Heterogeneous microstructure of duplex multilayer steel structure fabricated by wire and arc additive manufacturing

Wire and arc additive manufacturing (WAAM) is suitable for fabricating multimaterial dense structures in various additive manufacturing technologies. Watanabe et al. [1] reported that a unique two-stage stress-strain curve was observed during tensile testing of a duplex multilayer steel structure fabricated by WAAM. The material behavior was explained by the transformation-induced plasticity, which was attributed to a decrease in the austenite phase observed after testing using X-ray diffraction at three representative points of the tensile test specimen. However, the deformation mechanism was unclear from the viewpoint of the composite structure mechanics. In this study, the heterogeneous microstructure of the steel structure was investigated for the duplex layer region to infer the relationship between the material response and underlying deformation mechanism. The distributions of the phase and major alloy elements indicated that the layers melted and mixed during WAAM, and the multilayer structure subsequently changed compared to the design layout. The WAAM structure was composed of two dual-phase layers containing different volume fractions of the martensite phase. Hence, the austenite phase in the martensite-rich layer initially deformed and then transformed to the martensite phase during tensile testing. Consequently, the strength of the martensite-rich layer was recovered to the micro-mechanically estimated level and the two-stage stress-strain curve was generated. Thus, this paper presents the potential of multimaterial WAAM for controlling microscopic heterogeneities and their material responses by adjustment of the process parameters.