Influence of Additive Microstructure of AISI 316L on the Formation Mechanism of Pack Aluminizing Coatings

Abstract

This work investigates the influence of additively manufactured (AM) substrate microstructure on the microstructural evolution and formation mechanism of multilayer aluminide coatings formed by pack aluminizing on AISI 316L stainless steel. Substrates produced by different manufacturing routes—rolling, laser-directed energy deposition (L-DED), and plasma transferred arc directed energy deposition (PTA-DED)—were aluminized at 650, 850, and 1050 °C for 1 and 6 h. The substrates and aluminized coatings were characterized by optical microscopy, SEM, EDS, XRD, and microhardness measurements to evaluate the influence of the manufacturing route on substrate microstructure, phase constitution, elemental segregation, and the resulting coating phase evolution, thickness, and diffusion behavior. Regardless of the manufacturing route, pack aluminizing at a given processing temperature resulted in similar phase distributions among the substrates, although significant differences in coating thickness were observed. In contrast, different processing temperatures led to distinct phase constitutions, multilayer arrangements, and coating thickness. At 650 °C, the coating was predominantly composed of η-Fe 2 Al 5 , τ 1 -Fe 3 NiAl 10 , and Cr-rich γ 2 -Al 8 Cr 5 precipitates, while the internal layers remained limited in thickness. At 850 °C, the growth of internal layers containing lower-Al-content phases became more pronounced, resulting in well-developed β-FeAl and α-Fe(Al) layers. The rolled substrate exhibited thicker aluminide coatings than the AM substrates, which is attributed to the higher density of short-circuit diffusion paths in the AM microstructures that promote inward Al diffusion and reduce the Al flux available for sustained external layer growth. At 1050 °C, the increased atomic mobility promoted the consumption of high-Al-content phases, leading to a sequential phase arrangement consisting of a thinner γ 2 -Al 8 Cr 5 outer layer, followed by β-FeAl and α-Fe(Al) containing dispersed β-NiAl precipitates. These results highlight the decisive influence of substrate microstructure on aluminum diffusion pathways and phase transformations during the pack aluminizing of additively manufactured AISI 316L stainless steel.