Effects of heat treatment on microstructures and properties of a heterostructured alloy with dissimilar components fabricated by WAAM

Heat treatment significantly influences homogeneous material microstructures and mechanical properties, which can be improved by an optimal heat treatment process. However, heat treatment application to heterogeneous materials presents significant challenges due to compositional and microstructural heterogeneities. Herein, a laminated heterostructured alloy comprising alternating 316L stainless steel (SS) and 18Ni300 maraging steel (MS) layers fabricated using wire and arc additive manufacturing was investigated. A solution treatment was applied at 900 °C for 0.5 h. Subsequently, the solution-treated and as-fabricated (AF) samples were aged at 500 °C for 4 h; these samples were denoted SA and AT, respectively. The AT phase compositions and orientations were similar to those of AF. The SA 316L SS layer resembled that of AF, but the SA 18Ni300 MS layers exhibited a reduced austenite phase fraction and refined grain size, attributable to solid-state transformation. In the AT sample, aging induced the formation of nanoscale acicular ω phase and ellipsoidal Ni₃Ti, Fe₂Mo, and X precipitates in the 18Ni300 MS layers. Conversely, the SA precipitates contained acicular FeNi₂ and ellipsoidal ω, Ni₃Ti, and X precipitates, and their fractions were lower than those in AT precipitates. The 18Ni300 MS layer microhardness in the heat-treated samples increased due to nanoprecipitation, but the 316L SS layer microhardness resembled that of AF. The AT and SA ultimate tensile strengths increased to (1360 ± 50) and (1473 ± 41) MPa, respectively, attributable to precipitation strengthening. The SA 316L SS layer exhibited a high stress-induced martensite fraction, enhancing the ductility of heated samples.

Graphical abstract