【作者】 崔亚；

【导师】 张治民；

【作者基本信息】 中北大学 ， 机械设计及理论， 2013， 博士

【摘要】 镁合金变壁厚异型板类构件是弹箭装备的轻量化部件,其力学性能与尺寸精度直接关系到装备可靠性与打击精度。添加稀土元素的镁合金较常规镁合金具有更好的高温性能,但是这些耐热镁合金铸造产品成分偏析和夹杂严重,力学性能低,难以满足高速飞行弹箭装备的需求。塑性成形可大幅提高镁合金构件的力学性能,但镁合金为密排六方晶体结构,塑性较差。含稀土镁合金成形更易开裂,成形构件各向异性更加明显。国内研制的新型高强耐热Mg-13Gd-4Y-2Zn-0.6Zr合金,在航空航天、国防军工具有很好的应用前景,目前关于此合金的塑性变形特性研究较少。变壁厚异型板类构件通常采用稀土镁合金铸锭多向锻造开坯、通过多方向上大塑性变形实现细晶强化,这是改善成形构件各向异性与获得良好力学性能的重要手段；Mg-13Gd-4Y-2Zn-0.6Zr合金多向锻造过程中的塑性变形历史与组织响应规律,目前也没有研究涉及。变壁厚异型板类构件由于其苛刻的服役条件,力学性能与尺寸精度要求严格,必须采用控形控性一体化的塑性成形方法,实现筋条处金属充填、变形均匀性以及成形过程微观组织结构演变的协调控制,这也是当下研究的热点问题。针对Mg-13Gd-4Y-2Zn-0.6Zr合金,借助于热模拟压缩实验、光学金相显微分析、扫描电镜和能谱分析、硬度测试、室温／高温拉伸实验等研究手段,系统研究了不同变形参数下合金的流变行为,以及随着变形道次的变化合金内部显微组织的演化规律,为该类合金的塑性变形提供了实验依据。热模拟压缩试验表明：应变速率介于0.001～0.5s-1以及变形温度介于573-753K时,其真应力-真应变曲线都呈现动态再结晶特征。在合金热塑性变形过程中,动态硬化和动态软化的竞争贯穿始终；随着变形温度的升高,组织内部的动态回复和动态再结晶加剧,表现为流变应力的降低。基于Sellars方程,构建了Mg-13Gd-4Y-2Zn-0.6Zr合金不同应力状态的本构模型。基于动态材料模型建立了合金二维热加工图,发现Mg-13Gd-4Y-2Zn-0.6Zr合金具有两个峰值能量耗散区域：1)温度680～740K、应变速率0.001-0.01s-1；2)温度730-753K,应变速率0.01-0.1s-1；两区域能量耗散效率分别在30～40%和40～49%之间,这为成形工艺参数的选择提供了参考依据。研究了Mg-13Gd-4Y-2Zn-0.6Zr合金多道次变形特征。研究结果表明：随着变形道次的增加,变形过程中的动态软化程度减小,而道次间的静态软化程度持续增强。但由于变形过程中的加工硬化远远大于综合软化的效果,表现为真应力-真应变曲线的峰值流变应力快速上升。单道次变形相比,两道次相对软化了9.42%、三道次相对软化了33.07%、四道次相对软化了24.5%。可以看出,三道次变形相对软化程度最大,四道次后由于动态硬化程度加剧,软化程度降低。从塑性应变、晶粒尺寸、成形损伤的角度,研究了稀土镁合金变壁厚异型板类构件塑性成形的控性问题：从金属充填饱满时成形载荷的角度,研究了构件成形控形的问题。提出将洛伦兹曲线与基尼系数应用于评估构件变形均匀性；从应变、晶粒尺寸、损伤因子这三方面来考虑,提出了三个变形均匀性评价因子。以三个变形均匀性评价因子和成形载荷为多目标优化问题的目标函数,以两个分流孔的直径、挤压成形用坯料厚度、成形温度与成形速度为设计变量,通过正交试验设计、有限元模拟、灰色关联系数计算、基于层次分析法实现目标函数权重分配、灰色关联度计算,依据关联度大小获得了多目标优化的成形工艺参数组合。在优化的参数组合基础上,设计了各工序的控制成形模具,进行稀土镁合金变壁厚异型板类的多道次预变形及最终成形的工艺设计和实验验证,试制出了符合性能与尺寸要求的品质均匀的构件,通过了考核实验,为扩大稀土镁合金的应用提供了有力的技术支撑。更多还原

【Abstract】 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.更多还原