| Due to its good physical properties and molding properties,2219 aluminum alloy is widely used in various aerospace product structures.2219 aluminum alloy has been comprehensively applied in a new generation of large diameter rocket tank structure,in the tank structure of the new-generation launch vehicle,due to different bearing characteristics of different structures,the heat treatment status of 2219 aluminum alloy used in different structures in the tank is different.In this thesis,welding tests were carried out for three typical heat treatment conditions 2219-T62,2219-T852 and 2219-T87.Parameters optimization tests were carried out for the TIG suspension welding process of 2219 aluminum alloy with typical thickness(DC helium arc backing & AC argon arc covering surface),and the better process parameters were obtained.Then,on the basis of the optimized process parameters,the specimen welding in different heat treatment states was carried out,and the mechanical properties of the TIG welding joint in three different heat treatment states,as well as the macro and micro structure of the joint were analyzed.According to the fracture characteristics of the joint,the fracture mechanism was analyzed and the fracture type of the joint was determined.Finally,combining with the influence of assembly conditions on joint forming and mechanical properties,the welding test was carried out,and the ultimate assembly margin of 2219 aluminum alloy with typical thickness was obtained under the suspended TIG welding process.Aiming to 7mm 2219 aluminum alloy,this thesis first analyzed the process characteristics of its backing welding and covering welding,and the main process parameters affecting joint forming were studied.The results showed that: during the backing welding,the welding current was 240A±5A,the welding speed was 260mm/min,the diameter of the tip of the tungsten electrode was 1.5mm,the diameter of the tungsten electrode was 5mm,the length of the tip of the tungsten electrode was 8-10 mm,and the diameter of the tip was 1.5mm.During the covering welding,the welding current was 300±20A,the welding speed was 120mm/min,the wire feeding rate was 600-800mm/min,the pulse frequency was 0.8Hz,the pulse time was 60-70%,the AC frequency was 55-65 Hz,the AC balance was 65-75%,the base value current was 50-60%.In this case,the welding joint with high tensile strength and elongation could be obtained,the front of the joint was smooth and the back of the joint was full.The microstructure and fracture morphology of welding joints in different heat treatment states were studied.For the welding seam of 2219-T62 and 2219-T852 combination,the grains of 2219-T62 parent metal side was small and dense,2219-T852 base side was bulky,2219-T852 side heat affected zone was clearly organize grain coarsening,the joint average tensile strength was 286 MPa,the joint average elongation was 4.4%,the fracture was located in the fusion line area of 2219-T852 side,the fracture was mixed fracture;For the welding seam of 2219-T87 and 2219-T852 combination,2219-T87 base metal was lamellar structure,heat affected zone structure was ?(Al)solid solution,with intermetallic compound Al2 Cu and a small amount of eutectic structure,the average tensile strength of the joint was 278 MPa,and the average elongation was 5.1%.The joint was mainly fractured along the 2219-T852 fusion line,and the fracture was mixed type.For the welding seam of 2219-T87 and 2219-T87 combination,the grain structure of backing welding was thicker than that of cover layer,due to the addition of beneficial elements in the welding wire,the microstructure of the welding joint on the cover was fine equiaxial crystal,the average tensile strength of the joint was 296 MPa,and the average elongation was 4.9%.The fracture was a typical ductile fracture.The effects of assembly conditions on TIG welding joint forming and mechanical properties of 2219 aluminum alloy were studied.Welding gap test showed that: the ultimate welding gap of the 6mm and 8mm plate thickness was 0.6mm and 0.8mm,about 10% of the thickness of the sample,the back of the welding seam cannot be formed normally beyond this tolerance;Welding staggered seam test results showed that: the ultimate misalignment of the 6mm and 8mm plate thickness was 0.8mm and 1mm,about 13% of the thickness of the sample,when the limit was exceeded,the welding arc was unstable,prone to arc quenching,and the back weld was not well formed,the sharp transition and weld undercutting would happened. |
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