The effect of preheating on microstructure and mechanical properties of laser solid forming IN-738LC alloy

The microstructure and microhardness of Ni-base superalloy IN-738LC fabricated by laser solid forming (LSF) with the preheating was investigated. The results showed that with the increase of the preheating temperature (T-0) (25 degrees C, 800 degrees C, 900 degrees C and 1050 degrees C), the total crack length decreased significantly, the segregation of Ti and Al worsened, and the volume fraction of equiaxed grain increased, as well as the size of blocky carbides, gamma-gamma' eutectic and gamma' particles. The high-angle grain boundaries, at which the continuous liquid film can be easily formed, were found to be susceptible to cracking. Both the concentration of Ti and Al and the increase of dendrite spacing contribute to the growth of carbides and gamma-gamma' eutectic in the interdendrite while the size of the gamma' phases was largely affected by preheating. It was interesting to find that gamma' phase exhibited a bimodal distribution in the bottom of the deposit with the preheating at 1050 degrees C. The average microhardness of the deposits without preheating and with preheating at 800 degrees C and 900 degrees C is about at the same level, while the deposit with preheating at 1050 degrees C has the lowest average value. The crack-free deposit can be obtained when T-0 was up to 1050 degrees C, and its room temperature tensile properties are superior to cast IN-738LC alloy.