Defects and Quality Control of Friction Stir Welded Joints in Aluminum Bridge Decks

Aluminum vehicular bridge decks figuhave been increasingly used for the replacement of deficient bridge slabs made from reinforced concrete or steel. The design of these decks has been under increasing development since the late 90s and different types of vehicular bridge deck profiles have been industrialized. Aluminum vehicular bridge decks are mainly manufactured through joining several extrusions together by means of welding. Initially, traditional fusion welding techniques were used in the fabrication process. However, more recently, a relatively new technology, friction stir welding (FSW), was used in certain bridge projects. FSW is proving to enhance the welding quality and the overall mechanical performance of the joint compared to traditional welding techniques. However, unlike fusion welding techniques, FSW standards and specifications still lack important quality control criteria and tolerance levels for common FSW defects. In this context, this paper aims at defining tolerance levels for common FSW fit-up defects in aluminium bridge deck application. First, FSW trials of typical vehicular bridge deck extrusions were conducted to determine the welding parameters yielding a sound welding quality. Then, fit-up defects such as gap and tool offset were investigated experimentally, and tolerance levels were determined following a concise stage prequalification process. It was found that a gap of 1.5 mm and a tool offset in the retreating side (RS) of 1.5 mm were acceptable limits. It was also shown metallographically that the level of the tool offset in the advancing side (AS) of 5 mm presented a sound weld quality with a continuous remnant oxide layer within the weld nugget.