Achieving a Large-Area Bulk Ultrafine-Grained Titanium Alloy with High Thermal Stability Via Layer-by-layer Multi-Pass Overlapping Friction Stir Processing

It is difficult to prepare large -area bulk ultrafine-grained (UFGed) titanium alloys with uniform structure via multi -pass overlapping friction stir processing (MP-FSP) due to the low thermal conductivity and poor flow deformation ability of titanium alloys. Here, we proposed a new layer -by -layer MP-FSP method to control the defect and microstructure of the Ti-6Al-4V alloy. As a result, the as -cast coarse microstructure was changed directly to the defect -free large -area bulk UFG microstructure. The bulk UFG exhibited a uniform microstructure with average grain sizes of 0.7-0.82 mu m and over 90% of high angle grain boundaries. Moreover, the bulk UFG titanium alloy exhibited high hardness (357 HV) and great ultimate tensile strength (1177 MPa), which were much higher than those (308 HV and 739 MPa) of the base material. In addition, the bulk UFG had good thermal stability at a high temperature of 700 degrees C. Different from the bulk MP-FSP UFGed aluminium alloys where there is generally abnormal grain growth (AGG) at high temperatures, there was no AGG phenomenon in bulk MP-FSP UFGed titanium alloys even at 900 degrees C. This study provides an effective short -process method for preparing largearea bulk UFG titanium alloys with uniform structure and good thermal stability.