All EDM machining is a thermal/heat process that can alter or change the structure of the machined workpiece. Early EDM technology thermally damaged materials as a result of not controlling the discharge spark energy precisely enough. The resulting metallurgical quality produced by EDM can affect the workpiece function and lifecycle. It can even lead to component failure. The main metallurgical characteristics that are of most interest are:
- Recast: This is the re-solidification of the workpiece material back upon itself and can be harder and more brittle than the parent material.
- HAZ (Heat Affected Zone): This is the annealed layer of material that is located below the surface or outer skin of the parent material.
- Micro-cracking: Small cracks in the material on the outer surface.
While EDM is a thermal process, advances in generator technology have greatly improved and preserved the metallurgical integrity of the workpiece material. The main development driving the metallurgical quality improvement has been the drastic increase in intelligence and capability of the EDM generator’s adaptive power control system. Modern EDM machines are capable of sensing and adapting the discharge spark energy to finely controlled levels to produce consistent and predictable metallurgical results.
The HAZ and micro-cracking levels have been virtually eliminated with modern EDM equipment, but some form of recast will always exist. Generally speaking, the finer the surface finish that is produced by EDM, the smaller the recast amount. For most common EDM operations where some form of finishing is performed, the average recast layer value will be 0.0002” or smaller.
The EDM process in many cases might not provide the fastest means of material removal; however, it has matured into a very accurate, predicable and reliable process. This ends our first series Back to Basics series featuring the EDM process and its origin. Please comment or provide feedback if you’ve enjoyed this lesson. Happy EDMing everyone!