Improved formability of HMGF components by preforming in an upset bulging process

Abstract

Hot Metal Gas Forming (HMGF) is a well-established combination of a profile hydroforming operation and an in situ hardening process. Roof rails and camshafts are typical applications. A significant limitation of the component geometry is the strain-related material thinning. In the polar coordinate system of the tube-shaped parts, the strain can be divided into circumferential and longitudinal portions. To achieve higher expansion ratios in circumferential direction, a longitudinal compression is necessary. In a conventional, non-tempered process setup this is achieved by an axial feeding movement during the forming operation. Under tempered conditions, in particular by friction coefficients well above 0.3, this is no longer possible in a controlled process. Anyhow, a beneficial material pre-distribution is required to enlarge the circumferential strain before material failure occurs. In the proposed preforming operation, the longitudinal compression is achieved by offset bulging. Based on aspect ratios between initial diameter, wall thickness, initial length and compression stroke, a specific, controlled wave pattern is achieved on the specimen. Subsequently, the semi-finished parts undergo the HMGF process in an austenitisation, transfer, forming and quenching procedure. By means of continuous, finite element based simulation and related model calibration in experimental trials, a significant increase in geometric complexity is achieved.