【作者】 徐坚；

【导师】 熊惟皓；

【作者基本信息】 华中科技大学 ， 材料学， 2008， 博士

【摘要】 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(107K·m-1)、冷却速度极快(107 K·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。更多还原

【Abstract】 Electrical contact materials are the key component 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 automobile 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 was 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 signales.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 Volume 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, 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”metallurgy 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 (107K·m-1), high cooling rate (107K·s-1), porous microstructure and a tendency to chemical reactions.When the volume fraction of MeO was a constant, the effects of density, heat conductivity, heat capacity, and decomposition temperature and decomposition enthalpy on the arc 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 than 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 to 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 ( ?γ/ ?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 ?γ/ ?T of the fluid in the pool to zero would lead to not only the less possibility of materials sputtering in the pool but also the minimum volumetric erosion of electrical contact materials; therefore it would benefit the stability and uniformity of the electrical contact materials.Finally, the phenomenon that the MeO aggregating on the surface of electrical contact materials was discussed. The simulation results showed that the volume fraction of MeO on the surface of the electrical contacts increased due to the fluid flow in the molten pool. It would lead to a wider contact area while a lower welding strength for the more MeO aggregation on the surface, but the welding force would decrease. The welding force decreasing would benefit the arc welding resistance of the electrical contact materials.In summary, the research on decreasing the density of arc input power was the most important factor that would benefit the arc welding resistance of electrical contacts.When the mass faction of MeO was a constant, adding the MeO with low density would also benefit the arc welding resistance of electrical contacts.更多还原