| In this test, AZ31and ZK60wrought magnesium alloy as widely used in commercial applications are selected as study objects, the tensile, hardness, impact, welding and ultra-high-cycle fatigue properties of the two magnesium alloys and their welded joints are studied systematically.In this paper, the better heat treatment process for extruded AZ31magnesium alloy is300℃heat preservation45min, in this process, the extruded AZ31magnesium alloy has the most uniform and fine grain, tensile strength is about275MPa, yield strength is about209MPa, elongation is about12.4%. Tensile strength of AZ31magnesium alloy friction welded joint reaches98%of the base material, yield strength of the welded joint reaches122%of the base material, elongation has a slight increase, and the tensile fracture of the welded joint is the brittle-ductile mixed fracture.Fatigue limit dose not exist for extruded AZ31magnesium alloy during109cycles, corresponding to the109cycles, fatigue strength is about90MPa, the fatigue ratio is0.33, so the fatigue strength is in a medium state. Within the range of ultra-high cycle fatigue, crack initiation of extruded AZ31magnesium alloy mainly occurred in the surface and sub-surface. Fatigue limit dose not exist for extruded AZ31magnesium alloy welded joints during109cycles, corresponding to the109cycles, fatigue strength is about68MPa, lower than the base metal22MPa; between107cycles to108cycles, crack initiation of extruded AZ31magnesium alloy mainly occurred in the surface and sub-surface, more than108cycles, the crack initiation occurred in the sample internal.Extruded ZK60magnesium alloy after T5(150℃heat preservation24h) treatment, the grain is more uniform and fine, tensile strength and yield strength reach to the highest, tensile strength is about314MPa, yield strength is about264MPa, elongation compared to the extruded has a slight decrease; After T6alloy, the tensile strength, hardness and elongation of the alloy are decreased, while the yield strength increased. The tensile strength of extruded ZK60magnesium alloy welded joints reach102%of the base material, the yield strength lower than the base metal,, elongation decreased.Fatigue limit does not exist for extruded ZK60magnesium alloy after T5treatment within109cycles, corresponding to109cycles, the fatigue strength is105MPa, the fatigue ratio is0.33. Between105cycles and108cycles, the crack initiation occurred at the surface and sub-surface; when the cycles more than108, the crack initiation exists within the internal. Fatigue limit dose not exist for extruded ZK60magnesium alloy welded joints within109cycles, the fatigue strength is about97MPa, reaches92%of the very high cycle fatigue strength of the base material; between104cycles to108cycles, crack initiation of the welded joints mainly occurred in the surface and sub-surface, more than10cycles, the crack initiation occurred in the sample internal.Through the microscopic analysis of the fracture profile, twins crystals are produced during the ultrasonic fatigue experiment in stress-strain concentration area of the two magnesium alloys which may be the main reason for the crack initiation. |
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