Mechanics of the solid-state bonding under severe thermomechanical processes

Friction stir welding (FSW) has found increased applications in automotive and aerospace industries due to its advantages of solid-state bonding, no fusion and melting, and versatility in various working conditions and material combinations. The extent and quality of the solid-state bonding between workpieces in FSW is the ultimate outcome of their industrial applications. However, the relationship among processing parameters, material properties, and bonding extent and fidelity remains largely empirical, primarily because of the lack of the mechanistic understanding of (1) tool-workpiece frictional behavior, and (2) bonding formation and evolution.

In this dissertation, to study the underlying mechanism of tool-workpiece frictional behavior and bonding evolution at workpiece-workpiece interface during solid-state bonding process, firstly, a numerical model that take advantage of Coupled Eulerian Lagrangian (CEL) method is implemented to investigate the stick-slip behavior at tool-workpiece interface. An analytical model is also developed to correlate the stick-slip fraction to processing parameters such as the tool spin rate, and further to derive dimensionless functions for torque and heat generation rate predictions. These analyses provide the critical strain rate and temperature fields that are needed for the bonding analysis. Then, we note that the interfacial solid state bonding process under applied thermomechanical loading histories is a reverse process of the high temperature creep fracture of polycrystalline materials by grain boundary cavities, in this regard, a general modeling framework of bonding fraction evolution was derived, which directly depends on the stress, strain rate, and temperature fields near the interface. Finally, Based on the stick-slip contact analysis and the understanding of solid-state bonding mechanism, an approximate yet analytical solution has been developed to derive the bonding fraction field from the given processing, geometric, and material constitutive parameters, 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.