Microstructure Control Mechanism And Properties Of Wrought Magnesium Alloys Based On Multi-Directional Forging

Wrought magnesium(Mg)alloys are lightweight and high-strength,and have great application potential in aerospace,transportation,electronic products and other fields.However,the low strength,poor plasticity,and direction anisotropy of mechanical properties of commercial wrought Mg alloys greatly limit the expansion of its application fields.Grain refinement and texture control are important methods to enhance the comprehensive mechanical properties of wrought Mg alloys and to improve the directional anisotropy of their properties.The multi-directional forging process is simple and reliable.By optimizing the design of process parameters such as loading path,process temperature and single pass strain,the flexible control of the grain and texture of wrought Mg alloy can be realized.In order to better play the role of multi-directional forging in the preparation of high-performance Mg alloys,this paper selected commercial extruded Mg alloys as the research object.Based on high temperature uniaxial compression(UC)and plane strain compression(PSC)experiments,the microstructure evolution mechanisms under the influence of deformation temperatures and deformation modes were analyzed.Based on this,the isothermal multi-directional free forging(MDF)and isothermal cycle closed-die forging(CCDF)processes were designed and studied,and the effects of deformation temperature and accumulated strain(∑ε)on the microstructure and properties of isothermally forged alloys were revealed.Based on the above studies,a twinning segmentation and a temperature-dependent dynamic recrystallization microstructure refinement method,as well as a deformation mode dependent texture control method have been developed.Furthermore,the multi-directional forging with gradient cooling and high strain rate multi-directional forging with pass small strain(～0.05)for microstructure refinement and texture weakening were proposed,and the evolution mechanisms of microstructure and properties of the two processes were studied.The mechanical behavior and microstructure evolution during UC and PSC were comparatively studied.The results show that hot compression consisted of two stages: hardening and softening,in which the hardening stage was plastic hardening under the action of {10-12} twins or multi-system slips;the softening stage was mainly characterized by recr ystallization softening with multiple dynamic recrystallization competitions.Compared with the strain rate,the deformation temperature had a more significant effect on the {10-12} twinning and dynamic recrystallization behaviors.During the hardening stage,a large number of {10-12} twins were activated at lower temperatures,resulting in the refinement of the microstructure and the rapid formation of the <0001>// CD(compression direction)texture;in contrast,the activation of {10-12} twinning was suppressed at higher temperatures,and the changes in microstructure and texture state were small.Further studies showed that the activation of the {10-12}twin variants obeyed Schmid law.Affected by this,the texture components of UC and PSC were significantly different at lower temperature.In the softening stage,the recrystallization mechanisms at lower temperature s were twin-induced dynamic recrystallization(TDRX)and continuous dynamic recrystallization(CDRX),resulting in bimodal grain structure and strong <0001>//CD texture;in contrast,at higher temperatures,the recrystallization mechanism changed to discontinuous dynamic recrystallization(DDRX),forming a uniform equiaxed grain structure,and the diffusion degree of basal texture was significantly improved.In the study of isothermal MDF and CCDF,it is found that the effect of deformation temperature on the microstructure and properties of isothermal multi-directional forging was more significant than that of cumulative strain.After forging with ∑ε=0.9,the mixed grain structure and strong basal texture was formed at 200°C;the microstructure refinement effect was the best at 250°C,and the average grain size was reduced to about 6 μm;the texture regulation was remarkable at 300℃,resulting in bimodal and X-type basal texture.As the temperature increases,the dislocation strengthening and texture strengthening weaken,resulting in a large decrease in the compressive yield strength.At 300°C,after forging with ∑ε=0.9,as the cumulative strain continued to increase,the microstructure and texture characteristics of MDF and CCDF samples changed little,but the texture intensity increased slightly,which can cause various different trends of compressive yield strength in each forging direction.Using the gradient cooling multi-directional forging process,after forging with ∑ε=2.7 and the temperature from 300°C to 200°C,the microstructures of the MDF and CCDF alloys were refined from the initial 33.6μm to 2.1μm and 1.4μm,respectively,and exhibited bimodal and X-type basal texture characteristics similar to those of isothermal forging.The microstructure evolution mechanisms of the forging process changed obviously: when ∑ε<0.9,the refinement of grains depended on twinning and DDRX,and the evolution of texture was dominated by{10-12} twinning;when ∑ε≥0.9,the grain refinement mechanisms were dominated by CDRX,and supplemented by TDRX,and the evolution of basal plane texture was dominated by multiple slips.The compressive properties were investigated and found that the compressive yield strength along different forging directions more than doubled.The Hall-Petch formula including the effect of texture was constructed,and the comprehensive influence of grain size and texture state on yield strength was quantified.The high strain rate multi-directional forging with small strain of ～ 0.05 was adopted.After forging with ∑ε=6.0 and initial forging temperature of 400°C,the microstructure of forged alloy was refined from the initial larger than 43.8μm to5.3μm under the action of twin segmentation refinement and multiple rounds of DRX.Meanwhile,a unique four peak basal texture was formed under the combined action of {10-12} twins and basal slip.After forging,the tensile/compressive yield strength ratio along the ED as well as the tensile yield strength ratio along the ED and TD decreased from the initial 1.86 and 2.02 to1.02 and 1.05,respectively,and the directional anisotropy of the yield strength of extruded Mg alloy was significantly improved.