【作者】 任政；

【导师】 张兴国；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 镁及常用镁合金是密排六方结构,塑性成形能力差,所以铸造镁合金得到了大量应用,变形镁合金的发展严重滞后,很大程度上制约着镁合金的发展和应用。变形镁合金比铸造镁合金具有更优越的性能,具有更高的强度和更好的塑性,可以制备更多样性的产品和满足更多工况条件下应用的结构件。因此,开发和制备具有高强韧的变形镁合金显得尤为必要。本文工作重点是依据金属强韧化原理,开展变形镁合金的强韧化研究,探索变形镁合金强韧化的新思路,为制备高强韧的变形镁合金提供良好的基础研究。本文选用AZ31、AZ61和ZK60三种典型的变形镁合金为基础,依据金属强韧化原理,展开了变形镁合金微合金化、电磁连铸、电磁-悬浮成型、热挤压变形以及等温时效处理的实验研究。利用金相显微镜（MEF-4A）、X射线衍射仪（XRD-6000）、扫描电镜（JSM-5600LV）、电子探针（EPMA-1600）、透射电镜（TG220S-Twin JEM-100CXⅡ）、布氏硬度仪（HB-3000-1）和MTS实验机（MTS NEW-810）对变形镁合金进行了组织和性能分析,探讨了变形镁合金的强韧化机理。运用有限差分法（FDM）建立了变形镁合金AZ31圆柱锭电磁-悬浮连铸过程三维数学模型。论文研究得出如下主要结论:通过大量的实验研究,确定出镁合金电磁连铸的最佳工艺参数。利用微合金化和电磁场复合作用制备出表面光滑、无氧化夹杂和偏析瘤的高质量变形镁合金铸锭,并得到晶粒细小,析出相弥散的均匀组织。研究结果表明,微合金化和电磁场复合作用制备的电磁连铸锭力学性能较普通连铸锭均有较大提高。通过利用不同功率电磁场对变形镁合金ZK60进行电磁连铸的研究结果表明,当电磁场功率为10kW时,合金的微观组织和力学性能均达到最佳。自行设计了电磁-悬浮成型设备,实现了变形镁合金的电磁-悬浮铸造和电磁-悬浮连铸。制备了适合于加入非金属颗粒的复合悬浮剂线材,提高了非金属颗粒与金属熔体的润湿性。利用镁粉颗粒作为单一悬浮剂和ZrB₂/AZ31复合悬浮剂研究了变形镁合金电磁-悬浮铸造。研究结果表明,单一悬浮剂和复合悬浮剂均能改善合金的微观组织,单一悬浮剂能有效提高合金的力学性能,复合悬浮剂的加入提高了合金的抗拉强度,但合金的伸长率有所降低。利用ZrB₂/AZ31复合悬浮剂对变形镁合金AZ31电磁-悬浮连铸的实验研究表明,复合悬浮剂线材能实现变形镁合金的电磁-悬浮连铸,而且能改善合金的微观组织,得到铸锭内外均匀的细晶组织。利用电磁连铸温度场数值模拟的计算模型,对加入复合悬浮剂后合金的热物性参数进行了处理,同时对AZ31镁合金电磁-悬浮连铸过程的温度场进行了模拟计算,并验证了模拟程序的准确性。在此基础上进行了不同悬浮剂含量对温度场影响的分析,结果表明,随着悬浮剂含量的增加,合金初始温度降低,凝固时间缩短,导热能力下降,但冷却速度增加;随着悬浮剂含量的增加,稳态的液穴深度减小,凝固壳的起始位置升高,凝固壳的厚度增加,液穴坡度变小。通过对变形镁合金AZ31X、AZ61X和AZ61XE电磁连铸锭的热挤压研究结果表明,变形镁合金电磁连铸锭热挤压后均能得到由3-15μm的动态再结晶晶粒组成的均匀组织,同时合金的强度、硬度和伸长率均有很大提高。对变形镁合金AZ31X、AZ61X和AZ61XE挤压棒料进行时效处理的研究,结果表明,变形镁合金AZ31X时效处理后主要由弥散分布的不连续析出相组成,而变形镁合金AZ61X和AZ61XE主要由分布于晶界的不连续析出相和晶内的连续析出相混合组织组成。变形镁合金AZ31X力学性能表现出时效不敏感性,而变形镁合金AZ61X和AZ61XE表现出较好的时效热处理性。更多还原

【Abstract】 Due to hexagonal close-packed structure of magnesium alloys and their relatively poor workability, the main fabrication route of magnesium alloy parts remains die-casting, and the development of wrought magnesium alloys are relatively slow. Wrought magnesium alloys have higher strength and toughness, and can be made more diverse products to meet different conditions. So it is necessary to investigate wrought magnesium alloys with high strength and tougness. In this paper the focus of investigation is strengthening and toughening of wrought magnesium alloys, and finding a new method to produce high performance wrought magnesium alloys.In this paper, three typical wrought magnesium alloys, AZ31, AZ61 and ZK60 are choosed as the base alloys, the technology of microalloying, electromagnetic continuous casting （EMC）, hot extrusion, aging treatment and electromagnetic-suspension forming is employed. The microstructure and mechanical properties are investigated by MEF-4A Metallographic Microscope, XRD-6000 diffractometer, JSM-5600LV Scanning Electron Microscope, EPMA-1600 Electron Probe microanalyser, TG220S-Twin JEM-100CXII Transmission Electron Microscope, HB-3000-1 Brinell Hardness Tester and MTS NEW-810 Materias Test System, and the mechanism of strengthening and toughening is explored. calculated. A 3-D mathematical model of the electromagnetic suspension continuous casting casting process for AZ31 round billet has been developed using the finite differential method （FDM）.The results of the study are showed bellow:An optimized EMC technical parameters has been obtained after many experiment. The billets with EMC and microalloying have smooth surface, and the defects, such as segregation, trapped oxide on the billet surface can be eliminated. The refined grains andβ-Mg₁₇Al₁₂ phases with small size are observed. The electromagnetic field and microalloying can improve the mechanical properties of wrought magnesium alloys. The EMC of ZK60 magnesium alloy casting with different electromagnetic powers are investigated, the results show that the best refined grains and combination property are obtained when the electromagnetic power was 10 kW.The electromagnetic suspension casting and electromagnetic suspension continuous casting system are designed, and experiments for wrought magnesium alloys are carried out by the systems. The complex suspension particles easily adding non-metal particles are produced, the wettability between the non-metal particles and melting metal is improved. The electromagnetic suspension casting with magnesium powder and complex suspension particles can both impoved the microstructures of wrought magnesium alloys, properties are increased by simple suspension particles, UTS （Ultimate tensile strength） of magnesium alloy is improved by complex suspension particles, but the Elongation is decreased. The electromagnetic suspension continuous casting for AZ31 magnesium alloy is investigated by ZrB2/AZ31 complex suspension particles, the results show that the microstuctures are improved, and the refined grains from the boundary to the center of the billet.The mathematical model of the electromagnetic continuous casting process was utilized to predict the temperature in the billet of electromagnetic suspension continuous casting, after some thermophysical properties were processed. The model had been validated by comparing the calculated temperature profiles with the measured temperature profiles. The effect of amount of suspension particles on temperature field in the billet was studied. The conclusion was drawn that initial temperature of the alloy drops, solidification time of billet shortens, heat-transfer capability declines, comprehensive cooling ability enhances, solidification shell thickness increases, sump depth decreases, initial position of solidification shell enhances and slope gradient of sump profile decreases with increasing the amount of suspension particles.Hot extrusion of AZ31X, AZ61X and AZ61XE magnesium alloys are investigated, the results show that the microstructures of as-extruded magnesium alloys are made of 3-15μm fine homogeneous grains due to DRX （Dynamic recrystallization）, and the UTS, Hardness and Elongation are improved greatly. Discontinuous precipitation occurs for AZ31X magnesium alloy after aging heat treatment, but the microstructures of AZ61X and AZ61XE are form of discontinuous precipitation phase on the grain boundary and continuous precipitation phase in the grains. The aging heat effect of AZ61X and AZ61XE magnesium alloys are better than that of AZ31X magnesium alloy.更多还原