Abstract
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.
Highlights
Friction stir welding (FSW) is a relatively new-born solid-state welding technique free from scattering, flash arc, and fume
The total generated heat during underwater friction stir welding (UFSW)/FSW is dependent on the many mechanical parameters [61,62,63]
The used term ofheat production ratiohere refers to the ratio of the total heat generated by each part of the tool in the both the regular and underwater-FSW
Summary
Friction stir welding (FSW) is a relatively new-born solid-state welding technique free from scattering, flash arc, and fume. The mechanism of base metal (BS) welding is not related to an external heat source, and, for this reason, the properties of BS do not change highly. This feature caused, in some cases, the joint properties to be better than BS [3]. The welding heat is produced by friction at the contact area between the BS and the tool [4,5]. In this situation, the base metal undergoes thermo-mechanical deformation (TMD) by rotational movement of the FSW tool inside of BS. With TMD, fine and equiaxed re-crystallized microstructures form in the joint line and improve the final properties of welded samples [6,7,8,9,10,11]
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