Study On The Preparation, Microstructure And Mechanical Properties Of WE71 Mg Alloy Sheet

Rare earth wrought magnesium alloys have great potential to be widely used and developed, because its more extensive superiority meets the aero-industry, weaponry and automotive lightweighting requirement, especially indoor temperature, higt temperature and corrosion-resistance properties. At present, the rare earth wrought magnesium alloys mainly adopt the plastic deformation way of forging and extrusion, and its product is very limited by specifications and shapes. The rolling as the most cost-effective method of plastic working in the field of plastic processing of metallic materials has been widely used, and the study of the magnesium alloy sheet rolling is becoming more and more attention. With the increasing demand of magnesium alloy sheets, the strength of sheet put forward higher requirements and resulting in the development of high strength of wide magnesium alloy sheet become inevitable. According to the relevant literature of rare earth magnesium alloys, the results show that the percentage of rare earth elements is more than 10%, the strength is high, the plastic is low, the hot deformation processing window is narrow, and this alloy can only be prepared by a specific processing method. Therefore, appropriate reduction of rare earth content, a kind of rare earth magnesium alloy with high strength and high plasticity was developed based on the research of EW75, WE93, WE91 and WE83 alloy. Both the strength and plasticity, which provides the possibility for the development of high strength wide sheet of rare earth magnesium alloy.In this study, an Mg-7Y-lNd-0.5Zr alloy was designed successfully and prepared. There has been a lot of mainly research work about the microstructure and mechanical properties of Mg-Y-Nd series alloys, sush as as-cast alloy, as-homogenized alloy and deformated alloys. The valuable comclusion of the present work can provide theoretical and practical for the high strength sheets preparation processes.As-cast microstructure of WE71 alloy consists of a-Mg matrix, skeletal eutectic phase, cube of rich Y phase and Zr particles. After homogenization heat treatment, the skeletal eutectic phase in the as-cast alloy have been completely dissolved, the microstructure of as-homogenized alloy consists of a-Mg matrix and cube of rich Y phase. The structure and composition of cube of rich Y phase is identical with that in the as-cast alloy. After heat treatment at 537 "C × 16h,the alloys were directly extruded. The microstructure of the as-extruded alloy consists of fine equiaxed grains with a mean grain size of approximately 18?m, which demonstrated that the dynamic recrystallization had appeared during the extrusion process.Microstructure evolution during hot compression of as-homogenized WE71 alloy has been investigated. New dynamic recrystallization grains formed near grain boundaries. At 20% strains, the original grain boundaries are serrated and bulging at grain boundary regions can be recognized, which indicate that discontinuous dynamic recrystallization is operative. At 60% strains, new grains result from the gradual increase in mis-orientation between subgrains during plastic deformation. Continuous absorption of dislocations in the low-angle boundaries results in continuous dynamic recrystallization. The deformation activation energy of as-homogenized WE71 alloy is 212.37KJ/mol, the constitutive equation can be represented as ?=3.337×1012[sinh(0.01108|?|)]4.45exp[-(212.37×103)/8.314T].After hot rolling, the sheet was almost fully recrystallized and significantly refined, and produced a large number of twins, which induced recrystallization phenomenon, refining the organization, improving the ductility and reducing the deformation resistance. The dynamic recrystallization and static recrystallization are combined to control the microstructure and properties of WE71 alloy. In the rolling process, the eutectic structures were broken, increasing distortion energy of grains, grain boundary areas, and dislocation density, so that the diffusion of solute atoms increased. The homogenization time was obviously shortened. After rolling and homogenization, the grain size of the alloy is obviously smaller than the grain size of the as-cast directly homogenization alloy. The results show that the process of rolling and homogenization has good effect on the elimination of eutectic structure and grain refinement.As-extruded alloy through the rolling deformation, the microstructure was more uniform and finer grain size. With increasing the rolling temperature, the maximum density of basal texture first increased and then decreased, and the maximum pole density is 7.175 at 450?. With the increase of pass rolling deformation, the area of dynamic recrystallization increased, the grain size was obviously refined, and the uniformity of the microstructure is improved. With the increase of the total rolling deformation, the area of dynamic recrystallization increased, and the grain size was obviously refined. On the premise of effective rolling behavior, through the investigation of the microstructure and properties, it is concluded that the best WE71 alloy rolling process. The temperature is 500?, the pass rolling deformation is 10%, and the total rolling deformation is 50%.The precipitation sequence and strengthening mechanism in WE71 alloy during aging have been investigated. The precipitates of WE71 alloy aged at 175? was studied. It was found that the precipitated phase nucleation and growth process is slow during aging at low temperature.The precipitates of WE71 alloy aged at 200?are ?'and ? phases.A b.c.o. structure with lattice parameters of a=0.64nm, b=2.22nm and c=0.52nm can be identified for ?'phase. The ?'phases are totally coherent with the matrix, and formed on (1010). Coherent relation and joining of ?'phase are responsible for the good thermal stability of alloy ageing at 200?.High temperature tensile test was carried out on T5 alloy. With the increase of tensile temperature, the yield strength reduced rapidly, while the elongation increased; as the extension of heat preservation times, the strength values decreased, while the elongation increased.The new theories and processes developed in this work can be used to conduct rolling of the high strength of rare earth magnesium alloy sheets. It can strongly improve rollabiltiy and production efficiency.