The effect of annealing on the growth of Al3Ni and Al3Ni2 intermetallics at the interface of explosively welded Al/Ni clads

Abstract Explosive welding is an effective method for joining metals with dissimilar physicochemical properties. The Al/Ni system is particularly interesting due to the formation of intermetallic phases with unique mechanical and thermal properties, which makes these welds potentially applicable in industries, such as aerospace, energy, and chemical. Diffusion processes occurring at the interface between these metals play a significant role and can impact the durability of the connections. Therefore, this research focuses on the analysis of diffusion phenomena in Al/Ni welds formed by explosive welding and subjected to further annealing. It was investigated whether the technological parameters of the joining process, such as detonation velocity, which varied from 2000 to 2800 m/s or mutual localization of the colliding plates during the explosion, influence post-annealing interface transformations, in particular the sequence of the intermetallic phase formation, their thickness and growth mechanism. Welded clads underwent annealing at 500 °C for periods ranging from 0.5 to 168 h. After the heat treatment, microstructure and phase characterization of the interface zone were performed, with scanning and transmission electron microscopy, thanks to which the mechanisms and growth kinetics of the forming phases were determined. Additionally, for the first time, the anisotropy of the thermal expansion in explosively welded Al/Ni was examined with respect to the shock wave propagation and the accompanying microstructure evolution, providing a significant contribution to the engineering design of welds subjected to the thermal cycling. The thermal expansion coefficient was measured over the temperature range from ambient temperature to 500 °C and analyzed in relation to the welding conditions. Differences in the thickness of the two forming Al 3 Ni and Al 3 Ni 2 layers and their dominant growth mechanisms were observed for the individual clads. The effect of detonation velocity was particularly significant at 2000 m/s, where prolonged annealing led to weld degradation, thus limiting strength due to unfavorable welding conditions. The presence of porosity identified at the Al1050/Al 3 Ni 2 interface was attributed to the Kirkendall effect accompanying the annealing procedure. Measurements of the thermal expansion coefficient revealed minor differences between samples, with the largest discrepancies observed for clads produced at 2400 m/s, which may be due to the less uniform initial microstructure of the interface. The obtained results indicate a relationship between explosive welding conditions and changes in the microstructure and physical properties of Al/Ni welds after heat treatment.