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In twin-wire arc spraying (TWAS), the in-flight particles are atomized from a melting bath which generates an inhomogeneous spraying plume. This inhomogeneity is due to the fact that these particles are generated by the impingement of fast continuous flowing air upon the melting tips of electrically conductive wires. This work aims to contribute to the understanding of the initiation of such particles in the TWAS process. For this purpose, cored wires filled with W-rich particles were sprayed. After interrupting the TWAS process, the tips of these cored wires were imaged by 3D µCT and scanning electron microscopy in order to analyze how the filling powder interacts with the melted part of the sheath. The analysis of the 3D tomograms shows that the resolidified melting bath of the cored wires is interspersed with both spherical and irregular-shaped W-rich particles. This irregular shape suggests a partial melting of the W-rich particles. [ABSTRACT FROM AUTHOR]

Because of their excellent properties aluminum and its alloys cover a wide range of applications especially in the lightweight construction sector. In order to reach a higher strength and wear resistance metal matrix composites (MMCs) are used. Typically Al MMCs are manufactured by casting or extrusion processes. The disadvantage of these production routes is a cost-intensive and time-consuming finishing in terms of grinding and milling. The technique of thermal spraying provides the possibility to coat aluminum parts with MMCs close to their final shape. In addition to the shape accuracy the ductility and toughness of the coated parts are generally higher compared to extruded or casted parts. This study describes the development of detonation-sprayed boron carbide reinforced aluminum coatings on aluminum (EN AW 5754) substrates. The optimization of the coatings was focused on a homogeneous coating structure, a low coating porosity, a high deposition efficiency, a high number of embedded carbides, and a small percentage of oxides. In continuous tensile tests the influence of the MMC coating on the tensile strength was determined. Furthermore, the tensile strength was investigated in a discontinuous tensile test step by step. The different stages of deformation were analyzed by using micro computed tomography. This method enables the observation of tensile specimens in 3D, and consequently the site and moment of crack formation. [ABSTRACT FROM AUTHOR]

The powder injection parameters, the location of the injection port, as well as the metal matrix composites are important features, which determine the deposition efficiency and embedding behavior of hard materials in the surrounding matrix of the twin wire arc-spraying process. This study investigates the applicability of external powder injection and aims to determine whether the powder injection parameters, the location, and the material combination (composition of the matrix as well as hard material) need to be specifically tailored. Therefore, the position of the injection port in relation to the arc zone was altered along the spraying axis and perpendicular to the arc. The axial position of the injection port determines the thermal activation of the injected powder. An injection behind the arc, close to the nozzle outlet, seems to enhance the thermal activation. The optimal injection positions of different hard materials in combination with zinc-, nickel- and iron-based matrices were found to be closer to the arc zone utilizing a high-speed camera system. The powder size, the mass of the particle, the carrier gas flow, and the electric insulation of the hard material affect the perpendicular position of the radial injection port. These findings show that the local powder injection, the wetting behavior of particles in the realm of the molten pool as well as the atomization behavior of the molten pool all affect the embedding behavior of the hard material in the surrounded metallic matrix. Hardness measurement by means of nanoindentation and EDX analysis along transition zones were utilized to estimate the bonding strength. The observation of a diffusion zone indicates a strong metallurgical bonding for boron carbides embedded in steel matrix. [ABSTRACT FROM AUTHOR]

Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes. [ABSTRACT FROM AUTHOR]