NSF/DFG Collaboration to Understand the Prime Factors Driving Distortion in Milled Aluminum Workpieces
Final Report Abstract
The overall objective of the project was the prediction of the distortion caused by initial bulk residual stresses (IBRS) and machining-induced residual stresses (MIRS) when milling monolithic thin-walled aluminum workpieces and the development of methods to minimize the distortion. Therefore, first the effect of each RS type on distortion was analyzed individually before understanding their superposition. A repeatability analysis of the MIRS formed the initial position and showed that for stable machining repeatable MIRS led to repeatable distortions [Webe21a]. Furthermore, a set of different machining parameters were identified causing different MIRS, where more and deeper MIRS resulted in higher distortions. Hereby a simple experiment was developed highlighting the distortion potential of RS in the boundary layer of parts: A 1 mm thick wafer was removed at the milled surface. A static, linear elastic finite element model was developed to simulate the distortion due to the measured RS in a short time. The model considers all RS (IBRS and MIRS) contained in the entire part at different locations as well as the milling path. It was validated by various experiments (different geometries, RS, milling strategies). It could be shown that the shear MIRS are crucial and contribute much to distortion (when not compensated for), because they induce a torsional bending moment in addition to the bending moment of the normal MIRS. Furthermore, it could be shown that when making statements about which RS type (MIRS vs. IBRS) contributes more to the distortion, the entire milling strategy and not only the wall thickness, must be considered besides the magnitude and depth of RS: For low IBRS samples (stress relieved) e.g., the MIRS introduced in the surface layer of the pockets are driving the distortion when a zig milling path strategy is used. However, when milling the pockets in spiral form, the low IBRS dominate the distortion. For high IBRS samples the IBRS are driving the distortion. Nevertheless, there is a systematic influence of the combined effect of both RS types found for thin wall thicknesses (< 3 mm) and the zig milling strategy. Furthermore, a 3D FEM cutting simulation was developed to also predict the MIRS. Three main categories of precontrol distortion compensation techniques were identified: The process parameters, the part topology and the process strategy. Each of them influences either the MIRS, the IBRS or both and therefore the distortion. It could be shown that choosing an appropriate milling strategy, e.g. by applying opposite bending moments and aiming for the equilibrium of the shear stresses, which is given for a spiral milling path, is an opportunity to minimize the distortion. A minimization of the distortion by 41 % could be achieved by only changing the milling path from lines to a spiral form. In general, the distortion increases with decreasing wall thickness. But for smaller wall thicknesses the potential of minimizing the distortion by changing the direction of the milling path is higher. For high IBRS parts the distortion was compensated (77 %) by milling the predicted inverse distortion onto the backside of the sample. A conceivable follow-up research should investigate the effect of cryogenic machining on the distortion of thin-walled monolithic aluminum workpieces. The outcome of the project was awarded with the best paper award for the contribution on the 18th CIRP Conference on Modeling of Machining Operations 2021.
Publications
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Concept to analyze residual stresses in milled thin walled monolithic aluminum components and their effect on part distortion. Production at the leading edge of technology - Proceedings of the 9th Congress of the German Academic Association for Production Technology (2019): pp. 287-296
D. Weber, B. Kirsch, C.R. D’Elia, B.S. Linke, M.R. Hill, J.C. Aurich
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Analysis of machining induced residual stresses of milled aluminum workpieces, their repeatability and their resulting distortion. The International Journal of Advanced Manufacturing Technology (2021) 115: pp. 1089-1110
D. Weber, B. Kirsch, C.R. Chighizola, C.R. D’Elia, B.S. Linke, M.R. Hill, J.C. Aurich
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Finite Element Simulation Combination to Predict the Distortion of Thin Walled Milled Aluminum Workpieces as a Result of Machining Induced Residual Stresses. Open Access Series in Informatics (OASIcs) 89 - 2nd International Conference of the DFG International Research Training Group 2057 – Physical Modeling for Virtual Manufacturing (iPMVM 2020) (2021): pp. 11:1-11:21
D. Weber, B. Kirsch, C.R. Chighizola, J.E. Jonsson, C.R. D’Elia, B.S. Linke, M.R. Hill, J.C. Aurich
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Intermethod Comparison and Evaluation of Near Surface Residual Stress in Milled Aluminum. Experimental Mechanics 61 (2021): pp. 1309-1322
C.R. Chighizola, C.R. D’Elia, D. Weber, B. Kirsch, J.C. Aurich, B.S. Linke, M.R. Hill
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Investigation on the scale effects of initial bulk and machining induced residual stresses of thin walled milled monolithic aluminum workpieces on part distortions: experiments and finite element prediction model. Procedia CIRP 102 – Proceedings of the 18th CIRP Conf. on Modeling of Machining Operations (2021): pp. 337-342
D. Weber, B. Kirsch, C.R. Chighizola, C.R. D’Elia, B.S. Linke, M.R. Hill, J.C. Aurich
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Milling-Induced Residual Stress and Distortion Under Variations of Bulk Residual Stress. Proceedings of the 31st ASM Heat Treating Society Conference (2021): pp. 96-99
J.E. Jonsson, C.R. Chighizola, C.R. D’Elia, B.S. Linke, M.R. Hill, D. Weber, B. Kirsch, J.C. Aurich
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Oberflächenmorphologie gefräster Aluminiumwerkstücke. ZWF - Zeitschrift für wirtschaftlichen Fabrikbetrieb 7/8 (2021): pp. 452-455
D. Weber, B. Kirsch, K. Gutzeit, B.S. Linke, M.R. Hill, J.C. Aurich
