Abstract
Strategic utilisation of materials, particularly their effective integration, is a key factor in product innovation aimed at enhancing performance. However, assembling these materials poses significant challenges, particularly in ensuring that the joining processes do not compromise the individual materials' intrinsic properties or the final component's functionality. Cold welding offers a promising solution, in particular dealing with ductile metallic materials, as it requires no heat supply, thus preserving the original mechanical and electrical features of the materials involved. This process, which relies on plastic deformation to achieve atomic contact at interfaces, can produce joints that are often stronger than the base materials themselves while forming a hardened mechanically affected zone that enhances joint integrity.
Despite its advantages, the application of cold welding for joining dissimilar alloys poses significant challenges due to differences in mechanical properties and chemical composition. Materials with different mechanical behaviours present a mismatch in the flow stresses that must be overcome to induce simultaneous plastic deformation in both pieces. On the other hand, differences in chemical composition hinder spontaneous metallic bond formation and thus require atomic interdiffusion to establish strong multimaterial bonds. However, these metallic materials often exhibit low diffusion rates at room temperature, making atomic interdiffusion unlikely in the absence of an external heat source. Therefore, understanding the interplay between these mechanical and chemical factors is critical for advancing cold welding technology, particularly in microfabrication, where joint integrity and performance are crucial.
The study first addresses the mechanical challenges by investigating cold butt welding of aluminium wires with different mechanical properties, specifically soft AA1070 and hard AA6082 alloys. It demonstrates that during bonding, the harder alloy can penetrate the softer one as internal pressure builds up, allowing for effective plastic deformation and bonding. The successful joining of dissimilar alloys requires greater deformation and strain levels than similar joints, leading to severe grain refinement at the weld interface, ultimately strengthening the joint. Building on these findings, the study focuses on cold welding between the commercially pure AA1070 aluminium and pure Electrolytic Tough Pitch copper. Through an analytical model found in the recent literature, the work shows that plastic deformation can promote atomic interdiffusion without requiring elevated temperatures, supporting the feasibility of high-quality cold welding in diverse multimaterial applications, including microelectronics. To further explore these concepts, the research employs in situ mechanical testing and advanced microstructural examinations. It demonstrates that robust and reliable joints can be formed between dissimilar metals, such as aluminium and copper, and explains the mechanisms that make it feasible.
The study ultimately culminates in the development of a microscale cold welding testbed technique for aluminium and copper alloys, utilising a Focused Ion Beam-Scanning Electron Microscope (FIB) for real-time monitoring and process optimisation. Initial experiments demonstrate the feasibility of solid-state welding at the microscale, though the early experimental setup encountered challenges in achieving reproducibility and control. Subsequent refinements, including a more advanced setup and introducing a xenon Plasma-FIB, significantly improved process control and reduced defects. Optical, scanning and transmission electron microscopy techniques revealed the material flow and the mechanism by which superficial oxides are disrupted, while the presence of intermetallic phases proves that interdiffusion occurred. It is demonstrated that the efficiency of cold welding is also preserved at the microscale, as micro-welded interfaces appear more robust than the base metals themselves. The potential for industrial scaling of the optimised setup is also underscored, emphasising the need for automation and further refinement of process parameters to ensure consistent weld quality.
This thesis provides valuable insights into the mechanisms of cold welding, both at the macroscopic and microscopic scales. By investigating how different factors influence the cold welding process, the research offers a clearer understanding of how to join dissimilar materials effectively. The findings contribute to improving current welding techniques and open up possibilities for more reliable and efficient methods, particularly in areas like advanced manufacturing and microelectronics.