Parametric Study and Optimization of Wedm Process Parameters of Pure Titanium

Abstract

Titanium has been recognized as an element (symbol Ti; atomic number 22 and atomic weight 47.9) for at least 200 years. High strength, low density and excellent corrosion resistance are the main property that makes titanium attractive for a variety of applications. The major application of the material is in the aerospace industry, both in airframes and engine components. Non aerospace applications take advantage mainly of their excellent strength properties, for example steam turbine blades, superconductors, missiles etc. or corrosion resistance, for example marine services, chemical, petrochemical, electronics industry, biomedical instruments etc. Pure titanium offers good corrosion resistance in most environments, excluding those containing fluoride ions where it cannot compete with some ceramics, tantalum and various high-nickel alloys. In fluoride-free environments, titanium is cost effective when competing with high-alloy, corrosion-resistant materials such as Hastalloy. When compared with stainless steel, titanium has a much superior technical performance but would not be selected over commodity products such as ferritic and austenitic stainless steels as it is not cost-effective. Several problems such as chatter formation, lower cutting speed and generation of deformed machined surface are observed during conventional machining of titanium and its alloys. Thus, there is a crucial need for reliable and cost effective methods for machining of pure titanium. Over the passage of time, there have been great advancements in development of cutting tools including coated carbides, cubic boron nitrides and polycrystalline diamond. These tools have been successfully applied in machining of steels, high temperature alloys such as nickel based alloys and cast iron, but none of these is found truly applicable in machining of titanium alloys. Attempts have been made for cryogenic machining of titanium alloys by cooling the work piece or tools using a cryogenic coolant. But, even these approaches have inherent limitations. Some studies have proved that use of cutting fluids may improve machinability of titanium alloys using conventional machining. However, toxicity of cutting fluids seriously degrades the quality of machining environment. Keeping in view the difficulties associated with conventional machining of titanium, attempts can be made for machining of these using non conventional machining such as, electric discharge machining (EDM), abrasive water jet machining (AWJ), laser beam machining (LBM), ultrasonic machining (USM). Non conventional machining techniques such as AWJ, LBM can be used, but, the cost of vii equipment is high, height of the work-piece is a constraint as well as the accuracy and surface finish problems come into picture. Ultrasonic machining imparts better surface characteristics on the work piece; however, metal removal rate is very low, coupled with a relatively higher tool wear rate. On the other hand, a technique such as wire electric discharge machining (WEDM) seems to be a better choice as it can machine parts with complicated geometries and intricate shapes. This research work is mainly focused on WEDM of pure titanium (grade-2). An attempt has been made to model the eight response variables i.e. machining rate, surface roughness, material removal rate (MRR), overcut, dimensional deviation, wire wear ratio, surface crack size density and recast layer thickness in WEDM process using response surface methodology. The experimental plan is based on Box-Behnken design. The six parameters i.e. pulse on time, pulse off time, peak current, spark gap voltage, wire feed and wire tension have been varied to investigate their effect on output responses. These responses have been optimized using multi-response optimization through desirability. The ANOVA has been applied to identify the significance of developed model. The test results confirm the validity and adequacy of the developed RSM model. Finally, the optimum parametric setting has been designed for the optimization of process. An attempt has also been made to construct a micromodel for prediction of material removal rate and surface roughness using dimensional analysis. The present research work is also mainly focused on the investigation of integrity of the work surface and wire electrode surface after machining with WEDM. Experimental results showed that pulse on time, pulse off time and peak current significantly affected the surface integrity with the formation of deep-wide overlapping craters, pock marks, debris, micro cracks and recast layer. Both carbides and oxides were formed either in free form and/or in compound form due to decomposition of de-ionized water, machined samples and wire material. The compounds like titanium dioxide (rutile) (TiO2), (TiO0.325), Ti2O3, Ilmenite (Fe2Ti4O), titanium carbides (TiC) and copper titanium dioxide (Cu3TiO4) were formed due to phase transformations that were analyzed through X-ray diffraction and energy dispersive Xray method. The effect of process parameters on the wear of wire surface has also been considered.