Qingyu Shi

Residual stresses simulation for friction stir welded joint

Abstract As a solid state joining technique, friction stir welding (FSW) can produce high strength, low distortion joints efficiently. Compared to fusion welding, residual stresses in FSW joints are expected to be low due to a relatively low heat input. However, apart from the heat input, the force from the tool also plays an important role in the development of welding stresses. In the present paper, a semicoupled thermomechanical finite element model containing both thermal load and mechanical load was established to simulate the development of welding stresses during FSW process; an autoadapting heat source model was employed in the thermal analysis; the fixture was also included in the mechanical analysis model. The simulation results showed that due to the effect of the tool force, the longitudinal residual tensile stresses became smaller and were asymmetrically distributed at different sides of the weld centre; the peak of the tensile residual stresses at the retreating side was lower than that at the advancing side. Calculated and experimental results were compared.

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.

Experimental study on distortion of Al-6013 plate after friction stir welding

Abstract Friction stir welding (FSW) experiments with different panel dimensions and welding parameters have been designed to study the distortion of FSW. The FSW experiments were carried out with a load control facility to make the welding parameters reliable. The distortion of FSW is much smaller than that of arc welding, but it is still very significant. Three-dimensional distortion measuring system was applied to further study distortion trends. The results show that the distortion after FSW is in saddle shape, with convex bending in longitudinal direction and concave bending in transverse direction. This distortion pattern is in contrary with that of traditional arc welding. It is also found that increasing the panel length increases the longitudinal distortion but almost do not influence the transverse distortion. Increasing the rotation speed increases both longitudinal distortion and transverse distortion. The influence of welding speed on distortion is not very clear.

Sensitivity analysis of history dependent material mechanical models for numerical simulation of welding process

Abstract Yield stress of 6013-T6 aluminium alloy was tested on Gleeble 1500D thermal–mechanical system at predesigned temperatures during different typical thermal cycles, in order to accurately reflect the influence of weld thermal history on material properties. The typical thermal cycles were referred to the temperature field simulation results of real welding process. The changes of yield stress were obtained directly from the stress–strain curves generated by the tensile tests. The tests were more accurate than previous publications, where only the yield stresses at room temperature after thermal history were tested or calculated from microstructure evolution model. Experimental results showed that the changes of yield stress during the cooling stage of typical thermal cycles followed one set of curves. These yield stress–temperature curves were different from those during the heating stage. Temperature and temperature history dependent material model M2 and M3 were established based on the experimental results. M2 model was perfectly plastic model while work hardening effect was considered in M3 model. Compared with conventional temperature dependent material model M1, the distributions of longitudinal residual stress and strain obtained with temperature and temperature history dependent models fit better with published results. Yield stress of the material at the weld zone decreased a lot after having experienced weld thermal history and longitudinal compressive plastic strain at the weld zone recovered to some extent during the cooling stage in M2 and M3 models. These were the main causes for lower peak longitudinal residual tensile stress in M2 and M3 models.

Preventing welding hot cracking by welding with an intensive trailing cooler

Abstract Welding with an intensive trailing cooler has been investigated as a means to prevent welding hot cracking. Hot cracking initiated from the crater and extending backwards has been reproduced using a welding fixture with variable restraint. Welding with a trailing cooler can effectively prevent this kind of hot cracking and is feasible in welding production. The temperature field and the strain distribution have been analyzed using the non-linear finite element method. The results of experiment and numerical simulation expound the mechanism of the prevention of hot cracking.

Residual Stresses of Friction Stir Welded 2024-T4 Joints

Friction stir welding (FSW) is a solid-state joining technique which can produce high-quality joints efficiently. The residual stresses in FSW are generated due to the effect of both the uneven temperature field and of the tool force, which is different from that in fusion welding. In this study the residual stresses of 3mm-thick 2024-T4 aluminum alloy FSW joints have been investigated by using the Hole-drilling method. To reduce the influence of drilling upon the experimental results, annealed stress-free 2024 aluminum alloy plates were drilled; the relieved strains were measured and were subtracted from the total strains measured from the joints. The results showed that the longitudinal residual stresses in the joint were much larger than the transverse residual stresses; high longitudinal tensile residual stresses were concentrated near the tool shoulder direct affected zone and asymmetrically distributed at the different sides of the weld line; i-e, high at the advancing side and relatively low at the retreating side. Outside the tool shoulder direct affected zone, the longitudinal residual stresses decreased rapidly and became compressive residual stresses away from the weld line; the peak of the longitudinal residual stresses was 164.5MPa.The mechanism of the generation of the residual stresses was analyzed preliminarily.

Line Gauss heat source model: an efficient approach for numerical welding simulation

Abstract In view of the characteristics of the welding heat source, a new heat source model based on an established Gauss model, termed the line Gauss heat source, is developed to improve calculation efficiency. A typical welding process is simulated using both Gauss and line Gauss source models. The results show that using the new model, analysis time can be reduced greatly while high precision is maintained, and this method renders three-dimensional simulation of some real welding structures containing long welds practical. The cutting processes for the side plate of a bridge crane beam are simulated using the line Gauss source model, and the results agree fairly well with real measurements.

Microcavities accompanying a zigzag line in friction stir welded A6082-T6 alloy joint

Abstract An Al–Mg–Si alloy was friction stir welded (FSW), and the microstructure of the zigzag line in the welds was investigated using optical microscopy, energy dispersive X-ray spectroscopy and scanning electron microscopy. The effect of the zigzag line on the mechanical properties of the as welded joints was also examined. It was found that in the welds with high heat input, small discontinuous cavities or microcracks of several micrometres in size exist along the zigzag line, and the microcracked zigzag line was found to significantly affect the face bend properties of the FSW joint, but had limited influence on the tensile properties of the butt welds. In joints with low heat input, the zigzag line was only composed of oxide particles, no cavities or microcracks were detected at the zigzag line.

Auto-Adaptive Heat Source Model for Numerical Analysis of Friction Stir Welding

Heat generation is a dynamic process under solid state joining conditions during friction stir welding (FSW). In this article, a new heat source model in which heat generation, depending on yield stress and tool rotation speed, was established and was applied to simulate thermal field of FSW welded 2024-T3 panels. Results showed that for 2mm/s welding speed and 400rpm rotation speed, the simulated temperature-time curves on different positions corresponded well with measured results. With the increasing of rotation speed, temperature increased steadily but tended to a saturation state at high rotation speeds, which is consistent with the fundamentals of solid state joining.

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.