Microstructure and strength of solid-state joints of dissimilar nickel-based alloys

Solid-state joining of metallic materials is a promising technological method that allows joining various materials. It is especially important for hard to deform nickel-based alloys (superalloys). The paper presents the results of pressure welding experiments using superplastic deformation of dissimilar superalloys in different combinations and temperatures: EK61//EP975 and EK61//EP741NP at 850°C, EP741NP//VKNA-25 and EP975//VKNA-1V at 1125°C. It was shown that in all cases the solid-state joint zone was a transition diffusion zone. The width of solid-state joint obtained at 850°C is about 10 µm, and for the case of 1125°C it is about 14 –18 µm. In all joints no brittle particles are observed. In combinations EK61//EP975 and EK61//EP741NP at 850°C, and EP741NP//VKNA-25 at 1125°C, there are no significant microstructure changes in the solid-state joint. As for EP975//VKNA-1V at 1125°C, a γ'-layer about 10 –12 µm in thickness is formed in solid-state joint zone in VKNA-1V alloy side. In all cases an interface between joined alloys is observed. The strength of solid-state joints was evaluated by mechanical tensile tests at room temperature. The strength limit was σUTS =1170 MPa for the pair of EK61//EP975, σUTS = 604 MPa for EK61//EP741NP, σUTS = 500 MPa for EP975//VKNA-1V, and σUTS =1080 MPa for EP741NP//VKNA-25. The above results indicate that pressure welding using the effect of low-temperature superplastic deformation is a promising method for obtaining high-quality solid-state joint from nickel-based superalloys.