| Mg alloy has the advantages of low density,high specific stiffness,and high specific strength.Its use as a structural material can achieve lightweight in the aerospace and automotive industries.Among them,Mg alloy sheets are an important material for manufacturing highperformance Mg alloy shell components.However,Mg alloys have few independent slip systems that are easy to activate at room temperature,and their plastic deformation ability at room temperature is poor,making them difficult to process and form.Therefore,their widespread application is limited.To solve the problems of low plasticity and poor formability of Mg alloy sheets,this article takes traditional commercial AZ31 Mg alloy sheets as the research object.Based on the idea of pre-set tensile twinning and grain refinement to regulate the microstructure,improve mechanical properties,and improve formability,three constrained deformation processes and their combinations,namely "width-limited rolling","corrugated bending straightening and thickening",and "local deep drawing and flattening training",are adopted,Design a multi-pass bending width-limited rolling process to prepare large-sized Mg alloy thin sheets with good isotropy and excellent mechanical properties.Aiming at the different requirements of forming performance in different regions of complex components,Mg alloy stamping blanks with diverse microstructures and corresponding properties in different regions were obtained through local deep drawing training,meeting the requirements of integrated forming of shape and properties.At the same time,using XRD,SEM,TEM,EBSD characterization,in-situ tensile,in-situ tensile EBSD,and DIC experiments,mechanical properties and formability tests,combined with finite element analysis,the deformation behavior,microstructure,and performance evolution of Mg alloy sheets under different constraint deformation process conditions were studied.The main research content and results are as follows:(1)In this paper,width-limiting rolling,that is,by designing the groove on the rolls with the same width as the sheet,to limit the transverse spreading deformation of the edge of the Mg alloy sheet during rolling,is adopted,so that the sheet is subjected to bi-directional(rolling and transverse direction)compressive stress when passing through the rolls,which plays an effective role in preventing the edge cracking of the Mg alloy rolling.Finite element analysis shows that the edge of the free rolled sheet is subjected to stronger tensile stresses along RD than its middle part,which tends to lead to edge cracking.In contrast,the stress field between the edge and the middle part of the width-limited rolled sheet is the same,with no additional tensile stresses and simultaneous deformation of the edge and the middle part,which effectively reduces the tendency of edge cracking.The width-limited rolling can force to activation of the slip and twin system which is difficult to activate under the conventional deformation conditions to coordinate the deformation and weaken the basal texture.At the same time,the opening of the slip system corresponding to the dislocations distributed inside the matrix and twins shears the twins and avoiding a high degree of localization of material strain and effectively inhibiting crack initiation.In addition,multiple passes of width-limited rolling can effectively homogenize the microstructure and avoid the local strain incompatibility caused by the non-uniformity of the microstructure from aggravating the non-uniformity of the deformation while reducing the stress concentration area,thus preventing the occurrence of crack sprouting and fracture.(2)Adopting corrugated bending and straightening technology,that is,by placing the corrugated bent Mg alloy sheet in a fixed-width die for restricted straightening,the Mg alloy sheet produces transverse compression deformation,transverse shortening,and thickness increase,to achieve the efficient pre-setting of tensile twins in the Mg alloy sheet and effectively improves the stamping formability of the Mg alloy sheet,the IE value of the Mg alloy sheet was increased from 1.95 to 4.2 mm.In situ tensile test analysis shows that tensile twinning exhibits diverse evolutionary behaviors in coordinating plastic deformation of sheets:broadening and splitting and their inverse processes,selection of twin variants influenced by stress fields,de-twinning and re-selection of twin variants due to the change of local stress field.Tensile twinning causes changes in the grain orientation of the matrix grains,which weakens the basal texture and coordinates part of the plastic deformation.In addition,tensile twinning stimulates the activation of second-order pyramidal slip in the matrix,which better coordinates the plastic deformation between adjacent grains.(3)Adopting bending width-limited rolling technology,that is,the combination of bending straightening and width-limited rolling,so that the sheet with pre-set tensile twins through the width-limited rolling,and its multi-pass bending width-limited rolling to achieve the microstructure property control of Mg alloy sheet.The results show that after a single pass of bending width-limited rolling,the Mg alloy sheet shows excellent isotropy,as well as high strength and excellent tensile plasticity at room temperature,the elongation of the sheet along RD and TD,is significantly increased to 27% and the tensile strength reached 280 MPa along TD;After the two passes of bending width-limited rolling,the grain are further refined and the dislocation accumulation increases,the combined effect of grain refinement strengthening and dislocation strengthening increases the tensile strength of the sheet to 300 MPa based on guaranteed elongation.Microstructure analysis shows that the multi-pass bending width-limited rolling makes the sheet grain refinement,twins in the refinement of the adjacent grain transfer more quickly to form a "twin chain","twin chain" extended to form a "strain transmission belt" to better coordinate the local plastic deformation,but when the grain is refined to 2 μm,deformation twinning no longer occurs during the deformation process,the grain can not continue to pre-set tensile twins.In-situ tensile analysis shows that the bending width-limited rolling forcibly activates a variety of slip systems,and a large number of second-order pyramidal <c+a> dislocations and prismatic <a> dislocations exist in the grains,making it easier to activate second-order pyramidal <c+a> slip and prismatic <a> slip in the subsequent unidirectional stretching process,and improving the coordination of plastic deformation between grains.(4)Adopting local deep drawing and flattening training,that is,for specific regions of the Mg alloy sheet,using deep-drawing and flattening training to efficiently pre-set the tensile twins in the local microstructure and significantly refine the local microstructure.It is shown that the strain hardening index in the local area of the Mg alloy sheet is significantly increased after local deep drawing training,which can effectively prevent the local excessive thinning and cracking during its forming process.Among them,the IE value of the sheet after 6 mm local deep drawing and flattening training can reach 6.7 mm after holding and annealing at 200 ℃for 2h.The reason why local deep drawing and flattening training significantly improves the forming properties is that specific areas of the sheet are subjected to lateral compression along the entire cross-sectional direction during deep drawing and flattening,resulting in multiple variants of tensile twins.Compared with the c-axis deflection of grains in a single direction produced by corrugated bending and straightening,the multi-twin variant deflects the original grains’ c-axis and tends to be equally distributed in all directions within the sheet,which significantly reduces anisotropy and improves the stamping formability of the Mg alloy sheet.In this paper," corrugated bending and straightening ","width-limited rolling",and "local deep drawing and flattening training" constraint deformation process and their combinations are used to achieve the overall and local microstructure and property control of Mg alloy sheet.The combination of bending width-limited rolling and local deep drawing training can obtain Mg alloy sheets with good mechanical properties and excellent forming performance in specific areas,which can meet the requirements of integrated forming of complex shell components for Mg alloy sheets.Taking the car seat tubs as the forming object,the finite element simulation shows that the Mg alloy sheet is well-formed by bending width-limited rolling combined with the local deep drawing and flattening training,and the formed seat tubs have no cracks. |
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