| With the development of The Times,since entering the 21st century,the problem of aluminum alloy industrial application has been paid more and more attention,People's awareness of safety and environmental protection and aircraft and automobile manufacturing industry towards the direction of structural lightweight development,aluminum and aluminum alloy applications in aerospace and automotive manufacturing and other fields are more and more widely used,As a superduralumin alloy,the welding joint defects caused by laser welding and its mechanical properties do not meet the industrial requirements have been restricting the development of 7075 aluminum alloy in aerospace and automotive manufacturing industry.Therefore,the optimization of laser welding joint of 7075 aluminum alloy by means of post-welding heat treatment is the focus of this thesis.In this paper,4mm thick 7075 aluminum alloy was selected for laser welding with different process parameters,and the welded joints under the optimal process parameters were selected for different heat treatment methods.The similarities and differences of their microstructure and mechanical properties under different heat treatments were analyzed and compared,and the following conclusions were drawn: In the laser welding of 7075 aluminum alloy,the orthogonal experiment was firstly used to analyze the influence of different welding process parameters on the welding test indexes,and it was concluded that the influence of laser power was greater than the welding speed and the amount of defocus: The influence of laser power,welding speed and defocus on weld formation,weld microstructure and performance was analyzed based on the optimal process parameters obtained from orthogonal experiment.The optimized experiment showed that when the laser power was 4.2KW,the defocus was +4mm and the welding speed was 20mm/s,Under these parameters,the laser welding and joint welds have good appearance,moderate width ratio and good mechanical properties.The heat affected zone of the welding seam is narrow and long,and the grain structure is mainly columnar grain.The growth direction is perpendicular to the fusion line,and the center of the welding seam is uniform and fine equiaxed grain.The tensile strength of the welded joint reaches the peak value of 288 MPa,and the average microhardness reaches the highest value of 124.7HV.Through XRD and SEM analysis,it is found that there is only the diffraction peak of ?-Al matrix in the weld area,and the weld center is single-phase ?-Al supersaturated solid solution.Under the optimum technological parameters of laser welding welding joint for solid solution,aging treatment,analysis found that with the increase of solution temperature,soluble in the second phase particles in the aluminum alloy matrix also more,with the increase of aging time,the second phase precipitation and aging time were positively correlated,but the eta 'of metastable phase(MgZn2)could be transformed into stable state,eta(MgZn2),Finally,the optimal solution aging system is475?×1.5h+125?×24h.Under this heat treatment system,the weld center structure is equiaxed grain,the optimal mechanical properties of the weld reach 199.4HV,the tensile strength is 417 MPa,up to 70.1% of the base metal,the weld microstructure is uniform and the second phase particle dispersion is high.Most of the precipitated phase is ?' phase,followed by GP region.The best heat treatment system and the welding joint deformation processing,the center of the weld seam grain size decreases with the increase of deformation force,but the second phase distribution is very uniform,with the increase of deformation force,aging before the precipitation strengthening eta 'second phase(MgZn2)gradually transformed into a stable state of eta,deformation strengthening effect of weakening,so deformation force continues to grow,It will reduce the tensile strength of the welded joint,so the tensile strength of the welded joint reaches the highest 455 MPa when the deformation force is 25%.The fracture mode of welded joints under 5% deformation force is brittle fracture,and the fracture mode of welded joints under the rest deformation force is ductile-brittle mixed fracture. |
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