Welding Process for the Additive Manufacturing of Cantilevered Components with the WAAM

Abstract

Wire + Arc Additive Manufacturing (WAAM) is widely used in both research and industry and is now also being studied in the building industry. The deposition rates are higher than those of other steel printing processes, such as selective laser melting (SLM). WAAM in combination with topology optimisation is therefore also of interest for standard applications. The Institute for Steel Construction and Materials Mechanics has two robots (Comau) with Fronius welding attachments. These are used to assess various applications for WAAM in the construction industry. The question of whether to produce not only vertical structures with neutral welding torch positions but also cantilevered components with overhangs but without supporting structures is a recurring issue. In this way, e.g., bridges with cantilever construction can be manufactured additively, as it was done on the Campus of the Technische Universitat Darmstadt. The challenge in the additive manufacturing of structures with overhang is that the weld is not applied to a horizontal surface but sideways to the previous layer of weld. The molten pool then threatens to drop down before it hardens. Even without lateral dripping, clumps usually form in the vertical direction, making it difficult to produce the targeted geometry. Controlled solidification can be achieved by combining two welding process controls. On one hand, cold metal transfer (CMT) technology is used, which achieves controlled droplet separation by means of forward and backward movements. The process is supplemented by pauses in the welding process, which enables a defined partial solidification of the melt. The paper describes this new process in more detail and presents its use for the additive manufacturing of the world’s first in situ additively manufactured steel bridge. It is shown that very good material properties can be achieved with the process. In addition, a new cooling process is presented with water spray cooling, which significantly reduces the manufacturing and cooling times and does not influence the material properties.