Research On Process Optimization And Refinement Mechanism Of Microstructure Of TC11 Titanium Alloy During Continuous Variable Cross-section Recycled Extrusion

The grain refinement of metal materials is one of the effective methods to improve properties of materials. Currently the refined titanium alloy obtained by most SPD which still exist many problems including small size of the workpieces, and obvious tendency of texture. So it is difficult to widely apply in production. Continuous variable cross-section recycled extrusion(CVCE) provides a procedure for obtaining the bulk micrometer scale materials by severe plastic deformation. Compared with other severe plastic deformations, CVCE mainly has the following advantages in simple operation and high efficiency, it can control the specimen uniform deformation and is more advantageous to realize industrial production.The effects of CVCE deformation process parameters and the heat treatment on the microstructure and properties of TC11 titanium alloy have been analyzed systematically.The temperature field, stess field and strain field have been simulated. In this paper, under the condition the main research contents and conclusions are as follows:2(1) The microstructure of TC11 alloy which was deformed by CVCE has been investigated. The effects of different temperatures, cycles and speeds on the microstructure of TC11 titanium alloy have been analyzed. The result shows that near the recrystallization temperature, the grain size of the alloy decreases rapidly with increasing deformation cycles. However, at higher temperature(950℃), the refinement degree of the grain is increased firstly and then decreased with the increase of deformation cycles. Moreover, with the increase of deformation temperature, the grain size decreases firstly and then increases due to the recrystallization. When the specimen is deformed at 850℃with 5mm/s under 6 cycles, the size of the primary α phase is reduced from 10μm to 4 ~ 4.5μm and the microstructure is distributed homogeneously.(2) The effects of annealing temperature and annealing time on the microstructure of TC11 titanium alloy that deformed by CVCE have been studied. The temperature choosed for heat treatment is near the recrystallization temperature. The process parameters were 850 ℃, 6 cycles, 2mm / s and 850 ℃, 6 cycles, 5mm / s. They have been heat treated under 850 ℃,1h/AC.The primary alpha size is 3 ~ 4μm, and the grain boundaries are clear. In addition, the equiaxed degree of the secondary alpha increases greatly.(3) The microstructure of CVCE and heat treatment has been observed by transmission electron microscope. The mechanism of microstructure has been studied. In the process of heat treatment, the grain is mainly refined by recrystallization.(4) The temperature field,stress field and strain field of a set of parameter(850℃,6cycles,5mm/s)which is the better deformation effect has been simulated by Deform software. In the process of CVCE, the deformation energy turned to be thermal. With the increase of deformation cycles, the strain-effecive accumulates continuously.The effecive strain of the middle sample which is deformed under 6 cycles can reach5.83~7.20, and is distributied more evenly.