| 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.
|
PDF Full Text Request