Fatigue performance improvement of electrical discharge machined hardmetals by means of combined thermal annealing and surface modification routes

Abstract

Electrical discharge machining (EDM) provides an effective means of shaping hardmetals. However, it is also known that surface integrity of these materials may be affected by EDM, degrading their tribomechanical characteristics. This work focuses on assessing the use of two different surface modification routes: thermomechanical treatments (shot blasting, polishing and final high temperature annealing) and/or physical vapor deposition of hard coatings, for improving the fracture and fatigue strength of an EDM-shaped fine-grained hardmetal grade. In doing so, an optimal EDM's surface finish variant and a diamond ground and polished one are used. Mechanical behavior is evaluated under four-point bending and fatigue characterization is given in terms of fatigue limit following the staircase method. Experimental results indicate that both approaches markedly decrease the lessening effect of EDM on the mechanical strength of hardmetals, although a complete fracture and fatigue strength retention is only achieved by combining both of them. The improved mechanical behavior is rationalized on the basis of the beneficial changes induced by surface modification on the effective residual stress field at the surface of EDM-shaped hardmetals, as assessed by a linear elastic fracture mechanics analysis combined with fractographic examination.

Postprint (published version)