Research on the grain boundary liquation mechanism in heat affected zones of laser forming repaired K465 nickel-based superalloy

The damaged K465 nickel-based superalloy parts were repaired by laser forming repair technology. The cracking characteristics and grain boundary liquation in heat affected zones were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the cracks originated from the heat-affected zone and extended to the repaired zone. The calculation by Thermol-Cale software showed that the larger γ' particles at grain boundaries partly dissolved in the γ phase, which made the solutes' concentration at the γ'/ γ interface meet the eutectic-type liquation reaction condition of γ + γ' → L. Then, grain boundaries liquation occurred and liquid films appeared with the temperature increasing in a rapid heating process. However, the intragranular γ' phase completely dissolved into the γ phase with no devotion to the liquid film. The dissolution of M₅B₃ borides at grain boundaries could promote grain boundary liquation.