Microstructure evolution and failure mechanism of SP2215/Stellite-6 components prepared by laser directed energy deposition during high-temperature service

Additive manufacturing via laser directed energy deposition (L-DED) is an effective method for fabricating advanced coatings to enhance surface wear and corrosion resistance. However, cracking and spallation during high-temperature service of these coatings can significantly reduce their service life. This study systematically investigates the microstructural evolution and thermomechanical coupling behavior at the SP2215/Stellite-6 interface, where Stellite-6, a cobalt-based alloy, was deposited via L-DED onto austenitic stainless steel 22Cr15Ni3.5CuNbN (SP2215). L-DED-fabricated SP2215/Stellite-6 samples underwent prolonged thermal exposure at 650 °C for up to 5000 h to simulate service conditions. The decomposition of M₂₃C₆ along grain boundaries within Stellite-6 accelerates the diffusion of Cr and C atoms, leading to the reprecipitation and Ostwald ripening of M₂₃C₆ carbides along the SP2215/Stellite-6 interface and within Stellite-6 grains. These carbides act as crack initiation sites, promoting crack propagation and reducing fracture strength at both the interface and within the deposited Stellite-6 layer. The interfacial shear strength and impact absorbed energy decreased from 435.2 MPa and 82.8 J in as-built condition (0 h) to 112 MPa and 5.9 J after 5000 h high-temperature service, respectively. These findings can provide critical insights into the mechanisms of cracking and spallation failure in L-DED-fabricated cobalt-based alloy coatings under long-term and high-temperature service.