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1、华 中 科 技 大 学 博 士 学 位 论 文华 中 科 技 大 学 博 士 学 位 论 文 I 摘摘 要 要 电触头是继电器的关键部件之一其性能直接关系到继电器的工作寿命和可靠性随着欧盟环保指令的实施以及汽车供电系统由 14V 升高到 42V 的趋势汽车继电器的服役条件对电触头材料提出了更加苛刻的要求在触头的各种失效形式中最为严重的是电弧放电导致的触头熔焊对熔焊现象进行理论上的分析能够为触头材料的设计和生产提供指导 为便于后续对触头材料进行电性能测试本文设计了一个电接触模拟试验装置可用于模拟继电器触点工作状态测试触头材料的抗熔焊能力采用电磁铁和弹簧配合作用控制触头的往复运动触头材料装夹电流和
2、电压信号采集方便 本文在圆柱坐标系中建立了 AgMeO 复合触头材料在电弧作用下传热与熔池流动过程的数学模型并采用有限体积法计算了触头材料的温度场流速场和浓度场详细分析了触头材料熔池形貌及其演化过程特点讨论了基体材料及第二相金属氧化物性质对 AgMeO 触头材料抗熔焊能力的影响 温度场计算结果表明电弧能量作用于触头材料后触头材料经历了从室温至熔点熔点至沸点沸点至最高温度点然后开始冷却至沸点熔点开始凝固直至冷却到室温五个不同的状态阶段电弧作用是对触头材料再加工的物理化学冶金过程因此极大改变了触头材料原有的成分组织和性能触头材料凝固过程具有易于形核熔池温度梯度大107Km-1冷却速度极快107 K
3、s-1易形成孔洞疏松结构甚至会发生化学反应等五个突出的特点 对 MeO 体积分数一定的 AgMeO 触头材料改变 MeO 的热导率比热容密度分解温度和分解焓对触头材料的抗熔焊能力影响不大增大 MeO 的体积分数有助于提高触头材料的抗熔焊能力 但会增加触头材料的电阻率 为保证触头材料电性能MeO 体积分数宜控制在 20以内 不同电弧输入功率对触头材料熔焊力的影响巨大输入功率加倍其相应的熔池华 中 科 技 大 学 博 士 学 位 论 文华 中 科 技 大 学 博 士 学 位 论 文 II 接触面积亦加倍熔焊力加倍因此对不同材料产生和分散电弧性能的研究是触头材料抗熔焊性能研究的重点 在小电流条件下触
4、头材料熔池中流体的流动主要受表面张力驱动其次是电磁力作用而浮力的影响不大电弧作用下熔池中流体流动方向由表面张力温度系数所决定具有负表面张力温度系数的流体将从熔池中心向熔池边缘流动而具有正表面张力温度系数的流体将从熔池边缘向熔池中心流动添加能增大液态银表面张力温度系数至零左右的合金元素不仅可以减小触头材料喷溅概率而且可减小触头材料熔池体积从而有利于银基触头材料的均匀稳定 最后本文讨论了由于熔池流动造成 MeO 成分偏聚的现象计算表明熔池中流体流动会使触头表面的MeO平均含量升高 触头表面MeO的富集使熔化面积增加但熔焊强度下降熔焊力下降因此有利于触头材料抗熔焊MeO 的密度对成分分布的影响较小但
5、在 MeO 的质量分数一定时选择低密度的氧化物能够使其体积分数增加熔焊力下降 总之 提高 AgMeO 触头材料的抗熔焊能力 应优先考虑添加合金元素提高 AgMeO触头材料分散和熄灭电弧的能力其次在质量分数一定的情况下选择低密度的MeO 虽然使熔池的面积增加 但是能够降低触头的熔焊力 从而提高触头材料抗熔焊能力 关键词关键词复合材料AgMeO 电触头熔焊行为熔池流动表面张力数值分析 华 中 科 技 大 学 博 士 学 位 论 文华 中 科 技 大 学 博 士 学 位 论 文 III Abstract Electrical contact materials are the key compone
6、nt of relays. Their performance has great effect on the duability and reliability of relays. With the implement of environment restriction from the European Union and the voltage of power supply increasing from 14V to 42V in automobile, the requirements for the electrical contact materials used in a
7、utomobile relays become more and more rigorous. Arc welding of electrical contacts is the most fatal failure to the switch relays. The research on the behavior of arc welding of electrical contacts will serve the design and fabrication of them well. An apparatus which simulated the contact process w
8、as designed to test the arc welding resistance of electrical contact materials. The movement of the contacts was controlled by the electromagnet and springs. The contact force could be adjusted and the welding force could be measured. It was easy to clip the contact samples and to get the arc signal
9、es. The mathematical models for the heat conduction and the fluid flow in the molten pool of AgMeO electrical contact materials under arc were developed in the cylinder coordinates. The temperature, velocity and content distribution in the molten pool were calculated respectively by the Finite Volum
10、e Method. The geometrical profile of the molten pool and the characters in its evolution were analyzed. The effect of properties of MeO on the arc welding resistance of AgMeO was discussed in details. The numerical analysis showed that the electrical contact materials underwent five different stages
11、, i.e. from the room temperature, heating to the melting point, the boiling point, and the highest temperature and then cooling to the boiling point, the melting point and finally solidification to room temperature during and after arc process. The arc process could be seen as a “re-fabrication”meta
12、llurgy process to the electrical contact materials and the properties of electrical contact materials changed greatly after the process. The solidification of the molten pool in the electrical contact materials were characterized by easily nucleating, high temperature gradient (107Km-1), high coolin
13、g rate (107Ks-1), porous microstructure and a tendency to chemical reactions. When the volume fraction of MeO was a constant, the effects of density, heat 华 中 科 技 大 学 博 士 学 位 论 文华 中 科 技 大 学 博 士 学 位 论 文 IV conductivity, heat capacity, and decomposition temperature and decomposition enthalpy on the ar
14、c welding resistacne of the electrical contacts were very little. Increasing the volume fraction of MeO would be good to the arc welding resistance but would lead to a higher electrical resistivity of the electrical contacts. Therefore, it would be better to keep the volume fraction of MeO lower tha
15、n 20%.To reduce the density of the arc input power as much as possible was crucial to the arc welding resistance of AgMeO. The density of arc input power had the greatest effect on the arc welding resistance of the electrical contacts. The surface tension of liquid metal was the main driving force t
16、o the fluid in the molten pool of the electrical contacts under arc comparing with the Lorentz force, Buoyancy force when the arc current was low. The fluid flow direction in the molten pool was controlled by the sign of the temperature coefficient of surface tension (/ T): the fluid with the negative temperature coefficient would make the molten metal spread outward while that with the positive temperature coefficient would make the molten metal penetrate downward. Increasing/ Tof
