Grinding-aided electrochemical discharge machining of metal matrix composites

Abstract

Among the many non-conventional machining methods, electrical discharge machining (EDM), wire-EDM, and electrochemical machining (ECM), are perhaps, the most promising processes for shaping metal matrix composites (MMCs). Notwithstanding the merit of these machining methods, there are still problems which need to be solved and improvements to be made before they can be effectively utilised for shaping MMCs. The main problems encountered are low material removal rate (MRR), high risk of tool breakage and the presence of various forms of defects on the machined surface. In this research project, a new type of grinding-aided electrochemical discharge machining (G-ECDM) hybrid process has been developed, and a corresponding machining system has been designed and built. The tool-electrode has a composite coating containing a hard reinforcement phase of diamond particles, which can take various forms to suit different shape and profile requirements of the product. This process is capable of overcoming the problems encountered in machining MMCs. It is different from both the conventional electrochemical discharge grinding process, and the abrasive electrochemical grinding process. Unlike these two processes, the G-ECDM process functions under a combined action of electrochemical effects, electrical discharge erosion, and direct mechanical grinding. The material removal mechanism of the G-ECDM process in machining particulate reinforced aluminium alloy composites has been analysed both theoretically and experimentally. The first phase of the study involved the modelling, and verification by experiment, of the relation between electrochemical effects and spark discharge initiation, with an emphasis on the prediction of the critical breakdown voltage of hydrogen bubbles. A model to reveal the electrical field acting on a hydrogen bubble in ECDM process has been established. This model was found capable of predicting the position of the maximum field strength on the bubble surface as well as the critical breakdown voltage for spark initiation. A set of experiments was performed to verify the model and the experimental results agreed well with the predicted values. The experimental results also showed that an increase in current, duty cycle, pulse duration or electrolyte concentration would promote the occurrence of arcing action in ECDM. In the second phase of the research, the effect of the grinding action of the G-ECDM process on MRR was examined. When grinding was incorporated with the ECDM process, both the MRR and the molten material throw-out coefficient increased significantly. Moreover, an examination of the machined surface quality of the MMC workpiece showed that the G-ECDM process produced much better surface finish with less defects than the ECDM process. To further study the G-ECDM mechanism, a series of single pulse studies have been conducted. The results showed that for the G-ECDM process, the grinding action would remove the crater’s built-up edge. As a consequence, the MRR of G-ECDM was higher than that of ECDM and EDM. Although, the discharge waveform of the G-ECDM process resembles a form of abnormal arcing, the machining process was found to be stable. This is attributed to the rotational motion of the tool-electrode in constantly shifting the arcing position. The differences in the distribution of craters on the machined surface for the ECDM and G-ECDM processes can be satisfactorily explained by analysing the electrical field strength. By examining tool performance, it was found that machining chips clogged to the tool-electrode, produced by the grinding action could be removed by EDM sparks. The EDM spark thus serves the role of cleaning the tool and as a result, preventing short circuiting from occurring. Moreover, EDM sparks that occurred between the clogged material and the workpiece did not cause any noticeable damage at the interface between the diamond grit and the binding material of the tool-electrode. Therefore, the clogged material provides protection to the tool and as a consequence, a longer tool life is expected from the G-ECDM process.