Study of the Laser Remelting of a Cold Sprayed Titanium Layer

Properties of Cold Sprayed Titanium and Titanium Alloy Coatings after Laser Surface Treatment

Additive manufacturing (AM) has seen remarkable development in recent years due to relatively high efficiency of the process. Cold spraying (CS) is a particular method of AM, in which titanium and titanium alloy powders are used. CS is a very competitive technology enabling the deposition of coatings, repairing machine parts, and manufacturing new components. For specific applications, the surface of cold-sprayed materials may require further processing. This paper reports an attempt to employ laser surface treatment (LST) of cold-sprayed coatings on an aluminium alloy substrate. The influence of laser beam interaction time on the coatings' properties was analysed. The microstructure was investigated and observed employing scanning electron microscopy (SEM). To evaluate residual stress after CS and LST, the sin²ψ technique was used. Investigations were also performed on Vickers hardness, contact angle, and surface roughness. Significant changes in the surface morphology of the coatings and elevated residual stress levels dependent on the laser beam interaction time were observed. Increased Vickers hardness was recorded for titanium alloy Ti6Al4V. LST also led to increased surface hydrophilicity of the modified materials Ti and Ti6Al4V.

Modification of Cold-Sprayed Cu-Al-Ni-Al2O3 Composite Coatings by Friction Stir Technique to Enhance Wear Resistance Performance

An innovative hybrid process combining two effective surface modification techniques, cold spray (CS) and friction stir processing (FSP), was proposed to refine the microstructure of Cu-Al-Ni-Al2O3 composite coating material. FSP was performed under constant rpm using extensive cooling conditions to remove heat generated during the operation. Microstructural characterizations such as optical micrography (OM), scanning electron microscopy (SEM), Electron Backscatter Diffraction (EBSD), Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were performed to evaluate the microstructural evolution of the coatings before and after FSP treatment. Mechanical characterizations such as microhardness and elastic modulus were measured using micro-depth sensing techniques. Furthermore, sliding wear tests were performed to study the wear resistance of the as-sprayed and processed coatings. The findings suggest that after FSP, there is an improvement in microstructure of the coating layers with the elimination of particle boundaries, micro-pores and micro-cracks, and processed coatings showed an improvement in mechanical properties. Furthermore, there was a slight reduction in the wear rate of the deposited CuAlNi-Al2O3 composite coatings. Among all the test coatings, friction stir processed S1 coating showed the lowest wear rate, which was an almost two times lower wear rate than its unprocessed counterparts.

Cold spray micro-defects and post-treatment technologies: a review

Purpose

The deposition of particles onto a substrate during the cold spraying (CS) process relies on severe plastic deformation, so there are various micro-defects induced by insufficient deformation and severe crushing. To solve the problems, many post-treat techniques have been used to improving the quality by eliminating the micro-defects. This paper aims to help scholars and engineers in this field a better and systematic understand of CS technology by summarizing the post-treatment technologies that have been investigated recently years.

Design/methodology/approach

This review summarizes the types of micro-defects and introduces the effect of micro-defects on the properties of CS coating/additive manufactured, illustrates the post-treatment technologies and its effect on the microstructure and performances, and finally outlooks the future development trends of post-treatments for CS.

Findings

There are significant discoveries in post-treatment technology to change the performance of cold spray deposits. There are also many limitations for post-treatment methods, including improved performance and limitations of use. Thus, there is still a strong requirement for further improvement. Hybrid post-treatment may be a more ideal method, as it can eliminate more defects than a single method. The proposed ultrasonic impact treatment could be an alternative method, as it can densify and flatten the CS deposits.

Originality/value

It is the first time to reveal the influence factors on the performances of CS deposits from the perspective of microdefects, and proposed corresponding well targeted post-treatment methods, which is more instructive for improving the performances of CS deposits.

Molecular Dynamics Study on Regimes of Head-on Droplet Collision

The head-on collision of two water droplets with a diameter of 10 nm in an atmospheric environment was investigated via molecular dynamics simulations. The gas molecules between droplets were visualized, and the phenomena of gas extrusion and gas molecules being captured were found. By observing and analyzing the holes regime, a "periphery-sucking" mechanism was proposed to explain the thinning in the middle of the expanding disk and the holes appearing. It was found that the splattering regime can be divided into the limited splattering regime and the divergent splattering regime. The splattering modes and droplet characteristics of the two regimes are markedly different. The non-bonded interactions and intermolecular hydrogen bond were analyzed, and it was found that increasing the Weber number (We) can effectively promote the mixing of the two droplets, promote the formation of an intermolecular hydrogen bond between the two droplets, and reduce the average lifetime of the intermolecular hydrogen bond. The radial distribution function between the water molecules was plotted, showing that increasing the We makes the water molecules more dispersed as a whole in the collision process.

Thermal Spray Processes in Concentrating Solar Power Technology

Solar power is a sustainable and affordable source of energy, and has gained interest from academies, companies, and government institutions as a potential and efficient alternative for next-generation energy production. To promote the penetration of solar power in the energy market, solar-generated electricity needs to be cost-competitive with fossil fuels and other renewables. Development of new materials for solar absorbers able to collect a higher fraction of solar radiation and work at higher temperatures, together with improved design of thermal energy storage systems and components, have been addressed as strategies for increasing the efficiency of solar power plants, offering dispatchable energy and adapting the electricity production to the curve demand. Manufacturing of concentrating solar power components greatly affects their performance and durability and, thus, the global efficiency of solar power plants. The development of viable, sustainable, and efficient manufacturing procedures and processes became key aspects within the breakthrough strategies of solar power technologies. This paper provides an outlook on the application of thermal spray processes to produce selective solar absorbing coatings in solar tower receivers and high-temperature protective barriers as strategies to mitigate the corrosion of concentrating solar power and thermal energy storage components when exposed to aggressive media during service life.

Post-Process Treatments on Supersonic Cold Sprayed Coatings: A Review

Cold Gas Dynamic Spray or Supersonic Cold Spray, or simply ‘Cold Spray’, is an emerging technology for rapidly building thin films, thick coatings and large-scale additive manufacturing at relatively low temperatures. In a cold spray process, particles are accelerated to supersonic speeds by a propellant gas and impact a substrate, thus producing a strong bonding with the substrate and subsequently forming a deposit via layer-by-layer buildup. The scalability and low cost of this method make it promising for many applications in industry, such as metal component surface repair/enhancement/restoration and functional coatings for electrical, thermal, biomedical, energy storage, and nuclear plant applications. However, cold sprayed deposits usually require post process treatments to further modify their microstructures and mechanical properties in order to obtain the desired performances. A number of studies have been carried out on this topic. Here, recent progress in different post process treatments on cold sprayed deposits is reviewed, including heat treatment, friction-stir processing, shot peening, and laser re-melting. The effects of these post treatments on the microstructure, residual stress and mechanical properties of cold sprayed deposits are discussed.