Microstructure And Property Evolution Of 6061 Aluminum Alloy During Ultrasonic And Thermal Aging

6061 aluminum alloy has excellent properties such as high strength,good formability,and strong corrosion resistance,and is widely used in fields such as aerospace,automotive manufacturing,and electronics.After plastic forming,heat treatment,such as solution and aging treatment,is usually required to obtain higher strength of the aluminum alloy.Aging is the main process of controlling the precipitation of strengthening phases,which strengthens the alloy by promoting the precipitation of precipitation phases.In addition,during the aging process,in addition to the precipitation of precipitates,the plastic formed material is also accompanied by the elimination of residual stress,which is also known as aging stress relief.Since aging in heat treatment is usually performed at a lower temperature,there are problems such as long aging time,high energy consumption and low efficiency.Ultrasonic aging is a new and efficient aging process developed in recent years,but at present,ultrasonic aging is widely used in the field of stress relieving,and there is less research on the effect of precipitated phases.In addition,the evolution of the microstructure and properties of aluminum alloys during the process of ultrasonic aging for residual stress removal lacks research.To this end,this study investigated the effect of ultrasonic aging on the precipitation phase precipitation of 6061 solid-solution aluminum alloy,and further investigated the evolution of microstructure and properties during ultrasonic aging and thermal aging stress relief and its mechanism.The specific research content is as follows:(1)The effects of thermal aging on the residual stress,microstructure and its hardness of 6061 solid solution aluminum alloy were investigated.It was found that during thermal aging,the hardness of the aluminum alloy first rise to the peak and then decreased to the stable hardness value.Among them,the peak hardness at 180 °C aging temperature can reach 111.3±1.8 HV,which is about 50% higher than the hardness in solid solution state.Further analysis shows that the change in the hardness of the alloy during thermal aging is mainly attributed to two aspects: on the one hand,the precipitation of the precipitated phases,i.e.,the precipitation of Mg and Si solute atoms from the supersaturated solid solution during aging,which evolves to form the diffuse-reinforcedβ" phase as the main strengthening phase of the alloy and plays a major role in strengthening the alloy;on the other hand,the evolution of dislocations,i.e.,entanglement and accumulation,accompanying the residual stress release process also leads to the increase in the hardness of the alloy.(2)The evolution of residual stresses,microstructure and mechanical properties of6061 solid-solution aluminum alloy during ultrasonic aging and their interrelationships were studied.It is shown that,with the gradual release of residual stresses in 6061 solidsolution aluminum alloy during ultrasonic aging,the hardness ofthe aluminum alloy first rises to the peak and then gradually decreases to a stable value.During ultrasonic aging,the peak hardness of the alloy reaches 81.8±1.95 HV,which is about 45% higher than the initial state of 56 HV.When the residual stress is released by about 80%,the hardness of the alloy is stabilized at 63.75±1.65 HV.Further analysis shows that ultrasonic aging does not promote the precipitation of the reinforced phase of 6061 the solid solution aluminum alloy,and the change of the hardness of the alloy is mainly attributed to the evolution of dislocations during the release of residual stresses.During ultrasonic aging,the superposition of the dynamic stress generated by ultrasonic vibration and the internal residual stress of the alloy induces the movement of dislocations,which leads to the change of hardness,i.e.the superposition of ultrasonic dynamic stress and theinternal residual stress of the aluminum alloy firstly induces the multiplication and entanglement of dislocations,thus increasing the hardness of the alloy.With the further release of the residual stress,the dislocation multiplication rate slows down,the annihilation rate accelerates,and the corresponding dislocation density and dislocation entanglement decrease significantly,resulting in the reduction of the aluminum alloy hardness to a constant.(3)The effect of ultrasonic combined thermal aging(ultrasonic pre-aging followed by thermal aging)on the microstructure and hardness of 6061 solid-state aluminum alloy was investigated.The study showed that the changes in the hardness of aluminum alloy during ultrasonic combined thermal aging were similar to those during pure thermal aging,and the hardness curves of the alloy both experienced a rise to the peak hardness and then a decline to a stable hardness value.However,the peak hardness of ultrasonic combined thermal aging is 16% lower than that of pure thermal aging.The analysis shows that the ultrasonic pre-aging eliminates residual stresses in advance,resulting in a change in alloy hardness during ultrasonic combined thermal aging that depends only on the precipitation of precipitated phases.Therefore,the peak hardness during this combined ultrasonic thermal aging process is significantly lower compared to pure thermal aging.