Despite the further development of the forward rod extrusion process, it was not possible to achieve the desired strength of the joining zone. For this purpose, different possibilities to readjust the process are being evaluated. One possibility is to increase the joining area between the materials. This can be regulated by an adapted process control during the forming process. Further extrusion processes are possible, which introduce different load profiles into the joining zone. In addition, improved thermal process control can have a positive effect on the joining zone. The focus of this is to increase temperature gradients. Another possibility to improve the joining zone geometry is to adjust it already before forming. Furthermore, the material connection, which is created between the materials as a result of the friction welding process, can also be supplemented by an additional force and form closure on the basis of the joining zone design. The detailed improvement suggestions are discussed below.
In order to adjust the formation of the joining zone in hybrid semi-finished products before forming, the blanks must be machined before the friction welding process. The steel geometry does not undergo any macroscopic changes during friction welding and primarily the aluminium flows. Thus, the steel geometry determines the maximum joining surface. In consideration of these facts, different geometries can be taken into account to improve the strength of the joining zone. Three possibilities are presented below.
Geometry A shows a cone on the steel side, which significantly increases the surface. Comparable geometries are already part of various research projects and even show good joining zone properties after friction welding.
Steel-sided several holes were inserted in geometry B, which are filled with aluminium during the friction welding process. In addition to the significant potential increase in the joining surface, this geometry is particularly useful for absorbing rotational forces (Fig. 5).
Geometry C is designed to create a form fitting undercut. The aim is to adjust the steel geometry in a way that the aluminium is pressed into a steel die and expands. On the one hand, a stronger material connection can be generated due to the significantly larger contact surface; on the other hand, the undercut creates a form fit. This geometry can be further adjusted so that the aluminium encloses a steel-side pin during the friction welding process. This also generates a force fit connection (Fig. 6).
Thermal process control
Within the scope of the study, the focus is primarily on the joining zone development during the forming process. To improve this, two approaches were chosen. One of them is the blank temperature, which is adjusted by semi submerged inductive heating. Due to the hybrid design, the blank heating must be inhomogeneous. As mentioned in chapter 3, the current heating strategy is to heat the hybrid blank exclusively on the steel side. This creates a temperature difference in the blank, which is compensated over time, as a result of heat transfer between the materials. In consequence, the steel cannot be heated to its hot forming temperature, because the aluminium would melt. Currently, the steel is formed in the range of 380–850 °C, whereas the aluminium is about 380 °C, in the joining zone. However, this temperature gradient is constant and ranges between 850 and 220 °C. A larger temperature difference between the materials could lead to a better formation of the joining zone, as the yield stress difference between steel and aluminium is reduced. For this purpose, immersion cooling on the aluminium side is to be integrated into the heating process. This way it is possible to dissipate unwanted heat from the aluminium into the water, to keep it from melting, and thus enables the steel to be heated to a higher temperature. The semi submerged heating process is shown in Fig. 7.
Further extrusion processes
The stress state within the joining zone was identified as the main influence during the forming of the hybrid blanks. This results from the differences in yield stress, but can also be adjusted by counter pressure superposition, as described in Sect. 3.2. The system developed here reaches its limits in terms of process technology, since the counter pressure mechanism counteracts the punch force and high tool loads are occurring. In order to expose the joining zone to different load profiles, other extrusion processes are used for tailored forming. For this purpose, a hollow shaft was selected as a new demonstrator component, which is to be produced by three different impact extrusion processes.
The investigation of different impact extrusion processes serves to control the load profile of the joining zone during the forming process in different ways. Cup extrusion, for example, is characterised by high degree of deformation at high surface enlargements and contact pressures. Figure 8 shows the comparison of the simulative results.
The enlargement of the surfaces at the joining zone can lead to the intermetallic phases formed during friction welding being torn open, so that newly formed surfaces come into contact. If this is done at sufficiently high contact pressures, new positive fit connections can be created or the properties of existing connections can be improved by thermomechanical treatment. The hollow shaft geometry and the impact extrusion processes are shown in Fig. 9.