| Al-Zn-Mg-Cu series alloy is an important material in aviation,aerospace and military fields,and has an important strategic position in China's military and economy.With the increasingly complex international situation and the rapid development of the domestic aerospace industry,the demand for high-performance aluminum alloys is very urgent,and A1-Zn-Mg-Cu series alloys have many casting defects and poor weldability.This article takes Al-Zn6.1-Mg2.9-Cu2.0 alloy as the research object,using optical microscope?OM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and differential thermal analysis?DSC?,X-ray diffraction?XRD?and microhardness as testing method.The effects of Zr,Ti and Nb on the microstructure and mechanical properties of as-cast and heat-treated alloys were studied,and the heat treatment process of alloys with different compositions was optimized.In addition,this subject also used Gleeble thermal simulation to study the effects of Zr,Ti and Nb on the hot workability and weldability of the alloy.The above research has the following main conclusions:The addition of Zr?0.2 wt.%?,Nb?0.2 wt.%?alone and combined addition of Zr?0.2wt.%?+Ti?0.2 wt.%?can refine the grains of the as-cast alloy.Among them,the fine crystal effect of adding Zr and Ti is the most remarkable.The as-cast Al-Zn-Mg-Cu alloy mainly contains?phase?MgZn2?,S phase?Al2CuMg?and T?AlZnMgCu?phases.After adding trace elements,the phase composition of the as-cast alloy does not change significantly.The optimized T6 heat treatment process for alloys without adding trace elements,adding Zr,Nb and Zr+Ti is 480?/90 min+140?/4 h,470?/60 min+130?/4 h,490?/90 min+140?/4 h and 470?/60 min+130?/16h respectively.Among them,the nano-scale Al3Zr and Al3?Zr,Ti?dispersed phases which have the functions of refining alloy grains,pinning dislocations and hindering recrystallization were precipitated from the adding Zr and adding Zr+Ti alloys respectively after heat treatment.In addition,after three-stage aging treatment?130?/4 h+212?/30 min+170?/4 h?,the hardness of the alloy reaches 184 HV,which is 11%lower than the single-stage aging,but the conductivity rate was increased by 39%.The second phase distributed on the grain boundary is discontinuous,which is beneficial to the improvement of corrosion resistance compared to the single-stage aging continuous second phase.The flow stress of the four alloys decreases with the increase of hot working temperature,and increases with the increase of hot working rate.However,the constitutive equation of the four alloys is different.The deformation activation energy of alloys with Zr and Nb added is reduced,while the activation energy of alloys with Zr and Ti added is increased.The hardness of all as-cast and T6 alloys decreases with the welding heat cycle temperature increases.The as-cast alloy structure gradually changes from dendrites to equiaxed crystals as the heat cycle temperature increases and accompanied by recrystallization.While the T6 state the precipitation phase in the alloy gradually decreases with the increase of the peak temperature of the thermal cycle and becomes unevenly distributed. |
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