Research Of A Novel 6000 Series Aluminum Alloy With Enhanced Age-hardening Response For Automotive Body Panels

6000 series aluminum alloys are the most promising light weight material used in automotive body panels instead of steel. However, the stamping formability of aluminum alloy sheet is not increased significantly. Besides, the bake hardenability is not sufficient during paint baking process. In the present work, a novel 6000 series aluminum alloy with rapid age-hardening response for automotive body panels was systematically investigated, and the pilot production of the novel alloy at industrial level has been progressed on the basis of plenty of work in experimental laboratory. The main work and results were drawn as follows.Homogenization heat treatment and microstructural evolution of the novel alloy were investigated. The as-cast microstructure was typical dendrite structure, consisting of a-Al, Mg2Si, Si, Al15(FeMn)3Si2 and Al1.9CuMg4.iSi3.3 phases. Phases fully dissolved into Al matrix after homogenized at 430? for 10 h plus 550? for 18 h. The residual phase was Al15(FeMn)3Si2. Zinc-containing phase was not found in as-cast and as-homogenized microstructure.The deformation processing of the novel alloy was studied. According to the stress-strain curves, the flow constitutive equation and processing map were established, and microstructural evolution of deformed samples was also examined. The suitable hot rolling process for 6016 aluminum alloy was deformed under a strain rate of 0.1s-1 at 450? to 500 ?. The alloy presented a zonal distribution microstructure and grain elongated along rolling direction after hot rolling. Equiaxed grains with 51.2?m in size appeared when hot-rolled sheet was annealed at 400? for 1 h. The percentage of high-angle boundary was 89.1% and main grain orientation was Cube {001}<100>, this microstructure was in favor of improving the surface quality of sheet.Microstructural evolution during cold rolling and solution heat treatment of the novel alloy were explored. The alloy exhibited a fibrous microstructure along rolling direction after cold rolling. The matrix of the alloy was a-Al. Mg2Si, Si, Al15(FeMn)3Si2 and Zr-containing phases distributed in the matrix. According to DSC curves of as-rolled alloy, the optimum solution heat treatment was 550? for 15 min. The complete recrystallization occurred after solution heat treatment and equiaxed grains with an average size of 32.0?m were formed. The main grain orientation was Cube {100}<001>, which was beneficial for sheet metal forming.The mechanism and heat treatment process of pre-aging for the novel alloy was investigated. Compared with the alloy after natural aging, the alloy exhibited excellent bake hardening response after pre-aged at 130? for 10 min and 140? for 5 to 10 min, and the increment of yield strength after bake hardening was 120 MPa. Pre-aging effectively inhibited the forming of clusters during natural aging and improved the nuclei density of ?" phase. The formability of the novel alloy was evaluated and studied after pre-aged and then natural aged for one month. Bendability of the alloy was better than 6016 aluminum alloy. Erichsen value of the alloy was 9.7 mm, which met the requirements of as-received sheet metal. The alloy showed excellent hole expandability, limiting hole expansion ratio was 51.3%, which was higher than 40.6% of AC170PX aluminum alloy. Uniaxial extensibility and stretchability of the alloy were the same as the AC170PX aluminum alloy.Effect of Zn addition on aging precipitation behavior of the novel alloy was researched. Zn addition had no effect on yield strength of the alloy after solution treated and pre-aged. The alloy presented enhanced age-hardening response during paint baking process. The yield strength for the alloy with and without Zn was 250 MPa and 230 MPa, respectively. Meanwhile the increase of yield strength was 125 MPa and 98 MPa for the alloy with and without Zn, respectively. Zn atom distributed randomly during paint baking process of the novel alloy, and it did not form other strengthening precipitates. Zn element decreased the activation energy of ?" phase and promoted the precipitation of ?" phase. The activation energy of ?" phase for the alloy with and without Zn was 94 kJ/mol and 104 kJ/mol, respectively. At the early stage of aging, the volume fraction of ?" phase for the novel alloy was higher than that of the alloy without Zn.The fabrication processing, heat treatment regime, microstructure and properties of the novel alloy materials at industrial pilot scale were evaluated and investigated. Yield strength of the novel alloy at industrial pilot scale was low after pre-aged, and the sheet presented rapid age-hardening response after paint baking process. The increment of yield strength was 130 MPa. The novel alloy showed excellent formability after pre-aging. Bendability and drawability of the alloy at industrial pilot scale were better than 6016 aluminum alloy. Limiting hole expansion ratio of the novel alloy was the same as AC170PX aluminum alloy. The novel alloy exhibited excellent stretchability, FLDo of the alloy was higher than the alloy prepared previously. Owing to obvious anisotropy of the sheet after pre-aging, uniaxial extensibility of the alloy was poor.