High Cycle Fatigue Behavior Of AZ91 Series Magnesium Alloys

With the entrance to 21st century the resource and environment problems have become the principle problems in the sustainable development of human beings. The study and development on light castings has been an important research field and developing direction for the saving of materials and energy resource with the rapid development of automotive industry. Magnesium alloys are kind of the lightest structural metal alloys, which have many excellent and unique properties, such as low density, high specific strength and stiffness, good damping, easy processing and castability, high heat and electrical conductivity, excellent electromagnetic shielding capability, as well as good recovery capability, etc. So more and more magnesium alloy products have been used in automotive, electron communication apparatus and aerospace industries. The requirement of mechanical properties especially fatigue properties of magnesium alloy is higher and higher with the increasing of magnesium alloy usage. In practice, as any engineering structural materials, fatigue failure is one of the primary damage forms. It is not exceptional to magnesium alloy. So the research of fatigue behavior of magnesium alloy has not only academic value but also engineering practical importance.Today, magnesium alloys are used in making the parts under low stress of cars such as shells, wheels, transmission housings and pedals. The numbers of load cycles in automotive applications may be relative high, such as car wheels are stressed at variable amplitude with several 107 cycles in service, and the high cycle fatigue properties has been an important considerable factor. However, the high cycle fatigue data of the alloy can only be found sparsely in the literature. Therefore, in this paper, the high cycle fatigue behavior and micro-mechanism of fatigue fracture of the common die cast AZ91 alloy with the addition of different alloy elements and as cast AZ91 alloy with different heat treatment conditions have been investigated. The investigation provides the reliable academic foundation for improving fatigue properties of magnesium alloy. It also provides the essential support in theory for the reliable application in engineering and the safe design of magnesium alloy structural parts. The major research efforts of the present study are as follows:The microstructure observation and the experiments of tensile properties and hardness tests have been conducted about die cast AZ91 magnesium alloys with the addition of different contents Ce, Nd, Si and Ca elements. The tensile fracture surface and fracture pattern of the alloys have been observed and analyzed using SEM. The microstructure observation and analysis results showed that the microstructure of die cast AZ91 magnesium alloy are refined obviously with single and composite addition of Ce, Nd, Si and Ca.β-phase (Mg17Al12) become discontinues and dispersion distributing bones and granular shape. The rod-like and needle-like Al11Ce3 and Al11Nd3 phases are observed and the volume and size ofβ-phase are somewhat decreased after adding solely Ce and Nd in the alloys. More Ce and Nd additions cause further decreasing ofβ-phase and coarsening of Al11Ce3 and Al11Nd3 phases. The grain refined effect of Nd on the alloys is better than Ce. The microstructure of die cast AZ91 magnesium alloy is refined with the composite addition of Si, Ca and Ce, Ca. The effect of grain refined of the alloy containing Ce and Ca is best. The new Mg₂Si and Al₁₁Ce₃ phases are observed in two alloys, but no the phase of containing Ca.The conventional mechanical properties testing results showed that the tensile properties and hardness increase with the addition of Ce, Nd, Si and Ca. The tensile strength reach a high value with 1.0% Ce and Nd addition. The strength decrease slightly, and the hardness keep on increasing with more Ce and Nd additions. The effect of Nd on the strength and hardness of the alloy is more than the Ce effect. The tensile properties of the alloys are improved obviously with Si, Ca and Ce, Ca additions, and the elongation no obvious improvement. The tensile and yield strength of the die cast AZ91 magnesium alloy containing 1.0%Ce and 0.4%Ca is the highest in all tested alloys.The SEM analysis of the tensile fracture surface showed that the die cast AZ91 magnesium alloy indicates the brittle quasi-cleavage pattern. The fracture method of the alloys varies from the brittle to the tough-brittle mixed rupture after adding Ce and Nd. The dimple feature of the fracture surface is obvious with 1.0%Ce and 1.0%Nd addition. The dimple fracture translates into the mixed fracture with more Ce and Nd additions. The tensile fracture surface shows the mixed fracture characteristic in two alloys containing Si, Ca and Ce, Ca elements.The high cycle fatigue experiments of the die cast magnesium alloys with different addition elements were conducted at stress ratio R=0.1 and the number of cycles N=107. The fatigue strength of the tested alloys was calculated using up-and-down load method, and S-N curves drew. The fatigue fracture surface was analyzed in detail using SEM, the micro-mechanism of the die cast magnesium alloy fatigue fracture summarized. The results showed that the fatigue strength of die cast AZ91 magnesium alloy is improved with different contents Ce and Nd additions, and reach a high value with 1.0%Ce and 1.0%Nd. The fatigue strength decreases slightly with more Ce and Nd additions, the influence of Nd on the fatigue properties of the alloys be effective than Ce. The fatigue properties of die cast AZ91 magnesium alloy with Si, Ca and Ce, Ca additions are improved due to the microstructure refine and the new second phase effect. The fatigue life and strength with 1.0%Ce and 0.4%Ca die cast magnesium alloy is the highest (90.2MPa) than the alloys with single Ce, Nd and Si, Ca additions.The fatigue crack of the die cast AZ91 magnesium alloys with different element additions initiate at the gas pore and inclusion location of the alloys surface and inside. The fatigue crack propagation areas show a facet shape, and without obvious fatigue striations besides the alloy containing Nd element. The fatigue striations are not clear in AZ91-1.0%Ce alloys. The fatigue striations of AZ91-1.0%Nd alloy are the clearest and continuous due to its excellent plasticity deformation capability. The fatigue collapse fracture area of the tested alloys show the quasi-cleavage and dimple fracture characteristics as same as the tensile fracture surface, and exist large numbers of the rip ridges, dimples and second cracks.The heat treatment experiments about as cast AZ91 magnesium alloys were conducted. The influences of solution (T4) and aging after solution (T6) treatment on the microstructure, conventional mechanical properties and high cycle fatigue behavior of the metal gravity cast AZ91 magnesium alloys were investigated. The microstructure and properties of the as cast alloys were analyzed comparing with the die cast AZ91 magnesium alloys. The results showed that the grain size of the as cast magnesium alloy is larger than the die cast alloy. The microstructure comprise of primaryα-Mg, eutecticα+βandβphase. The structure of the as cast alloy become the single phaseα-Mg solid solution, andβphase dissolution inα-Mg after T4 treatment. After T6 treatment,βphase precipitates around the boundary and the inner of the grain continuously and non-continuously, the grains become bigger at the same time.The mechanical properties of the as cast magnesium alloy is lower than the one of die cast alloy. The tensile fracture surfaces show the brittle quasi-cleavage fracture. After T4 treatment, the tensile strength and elongation increase, and the yield strength and hardness decrease. The tensile fracture surfaces show obvious dimple feature. After T6 treatment, the tensile strength, yield strength and hardness of the as cast alloys are improved, and the elongation decreased. The fracture surfaces show the brittle cleavage characteristics. The fatigue testing results showed that the fatigue properties of the as cast alloy are improved, and the fatigue strength can be 71.5MPa at 107 cycles by T4 treatment. After T6 treatment, the fatigue strength is reduced to 65.2MPa, which a little higher than as cast of AZ91 alloy. The micro-morphology analysis of the fatigue fracture surfaces of the as cast AZ91 magnesium alloys with different heat treatment conditions showed that the fatigue crack initiate at the gas pore, inclusions and shrink locations of the alloys surface or subsurface. The fatigue striations are not observed in the fatigue crack propagation area, but it can be obviously found the feathery and fan-like characteristics. The morphology of the tested alloy at fatigue collapse fracture area similar with the tensile fracture surface.The microcosmic observation and analysis for near tensile and fatigue fracture surface were carried out using TEM technique. The strengthening mechanism and the effect of twins on the plastic deformation of magnesium alloys were investigated. The results showed that the strengthening mechanism of magnesium alloy involve the grain refinement strengthening, the second phase strengthening and solution strengthening. The dominant mechanism is not same for the different magnesium alloys. The mechanical properties of the alloy containing Nd are better than the alloy containing Ce because the grain refinement and second phase strengthening effect of Nd on the die cast magnesium alloy are greater than the one of Ce. The mechanical properties of the alloy containing Ce and Ca are the best because of the best grain refinement and solution strengthening effects. After T4 treatment, the yield strength of as cast magnesium alloy decrease due to the grain growth. The strength of as cast alloy is improved due to the second phase strengthening effect after T6 treatment. The plastic deformation of magnesium alloys contribute to the slide and twins. The effect is great for the plastic deformation ability improvement of the magnesium alloy due to the interaction of the twins, the twins and dislocation.