Investigation of mechanical and metallurgical behaviour of WAAM-CMT-fabricated SS308L quarter iso-grid structure with and without post-deposition heat treatment

Wire Arc Additive Manufacturing (WAAM) offers a cost-effective route for fabricating lightweight, high-performance structures; however, studies on complex iso-grid geometries remain scarce. In this work, a quarter iso-grid structure of SS308L stainless steel was successfully fabricated using Cold Metal Transfer (CMT)-based WAAM through a feature-specific path-planning strategy. Process optimization identified a wire feed speed of 10.5 m/min and a weld speed of 25 cm/min as the optimum deposition parameters, yielding stable bead morphology and superior geometric integrity. The influence of post-deposition solution annealing at 1100 °C for 2 h followed by water-quenched (HT1) and furnace-cooled (HT2) was systematically evaluated. The non-heat-treated structure exhibited a cellular-columnar γ-austenite matrix with interdendritic δ-ferrite. HT1 promoted recrystallization, grain refinement, and microstructural homogenization, whereas HT2 resulted in significant grain coarsening. X-ray diffraction confirmed γ-austenite as the dominant phase in all conditions, with reduced ferrite content after heat treatment. Mechanical characterization revealed that the as-deposited condition achieved the highest ultimate tensile strength (565 MPa), while HT2 exhibited superior ductility (42%) and impact toughness (172 J), corresponding to increases of 24% and 56%, respectively. HT1 provided the most balanced mechanical performance, attaining the highest hardness (230 ± 4 to 245 ± 5 HV), compressive strength (380–400 MPa), and flexural strength (570 MPa), representing improvements of 15% and 19%, respectively. The results demonstrate that post-deposition heat treatment effectively tailors the microstructure property relationship of WAAM-fabricated SS308L iso-grid structures, highlighting their suitability for lightweight load-bearing aerospace and advanced engineering applications.