Effect of solution cooling rates on microstructure and mechanical properties of K648 high chromium superalloy additive-manufactured by the extreme high-speed laser metal deposition

High chromium superalloy K648, additive-manufactured by extreme high-speed laser metal deposition (EHLMD) process, was heat treated by solution and then cooled at different cooling rates achieved by different conditions in this study. The investigation focused on the impact of solution cooling rates (water-cooling (WC), air-cooling (AC) and furnace-cooling (FC)) on the microstructure and mechanical properties of EHLMD K648 superalloy. As the solution cooling rates decrease, it was observed that grain sizes, carbide, and α-Cr grew larger. Simultaneously, the carbides at the grain boundary evolved from a discontinuous granular shape to a chain-like shape, while α-Cr transformed from short needle to a short rod shape. Concurrently, the quantities of secondary γʹ phase diminished, but the size of secondary γʹ phases enlarged as the cooling rate decreased. Additionally, a significant amount of small granular tertiary γʹ phase emerged in the FC superalloy. The microhardness of the EHLMD superalloy exhibited a substantial increase with the decrease of the solution cooling rate, to the point that the microhardness of FC superalloy reached 429 HV. The ultimate tensile strength and yield strength increase, but the elongation decreases significantly, with the ultimate tensile strength of FC superalloy reaching 1058.3 MPa. The fracture of EHLMD K648 superalloy gradually transformed into ductile fracture to ductile and brittle mixed fracture as solution cooling rates decreased.