Effect of Accumulative Strain on Grain Refinement and Strengthening of ZM6 Magnesium Alloy During Continuous Variable Cross-Section Direct Extrusion

In order to study the influence of die combination on continuous variable cross-section direct extrusion （CVCDE） in the extrusion process, the accumulative strain formula is derived, and it can be known that the extrusion ratio of various stages directly determines the size of corresponding stage strain by formula. In this paper, as an example of the two interim dies, three die combinations of different angles and extrusion ratio are designed. Aviation magnesium alloy ZM6 is studied, and the results show that dynamic recrystallization is even more complete when continuous shear deformation occurs, so that the refinement and homogenization of microstructure are obtained. By the use of different die combinations, the accumulative strain increases under the conditions of same total extrusion ratio. Thus, the refined crystalline strengthening effect of extrusion deformation can be further analyzed. Due to the dead-zone defects, the actual accumulative strain decreases significantly and the effect of microstructure and performance improvements also decreases with it. Therefore, the optimal design of die combination is the key to the process and product of CVCDE, which provides a scientific basis for the development of severe plastic deformation.

Microstructural characteristics and deformation of magnesium alloy AZ31 produced by continuous variable cross-section direct extrusion

Magnesium（Mg） alloy AZ31 was produced by continuous variable cross-section direct extrusion（CVCDE）to study its deformation behavior. Metallographic microscopy（OM）, transmission electron microscopy（TEM）, and scanning electron microscopy（SEM） were used to observe the variations in microstructure and fracture morphology of Mg alloy AZ31 as a function of processing methods. The results reveal that grains of Mg alloy AZ31 were refined and their microstructure was homogenized by CVCDE. The recrystallization in Mg alloy AZ31 produced by CVCDE with 2 interim dies was more complete than that produced by conventional extrusion（CE） and CVCDE with 1 interim die, and the grains were finer and more uniform.Plasticity of the AZ31 alloy was improved. Fracture mode was evolved from a combination of ductility and brittleness to a sole ductile form. In summary, a CVCDE mold structure with 2 interim dies can improve microstructure, plasticity, and toughness of Mg alloy AZ31.

Dual Strengthened Control of Recrystallization Behavior on CVCDE Magnesium Alloy Containing Characteristic Structure

Various characteristic structures in typical magnesium alloys,including dislocation cells,substructures and twins,have an important influence on the dynamic recrystallization behavior,and the DRX(dynamic recrystallization)behavior is closely related to the grain refinement and texture weakening of the hot deformed structure.Therefore,this study reveals the influence of the above characteristic structures on the dynamic recrystallization behavior of magnesium alloys,which have great significance for regulating the high-performance hot deformed microstructure of magnesium alloys and optimizing the macro mechanical properties.In this study,continuous variable channel direct extrusion(CVCDE)magnesium alloy was prepared by CVCDE,and its macro mechanical properties including hardness and uniaxial tension were characterized.The thermoplastic deformation behavior and texture evolution of magnesium alloy with characteristic structure were analyzed by electron back-scattering diffraction technology.It is found that the dislocation recombination was realized by deformation mechanism(slip,climb and cross slip),the formation of grain substructure in coarse grains and the induction of recrystallization by twins promote the recrystallization behavior in hot deformed structures more adequate,which effectively improves the degree of microstructure refinement and deformation uniformity.