| Automobile lightweight has become a common and basic technology for reducing energy consumption and emissions.Dissimilar aluminum alloy tailor welded blank(TWB)consists of two or more aluminum alloy sheets with different thicknesses or strength that have been welded together prior to forming.TWBs are subjected to forming process and form to final parts,which has the double lightweight effect of lightweight material and structural optimization.Friction Stir Welding(FSW)is a solid-state welding that can effectively connect lightweight materials such as aluminum alloys and has various advantages over conventional fusion welding techniques.However,there are some problems for dissimilar aluminum alloy FSW joints such as inhomogeneity and thermal instability of the microstructure which affect the formability performances of TWB and the serviceability of the formed parts.Therefore,it is necessary to carry out research on the microstructure,heat treatment process,forming property and deformation law of the dissimilar aluminum alloy FSW-TWB.The microstructure and properties of 2024/7075 dissimilar aluminum alloy FSW as welded joints was analyzed.It is found that the strength and elongation of the 2024-T4/7075-T6 joints are seriously reduced compared with the base metal,which becomes a weak area of the TWB.Strengthening phase will be redistributed with the post-welded solution and aging to improve strength of joint.Although the strength of the 2024-O/7075-O joint is higher than that of the base metal,the strength of the base material itself is low.Solution and aging are necessary to restore the strength.Therefore,considering the FSW connection efficiency,the necessity of post-weld heat treatment and the forming performance,it is decided to select the annealed dissimilar tailor welded blank for subsequent research.Changing the welding and the solution parameters is used to address the problem of Abnormal Grain Growth(AGG)in the joint microstructure during post weld solution treatment.The results show that the area of AGG increases when the solution temperature is improved,but decreases when the welding parameters being changed to increase the welding heat input.By analyzing the mechanism of AGG,a combined process window for welding and post-weld solution treatment that can control AGG was obtained.In order to compensate for the loss of joint strength during solution treatment at 450°C,the deformation aging was introduced after solution.It was found that when the deformation amount was 6%,the joint strength reached the maximum,at the level of 475°C.The investigation of formability and plastic deformation laws of the TWB was carried out.It was found that due to the inhomogeneity and asymmetry of the joint microstructure,the deformation was uncoordinated,resulting in poor forming properties at room temperature,which could not meet the needs of processing and forming.In the case of inhibiting the occurrence of AGG in the joint microstructure,the forming performance was improved by increasing the deformation temperature and then the hot stamping and in-mold synchronous quenching experiment of the TWB was carried out,and a well-formed sample was obtained.Finally the joint strength reached 425 MPa,91% of 2024-T4 base metal.Based on the dissimilar aluminum alloy FSW-TWB,this thesis aims to discover the microstructure and mechanical properties of the joints and reveal the influence of welding and solution parameters on the AGG.Then the heat treatment regulating the microstructure and properties of the TWB was adopted on the premise of ensuring good organization.Besides the forming property of the TWB was improved through high temperature forming,which providing theoretical basis and support for the application of the dissimilar aluminum alloy FSW-TWB for vehicle body components. |
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