Effects Of Cu And RE On The Microstructure And Mechanical Properities Of Al-Cu-Mn Casting Alloy

With the development of the aerospace, transportation, construction and other industyfields, the demand for reducing the weight of components has been increasinglyurgent. High strength cast aluminum alloys possess advantages of high ratio strength,short process and good formability. They could be used to replace common steel parts,which is in line with the current trend of lightweight in manufacturing industry. And itraises higher requirements on the performance of cast aluminum alloys.Al-Cu-Mn alloys have the highest strength in cast aluminum alloys, but theygenerally require aluminum with high purity of more than99.95%, and this increasesproduction cost. The Al-Cu-Mn alloys in this thesis are prepared by aluminum withpurity of99.7%. In this dissertation, optical microscopy, scanning electronmicroscopy, transmission electron microscopy are used to observe the microstructureof Al-Cu-Mn high-strength cast aluminum alloys after different heat treatments,whiletensile strength at room temperature and high temperature, hardness of the alloysunder different states are tested.Also, the strengthening mechanismes of the alloys arediscussed.The temperature range(530~535℃) of solution treatment is roughly determinedthrough the analysis of differential scanning calorimeter(DSC) curve of Al-Cu-Mnalloy cast sample, combining observation and analysis of the microstructure aftersolution treatment at different temperatures.The effects of Cu and RE content on the microstructure and mechanical propertiesof Al-Cu-Mn alloys are studied. The contents of Cu and RE of the Al-Cu-Mn alloysare optimized, and the strengthing mechanism of the alloys are discussed andanalyzed. After T6treatment, Brinell hardness of the alloy increases with increasingCu content, the tensile strength at room temperature reaches the highest value480MPa when the content of Cu is5.6%. However, the hardness reduces after theaddition of RE.The best content of RE is0.05%, while the tensile strength at roomtemperature is479MPa. Excess Cu and RE can not dissolve into matrix and remain inthe grain boundaries, which could not be conducive to improve the tensile strength of the alloys at room temperature, but hinder the sliding of grain boundaries anddeformation of grains at high temperature, thus slightly improving tensile strength ofthe alloys at high temperature.The effects of single-stage and multi-stage aging on microstructure andmechanical properties of the Al-Cu-Mn alloys are studied, and aging system of thealloys with Cu content of5.6%is established, and strengthening mechanisms arediscussed and analysed. Aging at different temperatures, the hardness increases withtime, but the changes have different features. The hardness of the alloy increasesslowly with time at120℃, the hardness reaches HBW123at the9th hour, and staysconstant substantially. At170℃and190℃, the curves of hardness demonstrate threedistinct parts, which are under-aging, peak-aging and over-ageing, the peak hardnessof the alloy are HBW140and HBW145respectively. Improving temperaturepromotes hardening rate, which makes the peak time to come early. The peak-agingprocess of the alloy is170℃×6h, its tensile strength reaches480MPa, and theelongation is5.5%. Optimum two-stage aging process is:100℃×12h+170℃×6h,the tensile strength of the alloy after this treatment is still slightly lower than peakaging.Well accepted precipitation sequence of the Al-Cu-Mn alloys are supersaturatedsolid solution→GP zones→θ’→θ"→θ（Al2Cu）. Precipitates grow up with aging.Improving aging temperature accelerates the growth of precipitation. The strength ofthe alloys reach peak through strain strengthening, bypass mechanism and shearingmechanism when a large number of fine θ’ precipitates come out in matrix.