Investigation of the joining zone formation of impact extruded hybrid components by varied forming sequence and partial cooling

Abstract

Hybrid material concepts enable the combination of beneficial properties of different materials to extend the limited potential of monolithic components. When it comes to steel and aluminium, a wear-resistant and a lightweight metal are combined to produce a weight-reduced high-performance component with load-adapted areas. A method to create hybrid gear shafts is a novel approach called Tailored Forming. The process chain consists of joining e. g. by friction welding and subsequent impact extrusion under elevated temperature. Before forming, an axial temperature gradient is set in the serial arranged semi-finished products to adjust the different yield stresses of the dissimilar materials through induction heating of the steel part. The subsequent forming is intended to positively influence the joining zone thermo-mechanically and geometrically. However, prior work indicated a limitation of the influence on the joining zone in forward rod extrusion. Therefore, approaches are being researched that enable a stronger formation of the joining zone geometry to influence the resulting bond qualities through surface enlargement. A forward rod extrusion process of friction welded hybrid semi-finished products made of 20MnCr5 (AISI 5120H) combined with EN AW-6082 (AA6082) was carried out experimentally. Complementary to prior investigations, in which mainly the aluminium section was reduced through the die angle followed by the steel, the forming sequence of the materials was reversed to increase the joining zone surface with variation of the forming path. Additionally, a cooling of the aluminium side was realized through an immersion cooling to adjust maximum temperature gradients and further equalize the different yield stresses. Hardness tests, metallographic and SEM images of cross-sections were taken to evaluate the bond quality with regard to the temperature influence, joining zone formation, occurring defects and the resulting intermetallic compound (IMC). Impact extrusion with initially steel formed followed by aluminium resulted in a spherical formation of the joining zone and consequently in greater surface area, but also lead to partial defects in the IMC. The partial cooling of the aluminium allowed higher temperature gradients to be set, thus reducing defects through improved material flow in the joining zone.