SOME STUDIES ON PRODUCT RECOVERY THROUGH ECH OF CLAD SURFACES

Abstract

The manufacturing of precise parts constantly deals with high labour intensive, most critical and least controllable nature. In the advanced industries like aerospace, automobile, nuclear power and turbine industries have been accompanied by the development of high strength, very hard, difficult-to-machine, non-ferrous materials and alloys. In order to enhance the tribological properties, fatigue strength and corrosion resistance, a sensibly better surface finish is required. Manufacturing complex part geometries and maintaining high precision in such advanced materials become extremely difficult with the conventional machining methods. Based on the literature review, the major disadvantages associated with the conventional surface finishing processes are (i) it suffers from low finishing rate, high tool wear, and high probability of surface damage due to existence of point forces on the workpiece surface, (ii) the hardness of the workpiece surface act as a determining factor and even sometimes such materials are very difficult-to-machine using conventional methods (iii) it suffers from low geometrical accuracy, which is almost impossible to attain up to the desire level for such advance materials by conventional methods. Electrochemical honing (ECH) was developed in last few decades to break the boundaries of traditional finishing processes in terms of higher tool hardness requirement and precise control of finishing forces during operation. It has a ability to finish harder materials with higher precision and surface finish. Owing to such capabilities, in many industries such as aerospace, automobile, petrochemical reactor, mould and dies, roller, gear industries, etc., ECH is used extensively. Almost all types of metals can be finished by this process. The unique capabilities of ECH process are that material of any hardness can be finished provided it is electrically conductive, high production rate: up to 5-10 times faster than conventional honing and up to 4 times faster than grinding, no tool wear occurs, no spark is produced and the temperature generated is low which does not cause metallurgical changes in the work surface material, less abrasive pressure required, nano finish achieve: of order of 0.05 μm, high tolerance: up to ± 2 μm. It is reported that the ECH is an ideal choice for finishing of hard and critical form of mechanical parts. The aim of the present work is to deal with ECH process with highly demanded advanced materials such as EN52, X40CrSiMo10-2 alloy steel and SS316. Based on the literature review, it is found that these material are used to recover the damage or worn- 31 out surfaces of the mechanical parts, such as engine valve face and crank shafts. To deposit the selected material on the work surface, many cladding techniques were established. From the recent research studies, it is found that the plasma transferred arc (PTA), high velocity oxy-fuel (HVOF) cladding for recovery of the engine valve face and twin wire arc (TWA) cladding for recovery of the shafts gives significant outcomes as compared to other material deposition techniques. Finally, research work has been attempted to investigate the process parameters, namely processing time, current, voltage, electrolyte composition, electrolyte concentration, electrolyte temperature, inter-electrode gap (IEG), rotating speed and honing pressure on the improvement of surface quality and geometrical accuracy of the recovered surface of the engine valve face and the external cylindrical shafts. The percentage improvement in average surface roughness (PIRa), maximum surface roughness (PIRt) and out-of-roundness (PIOR) have been used as the measures of ECH process performance. The effects of processing time, has been studied using graphical method. The effect of key input parameters have been studied using an appropriate design of experiment techniques. The experimental results provide useful guidelines for the user for proper selection of conditions for obtaining a good surface quality. The surface integrity aspect of ECH processed surface were also analysed using optical profile metre, scanning electron microscopy (SEM) and atomic force microscopy (AFM) test. It is an evident from the study that the process performance characteristics improve with increasing processing time up to 3 minutes and after 3 minute the rate of increment becomes marginal. Further, increase in processing time does not influence significantly to the process performance characteristics. It is found from the experimental investigation that the process performance characteristics improve with increasing voltage and achieve highest value near 30 V. Above 30 V, it is observed that the rate of increment slows down and the trend declines. While, the improvement in out-of-roundness still increase with increasing voltage. It is observed that the surface roughness continuously increases with increases in percentage of NaNO3 in electrolyte solution. But, the improvement in out-of-roundness marginally fall down with the addition of NaNO3. Hence, for the further study mixture of 80% NaCl and 20% NaNO3 has been selected as optimum electrolyte composition. It shows that the PIRa, PIRt and PIOR values increase with increase in electrolyte temperature up to a level of 340C and then it starts decreasing with a marginal difference. Also, the results show that 60 rpm selected as a rotating speed that gives more improvement in surface roughness. It is evident that the monitored outputs improve with 32 the increasing value of current and attain higher value at 40 A. Also, with increasing current, the rate of electrolytic dissolution increases and thus, the material removal rate improve. The electrolyte concentration has a significant effect on measures of process performance characteristics. It is observed that the electrolyte concentration at 8.5 % provides maximum improvement in the monitored outputs. The economical analysis between new part and recovered part shows that the recovered products cost only 34.05 % of new products, while 99 % of material. The comparative analysis between micro-grinding and ECH process shows that the ECH process is more efficiently and effectively finish the work surface. Also, it is established that the complete 3D profile of the workpiece has been finished using the ECH process in one setting, while it is impossible with the micro-grinding process. It is clear from the experimental study that the process performance characteristics improve with increasing processing time up to 120 seconds and after 120 seconds the rate of increment becomes marginal. Through the results analysis, it is justified that the identified optimum parametric combination is 78.82:21.18 for NaCl:NaNO3 electrolyte mixture. The value of voltage at 30 V, provide the maximum improvements in the monitored outputs based on the optimum results of the main experiments. The increasing value IEG initially helps to improve the process performance up to 0.55 mm and then starts declining. Whereas, increment in rotating speed decreases the process performance characteristics. It is observed that the rotating speed is found to have relatively less effective on the measures of the process performance. Results show that 60 rpm selected as a rotating speed that gives more improvement in surface roughness. Multi-objective optimization has been carried out on the basis of desirability analysis to find the optimum values of input process parameters and at the optimum setting of parameters, the process shows 80.65%, 56.42%, and 39.39% improvement in average, maximum surface roughness and out-of-roundness values respectively. The economical analysis between new part and recovered part shows that the recovered products cost only 42.12 % of new products, while 97.5 % of material. The comparative analysis between micro-grinding and ECH process shows that the ECH process is more efficiently and effectively finish the work surface. The effect of axial feed rate on the monitored outputs is observed that axial feed rate has been found to have significant effect on the measures of the process performance. This shows that the PIRa, PIRt and PIOR values decreases with the increasing axial feed rate. Hence, low value of axial feed rate, 2.85 mm/min is selected as input process 33 parameters for the further study. It is evident from the study that the process performance characteristics improve with increasing processing time or number of passes in the developed experimental setup up to 90 seconds and 3 passes. and after 3 passes the rate of increment becomes marginal. The surface roughness and geometrical accuracy, improve with the addition of NaNO3 in the NaCl solution initially, but the average roughness certainly fall with the increasing content of NaNO3 beyond to 26 percent. Multi-objective optimization has been carried out on the basis of desirability analysis to find the optimum values of input process parameters and at the optimum setting of parameters, the process shows 87.65%, 62.39%, and 38.32% improvement in average, maximum surface roughness and out-of-roundness values respectively. The microstructure of the ECH processed surface shows a glazed appearance and uniform texture. Keywords: Product recovery, High velocity oxy-fuel (HVOF) cladding, Plasma transferred arc (PTA) cladding, Twin wire arc cladding (TWA), Advanced machining process (AMP),Hybrid machining process (HMP), Electrochemical machining (ECM), Honing, Electrochemical honing (ECH), Mixture D-Optimal Design; Box Behnken Design (BBD), Analysis of variance, Desirability analysis, Surface integrity aspects, Economical aspects, Comparative analysis Organization of Thesis Thesis has been organized in following seven chapters. Chapter 1 provides brief introduction of product recovery techniquies, electrochemical honing, convetional finishing processes. A short introduction on the process performance characteristics of finished clad surfaces have been included. Methods to recover the product is also discussed. Chapter 2 presents up to date comprehensive review on the significant work reported in literature in the field of applications of product recovery of functional surfaces. Literature review is divided into three categories (a) based on the material deposited techniques, (b) based on the conventional and non-conventional techniques reported in the past research work, and (c) based on the electrochemical honing of mechanical components with a brief detail of the more inflencing process parameters. Hence, in the light of research gaps, objectives of the present research study are outlined at the end of this chapter. 34 Chapter 3 presents a brief detail about the experimental setup design and development for the selected parts. To attempt the objectives of the study, two experimental setup namely (i) ECH of the engine valve face, (ii) ECH of cylindrical shafts has been indigenously designed and developed on large scale, which ultimately give the strength to the research contribution. The design and material of the experimental setup is selectively based on the literature review, availibily and machine constraints. With the assistance of modular tool housing, experimental setups can provide versatility of running ECH, ECM and honing process in a single setup and to incorporate different sizes of workpieces with the minute setup changeover. Chapter 4 presents the brief detail about (i) process parameters of ECH and their suitable ranges, (ii) workpiece selection criteria, (iii) electrolyte selection criteria, (iv) research methodology (i.e. case study-I, case study-II and case study-III) and product recovery economics. The procedure and the measuring instruments used to analyze the output measures are also examined in this chapter. Chapter 5 describes the experimental outcomes of three attempted case studies. Each case study passes through three phases of experimentations namely pilot experimentation, main experimentation and confirmation experimentation with the aim to optimize the process parameters with respect to the material and the shape of the workpiece. The optimum process parameters have been selected based on the highest value of the monitored outputs. The surface integrity aspects of the ECH processed surface have also been discussed to highlight surface topographical and micro-structural changes of the workpiece occurred due to the process. Chapter 6 presents a study on the economics of the product recovery by comparing the present remanufacturing cost with the new parts taken from the open market. Also, to study the research outputs with the conventional higher precision process such as micro-grinding with same workpiece conditions has been included in this chapter. By using the both the results, a comparison data in a tabular form are also included. Chapter 7 presents the contributions originating from each of the objectives detailed in the thesis chapters. The thesis concludes with scope of future work.