Study On The Bendability And Deformation Mechanism Of AZ31 And AM30 Magnesium Alloy Tubes

The magnesium alloy tubes are easily to be fabricated, which have high precision of size, fine microstructure, good mechanical properties, and the replacement of tubular sections to stamped components could save weight of structure in most. The magnesium alloy tubes would be the potential industrial structuresin the future. But the application of tubes generally need further second forming, such as bending, expanding, and contraction. Magnesium, with its hexagonal close packed (hcp) crystal structure, has limited slip systems and limited formability at room temperature, and then the bending method can not copy from that for the steel and aluminum tubular sections. The aim of this paper is to investigate the bendability and deformation behavior of the popular wrought Mg alloy AZ31 and AM30 tubes, and the bending mechanisms of magnesium alloy tubes. The results would provide references and consultations to the further development and applications of the Mg alloy tubes.Two kinds of alloy AZ31 and AM30 tubes were extruded, and then the microstructure and mechanical properties of the tubes in room and high temperature were tested. The results indicate recrystallizaton following extrusion is near complete for the two kinds of alloy, and a majority of grain boundaries are high angle grain boundaries (HAGBs). A basal ring texture with the c-axes of grains perpendicular to the extrusion direction (ED) has formed. The grains of AM30 alloy tube became coarse with the raise of extrusion temperatures, and the tensile strength decreases but the elongation increases. The twinning was restricted during deformation for the fine-grain tube, and the crytal orientation is unfit for the operation of dislocation slips, so the tube has high tensile strength and low ductility. On the other hand, coarse-grain tube has low tensile strength but good ductility due to the harmony of dispersed twinning during deformation. For the tension and compression processes in high temperature, the yield strength and work hardening rate of AM30 alloy tube decrease and elongation increases with the raise of temperature and reduction of strain rate.A warmed rotary draw bender for Mg alloy tube was designed and manufactured. It can realize warmed temperature of 20-400°C, bend velocity of 0-5r/min and bend angle of 0-180°. The effects of temperature, velocity, tube microstructure etc. on the bendability have been studied using the wall thinning, cross-section ovality and spring back as evaluation criterion in the conditions of popular used two relative bend radius and 90 degree bend angle. The results indicate both the ovality and spring back decrease with the raise of temperature, but the wall thinning is smallest at 200°C. At certain temperature, the wall thinning and ovality increase, spring back decreases with the raise of bending velocity. The fine-grain tube could obtain smaller wall thinning and ovality during bending, and the grain size has little effect on the spring back. The lubrication between die and tube can make low wall thinning and ovality, but the spring back increased slightly. In the tested parameter range, the temperature of 150-200°C, the velocity of 8mm/s, fine microstructure and lubrication are most feasible for bending of AZ31 and AM30 alloy tubes. And the bendability of AM30 alloy tube is a little better than that of AZ31 alloy tube at same bending conditons.The finite element (FE) software MSC.Marc was used to simulate the bending process. The material in extrados and intrados of tube were defined with tension and compression curves respectively. The results of simulation were validated with actual experiments. The effects of bending angle degree, relative bending radius and tube size on the bendability were analyzed through FE simulation. The results suggest the maximum equivalent stress in tube surface increases shapely before 30 degree, then hold relative stable with the increase of bending angle degree. The maximum equivalent plastic strain increases all along with the raise of bending angle degree, but the increasing rate is reduced. At certain bending angle (90°), the distributions of equivalent stress and elastic strain are similar, namely, peak value appears both at neutral area, extrados and intrados. The peak value of equivalent plastic strain appears at the most far away of extrados and intrados, and the minimum value is at neutral area. The bendability of tube become worse rapidly and the equivalent plastic strain rises with the decrease of relative bending radius. The minimum bending radius is 1.5 times of tube outer diameter. With the raise of relative wall thickness (outer diameter/wall thickness), the bendability also become worse, and the maximum relative wall thickness is 30. The change of relative wall thickness has little effects on the plastic strain.The microstructure of tube wall (after bending at 150°C) at different strains were analyzed through electronic backscattered diffraction (EBSD) technology. The plastic deformation mechanisms of the two kinds of alloy during bending were summarized. The results suggest the material in extrados undergoes tension and the dislocation slip is the main deformation mechanism. And the material in intrados undergoes compression; {10-12} <10-11> tension twinning is an important deformation mechanism except for dislocation slips. The texture changes approximately 90 degree due to the extension twinning. The density of texture in AM30 alloy is smaller than that of AZ31 alloy, and the crystal orientation in AM30 is a little more dispersive, so the deformation process of AM30 alloy tube is more harmonious in bending.