A numerical model of pin thread effect on material flow and heat generation in shear layer during friction stir welding
Introduction
Friction stir welding (FSW) has been successfully used in joining various kinds of metals and alloys [[1], [2], [3]]. Tool geometry is the most influential aspect of the FSW process development [4]. The primary functions of the tool include the localized heating and material flow so that it plays a critical role in affecting the microstructures and mechanical properties of the weld joints [[5], [6], [7]]. A FSW tool typically consists of a rotating round shoulder and a threaded cylindrical pin, which generates heat locally by friction and drives the softened material to flow in the shear layer [8]. Features of the tool such as thread and flat on the pin surface are believed to improve material flow around the tool, promote oxide breakdown, increase the area of nugget zone, produce high quality welds and affect forces on tool [4,[9], [10], [11], [12], [13], [14]].
It is reported that for clockwise rotation, a left-hand thread could push the material downward adjacent to the pin. Besides, it also led to driving an upward motion of material of an equivalent amount far away from the pin surface in the shear layer, and enhance material mixing in the weld vicinity [11,14]. It has been experimentally proven that during FSW, the threaded pin profile can avoid several defects which are likely to occur for an unthreaded pin [6,15,16]. Zhao et al. [6] used four different pins (column threaded pin, taper threaded pin, column pin and taper pin) to make comparison, and concluded that taper pin with thread profile produced the best weld quality compared to others in terms of defect reduction and enhanced grain refinement. It is reported that wormhole defects are minimized or disappeared in weld nugget zone when threaded pin profile is used in place of unthreaded one for FSW [15,16]. The threaded pin promotes the material flow in a downward direction, thereby enhancing material transportation across the weld root. Ouyang et al. [17] found the presence of vortex-like structure formed by concentric flow lines for welding similar alloy and alternative lamellae of alloy components for welding dissimilar alloys, attributing to ‘stir action’ of a threaded pin. Therefore, a careful consideration of thread profile is vital for FSW tool design. However, for the thread design on pin side, there is a lack of scientific basis in practical application of engineering. Currently, the thread is designed empirically by trial and error. In addition, the underlying mechanism, in context to the variation of flow/thermal behavior with respect to threaded pin, is not properly documented.
In order to obtain dynamic information during FSW process, researchers attempted to observe the real-time plastic material flow around the threaded pin with the aid of sophisticated techniques such as computer tomography (CT) and tracer particles, etc. Schmidt et al. [18] investigated the material flow around the threaded tool by traditional metallography as well as X-ray and CT, and proposed the presence of different flow zones or layers across vicinity of the weldment. Morisada et al. [19,20] visualized the material flow around the pin by real-time X-ray radiography using a tiny spherical tungsten tracer, and obtained the three-dimensional material flow path by following the locus of the tracer. They found a comparatively higher material flow velocity at retreating side (RS) compared to advancing side (AS).
Even though great efforts have been made to observe and characterize the difference of heat generation, temperature profile, material flow and joint quality for unthreaded and threaded pins, most of the experiment-based methods just give some qualitative information. A quantitative analysis of pin thread effects on heat generation and material flow behavior during FSW process is essential to interpret the mechanisms involved. Numerical simulation has been widely used in analyzing the thermo-mechanical process and material flow behavior in FSW to explore the physical nature of the process or to guide tool design [[21], [22], [23], [24], [25]]. Generally, researchers employ computational fluid dynamics (CFD) [[21], [22], [23]] and computational solid mechanics (CSM) [24,25] to model FSW process. Due to the advantages in numerical simulation of material flow, the CFD method has been more widely used in modeling FSW process with complex pin shapes [[26], [27], [28], [29]]. In CFD models of FSW, velocity-based boundary condition [22,23] and shear stress-based boundary condition [30,31] are two categories to treat the contact condition at tool-workpiece interface.
In numerical simulation of the thermo-mechanical behavior and material flow in FSW process, some researchers did not take into consideration the thread features because the presence of threads brings the complexity and difficulty of modeling [[32], [33], [34], [35]]. Other investigators indirectly considered the thread feature by simply applying a uniform vertical velocity at pin side [22,[36], [37], [38]]. Such a presetting of vertical material movement may oversimplify the pin thread effect.
On the other hand, some researchers directly dealt with the thread feature in the geometric model of the calculation domain, and applied special boundary conditions at the tool-workpiece interface [31,[39], [40], [41]]. When the contact conditions at tool-workpiece interface were supposed to be full sticking, no obvious movement of material along the vertical direction due to the presence of thread is predicted [39,40]. Atharifar et al. [41] set the contact condition as partial sticking/sliding. Chen et al. established a CFD model of FSW process by considering an alternative frictional boundary condition [31], and took into account the pin thread features by adopting sliding mesh method in their model [42]. The thread feature was treated as an extra fluid volume which rotates with the same speed as that of the tool and is geometrically connected to the surrounding fluid domain by the mesh interface. The simulation results shown that the interfacial sticking is preferable at the inside of the thread groove, and pin thread causes a many-circle flow pattern around the threaded pin. However, the effect of pin thread on the in-process phenomena during FSW has not been completely understood, and there is still a lack of theoretical guidance in thread design and tool manufacture.
In this study, a new method is proposed to consider the pin thread effects on materials flow and heat generation in FSW. Based on the analysis of the interaction force between the thread groove and the workpiece material, special equations are derived to describe the effect of pin thread parameters on the material flow velocity inside thread grooves, which are taken as the boundary conditions for material flow velocity. Then, a three-dimensional transient CFD model is developed to quantitatively analyze the effect of threaded pin profile on the heat generation, temperature field and material flow characteristics in FSW. Finally, the numerical simulation results are verified by experimentally measuring the thermal cycles and the TMAZ boundary.
Section snippets
Governing equations
Fig.1 shows a schematic illustration of FSW process and its geometrical model features for numerical simulation. The origin of the Cartesian coordinate system is taken at the intersection point between the tool axis and bottom surface of the workpiece. The workpiece material is assumed as an incompressible single-phase non-Newtonian fluid. In order to simplify the FSW model, effects of tool shoulder concavity and tool tilt angle are ignored. The continuity, momentum and energy conservation
Driving effect of thread on workpiece material adjacent to the pin
Due to the influence of driving force produced by pin thread, a large percentage of material adjacent to the pin will move downward when FSW tool with right-hand threaded pin rotates in a counter-clockwise direction, and the material near the TMAZ (thermo-mechanically affected zone) in shear layer will flow upwards [27]. Therefore, it is vital to take into account the driving force of pin thread quantitatively to analyze material flow across thread groove region.
In this study, the pin used has
Effect of thread on material flow in shear layer
Fig. 6 shows a comparison of the material flow behavior for unthreaded and threaded pin profiles at the transverse cross-section of the weldment. Fig. 6 (a) presents the velocity vector at the transverse cross-section of the weldment for threaded pin profile, while Fig. 6 (b) and (c) are the magnified part of the local regions at RS and AS respectively. The red zone in Fig. 6 represents the predicted TMAZ. Fig. 6 (d) presents the velocity vector at the transverse cross-section of the weldment
Conclusions
- (1)
The interaction force between the thread groove and the workpiece material is analyzed, and special equations are derived to describe the effect of pin thread parameters on the material flow velocity inside thread grooves. These equations are combined with a three-dimensional transient CFD model to quantitatively analyze the effect of threaded pin profile on heat generation, temperature field and material flow in FSW.
- (2)
The results show that when tool rotates in counter-clockwise direction, a
Acknowledgement
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51475272).
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