【作者】 王宇；

【导师】 李峰；

【作者基本信息】 哈尔滨理工大学 ， 材料工程（专业学位）， 2021， 硕士

【摘要】 轻量化是航空航天、轨道交通、武器装备等领域实现节能减排的主要途径之一,镁合金具有质轻且优异的综合性能,是实现轻量化的理想首选材料之一。然而,镁合金具有密排六方(HCP)的晶体结构,在室温状态下能够启动的独立滑移系数目较少,致使其在成形性和延展性方面较差,这对镁合金在工程等方面的广泛应用起到很大程度的限制。挤压成型工艺是镁合金构件塑性成形的重要加工手段之一,在成形过程中变形部位的金属坯料在挤压筒内处于三向压应力状态,并在芯模口出发生剧烈的剪切变形,提高材料成形性能,能够获得具有细晶组织和优异力学性能的成形制品。但是上述工艺仍存在着很大局限性,在实际生产应用中存在诸多不便。变形过程中存在较大的死区,材料利用率低;微观组织分布在断面和挤出方向上都很不均匀;加工过程中载荷较高,模具寿命短。如何能够有效解决挤压成型工艺存在的诸多问题,实现镁合金组织形/性一体化精确调控已经成为镁合金研究的瓶颈问题。交替挤压工艺在此基础上应运而生,创新性的提出离散性的分体式凸模设计思路,代替常规挤压的凸模结构,分步加载模式改变了挤压筒内金属的流动顺序并产生了额外剪切变形作用。根据凸模加载周期的不同分为递进式和交互式,本文针对交互式交替挤压工艺展开系统性的研究。本文以铸态AZ31镁合金为研究对象,采用光学显微镜(OM)、电子背散射衍射(EBSD)及透射(TEM)等分析表征手段,研究了交互式交替挤压工艺制品的微观组织结构,揭示了不同成形温度对晶粒形貌的影响,研究发现:在573 K～673 K范围内,挤压过程中变形机制存在明显差异,局部粗大晶粒发生破碎形成的细小的晶粒带协调金属流动的变形方式,会形成不均匀的组织分布,但滑移能够促使组织更加均匀。值得注意的是,温度的升高促使动态回复效果增强,平均晶粒尺寸增加。交互式交替挤压变形过程中不同成形温度下主要动态再结晶机制有所差异,对晶粒形貌的影响存在诸多不同,但均发生连续动态再结晶行为(CDRX)、非连续动态再结晶行为(DDRX)、孪晶诱导再结晶行为(TDRX),对晶粒细化和圆整度提升有所改善。通过有限元模拟与工艺试验相结合的方式,以AZ31镁合金为研究对象,分析了交互式交替挤压过程中晶粒细化机制,发现离散性分体凸模的设计与分布加载模式会在坯料内部形成多道次固态搅拌,促使晶粒显著细化,此外,多道次加载致使晶粒c轴沿TD方向发生偏转,部分处于硬取向的粗晶难以转动从而发生破碎也是交互式交替挤压晶粒细化机制的主要组成部分,六道次加载变形后微观组织呈现细小致密且均朝向TD方向,动态再结晶行为成为主要细化机制。最后因交互式交替挤压工艺过程对晶粒取向产生持续性影响,织构分布相应产生变化,本文针对不同工艺参数对织构弱化和织构改性的影响展开研究,通过对滑移系开动的难易程度、晶粒偏转和动态再结晶机制分析织构分布规律,结果表明,成形温度和加载道次对(1-100)<11-20>柱面滑移系和(0001)<11-20>基面滑移系的开动影响较大;不同的动态再结晶机制均能在一定程度弱化基面织构,降低最大极密度,其中,孪晶的弱化效果最为显著。更多还原

【Abstract】 Lightweight is one of the main ways to achieve energy saving and emission reduction in aerospace,rail transit,weaponry and other fields.Magnesium alloy has light weight and excellent comprehensive properties.It is one of the ideal materials for lightweight.However,magnesium alloys have a hexagonal closepacked(HCP)crystal structure.At room temperature,there are few independent slip systems that can be activated by crystals,which show poor elongation,which limits the wide application of magnesium alloys.Extrusion process is one of the important machining methods for plastic forming of magnesium alloy components.During the forming process,the deformed part is in a state of three-dimensional compressive stress and is subjected to strong shear deformation,which is beneficial to eliminate a series of defects in the structure of the processed ingot,improve the forming performance of the material.The formed products with fine-grained and excellent performance can be obtained.However,the above process still has great limitations,and there are many inconveniences in actual production and application.There is a large dead zone during the deformation process,and the material utilization rate is low;the microstructure is unevenly distributed in the cross section and the extrusion direction;the load during the processing is high and the die life is short.How to effectively solve the many problems existing in the extrusion molding process and realize the integrated precise control of the structure and properties of magnesium alloys has become a bottleneck problem in the research of magnesium alloys.Alternate extrusion process came into being on this basis,innovatively proposed discrete split punch design ideas,instead of conventional extrusion punch structure,stepwise punch loading mode changes the metal in the extrusion cylinder The flow sequence and additional shear deformation.According to the different loading cycle of the punch,it can be divided into progressive and interactive.This paper conducts a systematic study on the interactive alternating extrusion process.This paper takes the as-cast AZ31 magnesium alloy as the research object,and uses optical microscope(OM),electron backscatter diffraction(EBSD)and transmission(TEM)analysis and characterization methods to study the microstructure of interactive alternating extrusion process products,revealing The influence of different forming temperatures on the morphology of the grains was investigated.The study found that: in the range of 573 K to 673 K,the deformation mechanism during the extrusion process is significantly different,and the small grain bands formed by the fragmentation of local coarse grains coordinate metal flow The way of deformation will result in uneven tissue distribution,but slippage can promote more uniform tissue.It is worth noting that the increase in temperature promotes the enhancement of the dynamic recovery effect,and the average grain size increases.During the interactive alternating extrusion process,the main dynamic recrystallization mechanism is different at different forming temperatures,and there are many different effects on the grain morphology,but both continuous dynamic recrystallization behavior(CDRX)and discontinuous dynamic recrystallization behavior occur(DDRX),twin induced recrystallization behavior(TDRX),improved grain refinement and roundness improvement.Through a combination of finite element simulation and process experiment,the evolution mechanism of the fine-grained structure of the magnesium alloy interactive alternating extrusion was studied,and it was found that the design and distributed loading mode of the discrete split punch would form a multi-pass solid state inside the blank Stirring promotes significant grain refinement.In addition,multi-pass loading causes the c-axis of the grains to deflect in the TD direction,and some of the coarse grains in hard orientation are difficult to rotate and break,which is also an interactive alternating extrusion grain refinement mechanism.The main component,the microstructure after six-pass loading and deformation is fine and dense and all face the TD direction,and the dynamic recrystallization behavior becomes the main refinement mechanism.Finally,due to the continuous effect of the interactive alternating extrusion process on the grain orientation,the texture distribution changes accordingly.This article focuses on the influence of different process parameters on the texture weakening and texture modification,and the slip system is activated.The degree of difficulty,grain deflection and dynamic recrystallization mechanism analysis of the texture distribution law,the results show that the forming temperature and loading pass are(1-100)<11-20> cylindrical slip system and(0001)<11-20>The activation of the basal slip system has a greater influence;different dynamic recrystallization mechanisms can weaken the basal texture to a certain extent and reduce the maximum pole density.Among them,the weakening effect of twins is the most significant.更多还原