Mechanical properties of wire and arc additively manufactured high-strength steel structures

Additive manufacturing with steel opens up new possibilities for the construction sector. Especially direct energy deposition processes like DED-arc, also known as wire arc additive manufacturing (WAAM), is capable of manufacturing large structures with a high degree of geometric freedom, which makes the process suitable for the manufacturing of force flow-optimized steel nodes and spaceframes. By the use of high strength steel, the manufacturing times can be reduced since less material needs to be deposited. To keep the advantages of the high strength steel, the effect of thermal cycling during WAAM needs to be understood, since it influences the phase transformation, the resulting microstructure, and hence the mechanical properties of the material. In this study, the influences of energy input, interpass temperature, and cooling rate were investigated by welding thin walled samples. From each sample, microsections were analyzed, and tensile test and Charpy-V specimens were extracted and tested. The specimens with an interpass temperature of 200 °C, low energy input and applied active cooling showed a tensile strength of ~ 860–900 MPa, a yield strength of 700–780 MPa, and an elongation at fracture between 17 and 22%. The results showed the formation of martensite for specimens with high interpass temperatures which led to low yield and high tensile strengths (Rp0.2 = 520–590 MPa, Rm = 780–940 MPa) for the specimens without active cooling. At low interpass temperatures, the increase of the energy input led to a decrease of the tensile and the yield strength while the elongation at fracture as well as the Charpy impact energy increased. The formation of upper bainite due to the higher energy input can be avoided by accelerated cooling while martensite caused by high interpass temperatures need to be counteracted by heat treatment.