Iron-based shape memory alloys have recently attracted increased attention due to their low material costs combined with good workability and high transformation strains. They show excellent welding properties, as shown by several studies and compared to non-iron-based shape memory alloys, and are potential candidate materials for large-scale application as damping elements in building structures. Since subsequent heat treatment is only possible to a limited extent for large-scale components, it is necessary to minimize the effects of processing and welding operations on the shape memory properties. Therefore, a suitable microstructure must be established in the heat-affected zone and the fusion zone during the welding process. Thus, industrially processed polycrystalline Fe-Mn-Al-Ni was joined by tungsten inert gas welding with matching filler material. The phases formed upon welding with different parameters were investigated using optical microscopy, scanning electron microscopy and X-ray diffraction. Shielding gas composition as well as mean arc linear energy have a huge impact on the γ-phase precipitation. Intercrystalline cracking can be supressed by increasing the γ content. Further, the α-fraction and grain size in the fusion zone can be controlled by the welding parameters. Ultimately, a hardness value of the fusion zone equal to heat-treated material was achieved which suggests that the fusion zone may be able to transfer the stress required for martensitic transformation.