| Magnesium and its alloys have a great potential as light-weight structural materials in many industrial fields,such as the aerospace,automotive and communications due to their excellent processing and performance characteristics.However,the Mg alloys always exhibit poor ductility and workability at room temperature for its hexagonal close packed(HCP)crystal structure.And coupled with the imperfect and immature plastic deformation theory and rolling technology,the application and development of Mg alloys have been subject to some restrictions.Therefore,it has great significance to study the theory of plastic deformation of magnesium alloy in depth and systematically,so as to explore the effective technologies to fabricate the high-performance Mg alloy sheets.Formability and mechanical properties of Mg alloys are linked with their microstructure characteristics,which were always affected by the rolling conditions processing method.Thus,we systematically investigated the microstructure and texture evolutions of AZ31 Mg alloy during hot rolling process with different conditions using optical microscopic(OM),X-ray diffraction(XRD)and electron backscatter diffraction(EBSD)analysis.And then a novel rolling process entitled "two-stage rolling" was proposed through selecting reasonable process parameters based on the previous results.It was demonstrated that a fine-grained microstructure could be obtained and corresponding the mechanical properties of AZ31 Mg alloy sheet could be improved by two-stage rolling.What's more,we explored an asymmetric reduction rolling(ARR)process to improve the performance of AZ31 Mg alloy sheet based on the results of finite element simulation and experimental study,and the effect of different ARR routes on the microstructure and texture characteristics of AZ31 rolled sheet were studied,too.The main conclusion can be reached as below:1.During the hot rolling of AZ31 Mg alloy sheets,the increase of rolling temperature will promote the dynamic recrystallization(DRX)and the microstructure uniformity of the rolled sheet,and the grain size is also increased,while the intensity of the basal texture is reduced.Increasing the rolling reduction will also promote the occurrence of DRX,while the size of the new recrystallized grains will be reduced and the basal texture is enhanced.A more uniform microstructure with finer grains can be achieved by multi-pass rolling compared with single pass rolling,despite the basal texture is slightly stronger.2.The deformation mechanism of the grains with different orientations of AZ31 Mg alloy sheet are distinct during hot rolling.The compression twins are always occurred in the large basal orientated grains,while the tensile twins are easily occurred in the grains which have about 90° with the basal orientation.Additionally,the slips of dislocation are usually activated in the grains which have about 45° with the basal orientation and promote the formation of DRX.Besides,the {10-12} tensile twins are easily occurred at the initial stage of deformation during rolling process,and this kind twins are stable and difficult to occur DRX once they are formed.In contrast,the {10-11} compression twins always emerged with the strain increased and are easily to act as the nucleation sites of DRX and significantly refine the grain sizes.3.When the AZ31 Mg alloy sheet rolled with little reduction,the twins-induced dynamic recrystallization(TDRX)is the dominant DRX mechanism in AZ31 Mg alloy sheets during rolling.And there are three main nucleation mechanisms:within the compression twins;the intersections of different twins and in the matrix near the twin boundaries.The TDRX grains reseverd the orientation of original grains which exhibit a typically basal texture.As the rolling reduction increased,the main DRX mechanism is still TDRX at 300?,however,it will be changed as rolled at higher temperatures.As the temperature increased to 400?,the main DRX mechanism will turn to be continuous dynamic recrystallization(CDRX).And at 500?,it can be observed two DRX mechanism,i.e.CDRX and discontinuous dynamic recrystallization(DDRX),both will weaken the intensity of the basal texture.Despite this,due to the stability of the basal orientated grains,all the rolled sheet show a strong basal texture characteristics.4.After two-stage rolling process,the largest rolling reduction of AZ31 Mg alooy sheet can be achieved to 89%,and the grain size can be refined obviously,where the final average grain size is stabilized around 3 ?m.Thus,the mechanical properties improved significantly,especially the yield strength that can be achieved to 220 MPa.It can be concluded that the sheet rolled by two-stage rolling exhibit a relatively high performance compared with other special rolling processes.5.As processed by asymmetric reduction rolling(ARR),a severe shear strain was introduced along the thickness direction of AZ31 Mg alloy sheet.Compared with the symmetrical rolling(SR),the microstructure of the ARR-ed sheet is more homogenous and with a weaker basal texture.Therefore,a significant improvement of the mechanical properties of sheets rolled by ARR are obtained.6.As processed by ARR with different routes,the sheet rolled by route D shows the most homogenous micro structure and the weakest intensity of basal texture as it can be introduced the uniform shear strain and the largest effective strain during rolling.So,the sheet D performs excellent mechanical properties,as its yield strength is about 185MPa,the ultimate tensile strength is about 285MPa,and a high failure elongation of 27%is also obtained. |
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