Study On Low-cycle Fatigue Behavior Of AZ31Magnesium Alloy

Magnesium and its alloys have the lowest density among the practical metals andalloys, with good machinability and recyclability. A recent push by the automotiveindustry to lower the fuel consumption and cost of automobile production is providingenhanced motivation for the study of lightweight materials for structural components.As structural materials in service, magnesium alloys always involve cyclic deformation,therefore, the low-cycle fatigue (LCF) properties of these materials need to besystemically investigated for safety reasons.The results showed, the plastic strain energy density model can be used to predictthe fatigue life appropriately by comparing four different fatigue life prediction models.The plastic strain energy density underwent a considerable variation through the entirefatigue process. The parameter is a combination of stress and plastic strain during thefatigue damage, thererfore, the energy-related model is suitable for the prediction oflow-cycle fatigue life of the extruded Mg alloy. There is an error between calculationvalue of the plastic strain energy density by formula and the true value, and errorincreases with the increased strain amplitude, twinning-detwinning process at higherstrain amplitude will change the shape of the hysteresis, resulting in errors:By analysising the fatigue deformation mechanism of AZ31magnesium alloy,when the strain amplitude increased from0.3%to0.4%, the volume fraction of residualtwins recorded in the area near the fracture surface increased from3.9%to32.7%, andthe type of most twins is tensile twin. The fatigue life reduced from18208N to3044N,Therefore, it is reasonable to deduce that strain amplitude of0.3%may be the turningpoint of deformation mechanism in the fatigue process.The higher fatigue frequency (1Hz) would delay the hardening of the strainamplitude (σa), which began to increase when N>100. The strain amplitude exhibited aconstant and stable cyclic hardening at lower fatigue frequency (0.5Hz), and hysteresisloop area was more lager. When the fatigue frequency was applied at0.5Hz, the yieldoccured earlier during the compression, and the hardening of compressive stressamplitude curve was more significant. Initial twin influenced the deformation behaviorand fatigue properties of magnesium alloy AZ31. The subsequent tensile deformationcould be fully accommodated by detwinning, because twins were activated duringpre-deformation, resulted in lower tensile stress amplitude. The longer fatigue life was obtained at pre-compression of2%, but the fatigue life reduced drastically whenpre-compression increased to4%. Therefore, appropriate considerations on this e ectare essentially required for product design.