Investigation of microstructure and hot crack formation mechanisms in the heat-affected zone of GTAW-repaired cast K4002 nickel-based superalloy with HGH3113 filler metal

Surface forming employing the gas tungsten arc welding (GTAW) with the HGH3113 filler metal was developed to repair gas-turbine investment castings composed of the non-weldable cast Ni-based superalloy K4002. This study mainly investigated the microstructure evolutions, fracture modes of the as-cast and repaired specimen, as well as the formation mechanisms of hot cracks. The results showed that as-cast K4002 alloy had millimeter-sized grains and various phases and solidified as follows: L → L ₁+γ ₁ → L ₂+γ ₁+(γ ₂+MC ₍₁₎) _eut →γ ₁+(γ ₂+MC ₍₁₎) _eut +(γ ₃+γ` <sub>1</sub>) <sub>eut</sub> +MC <sub>(2)</sub> → γ <sub>1</sub>+(γ <sub>2</sub>+MC <sub>(1)</sub>) <sub>eut</sub> +(γ <sub>3</sub>+γ` ₁) _eut +MC ₍₂₎ +γ` <sub>2</sub>. The phase transformations of repaired specimens were intricate. The cladding layer (CL) was constituted of columnar dendrites of varying dimensions, as well as the precipitated carbide particles and flocculent structures within the interdendrites. In the heat-affected zone (HAZ), the reprecipitated granular carbides coexisted with the initial blocky carbides, and the sizes and morphologies of the γ` phase were also significantly different. Compared with the as-cast condition, the volume fractions of the γ/γ` eutectic, MC carbide, and initial γ` exhibited a decrease from 14.7 % to 9.7 %, 2.5–2.0 %, and 62.1–43.1 %, respectively. The tensile strength after repair reduced slightly, but the ductility improved significantly. What’s more, liquation cracking, strain-age cracking (SAC), and ductility-dip cracking (DDC) all occurred in the HAZ. There was a partially melted zone (PMZ) between the CL and HAZ. The PMZ contained reprecipitated γ/γ` eutectics and fine carbides, and both liquation cracks and SAC arose from this region. The intergranular liquid film solidified through two modes (liquid film migration and normal solidification), which led to insufficient wetting of the grain boundaries, forming the liquation crack. Both SAC and DDC eventuated under great stress due to the reduction of grain boundary ductility. These two kinds of cracks were identified by the type of precipitated phases around the cracks. Moreover, high-angle grain boundaries decreased the susceptibility of liquation crack but intensified the DDC sensitivity.