Unconventional use of Ga+ FIB and Xe+ Plasma-FIB for performing and investigating microscale cold-welding of dissimilar aluminium alloys

Cold Pressure Welding (CPW) is an example of a solid-state welding method whose resulting joints are generally characterized by high metallurgical efficiency. The pieces to be coupled, typically wires or thin plates, are put in close contact under certain pressure and achieve bonding by severe plastic deformation and surface expansion. However, for the microscale, similar examples of techniques not involving heating cannot be found in the literature. Therefore, a CPW-inspired technique was developed and optimized for the microscale first by means of a Ga+ Focused Ion Beam microscope and successively optimized by employing a Xe+ Plasma-FIB during the sample preparation and the joints characterization steps. 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 on the micro-joints fabricated at the Ga+ FIB highlighted the presence of implanted gallium particles along the entire welded interface. Diffusion of gallium along aluminium grain boundaries is well known for heavily reducing aluminium’s strength and thus it needs to be avoided by employing a Xe+ PFIB for the phases before and after the coupling step. Future micro-mechanical testing is expected to show a better resistance behavior in the samples mainly fabricated at the FIB. A discontinuous oxide layer was also detected in correspondence with the lack of bonding regions, indicating that shear stresses occurring during the process should be enhanced to promote the expulsion of contaminants. Moreover, a variation of the setup was employed to perform an in-situ experiment that allowed to observe of the ongoing process, giving useful information on the deformation flow of the surfaces put in contact.