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Item Type: Tesi di dottorato
Resource language: English
Date: 7 December 2017
Number of Pages: 109
Institution: Università degli Studi di Napoli Federico II
Department: dep08
Dottorato: phd038
Ciclo di dottorato: 30
Coordinatore del Corso di dottorato:
Date: 7 December 2017
Number of Pages: 109
Keywords: Cold spray; Modelling
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/16 - Tecnologie e sistemi di lavorazione
Date Deposited: 08 Jan 2018 00:56
Last Modified: 07 Oct 2021 10:32

Collection description

The Cold Gas Dynamic Spray (CGDS) process, or generally referred Cold Spray technique, is an intriguing coating technique developed in the mid-1980s at the Institute for Theoretical and Applied Mechanics in the Soviet Union and firstly reported by professor Papyrin. In this process, fine powder particles (generally 1-50 µm in diameter) are accelerated to a high velocity normally ranging from 300 to 1200 m/s in a supersonic jet of compressed (carrier) gas and projected onto a metallic (or non-metallic) substrate in an entirely solid state. Upon impact with a target surface, conversion of kinetic energy to plastic deformation occurs, the solid particles deform and bond together. The supersonic gas jet is produced using a converging-diverging de Laval nozzle system, where the expansion of the gas in the divergent section of the nozzle will reduce the gas temperature, resulting in a process whereby the particles remain in a solid state prior to impacting the substrate. Generally, the reported advantages of cold spraying are high deposition efficiency, low residual stresses, minimal heat input to substrate, compositional stability and little need for masking. Furthermore, use of a cold spray process eliminates phase transformations, caused by melting and porosity formation caused by solidification, both of which are concerns with other thermal spray techniques. In cold spraying, the temperature of the gas is well below the melting point of the sprayed material and thus, particles are not melted in the gas flow. Therefore, as a solid-state method coating process, cold spraying avoids the formation of oxide inclusions, pores and other voids. About the CGDS apparatus, two kind of cold spray equipment are commercially available: High Pressure Cold Spray (HPCS) and Low Pressure Cold Spray (LPCS); these processes are based on the same principles but differ in terms of size and cost, as well as the achievable impact velocities and temperatures due to different spraying parameters used. Although the HPCS system is able to manage a larger materials selection and, moreover, to produce higher quality of the deposits, the investment costs for high pressure cold spray equipment are greater than those for low pressure cold spray. In this contest, the LPCS is often used to lay down successful materials for various functional properties and typical application fields: surface functionalization, restoration and bulk production. Prior to cold spray, many of the parts could not be reclaimed because there was not an existing technology that could be used for dimensional restoration. The aerospace industry is potentially the largest beneficiary of repair by cold spray; in particular, the LPCS, which is usually used in portable systems, could be a very useful and rapid method to repair in situ both the corrosion and mechanical damage of lightweight aircraft components. It could be also a very effective technique to create functional coatings in transportation industry. In this contest, it has been found that the cold spray technique is a valid potential method to replace titanium components with titanium coated aluminium components leading strong advantages in terms of both costs and weight saving. Because particles are heated in the gas stream only to a fraction of their melting temperature, the CGDS could be an effective technique to create coatings with materials sensitive to high temperatures. To this aim, a new method based on low pressure cold sprayed temperature sensitive powders to manufacture free shape precursors for metal foams was also proposed in literature. The key idea was to spray a powder mixture (made of both aluminium alloy powders as metal matrix and titanium hydride particles as foaming agent) through the LPCS on a free shape metallic substrate and then carry out the foaming process overcoming the limits of the available commercial manufacturing techniques. As for the bonding mechanisms, it has been accepted that the plastic deformation of the particle occurs by means of an adiabatic shear instability mechanism, which is activated only if the impact velocity is higher than a threshold value, called critical velocity. However, the actual bonding mechanism for cold spray particles is still not well understood, a high number of works has been carried out during the past decade, several theories have been proposed to explain the complex adhesion phenomena. Moreover, both the phenomena and the issues occurring during the long time deposition processes, the most interesting ones for industrial applications, were not adequately studied and identified. Aiming to fulfil this lack of knowledge, this PHD THESIS investigates in detail the behaviour of a LPCS system; in particular, the phenomena occurring during long depositions were studied through both fully experimental evaluations and computational fluid dynamics (CFD) analyses, pointing out the critical behaviour of LPCS systems under given working conditions. Numerical FEM (Finite Element Method) investigations about the impact of particles in cold spray based methods were also carried out in order to analyse the rebound phenomena and the bonding mechanism of a single particle impacting on a metallic substrate. For this purpose, an original 3D model was developed capable to simulate the bonding between the impacting particle and the substrate and, eventually, the failure under specific working conditions. The work done and the results obtained from these investigations could be very helpful to describe the cold spray process and, in particular, it could be a solid base to define optimized process design parameters. Moreover, fluid dynamics analyses could be used for the design of optimized nozzle geometries, thus resulting in more economical process conditions and better coating quality. The following thesis consists of four chapters and an abstract that provide a glance at the motivation and the problem this research has attempted to solve. The first chapter is an introduction to the CGDS technology, it brings forth ample knowledge from past to present research, vital to this report as well as to the continuation of this research. A detailed explanation of the process is reported in this chapter concluding with a specific section about the recent challenges and limitations. Moreover, a complete description about a new method based on low pressure cold sprayed temperature sensitive powders to manufacture free shape precursors for metal foams is given in order to highlight the real potentiality of this technology. In the second chapter, detailed procedures and investigations about the behaviour of an LPCS system are described, while attention is given to the phenomena occurring during long time deposition processes through experimental evaluations. Moreover, a CFD model was developed to analyse the fluid-dynamics behaviour of the cold spray nozzle governing the motion of the sprayed particles, thus the performances of the system. In the third chapter, numerical analyses are presented and discussed. In particular, an original FEM numerical model of a single particle impacting on a metallic substrate is proposed to better investigate about the rebounding and the deposition behaviour of particles during cold spraying. It is reasonable to state that one of the most key factors affecting the particles bonding or lack thereof is the residual energy available in the system due to impact during the elastic rebound phase. Therefore, the aim of this section is to numerically investigate about the possibility to cut down the particle impact residual energy, thus promoting the substrate-particle adhesion mechanisms. The complex rebound/adhesion phenomena, which govern the performances of the cold spray process, were studied in details. The thesis concludes with the fourth chapter that summarizes the entire study, it analyses the research activities discussed and provides a summary of the main results obtained along with the future work and perspectives.


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