镁合金非对称变形研究进展

Progress in Asymmetric Deformation of Magnesium Alloys

镁合金具有密度低、比强度高、比刚度高等优点,在航空航天、交通运输、电子产品等领域具有广阔的应用前景。但是,由于镁合金为密排六方结构,滑移系较少,塑性变形易产生较强的基面织构,导致合金塑性降低,严重限制了其应用。因此,提高镁合金的塑性对扩大其应用范围具有重要的意义。研究表明,非对称变形可引入剪切变形,激活非基面滑移系,削弱基面织构强度或产生新的织构组分,且热变形过程中产生的动态再结晶可细化晶粒,协同提升镁合金的室温强度和塑性。例如,通过等径角挤压温度、变形路径的探索,可在保证高塑性的前提下,使低合金含量的AZ31镁合金的屈服强度达到372 MPa,抗拉强度达到445 MPa,大大优于该合金常规轧制和挤压的力学性能。本文综述了异步轧制、等通道转角挤压、等通道转角轧制和非对称挤压等非对称变形方式在镁合金中的研究进展,叙述了变形温度、变形量、变形速度、变形路径等对非对称变形镁合金微观组织与力学性能的影响,以期为制备高强度、高塑性的镁合金材料提供参考。

Magnesium alloy has broad application prospects in aerospace,transportation,electronic products and other fields due to its advantages of low density,high specific strength,high specific rigidity.However,there are limited slip systems of magnesium alloy due to its nature of hexagonal close-packed structure.At room temperature,only basal slip and twinning are easy to be activated,which leads to the strong basal texture and the poor ductility.Therefore,enhancement in ductility of magnesium alloy is of great significance to expanding its application range.Studies have shown that asymmetric deformation can introduce shear strain and activate the non-basal slip systems,which may weaken the basal texture or generate some novel texture components.In addition,dynamic recrystallization generated during hot deformation can refine the grains of the alloy.The weakened texture together with the refined grains can improve the strength and ductility of magnesium alloys synergistically at room temperature.The asymmetric deformation methods including differential speed rolling,equal channel angular extrusion,equal channel angular rolling and asymmetric extrusion.In the aspect of differential speed rolling,shear strain can be introduced in deformation by changing the diameter or linear speed of the upper and lower rolls or by changing the contact friction.Different from conventional rolling,differential speed rolling can form a cross shear zone between the upper and lower rolls,resulting in shear strain.The current research on differential speed rolling of magnesium alloys mainly involves the influence of rolling path,reduction in per pass,differential speed ratio and rolling temperature on the microstructure and properties of the alloy.The intersection of the shear bands between different passes can be varied by changing rolling path.Studies have shown that when the plate is rotated 180°around its rolling direction,the shear bands can be crossed to form equiaxed,uniform,and fine grains,and AZ31 alloy with high yield strength of 295 MPa and high elongation of 21%can be obtained.The reduction during per pass has a significant influence on the microstructure of the alloy.As the reduction increases,the number of twins increases.However,when the reduction exceeds a certain value,the number of twins decreases or even disappears,and the deformed sheet exhibits good strength and ductility at room temperature.The differential speed ratio has the influence on the grain refinement,grain morphology and texture weakening.Although different researchers have obtained different experimental results on different alloys,the application of high differential speed ratio rolling is considered to be an effective method to achieve grain refinement and basal texture weakening.Researchers prepared commercial AZ31 magnesium alloy sheet with a grain size of 0.6μm and a high yield strength of 382 MPa by optimizing the process parameters of differential speed rolling with high differential speed ratio.The rolling temperature mainly affects the recrystallization behavior of the alloy.Dynamic recrystallization is more likely to occur at a high rolling temperature,which also increases the ductility of the alloy.In the aspect of equal channel angular extrusion,shear deformation were introduced to the processing process by the equal channel angular extrusion die,which leads to the refinement of the grains.Both of the extrusion passes and the deformation path are the factors to affect the microstructure of the magnesium alloy.The grain size of the alloy usually decreases first and then remains stable as the number of extrusion passes increases.Ultra-fine grains with the size of 1~2μm or smaller can often be obtained after multiple passes of extrusion.By changing the deformation path,both of grain refinement and texture modification can be obtained.The outstanding achievement involved that sub-micron grains were obtained through 12 passes of variable temperature extrusion based on the combination of different extrusion paths.Both of the grain refinement and texture strengthening were achieved,and the ultra-high strength AZ31 magnesium alloy with the ultimate tensile strength of 445 MPa and tensile yield strength of 372 MPa were fabricated.In the aspect of equal channel angular rolling,the shear strain is introduced through the rolling angular channel to weaken the texture.The principle of this method is that the sheet is sent into an angular mold channel through the friction force generated by the roller,and finally rolled out.Channel gap,rolling passes and bending after equal channel angular rolling have important effects on the microstructure and mechanical properties of the alloy.The number of twins and the texture of the sheet can be affected by changing the channel gap,and the formability of the sheet can be modified.As the number of rolling passes increases,it can significantly weaken the basal texture of the magnesium alloy and achieve grain refinement,thereby improving the mechanical properties of the alloy.By combining equal-channel angular rolling and bending,the basal texture of the sheet can be weakened or novel texture components can be formed,and the formability at room temperature of the sheet can be greatly improved.In terms of asymmetric extrusion,the billet exhibits different flow velocity in the thickness direction in the die during hot extrusion through the asymmetric design of the upper and lower inner surface of the traditional extrusion die.Therefore,a gradient strain in the thickness direction is achieved.The introduction of shear strain can effectively weaken the basal texture of the magnesium alloy or make the basal pole deviate from the normal direction,so the formability of the sheet can be improved.