Research On Equal Channel Angular Rolling And Drawability Of AZ31 Magnesium Alloy Sheet

As the lightest structural metal, magnesium alloys are praised as the most development perspective green materials in 21st century. Especially, wrought magnesium alloys sheet with numerous desirable features exhibit very extensively applicable opportunities and the research on the wrought magnesium alloys sheet with excellent integrate performance has been very important. And the key to expand the application of the sheet is to realize the stamping at low temperature even room temperature with high strain rate. But the drawability of magnesium alloys sheet is limited seriously by the basal texture induced by the normal processing technologies. Therefore, the development of new sheet forming techologies based on the study of plastic deforming theory in magnesium alloys are necessary.The objective of this dissertation is to explore the new approaches to enhance the drawability of AZ31 magnesium alloy sheet in order to improve the drawabiliy and realize the stamping at low temperature even room temperature with high strain rate. Based on the controlled grain orientation, a new processing, so-called equal channel angular rolling (ECAR) processing was proposed to process magnesium alloy sheet, which was a continuous shear deformation processing based on normal rolling and severe shear deformation, utilized the friction force among the sheet and the twin-roll during normally rolling to feed the sheet into the die channels with an oblique angle at the same height in a continuous manner. The evolving principle and mechanisms for the microstructure and drawability of AZ31 magnesium alloy sheet during the ECAR procedure and heat treatment were studied and the deep drawing processing was developed. The mainly conclusions can be drawn as the following.(1) The deformation status for equal channel angular rolling processing was analyzed through the slip line theory. The results indicated that the mainly strain at the die corner was shearing strain and the existence of the oblique radius widened the deforming area.(2) The basic principle for the ECAR deforming was investigated. The simulation used finite element method (FEM) analysis showed that the shearing angle and effective strain in one passage decreased with increasing of the oblique radius, the channel clearance and the oblique angle, occurring with the increased nonuniform deformation in the thickness direction. (3) The evolving law of microstructure in AZ31 mangesium alloy sheet during ECAR procedure was studied and the relationship among the processing parameters, microstructure and drawability during ECAR procedure was founded. According to the study on the ECAR processing parameters, including the rolling pass, the mould temperature, the channel clearance, the rolling route, the pre-heated temperature of sheet and the oblique angular, the microstructure and the drawability of AZ31 magnesium alloy sheet were mainly effected by deforming temperature, shear pattern, single passage strain, accumulative stain, annealing conditions during ECAR procedure and deformation area, etc. With increasing of rolling passes in Route A, the initial (0002) basal plane orientation in the as-received sheet evolved to {1011} pyramid plane orientation gradually after ECAR processing, followed with decreasing of yield ratio from 0.889 to 0.332 and increasing of strain hardening exponent from 0.125 to 0.378 and enhancing uniform elongation from 11.2% to 23.9% at the rolling direction. The tilted angle between basal plane and rolling plane decreased with increasing of the channel clearance and the oblique angle. And the microstructure of the ECARed sheets were correlated with the rolling route closely, which was minor affected by the die temperature and the pre-heated temperature of the sheet.(4) The ECAR processing for AZ31 magnesium alloy sheet was optimized. The optimized technological parameters used the die with the oblique angle of 115o were that the sheet was pre-heated at 350℃for 3min by one passage using the ECAR device at ambient temperature with the reduction in thickness during normal rolling of about 6% and the channel clearance of 1.01.(5) The evolving principle and influenced factors of the microstructure and drawability for AZ31 magnesium alloy sheets processed by normal rolling (NR) and ECAR processing were investigated under the different annealing conditions The results indicated that the grain was inclined to equiaxed and grew up gradually with the disappearance of twinning and the grain orientation was not changed obviously with the decreasing peak intensity of crystal plane with increasing of annealing time and elevating of annealing temperature. The grain of both type of sheets annealed isothermally under 300℃for 60min was equiaxed and homogeneous with the average grain size of 12.0μm and the yield ratio were 0.645 and 0.441, the strain hardening exponent were 0.263 and 0.458 and the uniform elongation were 21.6% and 32.3% at the rolling direction, respectively.(6) The relationship among the deforming behavior, drawability and deforming conditions were studied. The results showed that the deformation behavior of the NRed sheet were almost same with each other at the rolling direction and transverse direction, which tended to isotropy. And the difference of deformation behavior for the ECARed sheet between rolling direction and transverse direction was decreased with increasing of strain rate and decreasing of deformation temperature.(7) According to the Grosman model, the flow equation for AZ31 magnesium alloy sheets processed by both NR and ECAR associated with the drawability parameters were constituted according to the uniaxial tensile testing, which could give a fairly good fit to the measured values under the following deforming conditions of the elevated temperature among 150~300℃, the initial strain rate among 1×10-4s-1~1×10-1s-1 with the strain between 0.01~0.3.(8) The deep drawing tests at room temperature indicated that the Erichsen value and drawing ratio for the NRed sheet was only achieved as 3.78mm and 1.2, which could be up to 6.21mm and exceed 1.6 for the ECARed sheet, respectively. And for the ECARed sheet, the cupping workpiece of AZ31 magnesium alloy with the depth of 16.8mm could be successfully made using the punch with the diameter of 30mm and the shoulder radius of R8mm.(9) The proper hot deep drawing technologies were developed. The deep drawing tests indicated that the drawability for the ECARed sheet was similar with each other when the drawing temperature was above of 200℃. And the proper deep drawing technology for the NRed sheet was that the drawing ratio above 2.2 could be formed under the following conditions of drawing temperature between 210℃~240℃, the punch radius among 8~12mm and the die radius among 6~10mm, blank holder clearance between 1.10~1.17 and lubricated with the mixture of graphite + oil at the punch speed below 90mm·min-1. During the experiment, the most limiting drawing ratio of the hot drawing workpiece was up to 2.53.