Research On Microstructure And Mechanical Properties Of Friction Stir Processed Ultra-fine Grained AZ31 Magnesium Alloy

AZ31 magnesium alloy is one of the most extensively used deformation magnesium alloy at present. Studying on the plastic deformation behavior and characters of ultra-fine grain size of AZ31 magnesium is meaningful to push the application forward. In this work, Submerged Friction Stir Processing(SFSP) was used to prepare the microstructure with ultra-fine grain size. The technical methods, such as SEM, EBSD and XRD were used to test the grain size, grain boundaries characters, grain orientations and other microstructure feature. The tensile specimens were cut in the direction along Processing Direction(PD) and Tranverse Direction(TD) in room temperature and high temperatures, respectively. The deformation behavior of ultra-fine AZ31 magnesium alloys and high strain rate superplasticity were studied. The conclusions were as follows:The average grain size of Base Material(BM) was 16.56 μm. Three different parameters were applied to prepare ultra-fine grain size, the average grain sizes after SFSP were 0.61 μm, 0.86 μm and 0.95 μm. The grain size would decrease when the quantity of friction heat decreased. The microhardness of SFSP specimens increased by 10.2 Hv compared to BM, and the microhardness and grain size conformed to Hall-Petch eauation: HV=50.2+22.4d-1/2. The Specimens of Ultra-fine in room temperature tensile experiment showed anisotropy, the TD appears higher yield strength and lower elongation compared to PD, to which, the texture characters of(0002) was mainly responsible for it.The Ultra-fine grain magnesium alloy showed high srain rate superplasticity, as the strain rate was set as 1×10-2s-1 the elongation could be 888% at most. Compared to PD, the specimens of TD presented higher strength and lower elongation, which was the same to the phenomenon of room temperature. GBS was an important part of the deformation mechanism.