5777.2:
Metallurgical processes occurring at surfaces submitted to high melting and cooling rates during electrical-discharge machining (EDM) (Project of the TOP NANO 21 Innovation Committee)
Abstract
The goal of the present project is to prepare the way towards nano-scale EDM processing, i.e. towards the control of the average roughness of machined workpieces of a few tens of nanometer. At the same time, this goal also means the strong reduction of the top-to-bottom roughness fluctuation, which actually reaches up to 800 nm.
This will be achieved by a characterization of the EDM processed material surface at an unprecedented precision, reaching in the nanoscale range. The task consists of determining for the first time the precise amount of discharge energy which is transformed to heat, i.e. which really interacts with both, the material at the surface and the surrounding dielectric. The treatment of the energy conversion (heat losses) requires an extremely precise knowledge of the heat affected zone at a nano-scale. During the EDM processing, the heat affected zone undergoes a rapid heating/ultrarapid cooling cycle, leading to the formation of metastable crsytalline phases as well as of amorphous regions. This gives rise to strong local stresses, which have to be minimized.
The heat input is measured by determining the temperature at a workpiece surface submitted to a discharge. This can only occur by indirect measurements of the temperature profile at the backside of very thin steel foils, the surface temperature being determined by an inverse calculation method developed at EPFL. The temperature dependence will be measured by both, Pt/Pt-10%Rh thermocouples, which are deposited on the backside of the steel foils, and Infrared pyrometry. The complex modelisation procedure takes into account several parameters, i.e. heat conductivity of steel, evaporation rates, detaisl of phase mixture (including metastable phases) and local stress distribution. In order to reach this goals, it is mandatory to perform the characterization of the EDM processed regions at a nanometer scale. The analysis comprises advanced TEM techniques as well as nano-indentation, in addition to common X ray and SEM analysis.
Both cases of mono- and multidischarges will be considered, in order to get an accurate database for determination of the real surface temperature by the inverse calculation process. By performing EDM on thin steel foils with gradually increasing thickness, a temperature depth profile will be obtained. The present measurements will allow for the first time a sequence of the solidification processes in the regions close to the workpiece surface.
The precise knowledge of the nanostructure will allow to improve our knowledge of the EDM process dynamics, which in turn will lead to new EDM pulse parameter sets, required for nanoscale EDM processing.