预变形对汽车用Al-Mg-Si-Cu合金析出行为的影响

Abstract

To reduce the weight of car body, Al- Mg- Si- Cu alloys have been widely used to produce outer body panels of automobiles due to their favorable high strength- to- weight ratio, corrosion resistance, weldability and good formability. Al- Mg- Si- Cu alloys belong to age- hardenable aluminium alloys, whose strength derives mainly from the matrix precipitation during aging treatments. However, their bake hardening response still need to be further improved to enhance their dent resistance. A novel thermo- mechanical treatment consisting of conventional pre-aging, pre-deformation and re-aging was developed to enhance the tensile properties and bake hardening increment of Al-Mg-Si-Cu alloys. In this work, the effect of pre-deformation on the precipitation behaviors of AlMg-Si-Cu alloy was studied by DSC, mechanical property measurement and TEM. The results show that, the GPzone dissolution rate decreases with increasing pre-deformation during the slow heating up process for the pre-aged alloy, the corresponding activation energies of 0, 5% and 15% pre-deformed alloy calculated by Avrami-JohnsonMehl method are 137.1, 189.5 and 141.3 k J/mol, respectively. If the pre- deformed alloys are directly bake hardened at 185 ℃ for 20 min, precipitation and bake hardening increment can be greatly improved by pre-deformation(the highest bake hardening increment is 160 MPa), but the bake hardening increment rate gradually decreases if the pre- deformation is above 10%. In addition, the GP zone dissolution rates of pre- deformed alloys after bake hardening treatment are much lower when the heat treatment temperatures are below one certain value, but if the treatment temperatures above it, the corresponding GP zone dissolution rates are higher than that of alloy without pre-deformation, finally, the activation energy changes from high value to low value even can be observed in the ln[(d Y/d T)f/f(Y)]-1/T curve. For the β″ precipitation in the alloys, with increasing pre-deformation, its activation energy gradually decreases, corresponding gradually increase of precipitation rate.