Investigation On Process,Microstructure And Properties Of Magnesium Alloy Sheet Prepared By On-line Heating Rolling

As the most promising "green engineering material" of the 21 st century,magnesium alloys have a broad application prospect in the field of orbit transportation,aerospace,national defense,3C,etc.Compared with the casting parts,magnesium alloy sheets have limited industrial application.The extrusion technology of magnesium alloy sheets both at home and abroad is relatively mature and the cost is not high.However,the magnesium alloy sheets extruded under the current extrusion equipment have a width limited to 600 mm and the thickness larger than 2 mm,such size restricts its application.The rolling process for magnesium alloy sheets successfully breaks such width and thickness limits.However,during the conventional rolling process,the rolling temperature must be above 573 K,the sheets must go through several rolling passes with small reduction at each pass,and the annealing before each pass is required.These rolling processes and annealing lead to high cost of the rolled magnesium alloy sheets,which largely impedes their industrial application and development.In this study,the 4 mm thick magnesium alloy sheet have been continuously rolled using on-line heating rollers with four reversible rolls.The optical microscopy(OM),electron back scattering diffraction technology test(EBSD),scanning electron microscope(SEM),X-ray diffraction(XRD)analysis and other techniques were carried out to characterize the AZ31 B magnesium alloy thin rolled sheets.The intrinsic relationship between rolling temperatures,rolling passes,reduction and microstructure,macrotexture,mechanical properties,and forming properties were systematically studied.In addition,the on-line heating rolling process is compared with the traditional rolling technology with the aim to explore a new processing technology of magnesium alloy sheets.The mechanical properties and plastic deformation properties of magnesium alloy sheets were tested by the tensile test at room temperature and Erichsen test,respectively.The main results are as follow:(1)Prepared by on-line heating technology under constant rolling temperature,the as-rolled magnesium alloy sheets exhibit good comprehensive mechanical properties.For the sheet rolled at 443 K,the yield strength is 280-320 MPa,the yield ratio is 0.77-0.84,and the fracture elongation is 10.7-15.4%.When the rolling temperature rises to more than 563 K,no visible edge cracks were observed on the magnesium alloy sheets,the shape quality of the sheets is very good,and the yield strength is 220-240 MPa,the ultimate tensile strength is 330-350 MPa,and the fracture elongation is 16-20%.(2)It is found that the on-line heating continuous rolling technology can effectively induce dynamic recrystallization to occur at relatively low temperature.The multi-passes consecutive on-line heating technology can accelerate the dynamic recrystallization process,refine the microstructure of AZ31 B magnesium alloy sheets,and improve comprehensive mechanical properties and anisotropy.(3)The finishing rolling temperature has a great influence on the mechanical properties of magnesium alloy sheet under the on-line heating continuous rolling technology.On the varying temperature rolling process,the performance of “temperature ascending” sheet is similar to the sheet rolled at 623 K under constant temperature,and the performance of “temperature descending” sheet is similar to the sheet rolled at 503 K under constant temperature.(4)Rolled by the on-line heating continous rolling technology,the sheet rolled at low temperature and small pass reduction has severe anisotropy.The sheet prepared at 353 K contains a lot of {10 1(?)1}/56o compression twins,A large number of fine {101(?)2}/86o tensile twins appear in the sheet rolled at 433 K.(5)For the magnesium alloy sheets rolled under constant temperature(533K)and large pass reduction(25-30%)with the on-line rolling process,the grain size distribution of all the sheets matches the standard logarithmic normal distribution.The sheet from the third pass showed the best overall mechanical properties with the yield strength,ultimate tensile strength and elongation to fracture in the rolling direction reaching 232 MPa,347 MPa and 21%,respectively.