Effect Of Twinning On Shift Of Neutral Layer In AZ31B Magnesium Alloy Sheet During V-bending

In recent years, magnesium allys have been attracted by national defense 、aerospace、automobile、electronical industries and so on, which are regarded as ― green metal structure materials in 21 century.‖ However, due to its special closed-pack hexagonal structure, the independent slips are limited at room temperature. Twinning plays an important role on the deformation at this time. The activation of twinning results in the difference of neutral layer shifting of Mg alloys during bending with other common metal materials. Through the microstructure observing, this paper investigates the shift of neutral layer and deformation mechanisum of AZ31 magnesium alloys during bending; effects of deformation parameters(temperature, bending radius, strain rate) on springback and neutral layer shifting of Mg alloys; investigate effects of twinning-detwinning on the microstructure and mechanical properties of AZ31 Mg alloys; analyse effecs of detwinng on springback and shift of neutal layer during bending; Based on the results, the car seat bidet of magnesium alloys was processed at warm temperature with weak springback. Through the characterization technique of optical microscopy、electron back scattering diffraction(EBSD)、scanning electron microscope(SEM) and so on, the evolution of microstructure of magnesium sheet during bending is investigated system. The conclusions are as follows:(1) AZ31 B magnesium alloy sheet with thickness of 3mm is 90°V-bended at 150℃ with radius of 9.3mm. The coeficent of neutral layer(K-value) exceeds 0.5 and the bending neutral layer shifts to the outer tensile region. The bending microstructure between outer and inner region expresses diversity obviously. The equiaxed grains distrubut in outer region, slips dominate the deformation; while {101—2} tensile twins distribute in inner region and twinning dominate the deformation. The asymmetry of deformation mechanism results in the neutral layer shifting to outer region during bending. In middle region, the {101—2} twinning bands emerge and grow from inner side to outer along direction 45°.(2)AZ31B magnesium alloy sheets with thickness of 3mm 90°V-bended at 50, 100, 150, 200, 250, 300℃ with a radius of 9.3mm, the springback and coeficent of neutral layer decrease as temperature increasing. Due to higher temperature, dynamic recovery、recrystallization and non-basal slips active, the stress concerntration can be released easily. Therefore the twinning is restrained and the volime fraction of {101—2} twins decreases. The asymmetry of tension-compression between outer and inner region decreases so that coeficent of neutral layer(K-value) decreases.(3) AZ31 B magnesium alloy with thickness of 3mm is 90°V-bended at 100℃ with radius of 7.5, 8.1, 8.7, 9.3mm, the springback decreases and coeficent of neutral layer increases as the radius decreasing. The local bending strain increases as radius decreases which results in the stress concerntration and promotes twinning behaviors. The volume fraction of {101—2} tensile twins inreases as well and asymmetry tension-compression improved at the same time. Therefore coeficent of neutral layer(K-value) increases and the offset increases.(4) The tension and compression tests of AZ31 B magnesium alloy sheet with thickness of 3mm are conducted at 150℃ with strain rate of 10-2s-1, 10-3s-1 and 10-4s-1, the yield strength decreases, CYS/TYS increases and asymmetry of tension-compression decreases as the strain rate decreasing. During 90°V-bending, the coeficent of neutral layer decreases at the same time.(5) Pre-compression 1%, 3%, 5% along RD then inverse tension tests are conducted on AZ31 B magnesium alloys. Pre-stretch 1%, 3%, 5% along RD then inverse compression tests are conducted at room temperature as well. As the degree of compression increases, the volume fraction of {101—2} tensile twins increases. The detwinning behaviors carry out so that the yield strength decreases and elongation increases as the compression degree increasing. As pre-stretch degree increasing, the compression yield strength increases. {10 1—2} tensile twins are restrained and the volume fraction decreases as well.(6) In order to investigate the effect of detwinning on the deformation behaviors of magnesium alllys at different strain rate, inverse tension tests are conducted on pre-compression 3% AZ31 B Mg alloys at 150℃ with strain rate of 10-1s-1, 10-2s-1, 10-3s-1 and 10-4s-1, The yield strength decreases and ductility increases as the strain rate decreasing on as-received samples. However, the biggest elongation is obtained at 10-2s-1 on pre-compression samples. Besides, the elongation decreases as the strain rate decreases. Comparing with as-received samples during inverse tension deformation at 150 ℃, the dynamic recrystrallization is restrained and the volume fraction of recrystrallization grains decreases as well.(7) AZ31 B magnesium alloys sheet pre-compression 1%, 3%, 5% and pre-stretch 3%, 5% along extrusion direction at room temperature, then 90°V-bending is conducted at 150℃ on the pre-strained samples. The springback increases and coeficent of neutral layer decreases after pre-strain. V-bending on pre-compression samples, detwinning happens in outer region and twinning is restrained in inner region, so that the asymmetry of tension-compresson decreases. V-bending on pre-stretched samples, the {101—2} twinning is restrained in inner region while outer region is dominated by slips and asymmetry of tension-compression decreases. The decrease of tension-compression asymmetry results in the decreasing offset of bending neutral layer of Mg alloys.