Study On Welding And Bending Of ZM6 Magnesium Alloy Sheet For Thin Cylinder Forming

With the rapid development of aerospace and other fields,large thin-walled cylinders are widely and urgently needed in missile or rocket shells.ZM6 magnesium alloy,as a heat-resistant magnesium alloy,can meet the lightweight design of the structure due to its high specific strength and specific modulus.However,most of the current studies only analyze a single process in the forming of large thin-walled cylinders,and few investigate the changes of microstructure in the whole process systematically,especially the distribution and strengthening mechanism of rare earth elements in the process.Therefore,this paper relies on the technology of manufacturing,bending and welding plate.First,the effect of rare earth elements on the microstructure and properties during solution extrusion aging was investigated.Then,the extruded plates were welded by friction stir welding,and the microstructure and properties of the weld and the precipitation mechanism of rare earth elements during welding were analyzed.Finally,the microstructure and properties of the extruded plate and the welded plate were investigated,and the influence of rare earth elements and weld on the bending was revealed.According to the above,the relationship between process parameters,microstructure and mechanical properties was established,which provides technical support for the manufacture of large thin-walled cylinders.After solution at 530?for 16h,the Mg₁₂Nd at the grain boundary disappeared and the solution was basically complete.The alloy with solid solution was hot extruded at 390?,the grain size of the alloy was refined from 200?m as cast to 10?m.The precipitates,Mg₁₂Nd in bulk and Zn and Zr in rod shape,distributed parallel along the extrusion direction.Rare earth texture was formed in the extruded alloy,and the size of grain was formed by different recrystallization mechanism.After aging at 200?for 6h,the tensile strength of the extruded plate reached 283.4MPa,and the elongation was 19.1%.The original second phase of grain,texture and extruded structure did not change during aging,but the generation of nanoscale precipitates was found by TEM and the aging strengthening mechanism of rare earth elements was revealed.FSW of extruded plate showed that the weld joint was bowl shape macroscopically.The grain and the second phase distribution were different in different regions.The nanometer precipitated phase in the weld nugget was analyzed by TEM,and it was the flake Mg₃Nd precipitated along the cylinder surface of Mg matrix{1(?)00}.Tensile mechanical properties showed that the optimal parameters were 1200rpm and 60mm/min welding speed.After aging at 200?for 6h,the joint with the optimal parameters reached the yield strength of 158.4MPa,the tensile strength of 281.0MPa and the elongation of 13.7%,which was equivalent to 99%of the strength of the base metal.FSW results in the formation of a strong texture in the stirring zone,and the C-axis of the grain was perpendicular to the side of the stirring needle.This texture will affect the deformation and hardness distribution in the tensile process.According to the actual conditions of aging after bending or aging before bending,the extruded plates which were not aged and aged at 200?for 6h were bent at room temperature.The results showed that the internal bending Angle of the extruded plate increases from 127°to 150°after aging,and the nanometer precipitate promotes the formation of tensile twinning,but inhibits its growth.According to the actual welding seam in the inside or outside of the cylinder,the optimal parameters of the welding plate were bent in two ways.On the macroscopic level,the plastic deformation was not uniform,which was due to the difference of deformation mechanism between the center and both sides of the stirring zone.On the basis of the above research,relying on the sheet preparation,bending and friction stir welding,ZM6 magnesium alloy large thin-walled cylinder was finally formed.