Friction stir welding (FSW) is a quintessential solid-state-bonding technology, in which the workpieces are metallurgically bonded at the interface, under significant heating from tool-workpiece frictional sliding and plastic deformation in the workpieces. An inherent advantage of FSW is thus its immunity from defects and property deteriorations associated with solidification. From the viewpoint of materials processing, it has been found that strain rate, temperature, and stress fields are affected both by thermomechanical processing parameters in FSW and by the properties of the joining materials, a multitude of experimental and numerical studies have been conducted to understand the characteristics of these thermomechanical fields. A mechanistic understanding of the dependence of bonding evolution on processing parameters and materials behavior is of critical importance as it will eventually affect the structural integrity of the weldments. In this work, a numerical model will be proposed to study the temporal evolution of thermomechanical fields on the workpiece-workpiece interface during FSW process, aided with a theoretical analysis of the interface stick-slip behavior. A quantitative understanding of the solid-state bonding in FSW will be established and the evolution of the bonding fraction on the workpiece-workpiece interface will be predicted.
Friction stir welding process is simulated through Coupled Eulerian-Lagrangian (CEL) finite element method in the commercial software, ABAQUS. This CEL approach divides the entire control volume into Eulerian and Lagrangian domains, thus overcoming the difficulty in frictional modeling in CFD and the difficulty in large deformation in computational solid mechanics. The entire FSW procedures, including plunging, dwelling and welding, are considered in the simulation, hyperbolic sine law is used to describe the material behavior under extremely high stress and temperature conditions. In this work, the material of interest is Al6061-T6, corresponding simulated transient temperature, stress, and strain rate fields in FSW will be reported.
During FSW process, the interfacial bonding between workpieces is the shrinkage of the interfacial cavities, which can be regarded as the reverse process of the cavity growth in high temperature fracture. A kinematic constraint, Needleman-Rice length scale (LNR), is used to evaluate the competition between the diffusional and creep processes during bonding evolution, larger LNR represent interfacial diffusion governed cavity closure, a small LNR corresponds to the creep dominant closure of cavities. Based on the contour of LNR, deformation mechanism map and the thermomechanical history data on workpiece interface, dominant mechanism that controls the cavity closure could be determined, an appropriate model will be chosen to give a quantitative prediction on the evolution of interfacial bonding fraction.
Results and discussion
The thermomechanical histories of four representative reference points on the interface are obtained through CEL model. These thermomechanical histories are compared to both the deformation mechanism map and the contour plots of LNR, which upholds that creep-dominated cavity closure be the solid-state bonding mechanism. A quantitative prediction of the bonding fraction is presented for these four reference points, two different initial bonding fractions, which represents finely polished and rough surface separately, are chosen to avoid the need to conduct experimental characterization of interface morphology. According to the prediction, the area fraction of bonding, fb, remains almost unchanged until the reference point falls into the thermomechanical process zone. Before the tool moves right to the top of the reference point, fb rapidly increases to unity even with a very small initial bonding fraction, indicating that a full degree of bonding can be achieved mainly by creep before the reference point falls into the wake of the tool. Additional calculation which considered diffusion-driven reduction of fb has also be presented, the result indicates that the diffusion-driven part has little contribution to the overall evolution.
In summary, from the simulated thermomechanical histories of a number of reference points on the workpiece-workpiece interface, we find out the interface traverses in regimes with very low Needleman-Rice length scale, thus indicating the dominance of creep-controlled cavity closure. The evolution rate of the interfacial bonding depends primarily on the creep strain rate in the surrounding workpieces abutting at the interface, but not on the far-reaching temperature field. This study helps reveal design strategies in promoting the solid-state bonding in FSW by entering and staying in the creep-dominant interfacial cavity closure through tuning materials constitutive parameters, thermomechanical processing parameters, and geometric shape factors.