Influence of Open-circuit Voltage on Micro Electrical Discharge Machining of Ni-Al₂O₃ Functionally Graded Materials

Ni-Al₂O₃ functionally graded materials (FGMs) have become important for high temperature applications due to a continuous change in composition. However, the manufacturing of Ni-Al₂O₃ FGMs with conventional machining methods is a difficult process because of the high hardness and various graded compositions. In this paper, three-layerd Ni-Al₂O₃ FGMs, pure Ni layer (100% Ni), 70% Ni/30%Al₂O₃ layer, 30% Ni/70%Al₂O₃ layer, have been machined by micro electrical discharge machining (EDM). The machining characteristics of Ni-Al₂O₃ FGMs have been investigated, which include the effect of open-circuit voltage on material removal rate (MRR), electrical discharge status, surface microtopography, and element distribution. The experimental results indicate that with the increasing open-circuit voltage, the MRR of the pure Ni layer and 70% Ni/30%Al₂O₃ layer increases slowly and the surface quality becomes worse slowly. In contrast, when open-circuit voltage is changed from 110 to 170 V, the MRR of 30% Ni/70%Al₂O₃ layer increases rapdily. The surface quality of 30% Ni/70%Al₂O₃ layer becomes better before getting worse with the increase of open-circuit voltage. In addition, the quantity of normal discharges in the pure Ni layer and 70% Ni/30%Al₂O₃ layer has a little change, and the quantity of normal discharges in the 30% Ni/70%Al₂O₃ layer increases rapidly with the increase of open-circuit voltage. Finally, a small hole with fast MRR and good surface quality has been machined suceesfually based on the experimental analysis.