The present invention relates to a friction stir welding method that is applicable to panel welding used, for example, in aluminum alloy railway cars and buildings. 
A two-face structure (panel) for railway cars using hollow members is disclosed in Japanese Patent Laid-Open No.  246863/1 990, and another using laminated panels, such as honeycomb panels, is disclosed in Japanese Patent Laid-Open No.  106661/1994. 
The process of friction stir welding is performed by rotating a round rod inserted in a joint region to heat and plasticize the joint region thus forming a weld.  This type of welding is generally applied to a butt joint and a lap joint, and is described in WO 93/10935 (which is the same as EP 061 5480B1 and the Japanese Announcement laid-open publication No.  Hei 7-505090 and in the publication Welding & Metal Fabrication, January 1995, pp.  13-16.