Cold-Sprayed Aluminum Research Articles

Effect of 2D WS2 Addition on Cold-Sprayed Aluminum Coating

Tungsten disulfide (WS2) has excellent solid lubrication properties due to its 2D layered structure. This study focuses on depositing Al-2 wt.% WS2 composite coating by cold spray technique. The effect of WS2 addition on the microstructure, mechanical and tribological properties of the composite coatings is examined in the as-deposited and heat-treated conditions. After heat treatment, the coating density increased to 99% with improved intersplat bonding. The microhardness of the heat-treated Al-2 wt.% WS2 coating increased by 56% as compared to the as-sprayed coating. The wear resistance of heat-treated Al-2 wt.% WS2 coating improved by 75% with a synergistic reduction in the coefficient of friction (COF) by 51%. Transmission electron microscopy investigation reveals the presence of layered WS2 within aluminum splats with a strong interface. This study shows that cold spraying can be effectively used to integrate 2D layered WS2 as a solid lubricant in the metallic coatings.

Structures and Properties of Laser-Assisted Cold-Sprayed Aluminum Coatings

In the cold spray process, solid particles impact on a surface with high kinetic energy, deform plastically and form a coating. This enables the formation of pure and dense coating structures. Even more, coating performance and deposition efficiency can be improved by assisting the process with a laser. Laser-assisted cold spraying (LACS) has shown its potential to improve coating properties compared with traditional cold spraying. In this study, coating quality improvement was obtained by using a co-axial laser spray (COLA) process which offers a new, cost-effective laser-assisted cold spray technique, for high-quality deposition and repair. In the COLA process, the sprayed surface is laser heated while particles hit the surface. This assists the better bonding between particles and substrate and leads to the formation of tight coating structures. This study focuses on the evaluation of the microstructural characteristics and mechanical properties (e.g., hardness and bond strength) of LACS metallic coatings.

Optimization of the Quality Characteristics of Laser-Assisted Cold-Sprayed (LACS) Aluminum Coatings with Taguchi Design of Experiments (DOE)

This study employs Taguchi design of experiments to identify optimal process parameters (deposition temperature, transverse speed, powder feed rate, and operating pressure) associated with quality characteristics (porosity and hardness) of laser-assisted cold-sprayed (LACS) pure aluminum coatings. Optimal quality characteristics of the coatings were obtained by setting deposition temperature at 450°C, transverse speed at 5 mm/s, operating pressure at 30 bar, and powder feed rate at 25 rev/min. This study helps in improving the yield and stability of pure aluminum coatings produced via LACS process while it also reduces its variability which results in high rejection, rework, and warranty costs.

Effect of Surface Preparation on the Microstructure, Adhesion, and Tensile Properties of Cold-Sprayed Aluminum Coatings on AA2024 Substrates

Commercially pure aluminum coatings (CP-Al) were applied to AA 2024-T351 substrates utilizing the cold spray process using different surface preparation methods and carrier gases; the resulting microstructures and mechanical properties were investigated. Substrate preparation methods were examined to understand the effect of substrate roughness on coating properties, to minimize embedded grit, and to identify the surface preparation method that yielded the best combination of coating properties. Three substrate roughing preparations, glass bead, SiC grit, and alumina grit blast, were examined while utilizing both helium and nitrogen as carrier gases in the cold spray process. Coatings that were oxide free, possessing densities greater than 99% were achieved, with the mean coating porosity ranging 0.1-0.5%. The highest mean adhesion strength was 42 MPa for the nitrogen gas and 20 MPa for the helium gas, both using glass bead surface preparation. For the nitrogen process gas samples, the surface preparation methods that produced high pull strengths correlated to bend test specimens that showed no signs of cracking on surfaces or edges. The overall best combination of mechanical property results was achieved with coatings prepared by glass bead surface roughening using nitrogen as a carrier gas.

Scratch-Induced Deformation Behavior of Cold-Sprayed Aluminum Amorphous/Nanocrystalline Coatings at Multiple Load Scales

Scratch-induced deformation behavior of cold-sprayed Al amorphous/nanocrystalline alloy coatings at multiple load scales is reported. Micro-scratch tests are carried out at several loads (1000-4000 μN) in constant and ramp loading conditions. Macro/high load scratch tests are carried out at 5 N load in ramp loading conditions. The deformation of as-sprayed and heat-treated coatings is analyzed in terms of scratch morphology, and friction. A lower coefficient of friction (COF) is obtained for as-sprayed coatings. Despite lower COF, as-sprayed coatings show a higher wear volume loss (4 times in micro-scratch and 1.4 times in macro-scratch) as compared to heat-treated coatings. Fluctuations were observed in COF along the scratch length of as-sprayed coatings in micro-scratch tests, which are correlated to the shear band formation. Higher scratch wear resistance is observed in the heat-treated coatings as compared to as-sprayed coatings due to denser microstructure with embedded intermetallic phases. Scratch-induced deformation behavior changed from the shear band dominated brittle mode in as-sprayed to plowing type in the heat-treated coatings.

Post-heat Treatment Effects on Cold-Sprayed Aluminum Coatings on AZ91D Magnesium Substrates

Commercially pure aluminum (CP-Al) powder was deposited by the cold spray process onto AZ91D magnesium (Mg) substrates that had been subjected to three different heat-treatment conditions: namely, as-cast (F), homogenized (T4), and artificially aged (T6). The substrate hardness was measured to be 80.7 ± 1.8, 73.7 ± 4.0, and 103.6 ± 7.4 HV0.025 for the F-, T4-, and T6-Mg alloy substrates respectively. Thick (~400 μm) and dense (below 1% porosity) Al coatings have been obtained. After post-deposition heat treatment at 400 °C, the intermetallic Mg17Al12 (β) and Al3Mg2 (γ) phases with different thicknesses were found to have formed at the coating/substrate interface depending on the holding time. While no significant thickness differences of the intermetallic layers were detected in the cases of F- and T6-AZ91D substrates, thicker layers formed on the T4-AZ91D substrate. It is believed that the higher Al concentration in the T4-AZ91D solid solution within the α-Mg could diffuse and contribute more easily to the growth of the intermetallic phases. The hardness of the β- and γ-phase was measured to be 260.5 ± 10.7 HV0.025 and 279.6 ± 13.7 HV0.025, respectively. Shear strength test results revealed lower adhesion strength after heat treatment, which is attributed to the presence of brittle intermetallic layers at the coating/substrate interface.

Recrystallization in the particles interfacial region of the cold-sprayed aluminum coating: Strain-induced boundary migration

A cold-sprayed aluminum coating was fabricated using a mixture of atomized and cryomilled powder as the feed stocks. The microstructure in the particles' interfacial region of the atomized/cryomilled particles was investigated by TEM. The tongue-shaped grains with very low dislocation density were observed. The formation of the tongue-shaped grains is attributed to recrystallization dominated by strain induced boundary migration (SIBM). The driving force of SIBM in this work is an obvious difference in substructure on opposite sides of the grain boundary. A large boundary misorientation makes SIBM more viable and facilitates the grain boundary bulging into adjacent region of high stored energy. This partial reaction is also attributed to adiabatic shear instability due to severe impact during cold spraying, and it might be helpful to form ductile bonding in coatings and composite bulk materials.

Fabrication of Aluminum Coating onto CFRP Substrate by Cold Spray

Carbon fiber-reinforced plastic (CFRP) has a great potential application in aircraft fuselage due to its light weight, high specific stiffness and high specific strength. It is crucial to coat the CFRP surface with an electrically conductive material to avoid the damage from lightning strike. Cold spray process has been developed for metallic coating technique. In this study, aluminum coating was fabricated onto the CFRP substrate using interlayer was investigated. It was difficult to fabricate cold-sprayed aluminum coating directly on the CFRP substrate. Though smaller size aluminum particles could be deposited on the CFRP substrate, but the coating was peeled off when the thickness was around 30 μm. On the other hand, it was possible to deposit aluminum coating on the CFRP substrate by plasma spray process. Our proposed structure is using a thin plasma-sprayed aluminum interlayer on the CFRP substrate before doing the cold spray. The interlayer with larger contact area could retain on the substrate and able to facilitate the deformation of the next incoming cold-sprayed particles to build a thick coating. The volume resistivity of cold spray coating is lower than the plasma sprayed aluminum coating because of high process gas temperature in the latter case enhances the oxidation of sprayed particle. Therefore, lower process gas temperature should be used to fabricate lower volume resistivity coating on cold spray.

Correlation between nanoindentation and nanoscratch properties of carbon nanotube reinforced aluminum composite coatings

Abstract Nanoindentation and nanoscratch experiments are conducted on cold-sprayed aluminum composite coatings reinforced with and without carbon nanotubes (CNTs). Analytical model is developed to correlate nanoscale mechanical and tribological properties of the solid material. The geometry factor of Berkovich tip and its orientation during nanoscratch is taken into account to describe the deformation behavior of the solid material underneath indenter. Nanoindentation and nanoscratch experiments in conjunction with the computed results illustrate that the addition of carbon nanotubes into aluminum matrix contributes to increase in elastic modulus, hardness, yield strength, shear strength and nanoscale wear resistance. The coefficient of friction is unaffected by the addition of CNTs up to 1 wt.%.

Characterization of Low-Pressure Cold-Sprayed Aluminum Coatings

Aluminum alloys are widely used as materials for engineering components of automobiles and airplanes because of their light weight and high corrosion resistance. However, cracks may develop sometimes in aluminum components, which have to be repaired by welding. It is difficult to weld aluminum components due to its high specific thermal conductivity and high coefficient of thermal expansion. The low-pressure cold-spray technique can be used instead of welding for repairing cracks. However, the effects of surface conditions on particle deposition and the mechanical properties of cold-sprayed coatings have not been investigated thus far. In this study, the effect of surface conditions focusing on active newly formed surface on aluminum particle deposition is studied and the mechanical properties of low-pressure cold-sprayed aluminum coatings are investigated by four-point bending tests. It is found that for efficient particle deposition it was necessary to obtain active newly formed surface of the substrate and particle surfaces by several impingements because the existence of inactive native oxide films has an adverse effect on the deposition. Furthermore, the strength of a cold-sprayed specimen is found to be higher than that of a cold-rolled specimen under compressive loading.

Effect of Substrate Temperature on the Formation Mechanism of Cold-Sprayed Aluminum, Zinc and Tin Coatings

When describing the cold-spray process, one of the most widely used concepts is the critical velocity. Current models predicting critical velocities take the temperature of the sprayed particles explicitly into account, but not the surface temperature (substrate or already deposited layers) on which the particle impacts. This surface temperature is expected to play an important role, since the deformation process leading to particle bonding and coating formation takes place both on the particle and the substrate side. The aim of this work is to investigate the effect of the substrate temperature on the coating formation process. Experiments were performed using aluminum, zinc, and tin powders as coating materials. These materials have a rather large difference in critical velocities that gives the possibility to cover a broad range of deposition velocity to critical velocity ratio using commercial low-pressure cold-spray system. The sample surface was heated and the temperature was varied from room temperature to a high fraction of the melting point of the coating material for all three materials. The change in temperature of the substrate during the deposition process was measured by means of a high speed IR camera. The coating formation was investigated as a function of (1) the measured surface temperature of the substrate during deposition, (2) the gun transverse speed, and (3) the particle velocity. Both single particle impact samples and thick coatings were produced and characterized. Both the particle-substrate and interparticle bonding were evaluated by scanning electron microscopy (SEM) and confocal microscopy.

The Effect of a Simple Annealing Heat Treatment on the Mechanical Properties of Cold-Sprayed Aluminum

Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300°C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spray as a bulkforming process.