The Institute for Structural and Engineering Materials (ISEM) of National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, developed in collaboration with Hyogo Prefectural Institute of Technology and Sakurai Industries Co., Ltd. technology for manufacturing large-sized structural members from extruded noncombustible magnesium alloys, which had been developed by AIST, through frictional stir welding (FSW) or laser welding without degrading merits of materials. In this way, it has been made possible to build an automobile roof box, 2,000 (L) x 670 (W) x 270 (H) mm, from noncombustible magnesium alloys. The new technology allows reducing weight of a structure by about 25% from that of comparable size made from fiber-reinforced plastics (FRP). It is expected that the technology is applied to the development of products with complicated 3D geometry from extruded or wrought magnesium alloy materials, taking advantages of superior properties.

The work was carried out under the "Medium and Small Enterprise Supporting R&D Project of Fiscal 2003."

Photo: A roof box made from extruded incombustible magnesium material.

Magnesium alloy is the lightest among metal materials for practical use, with specific gravity 2/3 that of aluminum alloy and 1/4 that of iron. Owing to its excellent heat dissipation, vibration absorption and EM wave shielding, magnesium alloy is widely used for housings of various portable electronic gadgets, such as notebook PC, mobile phone and digital camera. Moreover, taking advantage of high specific tensile strength and light-weight, its application may be expanded to leisure goods, automobile parts, and home electric appliances, and for this purpose, R&D efforts have been made both in materials development and in molding and processing. The AIST has pursued a number of R&D works with magnesium alloys: development of "noncombustible Mg alloys" to remove one of serious defects, combustion; surface modification, and FSW-based joining.

On the other hand, most of Mg alloy members are currently fabricated by die-casting, while extruded and rolled alloys are rarely used, despite their favorable properties. In particular, large-sized products are hardly available, because fabrication technology for extruded and rolled alloys has not yet been established, and parts of complicated shape are difficult to realize by plastic forming with least deformable Mg alloys.

The present work aims at producing extruded and rolled materials of noncombustible Mg alloys, forming them into simple-shaped parts and combining them into a large block through welding.

However, magnesium alloy has generally high activity so that the conventional welding process often causes defects. For this reason, few welded structures of Mg alloy have been implemented. It has been attempted, therefore, to assemble Mg alloy parts through advanced welding process, such as FSW and laser welding, for establishing a reliable welding technique. The FSW method was developed by the Welding Institute (TWI), UK. It is characterized by welding without melting matrix, and hence, without degrading alloy properties. The method has been applied mainly to aluminum alloys, and not for welding Mg alloys.