STUDIES IN ELECTROLYTIC MAGNETIC ABRASIVE FINISHING PROCESS

Abstract

Production industries have several challenges. Processing of material to obtain desired size and shape by conventional techniques such as casting, welding, forming and machining is generally used in engineering industries. Machining enjoys the benefit of high accuracy and surface finish. It is required to machine variety of materials ranging from soft materials like plastics to very hard materials such as diamond. Metal-matrix composites, super alloys, ceramics and other high strength and high temperature resistant alloys are some of the materials which are difficult to machine by the traditional machining processes and these types of materials are known as difficult-to-machine (DTM) materials. The traditional machining processes are unable to provide required surface finish in the DTM materials at high economic cutting speed and involves high capital investment and labor cost. The need for finishing complex geometrical shapes with close tolerances at high production rates, miniaturization of engineering components and the problems faced in controlling the process output characteristics by use of the traditional machining processes are some of the driving forces behind development of the advanced machining processes (AMPs). The AMPs can deliver high accuracies as material is removed in the form of atoms or molecules individually or in groups unlike traditional machining processes where material is removed in the form of chips. The AMPs which use a single form of energy for material removal and finishing are limited by low finishing efficiency and restriction of the materials that can be ii machined. Combination of two or more advanced machining processes is used to incorporate the relative advantages of the constituent processes. Such combinations of two or more constituent processes for obtaining better surface finish are known as compound surface finishing methods. Magnetic field assisted machining processes are compound surface finishing methods which are effective in finishing, deburring and burnishing of metals and advanced engineering materials. Electrolytic Magnetic Abrasive Finishing (EMAF) process is one of the recent magnetic field assisted ultra-precision finishing technologies, which has not been fully investigated though it has become an linportant technique for processing of DTM materials (Kim et. al., 1998). EMAF process can simultaneously provide high material removal rate and good surface finish to the workpiece. Jin et al. (1992) were the first to propose this finishing process called Electrolytic Magnetic Abrasive Finishing which combined two processes namely electrolytic and abrasive machining assisted by a magnetic field to obtain high surface finish. In this process, the combined• action of the electric and magnetic fields is applied on the ferromagnetic abrasive particles so as to suitably change the, path of the anions in the electrolyte. This change in path of the ionic particles is responsible for enhanced process capabilities through selective dissolution of the crests- of surface irregularities with respect to the troughs. In addition, machining mechanism of the electrolytic process is dependent on electrochemical properties and not only on the mechanical properties of the workpiece materials. This allows finishing of tough, hard and high strength workpiece materials easily. Comparative study has shown that the EMAF process is 1.8 to 1.9 times faster than the allied MAF process (Yan et. al. 2003). Published literature on the EMAF process mainly deals with the qualitative description and the effect of magnetic field on material removal rate (MRR ) and surface finish. Thus, the effect of process parameters on process performance has been partially understood. Moreover, little published work is available on modeling and optimization of the process. In addition, optimization of the input process parameters on the performance characteristics like material removal rate (MRR), work surface finish, improvement in cylindricity of cylindrical parts, etc. has not been reported. The dynamics behind finishing of the surface irregularities by the EMAF process have not been well understood and little information is available about EMAFed worksurface characteristics. In view of the above, the present research work was undertaken with the following objectives: 1. To develop an EMAF setup for controlled variation of various EMAF input process parameters. 2. To investigate the effect of key input process parameters (inter-electrode gap voltage, rotational speed of the workpiece, inter-electrode gap, machining time, magnetic induction intensity, electrolyte concentration and electrolyte temperature.) on performance characteristics like MRR, worksurface roughness (Ra and Rt) and workpiece cylindricity by using Response Surface Methodology (RSM). iv 3. To develop empirical relationship between the input process parameters and the process output characteristics (response characteristics) by developing regression models. 4. To investigate the interactions between various input process and their effect on the process output characteristics like MRR, worksurface finish and workpiece cylindricity. 5. To develop physical model for material removal from surface for better understanding of material removal and surface finish related aspects. 6. To carry out multi-response optimization of the process parameters by using statistical technique and by artificial intelligent technique (neural network). An electrolytic magnetic abrasive finishing setup is designed and developed indigenously keeping in mind the research objectives and various design considerations and constraints. The setup consists of four major sub-systems: machining unit, power supply, electrolyte temperature control unit, and the drive for magnetic carriage. The machining unit and the magnetic carriage drive were mounted on a center lathe. The rotable work piece (anode) was located between two centers; i.e. live centre of the head stock of a centre lathe and the dead centre of an adjustable fixture The cathode tool which is made of brass in the form of a V-threaded screw is attached to the annular copper body of the machining unit The brass tool (cathode) can be precisely located with respect to the workpiece and locked at any position. The magnetic poles are likewise precisely located with respect to the cylindrical workpiece. The magnet solenoids, poles, cathode and the machining unit function as an assembly and are mounted on an insulated carriage. The carriage is given reciprocating motion by scotch-yoke mechanism though a connecting rod which ultimately results in reciprocation of the assembly in horizontal direction. Thus, the rotating cylindrical workpiece is subjected to a reciprocating magnetic field, and the combined effect of rotation (of workpiece) and reciprocation (of machining unit) is responsible for finishing of cylindrical workpieces. Two separate dc power supplies are used for the EMAF set-up; one for the electrolytic process and the other for generating magnetic field. The power supplies have a dc potential in the range of 0-30V (adjustable) and current adjustable in the range of 0-10A and operate as constant voltage sources. The positive terminal is connected to the workpiece through carbon-brushes, slip-ring, connectors and front locating pin. The negative terminal of electrolyte power supply is connected talthe cathode tool. The magnetic solenoids when energized can generate field strength upto 20000 Gauss. The electrolyte temperature controlling unit consists of a refrigeration unit, heating coil arrangement and a double walled electrolyte tank with stirrer pump, temperature sensor, thermostat and temperature controlling unit. This unit allows abrasive-laden electrolyte temperature to be maintained within ±1°C. A stainless steel pump capable of generating wide range of pressure and flow rate was installed for electrolyte supply to the machining zone. Electrolyte pressure and flow controls were employed at the pump discharge. Phase I of the present experimental study involved identification of the process performance characteristics and the key input process parameters. The determination of vi range of process parameters for the experimental studies were based on literature review, recent practices, pilot experiments, work material characteristics and kinematics of the developed experimental setup. Measurements of the performance characteristics were carried out with high precision instruments using standard practices. Hot—rolled AISI 302 stainless steel in the form of cylindrical bars was selected as workpiece material because of its common industrial application in light weight transportation equipment, furnace and heat-exchanger parts and components subjected to severe chemical environments, etc. Better sitrffice finish of these parts decreases the frictional and hydraulic losses of the components. The magnetic field strength in the machining zone (corresponding to various magnetic pole gaps) was correlated with the electric current applied to the magnetic solenoids with the help of digital Gaussmeter and Hall-Probe arrangement. Selection of experimental design matrix for conducting the experiments was made with the sole criteria that the design should provide all relevant information on the effect of the input process parameters on the process output characteristics but would not lead to excessive experimentation. Face Centred Central Composite design based on Response Surface Methodology was found to be one of the experimental designs that fulfills the above mentioned requirement, and the same was used in the present research. Phase II: In phase II the effects of various process parameters on the process output characteristics were studied through experimentation. The key input process parameters which would significantly affect the process output characteristics were identified as the applied voltage between the electrodes, work piece rotational speed, inter- vii electrode gap, machining time, magnetic induction intensity, electrolyte temperature, electrolyte concentration, type of electrolyte used and characteristics of abrasive (type, grit size). Of the above mentioned nine input process parameters the first seven parameters were simultaneously varied because these were believed to affect the mean and standard deviation of the process output characteristics i.e. MRR, worksurface finish (Ra and Rt) and workpiece cylindricity. The last two input parameters namely type of electrolyte and abrasive characteristic were kept constant. A mixture of equal weight of sodium nitrate and potassium nitrate are used to make up the electrolyte because sometimes a mixture of two or more chemicals is best suited for an electrolyte. Larger abrasive particles (50 to 7511m) and high concentration of ferromagnetic particles (42% by wt.) were selected as they were considered to provide good results as regards MRR and surface characteristics of the EMAFed workpiece. As first part of experimentation, sixty four experiments in randomized sequence were conducted according to coded principal 27-1 fractional factorial design matrix. The vi second part of experimentation involved the conduct of four experiments, keeping all the variable input process parameters at mid-level value (central points). Measurement of the process output characteristics was carried out using standard practices and the average of outputs of two parts of experimentation was taken. The degree of curvature which indicates non-linearity of the output characteristics was estimated from the difference in average outputs obtained from the two parts of experimentation. The third part of experimentation viii involved the conduct of two experiments for each parameter [one at high level (level III) and the other at low level (level I)] thereby leading to fourteen experiments for the seven variable input parameters and measurement of the output responses. Phase III: This phase of experiments was carried out to investigate the effect of seven selected input process parameters on the quality characteristics of the EMAF process. Attempts have been made to quantify the effect of each of the selected input process parameter on MRR, worksurface roughness (Ra and Rt) and workpiece cylindricity through optical piofilofnetry, scanning electron micrography (SEM) and atomic force micrography(AFM). Model adequacy test for each of the process output characteristics was conducted and thereafter analysis of variance (ANOVA) was carried out to determine the significant process parameters and the regression models for MRR, reduction in surface roughness (Ra and Rt) and improvement in workpiece cylindricity were developed. The material removal was measured by weight loss technique by using a digital weighing balance (METTLER; U.S.A) with least count of 0.1mg. The R. and Rt values were measured by the use of optical profilometer and the measurement of cylindricity was carried out with an electronic dial gauge with least count of 0.1 pm. Analysis: Preliminary analysis of raw data obtained from estimated MRR and worksurface roughness (Ra and Rt) show that high MRR and moderate rate of improvement in R. and Rt is obtained for a combination of high level of applied voltage, high workpiece rotational speed and inter-electrode gap while the other variable input process parameters are kept at ix low level. It has been observed that a combination of high applied voltage with high magnetic induction intensity favors high rate of improvement in Ra and Rt within the limits of experimental conditions employed. Preliminary uni-variate experiments indicate that the EMAF process is highly interactive in nature. The interactions amongst the electrolytic, magnetic and abrasive parameters play a major role in the process. A proper coordination of electrolytic action, magnetic induction intensity and abrading action is essential for achieving good surface finish. The nature, composition and concentration of electrolyte are crucial for optimal performance characteristics. Similarly, selection of abrasive grit size and mixing ratio of abrasive and ferromagnetic particles are crucial for effective removal of the passive layer formed on the worksurface. Increased differential dissolution, viscous layer theory, highly reactive chemical attack, diffusion of the reaction products, abrasive removal of the paSsive layer and vortex action of the magnetic field on the passive layer formed result in high finish-machining of the surface. Increase in applied voltage increases MRR but deteriOrate the surface finish. Uneven breakdown of the passive layer formed on the surface at higher voltage probably result in pitting type corrosion at localized regions which deteriorate surface finish at higher applied voltage. Change in applied voltage also results in changed magnitude and direction of Lorentz's force which significantly affects the output characteristics. Increase in workpiece rotational speed up to a certain limit increases MRR. Higher rotational speed results in better removal of material but probably results in gradual thinning down of the passive layer (viscous layer) formed on the worksurface. At higher rotational speed, the concentration difference between the reaction products formed on the worksurface and the electrolyte decreases, resulting in low diffusion velocity of the reaction products from the worksurface as well as lower anodic reaction rate at the worksurface. Inter-electrode gap decides the electrolytic current density and magnetic induction intensity in the machining zone and hence is crucial for optimal process performance. At low inter-electrode gap the EMAF process behave more like an ECM process and the effect of magnetic field in preferential cutting of the crests of surface irregularities is not obtained. Major= improvements in surface finish are obtained in the first few minutes of the EMAF operation and subsequently the rate of successive surface finish improvement diminishes. High magnetic induction intensity deflects the path of anions to an extent that they cannot reach the anodic worksurface and undergo electrochemical reaction. Mid-level combination of magnetic induction intensity and electric field results in appropriate Lorentz's force which increases MRR and surface finish. Electrolyte temperature determines ionic mobility and mass transport phenomenon of the electrolyte and thereby affects the reaction rate at the machining zone. The process can rectify a worksurface having minor form deviations in roundness and cylindricity. Multi-variable regression models have been developed for Ra and Rt. The optimal results for the different process performance characteristics are reported and validated through confirmation experiments. Analysis of Variance (ANOVA) was performed on the experimental output data. The significant variables were identified and their effect on responses was studied. xi Empirical relations between response and input process parameters in the form of regression models were developed to study the effect of different input process parameters and their interactions on the output characteristics. It has been observed that the applied voltage, the inter-electrode gap and the electrolyte concentration play a major role in governing the MRR. First order interaction of electrolyte temperature and electrolyte concentration and linear by linear interaction of electrolyte temperature also results in increased MRR. It has been observed that combination of applied voltage and workpiece rotational speed has considerable impact on average centerline worksurface roughness (Ra) value. The results also indicate that first order interaction of applied voltage and workpiece rotational speed may improve or deteriorate worksurface roughness (Ra) depending on the level of combination of these two input process parameters. Applied voltage, workpiece rotational speed, machining time, electrolyte temperature and electrolyte concentration were found to be the major input process parameters which affect the average peak-to- valley height of surface roughness value (Rt). j. EMAFed surfaces of R. value greater than 50 nm (0.051m) is considered as a rough surface. The EMAF process, under normal operating conditions, leaves no measurable adverse effect on surface integrity of the workpiece. Two surface integrity aspects viz. variation of the microstructure and microhardness were studied which show that the integrity of the workpiece is unaltered by the machining conditions of the EMAF process employed in the present investigations. It is an expected result because the process relies on formation and removal of passive layer by brushing action of the magnetic-field xii assisted abrasive particles A physical model of the system has been developed and a mathematical model has been formulated for parametric study of the decrease in peak-to-valley surface roughness height with machining time at the micro-geometrical and macro-geometrical level. It has been observed that the Lorentz's force, current efficiency, gram atomic weight of the metallic ions, conductivity of the electrolyte and density of the anode work material play a major role in super-finishing of worksurface at micro-geometrical level. For super-finishing the surface at macro-geometrical level of a work piece by EMAF process, the height of the surface irregularity and the inter-electrode gap are found to be the two important governing process input parameters. Process modeling and multi-response optimization has been carried out to obtain an optimal set of the process parameters that would result in the best performance of the process. The optimization module in Design-Expert (DX 6.0.8) searches for a combination of input process parameter levels that simultaneously satisfies the requirements placed on each of the responses characteristics and input process parameters. For optimization, each response was analyzed in order to establish the appropriate model. Multi-response objective function (desirability function) is formulated to obtain the numerically optimized solutions which give the different combinations of the input process parameters for obtaining the desired output process characteristics. The models are validated by conducting experiments at the predicted optimal settings. Artificial neural network is used for single response optimization of the output process characteristics. Back-propagation neural network using GDLM technique was also used to train the network using a sets of experimental input and output data for all the output process characteristics viz. MRR, reduction in surface roughness (Ra and Rt) and improvement in workpiece cylindricity. Another set of experimental data is considered for comparison with the simulated output from the network and the estimation error is found to be minimum indicating that the network is properly trained for inference. xiv