Fundamental Analysis of the Process Conditions during Electro Discharge Machining of Biodegradable Magnesium

Abstract

Magnesium is one of the most promising materials for the application as degradable biomaterial for load bearing implants due to the initial stability and extraordinary biocompatibility. But up until now magnesium degrades too fast before the ingrowing bone can support itself. One possible approach is the application of an interconnecting channel structure. This design increases the surface area to promote bone regeneration and gives the bone the possibility to build a supporting structure throughout the implant. In combination with a surface modification which increases the corrosion resistance the bone can regain enough stability to support itself before the implant has dissolved noticeably. Due to the low process forces EDM is very suited for the machining of the necessary filigree structures. But up until now there are no standard technologies for the machining of this material available. Therefore a fundamental analysis of the material specific influence of magnesium on the EDM process is necessary. In this paper the machining of three different magnesium alloys are compared to the machining of a tool steel. Due to the numerous influencing factors and their interdependencies it is normally not possible to analyze the impact of each input parameter of a typical EDM technology on the continuous process separately making the correlation of input and output parameters very difficult. Therefore the electrical signals are recorded during the machining process to identify and monitor the resulting discharge conditions and frequency. These results are compared to the fundamental influence of the material derived from single discharge experiments in which the boundary conditions are constant to a large extent. By this method the material removal rate as well as the resulting surface roughness of the continuous process can be linked to the material dependent process conditions. As a result technologies for the machining of new materials can be developed and optimized based on fundamental knowledge.