稀土镁合金大型构件循环镦挤预成形改性研究

发布时间：2018-03-07 17:23

本文选题：Mg-13Gd-4Y-2Zn-0.5Zr稀土镁合金　切入点：均匀性　出处：《中北大学》2017年硕士论文　论文类型：学位论文

【摘要】：作为大型设备的核心部件,大型构件工况特殊,受力复杂,质量要求严格,对力学性能提出了更高的要求。在各种成形方法中,塑性变形工艺能够有效提升工件力学性能,而循环镦挤作为一种大塑性变形工艺,具有显著改善工件组织均匀性和提升力学性能的优点。对于性能要求较高的大型构件,使用循环镦挤的方法进行预成形改性,使其在终成形前达到细化晶粒尺寸,提升力学性能的目的。本文以成形Mg-13Gd-4Y-2Zn-0.5Zr稀土镁合金大型构件为背景,对该合金多道次循环镦挤进行实验研究,重点研究了多道次循环镦挤工艺对应变量,力学性能,微观组织和均匀性的影响。研究了多道次循环镦挤对Mg-13Gd-4Y-2Zn-0.5Zr稀土镁合金室温力学性能的影响。结果表明,随着循环镦挤变形道次的增加,合金的抗拉强度和屈服强度不断增强,延伸率出现先上升,后下降,再上升的变化趋势,第二相的析出是延伸率下降的主要诱因,合金4道次变形后抗拉强度和延伸率达到了364MPa和10.32%,屈服强度和抗拉强度分别为铸态时的1.68倍和2.06倍。另一方面,随着循环镦挤变形道次的增加,合金的硬度值逐渐增大,1-3道次变形过程中,合金不同部位的硬度值曲线呈现出明显的发散状态,不均匀分布现象显著,4道次变形后硬度值曲线开始集束且基本与平均硬度值曲线相重合;力学性能均匀性获得显著提高。通过研究Mg-13Gd-4Y-2Zn-0.5Zr稀土镁合金循环镦挤变形后的微观组织,发现循环镦挤变形对合金具有强烈的细化作用,随着变形道次的增加,合金的细化效率逐渐降低,平均晶粒尺寸由50.6um细化到7.5um,晶粒尺寸数值跨度从1道次的27-38um,降低至4道次的7.2-9.8um,组织均匀性获得显著提高。同时,挤压温度的降低造成了第二相的大量析出,阻碍了位错运动的进行。在细晶强化和第二相强化的共同作用下,合金的力学性能得到显著提高。使用有限元模拟软件对多道次循环镦挤进行了模拟,揭示了变形过程中不同场变量的分布情况,结果显示,等效应力场的分布从缩颈区向顶部和底部逐级递减。在变形初期,工件应变由内而外存在明显的应变梯度,其中表面应变最大,为7.3,底部应变最小,为1.21,应变差值达到了6.09,应变分布不均匀,随着变形道次的增加,在累积应变的作用下,工件的大应变区域和应变均匀性区域在扩大;4道次变形后,工件不同部位的等效应变值范围达到了10.5-12.5,应变不均匀参数C值由2.481降低到0.151,应变均匀性获得显著提升。5-6道次变形后,工件应变均匀性没有进一步改善。通过本文的研究,掌握了循环镦挤工艺变形规律,确立了Mg-13Gd-4Y-2Zn-0.5Zr稀土镁合金循环镦挤的最佳变形道次,为该工艺的推广和应用奠定了基础,对丰富大型构件预成形理论与和实践提供了指导和参考意义。
[Abstract]:As the core components of large equipment, large components have special working conditions, complex forces and strict quality requirements, which put forward higher requirements for mechanical properties. Among all kinds of forming methods, plastic deformation technology can effectively improve the mechanical properties of workpieces. As a large plastic deformation process, cyclic upsetting extrusion has the advantages of improving the structure uniformity and improving the mechanical properties of the workpiece. In order to improve the mechanical properties of large components with high performance, the cyclic upsetting extrusion method is used to preform the large components. The purpose of refining grain size and improving mechanical properties is achieved before the final forming. In this paper, the multi-pass cyclic upsetting of Mg-13Gd-4Y-2Zn-0.5Zr magnesium alloy is studied in this paper. The effects of relative variables, mechanical properties, microstructure and homogeneity of multi-pass cyclic upsetting extrusion on the mechanical properties of Mg-13Gd-4Y-2Zn-0.5Zr rare earth magnesium alloys at room temperature were studied. With the increase of cyclic upsetting deformation pass, the tensile strength and yield strength of the alloy increase continuously, and the elongation increases first, then decreases, and then rises. The precipitation of the second phase is the main cause of the decrease of elongation. The tensile strength and elongation of the alloy reached 364MPa and 10.32MPa after 4 times of deformation, and the yield strength and tensile strength were 1.68 and 2.06 times of those of the as-cast alloy, respectively. On the other hand, with the increase of cyclic upsetting deformation pass, the tensile strength and elongation of the alloy reached 364MPa and 10.32MPa, respectively. When the hardness value of the alloy increases gradually, the hardness curves of different parts of the alloy show a distinct divergence state during the process of 1-3 pass deformation. After 4 times of deformation, the hardness curve began to gather and basically coincided with the average hardness curve, and the mechanical property uniformity was improved significantly. The microstructure of Mg-13Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after cyclic upsetting and extrusion was studied. It is found that cyclic upsetting deformation has a strong refining effect on the alloy, and with the increase of deformation pass, the refining efficiency of the alloy decreases gradually. The average grain size was refined from 50.6um to 7.5um. the numerical span of grain size decreased from 27-38um-1 to 7.2-9.8um. at the same time, the decrease of extrusion temperature resulted in a large amount of precipitation of the second phase. The mechanical properties of the alloy have been greatly improved under the combined action of fine grain strengthening and second phase strengthening. The finite element simulation software has been used to simulate the multi-pass cyclic upsetting extrusion. The distribution of different field variables in the process of deformation is revealed. The results show that the distribution of the equal effect force field decreases progressively from the necking region to the top and bottom. In the early stage of deformation, there is an obvious strain gradient from the inside to the outside of the workpiece strain. Among them, the surface strain is the largest (7.3), the bottom strain is the smallest (1.21), the strain difference is 6.09, the strain distribution is not uniform, with the increase of deformation pass, under the action of cumulative strain, When the strain region and the strain uniformity region of the workpiece are expanded for 4 times, the equivalent strain range of different parts of the workpiece reaches 10.5-12.5, the strain inhomogeneity parameter C is reduced from 2.481 to 0.151, and the strain uniformity is improved significantly. The strain uniformity of workpiece has not been further improved. Through the research in this paper, the deformation law of cyclic upsetting extrusion process is grasped, and the best deformation pass of Mg-13Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy is established, which lays a foundation for the popularization and application of this process. It provides guidance and reference for enriching the theory and practice of large member preforming.
【学位授予单位】：中北大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG306

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