Determination of the friction stir welding window from the solid-state-bonding mechanics under severe thermomechanical conditions

The vast literature and industrial standards state that most solid-state bonding techniques, particularly the diffusion bonding, are mainly governed first by the plastic crushing of rough surface asperities, and then by volumetric inter-diffusion that eliminate the interfacial pores. We shall demonstrate that the first stage plays an insignificant role and the second one is not relevant at all. In this work, first, we note that the evolution of interfacial cavities (or pores, voids, etc.) under applied thermomechanical loading histories is a reverse process of the high-temperature creep fracture of polycrystalline materials by grain-boundary cavities. The well-established knowledge in the latter suggests that the interfacial cavity evolution be governed by the Needleman-Rice length scale, dictated from the comparison between a lateral diffusive process on the bonded interface and the creep deformation. In this regard, we derive a general modeling framework of bonding fraction evolution, which directly depends on the stress, strain rate, and temperature fields near the interface. Second, the above bonding model is applied to the Friction Stir Welding (FSW) process. The full field information from our prior simulations is used as inputs to assess the evolution and extent of bonding fraction at the workpiece-workpiece interface. Based on the stick-slip contact analysis, an approximate but analytical solution has been developed to derive the bonding fraction field, and the predicted ultimate bonding extent with respect to these parameters becomes a figure of merit for the study of processing window for industrial applications and design of the FSW process.