Novel Approach to Keyhole Closure in Aluminum Alloys using Friction Spot Welding

The need for a method to refill keyholes has emerged in a wide range of applications, especially in industries that are using lightweight materials such as aluminum or magnesium. Major problems are damaged through holes, broken tools during machining processes or keyholes left by friction stir welding (FSW). This is particularly problematic in the case of parts with high standards of quality such as aluminum housings or pressurized tanks for aerospace applications. Conventional fusion welding techniques as well as friction based welding techniques that have been developed so far do not fulfill the requirements for high quality welds with respect to practicability, mechanical properties, and defect free keyhole closures.

We propose a new approach to refill keyholes using the friction spot welding (FSpW) technology. Keyholes in different precipitation hardening aluminum alloys are thus filled with a cylindrical plug of the same material. Afterwards the FSpW process is used to join the plug and the surrounding workpiece.

The advantages of this solid-state welding technology are the absence of defects associated with conventional fusion welding techniques like pores and hot cracks. Additionally, the FSpW process is not limited by the presence of the oxide layer on the surface of an aluminum workpiece. Therefore, all aluminum alloys are weldable. Furthermore the process produces a superior surface appearance (absence of keyholes or larger weld seams) that eliminates the need of additional surface treatment. The low heat input during FSpW minimizes the overaging phenomena in precipitation hardening alloys and therefore ensures an improved mechanical behavior compared to conventional fusion welding techniques.