The Microstructure And Properties On Post-weld Heat Treatment Of 2219 Aluminum Alloy

In order to meet the requirements for the new generation of aerospace vehicles with advantages of lightweight,large-size,and integrated forming,the bottom of the fuel tank is formed with friction stir welded tailor-welded plates of 2219 aluminum alloy,which avoids the shortcomings of large welds and poor performance caused by tailor welding of ordinary melon petals.The solution heat treatment will cause abnormal grain growth(AGG),which can make grain size reach a millimeter level,and cause its joint performance dropping sharply.In order to solve this problem,this article adopts post-welding heat treatment,including long-term annealing,graded annealing,high temperature shock heat treatment,FSW auxiliary heat treatment,to control the microstructure and properties of tailor-welded blanks.2219 aluminum alloy FSW tailor-welded blanks were used to study the influence of different heat treatments on the abnormal grain growth during the subsequent solution treatment and mechanical properties.The influence of heat treatment parameters on microscopic factors were analyzed.The best heat treatment plan and heat treatment parameters were explored.The results will provide theoretical guidance and technical support for the formulation of the actual process plan.Different welding heat treatment methods were studied.First,long-time annealing,graded annealing,high temperature shock heat treatment and FSW auxiliary heat treatment were performed on joints in sequence and then standard solid solution treatments were conducted on the joints.To count the grain sizes of the joints,a high resolution microscope was utilized.For long-time annealing and hierarchical annealing,results show the grain sizes increase with annealing temperature increasing from 300 ℃to 480℃;when the holding time increases from 16 h to 128 h,the grains are refined.And 300℃(128h)、 300℃(32h)-420℃(32h)-300℃(8h)are concluded to be the optimum process,with minimum average grain size reaching0.38 mm and 0.54 mm.For high temperature shock heat treatment,lower temperature amplitude,less cycles of shocks,and less holding time at peak temperature were conducive to suppress abnormal grain growth.Average grain size of 0.18 mm can be obtained by the appropriate process.For FSW auxiliary heat treatment,there is no significant difference on the grain sizes with temperature ranging from 200° C to350 °C.The minimum average grain size can be controlled to 0.16 mm.The mechanical properties tests,including tensile test and hardness test,were performed on the joints after different heat treatment conditions.The joint after the condition of 300℃(32h)-420℃(32h)-300℃(8h)as well as solution treatment exhibits a tensile strength of 290.7 MPa,an elongation of 18.34%,a strength coefficient of 0.83 and a plastic coefficient of 0.73.The hardness of the aging state was tested,and it is found that the hardness is slightly increased after the high temperature shock heat treatment,and the hardness value is about 135 HV.The joint after auxiliary heat welding at 250℃,holding for 30 min and solution treatment exhibits a tensile strength of 293.70 MPa,an elongation of 15.56%,a strength coefficient of 0.84 and a plastic coefficient of 0.62.EBSD,SEM and energy spectrum microscopic analysis test were employed to research the influence of heat treatment process parameters on the friction of large angle grain boundary,dislocation density and strain energy storage and mechanism of different heat treatments on abnormal grain growth was investigated.The results show that the proportion of large angle grain boundary rises,but both of dislocation density and strain energy storage decrease.Only high temperature shock heat treatment is able to reduce dislocation density and strain energy storage,while other methods exhibit no significant impact on them.