The Deformation Property Of Casting AZ31 Mg Alloy And Precision Forming Of Inner Spur Gear

Since the magnesium alloys have high specific rigidity, high specific strength,excellent electro-magnetic shielding ability, damping and heat-dissipating properties, theyhave wide application prospects in the fields of aeronautic and astronautic instruments,automobile panels and 3C products. However, the ductility of magnesium alloys is ratherpoor at room temperature. It is difficult to realize a large-scale production of Mg partsthrough press forming and stamping process, which greatly restricts their applications instructural field. Owing to the significant improvement of ductility with temperature, it ispossible to manufacture Mg alloy parts through forging and superplastic forming athigher temperatures.The purpose of this paper is through the studding on the uniaxial tensile & pressedbehavior, the microstructure evolution and the deformation properties of casting AZ31Mg alloy under different deformation conditions, and based on the model ofmicrostructure evolution, formula of stress and strain, the deformation process and themicrostructure evolution during the inner spur gear precision forming of casting AZ31Mg alloy will be simulated. With the studding on the deformation of inner spur gear usingthe device designed by the author, the regularity of flow and microstructure evolutionwill be researched and the best condition for casting AZ31 Mg alloy deformation will bedetermined. So we can provide more reliable basis for the development and widely usingof Mg alloy precision forming. For this, the following work has been conducted:Using the casting AZ31 Mg alloy produced by some factory, uniaxial tensile testswere carried out at the temperature range 300-450 ℃ and the strain rate range 4.25×10-4s^-1-1.0×10^-2s^-1 to evaluate the plastic and superlasticity. The threshold stresscharacterizing the difficulty for grain boundary sliding and the deformation activationenergy at various temperatures were calculated. The topography and fracture surfaces ofsuperplastically deformed specimens were observed and analyzed by using opticalmicroscope and scanning electronic microscope. It is shown that the alloy exhibitedsuperplastic deformation behavior when the strain rate less than 1.0×10-3s-1 in a range ofthe deformation temperature from 300 to 450 ℃. It was found that the elongationpercentage of the alloy at 400℃ withε = 4.25×10-4 s-1 is almost 200% and the strain ratesensitivity coefficient m=0.41. The microstructures undeformed and deformed specimenswere observed by optical microscope (OM). The initial grain size of tensile specimens isabout 25μm. The grain size growth near the necking zone is not obvious. Along thedirection of deformation, it is found that grains become a little elongated, but the grainshape almostly maintains equiaxed. Dynamic continuous recrystallization is the mainsoftening mechanism and grain stability mechanism in the superplastic deformation ofthe alloy.Uniaxial press tests were carried out at the temperature range 250-450℃ and thestrain rate range 0.001 s-1-10 s-1.with standard specimen, and the curves of stress-strain atdifferent conditions were obtained. A new flow stress pattern of AZ31 Mg alloy wasputted forward considered with the soft effect of strain and harden effect of strain rate, itis demonstrated that the new pattern can illustrate the tendency of stress by comparedwith the testing result. It can be used for the simulation of deformation andmicrostructure evolution.Based on the above, the AZ31 alloy flow process and the microstructure evolutionduring the isotherm precision forming of inner spur gear were simulated. It provideduseful parameter for the mould design and help us to find out the microstructure andmechanical property of the deformed parts.Based on the two-stage forming technology,a mould with its parameters revised forisotherm precision forming was manufactured, and the test of AZ31 Mg alloy inner spurgear deformation was carried out. It is demonstrated that the best temperature forisotherm precision forming of casting AZ31 Mg alloy is 280～380℃. It can improve theprecision of the deformed gear by using the correct amount and correct coefficient oftemperature and elasticity putted forward and calculated by the author. Themicrostructure of the parts is not well-distributed, we can fining the grain and improvingthe mechanical property by controlling the temperature, the rate and the way ofdeformation.