电磁搅拌及热处理对AZ91镁合金组织与性能的影响

发布时间：2018-03-06 02:15

本文选题：AZ91镁合金　切入点：电磁搅拌　出处：《沈阳工业大学》2017年硕士论文　论文类型：学位论文

【摘要】：镁合金的比强度高、材料可回收率高、电磁屏蔽性好,因此越来越受到现代工业的重视,但镁合金在室温下较差的塑性,使得其应用的广泛性不及其它金属材料。电磁搅拌可以细化晶粒,增加等轴晶,减少树枝晶,同时因电磁力而产生熔体的对流使其温度场均匀化,能减轻偏析。此外,镁合金也可通过固溶处理和时效处理提升力学性能。本文以AZ91为研究对象,采用不同电磁搅拌工艺参数和热处理方法,改善AZ91镁合金的显微组织,提高合金的力学性能;并通过第一性原理,对β相和Mg基固溶体进行热稳定性、电子性质和弹性性质的计算。结果表明:AZ91镁合金的力学性能和显微组织均随电磁搅拌频率的增加而得到改善,在电磁频率为10Hz、电流为150A时力学性能最好,合金的抗拉强度、伸长率和硬度分别为163MPa、5.3%和58.6HB;此时晶粒的细化最明显,平均晶粒尺寸为93μm,α-Mg相由树枝状转变为蔷薇状,Al元素固溶度提升,同时基体中出现了孪晶;原先粗大的β相得到细化和碎化。晶粒的细化,β相的碎化,Al、Zn的固溶强化以及孪晶,使得合金的力学性能得到提升。通过对电磁搅拌AZ91镁合金进行固溶处理,合金力学性能随着固溶时间的增加呈先增加后减少的趋势,在36h时达到最大值,此时合金的抗拉强度、伸长率和硬度分别为229MPa、8.8%和67.8HB;此时β相已大量溶解,剩余的β相呈弥散分布。通过对固溶36h的合金进行时效处理,合金的抗拉强度和硬度随着时效时间的增加呈先降低后增加再降低的趋势,在12h时达到最大值,此时合金的抗拉强度和硬度分别为247MPa和78.1HB,而伸长率一直降低;时效过程中晶粒呈长大趋势,而β相以不连续析出为主,伴随少量连续析出。第一性原理计算结果表明,β相可以自发形成并且稳定存在,并且具有较高的杨氏模量,呈脆性,因此当β相以细小状态弥散分布时,会对基体产生强化作用;Mg24、Mg23Al1、Mg23Zn1和Mg_(22)Al_1Zn_1掺杂固溶体中,Mg_(22)Al_1Zn_1同时具有相对较低的形成热和结合能,以及良好的塑性,说明Al、Zn元素的共同固溶利于Mg基固溶体的形成和稳定存在,并且提升其力学性能。
[Abstract]:Magnesium alloys have high specific strength, high recovery rate and good electromagnetic shielding, so they are paid more and more attention by modern industry, but the plasticity of magnesium alloys is poor at room temperature. Electromagnetic stirring can refine grain, increase equiaxed crystal, reduce dendrite, at the same time, the convection of melt caused by electromagnetic force can homogenize its temperature field and reduce segregation. The mechanical properties of magnesium alloys can also be improved by solution treatment and aging treatment. In this paper, different electromagnetic stirring process parameters and heat treatment methods are used to improve the microstructure and mechanical properties of AZ91 magnesium alloys. The thermal stability, electronic properties and elastic properties of 尾 -phase and Mg-based solid solution were calculated by first-principle method. The results show that the mechanical properties and microstructure of the 尾 -phase and Mg-based solid solution are improved with the increase of electromagnetic stirring frequency. When the electromagnetic frequency is 10 Hz and the current is 150 A, the mechanical properties of the alloy are the best. The tensile strength, elongation and hardness of the alloy are 163MPA 5.3% and 58.6 HB.The grain refinement is the most obvious. The average grain size was 93 渭 m, 伪 -Mg phase changed from dendritic to rose-like Al element, and twins appeared in the matrix, the 尾 phase was refined and shredded. The mechanical properties of the alloy were improved by the solution treatment of electromagnetic stirring AZ91 magnesium alloy. With the increase of the solution time, the mechanical properties of the alloy increased first and then decreased, and reached the maximum value at 36 h, when the tensile strength of the alloy was increased, The elongation and hardness of the alloy were 229MPA (8.8%) and 67.8 HB.The 尾 phase was dissolved and the remaining 尾 phase was dispersed. The alloy was treated by aging treatment for 36 h. With the increase of aging time, the tensile strength and hardness of the alloy decreased first, then increased and then decreased, reaching the maximum value at 12 h. The tensile strength and hardness of the alloy were 247MPa and 78.1 HBs, respectively, and the elongation decreased continuously. In the aging process, the grain size tends to be long and the 尾 phase is mainly discontinuous precipitation, accompanied by a small amount of continuous precipitation. The first principle calculation results show that the 尾 phase can spontaneously form and exist stably, and has higher Young's modulus and brittleness. Therefore, when the 尾 phase is dispersed in fine state, it will strengthen the matrix. Mg _ (24) mg _ (23) Al _ (1) O _ (mg _ (23) Zn _ (1) Zn _ (1)) and Mg_(22)Al_1Zn_1 doped solid solution have relatively low formation heat, binding energy, and good plasticity at the same time. The results show that the common solution of Al ~ (2 +) Zn is beneficial to the formation and stability of Mg-based solid solution and improves its mechanical properties.
【学位授予单位】：沈阳工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.22;TG166.4

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