Titanium alloys are widely used, while are typical hard-to-process materials. Improving its machinability is an important way to reduce manufacturing costs and improve machining efficiency. In addition to exploring appropriate processing technology and parameters, the research focus is also on improving titanium machinability by controlling the microstructural characteristics.
Common modification methods of titanium alloys have their own disadvantages, such as alloying element addition and processing, which are irreversible processes. However, the heat treatment of titanium alloy also has the problems of easy oxidation and poor dimensional stability. Due to the reversible alloying and high affinity of hydrogen in titanium alloys, thermal hydrogen processing (THP) can be used to improve the processability of titanium alloys. The main processes are hydrogen replacement, processing and dehydrogenation.Heat treatment with hydrogen under a hydrogen atmosphere effectively avoids the oxidation of titanium alloys. its processability after hydrogenation treatment is improved, and subsequent dehydrogenation treatment makes alloy restore its good comprehensive mechanical properties.
With the deepening of research, the mechanism of modification of hydrogen-induced titanium is mainly divided into hydrogen plasticizing and hydrogen-induced phase transition. However, when the set hydrogen content is supersaturated, the hydrogen-induced titanium shows "hydrogen brittleness".Zong et al. studied the deformation behavior of TC21 after hydrogen treatment at high temperature and found that the flow stress of TC21 alloy first decreases and then increases with the increase of hydrogen content (mass fraction of H), thus obtaining the optimum hydrogen content of 0.3%, at which time the flow stress decreases by 26%.Li et al. found that the optimum superplastic temperature of Ti-55 alloy decreased by about 125 C after adding 0.1% H, which the team attributed to hydrogen. β The phase transition temperature decreases and promotes dislocation movement, thus increasing the β volume fraction of the phase. The results of Losertov et al. show that the deformation resistance of Ti-6Al-4V alloy after hydrogenation decreases at 700~750 C, compared with the microstructure of the group without hydrogen. More hydrogen groups appear at the same temperature β Phase. The above research shows that proper hydrogen addition has the characteristic of modification gain for titanium alloy, so THP is also introduced to improve the cutting machinability .
