High Precision Micro-Grinding of Advanced Materials

Doktorsavhandling, 2021

The aim of this thesis is to advance a fundamental understanding of process mechanics and surface integrity in micro-grinding of advanced materials, such as additively and conventionally manufactured titanium and engineering ceramic. Grinding forces and specific grinding energy were chosen as the two most important indicators to investigate the process. The surface integrity was evaluated using X-ray diffraction measurements to quantify residual stresses, surface roughness measurements, SEM microscopy, and confocal images. At the first stage, the influence of different micro-grinding and dressing parameters and different tool specifications was examined. Then, via process modelling, the outcome of the micro-grinding process at different chip thicknesses and aggressiveness numbers was studied. Additionally, a hybrid laser-assisted micro-grinding process was developed to improve the process efficiency.

The results show that the flow stress of the material did not change with the changing of cutting speed and cutting strain rate. Moreover, it was observed that the depth of cut and grinding feed rate had the same (neutral) effect on the resultant grinding forces. Therefore, the efficiency of titanium micro-grinding could be highly influenced by changing the topography of the micro-grinding tool through different dressing parameters. However, using higher chip thicknesses resulted in a more efficient process in terms of cutting/chip-formation. The lowest specific energy, obtained in the single grain tests, was 11.5 J/mm3 for both-types of titanium materials. In contrast, a much higher minimum specific energy in real micro-grinding process with several (bonded) grains was observed – showing a higher amount of ploughing and rubbing/friction in the micro-grinding process. The build-up direction of additively-manufactured titanium, at low chip thickness, affected the process efficiency. In larger chip thicknesses, almost the same specific energy was measured independent of the material manufacturing method. The results of the XRD analysis showed that contrary to the specific grinding energy, the residual stresses of the ground surface changed by varying the cutting speed and feed-rate-to-depth-of-cut ratio, vw/ae. Higher cutting speeds resulted in lower compressive residual stress, and higher feed-rate-to-depth-of-cut ratios resulted in higher compressive stresses. This can be attributed to higher temperatures in the chip-formation process compared to the plastic deformation in micro-grinding at higher cutting speeds and lower vw/ae ratios, which was proved via SEM micrographs. A more efficient micro-grinding process was achieved via the Laser-Assisted Micro-Grinding (LAMG) process of a Si3N4 workpiece, where the values of the specific grinding energy, as well as the tool deflection by the LAMG process, were much lower than the Conventional Micro-Grinding (CMG) process.