Grain refinement of flux-cored wire arc additively manufactured duplex stainless steel through in-situ alloying of Ti

One of the major problems in the wire arc additive manufacturing (WAAM) of duplex stainless steel (DSS) is the epitaxial growth of coarsen columnar ferrite grains crossing the deposition layers, which leads to the mechanical property anisotropy and increases the risk of hot tearing. In this investigation, Ti was in-situ alloyed to the WAAM DSS to promote grain refinement of ferrite. The results revealed that the ferrite transformed from columnar to equiaxed morphology with the increase of Ti content. In the sample with 0.5%Ti addition, some fine equiaxed ferrite grains with an average grain size of 39 μm formed at the fusion line, which inhibited the epitaxial growth of columnar grains. With further increase of Ti addition to 1.0%, the ferrite grains in the as-deposited WAAM DSS completely transformed from columnar to equiaxed structure, with an average grain size of 12 μm at the fusion line and 19 μm at the interior of deposition layer. Ti2N-Al2O3 particles with an average diameter of 400 nm were found in the equiaxed grains at room temperature, which existed in the form of TiN-Al2O3 in the molten pool and promoted the heterogeneous nucleation of ferrite. The analyzation based on Interdependence Theory demonstrated that Ti refined the ferrite in WAAM DSS by promoting the formation rate of the constitutional supercooling zone ahead of the solid/liquid interface and by introducing effective nucleat particles into it.