Cooling strategies for friction stir additive manufacturing of EN AW-6082-T6

Friction stir additive manufacturing is a relatively new additive manufacturing technology with significant potential for fabricating large aerospace components (e.g., stiffeners or stringers). External cooling for this process is increasingly discussed because it can reduce mechanical degradation caused by reheating the lower layers during welding. In this article, an experimental study is described in which different cooling strategies (cooling plate, water spray, and water bath) were compared with a naturally cooled reference regarding their effects on workpiece temperatures, distortion, and hardness in specimens made of the aluminum alloy EN AW-6082-T6. In all specimens, distortion decreased with each additional layer, and hardness decreased from top to bottom, with several local maxima within the overlapping welds and in the welded part of the substrate. The cause of hardness loss was found to be grain and particle coarsening and a higher concentration of alloying elements in precipitation-rich areas. Compared to the naturally cooled reference, the water bath resulted in the lowest temperature during welding, lowest distortion, and highest hardness. The water spray also significantly affected the temperature and distortion, but its effect on hardness was small. In contrast, the cooling plate had the least effect on temperature and hardness, and it also negatively impacted distortion. Given the relatively low implementation effort and good results, future developments should focus on a system similar to the water spray, but with enhanced cooling of the trailing edge of the tool, e.g., cryogenic CO 2 with nozzles attached to the machine’s spindle.