AZ31镁合金NSA-TIG焊接溶池运动规律及接头性能强化机理研究

发布时间：2018-07-09 15:57

本文选题：AZ31镁合金 + 活性钨极氩弧焊　；参考：《重庆大学》2016年硕士论文

【摘要】：镁合金由于其优异性能(密度低、比强度和比刚度高、减震性好、电磁屏蔽性能优良、切削加工性好等),在交通运输、消费电子、航空航天、国防军工等领域都具有广阔应用前景。然而由于滑移系{0001}1120、孪生系{1012}1011和密排六方结构导致镁合金在室温下成型能力有限。因此,对于利用铸造和锻压工艺难以制备的镁合金复杂结构件,以及实现镁合金材料之间的连接,焊接是重要的技术手段,对其进行技术创新和科学机理研究十分重要。钨极氩弧焊(Tungsten Inert Gas Arc Welding,TIG)由于其经济性和适用性被主要应用于镁合金的焊接技术中。本文通过数值模拟、宏观形貌观察、显微组织分析和力学性能测试,系统研究了纳米SiC颗粒进入AZ31镁合金纳米增强活性钨极氩弧焊接(Nano-particles Strengthening Activing Flux TIG Welding,NSA-TIG)接头后的运动演变规律及其对接头宏观形貌、微观组织和力学性能的影响。通过模拟仿真与实验相结合,揭示了纳米颗粒在熔池中的分布规律及其主要影响因素。通过实验探究分析了纳米SiC颗粒溶入焊接接头后与基体发生的化学变化和产物,以及这些产物对AZ31镁合金NSA-TIG焊接接头微观组织和力学性能的影响。在实验事实的基础上,结合模拟和理论分析,得出以下主要结论:利用Fluent软件,建立三维瞬态移动热源作用下NSA-TIG焊焊接熔池的数学模型。从模拟结果可知:溶池内Marangoni对流主要受到表面张力的影响,其次是电磁力的作用,浮力的影响最弱。Marangoni对流的方向可以通过活性剂的成分来控制,在仅用纳米SiC颗粒做活性剂的溶池中,由于Marangoni对流从溶池中央向边缘运动并带动了纳米SiC颗粒,使其难以融入溶池中,故而纳米SiC颗粒主要分布于溶池表面。而在TiO2和SiC复合活性剂的焊接接头中,Marangoni对流带动纳米SiC颗粒向溶池中央和底部运动,但由于纳米SiC颗粒还受到电磁力的搅拌作用,虽然纳米SiC有一定的分布于溶池中央和底部的趋势。但其最终会均匀分布于溶池中。SiC颗粒溶入熔池后,大部分与基体中的Al反应,生成了Al4C3和单质Si,而单质Si进一步与Mg反应生成了Mg2Si相,其中Al4C3相是基体相α-Mg的形核剂,而Mg2Si颗粒则作为弥散相起到了增强NSA-TIG焊接接头的力学性能的作用。此外随着纳米增强活性剂表面涂覆密度的增加,接头的宏观形貌逐渐变差,熔池逐渐加深,焊缝加宽;焊缝中SiC数量增加,焊缝中α-Mg晶粒尺寸减小,第二相的β-Mg17Al12的体积分数减少;焊接接头的微观硬度逐渐增加,极限抗拉强度增加。当表面涂覆密度为20mg/cm2时,焊缝中α-Mg晶粒尺寸最小,第二相的β-Mg17Al12的体积分数最小,NSA-TIG焊接接头的力学性能达到最大。此后,随着表面涂覆密度的继续增加,焊缝中的SiC开始团聚,熔池中的α-Mg晶粒尺寸增大,第二相的β-Mg17Al12的体积分数增多,焊接接头的极限抗拉强度急剧降低,显微硬度继续变大。
[Abstract]:Magnesium alloys, due to their excellent properties (low density, high specific strength and stiffness, good shock absorption, excellent electromagnetic shielding performance, good machinability, etc.), are used in transportation, consumer electronics, aerospace, etc. National defense military industry and other fields have broad application prospects. However, due to the slip system {0001} 1120, the twin line {1012} 1011 and the dense hexagonal structure, the forming ability of magnesium alloys at room temperature is limited. Therefore, welding is an important technical means for the complex structure of magnesium alloy which is difficult to be prepared by casting and forging process, as well as to realize the connection between magnesium alloy materials. It is very important to study its technological innovation and scientific mechanism. Tungsten Inert Gas Arc WeldingTIG (Tungsten Inert Gas Arc Welding TIG) is mainly used in magnesium alloy welding technology because of its economy and applicability. In this paper, numerical simulation, macroscopic morphology observation, microstructure analysis and mechanical properties test are carried out. The motion evolution of nano-sic particles in nano-reinforced Activing flux TIG welding (NSA-TIG) joints of AZ31 magnesium alloy and its effects on the macroscopic morphology, microstructure and mechanical properties of AZ31 magnesium alloy joints were systematically studied. Through the combination of simulation and experiment, the distribution of nanoparticles in the molten pool and its main influencing factors were revealed. The chemical changes and products of nano-SiC particles dissolved in the welded joints and the effects of these products on the microstructure and mechanical properties of AZ31 magnesium alloy NSA-TIG welded joints were investigated by experiments. On the basis of experimental facts, combined with simulation and theoretical analysis, the main conclusions are as follows: using fluent software, the mathematical model of weld pool of NSA-TIG welding under the action of 3D transient moving heat source is established. The results show that the Marangoni convection is mainly affected by surface tension, followed by electromagnetic force, and the direction of buoyancy can be controlled by the composition of active agent. Because the Marangoni convection moves from the center to the edge of the solution pool, it is difficult to integrate the nano-SiC particles into the solution pool, so the nano-sic particles are mainly distributed on the surface of the solution pool. In the welded joints of TiO2 and sic composite active agents, Marangoni convection drives the nano-sic particles to the center and bottom of the solution pool, but the nano-SiC particles are stirred by electromagnetic force. Although nano-sic has a tendency to distribute in the center and bottom of the solution pool. However, after dissolved in the molten pool, most of the sic particles will react with Al in the matrix to form Al _ 4C _ 3 and Si, while the simple Si reacts with mg to form mg _ 2Si phase, in which Al _ 4C _ 3 phase is the nucleating agent of the matrix phase 伪 -Mg, and the Al _ 4C _ 3 phase is the nucleating agent of the matrix phase 伪 -Mg, and the Al _ 4C _ 3 phase is the nucleating agent of 伪 -Mg. The mg _ 2Si particles act as dispersion phase to enhance the mechanical properties of NSA-TIG welded joints. In addition, with the increase of the surface coating density of nano-reinforced active agent, the macroscopic morphology of the joint becomes worse, the weld pool is gradually deepened, the weld is widened, and the number of sic in the weld increases, and the grain size of 伪 -Mg in the weld decreases. The volume fraction of 尾 -Mg17Al12 in the second phase decreases, the microhardness of welded joints increases gradually and the ultimate tensile strength increases. When the surface coating density is 20mg/cm2, the grain size of 伪 -Mg is the smallest and the volume fraction of 尾 -Mg17Al12 in the second phase is the smallest. The mechanical properties of NSA-TIG welded joints can reach the maximum. After that, with the increase of the surface coating density, the sic in the weld began to agglomerate, the grain size of 伪 -Mg in the weld pool increased, the volume fraction of 尾 -Mg17Al12 in the second phase increased, the ultimate tensile strength of the welded joint decreased sharply, and the microhardness continued to increase.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG444.74

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