Fatigue and Fracture Behaviour of Steel Wire-Arc Additively Manufactured Structural Materials

Abstract

There is an increasing demand for automation that has influenced many industries to find ways to integrate it into their markets. Within the civil sector, the integration of automation into the various stages of a project unlocks new opportunities that were previously difficult to achieve. If the desires of the architects appeared unachievable due to high planning and manufacturing costs, this can now be resolved with the addition of automation and robotics embedded in the early stages of project development. The wire arc additive manufacturing (WAAM) process is an additive manufacturing (AM) process that allows efficient fabrication of structural elements. This process, also referred to as gas-metal arc additive manufacturing (GMAAM), uses directed energy deposition (DED) to create components. Specific to the WAAM process, a metal wire is fed into an electric arc, and then welded into a designed shape. For structural steel fabricators, this automated technology could allow for the reduction of supply chains, part inventories, and scrap waste, and will help improve the digitalization of the fabrication process. Moreover, the WAAM process allows the fabrication of customized connection nodes and unique structural shapes that are difficult to achieve with conventional subtractive manufacturing. Despite the many potential advantages of the WAAM process, research is needed for WAAM structural steel to be used in the civil engineering sector. Mechanical properties such as the elastic modulus, yield strength (YS), and the ultimate tensile strength (UTS) of WAAM material should be tested and validated. In addition, WAAM steel can have a very rough and wavy surface due to the additive manufacturing process. The rough surface can cause stress concentration zones within the material that may affect its fatigue performance. Although this can be mitigated by post-processing steps such as surface milling, it is important to study its properties in its as-built state as milling is an additional fabrication step, which takes time and cost and may not be necessary for some projects and applications. This thesis aims to explore the material properties, fracture toughness, and fatigue behaviour of WAAM steel components. Through experimental testing, mechanical properties such as the elastic modulus, yield strength, and ultimate tensile strength are determined. Further test results include Charpy v-notch impact tests to determine fracture toughness, as well as tests to determine crack propagation properties. Lastly, the experimental program included testing the fatigue behaviour of WAAM steel for both the smooth and rough (as-fabricated) specimens. The experimental program also examined the effects of weld direction by including tests on specimens oriented both perpendicular and parallel to the weld. The fatigue data collected from the experimental program was used to plot a stress-life (S-N) curve for WAAM steel. The data was then statistically analyzed and compared to current codes such as CSA S6 and S16. It was found that the fatigue behaviour of the WAAM steel was dependent on the weld direction. The specimens oriented parallel to the weld showed behaviour similar to CSA Detail Category B. Specimens oriented perpendicular to the weld showed behaviour similar to CSA Detail Category E. A metallurgical study of the WAAM steel showed that its microstructure showed resemblance to welded steel components. Looking at the microstructure, the grain sizes and boundaries indicated differences in the as-deposited zones and the reheated zones. The reheated zones, where the addition of new layers disturbed the microstructure, consisted of finer grains expected to exhibit greater toughness. Lastly, a linear elastic fracture mechanics (LEFM) model was used to predict the fatigue lives of the WAAM steel fatigue specimens in the perpendicular- and parallel-to-weld orientations. The model was able to predict the fatigue behaviour of the WAAM steel specimens, but the results were greatly dependent on the assumed surface stress concentration factors, Kt. More research is needed to obtain Kt values that will enable greater accuracy in determining the fatigue behaviour of WAAM steel.