Residual stresses in steel I-sections strengthened by wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) is a method of metal 3D printing that can be used to complement traditional steel manufacturing techniques (e.g. hot-rolling and cold-forming), to produce hybrid structural elements of enhanced efficiency. The case of hot-rolled steel I-sections strengthened by the addition of WAAM stiffeners at the flange tips is the focus of the present study. In addition to the improved local buckling resistance and increased flexural rigidity achieved by the addition of the stiffeners, a further source of enhanced load-bearing capacity arises from the inversion, through thermal action, of the adverse residual stress pattern typically associated with hot-rolled I-sections. Thus, in contrast to previous studies, the aim of the present investigation is not to eliminate but to take benefit from residual stresses, by prestressing (pretensioning) specific parts (i.e. the flange tips) of steel sections against subsequent applied compressive loading, thereby delaying yielding and enhancing the load-bearing capacity. The described hypothesis is demonstrated in the present study through the manufacture of a series of hybrid WAAM-strengthened hot-rolled I-sections and the subsequent performance of residual stress measurements. Stiffeners of different dimensions were printed using ER70S-6 welding wire. A detailed description of the adopted experimental technique, which utilised the sectioning method, is provided, while the magnitude and distribution of the measured residual stresses are reported. The addition of WAAM stiffeners at the flange tips of hot-rolled I-sections is shown to lead to the generation of desirable (for structural stability) tensile residual stresses in their vicinity, with peak values reaching the yield strength of the material; self-equilibrating compressive residual stresses are induced in the web and around the web-to-flange junction.