Strengthening and embrittlement mechanisms in laser-welded additively manufactured Inconel 718 superalloy

Although laser-welded additively manufactured Inconel 718 joints find numerous high-temperature industrial applications, their strengthening and embrittlement mechanisms remain underexplored. To bridge this gap, we herein prepared such joints by the laser welding of the as-built material (built-LW), laser welding of double-aging heat-treated as-built material (DAT-LW), and double-aging heat treatment of laser-welded as-built material (LW-DAT). The microstructures of the joint fusion zones (FZs) were examined using scanning electron microscopy (electron backscatter diffraction and secondary electron imaging), while nanoscale features were probed by transmission electron microscopy, and mechanical properties were evaluated using microindentation hardness (H_IT) measurements and tensile tests. The FZs of the built-LW and DAT-LW joints contained no strengthening precipitates, such as the Laves phase and γ′ and γ″ nanoparticles. In stark contrast, the FZ of the LW-DAT joint contained spherical nanoparticles of the γ′ and γ″ phases responsible for precipitation hardening. The DAT-LW joint displayed base metal (BM) strengthening and FZ softening (H_IT = 6.47 and 3.6 GPa, respectively), whereas the LW-DAT joint demonstrated BM and FZ strengthening (H_IT = 6.2 and 6.5 GPa, respectively). The built-LW joint exhibited the lowest ultimate tensile strength (UTS) of 833 MPa, primarily because of the absence of strengthening precipitates. The DAT-LW joint, despite experiencing FZ softening, exhibited a higher UTS of 1086 MPa and a limited elongation of 2%, while the LW-DAT joint featured the highest UTS of 1440 MPa, primarily because of the enhancement of nanosized γ′ and γ″ strengthening phases facilitated by postwelding double-aging heat treatment.