Formation of solid-state joints of dissimilar alloys based on Ni and Ni3Al

Nickel-based heat-resistant superalloys are unique materials that can operate at high temperatures. They are used for example, to make parts such as disks, shafts or blades. Superalloys are considered as “non-weldable”. Therefore, the task to obtain solid-state joint of such materials is relevant. This work is devoted to the study of the microstructure and properties of the solid-state joints in two combinations: VKNA-25//EP741NP and EK61//EP741NP. All the superalloys are heat-resistant: the EK61 is a wrought superalloy and the EP741NP is a powder one, both having different type of hardening phases, and the VKNA-25 is a cast monocrystalline intermetallic alloy based on Ni3Al. The solid-state joints were obtained by pressure welding under superplasticity conditions in a vacuum of P = 5 ·10−2 Pa. For the VKNA-25//EP741NP combination, pressure welding was carried out under EP741NP superplastic conditions at 1125°C. And for the EK61//EP741NP combination with stepwise temperature rise in 5 steps: 850°C → 925°C → 975°C →1025°C →1075°C →1100°C, which corresponds to the temperature (850 – 950°C) of the superplasticity conditions for the EK61 superalloy with ultrafine-grained microstructure and to the lower temperature limit (1000°C) of superplasticity for the EP741NP superalloy with fine-grained microstructure. The strength of the welded specimens was evaluated by mechanical tensile testing at room temperature. For the combination of VKNA-25//EP741NP, the microstructure both alloys remain. For the EK61//EP741NP combination, the microstructure of the EK61 superalloy becomes coarse grained (up to 50 μm) and microstructure of the EP741NP remains the fine-grained type. In both cases, a transition diffusion zone is formed between the dissimilar nickel-based superalloys; its thickness after pressure welding is 14 μm for the VKNA-25//EP741NP and it is 28 μm for the EK61//EP741NP. The ultimate strength of welded specimens for the combination of EK61//EP741NP was 620 MPa, and for the combination of EP741NP//VKNA-25 was 1080 MPa.