Effects Of Deep Rolling On Evolution Of Mechanical Properties And Residual Stress Of Pure Titanium

With the development of mechanical industry, the mechanical properties of metal materials became more and more important. Failure of materials usually occurred at the surface or initiated from the surface. In order to improve the service lives of components, surface modification technologies have been widely used. Mechanical surface enhancement (MSE) was one of the surface modification technologies to induce the surface microstructural change and residual stress. In such a case, the surface damage may be avoided.. However, the residual stress resulting from the mechanical surface enhancement may be not stable at high temperature due to the change of microstructure, such as grain size. Hence, when these components serviced at high temperatures, it is necessary to consider the changing of refined microstructures and the relaxation of residual stress. In this thesis, pure titanium was treated by using deep rolling. The evolution microstructure and mechanical properties due to deep rolling and after thermal annealing were investigated. The research contents and conclusions can be summarized as follows:(1) Optimal deep rolling parameters were selected. When titanium was treated by deep rolling, the surface microstructures can be refined, leading the increment of tensile strength. The compressive residual stress can be induced.(2) The refined microstructures were stable when annealing temperature was below the recrystallization temperature. With increasing the annealing temperature, the tensile strength decreased and the elongation increased gradually.. When annealing temperature reached recrystallization temperature, the grain size began to increase with increasing the temperature. However, the grain size after annealing treatment was generally smaller than that of as-received material. The strength of the deep rolled material after annealing treatment was almost equivalent to that of as received material, but the former exhibited higher proportional limit.(3) The annealing treatment would lead to the continuous relaxation of residual stress. A modified Zener-Wert-Avrami model was proposed to predict the stress relaxation behavior. The mechanism of residual stress relaxation was illustrated though the evolution of microstructures.