Study On Microstructure Properties And Thermo-mechanical Coupling Of 6061-T6 Aluminum Alloy Laser Welded Joint

Recently,with the improvement of people's awareness of environmental protection and energy conservation,light vehicles have become the trend of world.Aluminum alloy has the features of low density,high specific strength and abundant reserves.It is the preferred material for light vehicles.However,the unavoidable defects in laser welding such as blowholes are still difficult problems in experiment and engineering field.Therefore,it is of great practical value to study the influence of laser welding parameters on the weld properties of aluminium alloy.In this paper,6061-T6 aluminium alloy with 4 mm thickness is taken as the research object.Macroscopic morphology?microscopic characteristics,mechanical properties and corrosion properties of laser welded joints are tested,and the temperature and stress fields are simulated and analyzed by ABAQUS software.Conclusions are as follows:Experiments of laser welding on 4mm 6061-T6 aluminium alloy show that the change of welding parameters has a significant effect on the macro-morphology of the weld.Blowhole defects are unavoidable.Appropriate defocusing amount,laser power and welding speed can significantly reduce the porosity in the weld.The optimum technological parameters are P=4.8kW,?f=-1mm and v=30mm/s.The crystal structure near the fusion line is columnar and the growth direction is perpendicular to the fusion line.The center of the weld is fine equiaxed grain and dendritic structure,and the grain size is small and obviously different from that of the base material.Although the laser energy is concentrated,its heating speed and cooling speed are fast,so the heat affected zone of laser welding is very narrow.The results of X-ray diffraction and scanning electron microscopy analysis show that there is obvious segregation in laser welding seam of 6061-T6 aluminum alloy.The overall hardness distribution curve of the 6061-T6 aluminum alloy laser welding head is close to the "U" shape,the center hardness of the weld is low.In the tensile test,the fracture position of the specimen are all in the weld zone,and the tensile strength of the welded joint can reach to 71.5%of the tensile strength of the base metal.The fracture shows a 45° crack,and the fracture type belongs to quasi-cleavage fracture.The results of polarization curve measurement show that the corrosion resistance of laser self-welding weld is better than that of laser-MIG composite weld.The thermal cycle curves of the characteristic points simulated by the temperature field basically coincide with the temperature curves recorded by the paperless recorder,which indicates the correctness of the heat source model.Each characteristic point has experienced the process of rising to peak temperature and then falling to room temperatureBy changing the simulated output to stress and strain,the stress nephogram of the weld after cooling is obtained.When the welding process is stable at the beginning,the stress and strain output of the three characteristic points,which are perpendicular to the weld center,heat affected zone and far from the weld center are extracted.It is concluded that the longitudinal stress of the three characteristic points is compressive stress in the heating process,and the longitudinal stress far from the weld center is tensile stress.When the weldment is gradually cooling down,the stress value of each point tends to be a stable value.