MICROSTRUCTURE AND MECHANICAL PROPERTIES OF K4202 CAST NICKEL BASE SUPERALLOY FABRICATED BY SELECTIVE LASER MELTING

As a cast nickel base superalloy, K4202 is mainly used in aircraft engines due to its high strengths at elevated temperatures, excellent resistance to hot corrosion and favorable weldability. K4202 alloy is usually fabricated by the conventional casting method and mechanical processing, along with macro-segregation and excessive tool wear. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) is able to manufacture high-performance and complex components. According to the requirement of selective laser melting manufactured metal parts with complex structures in aerospace and other fields, K4202 alloy was used as material for SLM in this research and the forming technology, microstructure and mechanical properties of SLMed and heat-treated samples were studied. The results show that the microstructure of samples formed by SLM is composed of dendrites and isometric crystal. The growing direction of dendrites is nearly perpendicular to melt pool traces in most cases. The dendrite structures disappear completely after solution+ageing heat treatment on account of recrystallization and metal carbide precipitates in grains and at grain boundaries. The precipitates are able to improve prove the strength of the grain boundary due to the pinning effect. The microstructure has no significant changes after ageing heat treatment, but carbide precipitates at grain boundaries as well. The microhardness of SLM samples is uniform on cross section and vertical section. After solution+ageing and ageing heat treatment, there is a significant improvement on the microhardness. The mechanical properties for as -fabricated samples are superior to those of the cast K4202. Besides, the yield strength and tensile strength increase clearly after heat treatments and the mechanical properties is the highest after ageing heat treatment. This is because of the precipitation of gamma' strengthening phases. However, the obvious decrease in the ductility occurs at the same time.