Experimental Validation of an FSW Model with an Enhanced Friction Law: Application to a Threaded Cylindrical Pin Tool

This work adopts a fast and accurate two-stage computational strategy for the analysis of FSW (Friction stir welding) processes using threaded cylindrical pin tools. The coupled thermo-mechanical problem is equipped with an enhanced friction model to include the effect of non-uniform pressure distribution under the pin shoulder. The overall numerical strategy is successfully validated by the experimental measurements provided by the industrial partner (Sapa). The verification of the numerical model using the experimental evidence is not only accomplished in terms of temperature evolution but also in terms of torque, longitudinal, transversal and vertical forces.


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Friction Stir Welding (FSW) is a solid state joining technology in which friction and plastic dissipation are 28 sources of heat generation and material softening.

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The tool pin profile has a remarkable effect on the friction between the tool and the workpiece and the foremost 30 effect on the plastic deformation of the surrounding material. FSW pin tools are often featured with thread 31 forms as they are beneficial for improving the tool performance and contribute to an effective material 32 transportation near the weld and the generation of a defect free stir zone [1].

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Pin tools with threaded features are often used to investigate the relationship between the tool and the 34 microstructural properties obtained using different welding conditions.   In [1], the heat treatable AA 6061 and non-heat treatable AA 5086 aluminum alloys are welded by using three 39 different pin tools. It is found that FSW using threaded cylindrical pins provides better material flow between 40 two alloys among others.

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In [3], the influence of the tool geometries upon the axial and translational forces, temperature and mechanical 42 properties for AA7075-T6 is studied. In their experimental work, the threaded tapered, non-threaded triangular 43 and non-threaded cylindrical pins are considered.

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In [4], the effect of tool geometry on friction stir welding of polyethylene-polypropylene is investigated.

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Threaded cylindrical, squared, triangular and straight cylindrical pin shapes are considered. Interaction effects 46 of welding variables, including rotational speed and traverse speed are studied.

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In [5], a half-threaded pin tool to enhance the material flow at the lap interface is manufactured. The effect of 48 manufactured pin on the process is compared with that of full-threaded pin in terms of temperature, bonding 49 and material flow. It is observed, for instance, that the peak temperature during the process using the 50 half-threaded pin is lower than that using the full-threaded pin.

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A 3D finite element analysis is able to deal with several process complexities such as a concave shoulder, tool 63 tilt and threaded pin profiles. However, the large computational cost makes it inconceivable as a routinely used 64 design tool [8]. In previous works of the authors, a robust and fast numerical model was developed to study 65 FSW under different welding conditions [9][10][11][12][13][14][15]. A fully coupled thermo-mechanical model together with an 66 enhanced friction law was addressed to provide a more realistic thermo-mechanical response in comparison 67 with the existing models. The model took the benefits of an apropos kinematic framework combing Arbitrary

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Lagrangian Eulerian (ALE), Eulerian and Lagrangian formulations for the stir zone, the workpiece and the 69 pin-tool, respectively. A two-stage speed-up strategy was incorporated to reduce the simulation time while 70 preserving the accuracy of the results.

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In the present work, the model previously developed by the authors is adopted for the simulation of a FSW 72 process with a cylindrical threaded pin tool. The use of an apropos kinematic framework permits dealing with 73 arbitrary pin shapes as the threaded pin tool, without the necessity of using a re-meshing procedure due to the

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Also, the differences between threaded and featureless cylindrical pins of similar dimensions are studied in 85 detail.

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The paper is structured as follows: In the section 2, the overall solution strategy applied for simulation of FSW 87 process using cylindrical threaded pin tool is summarized. In section 3, the numerical assessment and the 88 calibration of the model using the experimental data are presented. Section 4 is devoted to the comparison of 89 the weld obtained using threaded and featureless cylindrical tool pins.
Heat flux e s  :

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The first stage consists of a "forced" transient analysis aiming to reach the steady-state quickly. This objective is

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The second stage performs a transient analysis in which the temperature and velocity field obtained in the first 111 stage are considered as initial condition.

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In the first stage, an Eulerian framework is adopted for the workpiece. Therefore, no periodic stage due to the 113 rotating movement of the tool is assumed. In the second stage, an apropos kinematic framework is adopted  The modified Norton's friction law reads: where T τ is the friction shear stress, 0≤q≤1 is the sensitivity parameter and T v ∆ is the relative sliding 133 velocity between the tool and the workpiece contact surfaces.

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Note that radiation is an important heat loss mechanism at the Heat Affected Zone (HAZ), due to the high

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Hence, the proposed framework for the numerical simulation of FSW process is capable of capturing accurately 199 the mechanical results (Table 3). This also vouches for the robustness of our friction model proposed for the 200 FSW.

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The total processing time on an Intel core i7 processor is approximately 10 hours.  218 Figure 7 shows the temperature field at steady-state on the workpiece surface. The temperature distribution 219 reveals a lower temperature at the head of the pin than the rear side. Thus, the flow stress is higher where the 220 material is hotter. Figures 8 and 9 show the velocity and plastic dissipation contour fills computed from the 221 numerical model. It can be clearly seen that the numerical model is able to represent the non-uniform

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The vertical velocity is 2.5 mm/s in the case of featureless tool pins in order to obtain the applied vertical 253 loading. It is slightly higher than the value applied for the threaded case.

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The results for forces and torque using both types of tool profile are presented in 263 Figure 11 presents the temperature contours under the tool on the workpiece for both threaded and unthreaded 264 tool pins. In the case of threaded pin, the difference in the temperature distribution on the retreating side and 265 advancing side is more visible than in the unthreaded case. Hence, the friction model proposed is able to 266 capture the non-uniformly distributed temperature around the tool.

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The distribution of the plastic dissipation under the tool shoulder on the workpiece using both tool pins is 268 compared in figure 12. The plastic dissipation is higher in front of the tool when using featureless pin and it is 269 higher in the rear of the tool if threaded tool pin is considered.

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The path of the two points which are not affected by the threaded pin movement passes around the featureless 277 pin. Hence, separation of the streamlines on the advancing side around the featureless pin is observed.

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Threaded pin Featureless pin The results of the FSW simulation using a threaded tool pin are presented in terms of longitudinal, transversal 294 and vertical forces, torque, as well as temperature distribution and compared with the experimental evidence.

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The agreement between the numerical and experimental results, both in terms of thermal and mechanical 296 behaviours, is remarkable.

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A comparison between the thermo-mechanical responses in FSW using threaded and featureless cylindrical 298 pins is also presented. Somewhat lower values of forces and torque are observed in case of threaded pin than 299 featureless one. The non-uniform distribution of heat generation around the tool using the enhanced friction 300 model is more visible in case of using a threaded pin. The threaded tool pin is found to increase the vertical 301 movement of the surrounding material.

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It is shown that the proposed numerical model for the simulation of the FSW process is capable of capturing the 303 thermo-mechanical responses with remarkable accuracy for both the featureless and threaded pin tools.