Study On Improving Tension-compression Anisotropy For ZK60 Extruded Rod Via Plastic Deformation And RE Alloying

Magnesium alloy with light weight and high specific strength is a kind of structural material with potential application.However,traditional processing techniques such as extrusion and rolling have resulted in strong anisotropy of mechanical properties of magnesium alloys.The anisotropy of mechanical properties is reflected in the difference of yield strength between tension and compression for as-extruded alloys.The tension-compression anisotropy is extremely detrimental to the application of magnesium alloys.Based on the above problems,in this paper,ZK60 is selected as the base material and 0.4 wt.%Y+0.2wt.%Gd mixed rare earth,1.2wt.%Gd pure rare earth as well 2.4wt.%Y pure rare earth are added into the alloy respectively.Mg-Zn-Zr-xRE alloys are extruded for the following investigating.The microstructure,mechanical properties of as-extruded alloys are tested in this work.The formation mechanism of tension-compression anisotropy is investigated in detail.And Mg-Zn-Zr-xRE alloys are cyclic extrusion and compressed and forging-extruded respectively to improve tension-compression anisotropy.The results show that the grains of the as-extruded alloys are both coarse and inhomogeneous due to insufficient dynamic recrystallization.The addition of rare earth plays a role in refining and homogenizing the grains.There is few second phase in ZK60 alloy and the quantity of second phase is greatly increased by the addition of rare earth.Both of the as-extruded rods exhibit intense extruded-fiber texture.Coarse grain and intense extruded-fiber texture,which is favorable to the activation of twinning result in the low yield strength when compressed along ED.In that case,tension-compression anisotropy is extremely serious for as-extruded alloys.Grain refinement is apparent by dynamic recrystallization after cyclic extrusion and compression.The quantity of second phase increases after cyclic extrusion and compression and the second phase becomes more refining and dispersive.Moreover,type of texture changes and after cyclic extrusion and compression.Most of grain is away from hard orientation.Moreover,the texture is weakened apparently.The variation of texture results in the decreasing of yield strength in tension and the increasing of yield strength in compression.Therefore,tension-compression anisotropy is improved by cyclic extrusion and compression to some extent.In the meanwhile,the elongation and uniform elongation of Mg-Zn-Zr-xRE alloys are both enhanced greatly.Grain refinement is apparent as well by dynamic recrystallization after forging-extrusion.But the effect is inferior to that of cyclic extrusion and compression.The quantity of second phase increases after forging-extrusion and the second phase becomes more refining and dispersive.There is still extruded-fiber texture in forging-extruded alloys but,the texture has been weakened.Twinning is suppressed effectively by grain refinement and texture weakening.Meanwhile,the basal slip is still hard to activate caused by the extruded-fiber texture.Therefore,the enhancement of compressive yield strength for forging-extruded alloys is higher than that for cyclic extrusion and compression.Fiber texture weakening,grain refinement and phase dispersing result in the only slight decreasing of yield strength in tension.Isotropy magnesium alloy rods with high strength and high ductility are fabricated by forging-extrusion.