The Influence of Diamond Addition to Ni-Al Powder on Oxidation Behavior of Ni-Al During Plasma Spraying for High Performance Oxide-Free Ni-Al Intermetallic Coating

Ni-Al intermetallics have excellent corrosion and oxidation resistance, but their use in thermal spraying has been limited due to issues with in-flight oxidation. In this study, a novel approach is proposed to remove oxide from Ni-Al droplets in-flight by adding a deoxidizer (diamond) to the feedstock powder. A mixture of nickel, aluminum, and diamond powders was mechanically alloyed using a combination of cryogenic and planetary ball milling. The resulting Ni/Al/diamond composite powder was then plasma sprayed via the APS process, forming Ni-Al coatings on Inconel 738 substrates. Phase composition, microstructure, porosity, and microhardness of the coatings were characterized by X-ray diffraction, scanning electron microscopy, image analysis, and hardness testing, respectively. Oxygen content measurements showed that the coatings contained significantly less oxygen than coatings made from ordinary Ni/Al powders. In-flight particle temperatures were also measured and found to be higher than 2300 °C. The low oxygen content in the coatings is attributed to the in-situ deoxidizing effect of ultrahigh temperature droplets which are also oxide-free.

Wearing Behavior of the α-Al9FeMnSi Intermetallic Compound Formed by Reactive Sintering onto AISI 304L Stainless Steel

In this study, using mixtures of pure Al, Si, Mn, and Fe powders, the α-Al9FeMnSi intermetallic compound was formed onto AISI 304L stainless steel samples by reactive sintering. The processing parameters were temperature (800, 850, and 900 °C) and applied pressure (15 and 20 MPa), using a constant holding time of 7200 s. In this paper, the influence of pressure and temperature on the microstructure, microhardness, and wear resistance of the formed layers was studied. Using X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness testing, and wearing measurements (pin on disc tests), the cross-section and top side of the coatings were observed and analyzed. We were able to determine the phase composition of cladded layers and interfaces as well as their morphology. The results indicated that several layers were formed during reactive sintering, i.e., an Al-diffusion layer on the top of the substrate, an interface, and the α-Al9FeMnSi coating itself. The microhardness values of the different layers formed were determined, ranging from 400 to 500 HV for the intermetallic coating, to 120 HV for the substrate. In this way, it was found that the formed intermetallic coating is suitable to increase the corrosion resistance of stainless steel. Additionally, all the coating showed high adherence to the substrate, exhibiting high microhardness and wear resistance. Pin on disc wearing tests showed the wearing mechanisms are predominantly delamination and ablation of the cladded layers and substrate.

Formation of Intermetallic Coating on 20880 Steel in the Liquid-Phase Inter-Reaction with Aluminum

The inter-reaction mechanism of ingot iron with a molten aluminum is proposed and experimentally confirmed. It is shown that the three-layer coating is formed on the surface of ingot iron under the conditions of liquid-phase inter-reaction and consists of bund interlayers of Fe2Al5 and FeAl3 intermetallics and a heterogeneous structure based on solid solution and FeAl3. The micro-hardness of the structural components of the heterogeneous structure ranges from 1 (for Al (Fe)) to 6–9 GPA (for FeAl3+Al (Fe)). An increase in the exposure time during the heat treatment is accompanied by an increase of the quantity of structurally free FeAl3+Al (Fe) fragments in its composition.

EFFECT OF DIFFUSION PROCESS ON COATING IN HOT-DIPPED ALUMINIZED STEEL

Aluminized steels possess excellent corrosion resistance due to the formation of Al-Fe solution phases and intermetallic compounds in coatings. Ni was added to baths to further improve the corrosion resistance of the coatings at high temperature. Here, the role of Ni in the formation of coatings and the effect of diffusion process on the developing of coatings were investigated. 45 steels were immersed in Al-Ni baths (Al-1mass% Ni, Al-3mass% Ni, and Al-5mass% Ni) and diffusion-treated at 1023 and 1123[Formula: see text]K for 20, 40 and 100[Formula: see text]min, respectively. The coatings of samples were analyzed via scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) was further used to confirm the types of phases that formed during diffusion treatment. The formations of intermetallic coating layers were also analyzed via the diffusion path. More continuous Al3Ni layer and compact coating were obtained with diffusion treatment at 1023[Formula: see text]K for 40[Formula: see text]min.

The Role of Ammonium Chloride in the Powder Thermal Diffusion Alloying Process on a Magnesium Alloy

The powder thermal diffusion alloying method could be utilized to fabricate Al-rich intermetallic coatings on magnesium alloys in the air. While the role of ammonium chloride powder in the diffusion alloying source is still to be investigated. This research took the AZ91D magnesium alloy as the substrate. Diffusion sources with various powders were utilized as the diffusion source. Microstructure observation and phase identification were enrolled to investigate the role of the ammonium chloride powder in the diffusion alloying process. Results indicate that HCl gas could turn some solid Al powder into gaseous AlCl3 to enhance the transport of active Al atoms, moreover, it reacts with the dense MgO film and converts it to a loose one, which enables the AlCl3 gas to penetrate MgO and arrive the matrix to form a protective coating. Furthermore, the ammonium chloride content should be confined to 10 wt. % of the diffusion alloying source. Too much ammonium chloride powder would result in a worse intermetallic coating.