Electrical Discharge Coating by Copper-Tungsten Composite Electrode Prepared by Powder Metallurgy Route

Electrical Discharge Coating by Copper-Tungsten Composite Electrode Prepared by Powder Metallurgy Route

Anshuman Kumar Sahu (National Institute of Technology Rourkela, India) and Siba Sankar Mahapatra (National Institute of Technology Rourkela, India)
DOI: 10.4018/978-1-5225-3035-0.ch010
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Abstract

Electrical discharge machining (EDM), a thermo-mechanical machining process, is used in producing complicated intrinsic cavity in difficult-to- machine materials with excellent surface finish. One of the major disadvantage of EDM process is the tool wear, which can be used advantageously for coating purpose. Coating is a unique method of EDM process by the use of electrode prepared via powder metallurgy route. Copper and tungsten powders in weight percentage of 30 and 70 respectively are used for the preparation of the tool electrode by varying the PM process parameters like compaction pressure and sintering temperature. The substrate on which coating is made is chosen as AISI 1040 stainless steel with EDM oil as the dielectric fluid. During coating, influence of parameters like discharge current, duty cycle and pulse-on-time on material deposition rate, tool wear rate and radial under deposition are studied. To find out the best parametric combination Grey Relational Analysis method combined with Harmony Search algorithm has been employed.
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Introduction

Electrical discharge machining (EDM), a non-conventional machining process, is extensively used in aerospace, biomedical, chemical, automobile and tool and mold making industries. EDM is normally used to produce parts with complex geometry made of difficult-to-machine materials having reasonable surface finish. In EDM process, both work piece and tool electrode are electrically conductive and immersed in a dielectric medium. The material removal occurs by electro-thermal process. When the potential difference is applied to the electrical circuit, an electric field is established due to the potential difference in the tool electrode and work piece gap. Therefore, free electrons on the electrode were emitted and accelerated towards the work piece by the electrostatic force. The accelerated electrons collide with the work piece surface and a high temperature around 8000-10000°C is generated. The material removal occurred by the melting and evaporation of tiny particles from the work piece surface due to the generation of successive sparks between work piece and electrode in the dielectric medium. Extensively high temperature generated at the work piece surface can easily melt and vaporize materials from difficult to machine materials with high hardness, strength and complex structural properties. The dielectric fluid closes the electrical circuit as well as swept out the removed material and enhanced the material removal rate (Mishra, 2012; Sahu, Mohanty, & Sahoo, 2017). During the process of EDM, material removals as well as tool wear occur due to the generation of very high temperature. The tool electrode wear phenomenon cannot be eliminated completely. But the wearing material from the tool can be used for coating purpose. Electrical discharge coating (EDC) is an exceptional method of EDM process where surface modification or surface alloying of the work piece occurs. The removed tool material is deposited on the work piece surface and form hard coated layer on the work piece surface. Hence, electrode prepared via powder metallurgy route is used for EDC process. By preparing electrode via powder metallurgy route, one can vary the properties of the tool electrode such as density, electrical conductivity as per requirement by varying different powder metallurgy process parameters like compaction pressure and sintering temperature. Therefore, tool material can be eroded sufficiently and transferred to the work piece surface. With the use hydrocarbon oil as dielectric fluid in EDC process, the dielectric fluid dissociated during the spark and combined with the tool material forming a hard-coated layer of metal carbide on the work piece surface. A hard ceramic layer of WC, TiC, SiC, TiN, B4C, Cu3Sn, CuSn, ZrB2 can form on the work piece surface after EDC according to the type of the tool electrode and the dielectric fluid used during the EDC process. The EDC process can be used to improve the corrosion resistance and hardness of the work piece that can be used in the industries like aerospace, automobile, biomedical and chemical where the work piece materials are used in a wide variation of temperature with extreme environmental conditions. The EDC process is less expensive as compare to other complicated and costly coating processes like chemical vapor deposition (CVD) and physical vapor deposition (PVD).

In the EDC process, multiple conflicting performance characteristics need to be optimized in order to maximize productivity of the process. For example, material deposition rate need to be maximized whereas tool wear rate and radial under deposition are minimized for improving the EDC process. Therefore, proper selection of the various process parameters is an important issue in EDC process. From recent literature, it is found that different optimizations techniques have been used for the optimization of the EDM and EDC process to enhance the performance of the processes. Various techniques used for the purpose include Grey Relational Analysis (GRA), Satisfaction Function and Distance Based Approach, Utility Concept and Quantum Behaved Particle Swarm Optimization (QPSO) (Sahu, Mohanty, & Sahoo, 2017; Patowari, Saha, & Mishra, 2010; Rahul, Datta, Biswal, & Mahapatra, 2017; Mohanty, Mahapatra, & Singh, 2017). In this work, grey relational analysis (GRA) based Harmony search (HS) algorithm has been used to get optimal parametric setting to get best output responses of EDC process. Here, GRA method used to convert the multi responses into single response i.e. grey relational grade (GRG) and harmony search algorithm has been used to find the optimum parametric setting.

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