| In the fields of aerospace,national defense and military industry,high-end equipment have a strong demand for lightweight.Magnesium alloy is the lightest metal structural material at present(the density of pure magnesium is about 1.74g/cm~3),which has a high specific strength and specific stiffness.The application of magnesium alloy can effectively reduce the structural quality of equipment.Besides,magnesium alloy has the characteristics of good damping,good electromagnetic shielding effect and easy recovery,which makes magnesium alloy the best lightweight material in the aerospace industry.However,due to the close-packed hexagonal crystal structure and limited slip system,the plasticity and mechanical properties of magnesium alloys are relatively poor at room temperature,which greatly limits the application of magnesium alloys.Plastic forming is an effective way to improve the strength and plasticity of magnesium alloy at the same time.However,the plastic deformation condition of magnesium alloy is harsh,and the single deformation of traditional deformation is too large,which is easy to cause material cracking.Therefore,to solve the problem of contradiction between formability and strength and ductility in the forming process of magnesium alloy,repetitive upsetting and extrusion are proposed.By accumulating small strain and repeated plastic deformation,the larger cumulative strain is obtained.Taking ZK60 magnesium alloy as the research object,the microstructure evolution law,refinement mechanism,texture evolution regularity,and second phase distribution characteristics of ZK60 Magnesium Alloy during repetitive upsetting extrusion under isothermal and cooling conditions were studied by OM,SEM and EBSD.The influence regularity of microstructure on properties was established by mechanical properties test,and the optimal heat treatment process has explored The strengthening and toughening mechanism of ZK60 alloy in repetitive upsetting extrusion was clarified,which provided a theoretical basis and technical support for the industrial application of repetitive upsetting extrusion process in magnesium alloy.The main conclusions are as follows:1.In terms of microstructure,texture evolution and mechanical properties response,the mechanical properties of the alloy after different passes of repetitive upsetting extrusion increase at first,reach the peak value and then decrease.In an isothermal process,the peak value of strength appears after three passes of deformation,and the plasticity and edge core uniformity are the best at this time(edge:tensile strength 297.5mpa,yield strength 176.4mpa,fracture strength 176.4mpa)In the cooling process,the peak value of strength appears after two passes of deformation(edge:tensile strength 294.8mpa,yield strength 169.9mpa,elongation after fracture 17.1%;Core:tensile strength 287.2mpa,yield strength 154.7mpa,elongation after fracture 18.4%).The results show that the isothermal and cooling repetitive upsetting extrusion can effectively refine the microstructure,and the microstructure has an obvious necklace-like bimodal distribution,and the grain refinement trend gradually decreases with the increase of passes.The initial refinement mechanism of the isothermal repetitive upsetting extrusion process is the DDRX mechanism dominated by grain boundary arched nucleus.With the continuous deformation,the CDRX mechanism of low-angle grain boundary transformed into high-angle grain boundary by continuously absorbing dislocations is observed;while the repetitive repetitive upsetting extrusion is dominated by twin induced dynamic recrystallization mechanism and supplemented by CDRX mechanism due to the decrease of subsequent deformation temperature.The two coexist to complete the refinement of the organization.The different refinement mechanism in the isothermal and cooling process leads to the difference of microstructure,which leads to the difference of mechanical properties after deformation.The results show that the optimum heat treatment parameters after deformation are 170?x 20 h,and no obvious grain growth occurs during aging.The change of the alloy properties is more determined by the precipitate phase.After aging,the strength of the samples increased more,which can be attributed to the fact that the decrease of deformation temperature leads to the retention of more substructures in the microstructure,promotes the nucleation of precipitates,and the aging strengthening effect is more significant.In the texture type,it is found that the"rare earth-like texture"is arc-shaped between<101~—1>to<101~—2>and<21~—1~—1>to<21~—1~—4>.Due to the introduction of friction resistance,the edge structure has a certain lag in the formation speed compared with the core,and the strength of deformation texture is weakened due to the appearance of recrystallized grains,which effectively reduces the overall texture strength The anisotropy of mechanical properties of the alloy is improved significantly.2.In the aspect of strengthening and toughening mechanism,although the grain structure is refined by repetitive upsetting extrusion,the yield strength is not significantly improved,mainly because the appearance of"rare earth-like texture"improves the overall Schmid factor,and the growth trend gradually slows down with the increase of passes.The increase of basal slip Schmid factor indicates that when tensile at room temperature along the ED,most of the tensile strength increases The difficulty of grain basal slip decreases with the increase of passes,which also leads to the decrease of axial yield strength and the improvement of plasticity.In the repetitive upsetting extrusion,texture softening is inevitable,but the effect of texture softening can be weakened to a certain extent by other strengthening methods such as fine-grain strengthening,to improve the strength and plasticity,and complete the strengthening and toughening of the alloy.Besides,the texture strength gradually weakens with passes,and the anisotropy of the alloy is also improved.However,the maximum fraction of the second phase after each pass deformation is only about 2%,which makes the strengthening effect not obvious.The effect of the second phase is more reflected in the pinning effect on the fine recrystallized grains.The evolution of mechanical properties of the alloy after repetitive upsetting extrusion is the result of multiple factors such as fine-grain strengthening,dislocation strengthening,second phase strengthening,and texture softening.There is a competitive relationship between each strengthening or softening mechanism,and the different weights will lead to the increase or decrease of mechanical properties. |
PDF Full Text Request