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

This study investigates the in-situ alloying of ER70S-6 and 316 L stainless steel using the WAAM-HW process through the fabrication of two walls with different wire proportions, employing wire feed speeds of 4 m/min for the primary wire and 2 m/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 316 L 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 316 L-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 316 L (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 0.529 V in the 316 L wall to below 0.042 V 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 316 L-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.