PSI - Issue 14

Sandeep Das et al. / Procedia Structural Integrity 14 (2019) 119–126

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Sandeep Das et al. / Structural Integrity Procedia 00 (2018) 000–000

1. Introduction High surface value components such as tools, turbine blades and internal combustion engine parts operating in stringent environmental conditions undergo high degradation of the surface such as wear, corrosion, etc. These lead to the increase in demand for advanced materials like nickel super alloys, aluminium/titanium alloys, high temperature steels possessing high hardness, high toughness, high fatigue, corrosive resistant, wear resistant, etc. However, bulky work-parts or components entirely made up of these materials are not economical. Therefore, application of coating with required surface properties can be an alternative cost effective process. Surface modification is the process of modifying the surface of a material by changing the physical, chemical or biological characteristics of the original surface of a material. It can be achieved by various techniques such as Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), magnetron sputtering, laser cladding, etc. However, these methods need design and development of costly equipment. Also, maintaining special processing condition viz. vacuum further limits their applicability. In view of this, electrical discharge surface modification is finding suitable for surface modification of high surface value components such as tools, dies and molds.

Fig. 1. Schematic diagram for surface modification by EDM.

The principle of surface modification using EDM is based on alloying of molten tool electrode elements with the work electrode. A schematic diagram of surface modification by EDM is as shown in Figure 1. The eroded material T from the tool electrode merges with carbon C from the decomposed hydrocarbon dielectric medium thereby forming a compound TC and deposits on the work surface. During EDM based surface modification process, the workpiece is cathode and the tool is anode. The tool and workpiece are submerged in a dielectric medium and are placed very close to each other at about 10 to 100 μm distance apart. An open-circuit voltage of about 200V is applied across the electrodes. The dielectric medium breaks down and gets ionized. This forms a plasma channel which melts and vaporizes metal from both work and tool electrode. Literature reports eminent research articles on electric discharge based surface alloying and surface modification. Jeswani (1978) carried out electron microprobe analysis for surface deposition and diffusion of material on mild steel, high carbon steel with copper and brass tool. Pulse energy was found to be the most important factor aiding surface deposition as compared to tool material, work material or the dielectric fluid. Tsunekawa et al. (1994) successfully deposited TiC-TiAl in situ composite layers by using green compact electrode of titanium and kerosene as dielectric fluid. The modified layers with an average thickness of 100 µm was developed. The TiC morphology was found to be coarse to fine dendritic particulates and its sizes become smaller towards the inside of modified layers. The hardness value close to the surface can be changed in the range of 3.5-1.5 GPa. Kruth et al. (1995) succeeded in depositing aluminium on steel and TiC on aluminium using Al and Ti-Al green compact electrodes respectively with traditional EDM machine. This was obtained by using porous electrodes with negative polarity favoring high tool wear. During