Automatic gap detection in friction stir butt welding operations

Friction stir welding (FSW) is a new solid-state welding technology that has been used successfully in many joining applications. A common problem that arises when welding two sheets is the presence of a gap between the sheets. Gaps may be due to improper fixturing, imprecision in the processes used to manufacture the sheets, etc. When the FSW tool encounters a gap, material can possibly escape from the processing zone and the welded part's effective cross-sectional area around the gap will decrease. Both of these effects can possibly cause an unsuitable weld. This paper develops a monitoring algorithm to detect gaps in friction stir butt welding operations in real time (i.e., during the operation). Experimental studies are conducted to determine how the process parameters (e.g., tool rotation rate and tool traverse speed) and the gap width affect the welding process; particularly, the plunge force (i.e., the force acting vertically down on the part). The proposed monitoring algorithm examines the filtered plunge force in the frequency domain to determine the presence of a gap. Several experimental studies are conducted for 2024 aluminum with a variety of process parameters and the monitoring algorithm is shown to be able to reliably detect the presence of gaps in friction stir butt welding operations for tool traverse speeds below 4.233 mm/s and gap sizes above 0.3048 mm.