Effects Of Retrogression And Re-ageing Treatment On Microstructure And Properties Of Al-7.6Zn-l.6Mg-l.7Cu Alloy

The Al-Zn-Mg-Cu aluminum alloy as the age hardening alloy has been widely used in aerospace industry because of its comprehensive performance with high strength,high toughness,corrosion resistance,fatigue resistance and high hardenability.In order to make the Al-Zn-Mg-Cu alloy possessing high strength,high toughness and good corrosion resistance together,foreigners have begun to use retrogression and re-aging(RRA)treatment,process known as T77.But the process still remain a secret.Focused on the Al-7.6Zn-1.6Mg-1.7Cu alloy which has the similar ingredients as 7085 alloy,the effects of retrogression and re-aging on microstructure,mechanical properties and corrosion resistance were investigated by means of hardness test,electrical conductivity test,ambient temperature tensile test,optical microscopy(OM)and transmission electron micocopy(TEM).The main conclusions are as follows:1?Without the hot deformation processing(extrusion or rolling)Al-7.6Zn-1.6Mg-1.7Cu alloy organization dense enough,there are many defects,which affect the toughness and corrosion resistance of alloy.2?Al-7.6Zn-1.6Mg-1.7Cu alloy organized burning temperature is about 480?.The best single stage solid solution system is 470?/2h.At this point,the second phase particles dissolve more thoroughly,grain growth is not obvious,and the organization is not overheating.3.The peak ageing process of Al-7.6Zn-1.6Mg-1.7Cu alloy is 120?/6h.At this condition,hardness and electrical conductivity is 202.5 HV and 18.8%IACS.Although alloy has high strength,but its stress corrosion cracking resistance needs to be further enhanced.Compared with the as-cast alloy,the peak ageing state of Al-7.6Zn-1.6Mg-1.7Cu alloy's strength and conductivity increased by 179.7%and 8.75%respectively.Though the strength of the alloy increase,the conductivity changed little.The conductivity still is less than 20%IACS,which shows that the alloy's corrosion resistance is poor.4?The RRA process of Al-7.6Zh-1.6Mg-1.7Cu alloy is 120?/6h+160?/120min+120?/6h.At this condition,hardness,tensile strength and conductivity of alloy is 198.5HV?382.4MPa and 20.3%IACS.RRA treatment not only improved electrical conductivity and SCC resistance but also obtained a tensile strength.Compared with the peak ageing state alloy,the RRA state alloy's hardness change little,and its conductivity increase by 8%,which shows that the alloy contains the better strength and corrosion resistance together.