Cyclic behaviour and constitutive modelling of wire-based laser directed energy deposited normal- and high-strength steels

Wire-based laser-directed energy deposition (DED-LB) is an additive manufacturing process with potential for structural applications owing to its competitive deposition rate and good surface quality. However, the cyclic mechanical behaviour of DED-LB steels requires detailed characterisation. This study presents an in-depth experimental investigation into the cyclic plasticity of DED-LB ER70S-6 normal-strength and ER110S-G high-strength steels. Specimens with different in-plane orientations are tested under as-built, machined, and heat-treated conditions using cyclic protocols with constant and ascending strain amplitudes. The results are evaluated in terms of hysteretic behaviour, stress evolution, and energy dissipation. Both materials are shown to exhibit stable cyclic response with notable ductility and energy dissipation. The normal-strength steel is shown to undergo initial cyclic hardening followed by stabilisation, whereas the high-strength steel is found to display gradual stress reduction after initial hardening. The response is shown to be primarily governed by strain amplitude, while in-plane orientation is found to have a limited effect. Post-processing is also shown to influence the cyclic response, with machined specimens exhibiting higher stress levels and improved stability and heat-treated specimens showing a more symmetric response. The evolutions of yield surface size and backstress indicate different contributions of isotropic and kinematic hardening mechanisms in the two steels. The test results are used to calibrate a Chaboche constitutive model, which is shown to capture the main cyclic behaviour within the investigated scope. The findings provide insights into the cyclic performance of DED-LB steels and a basis for future design-oriented studies on additively manufactured steel components.