Microstructure And Mechanical Properties Of AZ31 Alloy Plate Prepared By Multi-pass Friction Stir Processing

As the lightest structural metallic material,magnesium alloys have many advantages such as low density,high specific and good electromagnetic shielding ability,which have large potential in automobile,aerospace and 3C industry.However,most of magnesium alloys show low formability and ductility at room temperature due to the HCP crystal structure.In addition,their poor corrosion resistance seriously limit their application.Grain refinement is widely used to improve the mechanical properties of magnesium alloys.Exploring a new processing technique to produce fine-grained magnesium alloy plate is beneficial to the development of plastic forming of magnesium alloy.Severe plastic deformation(SPD),which brings severe deformation and heat input,can produce fine grains and ultrafine crystal via dynamic recrystallization.As a new SPD technique,friction stir processing(FSP)not only can significantly refine the grains,but also has the advantages of simple processing,automation and low energy consumption and so on.Furthermore,multi-pass friction stir processing(MFSP)can be used to expand the processing area to produce large-scale fine-grained plates by changing the overlap ratio.In this study,hot-rolled AZ31 magnesium alloy was selected as the base material.FSP as well as MFSP were conducted in air and under water cooling conditions.The effects of processing parameters,cooling conditions,and passes on the evolution of the microstructure were studied,and the relationships between grain size,grain distribution,texture and mechanical properties at room temperature were analyzed.Besides,the corrosion behavior of AZ31 plates with different cooling conditions were investigated and their corrosion mechanism were discussed.The research aims to provide technical theory for fine-grained magnesium plate prepared by FSP.Firstly,FSP in air cooling condition was carried out with different rotation speeds in order to understand the relationship between rotation speed and mechanical properties of AZ31 magnesium alloys,and the suitable parameters were used to produce MFSP AZ31 plate.The ductility was improved due to the grain refinement and the mechanical properties gradually decreased with the rotation speed and heat input increasing due to the recrystallized grains growth.And then MFSP in air cooling condition was performed on AZ31 magnesium alloy plate with the paraments of 1100 rpm and 60 mm/min.It was found that a transition zone with non-uniform microstructure was formed between adjacent processing passes due to the effect of passes and heat input.The grains can be further refined by subsequent processing passes,and the average grain size in processing zone was 12.9 ?m.From the tensile test in three directions of PD,TD and 45°,we found the ductility increased in all direction,especially in PD,which had an elongation of 46.6 ± 1.5 %.The change of ductility was mainly related to the grain refinement.In additional,MFSP AZ31 plate exhibited anisotropic tensile behavior due to the non-uniform distribution of microstructure and texture of the transition zone.Secondly,SFSP were carried out in underwater cooling condition in order to study the effect of enhanced cooling on microstructure evolution and mechanical behavior of AZ31 magnesium alloy.SFSP reduced the heat input,resulted in smaller grain size and better mechanical properties.The microstructure and mechanical properties also changed with the rotation speed like FSP cooled in air.M-SFSP was performed on AZ31 magnesium alloy plate successfully,which obtained the smallest grain size of 1.8 ?m in this study.Compared with MFSP plate,the M-SFSP plate had higher tensile strength and elongation,especially in PD,which had the tensile strength and elongation of 218 ± 5 MP and 49.7 ± 2.0 %.The main reason is that underwater processing had high cooling rate,which was beneficial to suppressing the coarsening of the grain caused by the subsequent heat input,the grain refinement significantly improved the mechanical properties of M-SFSP plate.In addition,the grain of 1?4 ?m distributed evenly in the transition zone,which reduced the negative effect of the transition zone on TD.The difference of mechanical properties between three directions was decreased,leading to the reduction of anisotropy on M-SFSP plate.Finally,the corrosion behaviors of base material,MFSP and M-SFSP samples were studied in 3.5 wt% Na Cl solution.The base material exhibited many pits at the beginning of the corrosion.After 48 hours,the surface of BM had been seriously damaged with obvious material loss.On the other hand,the MFSP sample still kept good surface due to the grain refinement,dissolution of the grain boundary elements and the protection of the small corrosion product covered the surface evenly.The M-SFSP sample obtained the lowest corrosion rate and weight loss among the three samples in the same immersion cycle,which meant the M-SFSP sample had the best corrosion resistance of the three samples.The main reason is that the fine and uniform microstructure reduced the difference of corrosion electric potential of M-SFSP plate.