The fabrication of Ni-based single-crystal (SX) superalloys through laser directed energy deposition (L-DED) is hindered by the high susceptibility of SX structures to hot cracking. Therefore, achieving crack-free SX superalloys during L-DED is crucial for advancing the application of this technology in SX turbine blade repair. Based on solidification shrinkage and solid-bridging theory, this study systematically investigated the formation mechanisms of hot cracks in a multi-pass multi-layer DD6 SX prepared by L-DED through microstructure characterisation and coupled thermal-mechanical simulations. The results demonstrate that the initiation and propagation of hot cracks are governed by the overlapping characteristics at the inter-pass and interlayer regions, which influence the formation of liquid films and localisation of the stress-strain concentration. Furthermore, the results revealed that the formation of stray grains and hot cracking mutually amplified each other. To address these challenges, a novel strategy for hot crack suppression is to optimise the dwell time at the inter-pass and interlayer regions to regulate the dendrite growth and elemental segregation. Consequently, a three-pass five-layer Ni-based single-crystal sample with a width of 2–3 mm was successfully prepared, which was free of cracks. Moreover, the hot crack suppression method was applied to single-pass thin-wall deposition, achieving a single-crystal structure with height exceeding 10 mm and a proportion of over 95 %. The repair quality surpassed the requirements for single-crystal blade repair. This study provides new insights into the thermal-mechanical mechanisms underlying hot cracking and establishes a scientific framework for mitigating the cracks in L-DED DD6 SX, thereby advancing their applicability in high-end component repair.
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