Fracture Toughness and Tribological Properties of Cemented Carbides Machined by Sinking Electrical Discharge Machining

Abstract

The quality of a forming process highly depends on the pressure applied to the workpiece. Consequently, the demand for higher workpiece qualities results in a demand for tools that can withstand high compressive stresses. Moreover, the tendency of using materials like high-strength steels as workpiece material, urges the need for tool materials that can withstand high compressive stresses and are resistant to wear. A class of materials that offer a combination of hardness or wear resistance and ductility are cemented carbides. However, these properties hamper their machining with conventional cutting technologies. Due to its electro-thermal working principle, Electrical Discharge Machining (EDM) is able to machine materials independently from their mechanical properties. On the other hand, the removal process is accompanied by thermal stresses, which can cause residual stresses and micro cracks near the machined surface. Due to their pre-existing stresses from the sintering process, cemented carbides are especially susceptible for these kind of damages. It is therefore necessary to identify the impact of EDM on the material. Different machining strategies are tested with two different types of cemented carbides and examined regarding their fracture toughness. The crack surfaces resulting from the three point bending test are microscopically inspected regarding failure initiation. Additionally pin-on-disc tests are conducted to determine the influence of the EDM strategies on the tribological properties of the machined cemented carbides.

Coefficient of friction of cemented carbides machined by sinking EDM

“…The experimental setup was chosen according to previous investigations, however, some alterations have been made. The process of specimen preparation was the same as previously [19]. The only difference lies in the geometry of the tested pin.…”

“…CTF30 consists of 15 wt% cobalt and tungsten carbide grains with a size of 0.8 -1.3 µm, whereas UMG01 does not contain a significant amount of binder material and consists of tungsten carbide grains with a size of 0.2 µm. The friction test was conducted according to previous investigations [19,21]. The setup can be seen in Fig.…”

“…1 on the right. However, because of the difficulties described in a previous publication [19], where the investigations were conducted according to the setup schematically depicted in Fig. 2, top left was changed.…”