Microstructure And Mechanical Property Of AZ31 Mg Alloy Sheet During Multi-pass Rolling Process

Magnesium?Mg?alloys have potential of being used as lightweight structural materials to substitute steel and aluminium alloys,especially in aerospace and automotive industries.However,the wide applications of Mg alloys have been restricted by their poor ductility and formability at room temperature owing to its hexagonal crystal structure.Therefore,developing an efficient method that is not only capable of obtaining excellent mechanical properties but also applicable to large scale industrial production is of great significance to promote the development of magnesium alloy.In this paper,the widely applied AZ31 Mg alloy was used as the initial sheet.The twin symmetry rolling was used as the research method.The multi-pass hot rolling process,multi-pass warm rolling process and large reduction cold rolling process of AZ31 Mg alloys were researched.The effect of rolling temperature and thickness reduction on microstructure and mechanical properties during one pass rolling process was analyzed.The effect of microstructure?grain size,dislocation density and twinning?,texture?texture intensity and average Schmid factor?on mechanical properties was researched in details.High strength AZ31 Mg alloy sheet with yield strength of 357 MPa and tensile strength of 393 MPa was successfully obtained.The AZ31 Mg alloy thin sheet with one pass cold thickness reduction with 41%was achieved.A rolling route which was suitable for industrial and mass production was explored in our work.As-cast AZ31 Mg alloy sheets with thickness of 26 mm were subjected to multi-pass hot rolling process with various thickness reductions?15%,20%,25%and 30%?to obtain as-rolled sheets with thickness of 4 mm.The multi-pass hot rolling process with different thickness reductions and the evolution of microstructure and mechanical properties in each pass have been investigated in details.The grains were obviously refined and the mechanical properties were significantly improved after hot rolling.The average grain size was smallest when the thickness reduction was 30%.The as-rolled sheet showed highest elongation with 24.7%,yield strength of 126.1 MPa and tensile strength of 242 MPa.The effects of rolling temperature at 150-300 ? and thickness reductions between 10-60%on the evolution of microstructure and mechanical properties of AZ31 Mg alloy sheets were investigated.Texture type changed from deformed texture ? mixed texture?deformed texture + recrystallization texture??recrystallization texture.The texture intensity was reduced with the increasing of rolling temperature,as well as decreased firstly and increased afterward with the increasing of thickness reduction.The sheet with finer microstructure would be fabricated by large thickness reduction and low temperature.The grains were significantly refined and the microstructure was uniform when rolling temperature was 250 ?.The sheet showed excellent mechanical properties when thickness reduction ranged between 30-40%.A high strength AZ31 Mg alloy sheet with yield strength of 357 MPa and tensile strength of 393 MPa was successfully fabricated.A revised Hall-Petch formula was proposed with the consideration of bothgrain size and average Schmid factor as:?_s-t = 0.3/m_t(?₀ +kd^-1/2).The contributions of solution strengthening,grain boundary strengthening and dislocation strengthening to the strength of as-rolled sheets were quantified by theoretical calculation.The reason for mechanical anisotropy of AZ31 Mg alloy sheet was also analyzed.Besides texture,the directivity of dislocation distribution also had the effect on anisotropy.Dislocation showed more obvious entanglement perpendicular to the orientation than parallel to it in the subsequent deformation,and then affected the strength anisotropy.Based on experimental fitting,theoretical analysis agreed well with the result of the experimental data when the directivity factor ? = f???was defined as 1 along RD and ? = 1 + ln?h/H?along TDIn order to enhance the rolling efficiency of AZ31 Mg alloy sheet,improve the sheet quality,Based on the control of texture,large thickness reduction of cold rolling process was researched.Excellent formability of AZ31 Mg alloy sheet with thickness of 2 mm was obtained by developing reasonable multi-pass hot rolling process.Compared with general as-rolled AZ31 Mg alloy sheet,the sheet obtained in this work showed weak basal texture,while the value was only 1/3-1/2 of general as-rolled AZ31 Mg alloy sheets.The maximum one pass cold thickness reduction could reach 41%in cold rolling process.To further ascertain the experimental results,large thickness reduction multi-pass cold rolling experiment was also conducted.A thickness of 0.55 mm AZ31 Mg alloy sheet with yield strength of 150 MPa,tensile strength of 300 MPa and elongation of 20%was fabricated by three passes cold rolling,intermediate annealing and final annealing.The one pass thickness reduction at room temperature was increased nearly three times.The problem existing in Mg alloy cold rolling process was solved by this process and enhanced the clod rolling efficiency.