Quench Sensitivity Of Novel 7xxx Series Aluminum Alloys With High Alloying Content

Higher strength and better combination of properties is the eternal target of the development of novel 7xxx series aluminum alloys with high alloying content.To date,increasing the alloying content has been an important method of chemical composition designation.However,it will increase the quench sensitivity and limit the application of novel 7xxx series aluminum alloys for aerospace as the integral structures.Direct quenching method,Jominy end quench method and interrupted quench method were carried out to study the stability of supersaturated solid solution and precipitation behavior of several novel 7xxx series aluminum alloys.The quench sensitivity of these alloys as well as how it was effected by the alloying elements were investigated.We hope it could provide technical support for the production and application of novel 7xxx series aluminum alloys.The suitable solution temper for these alloys were investigated and the lattice parameter of as-quenched alloys was tested.Scanning electron microscopy and optical microscopy were used to observe the microstructure so as to determine the suitable solution temper(470?/6h or 472?/6h).It was discovered that the increase of Mg,Zn and Zr content led to an increase of lattice constant of the alloys,while the increase of Cu content resulted in an opposite tendency.That variation reflected the increase of the lattice distortion degree of supersaturated solid solution as well as the decrease of stability of supersaturated solid solution.It can be concluded that the increase of Mg,Cu,Zn and Zr content in alloys will all decrease the stability of supersaturated solid solution.Cu element has a more significant influence compared with other element content if added with the same content.The variation of electrical conductivity and hardness,the depth of quenched layer and the precipitation behavior of ? phase experienced by Jominy end quench experiment were investigated.As the decrease of average cooling rate,the electrical conductivity of as-quenched alloy increased,while the hardness of as-aged alloy decreased.The depth of quenched layer of the studied alloys ranged from 40 mm to 75 mm.And the critical cooling rate ranged from 2.12 K/s to 4.78K/s.The increase of Mg,Cu,Zn and Zr content all increased the variation range of electrical conductivity and hardness,decreased the depth of quenched layer.exacerbated the precipitation tendency of ? phase.Quench-induced ? precipitates preferentially nucleated and precipitated at grain boundaries and grew up as the of cooling rate decreased.If the average cooling rate was lower than the critical one,coarse ?precipitates would nucleate on Al3Zr dispersoids inside grains.The variation of electrical conductivity and hardness,TTP curves and the precipitation behavior of ? phase experienced by interrupted quench experiment were investigated.Samples had the fastest response to the variation of soaking time at the temperature of 310?and 340?.The electrical conductivity of as-quenched alloy increased with soaking time,while the hardness of as-aged alloy decreased.The increase of Mg,Cu,Zn and Zr content all decreased the corresponding incubation period,but there was no regularity of the effect on the nose temperature.The nose temperature of studied alloys ranged from 300? to 320?.And the corresponding incubation period ranged from 0.8667 s to 3.358 s.Moreover,the increase of Mg,Cu,Zn and Zr content all exacerbated the precipitation tendency of ? phase.Quench-induced ? precipitates preferentially nucleated and precipitated at grain boundaries,and grew up with the soaking time.If the soaking time was long enough,coarse ? precipitates could nucleate on A13Zr dispersoids inside grains.After aging treatment,those coarse ?precipitates were surrounded by the precipitate-free zone and no strengthening phase were found nearby.Comprehensive results indicate that the increase of Mg,Cu,Zn and Zr content will all decrease the stability of supersaturated solid solution and increase the quench sensitivity of novel 7xxx series aluminum alloys with high alloying content.Cu and Mg element has a more significant influence compared with other element content if added with the same content.