The Effect of Argon as Atomization Gas on the Microstructure, Machine Hammer Peening Post-Treatment, and Corrosion Behavior of Twin Wire Arc Sprayed (TWAS) ZnAl4 Coatings

In the twin wire arc spraying (TWAS) process, it is common to use compressed air as atomizing gas. Nitrogen or argon also are used to reduce oxidation and improve coating performance. The heat required to melt the feedstock material depends on the electrical conductivity of the wires used and the ionization energy of both the feedstock material and atomization gas. In the case of ZnAl4, no phase changes were recorded in the obtained coatings by using either compressed air or argon as atomization gas. This fact has led to the assumption that the melting behavior of ZnAl4 with its low melting and evaporating temperature is different from materials with a higher melting point, such as Fe and Ni, which also explains the unexpected compressive residual stresses in the as-sprayed conditions. The heavier atomization gas, argon, led to slightly higher compressive stresses and oxide content. Compressed air as atomization gas led to lower porosity, decreased surface roughness, and better corrosion resistance. In the case of argon, Al precipitated in the form of small particles. The post-treatment machine hammer peening (MHP) has induced horizontal cracks in compressed air sprayed coatings. These cracks were mainly initiated in the oxidized Al phase.

Multiphysics Modeling and Simulation of an Arc-Jet Sprayer

Twin wire arc spraying (TWAS) is a plasma spraying process that offers low workpiece heating and high deposition rates at a lower cost. Variations in TWAS process conditions cause the substrate temperature to fluctuate and even melt. Therefore, the motivation of this project was to simulate the heat transfer from the TWAS torch to the substrate during spraying and layer formation of a coating. Simulations using ANSYS FLUENT Computational Fluid Dynamics (CFD) software were used to model the heat transfer in a TWAS system. The results of this paper are meant to augment and improve the database of TWAS technology. A CFD numerical heat transfer model is presented that was used to investigate the substrate surface temperature during the TWAS process. The results for the different pressure models showed that for a 3 second simulation, substrate surface temperatures increased as nozzle inlet pressure was decreased. For the upper and lower bound pressures of 75 psia and 29 psia, substrate surface temperature resulted in 946 °C and 1010 °C, respectively.

Mechanical and Microstructural Properties of Post-Treated Zn4Al Sprayed Coatings Using Twin Wire Arc Spraying

Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

Low Temperature Soldering of Laser Structured and Metal Coated Fiber Reinforced Plastics

Assemblies containing fiber-reinforced plastic (FRP) and metal parts are typically fastened together via mechanical joining or adhesive bonding. Mechanical joining processes tend to weaken FRP parts by cutting fibers, while adhesives require long cures and often lead to inseparable material compounds. This paper evaluates a new joining method in which plastic parts are laser treated, then metallized via wire-arc spraying, and finally soldered to mating metal parts using a low-temperature process. Due to the effective increase in interface area resulting from laser structuring, bond strengths of up to 15.5 MPa can be achieved.

Ni-5wt% Al Coatings Deposited by Twin Wire Arc Spraying for Molten Aluminum Attack Protection

Nickel-aluminum alloys are widely used in harsh environments due to their corrosion resistance, high melting temperature, and thermal conductivity. In this work, Ni-5wt%Al coatings were deposited by twin-wire arc spraying (TWAS) on tool steel using a design of experiments approach to study the effect of process parameters on coating microstructure and performance. Test results presented in the form of process maps show how N2 pressure, stand-off distance, and current affect in-flight particle velocity and temperature as well as coating thickness and oxide content. Using this information, optimized coatings were then deposited on test substrates and subjected, along with uncoated tool steel, to several hours of molten aluminum attack. The coated samples showed no signs of physical or chemical damage, whereas the uncoated substrates experienced oxidation, aluminum infiltration, and formation of Fe-Al intermetallics.

Development of Thermal Spray Processes for Depositing Coatings on Thermoplastics

Thermoplastics combine high freedom of design with economical mass production. Metallic coatings on thermoplastics enable power and signal transmission, shield sensitive parts inside of housings and can reduce the temperature in critical areas by functioning as a heat sink. The most used technical thermoplastics are polyamides (PA), while the described use cases are often realized using Cu. Consequently, several studies tried to apply copper coatings on PA substrates via thermal spraying; so far, this combination is only feasible using an interlayer. In this study, a new approach to metallize thermoplastics via thermal spraying based on validated state-of-the-art predictions of the thermoplastics’ material response at relevant temperatures and strain rates is presented. Using these predictions, high velocity wire-arc spraying was selected as coating process. Furthermore, the process parameters were adapted to realize a continuous coating while also roughening the substrate during coating deposition. The resulting Cu coating on PA6 had a sufficiently high coating adhesion for post-treatment by grinding. The adhesion is achieved by in situ roughening during the coating application. The results indicate that different process parameters for initial layer deposition and further coating buildup are required due to the low thermal stability of PA6.