Study on Machining Instability Phenomenon in Electrochemical Machining of Large Aspect-ratio Holes

Abstract

It is possible and very useful to process holes with large aspect-ratio up to 250 by using electrochemical machining (ECM). The aspect-ratio is defined as the ratio of the length to the diameter of a hole, namely L/D. In electrochemical machining, when the cathodic tool electrode approaches the anodic workpiece, dissolution reaction occurs at the interface between the material surface and solution. Therefore, the accuracy of ECM is poor and undercut occurs due to the stray current flowing over the workpiece surface. Moreover, one of the remarkable disadvantages of ECM is that there exists a possibility of short circuit because of the contact between the tool electrode and workpiece, which causes a fatal damage to the tool electrode and workpiece. In this research, the influence of the workpiece material, the electrolyte flow rate on the machining instability, including the occurrence of undercut and short circuit, in the ECM of large aspect-ratio hole were investigated. It was found that even in the same precipitation type of Ni base super alloy, the appearance of the dropout of the structure differs depending on the manufacturing methods such as casting and forging due to different amount of fusible γ phase. It was also found that even in the same material, the solubility in ECM differs due to the difference in precipitation of carbide grain boundary carbides. In addition, from the perspective of a difference in the flow rate, it was found that by increasing the electrolyte amount, the electrolyte flow in the vicinity of the electrode changes and causes different undercut.