Improved Cooling unit with Automatic Temperature Controller for Enhancing the Life of Ice Bonded Abrasive Polishing Tool

Transcription

1 Improved Cooling unit with Automatic Temperature Controller for Enhancing the Life of Ice Bonded Abrasive Polishing Tool S.Rambabu 1 and N. Ramesh Babu 2 * 1 Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai , ramu.sarimalla@gmail.com 2* Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai , nrbabu@iitm.ac.in Abstract This paper presents an improved refrigeration unit with in-built cooling bath and automatic temperature controller for Ice Bonded Abrasive Polishing (IBAP) system. The polishing tool is prepared by freezing a slurry of water and fine abrasives with coolant (Isopropyl alcohol) being circulated around the tool holder. In the earlier setup, the preparation of tool took about 5 to 6 hours, since ethylene glycol is circulated through heat exchanger of refrigeration unit in order to reduce its temperature from ambient to -12 C. The temperature of coolant was maintained by manually controlling the refrigeration unit. The current refrigeration unit with in-built cooling bath aids in pre-cooling the coolant before it is circulated around the tool holder in IBAP setup. This arrangement could prepare the tool in about 2 hours, thus permitting the tool to polish the specimen for a longer duration. Preliminary experiments on polishing of flat Ti-6Al-4V specimen demonstrated the effectiveness of IBAP process by producing very good improvement in. The experimental results showed the promise of nano level with removal of cracks existing on the surface of Ti-6Al-4V specimen. keywords: Ice bonded, polishing, refrigeration, controller 1. Introduction Ultrafine surface generation processes have become prominent to produce ultrafine surfaces with good surface integrity on components used in the fields of precision machinery, optical, semiconductor and medical applications. The surface quality of components is directly related to the service performance. Nano level generation on components is possible by using conventional polishing as well as unconventional polishing processes. Conventional polishing processes employ bonded abrasives, i.e., honing and loose abrasives used in lapping process. However, bonded abrasive tools are subjected to periodic dressing which thus interrupts production. On the other hand, unconventional polishing processes such as magnetic assisted polishing, fluidised abrasive polishing, water jet polishing and chemical mechanical polishing (CMP), make use of loose abrasives. Among them, CMP is preferred over other methods to generate smooth flat surfaces. In CMP, chemical and mechanical forces act on work surface during polishing. However, this process requires precise feeding of slurry into polishing zone, maintenance of slurry characteristics with polishing temperature, periodic replacement of polishing pad in order to achieve consistent results in polishing. In view of these shortcomings, attempts were made to develop a frozen abrasive tool by freezing the mixture of water and abrasives that can continuously expose fresh abrasive particles due to melting of frozen tool, during polishing. Initially, an approach was proposed by Belyshkin (1966) to use a frozen mixture of water and abrasives as self dressing tool for polishing of glass and crystals. Zhang et al. (2001) used frozen colloidal silica for polishing of silicon wafer and zerodur glass and have achieved nanometric level of on work surfaces by maintaining the temperature of frozen tool at about 40 C with the supply of appropriate quantity of dry ice around the tool and polishing the specimen for about 70 minutes. Yuan and Ming (2005) prepared non-abrasive ice polishing disk by freezing the deionized water in a commercial freezer for about 24 hours. This tool was used for polishing of LY12 aluminium alloy and YG6 hard alloys for about 50 minutes. Recently, Mohan and Ramesh Babu (2010, 2011, 2012) developed Ice Bonded Abrasive Polishing (IBAP) setup for in-situ preparation of polishing tool to produce ultrafine on copper and 304L stainless steel materials. In this setup, liquid nitrogen was poured around the tool space in order to prepare the polishing tool and maintain the tool temperature during polishing. However, the temperature could not be maintained at 704-1

2 Improved Cooling unit with Automatic Temperature Controller for Enhancing the Life of Ice Bonded Abrasive Polishing Tool particular value during the preparation of tool due to which the polishing tool could last for 15 minutes. Boopalan et al. (2013) develop an IBAP setup that includes the application of vapor compression refrigeration unit. In this setup, the mixture of ethylene glycol (70 % by weight) and water (30 % by weight) circulated through heat exchanger of refrigeration unit in order to reduce the temperature of a coolant from ambient to -12 C. The polishing tool was prepared by circulating the coolant around the tool space in IBAP setup, and the coolant temperature was controlled by manually operating the refrigeration unit. This cooling unit took about 5-6 hours for preparation of 200mm diameter and 30 mm height tool and the same could be used only for about 30 minutes. From the above discussion, it is clear that maintaining the temperature of polishing tool is one of the challenges in IBAP process. Hence, the IBAP system essentially requires a cooling unit that can prepare the tool in a robust way and can improve the life of IBAP tool. The present work is aimed at development of cooling system with automatic temperature controller for quick preparation of IBAP tool that can last longer. This paper presents the details of refrigeration unit with in-built cooling bath and automatic temperature controller for IBAP system. Preliminary experiments were conducted on flat Ti-6Al-4V alloy specimen with improved cooling unit and the effectiveness of IBAP process was shown by measuring the roughness on polished surface. 2. Refrigeration unit with in-built cooling bath The vapor compression refrigeration (VCR) system is employed to build the refrigeration circuit with inbuilt cooling bath for IBAP setup. The basic components of VCR system are compressor, condenser, expansion valve and evaporator coil. The VCR system shown in dotted lines in Fig.1, used R 404a as the refrigerant. The evaporator coil is wound inside the bath for transfer of heat between refrigerant and the coolant (Isopropyl alcohol), since the refrigerant cannot be supplied directly to the open atmosphere. Isopropyl alcohol was used as the coolant, since its freezing temperature is about -86 C. The coolant is pre-cooled to below zero degree Celsius in the bath, before its recirculation in annulus of IBAP system. After attaining the set temperature in the cooling bath, the coolant is circulated around the tool holder. This setup is also provided with a automatic temperature controller. The cooling unit is capable of providing -40 C under no load conditions and around -38 C when the same is coupled with IBAP setup. Compressor Pump Condenser 1. Temperature sensor (PT 100), 2. Evaporator coil 3. Stirrer, 4. Heater, and 5. Isopropyl alcohol. Figure 1 Schematic diagram of the refrigeration unit with in-built bath for IBAP system 2.1. Temperature control unit Expansion valve To maintain the temperature of polishing tool in IBAP process, Isopropyl alcohol was circulated around the tool space. The polishing tool temperature was maintained by monitoring and controlling the coolant temperature with a temperature sensor and a heater, which are in turn connected to microprocessor based PID controller. This unit has an accuracy of ± 0.5 C temperature control. The temperature sensor is capable of measuring the temperature up to -243 C. This setup enables close control of the temperature of IBAP process and enhances the life of polishing tool. 3. Ice Bonded Abrasive Polishing setup Figure 2 shows a schematic diagram of IBAP setup. It consists of a rotary drive system to provide relative motion between polishing tool and workpiece. The polishing pressure was applied using dead weights to ensure uniform contact between work surface and tool surface. The tool was prepared inside the inner cylinder of setup by circulating the coolant around the tool space, as shown in Fig.2. The setup is capable of polishing the specimen for a longer duration with the temperature of tool being adjusted to any temperature between -2 C to -40 C

3 Variable speed controller, 2. Pulley and belt drive, 3. PT 100 Temperature sensor, 4. Temperature display unit, 5. Outer cylinder, 6. Inner cylinder, 7. Isopropyl alcohol, 8. IBAP tool, 9. Work piece, 10. Work holder, 11. Dead weights, and 12. Motor, 13. Base. Figure 2 Schematic diagram of IBAP setup 4. Methodology 12 This section describes the method of slurry preparation, polishing tool preparation and workpiece surface preparation before polishing. 4.1 Slurry preparation 10 The first step in IBAP process is the preparation of polishing slurry. In this work, the slurry is prepared with silicon carbide abrasives, having a mean diameter of 15 microns with concentration of 30 percentage by weight, dispersed in water and thoroughly mixed by magnetic stirrer to avoid any agglomeration of particles in the slurry. 4.2 Preparation of Ice Bonded Abrasive Polishing Tool 12 The prepared slurry of 20 ml volume is poured into the inner cylinder of polishing setup and is frozen by circulating the coolant around the inner cylinder. After freezing each layer of slurry, another layer is formed by freezing the slurry. In this way, the IBAP tool was prepared by layer-by-layer method in order to ensure uniform distribution of abrasive particles in the polishing tool. During the preparation of IBAP tool, the temperature of freezing mixture is maintained by controlling the temperature of coolant being circulated around the inner cylinder. The inner cylinder of the setup was allowed to rotate at rpm in order to avoid the formation of uneven surface at the top of frozen layer. This unit has reduced the tool preparation time to about 2 hours and maintained the polishing tool temperature at a particular value during tool preparation and polishing. 4.3 Preparation of work surface A disk shaped, Ti-6Al-4V workpiece with 20mm diameter and 15mm thick were chosen for polishing studies. The surface was first machined on a CNC lathe and then ground on a surface grinding machine using Al 2 O 3 wheel, rotating at 2500 rpm, giving the depth of cut of 3 microns for 10 passes. The wheel was dressed with a diamond dresser before the grinding operation. All the ground surfaces are cleaned with pressurized water and dried with air. 5. Preliminary experimentation The preliminary experiments were conducted at different temperatures ranging between -2 C to -40 C in order to understand the role of tool temperature in polishing and to identify the best tool temperature for polishing of Ti-6Al-4V specimens. The results have indicated that the polishing tool is seen effective when its temperature is maintained at about -3.5 C. When the tool is maintained at much lower temperatures, it showed slippery behavior due to increased hardness thus hindering the polishing of work surface. A second set of experiments were conducted with a suitable choice of rotational speed of tool, polishing pressure, abrasive size and concentration in the slurry, in order to analyze the and quality of polished Ti-6Al-4V surface in IBAP process. The topography generated on the polished specimen is examined with the help of scanning electron microscope and perthometer. The parameters used in polishing process are listed in Table 1. Table 1 Process parameters employed for polishing of Ti-6Al-4V specimens with IBAP tool Abrasive type and size Silicon carbide, 15µm Concentration by weight (%) 30 Rotational speed of tool (rpm) 150 Rotational speed of workpiece (rpm) 200 Polishing pressure (kpa) 90 Eccentricity (mm) 65 Polishing temperature ( C) -3.5± 0.5 Polishing duration (hours)

4 Improved Cooling unit with Automatic Temperature Controller for Enhancing the Life of Ice Bonded Abrasive Polishing Tool 6. Results and Discussion (a) Figure 3 shows the variation in surface before and after IBAP process on Ti-6Al 6Al-4V specimens. The ice bonded abrasive polishing has improved the on each specimen. This trend clearly reveals the consistency of improvement in surface on polished specimens. Surface roughness (Ra in µm) 0.7 Before polishing After polishing Deep scratch (b) Sample 1 Sample 2 Surface crack Sample 3 Figure 3 Improvement of surface (Ra) on Ti-6Al-4V specimen Table 2 presents the measurements made on titanium alloy specimens before and after polishing with the same operating conditions. It shows about 87 % improvement of on polished specimens. Table 2 Percentage ercentage improvement of on Ti-6Al-4V specimen Sample 1 Initial surface (Ra, µm) Final surface (Ra, µm) % Improvement in surface Sample Sample Sample no. From the results presented in Figure 4 (a) and 4 (b), it is clear that the samples prepare prepared by surface grinding operation contained certain surface cracks, deep scratches, and have initial roughness (Ra) of µm. These cracks have disappeared after 3 hours of polishing with IBAP tool showed in Figure 4 (c) and have a roughness (Ra) of µm. However, shallow scratches existed on the polished surface. (c) Shallow Scratch Polishing pits Figure 4 Scanning Electron Microscope images of sample 1, (a) and (b) ground surface with cracks (c) polished surface without cracks 704-4

5 7. Conclusions The vapor compression refrigeration unit with in-built cooling bath and automatic temperature controller for IBAP system showed the capability of reducing the time for preparation of tool to about 2 hours and enhanced the life of tool to 3 hours. The present study showed the feasibility of this arrangement to attain fine on Ti-6Al-4V specimens in nanometer range. Future attempts are directed towards identifying the domains of freezing temperature for polishing of different materials. References Belyshkin, D.V. (1966), Using ice for polishing glass and crystals, Glass and Ceramics, Vol. 23, No. 10, pp Boopalan, M., Venkatarathnam, G and Ramesh Babu, N. (2013) An improved setup for precision polishing of metallic materials using ice bonded abrasive tool, Proceedings of 37 th International Conference on Manufacturing Automation and Systems Technology Applications Design Organisation and Management Research (MATADOR), Manchester, UK, pp Mohan, R. and Ramesh Babu, N. (2010) Experimental investigations on ice bonded abrasive polishing of copper materials, Materials and Manufacturing Processes, 25, pp Mohan, R. and Ramesh Babu, N. (2011) Design, development and characterisation of ice bonded abrasive polishing process, International Journal of Abrasive Technology, 4 (1), pp Mohan, R. and Ramesh Babu, N. (2012) Ultrafine ing of metallic surfaces with the ice bonded abrasive polishing process, Materials and Manufacturing Processes, 27, pp Mohan, R. (2011) Investigations on ice bonded abrasive polishing of metallic materials, PhD Thesis, IIT Madras. Yuan, D., and Ming, Z. (2005) Nano machining experiment of metal materials polishing with ice desk, Proceedings of the 7 th IEEE CPMT International Conference: High Density Microsystem Design and Packaging and Component Failure Analysis, Shanghai, China, pp.1-6. Zhang, F., Han, R., Liu, Y. and Pei, S. (2001) Cryogenic polishing method of optical materials, Proceedings of the 10 th International Conference: Precision Engineering, Yokohama, Japan, pp