Al-Si Coating Research Articles

Corrosion Resistance of Al-12Si Coatings on AZ91 Magnesium Alloy Prepared through Flame Spray

In this study, Al-12Si alloy coatings with different thickness were prepared through flame spray on the surface of the AZ91 magnesium alloy to improve its corrosion resistance. The corrosion resistance was characterized through corrosion potential using electrochemical methods. The Al-12Si alloy coatings were heat treated at 100 °C, 200 °C and 300 °C for 6, 12, 18 and 24 hours. The effects of heat treatment temperature and time on the coatings’ corrosion resistance were discussed. It was found that there were no phase changes during the deposition of Al-12Si coatings through flame spray and heat treatment. The greater the coating thickness was, the higher the corrosion potential was. After annealing, the inner microstructure of the Al-12Si coating was densified furtherly and the annealed coatings had higher corrosion potential and better corrosion resistance. The coating annealed at 100 °C for 18 hours had the highest corrosion potential and the best corrosion resistance in the same coating thickness.

Tribological performance of Al–12Si coatings created via Electrospark Deposition and Spark Plasma Sintering

Spark plasma sintering (SPS) and Electrospark deposition (ESD) were used as two manufacturing routes to create Al–12Si coatings with refined microstructures. The tribological performance of these coatings was tested using a linear reciprocating tribometer. The SPS and ESD coatings were compared to a conventionally as-cast ingot (AC) of Al–12Si to observe the effect of particle refinement and morphology changes. The ESD sample exhibited lower wear rates than the AC and SPS samples. During the wear process, the Al and Si phases on the surface of the AC sample were significantly refined. This refinement led to a microstructure near the surface that was similar to the sample generated by SPS, and therefore showed similar wear rates at the end of testing. Subsurface cracking, which is known to occur in the wear of powder metallurgy alloys, was observed for sample SPS. The refinement of the Al and Si phases achieved for ESD process was much greater than what resulted during the wear of the AC and SPS surfaces and as a result the ESD sample showed lower wear rates throughout the entire test.

Preparation and Oxidation Behaviour of an Al-Si Coating on a Ni3Al Based Single Crystal Superalloy IC21

An Al-Si coating was prepared on IC21 alloy by powder pack cementation. The cyclic oxidation tests were carried out at 1150 in air for up to 100 h. The results indicate that the oxidation resistance of IC21 alloy is significantly improved by the Al-Si coating due to the presence of Ni2Al3 and β-NiAl enriched outer layer, and Si can effectively supress the outward diffusion of Mo. The oxide scales mainly consist of α-Al2O3, which is the favorite to the oxidation resistance. Phase transformation occurred from β-NiAl to γ-Ni3Al and γ-Ni in the coating during oxidation. The coating still remained a certain amount of β phase after oxidation for 100h, which indicate a good protection. The microstructure change evolution was characterized, and the oxidation behavior of the coating was discussed.

Experimental and thermodynamic investigations into the microstructure of laser clad Al–Si coatings on AZ91D alloys

Al–Si coatings were fabricated on AZ91D magnesium alloys by laser cladding using Al–Si eutectic alloy powders. Microstructure of the coatings was examined by optical microscope (OM), scanning electronic microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometer (XRD). The phase fractions, phase compositions and solidification process of the coatings were calculated using Thermo-Calc software based on the experimental results. The results show that a dense, crack and porous free coating with good metallurgical bond was obtained. The top part of the coating consists of Mg2Si dendrites, Al12Mg17 matrix and bright needle-like Al3Mg2 phase. The bottom part of the coating is formed of dendritic α-Mg solid solution, irregular block Mg2Si phase and rosette eutectics (α-Mg+Al12Mg17). The calculation results obtained from Thermo-Calc software agree well with the ones from the experiment. It is beneficial to the coating design for desirable microstructural and mechanical properties.

The Tribological Behavior of Plasma-Sprayed Al-Si Composite Coatings Reinforced with Nanodiamond

Al-Si composite coatings reinforced with 0 vol.%, 0.5 vol.%, and 2 vol.% nanodiamond were synthesized by plasma spraying. The effect of the addition of nanodiamond on the microstructure, hardness, and tribological performance of the composite coatings is investigated. The addition of 2 vol.% nanodiamond results in 45% improvement in the wear resistance of Al-Si coating. Al-Si coating with 0.5 vol.% nanodiamond exhibited lower coefficient of friction (0.45) with a 12% improvement in the wear resistance. Plasma-sprayed AlSi coatings with nanodiamond have excellent potential as wear-resistant coatings in automotive applications.

Hot Corrosion Resistance Properties of Al-Si Coatings Obtained by Slurry Method

This paper presents the results of research on aluminide protective coatings manufactured on hightemperature creep resistant cast steel. The main purpose of these coatings is protection against the high temperature corrosion, at carburizing and oxidizing potential atmosphere. Coatings were obtained on cast steel type GXNiCrSi 3018 by slurry cementation in air atmosphere. The tests of carburizing and oxidizing were carried out. The structure of the coatings before and after carburizing and oxidizing is described in the present paper. The chemical composition, thickness and microstructure of coatings were determined. These coatings could protect equipment against hot corrosion at carburizing and oxidizing atmosphere and have thermal shocks resistance.

A comparative study of four modified Al coatings on Ni3Al-based single crystal superalloy

Four modified Al diffusion coatings (Al, Cr–Al, Al–Si and Cr–Al–Si coatings) were prepared on Ni3Al based single crystal superalloy IC20. The oxidation tests were carried out at 1 150 °C for up to 100 h. Cyclic hot corrosion tests were carried out at 950 °C for 50 h. The results indicate that the oxidation and corrosion resistances of IC20 alloy are improved significantly by the coatings, and both oxidation and hot corrosion resistances of the four coatings are rated in the order (from worst to best) of Cr–Al, Al, Cr–Al–Si, Al–Si coatings. It is found that the degradations of Al and Cr–Al coatings are very quick due to the serious inter-diffusion between the coatings and substrates. The inter-diffusion between Si-containing coatings and substrates is reduced since Si effectively retards the outward diffusion of Mo. The weak effects of Cr and benefit effects of Si to the oxidation and hot corrosion resistance were discussed. The hot corrosion degradation mechanism of superalloy IC20 was analyzed.

Phase Transformations in the Al-Si Coating during the Austenitization Step

Nowadays more and more hot stamped steel sheets dedicated for the automotive body-in-white structure are pre-coated to prevent the steel surface against iron oxidation and decarburization during the austenitization step. For these applications, the coating is deposited by continuous hot-dipping the steel in an Al-Si bath. The Al-Si coating, at the delivery state, contains Al-grains, Al-Fe-Si ternary phases, Al-Fe binary phases. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions. The mechanisms of Al-Fe-Si phase transformations at high temperature are almost unknown. The phase transformations occurring during austenitization define the final coating microstructure responsible for the in use properties of the product like spot welding, painting adherence or corrosion behavior. It is the aim of this paper to propose a new way of understanding the mechanisms of phase transformation in the Al-Si coating during the austenitization step (between 900 and 930°C) before the transfer into the hot-stamping press.

Al, Si, and Al–Si Coatings to Improve the High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Oxidation resistance of chromium steels is due to the formation of Cr2O3 on the surface. However, this surface layer destabilizes above 1,000 °C and does not protect the metal. In this study, three types of coatings were applied to AISI 304 stainless steel (SS), and the microstructure and oxidation resistance of the coatings were investigated. Aluminum coating, silicon coating, and the codeposition of Al and Si were deposited on an SS substrate by the pack cementation method. The microstructure of the samples was then examined by SEM and EDS, and phases were identified by XRD. The oxidation resistance of these samples was studied in air at 1,050 °C. The results showed that the best resistance to oxidation was obtained, in order, from the codeposition of Al–Si, Al coating, and Si coating.

The Effect of Corrosion Wear on the Recorded Texture of a Clad Al-Si Coating

The Effect of Corrosion Wear on the Recorded Texture of a Clad Al-Si Coating

Corrosion and wear resistance improvement of magnesium alloys by laser cladding with Al-Si

Laser cladding with Al-Si powders have been carried out on three different magnesium alloys (AZ61, ZK30 and WE54) in order to improve their wear and corrosion properties using Nd:YAG CW laser. Optimized parameters allow obtaining crack and pore free coatings with good metallurgical bonding. Special care in shielding atmosphere is required to avoid porosity and corrosion. The hardness of the coatings is higher (130-250 HV) than that of as-received alloys. Salt spray corrosion tests and pin on disc sliding wear tests were carried out confirming that Al-Si coatings improves the wear and corrosion resistance of the alloys.

Application of cold spraying for flux-free brazing of aluminium alloy 6060

In the present study, samples of aluminium alloy 6060 were coated by cold spraying with a powder of brazing alloy Al12Si. The influence of the process gas temperature on particle velocities and coating build-up was investigated. The coated samples were heat-treated in air and under argon atmosphere to investigate the wetting behaviour of the deposited Al12Si and the diffusion processes between Al12Si coatings and substrates. Coated samples were brazed flux-free under argon atmosphere by an induction heating system. The microstructure of the coated, heat-treated, and brazed samples was investigated. The shear strength of the brazed joints was determined. The results show that the brazing alloy Al12Si could be very well deposited on the substrate by cold spraying. The particle velocity increased with increasing process temperature. Correspondingly, the thickness of Al12Si coatings increased with increasing process temperature. The heat treatments showed that a very good metallurgical bond between the Al12Si coatings and the substrate could be realized by the deposition using cold spraying. The coated samples could be well brazed without fluxes. The coating thickness and overlap width influenced the shear strength of the brazed joints. The highest shear strength of brazed joints amounts to 80 MPa.

Microstructure and High Temperature Oxidation of the Hot-Dipping Al-Si Coating on Low Carbon Steel in Ethanol, Water Vapor and Air at 700°C

Low carbon steel was coated by hot-dipping into a molten Al-10%Si bath. The high-temperature oxidation was performed at 700oC for 1 h to 49 h in air, air +100% H2O, and air + 30% ethanol under atmospheric pressure. An elemental composition distribution, morphologies of the aluminide layer and the oxide scale were characterized by OM, XRD, and SEM/EDS. After hot-dipping treatment, the coating layers consisted of Al, Si, FeAl3, τ5-Fe2Al8Si, and Fe2Al5. The results of high temperature oxidation tests showed the oxidation rate were parabolic law in three different atmospheres. The polyhedral τ1-(Al,Si)5Fe3 formed at a short time oxidation completely transformed to FeAl2 and FeAl due to the composition gradient and the chemical diffusion. The effect of water vapor on the oxidation resistance of the Al-Si coating may be attributed to increase in Al and Fe ions transport, leading to loss of protective aluminide layer by formation of iron oxide nodules on the coating surface and at interface between aluminide layer and the steel substrate.

The Influence of CVD-FBR Coatings on HCM12 Corrosion Behaviour under Molten Salt Conditions

The influence of Al and Al-Si coatings on the corrosion behaviour of HCM12 in molten KCl-ZnCl2 mixture at 650°C in air has been characterized by electrochemical impedance spectroscopy (EIS). Al and Al/Si protective coatings were developed by chemical vapour deposition in fluidized bed reactor (CVD-FBR) at moderate temperature to respect to mechanical properties of substrate. Scanning electron microscopy (SEM) was used to analyse the damage on the HCM12 electrode surface. Al-Si coating was found to be more resistant to the molten chlorides attack than Al coating; and both coatings increased the corrosion resistance of HCM12 in these conditions. The surface composition has been determined by X-ray diffraction (XRD).

Investigation of the High Strength Steel Al-Si Coating during Hot Stamping Operations

To improve the low formability that HSS sheets exhibit at room temperature, innovative forming technologies like the hot stamping process are currently applied. In order to avoid scaling and decarburization during the heating step, metal sheets coated with a specially developed Al-Si coating are utilized. In the present work the coating characteristics in terms of morphology, surface roughness and tribological behaviour are investigated as function of heating temperature, holding time and cooling rate that are typical of hot stamping processes.

Flame Sprayed Al-12Si Coatings for the Improvement of the Adhesion of Composite Casting Profiles

In this study, flame sprayed Al-12Si coatings were produced on the surface of inlays (aluminum profiles) of composite castings parts. The aim was to enhance the strength between the joining partners inlay and cast. Due to the high surface roughness and the presence of pores in the coatings, combined with the formation of an intermetallic phase at the interface, the adhesion of flame sprayed inlays could be enhanced by a factor of 2 compared to blank inlays and by a factor of 1.3 when compared to sand-blasted inlays. However, results also show that gaps are present, mostly at the interface between the inlays and the flame sprayed coatings, and these gaps have a negative effect on the joining strength of the composite casting parts. Therefore, optimizing the adhesion of the coating on the Al profiles via an improvement in both the sand-blasting and the flame spraying parameters would be beneficial for further enhancement of the adhesion of composite casting parts.

Microstructure and Formation Mechanism of Aluminized Coatings on Nickel-Based Superalloys

Aluminized coatings prepared on nickel-based superalloys can provide good protection against high temperature oxidation and hot corrosion. This study investigated the simple aluminized and silicon-aluminized coatings on nickel-based superalloy K4104. The simple aluminized coating was prepared by pack cementation and the Al-Si coating was prepared by slurry aluminizing, respectively. The microstructure of simple aluminized and Al-Si coatings was analyzed by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA). And the formation mechanism of simple aluminized and Al-Si coatings was discussed. The results showed that the simple aluminized coating was about 49 um thick and consisted of three layers. The outer layer mainly consisted of Al-rich β-NiAl. The intermediate layer consisted of Ni-rich β-NiAl and Cr-rich. The inner diffusion layer consisted of Cr-rich and γ’-Ni3Al. The microstructure of Al-Si coating showed that the coating was about 70 um thick and consisted of five layers. The Al-Si coating consisted of CrxSiy, Al-rich β-NiAl, Ni-rich β-NiAl, Cr-rich and γ’-Ni3Al. The microstructure of simple aluminized coating was compared with that of Al-Si coating in order to find out the effect of Si. Owing to the effect of Si, there was a Transition layer in Al-Si coating. The Al-Si coating was thicker than simple aluminized coating. The declining trend of the aluminum concentration in the Al-Si coating was smoother than that of the simple aluminized coating.

Microstructure and hardness of laser clad SiC p–Al composite coatings on Al alloys

SiC reinforcing Al–Si coatings were fabricated on AlSi12 substrate by laser cladding using powder mixture of Al–12Si alloy and SiC. Microstructure and microhardness of the coatings were examined. The results show that, the coatings are divided into two sub-layers, the upper layer consists of needle-like primary Si, Al–Si eutectics, α-Al dendrites and small amount of SiC particles, and there are α-Al dendrites, Al–Si eutectics and many SiC particles in the bottom layer. Orientated growth dendrites and columnar grains, which are almost perpendicular to melting surface of the substrate, are observed in the bonding zone. The microhardness of the coatings is approximately 220∼280HV, 2–3 times as that of the AlSi12 substrate.

Degradation of Protective Al - Si Coatings during Exploitation of Gas Turbine Blades

Diffusion Al-Si coatings are often used to protect rotor blades of aircraft engines against high-temperature corrosion in environments containing sulfur compounds. Besides other microstructural changes, the degradation of AlSi layers can be indicated by an increasing amount of surface oxide phases and changes in parameters of the layer geometry. In practice, the timetemperature area beyond a critical temperature of the outgoing gas is used as an empirical exploitation parameter D indicating a degradation level. The efficiency of such approach was investigated by analyzing degradation features in the surface layers of rotor blades after exploitations corresponding to different values of D. Determined simple relationships between the relative thickness of degraded layer and the parameter D verify the methodology and yield its clear geometrical interpretation. However, this method fails to provide reasonable information in case when the gas is burning outside the combustion chamber due to a sudden decrease of turbine revolutions.

Effect of Al–Si coating fusing time on the oxidation resistance of Ti–6Al–4V alloy

A simple and inexpensive method to modify the surface of Ti–6Al–4V alloy by low oxygen partial pressure fusing (LOPPF) technology using Al–13 at.% Si alloy particles, has been proposed. The increase in the weight growth of the LOPPF Al–Si coating formation proceeds with fusing time in accordance with the parabolic law. After cyclic oxidation test at 1073 K for 150 h, a remarkable improvement of the oxidation resistance of the LOPPF Al–Si coating is obtained compared with Ti–6Al–4V alloy, however, for the LOPPF Al–Si coatings, prolonging the fusing time is adverse to improve their oxidation resistance.