Dragoș-Dumitru Vâlsan

The present work aims to investigate the microstructure and mechanical properties of laser beam spot welds in the superalloy Nimonic 80 A. Considering the importance of this innovative process in the manufacturing of engineering components for high-security industries, it is necessary to study the influence of the welding thermal cycle on the microstructure and mechanical properties of welded joints. The rapid heating/cooling, melting, and re-solidification phenomena that occur during welding modify the metallurgical characteristics of the weld compared with the microstructure of the base metal. Because the energy density is high and the process duration is very short, the microstructure obtained after solidification is fine dendritic in the central area of the joint and columnar in the weld–base metal transition zone. For the same reasons, the heat-affected zone (HAZ) is slightly extended. The increase in the size of the crystalline grains in the HAZ is negligible due to the low diffusivity of the nickel-based γ solid solution matrix, which inhibits the rapid migration of grain boundaries during the welding process. Metallographic analyses were performed using optical microscopy and scanning electron microscopy. The microhardness values, 152–168 HV0.05 in the weld and 180–190 HV0.05 in the base metal, together with the tensile–shear strength values (760–780 N/mm²) obtained at room temperature, demonstrate that the proposed welding process is appropriate and feasible for engineering applications involving Nimonic 80A superalloys.

The influence of phase transformations on the actuation capacity of microlayered architectures based on shape memory alloys is presented based on models developed based on the analysis of the phase transformations in each layer and on their contribution to the stress in the fil-substrate architecture. The analysis of the phenomena associated with the martensitic transformation allows the selection of the alloy composition to obtain a tuneable actuation in the desired temperature range as well as the profile of this operation based on the type of substrate used and its geometry as well as the fabrication parameters. The prediction of a microactuation profile is described for bimorph architectures with a film consisting of one or more shape memory alloy layers