Solid oxide fuel cells (SOFCs) offer numerous advantages in terms of high efficiency and clean electrochemcial energy conversion devices. However, owing to high operation temperature, this technology is restricted to stationary applications and leads to components degradation and long-term stability issues. The development of new design and their modifications for improving the electrochemical performance at intermediate temperatures and durability of the SOFC components are very important to bring this technology one step closer to the market. In this context, the current research on the development, properties, performance, and stability of geometrically modified flat-tubular (FT) SOFC cell and the stack is reviewed in detail. This advanced design exhibits higher performance compared to the tubular type and longer stability in comparison to the planar type SOFCs. The application of the interconnect material is emerging as the key factor influencing the electrical output of the FT-SOFC and operation at high temperatures and current density are the critical issues for cell durability. New stack designs are discussed in detail and experimental findings are summarized.

Vacuum electron beam welding is widely employed for the welding of titanium alloys using different beam oscillation patterns. Since these patterns influence the physical phenomenon in the weld pool, its effect on the microstructure, texture, mechanical properties and residual stresses is of prime interest. In order to understand this influence, electron beam welding was used to prepare Ti-5Al-2.5Sn weldments using beam oscillations of triangular and rectangular waveform. It was observed that a change of welding pattern had a strong influence on the residual stresses, impact properties and texture of weld zone while tensile properties were not significantly affected. A partial martensitic transformation was observed in both the triangular and rectangular waveform of oscillations. An increase in alpha lathe width was observed in the fusion zone and similar strength of the rectangular pattern as compared to triangular pattern. Despite of this, the observed higher Vickers hardness of the fusion zone of rectangular pattern as compared to triangular and no-oscillation was attributed to texture strengthening using rectangular waveform.

The microstructure and defects in the weld zone affect the weldment characteristics. One way to improve the microstructure and reduce the defects in the weld zone is by using a filler during welding which influences the physical, chemical, and mechanical properties of the manufactured component. In the present study, tungsten inert gas (TIG) was used to weld Ti-5Al-2.5Sn alloy using different titanium alloy fillers; Ti-6Al-4V, Ti-5Al-2.5Sn, and autogenous weldments were also produced. The welded joints were characterized in terms of their microstructure, mechanical properties, and residual stresses in its various regions. The weldment with Ti-6Al-4V as filler exhibited a higher proportion of α′ martensite in fusion zone, as compared to the welded joint with Ti-5Al-2.5Sn alloy as filler, owing to the higher proportions of β stabilizers present in Ti-6Al-4V alloy. The α’ martensite was present in basketweave and acicular morphology in all the weldments, with and without fillers. Ti-6Al-4V filler welded joint showed higher tensile strength (approximately 1144 MPa) and relatively higher hardness than Ti-5Al-2.5Sn filler welded joint (approximately 1027 MPa) and autogenous weldment (approximately 770 MPa), due to increased amount of martensite in its fusion zone. As compared to the weldment produced with Ti-5Al-2.5Sn filler, the welded joint produced without filler and with Ti-6Al-4V as a filler had more compressive residual stresses at surface (approximately 25% higher), leading to less amount of pile up after nanoindentation. This was attributed to the generation of compressive strains due to martensitic transformations in the fusion zone of both these weldments.

In the present work, an autogenous TIG welding technique was used to join 2 mm thick sheets of a TiNi binary shape-memory alloy at three different values of current (80 A, 90 A and 100 A). The effects of the welding current on microstructures, residual stresses, mechanical properties and nano-mechanical behaviour were investigated. Microstructure analysis revealed that with an increase in current, the columnar grain size decreased, which had a significant effect on mechanical properties. The tensile strength of the weldment at 100 A was ∼92% of that base alloy’s (BA) and elongation of approx. 14%. The reduction in elongation was due to the formation of Ti2Ni and Ti3Ni4 type precipitates. The microhardness profile in all the weldments showed an increase of approx. 30% between the base alloy and the fusion zone due to the formation of Ti2Ni and Ti3Ni4 type precipitates during welding. Residual stress analysis suggested the tensile residual stresses in the longitudinal direction are minimum in the weldment performed at 100 A. The nanoindentation results revealed that the weldment obtained at 100 A had the least plastic deformation (∼45.5% less than the 80 A weldment) owing to a decrease in inter-dendritic spacing and high proportion of IMCs: Ti2Ni and metastable Ni4Ti3.

In the present work, Inconel 625 and Incoloy 800 thin sheets were TIG-welded without using fillers and at three different welding currents (40 A, 50 A, and 60 A). The influence of variation in welding currents on various characteristics of the weldments was investigated using various characterization techniques. Weldments obtained at 60 A showed the highest tensile strength and plastic deformation due to the formation of cellular dendritic microstructure in the fusion zone. Observation of the fractured surfaces revealed ductile nature for all the weldments. In addition, impact tests were performed at 25°C, 150°C, and 250°C, and the results indicated that with the rise in temperature, impact toughness decreased due to coarsening of carbides at grain boundaries in all the weldments. Furthermore, impact properties and hardness of weldments were also affected owing to the differences in interdendritic spacing and grain size. Weldments obtained at 40 A depicted higher nanohardness, whereas 50 A weldment showed relatively greater pileup height as compared to other weldments. Moreover, 60 A weldments showed highest longitudinal and transverse residual stresses due to relatively higher volumetric changes.