Operando evaluation of passivation phenomenon during ECM/Laser-ECM: direct and on-machine evidence of passivation evolution

The performance of electrochemical micromachining (ECM) is compromised when processing highly passivating materials like Ti6Al4V, which can be improved through hybrid laser-ECM (LECM) which facilitates passivation weakening. To date, passivation phenomenon has been mostly analysed through metallography and potentiometric techniques. Metallography provides oxide formation details over a limited observation area after machining, which have limited relevance for manufacturing. Whereas, potentiometric techniques cannot replicate ECM conditions to provide accurate transpassive regime conclusions. The dynamic phenomenon of passivation during ECM requires an on-machine analysis technique to provide production-oriented process history, thereby improving fundamental understanding of passivation at different processing conditions. Therefore, in this study we propose a framework based on high frequency in-process current signals which reflects the operando change in passivation for on-machine evaluation of the passivation phenomenon. We validated this framework through experiments, which demonstrated that this approach can quantitatively capture the dynamic passivation behaviour and is sensitive to different processing conditions and microstructure of the same material. The results showed that passivation weakening improved with increasing voltage up to 30 V and with LECM. In addition, the samples prepared by selective laser melting (SLM) were more resistant to EC-dissolution compared to rolled samples and the material porosity led to inhomogeneous material dissolution. Therefore, since the proposed on-machine analysis technique provides both the overall influence of passivation and its process history, it is useful for mechanistic studies and has the potential to be further developed towards process optimisation and process control.