High Temperature Quick Plastic Forming Of Coarse Grained Magnesium Alloys

In this thesis, annealing conditions and high temperature quick plastic deformation and deformation mechanism of AZ31 and Mg-xAl alloys are investigated.Experimental data show that grain size of twin-roll casted AZ31 grow up from 9.5μm to 27.8μm at the annealing condition of 475℃( 12h ) + 425℃( 6h ). Coarse grained AZ31 reached the maximum elongation of 106.7% at 450℃and 10^-3 s^-1, which is less than fine grained material due to less contribution of grain boundary sliding. The coarse grained AZ31 alloy exhibits an activation energy of Q = 140.07 KJ/mol at 450℃and 10^-3 s^-1, which is similar to the diffusion energy of Al in Mg, Q = 143±10 KJ/mol. Combined with the stress exponent of n≈3, solute drag creep plays an important role during deformation at high temperature and low stress for Coarse grained AZ31. Under the same strain rate, fine grained AZ31 has higher activation energy than coarse grained material because that the high grain boundary density improves dynamic recrystallization.Mg-xAl ( x = 2,3,4 wt.% ) alloy ingot were made in vacuum furnace. Then the Mg ingot were hot rolled with an two roll reversible mill to produce plate with a thickness of 3.0 mm. The effects of annealing time on grain size of the rolled plate were studied. The average grain size reachs its maximum at 400℃for 60 min, under which condition, the average grain siz of Mg-2Al, Mg-3Al, and Mg-4Al are 120.4μm, 55.4μm, and 78.75μm, respectively.The effects of solute content of Al on quick plastic deformation were investigated for the Mg-xAl alloys. The peak tensile stress increases with an increase of Al content under constant temperature and strain rate.Elongations of Mg-2Al, Mg-3Al, and Mg-4Al are 122.1%,143.7% and 139.8% were consistently achieved at 10^-3 s^-1 and 450℃. The activation energy decreases with the increase of Al content under contant strain rate. The activation at high strain rate of (ε|') = 10^-2 s^-1 for the Mg-xAl alloy is similar to that for diffusion of Al atoms in Mg, Q = 141.34±10 kJ/mol. Combined with the stress exponent of n≈3 at 450℃, it is concluded that solute drag dislocation creep is the dominate deformation mechanism at the strain rate of 10^-2 s^-1 for the Mg - xAl alloys.