Abstract
Cold-spraying is a relatively new low-temperature coating technology which produces coatings by the deposition of metallic micro-particles at supersonic speed onto target substrate surfaces. This technology has the potential to enhance or restore damaged parts made of light metal alloys, such as Ti6Al4V (Ti64). Particle deposition velocity is one of the most crucial parameters for achieving high-quality coatings because it is the main driving force for particle bonding and coating formation. In this work, studies were conducted on the evolution of the properties of cold-sprayed Ti64 coatings deposited on Ti64 substrates with particle velocities ranging from 730 to 855 m/s using pure N2 and N2-He mixture as the propellant gases. It was observed that the increase in particle velocity significantly reduced the porosity level from about 11 to 1.6% due to greater densification. The coatings’ hardness was also improved with increased particle velocity due to the intensified grain refinement within the particles. Interestingly, despite the significant differences in the coating porosities, all the coatings deposited within the velocity range (below and above critical velocity) achieved a high adhesion strength exceeding 60 MPa. The fractography also showed changes in the degree of dimple fractures on the particles across the deposition velocities. Finite element modelling was carried out to understand the deformation behaviour of the impacting particles and the evolutions of strain and temperature in the formed coatings during the spraying process. This work also showed that the N2-He gas mixture was a cost-effective propellant gas (up to 3-times cheaper than pure He) to deliver the high-quality Ti64 coatings.
Highlights
Titanium (Ti) alloys, such as Ti6Al4V (Ti64), possess superb properties like low density, high specific strength and good corrosion resistance, and are ideal to be used in aerospace, chemical, and biomedical applications [1]
The particle velocity of the feedstock powder impacting onto the substrate or prior deposits provides the key driving force for bonding formation, which can be derived using the following equation [72,73]: (3)
It can be seen that the particle velocity is governed mainly by the molar mass, temperature and pressure of the propellant gas
Summary
Titanium (Ti) alloys, such as Ti6Al4V (Ti64), possess superb properties like low density, high specific strength and good corrosion resistance, and are ideal to be used in aerospace, chemical, and biomedical applications [1] As these Ti64 components suffer from wear and tear over the service period, it will be more cost-effective to repair them and restore their functionality instead of scraping or refabrication. A lighter gas, He or a mixture of N2 and He, with high gas pressure and a preheated temperature would generate a faster gas stream and provide a higher drag force onto each particle (for acceleration), which results in a more significant particle deformation upon impact and improves coating quality [39,40,41,42,43].
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