Research On Gas Blow Forming-cooling Contraction Bonding Process And Mechanics Of AZ31/7475 Bimetallic Composite Tube

The main purpose of this article is to study the gas blow forming and cold contraction bonding process of bimetallic shaped composite tube of AZ31 and 7475.The combination between two tubes is achieved by the difference of thermal expansion coefficients between the two materials.The main research methods include numerical simulations,experiments and mechanical analysis.In order to analyze the change of strain rate in forming process,we have simulated the gas blow bulging process of the AZ31/7475 bimetallic composite tube with hexagonal section was simulated by the finite element software MSC.Marc,and the effect of forming pressure,forming time,loading path,and gap between die and outer tube on the wall thickness distribution and corner radius was investigated.The suitable process parameters were summarized as follows: step pressure loading path,3MPa for initial forming pressure and 6MPa for maximum pressure,840 s for loading time,and0.5mm for the gap between die and outer tube.Based of numerical results,we have carried out the gas blow forming experiments for AZ31/7475 bimetallic composite tube at 450? and the effect of the maximum forming pressure and loading time on wall thickness distribution and corner radius was discussed,and the experimental results agree better with numerical results.Their maximum thinning differs about 0.35%,and corner radius differ 1mm.The wall thickness nonuniform rate of composite tube which formed with the optimal process parameters is 3.07%,the maximum thinning rate is 10.6%and the corner radius is7.0mm.Also the interface between the two tubes is combined closely and no diffusion bonding phenomenon.The gas blow forming-cold contraction bonding process of bimetallic shaped composite tube can be divided into four stage namely free bulging,side wall sticking,fillet filling,cold contraction bonding,and the stress analysis of magnesium tube and aluminum tube at each stage was carried out.Ther? was neglected during the bulging stage,sidewall sticking stage and fillet filling stage and the element was subjected to???,z.The free expansion stage was analyzed as tubes free bulging with fixed ends was in plane stress state,and the raised contour is considered as an ellipse curve which was die independent.During the sidewall sticking stage,the sticking point was regarded as the highest point on the free expansion tube whose bulge region was changed,then it can be analyzed as a free bulging tube;The thickness distribution of the element sticking on the die was regarded as right angle trapezoid in the fillet filling stage,and the wall thickness of no stick element was invariable.Based on the condition of volumeinvariance,the relation between corner radius and corner wall thickness in this stage can be established.In the cold contraction bonding stage,the hexagonal composite tube was divided into the fillet section and the straight wall section,considered as the corresponding bimetallic composite round tube subjected to shrinkage residual stress and spring back residual stress.It is proposed that the residual contact stress between the composite tubes should include two components: the contraction residual contact stress caused by the different shrinkage after cooling and the springback residual contact stress caused by the different springback between Mg and Al tubes.According to the hypothesis of interference fit between outer and inner tubes,a formula for calculating the residual contact stress between outer and inner tubes of bimetallic round composite tube and bimetallic hexagonal composite tube was derived.The residual contact stress between the bimetallic composite tubes formed at 400 and 450? were measured by compression tests and the differrence of theoretical and measured values were 27.9% and 19.2%respectively.The force condition at the straight wall of inner tube of as-formed composite tube is equivalent to the rectangular plate with fixed at both ends.Thus a mathematic model for calculating the buckling critical load of the inner tube straight wall was established,which the buckling stress is 70.34 MPa for the as-formed composite tube.