Research On Microstructure And Properties Of Heat-Affected Zone In 2219 Aluminum Alloy

High–strength aluminum alloy 2219 is a heat-treatable aluminum alloy.One of the major problems in welding joints is the softening of the heat-affected zone(HAZ),which deteriorates the mechanical properties of the welded joint.In this study,the effect of thermal cycle with different peak temperatures(Tp)on the microstructure of HAZ in 2219 aluminum alloy was investigated using welding thermal simulation.The microstructure of the HAZ was observed by means of optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),electron backscatter diffraction(EBSD)and X-ray diffraction(XRD).The mechanical properties of the HAZ were characterized by micro-hardness and micro-tensile test.Research results show that the number of the disc-precipitates(?” and ?'),which are the major strengthening precipitates,gradually decreased and even disappeared as the Tp increased.The coarsen ? phases first appeared when the Tp is 400°C and the size increased with further increasing the Tp.The elongated grains in base metal recrystallized to finer grains when the Tp raises up to 550°C.At the higher temperature,such as 500°C and 550°C,the small ?” and ?' phases re-dissolved into the matrix,causing in the decrease of the lattice parameter of the ?-Al.Meanwhile,the relative large ?” and ?' phases translated into the ? phases,which is non-coherent with the matrix lattice.As the Tp increased,the decrease of the number fraction of the low misorientation angles(<15°)is attributed to the recovery process.When the Tp is 550°C,the distribution of grain boundaries misorientation angles corresponds to the MacKenzie distribution.The microhardness and ultimate tensile strength of the HAZ gradually decreases as the Tp increases from 200°C to 500°C.With further increasing Tp to 550°C,the mechanical properties slightly improve.The softening of the different zones in HAZ is determined by the size,distribution and kind of precipitates.However,the grain size and the content of alloying elements in the matrix have little influence on the mechanical properties.2219 aluminum alloy was welded by the Variable Polarity Tungsten Inert Gas welding(VPTIG).Result shows that the changing tendency of mechanical properties in the simulated HAZ is consistent with that of VPTIG joints.