International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391 Volume 6 Issue 4, April 2017 www.ijsr.net Licensed Under Creative Commons Attribution CC BY Development of Copper-Chromium Electrical Contact Materials by Powder Forging Gotkhindikar Nitin N. 1 , Dr. V. V. Dabhade 2 1, 2 Indian Institute of Technology (Department of Metallurgical Engineering & Material science), Roorkee, Uttarakhand, India Abstract: The electrical contacts form an integral part of circuit interrupters for which the desirable properties of contact materials are high mechanical strength, high electrical and thermal conductivity, high wear and weld resistance. Despite of different manufacturing techniques like electro-slag crucible melting, vacuum arc smelting, vacuum continuous smelting etc.; the Powder Forging being a solid state process and having high densification factor is an efficient method for the production of electrical contact materials. In the present study Cu-Cr30 wt. % alloys are prepared from elemental powders by sinter-forging. Nb is also added in varying quantities to improve the electrical and mechanical properties and the forging temperature is also varied .The variation in contact material properties due to these changes are tabulated and compared. Keywords: Powder forging, SEM, XRD, EDS, Elemental mapping, Sinter-forging etc 1. Introduction Over past 50 years, contact materials based on Cu-Cr have become established for the use in vacuum interrupters for medium voltage switches. They are used as contactors and circuit breakers within a voltage range of 1 kV to 75 kV. The vacuum circuit breaker is such a kind of circuit breaking instrument where the arc quenching occurs in vacuum. For higher voltages vacuum technology has been developed but not commercially viable [1]. For current carrying contacts which material used plays vital role in performance of vacuum circuit breaker. Prerequisites of electrical contact materials therefore are best electrical and mechanical properties. Desirable properties of electrical contact materials can be broadly be stated as, high mechanical strength, minimum gas content, high weld resistance, optimum vapor pressure, low thermionic emission, low erosion, high thermal and electrical conductivity, high withstand voltage [2].The metals, satisfying these properties are Copper (Cu), Gold(Au) Silver(Ag), Tungsten(W).By alloying these metals to other elements make them fit for use by overcoming weaknesses. Copper suits for the application because of its high electrical and thermal conductivity, but it has strong tendency to form the weld. Tungsten is efficient thermionic emitter and Silver and Gold are very expensive [2]. Some of the desirable properties for electrical contact materials are satisfied by Cu like high electrical and thermal conductivity, minimal gas content, low thermionic emission except weld resistance and mechanical strength of Cu is low. By alloying one can overcome these drawbacks. Strengthening of Cu can be achieved by alloying with Zn,Cr,Bi,W,Te,Pb which in turn increases the hardness; reduces the conductivity of the alloy. On the relative scale, Cu alloys are cheap [2, 3]. Due to inadequate solubility of Chromium in copper in solid state, results in high thermal and electrical conductivities of a material. Cr precipitates in the form of dispersed fine particles. Another highly beneficial property of these contact materials high affinity to oxygen for Cr component in contact material which results in absorption of any oxygen content released during switch process in turn helps to protect vacuum in the interrupting chamber during service period of this device [4]. Owing to some of the properties mentioned above it is not that much easy to manufacture high quality Cu-Cr electrical contacts. A range of production processes are adopted for the manufacturing but mostly powder metallurgical processes. Most widely used procedure is mixing chromium with copper powder, compacting the mixture, sintering the green shape below the melting point of the Cu to produce blank for contact pieces. Another method can be varied as skeleton of Cr is sintered and Cu is infiltrated. Cu in liquid phase produces extremely strong bond between both the constituents [4]. Other methods like Vacuum continuous casting, Electro-slag crucible melting, Vacuum arc smelting, Explosive compaction are also the alternative production processes [1, 5]. Powder forging (P/F) is the technique to manufacture components essentially free from internal porosity. The density, mass and shape of the preformed component are controlled strictly to ensure consistency in the characteristics of the final forged component. The preform is generally sintered with conditions for the reduction of nonmetallic inclusions. The reheated sintered preform, placed in the forging die, and forged to aim at full density. Other advantages of powder forging are 1) Easily possible to produce Cu-Cr contact materials because with some processes difficulty arises due to large difference in their melting points(Cu-1091°C,Cr-1869°C) and densities(Cu-8.96g/cc,Cr-7.16g/cc). 2) It is a total solid state process and density around unity can be achieved by the process [6]. The aim of this study is to develop Cu-Cr electrical contact materials which involves powder forging of Cu-Cr .Powder forging was carried out by following premixing, compaction, sintering followed by forging. The detailed study of microstructural analysis, hardness and conductivity measurements were carried out for optimization of properties of contact material. Paper ID: ART20172484 879