【作者】 王野；

【导师】 李峰；

【作者基本信息】 哈尔滨理工大学 ， 材料加工工程， 2022， 博士

【摘要】 镁合金是目前工程实际应用中最轻的金属结构材料,它兼具多种优异的综合性能,可与钢、铝、塑料等材料实现优势互补,在航空航天、轨道交通、武器装备等轻量化急需领域有着广阔的发展潜力及应用前景。挤压过程中的塑性变形区内为三向压应力状态,能够提高金属的塑性变形能力,有助于变形量的不断累积,逐渐成为镁合金构件的主要加工手段之一。本文提出了细晶镁合金交替反挤压制备新方法,在此基础上挤压成形高塑性镁合金型材,揭示了加载过程中的变形流动规律,构建了载荷预测模型,分析了受力状态与晶体取向分布特征之间的关联性,系统研究了压下量和制备道次对交替反挤压制坯微观组织及性能的影响规律,阐明了镁合金型材的增塑机制,为剧烈塑性变形的准连续性加工提供新途径。基于平面应变假设,结合应力边界条件和流速场分布特征,构建了不同压下量条件下的滑移线场。利用虚功原理和最大塑性功消耗原理建立功耗平衡方程,采用上限法思想推导载荷预测模型,并根据亨盖应力方程对特征变形区的平均应力进行求解。成功地建立了有心扇形滑移线场,及与压下量、剪切应力有关的载荷预测分段函数,绘制出了载荷变化曲线,实现了应力分布的可视化表征。通过有限元分析了交替反挤压过程中金属流动规律,结合模拟载荷曲线变化特征进一步验证了预测模型的准确性。进行了镁合金交替反挤压制备实验,利用金相观察和EBSD测试研究了制备过程中的微观组织演变,明晰了变形区各点的应力状态与晶体取向关系,结果表明,材料的变形流动协调了交替加载时的局部变形,导致有限元模拟所得载荷曲线平稳值高出载荷预测值8.14%。制备道次可以调控制备组织晶粒尺寸的大小,8道次后平均晶粒尺寸在5.58μm左右,其中,最小晶粒尺寸能够达到1μm。材料内部产生了择优取向,基于滑移线场沿线特性求解出的应力分布状态有助于辨析坯料内部特征变形区的晶体取向分布。针对交替反挤压制坯后挤压成形的镁合金型材,结合XRD和极图结果,分析了织构演化规律。通过晶界数量、局部位向差角分析以及不同方向的Schmid因子探究了挤压成形镁合金型材的增塑机制。在实验过程中发现,仅需4道次交替反挤压制坯即可达到组织的最优细化,平均晶粒尺寸达到5μm左右。继续增大道次后,动态再结晶形核与高温下晶粒长大达到了平衡。随着晶粒尺寸减小,晶界数量增加,镁合金的变形协调能力升高,完全动态再结晶弱化了双峰织构,降低了位错密度,使得Schmid因子值增大,在室温条件下多种因素共同影响使得型材塑性大幅提高。更多还原

【Abstract】 Magnesium alloy is the lightest structural metallic material in current engineering applications,which has a variety of excellent comprehensive properties and can complement each other with steel,aluminum,plas tic and other materials.It has broad development potential and application prospects in the fields of aerospace,rail transit,weapon equipment and other urgent lightweight needs.In the process of extrusion,the plastic deformation zone is in the state of three-dimensional compressive stress,which can improve the plastic deformation ability of metal and contribute to the continuous accumulation of deformation.It has gradually become one of the main processing methods of magnesium alloy components.In this paper,a new method for the preparation of fine-grained magnesium alloy by alternating back extrusion was proposed,and high plasticity magnesium alloy profiles were extruded on this basis.The deformation flow law during the loading process was revealed,and the load prediction model was constructed.The correlation between the stress state and the distribution characteristics of crystal orientation was analyzed.The influence of reduction and loading passes on the microstructure and properties of the billet prepared by alternating back extrusion was systematically studied,and the plasticity enhancement mechanism of magnesium alloy profile was clarified,which provides a new way for quasi-continuous processing of severe plastic deformation.Based on the plane strain assumption,combined with the stress boundary conditions and the distribution characteristics of velocity field,the slip line fi elds under different reductions were constructed.The power balance equation was constructed by using the virtual work principle and the maximum plastic power dissipation principle.The load prediction model was developed by using the idea of upper bound approach,and the mean stress in the characteristic deformation zone was solved according to the stress equation of H.Hencky.The central sector slip line fields and the load prediction piecewise function related to reduction and shear stress were successfully established.The load variation curve was drawn,and the visual representation of stress distribution was realized.The metal flow law in the process of alternating back extrusion was analyzed by the finite element method,and the causes of metal lateral flow were clarified by combining with the stress distribution.The accuracy of the prediction model was further verified by simulating the variation characteristics of load curve.The preparation experiment of magnesium alloy by alternating back extrusion was carried out.The microstructure evolution in the preparation process was studied by metallographic observation and EBSD test,and the relationship between the stress state of each point in the deformation zone and the crystal orientation was clarified.The results show that the deformation flow of the material coordinated the local deformation under alternating loading,resulting in the steady value of the load curve obtained by finite element simulation was 8.14% higher than the predicted value.The loading passes could regulate the grain size of the prepared microstructure,and the mean grain size after 8 passes was about 5.58 μm,in which the minimum grain size could reach 1μm.The preferred orientation was generated inside the material.The stress distribution calculated based on the characteristics along the slip line is helpful to analyze the crystal orientation distribution of the characteristic deformation zone inside the billet.For the magnesium alloy profile extruded from billets prepared by alternating back extrusion,the texture evolution law was analyzed by combining with the results of XRD and pole figure.The plasticity enhancement mechanism of extruded magnesium alloy profile was explored by analyzing the number of grain boundaries,kernel average misorientation and Schmid factor in different directions.During the experiment,it was found that the optimal refinement effect of microstructure can be achieved only by 4 passes of alternating back extrusion,and the mean grain size reached about 5 μm.After increasing the preparation passes,the dynamic recrystallization nucleation and grain growth at high temperature reached equilibrium.With the decrease of grain size and the increase of grain boundary number,the deformation coordination ability of magnesium alloy increased.The complete dynamic recrystallization weakened the bimodal texture,reduced the dislocation density,and increased the value of Schmid factor.Under the joint influence of various factors at room temperature,the plasticity of the sheet was greatly improved.更多还原