Investigation On The CuWCr Composite Prepared By Mechanical Alloying And Its Vacuum Electrical Breakdown Properties

As one of the most important switchgears in power system, vacuum circuit breaker, which is used to control and protect the system, is widely used in many fields. With development of the power system, it is required for the breakers to have an increasingly stringent combination of properties for the contact materials with higher voltage, larger capacity, lower over-voltage and intellectualization. Since the CuW alloy has low breaking current and high chopping current and the CuCr has low erosion-resistant, fusion-resistant and breakdown strength, it is imperious to develop a new contact material with excellent combined properties.At present, the strengthening alloying elements addition and new technologies are mainly used to improve the properties. The WCr alloys and CuWCr composites are prepared by mechanical alloying and sintering and infiltration processes respectively in this investigation. Compared with the materials prepared by conventional metallurgy method, the relationship between microstructure and properties of those materials is studied. At the same time, the vacuum electric breakdown is investigated, and the relationships among the microstructure, properties and cathode spot motion are identified, and the important factors affecting breakdown voltage and chopping current are analyzed as well. From those investigations, it can be drawn the following conclusions.1.The W-Cr alloy powders with different Cr contents can be prepared by mechanical alloying. With increase of milling time, the grain size of the W-Cr powders decreases, the micro-strain increase, and the lattice parameter has a diminution tendency. The saturation of Cr in the W matrix increases and the full sosoloid is formed. The milling time for preparing W-Cr alloy powders is prolonged with increase of Cr content.2.In vacuum, the WCr alloys and CuWCr composites with fine grains, uniformcomposition and high density can be prepared by mechanical alloying. The hardness and density of both the materials prepared by the new method are higher than those of the materials produced by the conventional method.3.The WCr alloys and CuWCr composites prepared by the new method have good arc eroding endurance. Arc erosion zones are dispersive and uniform on the surfaces with some flat eroding pits. Both the materials have excellent electrical properties such as high breakdown voltage, low chopping current and long arc life and so on. The distribution of breakdown strength is dense due to the unobvious seasoning phenomenon.4.With increase of Cr contents, the hardness of both the materials prepared by mechanical alloying increases, the electrical conductivity decreases, the breakdown voltage increases, the chopping current decreases, and the arc life prolongs.5.A primary internal electric-field model of the WCr alloys is established. The calculated resluts show that the refined microstrucure enhance the additional electrical field intensity, thus lead to decrease the electrical escape function of W phase and increase electrical escape capability of W phase due to extra external electrical field. The breakdown of WCr alloys prepared by mechanical alloying is uniformly produced on the WCr electrode surface in the continuous and quasi-continous ways. Meanwhile, the weak breakdown phase of CuWCr composite prepared by mechanical alloying also changes, and the breakdown is uniformly produced on the CuWCr electrode surface in a continuous way.