Microstructure and hardness of S304/Ni25/TC4 functionally graded materials fabricated by laser solid forming

Functionally graded materials consisting of S304, Ni25, and TC4 were successfully prepared using laser solid forming technology. The microstructure, composition, and microhardness of the gradient materials were analyzed. As the proportion of Ni25 increased in the S304/Ni25 gradient range, the microstructure in the deposit mainly consisted of columnar dendrites, with an increasing amount of white and bright precipitates between the dendrites. In the Ni25/TC4 gradient range, the volume of eutectic structure between dendrites increased as the proportion of TC4 increased, with a change in morphology from fishbone-shaped to typical basketweave and eventually to petal-shaped. The gradient materials also exhibited numerous low melting point eutectic phases near cracks, showing epitaxial growth along the deposition direction. The Scheil solidification curves of gradient materials with different alloy composition ratios were calculated using Thermo-Calc thermodynamic software. It was observed that as the proportion of Ni25 increased in the S304/Ni25 gradient range, the γ′ reinforcing phase gradually precipitated between dendrites, resulting in an increase in hardness from 207.5 HV to 332.8 HV. The addition of TC4 significantly increased the hardness, reaching its peak at 20% TC4 content. However, a further increase in TC4 content led to the precipitation of Laves phase in gradient materials with a high TC4 proportion, resulting in a decrease in the content of γ′ reinforcing phase between dendrites and ultimately resulting in a decrease in hardness.