Electro discharge machining of nickel–titanium shape memory alloys

Abstract

Electro discharge machining (EDM) is a common fabrication process for miniaturized components in medical technology and micro engineering today. As EDM induces material changes in the near surface zone, the surface integrity becomes ever more important, the smaller the components are. In order to characterize the influence of EDM on the near surface zone, basic metallurgical investigations on pseudo-elastic NiTi shape memory alloys (SMAs) were carried out. Material removal in EDM depends on the electric discharge processes between the tool and the workpiece electrode in a dielectric fluid. Material is removed by melting and vaporization in single sparks. This results in craters with varying size and depth depending on the discharge energy. The microstructure of this melting zone is characterized by hollows, cracks and precipitation. Cracks open at the surface in consequence of randomly and locally overlapping thermal shocks. The cracks grow vertically into the material, starting at the surface. In the melting zone, significant precipitations were detected and subsequently identified by EDX as titanium carbides. The material removal rate, which is an important process factor in manufacturing, approximately increases in linear proportion with the discharge energy, and achieves commercially interesting values by using an electrode made of copper and tungsten. The results of the microstructure analysis require the removal of the near surface zone to ensure the properties of the components. This is possible via a smooth EDM-process, followed by electrolytic polishing.