Modeling of microstructure evolution of AZ80 magnesium alloy during hot working process using a unified internal state variable method

In this paper, a unified internal state variable(ISV) model for predicting microstructure evolution during hot working process of AZ80 magnesium alloy was developed. A novel aspect of the proposed model is that the interactive effects of material hardening, recovery and dynamic recrystallization(DRX) on the characteristic deformation behavior were considered by incorporating the evolution laws of viscoplastic flow, dislocation activities, DRX nucleation and boundary migration in a coupled manner. The model parameters were calibrated based on the experimental data analysis and genetic algorithm(GA) based objective optimization. The predicted flow stress, DRX fraction and average grain size match well with experimental results. The proposed model was embedded in the finite element(FE) software DEFORM-3 D via user defined subroutine to simulate the hot compression and equal channel angular extrusion(ECAE) processes. The heterogeneous microstructure distributions at different deformation zones and the dislocation density evolution with competitive deformation mechanisms were captured.This study can provide a theoretical solution for the hot working problems of magnesium alloy.

Microstructures and tensile properties of submerged friction stir processed AZ91 magnesium alloy

6 mm thick AZ91 casting alloy plates were subjected to normal friction stir processing(NFSP,in air)and submerged friction stir processing(SFSP,under water),and microstructures and tensile properties of the experimental materials were investigated.After FSP,the coarse microstructures in the as-cast condition are replaced by fine and equiaxed grains and the network-like eutecticβ-Mg_(17)Al_(12)phases disappear and are changed into particles pinned at the grain boundaries.SFSP results in further grain refinement in comparison with NFSP,and the average grain sizes of the NFSP and SFSP alloys are 8.4±1.3 and 2.8±0.8µm,respectively.XRD results reveal that the intensity ofβ-Mg_(17)Al_(12)diffraction peaks in the SFSP specimen decreases compared with NFSP.Due to significant grain refinement,the tensile strength and elongation of the SFSP AZ91 alloy are increased from 262 MPa and 18.9%for the NFSP material to 282 MPa and 25.4%,and the tensile strength(282 MPa)is nearly three times that of the BM(105 MPa).SFSP is an effective approach to refine the grain size and enhance the tensile properties of AZ91 casting alloy.

Superplasticity of AZ31 magnesium alloy prepared by friction stir processing

Microstructure and tensile behaviors of AZ31 magnesium alloy prepared by friction stir processing（FSP） were investigated.The results show that microstructure of the AZ31 hot-rolled plate with an average grain size of 92.0 μm is refined to 11.4 μm after FSP.The FSP AZ31 alloy exhibits excellent plasticity at elevated temperature,with an elongation to failure of 1050% at 723 K and a strain rate of 5×10-4 s-1.The elongation of the FSP material is 268% at 723 K and 1×10-2 s-1,indicating that high strain rate superplasticity could be achieved.On the other hand,the hot-rolled base material,which has a coarse grain structure,possesses no superplasticity under the experimental conditions.

Application of cyclic upsetting-extrusion to semi-solid processing of AZ91D magnesium alloy

The microstructural evolution of AZ91D magnesium alloy prepared by means of the cyclic upsetting-extrusion and partial remelting was investigated. The effects of remelting temperature and holding time on microstructure of semi-solid AZ91D magnesium alloy were studied. Furthermore, tensile properties of thixoextruded AZ91D magnesium alloy components were determined. The results show that the cyclic upsetting-extrusion followed by partial remelting is effective in producing semi-solid AZ91D magnesium alloy for thixofonning. During the partial remelting, with the increase of remelting temperature and holding time, the solid grain size increases and the degree of spheroidization tends to be improved. The tensile mechanical properties of thixoextruded AZ91D magnesium alloy components produced by cyclic upsetting-extrusion and partial remelting are better than those of the same alloy produced by casting.

Corrosion behavior of electron beam processed AZ91 magnesium alloy

The increasing use of light alloys owing to their high performance makes magnesium alloys very attractive for the use in automotive and biomedical applications.However,it is well known that magnesium and its alloys have poor corrosion resistance in different atmospheric and aqueous environments.As a means of improving corrosion resistance through the microstructure modification,electron beam processing(EBP)was applied on the as-cast AZ91 magnesium alloy.To evaluate the microstructure influence on the corrosion-resistant,the EB processed samples underwent a solution heat treatment and an artificial aging heat treatment.Four different obtained microstructures were investigated by standard microscopy and electrochemical corrosion tests to evaluate the microstructure and its effects on the corrosion resistance of AZ91 alloy.The EBPed specimens show a significant microstructure refinement and homogenous distribution ofβ-phase at the grain boundaries surrounded by supersaturatedα-Mg which acts as a barrier against corrosion.The electrochemical corrosion test of the samples immersed in 3.5 wt%NaCl after 4 weeks indicates that the EBP improves the corrosion resistance of the alloy due to the nobler corrosion potential of supersaturated a-Mg and more stable protective hydroxide films compared to the heat-treated and as-cast conditions.

Development of AZ91/SiC surface composite by FSP:effect of vibration and process parameters on microstructure and mechanical characteristics

A surface composite layer enhances the mechanical characteristics of a surface while retaining the properties of the base material.Friction stir processing(FSP)is a method for forming surface metal matrix composites(SMMCs)that reinforce a surface with particles.In the current study,a new method entitled friction stir vibration processing(FSVP)was applied to form SMMCs on the surface of AZ91 magnesium alloy with SiC particles as the reinforcing particles.Contrary to FSP,in FSVP,the workpiece was vibrated normal to the processing line while the tool rotated and traversed.The microstructure and mechanical properties of friction stir(FS)and friction stir vibration(FSV)processed specimens were evaluated.Additionally,the effects of vibration frequency and process parameters on different characteristics of FS and FSV processed specimens were studied.The results showed that the stir zone grains for FSV processed specimens were finer than those for FS processed specimens,and the second phase particles(SiC particles)had a more homogenous distribution in the former specimens than in the latter specimens.This was related to the effect of workpiece vibration during FSVP,which increased the material deformation and led to enhanced dynamic recrystallization and the breakdown of agglomerated SiC particles.The results indicated that the stir zone grain size decreased,and the distribution homogeneity of the SiC particles increased as vibration frequency increased.It was also observed that the stir zone grain size increased,and the mechanical properties of the processed specimens decreased as tool rotation speed increased.

Microstructural characteristics of near-liquidus cast AZ91D alloy during semi-solid die casting

Near-liquidus cast ingot was reheated to semi-solid firstly, and then a bracket of motor was prepared by die casting the semi-solid ingot into mould. The microstructural characteristics of AZ91D alloy in these processes were investigated. In the process of near-liquidus casting, primary α-Mg grains tend to be rosette-like because of the increase of plentiful quasi-solid atom clusters in molten alloy with the decrease of pouring temperature. These rosette-like a-Mg grains in ingots fabricated by near-liquidus casting are fused off and refined into near-globular structure owing to the solute diffusion mechanism and the minimum surface energy mechanism during reheating. After semi-solid die-casting, a-Mg grains, located in biscuit, impact and connect with each other; α-Mg grains, located in inner gate, congregate together; while α-Mg grains, located in component, distribute uniformly and become into globularity or strip. Because the inner gate limits the flowing of semi-solid slurry, and the pressure acted on the semi-solid slurry decreases gradually along the filling direction of semi-solid slurry in Cavity, microstructural segregation of unmelted a-Mg grains appears along this direction. Shrinkage holes in casting are caused by two different reasons. For biscuit, the shrinkage holes are caused by the blocked access of feeding liquid to the shrinkage zone for the agglomerated unmelted α-Mg grains. For component, the shrinkage holes are caused by the lack of feeding of liquid alloy.