Modelling, simulation and experimental investigation of the effects of material microstructure on the micro-endmiling process

Recently it has been revealed that workpiece microstructure has dominant effects on the performance of the micro-machining process. However, so far, there has been no detailed study of these effects on micro-endmilling. In this research, the influence of the microstructure on the matters such as cutting regime, tool wear and surface quality has been investigated. Initially, an experimental investigation has been carried out to identify the machining response of materials metallurgically and mechanically modified at the micro-scale. Tests have been conducted that involved micro-milling slots in coarse-grained (CG) Cu99.9E with an average grain size of 30 μm and ultrafine-grained (UFG) Cu99.9E with an average grain size of 200 nm. Then, a method of assessing the homogeneity of the material microstructure has been proposed based on Atomic Force Microscope (AFM) measurements of the coefficient of friction at the atomic scale, enabling a comparative evaluation of the modified microstructures. The investigation has shown that, by refining the material microstructure, the minimum chip thickness can be reduced and a better surface finish can be achieved. Also, the homogeneity of the microstructure can be improved which in turn reduces surface defects. Furthermore, a new model to simulate the surface generation process during micro- endmilling of dual-phase materials has been developed. The proposed model considers the effects of the following factors: the geometry of the cutting tool, the feed rate, and the workpiece microstructure. In particular, variations of the minimum chip thickness at phase boundaries are considered by feeding maps of the microstructure into the model. Thus, the model takes into account these variations that alter the machining mechanism from a proper cutting to ploughing and vice versa, and are the main cause of micro-burr formation. By applying the proposed model it is possible to estimate more accurately the resulting roughness owing to the dominance of the micro-burrs formation during the surface generation process in micro-milling of multi-phase materials. The model has been experimentally validated by machining two different samples of dual-phase steel, AISI 1040 and AISI 8620, under a range of chip-loads. The results have shown that the proposed model accurately predicts the roughness of the machined surfaces with average errors of 14.5% and 17.4% for the AISI 1040 and AISI 8620 samples, respectively. The developed model successfully elucidates the mechanism of micro-burr formation at the phase boundaries, and quantitatively describes its contributions to the resulting surface roughness after micro-endmilling. (Abstract shortened by UMI.)