Numerical simulation of solidification and liquation behavior during welding of low-expansion superalloys

Low-expansion superalloys are susceptible to weld solidification cracks and heat-affected zone (HAZ) microfissures. To predict solidification cracking, QBasic procedures were developed and solidification reaction sequence, type, and amount of eutectic product were calculated. As manifested, primary solidification is followed by L → (γ + NbC) and L → (γ + Laves) eutectic reaction sequentially for GH903 and GH907; hence, the terminal eutectic constituents are made up of γ/NbC and γ/Laves. While for GH909, only reaction L → (γ + Laves) occurs and more γ/Laves eutectic forms. Therefore, GH909 is more sensitive to solidification cracking. To predict HAZ liquation, cracking Visual FORTRAN procedures were developed, and constitutional liquation of NbC was simulated. As shown, solid dissolution of NbC prior to liquation decreases, and initial liquid film increases with the rate of thermal cycle. Higher rate of thermal cycle promotes the melting of the matrix adjacent to the liquid film and postpones the solidification of the liquid by the liquid-to-γ mode. Thus, more residual liquid film remains at the eutectic point, which will promote HAZ microfissuring. The increase in original grain size and peak temperature also promotes liquation. Finally, these conclusions were verified indirectly by hot ductility tests.