Evolution Of Cavity Closure In AZ80 Magnesium Alloy During Upsetting

As the lightest metal structural material in engineering application,magnesium alloy has a wide application prospect in aerospace,transportation,3C products and other fields,but there will inevitably be some empty defects in magnesium alloy.These void defects can be eliminated by plastic deformation process,and then improve the life of parts made of magnesium alloy.Therefore,it is of great significance to eliminate the internal void defects of as-cast magnesium alloy in the process of plastic forming.In this paper,the evolution process of internal void defects in AZ80 magnesium alloy cylindrical specimens during hot upsetting is studied by combining physical experiments with numerical simulation,and a general understanding of the law of cavity closure evolution of AZ80 magnesium alloy is obtained.It has a certain reference value and guiding significance for the reasonable setting of upsetting process parameters.The numerical simulation is based on the typical volume element finite element model of RVE.First of all,The hot upsetting process of single or multiple voids at the center of AZ80 magnesium alloy cylindrical specimen under the parameters of compression velocity,deformation temperature,reduction ratio and height-diameter ratio was simulated.The effects of them on the evolution of cavity closure are studied,and the process of cavity closure is also observed.Then,the effects of different sizes and shapes of voids and different friction coefficients between dies and blanks on the closure of voids in the center of AZ80 magnesium alloy during hot upsetting deformation were discussed.In addition,the cavity closure caused by the interaction between voids and multiple voids in AZ80 magnesium alloy was studied by numerical simulation.the equivalent stress,the equivalent strain rate and the distribution characteristics around the cavity are analyzed by DEFORM11.2-3D.Finally,the physical testsare carried out by setting up artificial voids in the cylindrical specimen and hot upsetting deformation on the INSTRON universal tensile testing machine,in order to verify the reliability of the numerical simulation.The following results are obtained by numerical simulation and physical experiments.From the initial stage of upsetting deformation to the realization of final closure,the internal void defects of AZ80 magnesium alloy will go through the following process:spherical ellipsoid crack closure;equivalent stress in the inner cavity center of AZ80 magnesium alloy.The three equivalent transformation rates are symmetrical and uniform distribution.And from the center of the cavity to the outside direction of the cylindrical specimen gradually decreased,there is a certain gradient;The ratio of height to diameter and reduction ratio are the main factors affecting the closure of void defects in AZ80 magnesium alloy.When the ratio of height to diameter is 1 and the reduction rate is close to 40%,the void defects are the easiest to close.The compression velocity,friction coefficient,deformation temperature,initial shape and size of the cavity have little effect on the closure of the cavity.At the same time,the initial position of the cavity in AZ80 magnesium alloy also has a great influence on the closure of the cavity.When the cavity defects are distributed in the center of the cylindrical axis,it is easiest to close.When the cavity defects are distributed in the symmetrical cross section of the cylinder,the closer the cavity position is to the side of the cylindrical specimen,the more difficult it is to close the cavity.Compared with the numerical simulation,the error of the physical experimental results is less than 10%,and the two results are basically consistent.