Study On The Shape And Performance Controllability Of Special-typed Panel Component With Variable Wall Thickness Made By Magnesium Alloys Containing Rare-earth Metals During Forming Procesing

The special-typed panel component produced by magnesium-based alloys containing rare-earth (RE) metals with variable wall thickness is one of the important lightweight parts of projectiles and rockets. Its mechanical properties and dimensional precision have a direct influence on projectiles and rockets’reliability and target accuracy. RE-Magnesium-based alloys have better high-temperature resistance than the common magnesium alloys. However, the cast products by magnesium-based alloys containing rare-earth metals cannot satisfy the demands of high-speed flying projectiles and rockets, due to their low mechanical performances caused by composition segregation and inclusion defects. Although plastic forming could be an efficien way to improve the mechanical properties of magnesium alloy components, magnesium alloy has low plasticity because of its hexagonal close-packed structure. However, magnesium alloy added with RE metals is even more easily to crack during plastic forming, and the forming components have more obvious anisotropic character.Mg-13Gd-4Y-2Zn-0.6Zr alloy has good properties for its high strength and heat resistance. This makes it particularly attractive for aerospace, national defense and military and other fields. However, there are few studies on the alloy’s plastic forming. The panel component with variable wall thickness is usually cogged for cast ingot by multiple forging. The fine grain strengthening can be obtained by the severe plastic deformation in varying directions, which is the important method to improve the component’s anisotropy and acquire high mechanical performance. Moreover, at present, no studies involve microstructure response law about the plastic deformation history during the multiple forging for Mg-13Gd-4Y-2Zn-0.6Zr alloy. The mechanical performance and dimensional precision of the panel component with variable wall thickness are strictly required, owing to the alloy’s tough serving condition. The method of shape-and-performance controllability integration is applied to realize the coordinated control of metal filling in the rib, forming uniformity and the microstructure evolution, which are also the research focus in this field.Mg-13Gd-4Y-2Zn-0.6Zr alloy was analyzed in this paper by means of isothermal compression, optical microscopy, scanning electron microscopy, energy spectrum, hardness testing, tensile test at room temperature and high temperature etc. The deformation behavior of the alloy with different deformation parameters is systematically studied, as well as the microstructure evolution law in the alloy during the deformation passes, which provides the experimental basis for the plastic deformation of the alloy. The isothermal compression test of Mg-13Gd-4Y-2Zn-0.6Zr alloy shows that, when the strain rate is0.001～0.5s-1and deformation temperature is573-753K, true stress-true strain curve has the character of dynamic recrystallization. During the process of thermo-plastic deformation, the competition between the dynamic hardening and softening run all along. With the deformation temperature rising, true stress-true strain curve show the character of low flow stress because the dynamic recovery and dynamic recrystallization increase.Based on the equation of Sellars, the constitutive model of the alloy under the different stress states was established. And based on the dynamic material model, the alloy’s two-dimension processing map was created, which showed that the alloy has two peak energy dissipation areas:1) when the temperature range is680-740K and strain rate range is0.001-0.01s-1,2) when the temperature range is730-753K and strain rate range is0.01-0.1s-1. The energy dissipation efficiencies are about30-40%and40～49%, respectively. This provides the reference for choosing the forming parameters.The multiple passes forming features of Mg-13Gd-4Y-2Zn-0.6Zr was analyzed in this paper.The results show that dynamic softening decreases when the passes increase, while static softening strengthens continuously. However, the work hardening during the deformation process is far beyond the combined softening, as shown as the peak flow stress rises rapidly in true stress-true strain curve. Compared with the single pass deformation, relative softening degree is9.42%after two passes,33.07%after three passes and24.5%after four passes. It can be investigated that the relative softening degree is the maximum after three passes. The reason for the fall of softening degree after four passes is that the dynamic hardening increases at the same time.The performance controllability of the panel component produced by rare-earth metals added into magnesium-based alloy with variable wall thickness was studied from the prospectives of plastic strains, grain sizes and forming damage, while the shape controllability of the component was analyzed by the means of forming load when the metal filling reaches full. This paper applies the Lorentz curve and Geordie Coefficient into the deformation uniformity of the forming component. Three aspects including straining, grain size, and damage factor are considered as evaluation factors of the deformation uniformity for the forming component. The objective function of multi-objective optimization problem with three forming uniformity evaluation factors and forming load is established based on the diameters of two Diversion holes, blank thickness for extrusion, forming temperature and speed as variables. By means of orthogonal test, finite element modeling, grey incidence coefficient calculation and AHP (analytical hierarchy process), the weight distribution and grey correlation degrees of the objective function can be realized. And the forming parameters combination from multi-objective optimization can be obtained according to the numerical value of correlation degree.Finally, on the basis of the optimized parameters combination, the forming controlling dies were designed, and the multiple passes pre-deformation and final forming technology of the panel component produced by rare-earth metals added into magnesium alloy with variable wall thickness were fabricated, and then the technology was validated. The forming component with even properties meets the properties and dimensional requirements in the design, which is a strong technical support for the wider applications of magnesium alloys added with rare-earth metals.