Microstructural Evolution and Mechanical Behavior of Dissimilar Nickel-Based Superalloy 718/Stainless Steel 321 Joints Fabricated via Pulsed Laser Welding
摘要
This study presents a novel approach to the dissimilar joining of nickel-based superalloy 718 and stainless steel 321 through autogenous pulsed Nd:YAG laser welding. For the first time, critical process parameters—including welding speed, pulse energy, and pulse duration—were systematically optimized to achieve a defect-free joint with enhanced mechanical properties through minimizing intermetallic formation and improving joint integrity. Microstructural evolutions were characterized via optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Mechanical properties were evaluated through tensile testing and Vickers microhardness measurements. Results revealed the formation of a single austenite (γ-FCC) phase in the weld metal center of most joints, with no elemental segregation observed in the weld zone. Insufficient penetration was observed with reduced heat inputs. The weld metal microstructure exhibited equiaxed grain morphology, where increasing the heat input to 47.44 J mm−1 led to the coarsening of equiaxed grains up to 93 μm. Consequently, the weld hardness and ultimate tensile strength (UTS) decreased to 208 HV and 477 MPa, respectively. In the heat-affected zone (HAZ) of the IN 718 superalloy, hardness increased with higher heat input due to the formation of the Laves phase. Conversely, insufficient heat inputs resulted in incomplete alloying and diffusion, reducing the UTS to 394 MPa. The highest tensile strength (601 MPa) was achieved at a welding speed of 5 mm s−1, a pulse energy of 12 J, and a pulse width of 6 ms. The presence of equiaxed FCC austenite grains, along with numerous active slip systems, led to crack-free weld joints exhibiting superior resistance to cracking. The results confirm that dilution rate critically controls grain size in laser welds, but secondary phases (e.g., Laves) can override Hall–Petch strengthening. Optimal properties occur at 30-50% dilution where grain refinement and austenite phase exist.