Partial transient liquid phase diffusion bonding of zirconium alloy (Zr–2.5Nb) to stainless steel 321

Abstract

Partial transient liquid phase diffusion bonding was applied for bonding zirconium alloy (Zr–2.5Nb) to stainless steel 321 using one active titanium based interlayer and two zirconium based interlayers. Specimens were joined in a vacuum furnace at different temperatures under 0.5MPa dynamic pressure of contact. The influence of different bonding temperatures and bonding times on the microstructure, microindentation hardness, shear strength and interlayer thickness has been studied. The diffusion of Fe, Cr, Ni, Nb and Zr has been investigated by scanning electron microscopy and energy dispersive spectroscopy elemental analyses. The wettability of the interlayers on the zirconium alloy and stainless steel 321 was studied. Sessile drop test was performed for the specimens before the bonding operation at a temperature range between 820 and 865°C for 5, 10 and 15min. It is found that the titanium based interlayer with higher content of titanium has better wetting behaviour on the surface of the zirconium alloy at 850°C. Further studies showed that the increase in the bonding temperature caused significant reduction in the contact angle, spread ratio, spread factor and radius of the wetting. However, the height of the interlayer was decreased on the surface of the substrates by increasing the bonding temperature. Results indicated that the bonds prepared at 850°C for 15min for all the interlayers give suitable wetting behaviour, but due to the effect of bonding temperature the mechanical properties of the joints were reduced and brittle intermetallic compounds were formed at the interface of the joints. The titanium based interlayer operates as a barrier for diffusion of Fe and Cr and prevents the formation of brittle intermetallic compounds like Zr3Fe2, Zr2Ni and Zr3Fe.