Hits:

Indexed by:期刊论文

Date of Publication:2008-08-01

Journal:WELDING JOURNAL

Included Journals:SCIE

Volume:87

Issue:8

Page Number:202S-211S

ISSN No.:0043-2296

Key Words:Friction Stir Spot Welding; Friction Stir Welding; Liquation; Liquation Cracking; 2219 Aluminum; 6061 Aluminum; AZ91 Magnesium

Abstract:Friction stir spot welding (FSSW) is considered to be solid-state welding, but recent studies have shown that liquation (i.e., liquid formation) can occur in FSSW of some Al and Mg alloys such as 2024 Al, 7075 Al, and AZ91 Mg. Instead of these more complicated multicomponent alloys, a simple binary alloy with an easy-to-understand microstructure, that is, 2219 Al (similar to Al-6.5Cu), was used to study liquation in FSSW and its effect on the resultant weld. The first part of the study focused on assessing the liquation tendency of 2219 Al. An augmented-torque test was used, and the extent of liquation in 2219 Al was compared with those in 6061 Al and AZ91E Mg. The liquation tendency increased in the order of 6061 Al, 2219 Al, and AZ91 E Mg. The torque measured during FSSW showed that the heat input decreased in the same order. The liquation tendency was explained by using a method based on both the phase diagram and the curves of temperature vs. fraction solid during solidification, which indicated that the amount of liquation-inducing constituent increased and the liquation temperature decreased in the same order. The second part focused on examining liquation in 221.9 Al. The amount of material stuck to and removed by the tool increased with increasing tool rotation speed. The evidence of liquid-film formation observed included 1) a thin eutectic film at the interface between the weld and the residue of the material removed, 2) thin eutectic films in the stir zone, 3) a mirror-like weld surface or fracture surface, and 4) a high concentration of eutectic particles in the stir zone at a location corresponding to the fracture surface. In addition to material removal by the tool, liquation during FSSW caused erosion or fracture at the weld surface and cracking in the stir zone.