Evolution behavior of liquid film in the heat-affected zone of laser cladding non-weldable nickel-based superalloy

Laser cladding with a paraxial powder-feeding device was used to repair as-cast K447A nickel-based superalloy, a kind of non-weldable alloy with a high Hf content and working as gas-turbine hot-section components. In the present study, the metallurgical evolution behavior of liquid film and liquation cracking mechanisms are studied. The results showed that a kind of uncommon Ni₂₁Hf₈ phase was found in the incipient melting regions (IMR) of the intergranular region. In the thermal cycle, γ/Ni₂₁Hf₈ eutectic liquefaction, M₅B₃ dissolved, and γ/γ’ eutectic liquefaction successively occurred in the heating process in IMR, and liquid → γ + γ/γ’ + M₅B₃ + γ/Ni₂₁Hf₈ → γ + γ’₍₃₎ + γ/γ’ + γ/Ni₂₁Hf₈ + M₅B₃ occurred in the cooling process. The inhomogeneity of the intergranular structure leads to the liquid film with the characteristics of discontinuous distribution and uneven width. The cracking criterion of the liquid film was established based on pressure drop. When the closed liquid film meets the condition of 2γ_sl<1/K*(ρ_s/ρ_l−1*R*dt+∆h_ε), the liquid film with a thickness that is greater than critical thickness will be induced into a liquation cracking.