Influence Of Rolling On Microstructure Evolution Of Direct Smelting AZ91Magnesium Alloy

Magnesium alloy is the lightest metal structure material, whose specific strength ishigher than that of aluminum alloy and steel and has ascendancy to realize the lightweight of products. But magnesium alloy were close-packed hexagonal structure, withlow symmetry, less independent slip, low room temperature plasticity and badmalleability. AZ91magnesium alloy was widely used for its excellent castingcharacteristic. The microstructure and kinetics of β-Mg₁₇Al₁₂phase transformationduring soluting process for warm rolling AZ91magnesium alloy and during agingprocess for warm rolling AZ91magnesium alloy were investigated. The results show thatcrystal grain of cast AZ91magnesium alloy deformed along rolling direction duringrolling process. During soluting process the original bulky organization was replaced byrecrystallization grain, compared with the cast, grain refinement was obvious. Combinedwith the experimental data and JMAK equation, fitting kinetics of β-Mg₁₇Al₁₂phasetransformation, the formula is x（t）=4.77[1-exp（-6.41t-2.24）]. The results of aging partshow that few twins were found in solid state AZ91magnesium alloy after warm rolling,which provides energy for β-Mg₁₇Al₁₂to form and growth; the β-Mg₁₇Al₁₂grains wereprior to form at α-Mg and twin grains boundary and especially easy to form at the placeof high energy; by the aging time extending, the quantity of β-Mg₁₇Al₁₂were increasing;combined with the JMAK equation, fitting equation of kinetics of β-Mg₁₇Al₁₂phasetransformation: x（t）=1-exp(-2.0×10^-2t^0.11) and x（t）=1-exp(-1.1×10^-2t^0.17); during agingprocess, the twins after warm rolling were disappeared, the original bulky organizationwas replaced by recrystallization grain, the size of grain was reduced to20-50μm, grainrefinement was obvious.