Microstructural Evolution and Through‐Thickness Mechanical Property Gradients in a 70‐mm‐Thick 316LN Stainless Steel Joint Produced by Narrow‐Gap Laser Welding With Pre‐Placed Filler Plates

Reliable joining of 70‐mm‐thick 316LN stainless steel is essential for fusion reactor applications. A high‐efficiency narrow‐gap laser welding strategy combining beam oscillation with pre‐placed filler plates was developed. A sound joint was produced in five passes, including one autogenous root pass and four approximately 13‐mm‐thick filler passes. The weld metal solidified fully austenitic and exhibited pronounced through‐thickness gradients. Electron backscatter diffraction showed that the maximum texture intensity decreased from 4.87 in the newly deposited zone to 3.97 in the interlayer transition zone (TZ), which also contained more Σ3 twin boundaries and lower local lattice misorientation. Mean microhardness increased from 224.8 ± 4.1 HV in the base metal to 268.7 ± 5.6 HV at the weld center. The TZ reached 261.2 ± 5.3 HV, about 21 HV above the adjacent newly deposited region. The root‐pass group achieved 612.4 ± 8.6 MPa ultimate tensile strength and 52.8% ± 2.3% elongation, whereas intermediate filler layers showed 548.9–556.8 MPa and 28.9%–31.5%. Mean absorbed impact energy decreased from 225.91 ± 7.36 J at the root to 133.83 ± 4.15 J at the cap. These results demonstrate efficient few‐pass joining with periodic interlayer hardening and an overall through‐thickness toughness gradient.