Evaluation Of Microstructrue And Mechanical Property Of Mg Alloys During Close-die Hot-pressing Of Extruded Preforms

The one-shot closed-die pressing for extruded preform is a new process to produce magnesium forging products with high material utilization, short process, high efficiency as well as high quality. The current research are focused on the law of Mg deformation, tests of microstructure and properties and the theory of process, and the extensive literature is consequently presented. However, it is rare to see reports about the experimental observations and theoretical research on the evolution of the microstructure and performance for magnesium extruded preforing profile in the process of closed-die pressing.The microstructure evolution for typical Mg extruded profiles in hot closed-die pressing and the effects on mechanical properties in this paper were experimentally investigated and theoretically analyzed. And the evolution of microstructure and performance on extrude Mg profile during pressing were eventually discovered, which laid a theoretical and experimental foundation on the technology of closed-die pressing process for extruded preform.Therefore, a preform extrusion profile with the 2D geometrical feature of the specimens was first produced in use of AZ81, Then, the profile was sectioned, and net-shapely pressed into the specimens with the required geometry. The Gleeble1500 thermo-simulation machine was adopted to conduct analytical investigation on the hot deformation behavior of the as-extruded AZ81 in preparation for process parameters of pressing forming. Instruments such as metaloscope, SEM,EBSD, tensile testing machine as well as hardness test equipments were used during the whole experiment to deeply analyze the evolution of microstructure and mechanical properties of the profile during pressing forming. The economy and feasibility of near-net-shape pressing for extruded preform were also discussed.The major contests and conclusions are as follows:(1) The influence of extrusion preforming process on mechanical performance together with on fracture mechanism for Mg alloys through SEM were studied, and preventive measures for avoiding material defects in extrusion preforming were also presented to guarantee the profiles with uniform and fine microstructure. Study shows that dynamic recrystallization (DRX) and grain refinement are occurred during hot pressing. With same extrusion ratio, DRX increases as temperatures elevates. The finest grain size and excellent mechanical properties can be obtained with temperature at 370℃and extrusion rate 6~20mm/s, while the size of recrystalized grain becomes larger as temperature continues to increase. Under the same temperature, the larger the extrusion ratio is, the more the DRX happens, and the more uniform the grains are. Meanwhile, the grain size is coherently decreased, and tensile strength, yield strength as well as elongation are improved. The extrusion preforming process not only increases the yield strength of Mg alloys but also improves the plasticity in comparison with as-cast alloys, in order that following press forming experiment can be done with the high quality profiles.(2) The hot deformation behavior of the as-extruded AZ81 were also investigated, and microstructure during hot compressive deformation is mainly characterized by DRX. The peak stress of flow curve is higher either at lower temperature or faster strain rate. While at the same rate, the peak strain obviously decreases as temperature increases. The relationship amongst flow stress, strain rate and temperature can be described as , where the average activation energy Q=182.17KJ/mol, the stress exponentα= 0.0043Mpa-1, the strain exponent n=4.4865, And the constitutive equation for high-temperature plastic flow behavior is in which the peak strain and deformation condition(Z) suit to(3) Analysis on the hot deformation mecahnism and processing Map of as-extruded AZ81 magnesium alloy, according to power dissipation map and instablility map developed molding forming process. The microstructure and mechanical properties of specimens after pressing forming were researched. The results show that after pressed at 400℃, the equiaxed grains are elongated along pressing direction or fragmented into numerous small unit particles under three-dimensional compressive stress and extrusion force. A strong texture structure is formed in the fracture area, and mechanical properties are increased a certain degree but not too much compared with that of as-extruded AZ81. The tensile strengthσ_b, the yield strengthσ_s, the elongationδof as-extruded AZ81 are 331MPa, 236.7MPa and 16.6% respectively, while that of after-pressed AZ81 are 350.2MPa, 244.8MPa, and 16.1% respectively. After T6 heat-treated alloy, AZ81 magnesium alloy to improve the tensile strenght is not obvious, while the yield strength has been significicantly improved, but the extension rate was significantly, its mechanical properties ofσ_b = 358.5Mpa,σ_s = 270.7MPa;δ= 9.8%.(4) Remarkable grain refinement occurs during pressing process, while the average grain size in as-extruded is 3.5μm. With small deformation ratio, the grain tends to grow, whereas the grain is refined to 1~2.5μm when deformation ratio increases to 50%~60% together with outer force is applied. The profiles of extruded preforming is dominantly on basal texture {0001}<10 1 0>, and the basal plane is parallel to extrusion direction(ED). As deformation continues, the inner shear stress gives rise to grain rotation along boundaries with certain angles and initial basal texture and grain orientation vary remarkably, though the texture intensity is weaker than that of as-extruded. The basal preferential orientation deviates from extrusion direction (ED) and aligns with the FD. With deformation ratio increased and non-basal slip activated, the texture orientation turns away from extrusion direction(ED) and deflects along pressing direction. And the grain misorientation gradually turns into the direction of the larger angle. Consequently, it is prone to create a texture perpendicular to pressing direction. The grain refinement and texture for extruded preform and pressing compound process make great contributions to the mechanical properties.(5) The economy and feasibility of the pressing for extruded preform were finally analyzed. Through this technique, the near-net shape products with remarkable refined and uniform grains and excellent comprehensive mechanical properties can be obtained. It is a plastic process that combines with high efficiency, low energy consumption, high quality as well as high yield ratio, which can be put into industry in the future.