In-Situ Alloying of 316L Stainless Steel and ER70S-6 by WAAM-HW: Fabrication, Microstructure, and Corrosion Performance of Bimetallic Walls

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 4 m/min for the primary wire and 2 m/min for the hot wire. When ER70S-6 served as the main wire, its higher thermal and electrical conductivities promoted stable arc behavior, efficient melting of both feed materials, and improved compositional homogeneity. 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 reflected in the microhardness results, with the martensitic wall showing higher and more uniform hardness values, whereas the heterogeneous wall displayed pronounced local variations. Both alloyed walls demonstrated significantly enhanced corrosion resistance compared with ER70S-6, reaching similar values to 316L (I _CORR on the order of 10 ^− 7 A/cm ² ), although the passive behavior characteristic of stainless steel was not fully preserved, reducing the passive range from 0.529 V in the 316L wall to less than 0.042 V in the hybrid walls. Corrosion preferentially initiated in Cr and Ni depleted regions, with the higher segregation and structural defects in the 316L based wall contributing to reduced performance. Overall, the results highlight the potential of WAAM-HW in-situ alloying for the development novel alloy compositions while emphasizing the need for precise process control to ensure adequate melting, mixing, and microstructural uniformity.