Microstructural Evolution Mechanisms Of Titanium And Aluminum Alloy Due To Deep Rolling

Lots of surface treatment techniques have been developed and used to extend the serving lives of components. Among these techniques, surface modification techniques through cold working, such as shot peening, laser shot peening and deep rolling, have received considerable attentions. By using these techniques, nanocrystalline and sub-microcrystalline may be generated in the surface layer due to plastic deformation. Nevertheless, up to now, there are three main contents need to be systematically studied, namely mechanism of formation of ultrafine crystallites due to plastic deformation, relationship between the microstructure and mechanical properties of the surface layer, the thermal stability of the surface layers. This thesis was aimed to solve the above problems. Evolution of microstructure and residual stress and the thermal stability of the surface layer of pure titanium and2024aluminum alloy after deep rolling treatments were investigated. The main research contents and conclusions can be summarized as follows:(1) Equiaxed nanograins were obtained on the top surface of pure titanium due to deep rolling. The mechanism of surface grain refinement can be divided into four steps, namely the onset of twins and the intersection of twin systems, the formation of dislocation and lamellae, the subdivision of lamellae into blocks and submicrocrystalline, and the formation of nanocrystalline.(2) The thermal stability of surface nanocrystalline layers of pure titanium after deep rolling treatment due to annealing heat treatment was investigated through observing the microstructural change. The microstructure had no apparent change when annealing teamperature was lower than500℃. As the annealing temperature reached600℃, the size of nanocrystalline increased rapidly..(3) The microhardness near the surface of pure titanium and2024aluminium alloy significantly increased after deep rolling treatments. with increasing distance from the surface, the magnitude of microhardness decreased gradually. The residual compressive stress increased, reached a local maximum and then decreased in the depth direction. Moreover, the deep rolling treatments can lead to the obvious decrement of the surface roughness.