Modelling the cross-sectional profile of the kerf generated in overlapped pass erosion in abrasive waterjet milling of Al6061-T6 alloy

Abstract

Machining complex surfaces on difficult-to-cut materials by conventional methods is challenging due to the issues that arise from both the ‘freeform surface generation’ and the ‘machining difficult-to-cut materials’. On the other hand, abrasive waterjets (AWJs) have proven their unique capabilities in machining a wide range of materials and 3D shapes by maneuvering the jet strategically. In realizing 3D shapes through AWJ milling, the cross-sectional profile (CP) of the kerf, i.e. (i) trench — formed in a single pass erosion and (ii) cavity — formed in an overlapped pass erosion, need to be predicted apriori. However, the stochastic and aggressive behaviour of the AWJ, and the lack of complete understanding of the kerf generation, make the accurate prediction of its CP a difficult task. Furthermore, the cavity generated (by overlapping two trenches) is not a simple linear combination of trenches generated in two single AWJ passes. Also, the modelling efforts that consider both the dynamic jet characteristics and the non-linearity in the kerf formation are very limited. In this work, analytical models are proposed to predict CP the kerf (trench and the cavity) milled by the AWJs by considering the experimental understanding gained on the kerf generation in single and overlapped pass erosion. The modelling strategy evaluates the local erosion volume and sweeps this over the CP along the top width of the kerf. This strategy enables the incorporation of region-wise physical phenomenon in the material removal. Furthermore, it considers the (i) aggressive and stochastic nature of the AWJ, and (ii) particle erosion theories applicable to ductile materials, which pose multiple challenges during conventional machining. The model developed for the trench considers the effective jet divergence, spatial abrasive mass- and velocity-distributions in the jet plume, as inputs. Towards capturing the non-linearity in the cavity generation, the non-flat surface from the previous pass and its effect on the variation of local particle impact angles, uneven jet deflection during subsequent passes around the jet axis, and the degree of overlap were considered. From the experimental validation, it was observed that in the single pass and overlapped passes at different degrees of overlaps (5 %–80 %), the proposed model predicts the kerf geometry with a maximum mean absolute error (MAE) of 36 μm and 49 μm, and the maximum depth of CP with a maximum error of 7 % and 11 %, respectively.