Feasibility of producing mini-tubes from hydrostatically extruded rods of biodegradable pure zinc by EDM process

Abstract

Mini-tubes with a 180 µm wall thickness were prepared by electrical discharge machining (EDM) from hydrostatically extruded pure zinc. Given zinc’s low thermal stability, microstructural analysis of the EDMed tubes was performed using electron backscattered diffraction (EBSD). The impact of variable pulse currents on microstructure and surface quality was assessed, considering different material states before EDM, including solid and predrilled rods. Surface quality was determined based on scanning electron microscopy observations and roughness measurements. Finally, mechanical properties were evaluated using static tensile tests. To observe the effect of different EDM parameters, the microstructure and mechanical properties of the mini-tubes were compared to pure zinc in rod form. The study revealed that pulse currents in the range of 3.2–2.6 A resulted in the formation of new grains, which were found across the cross-section of the mini-tubes, suggesting that the entire wall thickness was a heat-affected zone where recrystallization processes occurred. In the case of the lowest applied pulse current, grain size remained unchanged; however, some twins and an increased share of low-angle grain boundaries were observed. Surface quality deteriorated with increasing pulse current, with the thickest recast layer and highest roughness observed in hydrostatically extruded pure zinc, machined with the highest pulse current. It was also demonstrated that the initial form of the material had a slight effect on these features. The mechanical properties of the mini-tubes remained comparable to those of the rods. Overall, the EDM process shows promise for fabricating semi- or final products of absorbable zinc-based stents.