Effects Of Surface Deep Rolling On Titanium, Nickel Surface Integrity And Its Thermal Stability

With the development of modern industry, the properties of the metal material become more and more important. Material failure generally occurs on the surface. Therefore, surface strengthening technology has become an effective method to improve the service life of components. The surface mechanic enhancement can improve the strength of components without reducing the toughness through grain refinement. In this way, the service life of parts can be extended effectively. However, under the high temperature environment, the reinforced organization is often on the unstable state. So the evolution of microstructure and relaxation of residual stress at high temperature need further analysis. Through a new surface self-nanocrystallization method--surface deep rolling(SDR) treatment, the gradient ultra fine grain structure whose grain sizes are less than 500nm is prepared on pure titanium (TA2) and pure nickel (N4). And the microstructure, residual stress field and mechanical properties of the material are studied. The surface integrity and thermal stability are also considered. The research results show:(1) After the surface deep rolling, the severe plastic deformation has occurred on the surface and subsurface of material. Therefore, a large number of dislocations are generated, and the grain is obviously refined. Finally ultrafine grain and nanocrystalline are formed.(2) Due to the differences of crystal structure, the refining mechanisms of pure titanium and pure nickel are slightly different. In pure titanium, refinning is concerned with twinning. While the evolution of dislocation structure plays a major role on refining of pure nickel. And the transformation from small angle grain boundaries to large angle grain boundaries bases on the continuously dynamic recovery and recrystallization.(3) After the surface deep rolling, the residual compressive stress field with a certain thickness is formed near the surface. Compared with the as-received material, the hardness of the material increased after SDR treatment. The thickness, hardness, surface roughness of refined layer and residual stress field within it can be adjusted through proper selection of treatment parameters(4) In the high temperature environment, the residual stress induced by deep rolling treatment will be released, and the grain size can be changed.