| Superplastic gas blow forming is the typical application in the field of superplastic forming technology. Interference fit process has become the main way of shaft coupling. Two kinds of characteristics above were made use of in this paper.For difference in the ability of cold shrinkage between AZ31 magnesium alloy and AA5083 aluminum alloy, gas blow forming and cold contraction bonding process for bimetal square box was carried out, which was deeply discussed by numerical and experimental studies.Structure and heat field was explored by commercial finite element software MSC. Marc. The whole gas blow forming process can be divided into three stages,namely free bulging, contacting at bottom of mold and filling round corners. The most serious area of thickness thinning was transferred from dome to corners between surfaces where the final thickness thinning ratio is 58.85% and 60.18%respectively. The maximum relative error of thickness between numerical and experimental dates is 10.92%. Moreover, wall thickness distribution of square box was analyzed with different modal ratio of length and depth: the minimum thickness and their positions were decided by forming degree and cavity size of mold.Rounded corner and rounded turning radius were 7.21 mm and 9.18 mm when ratio of length to depth of mold was 2.30, which was considered the reasonable value with good forming results. The result of heat field showed that bimetal sheets of AZ31/AA5083 with radius of 45 mm and thickness of 1.0mm cooling from 435 o C to room temperature spends 8 min.Meanwhile, the experiments on AZ31 magnesium alloy and AA5083 aluminum alloy sheet with thickness of respective 1.0mm bimetal square box forming was performed. The suitable forming temperature for square box with length of 56 mm and depth of 20 mm was 435 o C; step pressure profile was used, which pressure increased 0.25 MPa every 10 min with initial pressure of 1.00 MPa and final pressure of 2.00 MPa and the total forming time is 2700 s. Maximum strain rate of simulation ranged from 2.259E-4 to 3.808E-4s-1.The process parameters above could match plasticity of AZ31 and AA5083 sheets and coordinate deformation, under which square box with rounded corner radius of 5.1mm and rounded turning radius of9.4mm was obtained. Thickness distribution was affected by various process parameters through its impact on forming degree of square box. With observation of microstructure of as-formed AZ31 magnesium alloy sheet, grain coarsening phenomenon was found. Grain size with highest deformation degree increased about1.78 times than the original grain and 67% than undeformed heated sample, which was considered due to the effect of Deformation Induced Grain Growth(DIGG).Comparative experiments for single layer and bilayer AA5083 sheets with equal thickness of 1.6mm were conducted. The results demonstrated that the plasticity of bilayer sheets is much lower than single layer sheet with the same thickness because of the gap between sheets.The degree of bonding of bimetal square box is the important indicator for evaluating gas blow forming-cooling contraction bonding process. Therefore, finite element analysis and theoretical derivation were performed to obtain bonding force value of bimetal square box. Bonding force simulation value of square box ranges from 32 MPa to 51 MPa in sidewall and ranges from 24 MPa to 48 MPa in bottom,corresponding theoretical derivation value is 42.878 MPa. For possibility of instability behavior during cooling shrinkage, inner sheet of bimetal cylinder with diameter of 80 mm and bimetal square box that this paper studied were analyzed theoretically, whose critical compressive stress are 24.75 MPa and 84.76 MPa respectively. |
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