Rare Earth And The Impact Of Aging Process Of Al-mg-si Car Plate Organization And Performance

In this paper, aiming to obtain the Al-Mg-Si auto sheet, four kinds of auto sheets were produced with electrolytic low-titanium aluminium alloys. The effect of rare earth content and heat treatment process on the microstructure, mechanical properties and corrosion resistance of the Al-Mg-Si sheets are investigated using Microscope, Transmission Electron Microscope(TEM), Scanning Electron Microscope (SEM), X-Ray difraction methods, micro-hardness measurement, tensile testing and the Anodic polarization curves.The results show that with the increasing of rare earth content in the alloys, the density of theβ" phase is increased, its size is also decreased and its distribution is more homogeneous. Some fine and continuous precipitated phasees on the grain boundary can be found. After treated with different aging process, the spiondal decomposition structure, which may beβ" phase, are appeared in alloys with 0.2% RE. Therefore, it is beneficial to form theβ" phase when alloys contain certain rare earth element. The high densityβ" phase and grain boundary precipitated phase made the alloys which contain RE have a higher strength and elongation than the alloy without RE after baking. The strength and elongation is decreased after the T4 and T4P treatment, so it is beneficial to the formability of sheets when alloys contain certain rare earth.The uniform microstructure and the high densityβ" phase in alloys containing rare earth make the alloys have a lower E_corr and a biggerΔE, therefore, the spot corrosion resistance of alloys is increased. But the grain boundary precipitated phase made the alloys have a high resistance of recrystallization, the alloys with RE have a lower exfoliation corrosion resistence because some elongated grains is still kept.Nature aging results in the aggregation of solute atoms and the formation of the larger cluste. These larger clusters don't be dissolved when the alloys are artificial aged. Therefore, it is harmful to improve the properties of alloys after simulate baking. Pre-aging (140℃×10min) increases the nucleus ofβ" phase and restrains the aggregation of Mg, Si atoms clusters during nature aging, which increases the number ofβ" phase after simulate baking at 180℃. On the one hand, pre-aging process almost has any effect on the mechanical properties of the sheets before baking. The sheets have similar yield strength, tensile strength and elongation as that of sheets treated by T4 process. Thus the sheets have similar formability as those by T4 process. On the other hand, pre-aging process can effectively decrease the deleterious effect of nature aging on the precipitate hardening of sheets. The mechanical properties of the sheets treated by T4PBaking process are obviously improved. The tensile strength of sheets is close to that treated by T6 process. And the sheets almost have the same elongation as those treated by T6 process. Therefore, the T4PBaking process combines the advantages of T4 and T6 processes. The higher performance of the sheet is achieved.The mechanical properties and formability of auto sheets can close or even exceed some of the traditional 6000 auto sheets by the optimization of the alloying composition and heat treatment process. The auto sheets with 0.2%RE have the optimal mechanical properties after treated by pre-aging treatment at 140℃for 10min and simulate baking.