Induction assisted aluminizing of 316L and Inconel 718 alloys

Abstract

This doctoral study encompasses the application of three different aluminizing methods hot-dip aluminizing (HDA), slurry aluminizing (SA), and pack induction aluminizing (PIA) on AISI 316L stainless steel, Inconel 718 (IN718) superalloy, and IN718 produced by Cold Spray Additive Manufacturing (CSAM), along with the enhancement of these coatings through rapid induction heating. The common objective of all studies is to determine the effectiveness of induction heating in accelerating the formation of aluminide phases within very short durations and to evaluate the resulting coatings in terms of their microstructural, mechanical, tribological, and high-temperature oxidation behaviors. In first stage of this thesis is summarized. This study investigates the wear and high-temperature oxidation behavior of AISI 316L stainless steel (SS) subjected to two distinct aluminizing processes hot-dip aluminizing (HDA) and slurry aluminizing (SA) both followed by rapid induction heating. The objective was to assess the efficiency of short-time induction heating as a diffusion treatment and to compare the resulting structural and functional properties of the coatings. Microstructural characterization was carried out using SEM, XRD, and EBSD, while mechanical and tribological properties were evaluated by nanoindentation and wear testing. The HDA coating exhibited a uniform outer morphology, whereas the SA coating developed a lamellar structure due to localized thermal gradients during induction heating. Despite these morphological differences, both coatings consisted of Fe₂Al₅, FeAl, and α-Fe (Al) phases. The SA coating demonstrated a higher surface hardness (13.2 GPa vs. 10.8 GPa for HDA) and a lower coefficient of friction (0.40 vs. 0.52), resulting in a markedly lower wear rate (3.2 × 10⁻⁵ mm³/N·m vs. 6.5 × 10⁻⁵ mm³/N·m). Isothermal oxidation at 1000 °C for 24 h and 96 h revealed that both coatings transformed toward a protective α-Fe (Al) matrix with a continuous Al₂O₃ scale. However, the coating thickness increased more significantly in SA samples from 35 μm to 400 μm after 96 h compared to 230 μm for HDA, indicating superior Al diffusion kinetics in the SA process. Overall, the SA process combined with rapid induction heating exhibited superior wear resistance compared to the HDA route, whereas the HDA process combined with the same thermal treatment demonstrated enhanced oxidation resistance relative to the SA. In second stage of this thesis is summarized. This study compares the effect of very fast induction heating for two competitive and cost effective aluminizing process, namely, hot dip aluminizing (HDA) and slurry aluminizing (SA) of Inconel 718 superalloy. Each process produced well adhered coating layers, which comprise two or more layers. Morphological, structural and chemical characterizations of each layer were extensively studied, and formation mechanisms of the coatings were discussed in comparison to each other. The HDA process performed in a molten Al-11 wt. Si bath at 700 °C formed a 120 μm coating layer, mostly comprising of NiAl3. The induction heating for 20 s at 1000 °C increased the coating thickness more than two times (250 μm), transformed the aluminides to Ni2Al3, made the elemental distribution more uniform within the coatings, and also formed CrSi2 precipitates, which might be beneficial for an improved oxidation resistance. In the SA process, the induction heating forms a coating of 43 μm in thickness, which is more uniform than the HDA coatings, and also comprises of two layers having Ni2Al3 and NiAl type aluminides. EBSD examination revealed that the SA coating has a uniform grain size and random grain orientation, which are promising for improved oxidation resistance. Evaluation of the results in comparison to each other provided a better understanding of the effect of induction heating on different aluminizing processes. For example, a coating uniformity similar to that of the SA coating can be achieved only after the application of the induction heating in the HDA coatings. Also, the SA process assisted with the induction heating can be applied to various substrates without subjected them to a high temperature for a long time, and would be a promising method for partial and complete aluminizing of Ni-based substrates. In thirth stage of this thesis is summarized. This study investigates the development of aluminide coatings using a Pack Induction Aluminizing (PIA) process on Inconel 718 (IN718) superalloy, which was produced by Cold Spray Additive Manufacturing (CSAM). Benefit of the proposed method is to significantly reduce the aluminizing time, from a few hours as for the conventional furnace aluminizing conducted in a furnace to a few ten minutes as for the prosed method. The research also aimed to characterize the resulting coating's microstructure, phase evolution, and tribological performance in comparison to the conventional pack aluminizing furnace. The results confirmed the formation of a dense, dual-layer aluminide coating that grew in thickness from approximately 10 µm to 19 μm with the increase in process time from 5 min to 10 min, respectively. XRD and EDS analyses identified the coating's primary constituents as aluminum-rich intermetallics, with an outer layer of Ni(Fe,Cr)Al₃ and an inner layer of Ni₂(Fe,Cr)Al₃, characteristic of a high-activity aluminizing process. The PIA treatment significantly enhanced the tribological properties of the IN718 substrate. It reduces friction coefficient, and improves wear resistance reducing the wear track depth by over 80% for the samples treated for the process durations of 10 and 15 min, as compared to the uncoated alloy. These findings demonstrate that the PIA method is a highly effective and rapid technique for producing protective, high-performance aluminide coatings on IN718 superalloy.