Key Engineering Materials Vol. 813

Abstract: Cold spray (CS) is a deposition technique to form a coating from the particles with temperature lower than their melting point. In this technique, particles are accelerated by a supersonic flow of a carrier gas such as air or nitrogen. Upon impact, particles undergo significant plastic deformation that bonds them to the substrate. Since the particles are not molten, this deposition method does not apply a lot of heat to the substrate and this makes CS the best candidate for temperature sensitive and oxygen sensitive materials. CS can be adapted to form 3D objects following layer-by-layer approach. This is called cold gas dynamic manufacturing (CGDM) or cold spray as additive manufacturing. Developing complex shapes by CGDM may result in formation of inclined surfaces, corners and sharp edges. Deposition in those regions is often accompanied with challenges that affect the accuracy and efficiency of the manufacturing. In this study, CGDM for two typical shapes such as cylinder and frustum on a flat substrate has been simulated to represent the additively manufactured parts. Particle trajectories and impact conditions i.e. velocity and size distributions have been compared. The results of numerical modelling provided useful information for understanding the limitations and challenges associated with CGDM that can help us to improve the quality and precision of particle deposition.

Abstract: Tantalum as a transition element possesses good corrosion resistant properties, along with ductility and hardness. It is also one of the best heat-resistant material (melting point 2996°C) and is known for its high heat and electrical conductivity. In this research, Tantalum is deposited on stainless steel substrate using high-pressure cold spray (HPCS) method. Cold spray coating technology enables the deposition of powder feedstock without melting. Feedstock particles are propelled through a nozzle at supersonic velocities and they deform plastically on impact, resulting in good bonding strength to the substrate. The low temperature and solid-state deposition associated with cold spray allows refractory materials such as Ta, Mo, and W to be deposited without high temperature requirements. The objective of this work is to achieve a dense and nonporous coating microstructure with a high deposition efficiency. The hardness of as-received tantalum particles is found to be 279 HV_0.3 and the microstructure is very dense. Tensile testing carried on the sample coated at a stagnation gas pressure of 50 bar and gas inlet temperature of 900°C exhibited an ultimate tensile strength of 442 MPa and adhesion strength of 77 MPa. Further mechanical properties of the coating in terms of hardness is carried out by nanoindentation. These results will be correlated with microstructural imaging and elemental analysis including morphology and composition using scanning electron microscopy and X-ray diffraction techniques.

Abstract: The physical behavior of metal powders during laser-based additive manufacturing processes has been investigated. In particular, an experimental campaign of direct metal deposition has been carried out to evaluate the effect of the laser power and spot size on the powder/substrate interaction and on the surface morphology of the final piece. A fast-camera has been used to evaluate the interaction phenomena during the printing process, while confocal microscopy has been carried out to measure the surface morphology of the samples. Results highlighted that increasing the laser power and laser spot size, the particle impact velocity is about constant, while the powder/laser/substrate interaction zone increases. As a consequence, the mean thickness increases, as confirmed by surface characterization.

Abstract: In this study, the effects of surface treatment of a low-temperature atmospheric oxygen plasma on basalt/epoxy composites were investigated to improve the hydrophobility of the composite surface. After the plasma treatment, the unmodified and surface treated composite laminates have been experimentally characterized by performing contact angle measurements, low-velocity impact tests and indentation depth on the impacted laminates. Results have showed a dependence of such composite properties on the plasma coating deposition and on the treatment parameters outlining the need to optimize both the plasma power and exposition time to plasma in order to assess the efficiency of the plasma treatment and establish the optimal processing conditions.

Abstract: Fused deposition modeling (FDM) is an additive manufacturing technology where three-dimensional physical models are manufactured by layer-by-layer deposition. However, the layered surface built with FDM suffers from poor surface quality and dimensional accuracy even for basic part geometries. This proves to be unacceptable and not satisfactory for most general purposes with the consequence of a decreased value of the final product. Several methods for post-processing were proposed to achieve fine surface of manufactured components. In particular, for components manufactured with polylactic acid (PLA) the chemical post-processing with dimethyl ketone (acetone), named vapor smoothing process, seems to be very promising to significantly improve the surface roughness. Moreover, acetone has the main advantage to have a low cost, low toxicity and high diffusion rate. However, this polishing procedure may dissolve the outer surface of the parts affecting the structural reliability of the part. In this work, a novel device, consisting of a cylindrical chamber in Pyrex, is set-up for the vapor smoothing process with acetone. The system is designed to permit the injection of a gas containing acetone at different concentrations and at different operating conditions (temperature, contact time). The samples used for the test are truncheon design manufactured using different printer settings; each truncheon is built at inclination angles varying from 0° to 45° in step of 5°. The variation of the surface roughness was investigated using a confocal microscope Leica DCM3D, equipped with the software LeicaScan and LeicaMap.

Abstract: This work aims on the study of the wear resistance of a commercial polyester composite system under severe wear conditions. It is an example of composite engineering material manufactured from synthetic fibers and thermosetting resins with appealing physical and mechanical properties that make it very interesting for products like sheets, rods, tubes, flange, washers and wear ring. In order to make it suitable for applications where damage by metal roll surfaces can occur, the study of the wear behavior under specific conditions is then necessary. At this aim, a detailed experimental campaign, including tribological tests and microgeometrical measurements, was carried out. In particular, the tribological behaviour was studied through the pin-on-disk tests conducted at 209 mm/s as peripheral speed under an applied load comprising in the range of 10 - 70 N. The tests were followed by microgeometrical measurements by using a confocal microscope in order to critically observe the wear tracks, evaluate their depth, width and then to calculate the final less of volume. The results proved that the wear mechanisms founded for the composite under investigation differ from those commonly associated to metals; the surface asperities of the fibres allow to hold the lubricant inside the surface and thus assist in the maintenance of a continuous film on the working surface involving in very low friction coefficient.

Abstract: Recent studies showed that cold spray is a suitable way to produce metallic coatings on non-metallic surfaces such as polymers and composites for engineering applications. Mechanical bonding and particle anchoring process onto these substrates have not been totally understood yet. This study is focused on the creation of conductive coatings by cold spray onto short carbon fiber reinforced PEEK. For the requirements of the industrial application, the deposit should be able to tolerate both electrostatic and electric currents (higher than 10A). Because of their high conductivity, aluminium and copper powder seemed to be the best candidate materials. To promote adhesion and coating build up, copper-zinc and copper-PEEK mixed powders were also tested. A correlation between experimental and numerical tests has been adopted to assess and improve the possibilities of this material coupling. Both low pressure and high-pressure cold spray equipment were used for the spraying. Since polymer matrix-based composites are thermosensitive materials, it was necessary to develop a computational fluid-dynamics tool to investigate the interaction of the hot gas on the substrate and the correlation with the deposition process. A series of finite element simulations of single and multi-particle impacts onto local structures of the composite, as observed from SEM images of the specimens, is on-going. A classical pull-off test of the specimen will be adopted to collect data about coating adhesion strength. Finally, the four points measurement by Van der Pauw method will be used to assess the conductivity of specimens for different powders and coating thicknesses.