Investigation of microstructure and mechanical characteristic of underwater friction stir welding for Aluminum 6061 alloy – Silicon carbide (SiC) metal matrix composite

Demand for metal matrix composites (MMCs) is expected to increase in these applications, such as ‎in the aerospace and automotive sectors. Adequate joining techniques, which are important for ‎structural materials, have not yet been developed for Metal Matrix Composite (MMCs), however. ‎This work aimed to demonstrate the feasibility of ‎friction stir welding (FSW) and ‎underwater friction stir welding (UFSW) for joining Al 6061/5, Al 6061/10, and Al ‎‎6061/18 wt. %SiC composites have been produced by utilizing reinforce stir casting technique. Two ‎rotational ‎speeds,1000and 1800 rpm, and traverse speed 10mm \ min were examined. Specimen ‎composite plates 10 mm thick have been successfully welded by FSW. For FSW and UFSW, a tool ‎made of high-speed steel (HSS) with a conical pin shape was used. The result revealed that the ‎ultimate tensile strength of the welded joint by FSW and UFSW at rotation speed 1800 rpm for (Al ‎‎6061/18 wt. ‎‎% SiC composites) was 195 MPa and 230 MPa respectively. The ultimate ‎tensile ‎strength of the welded joint by FSW and UFSW (Al 6061/18 wt.% SiCe composites) was 165 MPa ‎and 180 MPa at rotation speed ‎‎1000 rpm respectively. The microstructural assessment showed that due ‎to larger grain sizes at FSW and UFSW, most of the fractures are located in the thermal ‎mechanically affected zone (TMAZ) adjacent to the weld nugget zone (WNZ). It is observed that in ‎failure, most of the joints show ductile features. As the volume fraction of SiC (18 wt.%) increases, ‎the friction stir welded and underwater friction stir welded efficiency decreases.

Thermo-Mechanical Simulation of Underwater Friction Stir Welding of Low Carbon Steel

This article investigates the flow of materials and weld formation during underwater friction stir welding (UFSW) of low carbon steel. A thermo-mechanical model is used to understand the relation between frictional heat phenomena during the welding and weld properties. To better understand the effects of the water environment, the simulation and experimental results were compared with the sample prepared by the traditional friction stir welding (FSW) method. Simulation results from surface heat diffusion indicate a smaller preheated area in front of the FSW tool declined the total generated heat in the UFSWed case compared to the FSWed sample. The simulation results revealed that the strain rate of steel in the stir zone (SZ) of the FSWed joint is higher than in the UFSWed case. The microstructure of the welded sample shows that SZ’s microstructure at the UFSWed case is more refined than the FSWed case due to the higher cooling rate of the water environment. Due to obtained results, the maximum temperatures of FSWed and UFSWed cases were 1228 °C and 1008 °C. Meanwhile, the simulation results show 1200 °C and 970 °C for conventional and underwater FSW samples, respectively. The maximum material velocity in SZ predicted 0.40 m/s and 0.32 m/s for FSW and underwater FSWed samples. The better condition in the UFSW case caused the ultimate tensile strength of welded sample to increase ~20% compared to the FSW joint.

Study on the effect of the welding environment on the dynamic recrystallization phenomenon and residual stresses during the friction stir welding process of aluminum alloy

Various methods have been proposed to modify the friction stir welding. Friction stir vibration welding and underwater friction stir welding are two variants of this technique. In friction stir vibration welding, the adjoining workpieces are vibrated normal to the joint line while friction stir welding is carried out, while in underwater friction stir welding the friction stir welding process is performed underwater. The effects of these modified versions of friction stir welding on the microstructure and mechanical characteristics of AA6061-T6 aluminum alloy welded joints are analyzed and compared with the joints fabricated by conventional friction stir welding. The results indicate that grain size decreases from about 57 μm for friction stir welding to around 34 μm for friction stir vibration welding and about 23 μm for underwater friction stir welding. The results also confirm the evolution of Mg2Si precipitates during all processes. Friction stir vibration welding and underwater friction stir welding processes can effectively decrease the size and interparticle distance of precipitates. The strength and ductility of underwater friction stir welding and friction stir vibration welding processed samples are higher than those of the friction stir welding processed sample, and the highest strength and ductility are obtained for underwater friction stir welding processed samples. The underwater friction stir welding and friction stir vibration welding processed samples exhibit about 25% and 10% higher tensile strength compared to the friction stir welding processed sample, respectively. The results also indicate that higher compressive residual stresses are developed as underwater friction stir welding and friction stir vibration welding are applied.