Cold Pressure Welding of aluminium: conventional and FIB-assisted microscale techniques

Cold Pressure Welding (CPW) is an example of a solid-state welding method whose resulting joints are generally characterized by a high metallurgical efficiency. The pieces to be coupled, typically wires or thin plates, are put in close contact under a certain pressure and achieve bonding by severe plastic deformation and surface expansion. These are two of the many phenomena contributing to bond formation in the HYB process (Hybrid Metal Extrusion and Bonding), which is a cold-welding technique patented by researchers of the group. To better investigate the bonding mechanisms happening during HYB welding, CPW has been taken as a simplified model for reproducing and monitoring HYB at the microscale. Therefore, a CPW-inspired technique has been developed and optimized for the microscale by means of a Focused Ion Beam microscope. By employing a tailored setup of the instrument, it was possible to recreate CPW bonding conditions between two aluminium pieces: an AA1070 aluminium plate as Base Metal (BM) and an AA6082 wire used as Filler Metal (FM) (Figure 1). Cross-sections of the resulting axisymmetric V-shaped micro-joint were investigated by high resolution SEM imaging and compared with reference CPW samples. EDS analysis highlighted the presence of implanted gallium particles and a relatively thick oxide layer at the materials interface, which being both detrimental for the joint mechanical properties have worked as guide for further technique improvements. Moreover, a variation of the setup was employed to perform an in-situ experiment that allowed to observe the on-going process, giving useful information on the deformation flow of the surfaces put in contact. A plan for future micro-mechanical testing is shown and discussed.