Wenxian Wang

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.

Laser surface melting AZ31B magnesium alloy with liquid nitrogen-assisted cooling

Abstract Laser surface melting (LSM) is a high-energy surface treatment that allows modification of the microstructure and surface properties of Mg alloys. In the present work, an attempt of LSM on magnesium alloy with liquid nitrogen-assisted cooling (LNSC) was carried out to get the higher cooling rate and improve the surface properties. The experimental results were compared with those of Ar gas protection at room temperature. The samples after LSM with LNSC resulted in a thinner melted layer, a highly homogeneous, refined melted microstructure and formed a lot of worm-like nanocrystals and local amorphous structures. Microhardness of the melted layer with LNAC was improved to HV 90–148 as compared to HV 65–105 of the samples with Ar gas protection. The corrosion resistance of the melted layer in a 3.5% NaCl solution (mass fraction) was improved because of the grain refinement and redistribution of β -Mg 17 Al 12 phases following rapid quenching associated with the process.

Effect of environmental media on ablation rate of stainless steel under femtosecond laser multiple raster scans

We investigate the influence of environmental media on ablation rate of AISI 443 stainless steel under femtosecond (fs) laser single raster scan and multiple raster scans in air, water, and methanol. Meanwhile, the development of ablation rate with the change of fs laser-induced surface morphology in the three environmental media is comparatively studied. The results show that environmental media as well as fs laser-induced morphology control the ablation rate with the increasing number of raster scans (N). Under single raster scanning (N = 1), the ablation rate is higher in liquid than in air due to the confinement of plasma, laser-induced shockwaves, and bubble-related mechanical forces. However, under multiple raster scans, the variation in ablation rate with the increase in N in these three environmental media is complicated and is largely determined by the surface morphology induced by previous fs laser ablation. When N > 20, the ablation rate is much higher in air than in liquids due to preferential ablation caused by the formation of nanostructures-textured mound-shaped microstructures in air. Besides, the redeposition of ejected ablated materials is also an important factor that affects the ablation depth.

Theoretical prediction of forming limit diagram of AZ31 magnesium alloy sheet at warm temperatures

Abstract A theoretical prediction on forming limit diagram (FLD) of AZ31 magnesium alloy sheet was developed at warm temperatures based on the M–K theory. Two different yield criteria of von Mises and Hill’48 were applied in this model. Mechanical properties of AZ31 magnesium alloy used in the prediction were obtained by uniaxial tensile tests and the Fields–Backofen equation was incorporated in the analysis. In addition, experimental FLDs of AZ31 were acquired by conducting rigid die swell test at different temperatures to verify the prediction. It is demonstrated from a comparison between the predicted and the experimental FLDs at 473 K and 523 K that the predicted results are influenced by the type of yield criterion used in the calculation, especially at lower temperatures. Furthermore, a better agreement between the predicted results and experimental data for AZ31 magnesium alloy sheet at warm temperatures was obtained when Hill’48 yield criterion was applied.

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 (ΔσeSNu200a=u200a90·3 MPa) derived from the traditional method through 107 cycles were compared with the values predicted by the infrared thermographic method (ΔσeTMu200a=u200a87·3 MPa) and the energy approach (ΔσeΦu200a=u200a86·2 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.

Laser Cladding Al-Si/Al2O3-TiO2 Composite Coatings on AZ31B Magnesium Alloy

To improve the wear resistance and corrosion resistance of magnesium alloys, a 5 kW continuous wave CO2 laser was used to investigate the laser surface cladding on AZ31B magnesium alloys with Al-Si/Al2O3 -TiO2 composite powders. A detailed microstructure, chemical composition, and phase analysis of the composite coatings were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The laser cladding shows good metallurgical bonding with the substrate. The composite coatings are composed of Mg17Al12, Al3Mg2, Mg2Si, Al2O3, and TiO2 phases. Compared to the average microhardness (50HV0.05) of the AZ31B substrate, that of the composite coatings (230HV0.05) is improved significantly. The wear resistances of the surface layers were evaluated in detail. The results demonstrate that the wear resistances of the laser surface-modified samples are considerably improved compared to the substrate. It also show that the composite coatings exhibit better corrosion resistance than that of the substrate in 3.5wt% NaCl solution.

Calculation of Second Phase Particle-Grain Boundary Interaction Range

On the basis of the grain boundary equation by Hellman and corresponding analysis of Worner, this article deals with the interaction range between the second-phase particle (SPP) and grain boundary (GB) as viewed from the applicability of grain boundary equation. Also, a new expression describing the interaction range has been derived, which solves the problem in theory that the interaction range between SPP and GB can only be qualitatively analyzed previously. It is shown that given the interaction position between SPP and GB, the interaction range can be quantitatively determined by use of this expression.

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.