On the stray grains in single-crystal Ni-based superalloy by additive manufacturing: formation mechanism and elimination strategy

Single-crystal nickel-based superalloys are indispensable for aerospace engine turbine blades due to their superior high-temperature performance. Traditional manufacturing methods are complex and costly, with additional expenses from scrapping damaged blades. Directed energy deposition (DED) offers an effective solution for blade repairs and a promising approach for direct blade production. This study is dedicated to understanding how processing parameters affect the formation of stray grains (SGs) and cracks in flat-top laser DED, and proposing feasible strategies to eliminate SGs and cracks. Results show that surface SGs are mainly due to the excessive powder feeding, and the SGs formed in the sample interior near to the substrate result from the occurrence of constitutional undercooling during the deposition of the first track. Unidirectional laser scanning can also trigger the nucleation and growth of SGs, but this can be significantly reduced through bidirectional laser scanning. For DED multi-track bulk samples, the hatch spacing is found to be the determining factor for the formation of SGs. Cracks can be completely eliminated if formation of high angle grain boundaries are inhibited during DED. Finally, crack-free multi-track samples were achieved using the optimized process parameters, demonstrating the feasibility of proposed approach in enhancing the integrity of additively manufactured single-crystal nickel-based superalloys.