Wettability and microstructural evolution of copper filler in W and EUROFER brazed joints

Abstract

In terms of wettability, active systems are characterized by a reduction in interfacial energy as the time at specific conditions is increased. This article aims to investigate the evolution of wettability and microstructure, which undergoes a critical transformation at temperatures and dwell times near brazing conditions due to their significant impact on resultant mechanical properties. The objective is to enhance wettability and prevent the formation of different phases that can occur rapidly within the brazing window conditions. Up to 1105 °C, complete fusion of the filler does not occur. However, once it happens, the expansion of the copper filler in EUROFER increases up to 400%, and the contact angle reduces from 100° to 10°, indicating an active wetting behavior. On the other hand, when copper is used with tungsten, an inert behavior is observed, maintaining the contact angle around 70°. Brazed joints carried out under the most promising wetting conditions demonstrated that at 1110 °C-1 min, various phenomena began to occur. This includes solid-state diffusion of copper in the EUROFER, following the austenitic grain boundaries, and partial dissolution of Fe in the copper braze. Increasing the brazing time from 2 to 5 min achieved high interfacial adhesion properties and controlled the diffusion layer and Fe-rich band formed at the W-braze interface, resulting in the best mechanical results (295 MPa).

Exploring FAST Technique for Diffusion Bonding of Tungsten to EUROFERE97 in DEMO First Wall

“…On the other hand, the use of liquid state joining techniques usually reports higher interactions at the bonding interfaces. For example, I. Izaguirre et al reported copper penetration of the EUROFER97 grain boundary at the ERUFOER97/braze interface and the formation of a reaction layer at the W–braze interface, giving rise to a more complex microstructure [ 32 , 33 ].…”