Experimental Investigations into Machining Characteristics of Niobium Carbide Cermet with µECM / hybrid laser-µECM.

Abstract

Niobium carbide (NbC) is a high hardness 1960 (HV30) cermet material which finds applications in the moulds for ceramic injection moulding as well as inserts for cutting tools. This material possesses excellent combination of hardness, toughness and thermal shock resistance. The use of nickel (Ni) binder makes it a promising material for such applications, especially in view of rising cobalt (Co) metal prices used as binder in conventional tungsten carbide (WC-Co) cermet. The tool-wear and tool-failure issues make it difficult for mechanical machining technologies to process this material, especially in the micromachining regime (< 2 mm). To address machinability challenges, electrically conductive Nb cermet has been developed inhouse with Ni as the binder material. This makes it a suitable workpiece candidate for electrochemical machining (ECM) and electrical discharge machining (EDM) processes.Therefore, experimental investigations are presented in this work to evaluate the machining characteristics of NbC cermet with micro-ECM process. Channels have been used as test-features for evaluating machinability. Furthermore, two different grades of NbC cermets have been machined to identify the effect of WC addition on the dissolution characteristics and possible passivation during ECM. A micro-second pulsed voltage source has been utilized for controlled and localized material removal. To further enhance removal mechanisms in terms of processing speed and weakening of passivating layer, the micro-ECM process has been assisted with a nano-second pulsed laser on an in-house developed hybrid machine-tool. The aim is to uniformly dissolve the NbC matrix and Ni binder with ECM process for a high integrity machined surface. The dissolution and passivation characteristics have been further analysed through a combination of current pulse acquisition and channel profile characterisation. Linear sweep voltammetry and channel machining revealed that the addition of WC has a significant impact on the stability of passive layer. Laser assistance could enhance the material removal for both the NbC grades and promote uniform dissolution, with the effect being more pronounced in the NbC grade without WC (41 % increase in average pulse peak current) due to the weaker passive film. The material dissolution was more localized in LECM of NbC grade with WC due to the strong passivation along the channel walls.