Research on stamping formability of pre-compressed magnesium sheets annealed at different temperatures
To investigate the effect of twins on static recrystallization of magnesium alloy,{ 10(1)2 } tensile twins were introduced by pre-compression 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 temper-ature on mechanical properties and stamping formability were discussed.The results show that recrystallization for magnesium sheet an-nealed 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 recrystal-lized 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)increa-ses 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.
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