Fracture and fatigue behavior of electrical-discharge machined cemented carbides

Electrical discharge machining (EDM) is an alternative shaping route for manufacturing complex component shapes of hard and brittle materials such as WC–Co cemented carbides (hardmetals). However, it results in a poor surface integrity that often leads to mechanical degradation of these materials. In this investigation, the influence of multi-pass sequential EDM on the fracture and fatigue behavior of a WC–10wt%Co cemented carbide is studied. It is compared with the behavior exhibited by a reference condition, attained through conventional mechanical grinding and polishing using diamond as abrasive. Considering that rupture is related to existing defects, either introduced during sample fabrication or induced by machining, a detailed fractographic examination is conducted to discern failure origins. The experimental findings indicate that flexural strength of WC–Co hardmetals, under both monotonic and cyclic loading, is strongly affected by EDM. An analysis of the results using a linear-elastic fracture mechanics approach allows to establish a clear connection between surface integrity and mechanical strength. Quantitative discrepancies between the estimated and the experimentally measured critical flaw sizes for all the EDM samples are rationalized through the existence of residual tensile stresses of considerable magnitude at the shaped surface. From a fatigue viewpoint, these residual stresses are even more detrimental because they imply an additional mean stress; thus, higher effective load ratio at the EDMed surface. As a consequence, fatigue sensitivity of the EDMed specimens is higher than for the reference condition. Relief of these stresses through annealing treatments is assessed and shown to be a relatively effective alternative for improving the fracture and fatigue behavior of WC–Co cemented carbides shaped by EDM.