Electric Discharge Machined Surfaces
During Electric Discharge Machining (EDM), small volumes of metal are removed from the machined surface by a series of random electric discharge events (breakdowns) along the surface. The removal process involves rapid thermal evaporation of the work material and its consequent quench-solidification in a liquid medium (de-ionized water in wire EDM and a Kerosene base in conventional or ram EDM). The evaporation event from a micro-pool of molten work material on the surface is followed by fast cooling through heat transfer to the surrounding, comparatively large and cold work piece.
In heat-treatable metals, such as high-speed steels and tool steels, the surface temperature is raised momentarily above the austenitic transformation temperature followed by rapid cooling, generating a glassy and brittle, untempered martensite layer on the surface. Deeper under the surface there still can be an increase of the temperature, sufficient to cause some softening to below the original specified hardness of the steel. This structure is called the ‘re-cast layer’.
Since cutting edges are formed by the intersection of two surfaces, their properties are limited by the inferior one of these two. If one surface covered with a re-cast layer, then the edge formed will have a high propensity of brittle fracture or micro-chippage, and will yield a tool with a substantially shortened life and/or impaired performance.
In cemented carbides, EDM often results in a damaged surface layer, with altered chemistry and structure. The surfaces can suffer from eta phase formation or cobalt leaching, with the end result of inferior mechanical characteristics and impaired performance.
Although the coating quality on EDM-d tools is unpredictable, their performance can be optimized by observing the following simple guidelines:
- Keep the recast layer thickness to a minimum by specifying low burn currents and/or low energies/pulse. For specifics, consult the manufacturer of the EDM machine used.
- The finishing burn should produce as smooth a surface as practical, and remove the heavily damaged para-surface layer generated by the roughing burn
- Subject high speed steel EDM-d tools to an additional draw/stress relieve heat treat cycle to temper the untempered martensite of the re-cast layer
- Make sure that the liquid in the tank of the EDM machine is in good condition and clean
- Remove surface contaminations; achieve a clean and shiny metallic appearance. Use IVAC’s Standard Surface Preparation Instructions as a guideline to obtain optimum finish
Call us for more detailed instructions or explanations about any aspect of these instructions.
It should be noted, that adherence to correct EDM technology and tool preparation will improve the EDM-d tool performance, whether or not it is PVD coated.