Process Operational Windows and Robotic Scenarios for Assembly of Large Aluminium Structures by Robotic Friction Stir Welding

摘要

In response to world-wide societal demands for environmental and ecological stewardship and consumer demands for improved fuel efficiency and economy, lightweight design has become a key strategy for manufacturing in the transportation sector. Of the prevalent material substitution strategies for lightweighting, increased application of aluminum alloys in the architecture of next-generation transportation aircraft and vehicles for aerospace, automotive and rail industries can leverage the unique combination of properties including low density, high strength-to-weight ratio, good formability, corrosion resistance and recyclability. However from a manufacturing perspective, the weldability of aluminum alloys by means of conventional technologies has been a limiting factor to more widespread application. For instance, 6xxx series wrought aluminum alloys, such as AA6061, require special welding procedures to prevent solidification cracking and minimize residual stresses and distortion, which can be especially challenging for long butt or lap joints. In this respect, friction stir welding (FSW), a solid-state joining process, presents a tremendous potential for assembly of 6xxx series aluminum structures for the transportation industry due to the low heat involved, which is likely to translate into better joint mechanical performances and lower welding-induced distortion. The application of conventional gantry machines for FSW involves capital intensive investments for an inflexible and small work-envelop that renders a significant disadvantage for the industrialization of the technology for Canadian companies. Serial industrial robots could prove to be a cost-effective and viable option for industrial application of FSW on large structural elements. FSW process development and industrial implementation should however consider the limitations inherent to this robot architecture, the main challenges being the robot flexibility and limitedforce capability. In this work, FSW of 3.18 mm thick AA6061-T6 sheets in the butt and lap joint configuration was investigated with the objective of industrializing the process using low-cost serial industrial robots. The influence of weld pitch on the welding defects, microstructure, hardness and bend performance of butt and lap welds was examined to identify process operational windows for both joint types. In parallel with these trials, a methodology based on kinetostatic analysis was developed to identify and evaluate robotized scenarios for FSW. This methodology was then applied to identify optimized FSW scenarios for the fabrication of large integrated AA6061 structural components with stringer-to-skin and skin-to-skin joints, considering the findings from the experimental study. Candidate synthesized workcell layouts are also presented.