Comparison of the Joining Zone development of Hybrid Semi-Finished Products after different Extrusion Processes

Abstract

The use of hybrid semi-finished products made of aluminium and steel enables the production of components with locally adapted properties, i.e. high strength and wear-resistance with reduced weight. In the scope of this work, different impact extrusion processes for the forming of friction-welded hybrid semi-finished products consisting of steel (20MnCr5) and aluminium (EN AW-6082) were developed and experimentally implemented. The resulting material flows were intended to enable different joining zone geometries as well as to evaluate the influence of a thermo-mechanical treatment during the impact extrusion process on the quality of the joining zone. For this purpose, a full-forward extrusion, cup-backward extrusion, combined cup-backward-full-forward extrusion and a hollow-forward extrusion process were investigated. The evaluation of the resulting component quality was carried out based on metallographic images, which provide microstructural information about the forming-related influence on the friction welded joining zone. Based on the characteristic values determined, a correlation between the reproducibility and quality of the joining zone properties and the type of impact extrusion process is deduced. The backward extrusion processes have proven to be the best processes in terms of influencing the joining zone geometry. Further, the effect of forward extrusion showed no significant influence on the joining zone geometry, even resulting in a reduction of the joining zone formation in the combined cup-backward-full-forward extrusion process.

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

“…If the yield stress difference between steel and aluminium is too high, the local deformation is not sufficient and the different materials flow one after the other into the extrusion shoulder with only a parallel displacement of the interface. In previous work [17], the influence on the joining zone properties after friction welding and impact extrusion under elevated temperatures of steel-aluminium billets was investigated. In various forming processes such as backwards cup (full forward) extrusion or hollow forward extrusion, it was possible to geometrically influence the joining zone so that larger joining zone surfaces were created.…”

“…1). Since in forward rod extrusion no additional compressive stress is initiated by forming tools as in the other processes investigated, the axial temperature gradient is particularly important in the case in order to set the previously mentioned local deformations resulting in a spherical shape of the joining zone [17]. Due to this, a larger temperature gradient must be set in order to further equalise the yield stresses and also to influence the joining zone geometry.…”

“…Particularly in cold extrusion of hybrid materials, fractures occur inside the component due to differing yield stresses, which can be significantly reduced by this measure with the similar effect of the selection of material sequence [11][12][13]. However, this does not have any influence on the surface enlargement [17], but is required to ensure the reduction of defects.…”

Influencing the mechanical properties of pre-JOINED hybrid semi-finished products by impact extrusion

“…The materials are welded together at the end faces in the first step using rotary friction welding. This is followed by either cup backward extrusion (CBE), cup backward full forward extrusion (CBFFE) [10] or hollow forward extrusion (HFE) [10,11] under elevated temperatures to produce a near-net-shape preform, which is then given its functional surfaces by machining.…”

“…First, the semi-finished products are joined together in a serial arrangement by rotatory friction welding on the KUKA Genius Plus friction welding machine with parameters chosen according to a previous study [10]. To achieve a defect-free bond, the welding surfaces are machined and cleaned of residues with ethanol.…”