A Study On The Deformation And Aging Process In Al-Mg-Si-Cu Alloys

Compared to the traditional metal materials such as steel, Al alloys are charactesistic of low density, high strength, good corrosion resistance and easy recycling. Therefore, Al alloys are widely used in building constructions, electronic products and food packaging, they also serve as the main materials for manufacturing aerospace and automotive vehicles. In order to save enegy and reduce emissions, the application of Al-Mg-Si-Cu alloys(6xxx series) in the aerospace and automotive industry is becoming more and more popular. Compared with the 2xxx series and 7xxx series Al alloys, the relatively low strength of 6xxx series Al alloys limits their applications. Therefore, it has been a common concern for both industry and academia to improve the strength of Al-Mg-Si-Cu alloy while maintaining the ductility.By tuning the initial grain size before cold rolling, or changing the temperature and deformation ratios in rolling process, this work aims to find the optimized parameters for improving the mechanical properties of Al-Mg-Si-Cu alloys processed by combined deformation and aging. The effect of processing parameters on the mechanical properties and microstructure evolution in Al-Mg-Si-Cu alloys are investigated by hardness tests, tensile tests, electron back-scattering diffraction(EBSD), transmission electron microscopy(TEM) and differential scanning calorimetry(DSC) and so on.Three kinds of Al-Mg-Si-Cu alloys, L1(Al-0.75Mg-0.75Si-0.8Cu, wt.%), L2(Al-1.0Mg-0.5Si-0.8Cu, wt.%) and L3(Al-1.0Mg-0.5Si-0.8Cu-0.1Ti, wt.%), were used in this work, and the experiment include three parts. L2 and L3 alloys with the same primary alloying elements were chosen in the first part, trace Ti was added in L3 alloy to refine the initial grain sizes. These two alloys were then treated by the same thermal processing to explore the effect of initial grain size on the Al-Mg-Si-Cu alloy. In the second part, L1, L2 and L3 alloys experiencing the same heat treatment process were studied to reveal the effect of rolling temperature on the mechanical properties of Al-Mg-Si-Cu alloys. The third part investigates the effect of deformation ratios in rolling process on the mechanical properties and microstructure evolution for Al-Mg-Si-Cu alloys(L1 and L2). The main conclusions are listed as follows:(1) When the alloys are pre-treated by natural aging for 1 day(refered to NA1D) before cold rolling, reducing the initial grain size can't improve the hardness of alloys after deformation and post-aging, and the strength and elongation will also decrease slightly. When the alloys are pre-treated by artificial aging for 10 min at 180? before cold rolling, refining the initial grain size has no effect on the hardness and strength of the alloys, but the elongations of alloys will decrease slightly. For the combined deformation and aging process, refining the initial grain size before cold-rolling is fultile in improving the mechanical property.(2) For L1 and L2 alloys, no matter the pre-treatment before rolling is natural aging or artificial aging, the hardness, strength and elongation of the alloys subjected to liquid-nitrogen cold rolling will decrease slightly compared with room temperature rolling. For L3 alloy, when the pre-treatment before rolling is artificial aging, the effectts of these two kinds of rolling treatment on the mechanical propertites show no obvious difference. When the pre-treatment before rolling is natural aging, liquid-nitr ogen cold rolling has a positive effect on the hardness of the alloy compared with room temperature rolling. The values of hardness and strength are similar to the L2 alloy pre-treated by NA1 D and rolled at room temperature. Liquid-nitrogen cold rolling could exert additional cost and difficulty on the rolling proces, thus there is no need to perform liquid-nitrogen rolling for the combined deformation and aging process.(3) With the increase of deformation ratios, the as-rolled hardness of the Al-MgSi-Cu alloys will increase gradually and the hardness of the alloys will increase further in subsequent aging process, however, the age hardening potential will decrease continually. With the increase of deformation ratio, the yield strength and tensile strength of the alloys rise gradually, while the elongation has a tendency to decrease at first and then rise. Even when the deformation ratio reachs 80%, Al-Mg-Si-Cu alloys still has high strength and good elongation.(4) The ratios of deformation have a significant effect on the microstructure of the alloys. The grains elongate along the rolling direction during deformation and finally have a lamellar structure. Fragmentation and extensive defects like sub-grain boundaries occurs inside the grains. The dislocations become denser inside the alloy with the increase of the deformation ratio. When the deformation ratio is large(above 60%), formation of dislocation tangling and sub-grains are observed. Deformationinduced change of the dislocation configuration affects the precipitation significantly. Our findings are fruitful for tuning deformation and ageing temper to produce Al alloys with excellent balance of strength and ductility.