Warm Spray Process Research Articles

Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings

Ti-Al-based intermetallics are promising candidates as coating materials for thermal protection systems in aerospace vehicles; they can operate just below the temperatures where ceramics are commonly used, and their main advantage is the fact that they are lighter than most other alloys, such as MCrAlY. Therefore, Ti-Al-Si alloy coatings with five compositions were manufactured by spraying pure Ti and Al-12 wt.% Si powders using warm spray process. Two-stage hot pressing at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical composition, and phase composition of the as-deposited and hot-pressed coatings were investigated using SEM, EDS, and XRD. Applying of hot pressing enabled the formation of dense coatings with porosity around 0.5% and hard Ti5(Si,Al)3 silicide precipitates. It was found that the Ti5(Si,Al)3 silicides existed in two types of morphologies, i.e., as large particles connected together and as small isolated particles dispersed in the matrix. Furthermore, the produced coatings exhibited good isothermal and cyclic oxidation resistance at a temperature of 750 °C for 100 h.

Manufacturing and Properties of Al-Al2O3 Composite Coating Layer Using Warm Spray Process

Properties of coatings produced by warm spray were investigated in order to utilize this technique as a repair method for Al tire molds. Al-(0-10 %)Al2O3 composite powder was sprayed on Al substrate by warm spraying, and the microstructure and mechanical properties of the composite coating layer were investigated. For comparative study, the properties of the coating produced by plasma spray, which is a relatively high-temperature spraying process, were also investigated. The composite coating layers produced by the two spray techniques exhibited significantly different morphology, perhaps due to their different process temperatures and velocities of particles. Whereas the Al2O3 particles in the warm sprayed coating layer maintained their initial shape before the spray, flattened and irregular shape Al2O3 particles were distributed in the plasma sprayed coating layer. The coating layer produced by warm spray showed significantly higher adhesive strength compared to that produced by plasma spray. Hardness was also higher in the warm sprayed coating layer compared to the plasma sprayed one. Moreover, with increasing the fraction of Al2O3, hardness gradually increased in both spray coating processes. In conclusion, an Al-Al2O3 composite coating layer with good mechanical properties was successfully produced by warm spray.

Numerical investigation of dual-stage high velocity oxy-fuel (HVOF) thermal spray process: A study on nozzle geometrical parameters

The present study takes the advantage of computational fluid dynamics (CFD) methods to model steady-state, two-dimensional, axisymmetric, turbulent, compressible and combusting flow in a dual-stage high velocity oxy-fuel (HVOF) thermal spray system. The Eulerian method is used to solve the continuum gas phase and the Lagrangian method is utilized for tracking the particles. The effects of particle loads on the continuous gas phase are included in the simulation. Thus, compared to the previous studies, we investigate the influence of coupling between the particle and gas phases in modeling of the dual-stage HVOF process. It is found that decouple modeling of the particle and the continuous phase causes a significant error in velocity of particle at the impact moment, even for low powder particle loading. We further investigate the effects of four geometrical parameters on the behavior of gas phase and consequently the particle phase. Results also show that the turbulent intensity of flow at different sections of the warm spray process is the most important factor determining the radial distribution of nitrogen and temperature in the barrel. It also determines the radial distribution of oxygen in the free jet outside of the barrel. It is further found that reduction of the first nozzle diameter and increasing the length of the divergent section (for a fixed divergent angle) of the convergent-divergent nozzle reduce the particle temperature while these changes do not affect the particle velocity. In other words, changing these geometrical parameters has a desirable effect on the particle temperature without causing an undesirable change on the particle velocity.

Manufacturing and Macroscopic Properties of Warm Sprayed Cu-36at.%In-15at.%Ga Coating Layer

A Cu-36at.%In-15at.%Ga coating layer is manufactured using a warm spray process and ternary alloy powders. The powder used had a particle-size distribution of 5~22μm with a spherical shape and average particle size of 11.6μm. Pure aluminum plate was used as a substrate. As a result, a CIG coating layer with 120μm thickness was successfully obtained. In order to improve the density of the coating layer, post heat treatment was applied for 1 hour in each condition of 200, 300 and 400°C. Phase analysis of the powder initially used identified Cu9In4, Cu9Ga4 phases, and Cu9In4, Cu9Ga4, Cu3Ga phases were identified after warm spraying. As heat treatment was applied, porosity decreased from 0.98% (as sprayed) to 0.8% (400°C), showing an increase in density, and hardness also decreased from 304Hv (as sprayed) to 258Hv (400°C).

Effects of Al Content and Addition of Third Element on Fabrication of Ti-Al Intermetallic Coatings by Heat Treatment of Warm-Sprayed Precursors

Four powder mixtures of titanium and aluminum with 50:50, 40:60, 30:70, and 20:80 atomic ratios were used as feedstock for Warm Spray process to produce composite coatings. A two-stage heat treatment at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical, and phase composition of the as-deposited and heat-treated coatings were investigated using SEM, EDS, and XRD. It was found that the Al content affects on the thickness expansion of the heat-treated Ti-Al coatings significantly and also has a major influence on the porosity development, which is caused by the Kirkendall effect. The effects of adding a third element Si and heat treatment with pressure to produce denser Ti-Al intermetallic coating were also examined. The investigated hot-pressed coatings with addition of Si exhibited much denser microstructure and contained Ti-Al intermetallic phases with titanium silicide precipitates.

Warm Spray 공정과 Cu-Ga 및 Cu-In 혼합 분말을 이용한 CGI계 복합 코팅층의 제조 및 특성

This study manufactured a CIG-based composite coating layer utilizing a new warm spray process, and a mixed powder of Cu-20at.%Ga and Cu-20at.%In. In order to obtain the mixed powder with desired composition, the Cu-20at.%Ga and Cu-20at.%In powders were mixed with a 7:1 ratio. The mixed powder had an average particle size of 35.4 µm. Through the utilization of a warm spray process, a CIG-based composite coating layer of 180 µm thickness could be manufactured on a pure Al matrix. To analyze the microstructure and phase, the warm sprayed coating layer underwent XRD, SEM/EDS and EMPA analyses. In addition, to improve the physical properties of the coating layer, an annealing heat treatment was conducted at temperatures of 200 o C, 400 o C and 600 o C for 1 hour each. The microstructure analysis identified α-Cu, Cu4In and Cu3Ga phases in the early mixed powder, while Cu4In disappeared, and additional Cu9In4 and Cu9Ga4 phases were identified in the warm sprayed coating layer. Porosity after annealing heat treatment reduced from 0.75% (warm sprayed coating layer) to 0.6% (after 600 o C/1 hr. heat treatment), and hardness reduced from 288 Hv to 190 Hv. No significant phase changes were found after annealing heat treatment.

Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures.In this study, Ti–Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 °C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl3 was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl3 layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity.

Preparation and Characterizations of NiTi Intermetallic Coatings

Much attention has been paid to NiTi intermetallic coating for enhancing cavitation erosion resistance property due to its shape memory effect. In this paper two processes were used to deposit NiTi intermetallic coatings. Process 1 was the deposition of NiTi intermetallic coating using warm spray process with NiTi intermetallic powder as feedstock, and solution treatment was subsequently performed. Process 2 was a mixture process of warm spray and laser treatment. Ni-Ti coating was first deposited via warm spray process with Ni-cladding Ti as feedstock, and laser-treatment was performed to realize the alloying of Ni-Ti. The microstructure and phase compositions for NiTi intermetallic coatings were characterized by means of SEM and XRD. The fundamental properties for NiTi intermetallic coatings were also analyzed.

Development of WC-Co Coatings Deposited by Warm Spray Process

The high-velocity oxy-fuel (HVOF) process is commonly used to deposit WC-Co coatings. There are some problems with this process; especially the decomposition and decarburization of WC during spraying make a coating brittle. To suppress such degradation, the warm spray (WS) process was applied to deposit WC-Co coatings, which is capable of controlling the flame temperature in the range of 500-2000 °C. The microstructure and phases of the deposited coatings were characterized by using SEM and XRD, and the mechanical properties such as hardness, fracture toughness, and wear properties were also investigated. WS process successfully suppressed the formation of the detrimental phases such as W2C and W, which are usually observed in HVOF coatings. The WS coatings showed the similar trend of the hardness variation for Co content with a sintered bulk material. Improvement of toughness and wear behavior was also observed in WS coatings.

Fabrication of nano-sized oxide composite coatings and photo-electric conversion/electron storage characteristics

Titanium dioxide TiO 2 can be used as a photo-anode to give generated electrons to the metal substrate under illumination. The transition metal oxide such as iron oxide Fe 2O 3 can be used to store electrons generated by the photo-electric conversion function of TiO 2 under the illuminated situation while the electrons are discharged from the transition metal oxide to the metal substrate in the dark. In this paper, coatings of nano-sized composite of TiO 2 and Fe 2O 3 were fabricated by the Warm Spray process, in which the feedstock powder is accelerated by a supersonic gas jet with speed above 1.0 km s - 1 and temperature between 800 and 2500 K, and then impacted onto the target substrate continuously to form coatings. The coatings of TiO 2 and Fe 2O 3 nano-composite fabricated by Warm Spray showed no thermal deterioration such as phase transformation and particle growth of the feedstock during the spray process. The coatings fabricated by the Warm Spray had larger photo-current and the electron charge/discharge capacity than that by a conventional HVOF process. In addition, these characteristics were improved by decreasing the primary particle size of TiO 2 and Fe 2O 3.