Fatigue crack susceptibility of electrical discharge drilled holes in nickel based heat resistant alloy

Electrical discharge (ED) drilling is one of the prevalent processes that is utilised to drill deep holes in hard to machine materials such as nickel based heat resistant alloys used for turbine applications. In order to satisfy the high durability needs of heavy duty engines applicable to power generation industry, cooling design for hot gas path components has become complicated. ED drilling technique has been employed frequently to machine holes in complex parts. Thus, the effects of ED drilling and post electrical discharge machining (EDM) processes on the fatigue durability of the materials need to be clearly understood. However, the characteristics of the altered material zone (AMZ) created by ED drilling has not been fully analysed in terms of fatigue crack susceptibility. In order to investigate the effects of ED drilling process, fatigue test specimens with ED drilled through holes machined under four EDM conditions were fabricated. For some of the specimens, subsequent heat treatment was performed in order to verify the effect of the AMZ. Also two configurations of the hole modifications were attempted to assess whether getting rid of the AMZ from the weakest spot of the hole edge was effective to extend the fatigue durability. After the fatigue test, crack behaviour of the ED drilled inside hole was analysed using scanning electron microscope (SEM) fractography. Based on the findings of the present study, desirable ED drilling parameters in association with the post treatment processes are suggested.