Studies On Microstructure And Characteristics Of Al12Si Alloy Under The Condition Of Thermal Stress

Al-Si is one of the most widely used and the most important industrial alloy,which has small density and thermal expansion coefficient, stable dimension, goodwear resistance and so on. Also, Al-Si is widely used in automotive, aerospace,transportation and other industries. The conventional cast aluminum-silicon alloyorganization contains big silicon phase, which severely splits alloy matrix and reducesthe alloy performance. Rare earth can refine grains, improve the crystal structure, andstrengthen the wear resistance to high temperature. The rare earth has been widely usedin the Al-Si alloy. The rare earth heat phase morphology and distribution and heattreatment process will be the focus of future research.In this thesis, Al12Si alloy was chosen for the investigation. Different contents ofrare earth alloys were melted and the processed samples were respectively loaded thestress of3MPA,5MPA and8MPA by the self-designed mechanical stress device. Thenwe placed the device and the specimen into the resistance furnace at100°C,200°C and300°C for10h insulation processing. Then we took photos of microstructure, testVickers hardness and Brinell hardness, and analyzed the impact of the rare earth andthermo-mechanical stress on the morphology and performance of the alloy Al12Siorganizations. Finally, the stress and strain field distribution of Al12Si alloy wassimulated by FEM software, through establishing reasonable model and settingreasonable boundary and setting process condition.The main experimental results of this thesis are shown as follows:When the RE content is less than3%of the total alloy, the RE has metamorphismfor the Al12Si alloy; When the RE content is more than3%, the RE modification effectnearly disappeared, and meanwhile, white rich RE phases appeared in the matrix.Mechanical thermal stress could cause the micro-cracks of the coarse secondphase or even broken the big organizations. The large number of small precipitatesappeared in the matrix. With the increasing of the mechanical stress and the incubationtemperature, the number of precipitates increased and the sizes of organizationsreduced and distributions were more uniform.The hardness of Al12Si alloy increased substantially with the RE contentincreasing. Mechanical stress influences were different for the second phaseorganization and the matrix organization; the Vickers hardness of the second phasestructure was biggest at3MPA; the Vickers hardness of the matrix organization was maximum at5MPA; the Brinell hardness reached the maximum at8MPA. The Vickershardness of the second phase structure reached maximum when the temperature at200°C; the Vickers hardness of the base body tissue reduced with the temperaturerising constantly. Brinell hardness of the alloy reached maximum when the temperatureat100°C, and appeared the hardening phenomenon; after that the Brinell hardnessgradually decreased and appeared the softening phenomenon.The result of the stress and strain numerical simulation showed the casting lateralstrain distribution is more uniform and longitudinal stress first increased from top tobottom, then reduced.