Experimental analysis of subsurface integrity during fine turning of OFE copper for radiofrequency cavity manufacturing

Machining oxygen-free electronic (OFE) copper could be challenging but is not widely studied because few industrial or critical components requires to master the machined sub-surface characteristics. CERN radio frequency cavities are one of the applications, especially because the turned surface is not the functional one of the final products. The niobium coating post process, which gives superconductive properties to the cavity, largely depends on the machined surface characteristics. The present study relies on an experimental approach of the cutting process, through thermal and mechanical probing of high precision, pollution free, turning. Cutting forces and thermal load on the tool are detailed for finish turning. The critical uncut chip thickness, defined at macroscale as the limit between cutting and ploughing behavior, is also a frontier at microscale. Consequently, surface integrity is evaluated by advance microstructural analysis (EBSD and FIB), imposed by the thinness of the affected layer. Grain recrystallization appears in the first 0.6 micrometers below the surface and deformed grains are observed up to 4 micrometers for cutting regime, while the thickness of the layers is three time larger in case of ploughing regime. Hence surface integrity of OFE copper finish turning is characterized and optimal cutting conditions are defined. The research shows that simple cutting tests can quickly narrow down to optimal cutting condition, which are then confirmed through metallurgical analysis, even in the edge case of pure OFE copper, hence relevant to other material.