Butt Joining of Aluminum Sheets Using Face Filler Friction Stir Spot Welding

Abstract

Friction stir spot welding FSSW, as a general rule, is adopted and limited for joining sheets of similar or dissimilar metals in a lap joint configuration, and it cannot be used in butt jointsss. The idea of this study is to weld two AA 1050 aluminum sheets in a butt joint configuration, having a thickness of 2 mm each, by using a face filler disk from the same AA 1050 alloy of 1 mm thickness via friction stir spot welding. A stir tool (rod or welding tool) is made from HSS high-speed steel. A special fixture is manufactured to hold and weld the sheets with the facial filling in a butt configuration. Three different rotational speeds (1120, 1400, and 1800 revolutions per minute) with a linear movement in a direction perpendicular to the facial filling and then to the sheets were used. The samples were welded with and without preheating, with the stirring tool lowered by 1 mm in a vertical orientation. The best result obtained regarding the force applied to break the welded sample is 4000 Newtons as a tensile force, with a joint efficiency of 72% compared to the strength of the original metal. Also, we obtained a tensile strength to break the welded joint of 125 MPa compared to the tensile strength of the original metal, with a joint efficiency of 113% of the tensile strength of the original metal. The cross-sectional area of the fracture was also observed, which was acceptable from the point of view of microscopic examination. No traces of the contact area between the two sheets were seen, and the fusion of the two sheets was perfect. With the elimination of the keyhole, considered a flaw in the standard friction stir spot welding procedure, it was possible to obtain keyhole-less areas while maintaining adequate joint strength. The metallurgical bonding between the face filler disc and the lower sheet causes the disc to become a part of the spot when inserted into the lower sheet.