.7050 Aluminum Alloy Hardenability End Quenching

7050 aluminum alloy is an Al-Zn-Mg-Cu-based high strength alloy developed by replacement of dispersoid-forming Cr and Mn with Zr and changing contents of Zn and Mg in the 7075 aluminum alloy in the 1970s. Typical applications for 7050 Al alloy thick plate are fuselage frames and bulkheads in active service large airplanes.It is a key to control penetration as quenching(hereafter hardenability)because of its quench sensitivity.However,there is few literature quantitatively reports the hardenability about 7000 series aluminum alloy.In fact,the quantitative hardenability is very important to maximize magnitude properties and uniformity of microstructures and minimize residual stress in quenched high strength aluminum alloy thick plate.In present paper,a demonstration quenching testing system of Jominy End Quench method adapts to aluminum alloy materials is established. Hardenability of samples cut from a 7050 Al alloy hot-rolled thick plate is investigated by testing the hardenability curves after being quenched by spraying various reagents.The microstructures of different cooling rates are characterized by optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).As a result,the conclusions can be gotten:(1)The demonstration method of non-linear relationship between surface heat-transfer coefficient and temperature of the sample has been established.This non-linear surface heat-transfer coefficient by spraying water at 20℃is used to simulate the temperature-field of the quenched sample.The simulated result is consisted with the measured one.(2)The hardenability curves of the 7050-T6 samples are determined by spraying water,10-wt%brine and PAG regent at 20℃,respectively, the depths corresponding to HV3=175(hardness approximately decrease by 10%)are 60mm,60mm,45mm,respectively.The most stable hardenability curve is contributed by spraying PAG reagent.(3)The precipitation of.(MgZn₂)phase,remotion of grain or/and subgrain boundaries,and precipitation free zone(PFZ)are affected by the cooling rates.The critical average cooling rate of precipitating phase is about 40～60℃/S.