Experimental determination of the limiting bulge height in hydromechanical deep drawing

Abstract: Owing to fluid pressure, the unsupported blank between the punch and the drawing ring is bulged in the direction opposite to the punch movement. The blank is thus drawn over a fluid cushion and does not contact the drawing ring profile radius. The bulge is hence advanta-geous for ensuring class A surface quality of the workpiece. The blank is stretched bi-axially in the bulged region owing to the acting fluid pressure. This can result in blank fracture at high-fluid counter pressures. High-fluid counter pressures are, however, necessary to achieve higher geometrical accuracies and lower springback. High pressure can also be used to transfer small contours from the punch to the drawn cup. A high-pressure die set for hydraulic bulging has been designed in order to emulate the bulge formation in hydromechanical deep drawing. A series of experiments have been carried out in order to determine the limiting bulge height for different sheet materials. Different punch diameters have been used to determine the influ-ence of drawing clearance on the limiting bulge height. The effect of fluid pressure on the smallest transferable radius has also been investigated. Results thus obtained can be used in determining the dedicated optimal counter pressure curve for a given workpiece geometry, material, and die set combination in order to design a robust computer-aided engineering (CAE) chain for the hydromechanical deep drawing process. Selected representative experi-mental results for hydromechanical deep drawing of complex part geometries have been shown. The counter pressure curve for hydromechanical deep drawing of the shown cups was selected in conformance to the allowable bulge height for the considered instantaneous sheet material, part shape, and process parameter combination