hot cracking. Through gradually reducing laser energy input, the grain size is significantly decreased from 27.86 μm (47.40 J/mm3) to 14.66 μm (31.81 J/mm3). Moreover, the calculated cooling rate |dT/dt|and temperature gradient |dT/ds| gradually increase with decreasing energy input, which reduces the duration of dendrite merging and shortens the length of the liquid film. Compared with cracked samples, the optimized sample showed superior mechanical properties, including high yield strength (678 MPa), ultimate tensile strength (943 MPa), and elongation to failure (19.2%), which increased by 16.1%, 9.7%, and 77.7%, respectively.