Surface Nanocrystallization On Copper By Burnishing And CPFEM Simulation

The research of traditional burnishing places emphasis on surface finishing and stress strengthening. In fact, surface self-nanocrystallization(SNC) can be realized by the improvements of traditional surface strengthening techniques. On one hand, the improvements make it possible to fabricate a nanocrystalline surface layer with a peculiar structure on traditional engineering materials, leading to a dramatic increase on material properties. On the other hand, the improvements offer a new approach and idea for the preparation of nanocrystalline materials. Meanwhile, extruded-typed SNC technique will be helpful to improve the traditional burnishing. Therefore, in our paper, a surface strengthening technique called Severe Plastic Roller Burnishing(SPRB) is proposed. The grain refinement mechanism of gradient nano-micro structure and the properties of nanocrystalline surface layer induced by SPRB have been studied extensively. Crystal Plasticity Finite Element Method(CPFEM) is a powerful tool to simulate the mechanical response and texture evolution of materials under local severe plastic deformation. Given this, the SPRB process is simulated by CPFEM to research on the mechanical response and orientation stabilities. The main contents of our paper are as follows:(1) Use SPRB to realize the surface nanocrystallization on copper. The grain size of nanocrystalline surface layer is about 4~18 nm. The microstructure characteristics and surface properties of samples at various depths from the topmost surface and cross section were investigated by SEM, TEM, XRD and OM. The grain refinement mechanisms along different depths were revealed. In the nanocrystalline layer(depth from 0~30 μm), the formation of nanostructure is dominated by dislocation activities accompanying with the rotation of grains in some local regions; in the submicron layer(depth from 30~100 μm), deformation twinning is found to be the primary form for the formation of submicron grains;. in the micron layer, the coarse grains are refined into the few micro-sized grains by the dislocation activities.(2) Research the influence of process parameters on mechanical properties and surface performance on the burnished copper. The fretting tests in the form of ball-to-plane on the burnished copper and CG copper under dry condition were performed. The results indicate that SPRB can enhance the hardness of samples. The surface hardness is 70% upon that of matrix. The burnished copper has a better friction and wear performance in the whole load range than that of CG copper. Meanwhile, SPRB can improve the surface quality of samples. The research results on surface finishing effect of SPRB indicate that SPRB can reduce the surface roughness up to 2% that of the original roughness; exist a proper burnishing force and burnishing speed to get the lowest surface roughness; burnishing force is the main factor that influences the surface roughness and microhardness, burnishing speed comes the second, turning feed rate is the lowest.(3) Build the shape-controllable 2D and 3D Voronoi geometries, then extract their geometric topology information. Build the finite element model of single(poly-) crystalline by reconstructing the geometric features in ABAQUS through the interactive environment between Python and Matlab. Design an orthogonal grid algorithm to mesh the polycrystalline geometry model. Develop the user defined material constitutive relation subprogram to realize the CPFEM simulation in ABAQUS/CAE. Then verify the UMAT subprogram through the CPFEM simulations on uniaxial tension, uniaxial compression and simple shear. The results of stress-strain response, slip systems deformation and crystal rotation are consistent with other examples.(4) Orientation stabilities of FCC ideal orientations during SPRB was predicted using CPFEM. The result of the simplified model under the condition of compression on normal direction(ND) and tension on transverse direction(TD) indicates that rotated Goss orientation is stable and Cube orientation is metastable, which is consistent with the texture results of the burnished nanocrystalline copper in surface layer. The stabilities of orientations in the model analogous to rolling are similar with those in cold rolling on pure copper and aluminum.