NSF/DFG Collaboration to Understand the Prime Factors Driving Distortion in Milled Aluminum Workpieces

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Thin-walled workpieces of aluminum are distorted during milling due to the residual stresses inside the workpiece. Distortion reduces the machining accuracy. Distortion necessitates time-intensive and expensive rework or can result into rejection. Within this proposal, we will investigate how the distortion of thin-walled monolithic workpieces can be controlled when milling. Experimental investigations are carried out and a finite element model for the prediction of the distortion of the workpiece is developed. Dry milling experiments are conducted to generate fundamental knowledge about the distortion phenomena. These experiments provide a quantitative database for the finite element simulations. The distortion of the workpiece is analyzed with regard to multiple parameters. Those are the cutting condition, the workpiece geometry, the clamping device, as well as the magnitude and distribution of the initial bulk residual stress. The experimental results allow for the specification of required boundary conditions for the finite element model. Moreover, experimental and numerical results can be continuously compared in order to evaluate the capability of the finite element model. The x-ray diffraction method and contour method will be applied to measure the residual stresses. The comparison of the results will facilitate the evaluation of the capability of both methods. This helps to establish or further improve the methods. Workpiece distortion is induced by both bulk and machining residual stresses. Both will be considered in the finite element model. With the numerical and experimental results we will explore the significance of the bulk and machining induced stress for the distortion. Possible stress regimes will be determined, where bulk residual stresses only, machining residual stresses only, or a combination of bulk residual stresses and machining residual stresses define the distortion. Numerically generated milling strategies are experimentally used to investigate the possibility to compensate or counteract bulk residual distortion. The result of the research project is a significant improvement of the machining accuracy, the efficiency, and sustainability in milling of thin-walled monolithic workpieces.