Precipitation Characterization And Behavior Research Of Single And Double Aged Al-Zn-Mg-Cu Alloy

With higher and stricter demands of lightweight engineering in automotive industry,a lot of attention has been drawn to Al-Zn-Mg-Cu high strength aluminum alloy.However,the low efficiency of heat treatment and high price of this alloy have narrowed its application.Therefore,this research is aimed at improving its treatment efficiency and then promote its use in automotive industry.Firstly,the solid solution treatment has been studied in order to shorten the treatment time when the alloy is completely solutionized.For 7075 aluminum alloy,heating at 480?C for 20 min is enough for the redissolving of solute atoms in the Al matrix and therefore forming the saturated solid solution.And if the solid solution treatment time is shortened to 10 min,the dissolving of atoms is not so efficient but its influence on the following age hardening peak is not very obvious.Then the effect of different aging temperatures on the single aging treatment is researched.It terms out that the heating temperature is significant for the age hardening effect.The higher the treating temperature,the faster the peak emerges but the lower the hardness peak value.According to HAADF-STEM observation and analysis,large dispersoids of 100 to 200 nm,precipitates of η and η’ are all found in higher temperature treated sample and lower temperature treated sample.However,in higher temperature aged sample,larger dispersoids are more,forming large area of non-precipitation zones.Moreover,in precipitation phases,η particles are more,making the alloy even less strengthened.However,regardless of the single aging temperature,single aging treatment cannot shorten the heat treatment time while maintaining the age hardening effect.Therefore,double aging treatment is then studied,with its 1st aging temperature,1st aging time and 2nd aging temperature as variables.An improved aging method is then developed,which starts with a solid solution treatment at 480?C for 20 min and follows a double aging treatment – 1st aging at 125?C for 60 min and 2nd aging at 165?C for 130 min.The total treatment time counts only 2 h and 10 min,comparing with the origin treating time,13 h and 50 min have been saved.Moreover,sample treated under this method has only small decrease in alloy mechanical property.This double aging treatment method might have important meanings in automotive industry.