Zhifeng Yan

Temperature evolution and fatigue life evaluation of AZ31B magnesium alloy based on infrared thermography

Abstract The surface temperature of extruded AZ31B alloy plate was measured by infrared thermograph in air during tension and high-cycle fatigue tests. The mechanism of heat production was discussed and the value of critical fatigue damage temperature was calculated according to the P —Δ T curve. Results show that the variation trend of temperature is different between tension and fatigue tests. The temperature evolution in tension test consists of four stages: linear decrease, reverse linear increase, abrupt increase, and final drop. The initial decrease of temperature is caused by thermal elastic effect, which is corresponding to the elastic deformation in tension progress. When cyclic loading is above the fatigue limit, the temperature evolution mainly undergoes five stages: initial increase, steep reduction, steady state, abrupt increase, and final drop. The peak temperature in fatigue test is caused by strain hardening that can be used to evaluate the fatigue life of magnesium alloy. The critical temperature variation that causes the fatigue failure is 3.63 K. When Δ T ≤3.63 K, the material is safe under cyclic loading. When Δ T >3.63 K, the fatigue life is determined by cycle index and peak temperature.

Improved properties of friction stir-welded AZ31 magnesium alloy by post-weld heat treatment

Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200 rev min−1 and welding speed of 300 mm min−1, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5 MPa, 199.1 MPa and 7.3%, respectively. It was found that (300°C – 1 h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4 MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.

Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in gear oil

Abstract The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in gear oil was investigated. The corrosion and the fracture surfaces after fatigue test were analyzed by scanning electron microscopy (SEM) and the corrosion compositions were measured by energy-dispersive spectrometry (EDS). The fatigue properties and the crack initiation mechanisms of the specimens before and after epoxy coating treatment were discussed. The results indicate that the fatigue limit after epoxy coating treatment in gear oil is higher than that of the uncoated specimens. The epoxy coating is an excellent way to prevent direct contact between the Mg–3Al–1Zn alloy and surrounding environments. The mechanical properties of the epoxy coating layer are lower than that of magnesium alloy, which is the main reason for the fatigue crack initiation on the epoxy coating layer. In addition, the gear oil lubrication could lead to the flaking off of the epoxy-coated layer.

High-frequency corrosion fatigue behavior of AZ31 magnesium alloy in different environments

The high-frequency corrosion fatigue behavior of extruded AZ31 magnesium alloy was investigated in different environments: air, gear oil and 3.5% NaCl solution. Compared with that in air, the corrosion fatigue limits were degraded by approximately 3.52% and 58.91%, respectively, and the corrosion fatigue lives were shortened by about 13.43% and 89.36%, respectively. In the same environment, the high-frequency fatigue limits are all higher than those tested at low frequency. The specimen geometrical shape plays a certain factor on stage characteristics of S–N curves. Compared with that of arc transition specimens, the stress sensitivity of line transition specimens is only reflected in a relatively high cycle fatigue life region. Different environments influence the corrosion failure kinetics processes (the crack initiation mechanism), but do not change the fatigue fracture mechanism of the alloy, and the higher loading frequency only accelerate the crack nucleation processes.

Rapid determination for fatigue parameters of AZ31B magnesium alloy based on evolution of temperature under high cyclic fatigue

Abstract Based on the infrared thermography method, experiments are carried out to investigate the evolution of temperature field of the extruded AZ31B magnesium alloy specimens under high cyclic fatigue load. The experimental results show that the superficial temperature of specimen under cyclic fatigue load changes with the number of cycles. According to the characteristics of surface temperature change, we propose a formula to calculate the residual fatigue life using energy approach. The proposed formula to assess the fatigue parameters (fatigue limit, residual fatigue life, fatigue life and S–N curve) achieves good results for AZ31B magnesium alloy. Furthermore, the fatigue limits (ΔσeSN = 90·3 MPa) derived from the traditional method through 107 cycles were compared with the values predicted by the infrared thermographic method (ΔσeTM = 87·3 MPa) and the energy approach (ΔσeΦ = 86·2 MPa), and the comparison results of percentage differences are 3·3 and 4·5% respectively.

Fatigue fracture behaviour and thermographic analysis of friction stir-welded AZ31

The fatigue fracture behaviour and thermographic analysis of friction stir-welded (FSW) AZ31 was studied. It showed that fatigue fracture at the advancing side (AS). Heat-affected zone (HAZ) contained a greater fraction of coarse grains and a small amount of twins. The grains in the thermo-mechanically affected zone were smaller, indicating that grains may originate from incompletely dynamic recrystallisation during the FSW. The nugget zone is composed of equiaxed grains caused by the dynamic recrystallisation. During cyclic deformation, hysteresis loops from the AS of the FSW joints with the stain amplitude were higher than the retreating side of the FSW joints, the temperature of HAZ at AS is higher than other regions.

Temperature evolution in magnesium alloy during static and cyclic loading

Abstract The temperature evolution in an AZ31B magnesium alloy plate was measured during static and cyclic loading via infrared thermography. The relationship between loading process and temperature evolution was established. The yield limits during static and cyclic loading were predicted. The temperature variation on the specimen surface was closely related with the applied load. The initial decrease in temperature during tension was caused by the thermoelastic effect, and the minimum temperature corresponded to the yield limit. During cyclic loading, the thermoelastic effect, viscous effect and plastic work had an effect on the temperature evolution. The cyclic yield limit was <1/6 of the yield limit obtained in the tension test.

Anisotropy of fatigue behavior and tensile behavior of 5A06 aluminum alloy based on infrared thermography

The fatigue behavior during high cycle fatigue testing and the tensile behavior of 5A06 aluminum alloy considering the anisotropy were studied. Two types of specimens including longitudinal specimen (parallel to the rolling direction) and transverse specimen (perpendicular to the rolling direction) were prepared. Infrared thermography was employed to monitor the temperature evolution during the fatigue and tensile tests. The temperature evolution curves in the two directions were contrastively analyzed. It is found that the temperature evolution during fatigue process possesses four stages: initial temperature rise stage, slow temperature decline stage, rapid temperature rise stage, and finial temperature decline stage. The heat generating mechanisms of the four stages are discussed. Obvious differences can be found between the longitudinal specimen and transverse specimen in fatigue strength and fatigue life. The fatigue strength and fatigue life of longitudinal specimen are higher than those of transverse specimen. During the tensile and fatigue testing process, the fracture temperature in the transverse direction are higher than that in the longitudinal direction. The fatigue strength prediction by means of infrared thermography has a good consistency with that by the traditional method.

Temperature evolution analysis of AZ31B magnesium alloy during quasi-static fracture

Fracture toughness of AZ31B magnesium alloy subjected to quasi-static loading was investigated by infrared thermography. The results showed that temperature evolution around the crack propagation path during fracture underwent three stages: initial steady stage, monotonic increase stage and final steady stage. The temperature increase at the beginning of stage II is nearly corresponding to the initiation of unstable crack propagation. And based on this phenomenon, a method applying infrared thermography to estimate fracture toughness of AZ31B magnesium alloy was proposed. Fracture toughness was calculated through infrared thermography, which was in good agreement with the result determined by traditional standard method. Finally, the fracture mechanism was investigated.

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.