Tungsten Copper and EDM
Electro Discharge Machining (EDM) is a well-established nontraditional machining process s. It is widely used for machining complicated c contours in hard materials. Thus, it is a well accepted pr active in die and mold making industries for quite a few decades. There are two major disadvantage s of conventional die-s inking EDM process; one is tool wear and the other is the formation of brittle and cracked white layer on the machine d surface. The tool wear can be controlled to some ex tent in EDM, but to achieve a condition without tool wear is almost impossible. Since EDM is an electro thermal process, so some alteration in surface integrity takes p lace due to the formation of recast or white layer. It has been also shown that the resolidified layer is actually an alloy composed of both electrode material s and decomposed product s of dielectric like carbon. Due to the presence of resolidified alloyed mat erialon the surface and the heat- affected zone, the surface integrity of the work piece changes. This phenomenon can be turned into an innovative and fairly cost- effective technique for the modification of surface proper ties. Considering this aspect of EDM, attempts have been made to determine the possibility of using tool electrodes as feedstock materials.
This is an effort to produce significant alloying on work piece surface and, in doing so, enhance the surface integrity of the machined surface.
The above ph e nom en o n ha s in spire researches her s to explore the possibility of new method of surface modification by EDM. By using an ordinary EDM machine tool and hydro carbon oil as dielectric fluid, a hard layer can be created on the work piece with a powder met allergy (P /M). This novel method is also called as electro-discharge coating (EDC).
A schematic diagram of the principle of EDC is show n in Fig. 1. The process of EDC begins with erosive on of the tool during EDM, followed b y creation of hard carbides through the chemical react ion between the worn electrode materials (M) and the carbon particles (C) decomposed from hydro carbon fluid under high temperature. The carbide (MC) is gradually pile d up on the work surface and becomes a thick hard layer in a few minutes.
Electro-discharge coating is basically a process of mass transfer to the work surface. So, the tool materials should erode sufficiently to get transferred to the work surface in this type of treatment. P /M compact, either green or semi sintered, can play a vita l role as EDM tool, which can supply required material s to the work piece surface. The weak bonding among the powder particles helps in this regard. The other advantages of P/M tool s lie in the facts that they can be fabricated easily by mixing powders of any composition and can be given various shapes with less effort. The proper ties of P/M tool s can be controlled by varying compaction pressure and sintering temperature.
Thus, the P/M tools help in modifying the surface integrity of a work surface.
The EDM is a complex phenomenon which is depends upon a number of variables. A small change in one variable can abruptly change the output. So it is very difficult to establish models that accurately correlate the processing condition s with processing results by mean s of math emetic relational expressions [2 – 8]. Even though some efforts have been made, a complete model that takes account of the physical process has not yet been described in detail. Prediction of material removal rate (MRR) and surface finish in EDM process based on physical and empirical model s have been tried by some researchers.
End eaves have been made by many researchers to intentionally modify the work surface using ED M by depositing a layer on it. But any mathematical, physical or empirical model to depict the change in surface integrity by h eats and mass transfer due to discharge, breaking up of dielectric fluid, and diffusion of electrode material s ha s not been reported until now. Such studies need to combine two complex phenomena; one is electro discharge and another is mass transfer or diffuses on process in electrical discharges and that is again extremely complicated to model. Attempt to model the entire phenomena even by the most elegant numerical technique would require many assumptions which might b e far away from reality.
The aspect of surface modification by EDM ha s enough scope to be explored. Application o f artificial neural network (ANN) in this process h as not been reported yet.
In the present paper, ANN has been applied to model the surface modification phenomena by EDM with tungsten –copper (W – Cu) P/M sintered electrodes. An attempt has been made to correlate the in put parameters such as compaction pressure (C P), sintering temperature (ST), peak current (Ip), pulse on time (Ton), pulse off time (T off) with output measures like material transfer rate (MT R), and average layer thickness (LT) . Thus, it would help in predicting the MTR and LT with various input parameter combinations.
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