不同退火温度下预压缩镁板冲压成形性能研究

Research on stamping formability of pre-compressed magnesium sheets annealed at different temperatures

为探讨孪晶对镁合金静态再结晶的影响机制,首先对1 mm厚AZ31镁板沿轧向预压缩7%引入{1012}拉伸孪晶,然后对预孪晶镁板在200、250、300和350℃下进行3 h的退火处理,研究了镁板显微组织演变,探讨了温度对力学性能和冲压成形性能的影响规律。结果表明:200℃退火镁板未发生再结晶,保留了孪晶片层结构,同时去除了残余应力,降低了位错密度,导致基体软化。随着退火温度的升高,再结晶发生完全,晶粒尺寸先增大后减小,300℃的退火试样晶粒尺寸达到峰值。孪晶结构导致了晶格畸变,在基体中引入了较高的储存能,利于再结晶晶粒的形核及生长。随退火温度的升高,镁板的屈服强度(YS)和极限抗拉强度(UTS)先降低后升高,在300℃时最低;伸长率(EL)先升高后降低,在300℃最佳。随着退火温度升高到300℃,因再结晶晶粒的逐渐长大,晶粒取向变得有利,导致基体软化,从而YS和UTS降低,EL升高。而350℃退火的试样晶粒尺寸变得更加均匀细小,利于强度的改善。随着退火温度的升高,镁板的加工硬化指数n先增加后减小,屈强比ReL/Rm和塑性应变比r先减小后增加,300℃退火试样的n值最大、Re L/Rm和r值最小,使其达到了优异的杯突成形性能。

To investigate the effect of twins on static recrystallization of magnesium alloy,{1012}tensile twins were introduced by precompression 7%along the rolling direction for AZ31 magnesium sheets with the thickness of 1 mm.Then pre-twin magnesium plate was annealed at 200,250,300 and 350℃for 3 h to study the microstructure evolution of the magnesium sheet.The influence laws of temperature on mechanical properties and stamping formability were discussed.The results show that recrystallization for magnesium sheet annealed at 200℃does not occur,and the twin lamellar structure is retained.Moreover,the residual stress is removed and the dislocation density is decreased,resulting in softening of the matrix.With the increase of annealing temperature,recrystallization occurs completely,and the grain size increases first and then decreases.The grain size of annealed specimen reaches the peak at 300℃.Lattice distortion is resulted by twins,and the higher storage energy is generated in the matrix,which is conducive to the nucleation and growth of recrystallized grains.With the increase of annealing temperature,the yield strength(YS)and ultimate tensile strength(UTS)of magnesium sheets decrease first and then increase,and the lowest strength for magnesium sheets is annealed at 300℃.The elongation(EL)increases first and then decreases,and the EL for magnesium sheets annealed is the best at 300℃.With the increase of annealing temperature to 300℃,the grain orientation becomes favorable due to the gradual growth of recrystallized grains,which leads to the softening of the matrix,resulting in the decrease of YS and UTS and the increase of EL.The grain size for magnesium sheets annealed at 350℃becomes more uniform and fine,which is conducive to the improvement of strength.With the increase of annealing temperature,the work harden-ing index n of magnesium sheets increases first and then decrease,the yield ratio ReL/Rm and plastic strain ratio r decrease first and then increase.The annealed magnesium sheet at 300℃has the highest n value,the lowest ReL/Rm and r values,resulting in excellent cup formability.