Gaoqiang Chen

An Alternative Frictional Boundary Condition for Computational Fluid Dynamics Simulation of Friction Stir Welding

For better application of numerical simulation in optimization and design of friction stir welding (FSW), this paper presents a new frictional boundary condition at the tool/workpiece interface for computational fluid dynamics (CFD) modeling of FSW. The proposed boundary condition is based on an implementation of the Coulomb friction model. Using the new boundary condition, the CFD simulation yields non-uniform distribution of contact state over the tool/workpiece interface, as validated by the experimental weld macrostructure. It is found that interfacial sticking state is present over large area at the tool-workpiece interface, while significant interfacial sliding occurs at the shoulder periphery, the lower part of pin side, and the periphery of pin bottom. Due to the interfacial sticking, a rotating flow zone is found under the shoulder, in which fast circular motion occurs. The diameter of the rotating flow zone is smaller than the shoulder diameter, which is attributed to the presence of the interfacial sliding at the shoulder periphery. For the simulated welding condition, the heat generation due to friction and plastic deformation makes up 54.4 and 45.6% of the total heat generation rate, respectively. The simulated temperature field is validated by the good agreement to the experimental measurements.

Recent Development and Applications of CFD Simulation for Friction Stir Welding

Friction stir welding (FSW) has been successfully applied in fabricating many critical structures, e.g. rocket fuel tanks. Generally, CFD simulation is required to better understand the in-process material flow during FSW. In this paper, we discuss the concepts and the approaches that have been employed in the recent development and application of the CFD simulation for FSW. First, special considerations on friction, heat generation and transient tool motion have been adopted to capture the fully-coupled heat-and-mass-flow phenomenon during FSW. Second, temporal evolution of the material state during welding is analyzed by interpolation and integration along the flow paths, which is further used to predict the typical feature and defects in the welds. Third, the CFD-based predictions on the temperature and the material flow are validated by experimental measurements. Finally, the current concepts and approaches in the simulation of FSW could be applied in CFD-based studies on other similar thermal-mechanical processes.

On the Material Behavior at Tool/Workpiece Interface During Friction Stir Welding: A CFD Based Numerical Study

The material behavior at the interface between the welding tool and the workpiece in friction stir welding (FSW) is a critical phenomenon governing heat generation and material flow. Previous computational modeling studies tend to simplify the material behavior at the interface, generally assuming simple stick or frictional slip conditions. In this paper, a new approach is developed that allows for dynamic friction between tool and workpiece, based on considerations of relationship between frictional force and material velocity at the interface. This new approach is implemented in a computational fluid dynamics (CFD)-based model to simulate the heat transfer and material flow. The simulated temperature field and velocity distribution are validated by experiments. The simulation results show that material velocity on tool/workpiece interface is significantly different from the tool velocity and increase with the interfacial frictional stress. It is found that when frictional stress is not large enough to enable large extent plastic flow, asymmetrical material flow appears. Material on the advancing side tends to flow at lower velocity than that at retreating side.

Effect of Strain Hardening Constitutive Relations on Weld Residual Stress Simulation of Dissimilar Metal Weld

Weld residual stress (WRS) in dissimilar metal welds (DMWs) has been identified as an important driver for primary water stress corrosion cracking, which is observed in nuclear power plant safety-related components. In this work, a newly developed dynamic strain hardening rule is implemented in finite element (FE) thermal-mechanical model to simulate the residual stress distribution in a dissimilar metal weld studied in a recent NRC/EPRI Round Robin study. This new dynamic strain hardening constitutive rule takes into account the effect of dynamic recovery and recrystallization at elevated temperatures on the strain hardening behavior during welding. Weld residual stresses calculated using the new dynamic strain hardening rule are compared to those with the conventional strain hardening ones (isotropic and kinematic), as well as the experimental measurement data. The new dynamic strain hardening rule results in improvements in WRS prediction.Copyright

The influence of stored energy on the fatigue crack propagation of friction stir welded aluminum alloy

Fatigue crack propagation (FCP) rate was tested in friction stir welded (FSW) joint. The high FCP rate was detected in the stir zone (SZ) with refined grain size. Differential scanning calorimetry tests were conducted. The results showed that there was relatively higher energy stored in SZ. After releasing the stored energy, the resistance of FCP in SZ improved significantly while the grain size kept the same and the fatigue fractographs of crack propagated in SZ had translated from quasi-cleavage to plastic fatigue striation fracture. Those prove that the key reason leading to high FCP rate in SZ is the stored energy.