Study On Preparation And Performance Of Surface Nanocrystallization Of Commercial Pure Al And Cu By Rotation Accelerated Shot Peening

The major failure modes of metallic engineering materials are facture, corrosion and wear. The wear failure mode occurred directly on materials surface, and the corrosion and fatigue crack are also originated and started from the surface of materials. Therefore, surface engineering-tailoring the surface microstructures and compositions via physical and chemical techniques to satisfy the industrial requirements for the surface properties- is one of the important fields in materials science and engineering.In this dissertation, by using of rotationary acceleration shot peening equipment (RASP), nanocrystalline and ultra-fine grained surface layers with certain thicknesses were prepared for industrial pure copper and aluminum.Moreover, optimum production technology parameters were obtained by changing the process parameters and orthogonal testing designation.The microstructure evolutions of the deformed surface layers were performed by metallographic microscope, XRD, TEM and other testing technology. The processing properties of samples were analyzed by means of portable roughness tester and hardness-testing device. The experimental results are as follows:1. After treatment with RASP, the surface regions of both Al and Cu were deformed and the initial coarse grains were refined: the surface of pure copper material formed nanocrystall-ine, with increasing thickness from surface to center, the grains in the deformation layers were c hanged from nanocrystalline to ultrafine grains and further to coarse grains, pure Al surface formed the superfine crystal,also forming a gradient grain structures. For copper, the greater projectile diameter, the better effect of grain refinement. For Al, the higher projectile energy (related to the projectile diameter, velocity and time), the larger surface roughness and hardness, and the more grain refinement effect. However, if the projectile energy is too high, i.e. the processing time is the 60min, dynamic recrystallization occurs on the surface deformation layer, caused a grain growth and the hardness decrease.2. Microstructural observation on pure aluminum indicates that the grain refinement was realized via dislocation activities:dislocations were first accumulated within the coarse grains and tangled, formed dislocation wall, and then gradually turned into a dislocation cell structure and further to a small angle sub-grain boundaries. With further dislocation accumulation, the small angle grain boundaries were turned into high angle boundaries, forming equiaxed grain with random orientations. This grain refinement mechanism is attributed to the high stacking fault energy of pure Al.3. For pure copper, when the. strain rate is high, mechanical twinning and dislocation slipping are main deformation mechanisms. We observed a high density of twins in coarse grains which divided the coarse grain into lamellar structure. With increase deformation strain, dislocation slipping broke the twin lamellar and refined them into nanometer scale, and ultimately formed the nanocrystalline geains with random orientation. At lower strain rate, dislocation motion is the main deformation mode. The grain refinement of Cu is similar with that of Al. The Cu grain refinement mechanisms are caused by the middle of the stacking fault energy of Cu.