Hybrid Laser-Electrochemical Micromachining Technology: Machine Design and Process Analysis by Simulation and Experiments

This thesis proposes a novel tool-based hybrid laser-electrochemical micromachining (laser-ECM) process which exploits synergy of laser and electrochemical process energies along the same machining axis, thereby enhancing the application potential of both processes while compensating and minimizing their limitations. This process combines features from jet-electrochemical machining (jet-ECM) and water jet guided laser processes into a new tool based hybrid laser-ECM process. It is a fast and force-free processing technique to machine difficult-to-cut multi-materials with conductivity variations with added advantage of accelerated material dissolution, oxide layer weakening and improved surface quality. In this process, the laser can assist electrochemical dissolution or participate in material removal depending on the fluence available on the workpiece surface.  

To experimentally realize this tool-based hybrid laser-ECM technology, a prototype hybrid machine tool is developed which employs a nano-second laser and a micro-second pulsed voltage source for precision micromachining. A hybrid tooling concept is proposed for a novel process scheme of precision hybrid laser-electrochemical micromachining. The tool serves the function of both an ECM electrode as well as a multimode waveguide for the laser and delivers laser homogeneously together with the electrolyte on the workpiece surface without requiring laser to be focused on the workpiece surface.

Furthermore, experimental investigations on process mechanisms and synergistic effects are performed using Inconel IN718 as workpiece material. It has been observed that while the process response is material-dependent as well as ECM parameter dependent, the effective laser pulse energy reaching the workpiece surface is the main factor influencing the surface characteristics. Additionally, the electrolyte flow rate affects material removal and also influences laser coupling into the tool-electrode. According to the experimental results, material removal rates of the order of 0.6 mm3/min are obtained. Furthermore, metallographic investigations on the machined surface reveal presence of multiple removal mechanisms such as laser removal, laser assisted electrochemical removal and electrochemical removal depending on the applied laser pulse energy.

A multidisciplinary model scheme is proposed using global modelling approach where the model mimics several microscopic physical and electrochemical phenomenon involved during novel tool-based hybrid laser-ECM process. The model takes into account several physics inherent to the hybrid laser-ECM process such as electric currents, fluid dynamics, modelling of laser source, hydrogen and oxygen gas generation and heat transfer in solids and fluids. The model is capable of capturing temperature effects on each of the physics in the model. Material removal is simulated by using deformed geometry feature and automatic remeshing technique generates a new boundary every time after removal for subsequent solution steps. This model also provides insights into several multiphysics phenomenon occurring in the interelectrode gap which are difficult to characterize experimentally and therefore acts as a virtual sensor.

 With the tool-based hybrid laser-ECM process, a hierarchical cavity structure is observed where the central region is dominantly exposed to laser-ECM interaction and the region towards the edge is mainly exposed to the electrochemical machining process. Thus, a surface roughness gradient exists within the cavity due to application of process energies of different nature at different scales. This work presents a methodology and the results of areal surface roughness measurements (Sa, Sz, Sq and Sdq) in different zones of the cavity. To get the information of surface quality in different zones of the cavity, Matlab® code is written for dedicated filtering and extraction of regions of interest. The extraction and filtering methods are discussed.

Real time observations are carried out close to interelectrode gap during tool-based hybrid laser-ECM process.  A combination of high-speed imaging and Large Scale Particle Image Velocimetry (LSPIV) is employed to visualize the newly developed tool-based hybrid laser-ECM process in real time. It also allowed to carry out experimental investigations on the by-product and bubble generation which has a direct effect on process performance in terms of accuracy and efficiency.