Macro and micro-scale material removal mechanisms during ECM/hybrid laser-ECM of a passivating multiphase NbC–Ni cermet

Abstract

Electrochemical machining (ECM) is a non-contact and athermal machining process where the material removal is accomplished through controlled anodic dissolution of the workpiece governed by Faraday laws. ECM process has been hybridized with several other processes for improving material processing windows. Hybrid laser electrochemical machining (LECM) synergistically applies electrochemical and laser process energies with added benefits of escalated reaction kinetics leading to enhanced transpassive dissolution, weakening of passivation layer, process localisation and uniform dissolution. The laser energy acts as a localised and controllable heat source thereby offering multi-fold processing benefits. For alloys and cermets, a characteristic surficial fingerprint is the presence of inhomogeneous multiphase dissolution and sporadically distributed passivation layer, necessitating addition of aggressive reagents in electrolytes. LECM has the potential to addresses these challenges while processing in pH neutral electrolytes. Previous works have very limited analysis on the macro and micro removal mechanisms while processing relevant strategic materials and multitude of applications of LECM remain unexploited. Therefore, this work presents in-depth investigations into macro and micro-scale material removal mechanisms of ECM/LECM on sintered niobium carbide with nickel binder (NbC–Ni), which is a potential cobalt-free alternative to tungsten carbide. The results revealed new insights into the removal behaviour of the constituent phases which differed from the first principles and their interaction with the laser. During ECM, the Ni phase dissolved preferentially and influenced the surface pattern and particle breakout which was reduced with laser assistance. The surface evolution characteristics were also analysed based on the ridge-crevice pattern. Additionally, the weakening of passive layer was correlated with the pulse analysis that revealed quantitatively the different process regimes occurring during ECM and LECM. The grain level study revealed that orientation effects still exist during LECM and the grains with higher surface energy (FCC (001) vicinal planes) passivated more and dissolved less. Furthermore, the improvement in surface quality, overcut and reduction in particle breakout with LECM process makes it promising for machining newer recipes of metal carbides.