The Heat Treatment Influence on the Microstructure and Hardness of TC4 Titanium Alloy Manufactured via Selective Laser Melting

In this research, the effect of several heat treatments on the microstructure and microhardness of TC4 (Ti6A14V) titaniumalloy processed by selective lasermelting (SLM) is studied. The results showed that the original acicular martensite alpha'-phase in the TC4 alloy formed by SLMis converted into a lamellar mixture of alpha + beta for heat treatment temperatures below the critical temperature (T-0 at approximately 893 degrees C). With the increase of heat treatment temperature, the size of the lamellar mixture structure inside of the TC4 part gradually grows. When the heat treatment temperature is above T-0, because the cooling rate is relatively steep, the beta-phase recrystallization transforms into a compact secondary alpha-phase, and a basketweave structure can be found because the primary alpha-phase develop and connect or cross each other with different orientations. The residence time for TC4 SLMparts when the treatment temperature is below the critical temperature has little influence: both the alpha-phase and the beta-phase will tend to coarsen but hinder each other, thereby limiting grain growth. The microhardness gradually decreases with increasing temperature when the TC4 SLM part is treated below the critical temperature. Conversely, the microhardness increases significantly with increasing temperature when the TC4 SLM part is treated above the critical temperature.