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TWI, a British welding research institute, developed friction stir welding in 1991. Hitachi, Ltd., as a member of the research and development team, worked on the practical applications of the technology, developing a device for FSW welding of 25m aluminum material by FSW.
The A-train's strong and attractive-looking body is the result of the use of revolutionary technology called FSW.
FSW is a welding method that makes use of the plastic flow of metal. In this method, the FSW tool is inserted between the welding line of the aluminum double skin, and the tool is rotated as it moves along the welding line. The faces are welded due the plastic flow of metal generated because of friction between the tool and the parent metal. Since FSW is a welding method through mechanical friction heat that does not make use of filler materials, the composition of the weld is almost the same as that of the parent metal, therefore degradation in strength due to thermal effects and heat distortion are extremely small. As a result, high strength and high quality joints can be attained. The joint is clean and safe, with no sparks or gas generated during the joining process. As the finish of the joint is clean and attractive, the outside finishing of car body can be no paint finishing or painted over with extremely small distortion. The surface finish is smooth and no putty is necessary before painting the surface.
Figure 1 : Principles of FSW
The maximum temperature that FSW welds at is 480 degree Celsius, far lower than the 660detemperature of MIG (Metal Inert Gas) welding. The amount of distortion is only one twelfth of that in MIG welding. A comparison of the amount of distortion and contraction for the two welding methods is shown in Figure 2.
Figure 2 : The Amount of Distortion / Contraction in FSW and MIG Welding
Since FSW is based on the low-temperature-plastic-flow characteristics of aluminum, the required heat input is also low;consequently, the welding distortion and contraction are low.
FSW welds were shown to be equal to or better than MIG joints on tensile strength tests. All ruptures were in sections affected by the heat. FSW has better mechanical features than MIG welding because the welding temperature is lower, there is less distortion, and there are no flaws.
The results of Charpy Impact Test carried out at room temperature show that the impact strength of the joints welded by FSW is 1.7 times greater than that of the parent metal, and 2.4 times stronger than that of joints welded by MIG. The "stirring" of the aluminum alloy during FSW gives it a finer micro-structure, which improves its resistance to impact. In contrast, structure, and in the post-extrusion parent material, a rougher micro-structure is formed as a result of re-crystallization and this columnar crystalline structure. The results of the Charpy Impact Tests are shown in Figure 3.
Figure 3 : Impact Strength of FSW and MIG Welds
Impact strength of joints welded by FSW is 1.7 times greater than that of the parent metal, and 2.4 times stronger than that of joints welded by MIG