Research On High Cycle Fatigue Properties Of Magnesium Alloys With Different Forming Processes

Magnesium alloys have been widely used in a lot of industries due to their low densities, high specific strength and specific stiffness, good damping characteristics, high electromagnetic shielding and radiation resistance, excellent machinability and easily recycled, etc. There is a prediction that the applications of magnesium alloys will increase quickly by 25-30% from the year 2004 to 2007. The request of mechanical properties especially the fatigue properties becomes higher as the applications of magnesium alloys increase, and most of the parts used in engineering work under the conditions of high cycle fatigue. However there is very little report about high cycle fatigue of magnesium alloys. In the present work, the research on high cycle fatigue properties of die-cast AZ91 magnesium alloys with addition of Ce, Si and Ca and cast AZ91 magnesium alloys with different heat treatment conditions are investigated. At the same time, it builds the connection between fatigue strength and cycle load, discusses the micro-mechanism of fatigue fracture and provides the reliable academic foundation.The investigation indicates that with addition of Ce, Si and Ca, the microstructure of die-cast AZ91 magnesium alloys are refined obviously especially the alloys with addition of Ce and Ca. The new rod-like compound Al4Ce phase and particle-like compound Mg2Si phase are formed in the alloys after adding Ce, Si and Ca, however Ca is mainly dissolved inβ-Mg17Al12 phase; The mechanical properties testing indicates that the values of tensile strength, yield strength and fatigue strength of die-cast AZ91 alloy with addition of Ce and Ca are the highest, its value of fatigue strength can be up to 90.2MPa. The fatigue fracture surfaces of three kinds of alloys appear a lot of rip edges and dimples, and the fatigue crack propagation area contains many small and plane sections, but the fatigue stripes can not be observed.The fatigue properties of magnesium alloys can be improved with heat treatment. Microstructural observation indicates that cast magnesium alloys AZ91 chiefly containsα-Mg matrix andβ-Mg17Al12 phase. After the solid solution treatment (T4),βphase almost dissolves in Mg matrix. The mechanical properties testing results show that the tensile strength and elongation are improved obviously, but the yield strength and hardness are reduced. Meanwhile the fatigue limit is improved notably and the value of fatigue strength can be up to 71.5MPa with 107 cycles to failure; after the solid solution and ageing treatment (T6),βphase precipitates around the grain boundary and intragrain continuously and non-continuously, the grains become bigger at the same time. The mechanical properties testing results show that the tensile strength, yield strength and hardness of cast magnesium alloys AZ91 are improved obviously, but the elongation and fatigue strength are reduced, the value of fatigue strength can be up to 65.2MPa, a little higher than as-cast of AZ91 alloy. The analysis of fatigue fracture surface morphology indicates that the fatigue crack initiates from the flaws like cavities and impurities on or under the surface of the samples. The fatigue stripes can not be observed on the fatigue crack propagation area, but it can obviously be found the feathery and fan-shaped propagation characteristics; The fatigue fracture area morphology of three kinds of alloys are different, the as-cast AZ91 alloy is quasi-cleavage fracture, the cast AZ91 alloy with T4 is toughness fracture and the cast AZ91 alloy with T6 is brittle fracture that is mainly composed of cleavage-type fracture.