Efficient Build-Up of High-Strength Aluminum Structures Using Friction Stir Additive Manufacturing

Abstract

Friction Stir Additive Manufacturing (FSAM) is a novel process with which large-scale aluminum structures can be produced from high-strength alloys such as the 7xxx series. Due to the prevalence of these alloys in airplanes and rockets, the process offers high application potential, for example in fabricating stringers and stiffeners. The building process in FSAM is characterized by sequentially stacking and friction stir lap welding (FSLW) metal sheets. Before adding the next layer, the surface is machined (i.e., by milling). So far, this is a necessary step to enable gap-free welding of the layers, which results in increased costs and reduced layer heights. The investigations described in this paper were aimed at improving the weld surface quality to enable defect-free FSAM without the additional machining step. For this, FSLW was conducted using different welding tools. The resulting welds were evaluated based on superficial and internal characteristics as well as the mechanical properties (shear strength). With a welding tool in which both a rotating and a stationary shoulder were combined, defect-free weld seams with a mean underfill and a mean flash height of 0.07 mm were produced. In a subsequent study, it was proven that defect-free FSAM without surface machining is possible up to the fifth layer using the combined welding tool.