The effect of metal inert gas welding on the microstructure, abnormal grain growth and the pseudoelastic properties of industrially processed Fe–Mn–Al–Ni shape memory alloy sheets were studied. Square-butt welds were manufactured using similar filler material. The influence of alternating mean arc linear energy on the microstructure of the individual zones is shown. A correlation between the process parameters, the associated heat input, the grain morphology and the α/γ-ratio could be deduced. As the mean arc linear energy increases, the α/γ-ratio in the fusion zone and the heat-affected zone increases. To evaluate the influence of the welding process on abnormal grain growth, a post-weld cyclic heat-treatment was carried out. Although no loss of major alloying elements in the fusion zone was observed after welding, metal inert gas welding has a significant effect on grain growth upon cyclic heat treatment. While abnormal grain growth occurred in the base material, a polycrystalline microstructure characterized by significantly smaller grain sizes was visible in the former fusion zone. Incremental strain tests revealed severe plastic deformation near the grain boundaries in the former fusion zone. However, the grain boundaries of the bamboo-like grown grains in the base material turned out to be more critical for structural failure.

In the present study, tungsten inert gas wire and arc additive manufacturing was used to process an iron-based FeMnAlNi shape memory alloy. By a layer-by-layer method, a wall structure with a length of 60 mm and a height of 40 mm was generated. Bidirectional welding ensured grain growth parallel to the building direction. To maintain a nearly constant temperature–time path upon cooling, the structure was fully cooled after each weld to room temperature (298 K). With this approach, an anisotropic microstructure with a grain length of up to 8 mm (major axis) could be established. The grain morphology and formed phases were investigated by optical microscopy and scanning electron microscopy. The images revealed a difference in the orientation with respect to the building direction of the primarily formed γ grains along the grain boundaries and the secondarily formed γ grains in the heat-affected zones. Subgrains in the α matrix were observed also by scanning electron microscopy. With X-ray diffraction, the preferred orientation of the α grains with respect to the building direction was found to be near ⟨100⟩. Overall, an anisotropic polycrystalline material with a columnar texture could be produced, with a preferred grain orientation promising high values of transformation strains.

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.

As a lightweight construction material, aluminum plays a key role in weight reduction and, thus, sustainability in the transport industry. The brazing of aluminum and its alloys is impeded by the natural passivating oxide layer, which interferes with the brazing process. The presented study investigates the possibility of using a thermal silane-doped argon plasma to reduce this oxide layer in situ and thus eliminating the need to use hazardous chemical fluxes to enable high-quality brazing. Using plasma spectroscopy and an oxygen partial pressure probe, it was shown that a silane-doped argon plasma could significantly reduce the oxygen concentration around the plasma in a thermal plasma brazing process. Oxygen concentrations below 10⁻¹⁶ vol.-% were achieved. Additionally, metallographic analyses showed that the thickness of an artificially produced Al₂O₃-Layer on top of AlMg1 samples could be substantially reduced by more than 50%. With the oxide layer removed and inhibition of re-oxidation, silane-doped plasma brazing has the potential to become an economically efficient new joining method.