Analysis of Friction Stir Welding Using Tools with Different Pin Profiles

Welding is an important process in manufacturing and fabrication where improving the weld quality and reducing the cost are the main targets. In view of that, extensive research is still being carried out to improve efficiency of the existing welding processes and developing alternative welding techniques. One such technique is friction stir welding which has the capability to produce high strength joints particularly for the materials that cannot be welded by the conventional welding processes. Controlling the FSW process parameters to improve the quality of welded joint is still a subject of active research. Controlling the welding thermal cycle through the geometrical and operational parameters may lead to produce sound weldments where defects can be avoided. Various studies have been carried out considering the various tool pin profiles to improve the mechanical strength of welded joint, but the influences of the pin profiles on the thermal characterisations have been rarely reported. Besides, the experimental work has often showed that it is cost and time consuming particularly when the effect of geometrical variables is investigated.

The present work focuses on the employment of modeling approaches to simulate the thermal field within the weldments which allow quantification of the thermal behaviour corresponding to the different proposed tool pins. A three – dimensional transient model has been developed to simulate the physical model of friction stir welding. It has then been employed to carry out the investigations under various geometrical and operational parameters. An analytical approach has been integrally used to develop novel mathematical equations that calculate the heat generation for all cases of welding tools to be studied. Then, to accomplish the CFD simulations of the cases under consideration, the heat generated amount has been applied to the pin and shoulder surfaces as a heat flux which represents the process heat input.

Both qualitative and quantitative analysis has been carried out for the temperature distribution within specimens welded by FSW using tools with different polygonal pins. In order to cover a wide range of geometrical and operational conditions, the numerical simulations have been conducted for various shoulder diameters, pin diameters, pin heights, material thicknesses, rotational speeds and axial loads. The attained results have given a clear picture of the transient and the spatial temperature behaviour as well as the effect of presence of the polygonal pin on the thermal field.

In the same way, a detailed CFD based investigation has been carried out on the thermal field for those FSW processes using tools with eccentric polygonal pins and tapered eccentric polygonal pins covering the same range of the geometrical and operational parameters. Furthermore, the current study made use of the peak temperature values for various cases under investigation in order to develop a semi-empirical correlation, which predict the maximum temperature in the weldments for various welding conditions. The developed prediction models of the peak temperature have shown a good accuracy particularly no single study has considered a wide range of investigations on the FSW thermal field which would be beneficial in selection of the welding parameters. Lastly, the advanced CFD modeling technique and the novel mathematical equations of heat generation being incorporated in this study have made it possible to spatially and transiently mapping of the thermal field even on the small scale of variations which led to a better understanding of the physical behaviour.