DEVELOPMENT OF MAGNETIC ABRASIVE POWDERS FOR POLISHING OF METALLIC COMPONENTS

Abstract

Worldwide, manufacturers felt the need for high precision in manufacturing to improve consistency of components, better quality control and for longer wear/fatigue life. On the basis of achievable accuracy, finishing processes can be classified into two categories: conventional finishing processes and advanced finishing processes. Conventional finishing processes like grinding, lapping and honing use multipoint cutting edges in the form of abrasives, which may or may not be bonded to perform cutting action. The major limitation in the conventional finishing processes is about the accuracy of shape and size of a part that they can finish. Also in-process control of the force acting on the workpiece is not possible and hence low degree of control on the finish. All these demands led to the development of second category of finishing processes i.e. advanced finishing processes. The advanced finishing processes can be further divided into two classes for the better understanding of their working principles. The first one includes Abrasive Flow Finishing (AFF), Elastic Emission Machining (EMM) and Chemo Mechanical Polishing (CMP) in which the force acting on the workpiece during the finishing operation is not controlled externally. The second one includes Magnetic Abrasive Finishing (MAF), Magneto rheological Finishing (MRF), Magneto rheological Abrasive Flow Finishing (MRAFF) and Magnetic Float Finishing (MFP) in which it is possible to externally control the force acting on the workpiece by varying the magnetic field. Among these processes MAF has achieved remarkable results in the area of finishing of metallic (ferrous and non ferrous) as well as non metallic (ceramic) components. In MAF, magnetic abrasive particles which play the role of cutting tools are very crucial in ensuring finishing of required quality and accuracy. Most of the existing studies on MAF try to concentrate on the development of process and enhancing the capabilities and applications of the process. The literature reveals that very few studies have been done in the direction of development of magnetic abrasive powders. The existing methods for preparing magnetic abrasive powders are complex and most of them are patented. In fact MAF technology has not been fully utilized due to this fact. In view of the above, the present research work has been carried with the following major objectives. iii 1. Development of an alternative method to prepare magnetic abrasive powders using Mechanical Alloying process. 2. Development of an internal MAF setup for performance evaluation of proposed magnetic abrasive powders. 3. Development of a flat plate polishing setup. 4. Polishing and revealing the microstructure of metallic specimens. This aspect is a new one which evolved itself during the progress of the work extending the scope of MAF in preparing metallurgical microstructure. The present research work has been divided into three phases: Phase1: Preparation of magnetic abrasive powders by Mechanical Alloying process: The magnetic abrasive powders were prepared by mechanical alloying process. Mechanical alloying is a solid state powder processing technique involving repeated welding, fracturing and rewelding of powder particles in a high energy ball mill or attritor at room temperature. Three different batches of magnetic abrasive powders were prepared: iron with aluminium oxide, iron with quartz and iron with silicon carbide. In the present work an attritor was used and mechanical alloying was performed at room temperature using high carbon high chromium steel balls as milling media. The composition was 80 % by volume of iron powder and 20 % by volume of abrasives. A magnetic separation test was performed to ascertain the strength of bonding between iron and abrasive particles after mechanical alloying. FESEM was carried out to study the structure of produced magnetic abrasive powders. Also particle size analysis was performed to obtain the size of grains after mechanical alloying. Phase 2: Development of internal MAF setup: An indigenous internal MAF setup was developed to study the performance of above prepared magnetic abrasive powders. The main subsystems of the developed setup are DC electromagnet with variable power supply and workpiece holding and rotating system. The polishing parameters required for evaluation of newly developed magnetic abrasive powder were divided into two categories as: 1. Process related parameters like rotational speed of workpiece, workpiece material, workpiece diameter, pole-workpiece gap, magnetic flux density and machining time. iv 2. Magnetic abrasive based parameters are quantity of abrasive powder, percentage of machining fluid and proportion of ferromagnetic and abrasive components. Stainless steel (SS 304) tubes of 38 mm diameter and 1.5 mm thickness were used as workpiece. The key input parameters like quantity of magnetic abrasive powder, percentage of machining fluid, magnetic flux density, speed of rotation of workpiece and machining time which significantly affect the response parameter (surface roughness) were studied. Experiments were performed to determine the range of these parameters. The response parameter was measured with the help of AMBIOS XP 200 surface profilometer. Out of the three magnetic abrasives prepared, iron and aluminium oxide based magnetic abrasives were found to give overall better results and quartz based magnetic abrasives gave highest improvement in surface finish. Atomic Force Microscopy was carried out on the selected optimized samples. In case of quartz based magnetic abrasives the surface roughness was reduced to around 23 nm. The Design of Experiments methodology was used to study simultaneous variation of parameters. Since stainless steel tubes are widely used in industry, the finishing of SS tubes improves the applicability of developed magnetic abrasives in industry. Phase 3: Development of flat plate polishing equipment: Equipment for polishing flat specimens was developed in house. The specimen was fixed in the stationary workpiece holder. Two permanent magnets of 1 Tesla were rotated with the help of a PMDC motor. After several hours of polishing it was possible to observe the structure of 60-40 brass and aluminium silicon alloy specimen. This method developed for revealing microstructure does not require any etching and is comparable with the microstructure obtained by usual cloth polishing and etching procedure. Microstructure of both namely as developed in the present investigation involving magnetic abrasives and the one developed conventionally by mechanical and cloth polishing have been recorded to prove that the present method develops identical microstructure with much more simplicity and economy. Since this is entirely new finding, not hitherto reported so far, the reason for development of microstructure by magnetic abrasive polishing alone (i.e. not doing chemical etching to reveal the structure) has been offered and this structure can be a better representation when compared to chemically etched structure.