Study On Plastic Forming Of VW84M Magnesium Alloy Conical Cylinder

Magnesium alloys with excellent mechanical properties and good corrosion resistance,which are currently important lightweight materials in aerospace,military,and other fields,can be obtained by adding rare-earth elements,such as Gd and Y,to magnesium.Traditional rare-earth magnesium alloys have a relatively low plasticity but high strength,which makes it difficult to produce vital conical cylinder parts with large shaft diameter ratios for the military industry.This study found that the addition of Zn can form a long-period ordered stacking structure phase in rare-earth magnesium alloys;furthermore,the plastic processing ability is improved,and the plastic toughness itself is significantly enhanced.Research on the plastic forming of such rare-earth magnesium alloys is relatively rare.To further expand the application of rare-earth magnesium alloys in the development of high-end equipment and enhance the status of magnesium alloys in the field of non-ferrous metals,this paper uses a Mg-8.3Gd-4.4Y-1.5Zn-0.8Mn(wt.%)alloy(VW84M hereinafter)as the object to perform relevant research.The goal is to develop a conical cylinder part with a large axial diameter ratio.The following work was conducted.Two heat treatment systems(500?/48 h and 500?/48 h+furnace cooling+460?/2 h)were used to obtain homogenized and annealed VW84M magnesium alloys.The alloys contained massive LPSO structural phases and a layered LPSO structure phase.Through isothermal thermal compression experiments,the flow stress curves of the two state alloys were obtained,and the constitutive relationship of the hyperbolic sine function model was established.The constitutive relationship was drawn on the basis of dynamic material model theory.A material thermal processing diagram showed that the ideal processing window was 460?,around 0.01 s-1.Observation of the changes in the microstructure indicated that the bulk LPSO played a role in promoting the dynamic recrystallization of the alloy,the layered LPSO phase could be deformed in coordination through kink,and the kink can be used as a nucleation point for recrystallization.The annealed VW84M alloy is an ideal material for subsequent processing.According to the constitutive equation,stress prediction was conducted on the annealed VW84M alloy using the software DEFORM,which verified the validity of the equation.A standard of alloy failure in the multidirectional forging process was given.Multidirectional forging was simulated with different passes and different pass deformations.The results showed that the deformation pass was six passes,and the scheme with a pass deformation of 45%was the best.A backward extrusion die was designed for conical cylinder parts with large shaft diameter ratios.In this process,the blank is first pressed,and backward extrusion is performed.The effect of the friction factor of the upper and lower molds and the extrusion speed on the back extrusion process was simulated and analyzed.Findings showed that the extrusion speed and the friction factor of the upper mold were positively correlated with the maximum principal stress during the back extrusion process.The lubrication of the upper die and a low extrusion speed must be ensured during the deformation process.Given the results of finite element analysis and the hot working diagram,the temperature of multidirectional forging and backward extrusion was set as 460?,the number of multidirectional forging passes as six,and the pass deformation as 50%.After the multidirectional forging was completed,the billet was heated and left to sit for half an hour before the pressing and backward extrusion.There was no cracking of the billet during the multidirectional forging and backward extrusion,which contradicted the simulation results.The prepared conical cylinder parts had excellent comprehensive mechanical properties.