Solidification microstructure of electrodischarge machined surfaces of tool steels

Abstract

Some recent advances in the understanding of the solidification behaviour of molten metal during electrodischarge machining (EDM) of a range of commonly used tool steels are presented. In general, the recast layers produced by EDM can be classified into three structural types: thick multilayer, intermediate single layer, and thin featureless. The discharge energy affects the average thickness of the recast layer, and hence the distribution of the different types of recast layer formed during EDM. Detailed microstructural studies have shown that during EDM, solidification of the molten metal takes place simultaneously from the top interface with the dielectric and the bottom interface with the underlying metal into the melt, as well as from within the melt towards both interfaces. This leads to the formation of three distinctive sublayers withing each recast layer resolidified from a given pool of molten metal. The effects of steel composition and thermal properties on the solidification microstructure of recast layers are discussed. Attempts are also made to characterise the globule appendages produced by EDM.