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Friction-stir Welding

A researcher talks about some recent technical issues


Name: HIRANO Satoshi
Joined: 1987
Specialty: Joining technology

We are carrying out research on a new joining technology called "friction-stir welding" (FSW)-which is a way of joining materials without melting them. This joining method has advantages such as the shape of the sections to be joined changes little after joining (compared to the deformation after conventional fusion welding) and the strength of the joint is high. Accordingly, FSW is being used for forming joints that require high reliability (such as railway vehicles and automotive component).

Q 1: What sort of joining technology is FSW?


A 1: "FSW" is short for "friction stir welding". A so-called "joining tool" is pushed onto the materials to be joined while it is rotated on a cylindrical member. At the time of contact, frictional heat is generated at the contact surface between the tool and the materials to be joined. As a result, the temperature of the materials rises, and the materials thus soften and become easy to deform. The easily-deformable materials are then joined fused together by the turning force of the joining tool, thereby joining them. And since the materials do not actually melt, there is little deformation after joining. Moreover, the metal structure of the joined section forms as a fine, equiaxed grain structure, so it is characterized by excellent mechanical strength.

Q 2: What products do you use FSW for?


A 2: For ten years or so, we have used FSW for a variety of products of Hitachi Group. As for materials, aluminum alloys (in which defects are easily generated by fusion welding) and copper alloys (which are difficult to weld) are two examples. The diagram schematically shows an example of a "hog-backed member" in which the joining line follows a curve in three dimensions. As for products, for example, FSW is being used for components of railway vehicles and sputtering equipment as well as motorcycle brakes and lithium-ion batteries. In regards to railway vehicles, FSW is being applied for bullet trains such as the N700 Series. Please take care to find exactly where the sections are being joined.

Q 3: What is the difference between FSW and conventional welding?


A 3: As I mentioned in A 1, the key difference is that FSW does not melt the materials being joined. And since no melting occurs, there is no solidification shrinkage when the joined sections solidify. In addition, the maximum temperature during joining is lower than that during fusion welding, so the drop in temperature down to room temperature after joining is smaller too. Consequently, the eventual shape deformation is also less. And since no light is radiated as bright flashes, no sparks fly, and no smoke is emitted, FSW is considerably more environmentally friendly than conventional "fusion" welding.