Microstructure and corrosion performance of WAAM-HW 316L stainless steel and ER70S-6 in-situ alloyed bimetallic walls.

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This study investigates the in-situ alloying of ER70S-6 and 316L stainless steel using the WAAM-HW process through the fabrication of two walls with different wire proportions, employing wire feed speeds of 4m/min for the primary wire and 2m/min for the hot wire. When ER70S-6 served as the primary wire, its higher thermal and electrical conductivity promoted stable arc behavior and efficient melting of both feed materials. In contrast, the wall produced with 316L as the primary wire exhibited porosity, lack of fusion, and elemental segregation, associated with its lower thermal and electrical conductivity as well as its lower melting temperature compared to ER70S-6. Metallographic analysis revealed distinct microstructural regimes: a predominantly martensitic structure in the ER70S-6-rich wall and a heterogeneous mixture of martensite, austenite, and ferrite in the 316L-rich wall. These differences were also reflected in the microhardness results, with the martensitic wall exhibiting higher and more homogeneous hardness values (378.79 ± 17.15 HV), whereas the heterogeneous wall showed pronounced local variations (264.21 ± 42.36 HV). Both alloyed walls demonstrated significantly enhanced corrosion resistance compared with ER70S-6 (Icorr 3.69 × 10⁻⁵ A·cm⁻²), reaching values comparable to 316L (Icorr on the order of 10⁻⁷ A·cm⁻²). However, the passive behaviour characteristic of austenitic stainless steels was not fully retained, as evidenced by a reduced passive range, decreasing from 0.529V in the 316L wall to below 0.042V in the hybrid configurations. Corrosion was observed to initiate preferentially in Cr- and Ni-depleted regions, with increased chemical segregation and microstructural heterogeneity in the 316L-rich wall contributing to reduced electrochemical performance. Overall, the results highlight the potential of WAAM-HW in-situ alloying for the development of novel alloy compositions, while emphasizing the strong influence of process-dependent thermal conditions on melting efficiency, chemical homogeneity, and microstructural uniformity.