Study On Surface Mechanical Grinding AZ31B Magnesium Alloy And Its Properties

Surface mechanical attrition treatment (SMAT) is a new kind of material processing technology which uses method of severe plastic deformation caused by the impacting from steel balls to generate gradient structure in the surface layer. At the same time the central part maintains the original coarse crystalline. So a gradient structure is produced by this method and the special structure has a great impact on surface properties and overall performance. And so SMAT will become a very promising method to improve material properties.In this work, AZ31B magnesium alloy was selected to be treated by SMAT. Various parameters were applied in this experiment to explore the factors affecting the effects of the gradient structure on AZ31B magnesium alloy. Optical microscope, XRD, AFM were used to characterize the gradient structure in the surface layer generated by SMAT. The tensile properties, microhardness, corrosion resistance and thermal stability of the samples processed by SMAT were tested respectively. The results showed that gradient structure was formed in the surface layer of the samples and the grain size of the gradient structure was refined. The grain size of the surface layer was decreased as the increasing time and then tended to be stable while the grain size increased gradually from the surface to the center creating a gradient structure. The thickness of the gradient structure increased with treatment time increasing. The distribution of microhardness in the cross section is consistent with the distribution in grain size. It appears that the dependence of yield stress on grain sizes follows the Hall-Petch relationship in the gradient structure generated by SMAT. The gradient structure induced by surface severe plastic deformation owned certain thermal stability below 200℃ and higher temperature will lead to grains growth making the SMAT useless. The gradient structure can improve the mechanical properties of AZ31B samples resulting in excellent strength and ductility simultaneously. The proportion of the gradient structure was the key point of mechanical properties of materials. When thickness of the samples was too small, the deformation caused by the impact resulted in an overall hardening reducing the effect of grain refinement with the samples owning low strength and plasticity. When thickness of the samples was too lager, the effect of grain refinement was obvious, but the proportion of the nano-gradient structure was too low resulting limited strength enhancement. When this proportion was 4%-7%, best mechanical properties were received. The process of SMAT was still applied on AZ31B at liquid nitrogen temperature. The degree of grain refinement in the surface layer was improved in short time. The main reason was that liquid nitrogen was added effectively eliminating impact of heat generated by impacting and suppressing recovery and recrystallization. At the same time ultra-low temperature also led to samples deformed in the form of twins which proved twins were important for the generation of the gradient structure. For the samples treated by small balls, hardness was slightly improved and hardened layer was thin which proved the high strain rate generated by high-energy impact was necessary. Electrochemical corrosion of all the samples treated by SMAT decreased significantly which was mainly due to magnesium alloy owning no passivation, the refined grains with higher activity, residual stress and element diffusion form impacting.