Exploration of Stability of 3D-Printed Steel Members

Abstract

Wire and arc additive manufacturing (WAAM), a relatively new metal 3D-printing technology, allows for fabrication of shapes that were not possible to produce a few years ago. Using 6-axis industrial robots with a gas metal arc welding (GMAW)-head, Dutch start-up MX3D prints metal in every direction mid-air. By welding small parts of material at a time along a laser guided - CAD-generated path, self-supporting cross-sections of very complex shapes, such as internally reinforced non-prismatic hollow sections can be manufactured fully automatically. Unlike powder-bed based manufacturing, this technology is not bounded by size limits of a final product, which creates opportunities for fabrication of large-scale structural components and finally entire structures. It is MX3D’s aim, in collaboration with key industry partners, to 3D-print world’s first fully functional bridge spanning 12 metres, to be installed in the city centre of Amsterdam in the course of 2019. The bearing elements of the bridge consist of tubular shaped elements, together building up to a complex-shaped free-form steel structure. These tubular members mainly behave as axially compressed bearing elements, nonetheless it is yet not known how the stability of WAAM-members can be assessed. Several standards for testing (and characterisation) of additively manufactured materials are published by different national and international committees, e.g. ISO/ASTM 529 and CEN-CT 438, but a comprehensive set of international standards has not been achieved yet. It is widely recognised that there is a strong link between manufacturing process parameters and material properties, which requires special attention in this research area. So far there is a lack of sufficient test results to cover a specific manufacturing process. In this research the material properties of MX3D’s dot-by-dot and continuously printed members are studied. This pioneering research provides insight in material and geometrical properties relevant for the stability of (ER)308LSi stainless steel tubular columns. A tubular column is used as existing basic structural element to assess the stability of wire and arc additively manufactured steel members. Advanced 3D-laserscanning technology is applied to characterise and analyse geometrical imperfections. A clear understanding of bending stiffness and buckling behaviour is established by performing four-point flexural bending tests and flexural buckling tests on tubular columns, diameter 33.8mm, thickness 3.7 mm. Tensile properties of milled WAAM specimens are analysed both in - and perpendicular to - their print direction for dot-by-dot and continuously printing. It is verified whether or not the structural properties are in compliance with the existing steel standard (EC3). Additional Vickers hardness measurements and metallographical analyses are performed to examine the microstructure of column samples. Finally, a buckling design graph is proposed based on own experiments, aiming towards a safe model suitable for stability calculations of additively manufactured tubular columns.
It is concluded from experiments that the material properties of 3D-printed steel differ from that of conventionally produced steel. Following the thermal gradient of the weld pool, large columnar grain structures are detected giving rise to anisotropy. Stiffness, strength and ductility prove to be dependent on the print direction.
Whereas the tensile strength top expected values, the obtained stiffness is significantly lower than that of commonly applied steel grades. Inaccuracies of the printing process result in local wall thickness variations and a relatively high out-of-straightness, both negatively affecting the buckling capacity of printed tubular columns.
Yet it should be acknowledged that due to rapid advancements in improving the printing process, the geometrical imperfections are soon expected to reduce drastically. It is recommended to apply active cooling during manufacturing to enhance mechanical properties even further. By combining topology optimisation and WAAM, an optimal material layout can be found ánd manufactured, consequently an excellent material efficiency can be achieved. A promising prospect for future construction projects.