Microstructural evolution in dissimilar Al/Ti interfaces generated under non-equilibrium conditions by solid-state processing

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Multi-material structures are possibly the best alternative to achieve structural weight reduction in the transportation sector. Such multi-material structures are currently joined by either adhesive bonding or in form of mechanical connections, such as riveting. The former brings an environmental burden and the latter a weight penalty. Solid-state joining processes are environmental friendly technologies, capable of producing defect free joints in dissimilar material combinations with superior mechanical properties. These processes are characterized by transient temperature cycles (i.e. high heating rates and cooling rates where the maximum temperatures stays below the melting point of the materials being joined) and high strain rates, causing intensive material flow in the joint region. As a result, non-equilibrium structures are formed in the interface of the joints. Such microstructures have been shown to be directly responsible for the mechanical performance of lightweight structures. A few isolated studies in the literature have investigated the microstructure evolution resulting from thermo-mechanical processes typical of solid-state joining processes for specific materials or material combinations. However, no systematic effort is reported on isolating the effects of thermal cycle and material flow on joint formation in dissimilar materials joints. Furthermore, the effect on specific alloying elements on diffusion phenomena and IMC phase formation in interfacial regions is sparsely reported for joints produced by solid-state processes. The present project addresses the above mentioned knowledge gaps in the understanding of microstructural evolution in bi-metallic interfaces generated under non-equilibrium conditions. To achieve this goal, friction stir welding and refill friction stir spot welding will be employed to produce dissimilar overlap joints between experimental Al-Si-Mg alloys (based on the AA6013 composition) and a Ti-6Al-4V alloy, supported by corresponding numerical process simulations. The experimental Al alloys will contain systematically varied alloying contents to allow for the investigation of the effect of these key elements on microstructural evolution.