Researchers at the Institute of Physical and Chemical Engineering of Nuclear Industry have published a systematic study in the journal Materials Characterization (DOI: 10.1016/j.matchar.2026.116462) examining how electron-beam welding (EBW) beam current and scanning speed shape the weld microstructure of Allvac 718Plus, a precipitation-hardened nickel-based superalloy widely used in load-bearing joints of aero engines and gas turbines at service temperatures up to 704 °C.
The study is motivated by a well-known metallurgical challenge. Under the extremely high thermal gradients and fast thermal cycles of welding, the solidification path and non-equilibrium segregation in such highly alloyed materials readily drive niobium, titanium and molybdenum toward the interdendritic regions, forming Nb-rich Laves phases and constitutional-liquation products. These features are the root cause of fusion-zone (FZ) and heat-affected-zone (HAZ) cracking and property inhomogeneity. Because Allvac 718Plus shows a more complex γ/γ′/γ″ precipitation response than conventional Alloy 718, extrapolating from legacy data on Alloy 718 is unreliable.
Electron-beam welding offers high energy density, a clean vacuum environment, concentrated heat input and a narrow HAZ, giving it a clear advantage in controlling overall thermal damage. However, the EBW pool geometry is extremely sensitive to beam current and scanning speed: together they set the temperature-gradient-to-solidification-rate ratio that governs epitaxial columnar growth, dendrite-arm spacing, segregation extent and terminal solidification feeding.
The team's key innovation lies in quantifying these links with high-resolution characterization. Using EBSD to map texture, orientation and low-/high-angle grain boundaries, and EPMA to localize niobium segregation across a matrix of beam-current and scanning-speed combinations, they showed that an excessively large beam current deepens the weld pool and strengthens directional growth — and it is this mechanism, rather than simply "too much heat," that induces centerline interdendritic solidification cracks and porosity. In the widest HAZ region at the pin-head neck zone they also captured boundary liquefaction and eutectic re-solidification of (Nb,Ti,Mo)(C,N) particles, yet observed no HAZ cracking — evidence that the presence of liquefaction does not necessarily mean inevitable cracking, given Allvac 718Plus's comparatively good weldability.
Because the fine γ′ precipitates that give the base metal its hardness dissolve during the weld thermal cycle and cannot fully reprecipitate in the re-solidified FZ, the joints exhibit localized FZ/HAZ softening bands. The work clarifies the process–solidification-condition–defect/property correspondence and provides a practical basis for optimizing EBW process windows — beam current and scanning speed in particular — for high-temperature nickel-superalloy components, helping to suppress solidification cracking and porosity while improving joint reliability.