Study on the characteristics of functionally graded materials from Ni-20cr to Ti-6Al-4V via directed energy deposition

Abstract

Functionally graded materials (FGM) have been developed by gradually combining two or more materials and are used to improve the properties of metallic materials, such as heat resistance, corrosion resistance, and hardness. FGM can be effectively fabricated using additive manufacturing. In this study, FGM were fabricated using a direct energy deposition method with Ni-20Cr and Ti-6Al-4V metal powders. The initial layer contents were 100% Ni-20Cr and 0% Ti-6Al-4V, and the final layer contents was 100% Ti-6Al-4V and 0% Ni-20Cr, for a total of 11 layers stacked sequentially. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and phase diagram analyses were performed to examine the microstructure, composition, microhardness, and crack generation. Intergranular cracks were observed owing to the Cr depletion zone in the Ti alloy contents 40 ∼ 60% region. Cr carbides, such as Cr23C6, are responsible for the Cr depletion zone. A calculated phase diagram was used to investigate the Cr carbide precipitation temperature, the deposition conditions were suggested to prevent intergranular cracks and the effect was verified through a verification experiment. These experimental results can be used to develop other stable FGMs.