Modelling the abrasive flow machining process on advanced ceramic materials

Abrasive flow machining (AFM) is a unique machining method used to achieve high surface quality on inner, difficult-to-access and on outside contours. Using AFM, it is possible to realise predefined edge rounding on any brittle or hard marerial. AFM is easy to integrate in an automated manufacturing environment. The abrasive medium applied during AFM is a fluid consisting of a polymer which carries silicon carbide or super-abrasive grains. With a specified pressure and temperature, this fluid flows in alternating directions along the contours of the workpiece resulting in an abrasive effect. AFM is also well suited to process advanced ceramic materials. Especially advanced ceramics increasingly are playing a significant role as a substitute for metals. However, the high costs for the inevitable finishing process on ceramics prevent a more frequent use. This paper represents the technological results of a research project discovering the fundamental principles of AFM on advanced ceramic materials such as a correlation between flow processes, surface formation and edge rounding. Furthermore, an insight into a process model is given which was developed using modern simulation techniques. The overall objective of this approach is to anticipate work results like surface quality and edge rounding on any user-defined geometry. The material removal process behaviour of ceramic materials is mainly ductile, hence the smooth surface is extending. As a result of the machining process, typical washed-out surface textures occur. Grain boundaries as well as edges of micro cracks have been smoothed out. By removing the surface layer by layer, existing damages beneath the surface have been uncovered with AFM. Only when a failure of the grain boundary is approached, grains are breaking off. The investigation shows that for advanced ceramics a ductile material removal mechanism is achieved by using abrasive diamond grains with average grain size under 44,5 micron, whereas a brittle material removal mechanism could be achieved with grain size upon 185 micron. It has been proved that in most cases a weight ratio of 1:2 between carrier fluid to abrasive grain is attained to success. Rising temperature leads to descending viscosity of the grinding medium, hence the abrasive removal rate is sinking. By raising the processing pressure and reduction of the flow cross-section, the fluid velocity and the removal rate are increasing.