纯镁室温包套等通道转角挤压工艺设计及变形机制

发布时间：2018-11-26 09:15

【摘要】：镁合金作为目前工程应用中密度最轻的结构材料，在汽车、电子、航空、航天等领域有重要的应用价值。然而，镁及镁合金作为密排六方结构的晶体，在室温下只有有限的滑移系能够开动，所以镁及镁合金的室温塑性和延展性极差，使得镁及镁合金在实际应用中受到了很大的限制。细化晶粒是一种有效提高材料综合性能的重要手段，它不仅能提高材料的塑性和延展性，同时还能进一步提高材料的强度，使得材料的综合性能得到一定的提高。等通道转角挤压变形是细化晶粒的一个有效手段。为避免材料在挤压过程中开裂，镁及绝大部分镁合金的等通道转角挤压均需在200℃以上进行，使得在变形过程中，晶粒细化的同时又伴随一定程度的长大，减弱了晶粒细化的效果。为了充分发挥等通道转角挤压细化晶粒的作用，避免其高温长大，本实验室提出并开发了包套等径角挤压技术，实现了大块纯镁及镁合金在室温下的多道次等通道转角挤压变形，使进一步细化纯镁及镁合金组织成为可能。在此基础上，本文进一步优化了包套结构，顺利实现了纯镁室温下更多道次（8道次）的等通道转角挤压，同时研究了纯镁在室温包套等径角挤压过程中的变形机制。该设计将经过热挤压并退火处理后，，尺寸为867mm的纯镁进行铁套包覆，由此增大了纯镁在变形过程中所受三向压应力状态，从而对纯镁起到了一定的保护作用，进而对包套纯镁以Bc路径、4mm/min的速度对其进行了室温八个道次的等通道转角挤压变形，从而使得纯镁组织晶粒尺寸得到了明显细化。利用电子扫描显微镜（SEM）结合背散射取向成像技术（EBSD）和显微硬度计对热挤压前后及退火处理后的纯镁和等通道转角挤压变形前后纯镁的组织、织构、性能及变形机制做了分析，同时，在250℃下对包套纯镁做了一个道次的等通道转角挤压变形，并对其变形后的组织、性能和变形机制进行了分析，与室温下包套纯镁一道次等通道转角挤压后的组织、性能和变形机制做了对比。纯镁在300℃下，经过热挤压后，组织晶粒得到了明显的细化，并形成了与挤压方向约10°～15°的基面织构，同时还有一些漫散的非基面织构所形成，挤压后的纯镁硬度有了明显的增大。随着对挤压态纯镁的退火处理，织构没有发生明显的变化，硬度值有所降低。对课题组之前包套纯镁等通道转角挤压变形工艺进行改进后，成功实现了室温纯镁八个道次的等通道转角挤压变形，获得了表面无裂纹的纯镁试样。通过在室温下对纯镁的等通道转角挤压大塑性变形后，纯镁组织晶粒得到了明显的细化，随着挤压道次的增加，纯镁组织变形量也在增加，从而更加的细化了纯镁组织的晶粒，当经过八个道次的室温等通道转角变形后，纯镁组织晶粒已经得到了相当显著的细化，并且晶粒大小均匀，晶粒尺寸大约为几百纳米。随着挤压道次的增加，纯镁的硬度值也不断的增大。在250℃下对包套纯镁进行一道次的等通道转角挤压变形后，与室温下对包套纯镁进行一道次等通道转角挤压变形后纯镁对比分析后发现：纯镁的组织晶粒要比在室温下一道次等通道转角挤压后的纯镁组织晶粒大。然而，在两种情况下，纯镁变形过程中的变形机制相同，都是主要以基面滑移为主，非基面滑移和孪生也同时发生。同时，在两种条件下，所得到的纯镁硬度也几乎相同。
[Abstract]:As the most light structural material in the current project, the magnesium alloy has important application value in the fields of automobile, electronics, aviation and aerospace. However, the magnesium and the magnesium alloy are used as the crystals of the hexagonal structure, and only a limited slip system can be started at room temperature, so that the room temperature plasticity and the ductility of the magnesium and the magnesium alloy are very poor, so that the magnesium and the magnesium alloy are greatly limited in the practical application. The refined crystal grain is an important means to effectively improve the comprehensive performance of the material, which not only can improve the plasticity and the ductility of the material, but also can further improve the strength of the material, so that the comprehensive property of the material can be improved. Equal channel angular pressing and deformation are an effective means to refine the crystal grains. In order to avoid the cracking of the material during the extrusion process, the equal channel corner extrusion of the magnesium and most of the magnesium alloy need to be carried out above 200 DEG C, so that during the deformation process, the grain refinement is accompanied by a certain degree of growth, and the effect of grain refining is reduced. In order to give full play to the effect of the equal channel angle extrusion to refine the crystal grain, and to avoid the high-temperature growth of the crystal grains, the laboratory puts forward and develops the diameter-angle extrusion technology such as the bag sleeve, and realizes the multi-channel angular pressing of the bulk pure magnesium and the magnesium alloy at room temperature the deformation is so as to further refine the pure magnesium and the magnesium alloy tissue to be On the basis of this, this paper further optimizes the structure of the package, and realizes the equal-channel angular extrusion of more times (8 times) at room temperature of pure magnesium, and also studies the deformation of pure magnesium in the process of extrusion of the room temperature package. The mechanism is that after hot extrusion and annealing treatment, the pure magnesium with the size of 867mm is coated with the iron sleeve, so that the three-way pressure stress state of the pure magnesium in the deformation process is increased, so that the pure magnesium plays a certain protective role, At the speed of the Bc path and the speed of 4mm/ min, the equal-channel angular pressing and deformation of eight channels at room temperature were carried out, so that the grain size of the pure magnesium was obtained. The microstructure, structure, properties and deformation mechanism of pure magnesium before and after hot extrusion and before and after hot extrusion and after annealing treatment were made by means of electron scanning microscope (SEM) and back-scatter-oriented imaging (EBSD) and microhardness tester. At the same time, at the same time, at the same time, the pure magnesium of the package is subjected to a channel angular extrusion deformation at 250 DEG C, and the microstructure, the performance and the deformation mechanism after the deformation are analyzed, and the tissue, the performance and the deformation machine after the passage angle extrusion with the pure magnesium at room temperature and the like are carried out compared with that of pure magnesium at the temperature of 300 DEG C, after hot extrusion, the grain of the tissue is obviously refined, and the base surface is formed in the direction of about 10-15 DEG with the extrusion direction, With the annealing treatment of the pure magnesium in the extruded state, there was no obvious change in the texture. and the hardness value is reduced, and after the improvement of the extrusion deformation process of the passage angle of the pure magnesium and the like before the research group, the equal-channel corner extrusion deformation of the eight channels of the room temperature pure magnesium is successfully realized, A pure magnesium sample with no crack on the surface is obtained. After the plastic deformation of the equal-channel corner of pure magnesium at room temperature, the grain of the pure magnesium alloy is obviously refined. With the increase of the extrusion channel time, the deformation amount of the pure magnesium alloy is also increased, the crystal grains of the pure magnesium organization are refined, and when the channel angles such as the room temperature and the like of the eight channels are deformed, the grain of the pure magnesium organization has been greatly refined, and the grain size is uniform, the grain size is about a few hundred nanometers. The hardness value of the magnesium is also continuously increased. When the pure magnesium of the bag is subjected to the equal-pass angular pressing and deformation at the temperature of 250 DEG C, the pure magnesium of the bag is subjected to a secondary equal-passage corner at the same temperature with the room temperature After the comparative analysis of pure magnesium after extrusion, it was found that the microstructure of pure magnesium should be more than that at room temperature. However, in both cases, the deformation mechanism in the process of pure magnesium deformation is mainly based on the base surface. The non-base slip and twinning also occur at the same time, at the same time, under both conditions
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG379

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