Effect Of Heat Treatment On Micro-structure And Properties Of Laser Welding Joint Of 7075 Aluminum Alloy

7075 aluminum alloy is an Al-Zn-Mg-Cu high-strength aluminum alloy.It is also a typical age-hardenable aluminum alloy.It has high specific strength,good fracture toughness,excellent low cycle fatigue resistance,and welding performance.Characteristics are the main welding structural materials for aerospace,construction,and military facilities.Compared with the traditional welding methods,the fiber laser welding has the characteristics of fast welding speed,small formed weld width,fine grain,and small deformation of the weldment.However,there are still joint softening problems when using laser brazing laser welding of aluminum alloys.Good heat treatment process can improve the welding softening of 7075 aluminum alloy.Taking a 3mm thick 7075 aluminum alloy as the research object,YLS-10000 fiber laser is used for welding.Firstly,an ideal welding heat input range is determined by self-welding.Based on this,based on the tensile strength of the joint,the optimal welding parameter combination for butt welding is determined by orthogonal test,the laser power is3.2 kW.welding speed 38mm/s,defocus amount-1mm.The degree of influence of the welding parameters finally determined by the range analysis method on the tensile strength of the joint is descending from the focal point>laser power>welding speed.The joint tensile strength under this welding parameter combination is 336 MPa,and the joint strength coefficient reaches At 60%,the fracture of the joint is a brittle fracture.Using the obtained optimal welding parameter combination,solution treatment was carried out at solution temperatures of 460°C,470°C,480°C,490°C,and 500°C for 30minutes,60 minutes,90 minutes,120 minutes,and 150 minutes.Solution materials were found.The microhardness change rule in the center of the weld is basically the same.With the increase of solution temperature,the peak time of microhardness becomes shorter,the microhardness of the base metal reaches its maximum at 490°C×60min?182HV?,and the microhardness of the weld reaches its maximum at 470°C×60min.?177.6HV?.The tensile strength at 470°C is significantly better than the tensile strength at other temperatures.Through the metallographic observation and SEM,it was confirmed that the second phase in the 470°C×60min phase was uniformly distributed,and the number of the second phase was small and the second phase of different shapes mainly contained MgZn₂ and Al₂Mg₃Zn₃.Through the mechanical properties and microstructure can be determined that470°C×60min is the best solution solution of 7075 aluminum alloy fiber laser welding.Through the better solution treatment system obtained,single-stage aging treatment was performed at aging temperatures of 100°C,120°C,140°C,160°C,and 180°C for 0h to34h.The change of the microhardness of the base metal and weld center was found.basically the same.Through metallographic observation and SEM,it was confirmed that the second phase in the tissue at 120°C×24h was evenly distributed,with numerous and small phases.The second phase of different shapes was mainly Al₂Mg₃Zn₃.Through the mechanical properties and microstructure can be determined 120°C×24h is 7075 aluminum alloy fiber laser welding better aging treatment system.Through the solution treatment and aging treatment to determine the final better heat treatment system is 470°C×60min+120°C×24h,in this heat treatment system,the joint strength coefficient can be increased by about 25%.