STUDIES ON ELECTRO CHEMICAL HONING (ECH) OF EXTERNAL CYLINDRICAL SURFACES

Abstract

Surface roughness has become the most significant functional requirement and it is an index of product quality. Nowadays, the manufacturing industries are specially focusing on dimensional accuracy and surface finish. With the new development of technology, more and more challenging problems are faced by the scientists and technologists in the field of manufacturing. The rapid development in advanced technological industries like aerospace, automobile, nuclear power and turbine industries has been accompanied by the development of very hard, high strength, difficult-to-machine materials and non-ferrous materials such as super alloys, stainless steel, titanium and its alloys, inconel, incolay, nimonics, composites etc. having high strength to weight ratio and low machinability [1]. Producing complicated geometries and maintaining high dimensional accuracy in such materials become extremely difficult with the conventional machining methods. The growing demand for these parts with close dimensional and geometrical tolerances necessitates an advance and efficient finishing method like electrochemical honing (ECH) which can effectively meet in order to achieve the best possible surface finish and highest material removal rate at an atomic scale by electrolytic dissolution and abrasive honing action [2]. In last few decades, electrochemical honing has emerged as a most potential hybrid micro-finishing technique. The concept of taking help of electrolyte to enhance the work and productivity of mechanical honing developed during 1963-1965 [3-4]. Though the concept dates back to 1970s, and that time most of the research work was focused on electrochemical honing of internal cylinders and also ECH of gears. This ECH process was picked up by Chen et. al. in 1981 after it was initiated by Capello and Bertoglio in 1979. ECH is one of the latest advanced finishing process which can be used for finishing of gears and cylindrical surface components. The requirements of fine finishing of work surface also pull the research interest towards the use electrochemical honing process. However, so far the investigations were carried out to provide detailed study on precision finishing of internal cylinders as well as various gears. Now it is an one attempt to explore the process capabilities of Ti alloys as well as EN8 alloys to explore the effect of influencing process parameters on surface topography and surface integrity of machined surface and process capability. Cylindrical shafts are used to provide motion to any mechanical devices and employed to transmit power motion efficiently for smooth and noiseless operations. These round shafts are very vital element for noiseless and efficient power transmission for any mechanical equipment because of smooth and error free surface. For smooth and noiseless operation of any mechanical device a smooth finishing of shafts are desirable. The finishing of external surface of cylindrical shaft plays a crucial role in overall service performance and to improve the life span of any mechanical device. Conventional finishing processes such as turning, facing, super finishing, honing, lapping, etc. are presently popular in industries. In these processes, material is removed by shearing action owing to severe plastic deformation caused by relative motion between tool and workpiece and therefore, the tool material should be harder than workpiece material. Thus, these processes are to some extent limited by hardness of workpiece material. In addition, these are costly and time consuming. These shortcomings necessitate the 32 exploration of cost effective and advanced super finishing processes in which the energy sources used for material removal is quite different from the traditional processes. Generally, honing process is applied for finishing of internal and external cylindrical component surfaces by honing action due to which high smooth surface takes place. Many engineering components like engine valves, gun barrels, cylindrical shafts, resurfacing of cylinder lines, internal cylinders, hydraulic cylinders, pistons, bearing bores, pin holes etc., requires finishing to finer levels than honing which ECH can replace [5]. It is generally used for removing surface irregularities, burrs, reducing surface stress and producing high wear resistance surface. Other advantages include the ability to create round and straight linear holes in relatively long cylindrical work pieces and improve the surface micro irregularities like waviness , cylindricity, circularity and out of roundness etc. The abrasive action of honing tool and rotary and reciprocation motion of tool leads to achieve the closer tolerances of desired work pieces. Titanium is one of the most conductive material which is very difficult to machine by traditional as well as non-traditional machining methods and is highly potential among other super alloys having high strength to weight ratio and low machinability [6]. Titanium and its alloys are widely used in aerospace, marine and automotive industries, prosthetic devices, nuclear industries, chemical processing equipments etc. due to their specific strength, strong corrosion resistance, biological compatibility and ability to retain high strength at elevated temperatures . However, titanium and its alloys have poor machinability due to their low thermal conductivity, high chemical reactivity and low modulus of elasticity. During machining of titanium and its alloys by conventional processes, the above characteristics lead to high temperature & rapid tool wear [7-8]. The main issues related to poor machinability of titanium by conventional machining is replaced by non conventional machining of ECH process. Keeping in view the severity of machining of titanium and its alloys by conventional machining processes, the need of development of newer concept for machining of titanium by ECH process is explored and has given rise to design and development of an indigenized tooling setup for ECH of external cylindrical components. The process can produce a good surface finish and moreover, it has no damaging effect on the mechanical properties of the metal (Burr & Oliver, 1968) [9]. In last few decades, Electrochemical Honing (ECH) has emerged as a most potential hybrid micro-finishing technique. The present study discusses the precision finishing of external cylindrical surfaces of Titanium alloys as well as EN8 steel alloys by ECH process to explore the effect of influencing process parameters on surface topography and surface integrity of machined surface, and process capability. For the present study, an experimental setup with tool drive and tooling system has been indigenously developed. With the assistance of this tooling system, the developed machine setup can provide the versatility of running electrochemical honing, Electrochemical machining (ECM) and honing process in a single setup beside incorporating different diameters of cylindrical shafts with minimum standard length. The experimental investigation has been 33 conducted into three phases: pilot experiments, main experiments and confirmation experiments. Pilot experiments have been carried out to study the effect of processing time, electrolyte concentration, electrolyte composition, electrolyte temperature, inter electrode gap, electrolyte flow rate, electrolyte pressure, abrasive grit size and current on measures of process performance i.e. percentage Improvement in average surface roughness (PIRa), percentage improvement in root mean square surface roughness (PIRq), percentage improvement in highest peak level surface roughness (PIRz), percentage improvement in cylindricity (PICyl), percentage improvement in circularity (PICir) for Titanium alloys and EN8 steel alloys. The surface roughness, cylindricity and circularity values of the samples before and after the process are measured with a Wyko NT 1100 optical profilometer interfaced with Vision 32 software to find out the percentage improvement in surface roughness , cylindricity and circularity values. Ten separate measurements have been taken on cylindrical shaft up to 65 mm length along the face and the average value is used for further analysis. Material removal rate is measured by calculating amount of metal removed per unit processing time. The experimentation to find out the optimal values of processing time, electrolyte concentration, electrolyte composition, electrolyte temperature, inter electrode gap, electrolyte flow rate, electrolyte pressure, abrasive grit size and current have been conducted following One-Factor-At-a-Time (OFAT) technique and the optimal values are identified by graphical analysis. Based on the results of pilot experiments of Titanium alloys, 08 minute processing time, 15.00% concentration of pure NaCl electrolyte solution and 10.00% concentration of pure NaNO3 electrolyte solution, 300C electrolyte temperature, 0.50 mm inter- electrolyte gap, 30 l/min electrolyte flow-rate, 1 MPa electrolyte pressure, SiC abrasive grit size of 600 and 70 A current are found optimum. The main experiments have been carried out to explore the influence of input process parameters; voltage, rotating speed of ECH tool and electrolyte concentration on process performance characteristics. The trial runs are designed according to the Box Behnken Design (BBD) of Response Surface Methodology (RSM) in which voltage, rotating speed of ECH tool and electrolyte concentration are varied in the ranges of 25-35 V, 60-100 rpm, and 5% - 15% concentration respectively. Analysis of Variance (ANOVA) and parametric optimization are carried out with the help of Design Expert (Version: 5.0.8) software for analyzing the results in order to identify the optimum process performance characteristics with a particular combination of input process parameters. Voltage and electrolyte concentration have been found to have significant effect on ECH process performance characteristics while rotating speed of ECH tool is found to have insignificant effect. The surface finishing and material removal rate are found to improve with the simultaneous increase in current and electrolyte concentration and attain higher value at voltage of 35 V and electrolyte concentration of 15% pure NaCl solution. However, it is evident that process performance characteristics initially improve with increasing rotating speed upto 80 rpm and then starts declining indicating the presence of an optimum value of rotating speed for ECH tool is between 60 rpm and 100 rpm. The input process parameters used in the study are independent of each other and as a result of this the interactive effects of input process parameters on process performance characteristics are found insignificant. Regression models for PIRa, PIRq, PIRz, PICyl and PICir have been developed. The coefficient of determination for 34 developed regression models for PIRa, PIRq, PIRz, PICyl and PICir are found as 0.9738, 0.9910, 0.9880, 0.9931 and 0.9942 respectively which can provide flexibility to the process users in deciding potential application of ECH of external cylindrical surfaces of Ti alloys. It is obvious that the developed models are suitable in predicting the results with reasonable accuracy (3.91% prediction error for PIRa, 4.92% prediction error for PIRq, 5.32% prediction error for PIRz, 3.64% prediction error for PICyl and 7.89% prediction error for PICir). Multi-objective optimization has been carried out on the basis of desirability analysis and 35V voltage, 78 rpm rotating speed of ECH Tool and 15% electrolyte concentration of NaCl pure solution are found optimum for desirability value of 0.987. Experimental investigation has been also carried out for EN8 Steel alloy using continuous power supply to explore the effect of surface quality improvement and material removal rate of the process. The trend showing the effect of processing time using continuous current on process performance characteristics is found similar to that of ECH of external cylinders of Titanium alloys. Though it is evident that time taken for electrochemical honing process of EN8 steel alloy is on higher side compared to the ECH process of Titanium alloys. The process performance characteristics are found to improve with increase in processing time upto processing time of 12 minutes and after 12 minutes the rate of increment becomes marginal. The range of finishing time is 03 -15 Min. It is evident from the study that voltage and rotating speed of tool and have significant influence on measures of process performance. However, it is evident that process performance characteristics initially improve with increasing rotating speed upto 65 rpm and then starts declining indicating the presence of an optimum value of rotating speed for ECH of external cylinders in between 50 rpm and 80 rpm. The input process parameters used in the study are independent of each other and as a result of this the interactive effects of input process parameters on process performance characteristics are insignificant. This is also evident from percentage contribution of interacting factors. Regression models for PIRa, PIRq, PIRz, PICyl and PICir have been developed which can provide flexibility to the process users in deciding potential application of ECH of external cylinders of EN8 Steel alloys. The coefficient of determination for developed regression models for PIRa, PIRq, PIRz, PICyl and PICir are found as 0.9472, 0.9792, 0.9636, 0.9839 and 0.9646 respectively. It shows that all the values are closer to unity indicating a good degree of fit between experimental data and developed models. It is obvious that the developed models are suitable in predicting the results with reasonable accuracy (2.32% prediction error for PIRa, 4.65% prediction error for PIRq, 3.16% prediction error for PIRz, 4.35% prediction error for PICyl and 4.68% prediction error for PICir). Multi-objective optimization has been carried out on the basis of desirability analysis and 30 V voltage, 65 rpm rotating speed and 10% electrolyte concentration of 3/4th NaCl and 1/4th NaNO3 electrolyte composition are found optimum for desirability value of 0.993. Using these optimum values of input process parameters, confirmation experiments have been conducted to validate the analytical part to study and very low value of average error in prediction is found. It is obvious from the study that proper combination of both the salt is required to obtain optimal electrolyte composition for improved process performance 35 characteristics. It is evident from the study that the optimal solution must contain approximately 75% NaCl and 25% NaNO3. It is observed that at optimum setting of electrolyte composition, the process provides approximately 7% and 21% higher improvement in PIRa value than the process runs using pure NaCl and pure NaNO3 respectively. It is obvious that ECH of EN8 alloys with 75% NaCl and 25% NaNO3 solution confers better surface finish with higher material removal rate than ECH of EN8 steel alloys with single salt solution NaCl or NaNO3. However It is also observed that ECH of Ti alloy provides better surface finish with higher material removal rate than ECH of EN8 steel alloys. In short ECH of Titanium alloys, 08 minute processing time, 15.00% concentration of pure NaCl electrolyte solution and 10.00% concentration of pure NaNO3 electrolyte solution, 300C electrolyte temperature, 0.50 mm inter- electrode gap, 30 l/min electrolyte flow-rate, 1 MPa electrolyte pressure, SiC abrasive grit size of 600 and 70 A current are found to be optimum setting parameters from pilot experiments. From main and confirmation experiments, 35 V voltage, 78 rpm rotating speed, 15% electrolyte concentration of pure NaCl electrolyte solution are found as optimum and at the optimum setting of parameters the process shows 74.52% as PIRa, 73.95% as PIRq, 55.89% as PIRz, 68.52% as PICyl and 60.12% as PICir of cylindrical parts respectively. Similarly, in short ECH of EN8 steel alloys, 12 minute processing time,10.00% electrolyte concentration of 3/4th NaCl and 1/4th NaNO3 electrolyte composition, 350C electrolyte temperature, 0.50 mm inter- electrode gap, 25 l/min electrolyte flow-rate, 1.5 MPa electrolyte pressure, SiC abrasive grit size of 1200 and 50 A current are found to be optimum setting parameters from pilot experiments. From main and confirmation experiments, 30 V voltage, 65 rpm rotating speed, 10% electrolyte concentration 3/4th NaCl and 1/4th NaNO3 electrolyte composition are found as optimum and at the optimum setting of parameters the process shows 68.45% as PIRa, 60.78% as PIRq, 59.23% as PIRz, 66.45% as PICyl and 58.24% as PICir of cylindrical parts respectively. A comparative study between electrochemical honed external cylindrical surfaces of Titanium alloys and EN8 steel alloys has been carried to demonstrate the capability of the process in improving surface quality of cylindrical parts. It is found that in case of ECH the electrolytic dissolution enhances the process capability by improving surface quality, cylindricity and circularity. It is evident that PIRa, PIRq, PIRz, PICyl and PICir values of ECH of Ti alloys are approximately 1.5 to 2.0 times higher than respective values of ECH of EN8 steel alloys. The surface characteristics of electrochemical honed surface have been analyzed and it is found that ECH improves the surface quality by removing the small pits, micro-burrs, feed marks and irregularities. No significant changes in surface / sub-surface micro-structure and micro-hardness have been noticed. A significant reduction in surface roughness value has been observed in processed surfaces. 36 ANN modeling of process performance characteristics indicates good agreement between neural network predictions and experimental values. It is believed that ANN requires much more number of experiments than RSM to form a well structured model. ANN modeling has been found to work well even with relatively less data if data is statistically well distributed in input domain. A comparative study of ANN modeling and RSM modeling has been carried out and it is observed that the ANN process is best suited than RSM for modelling the process performance characteristics of ECH of external cylindrical surfaces of Ti alloy as well as EN8 steel alloys even with less number of experimental runs. The optimized values of input process parameters are given in tabular form at the end of the thesis to help the potential users of the process. Keywords: Advanced machining process (AMP); hybrid machining process (HMP); electrochemical machining (ECM); honing; electrochemical honing (ECH); titanium alloy (TI 6Al 4V); EN8 steel alloy (EN8); external cylindrical surfaces; Box Behnken Design (BBD); analysis of variance (ANOVA); response surface methodology (RSM); desirability analysis; surface integrity aspects; ANN modelling. Organization of Thesis Thesis has been organized in following eight chapters. Chapter-1: It presents brief introduction of advanced machining processes and hybrid processes. Electrochemical honing process and its constituent process i.e. electrochemical machining and honing have also been discussed. Chapter- 2: It contains the comparative study of various internal cylinders and gear finishing processes and comprehensive review of past research work. On the basis of literature review, gaps in past research work are identified and the objectives of the present research work have been outlined. Chapter- 3: It describes the details of designed and developed experimental setup and the material selection criteria for different components of the machine setup. Chapter- 4: It provides the detailed discussions about electrochemical honing process parameters, workpiece selection, electrolyte selection, parametric study to find out ranges and levels of input process variables and fixed parameters, process performance characteristics and design of experiments technique to plan the experimental runs. Chapter- 5: It contains the experimental outcomes of pilot, main and confirmation experimentation and analysis of the outcomes to investigate the optimum parametric combinations for electrochemical honing of Titanium alloys and to obtain the better improvement in surface quality , cylindricity and circularity with higher material removal rate. Chapter- 6: It describes the experimental outcomes of pilot, main and confirmation experimentation and analysis of the outcomes to investigate the optimum parametric combinations for electrochemical honing of EN8 steel alloys to obtain the better improvement in surface quality, cylindricity and circularity with higher material removal rate. Chapter -7: It describes the artificial neural network (ANN) modeling of electrochemical honing process parameters for predicting the values of process performance characteristics. A 37 comparative study between ANN and RSM has also been discussed in this chapter for Ti alloy as well as EN8 alloy. Chapter- 8: It contains the conclusions of present research work and future scope for further study.