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Forefront cold spray additive manufacturing development by leveraging new spraying strategies

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Universitat de Barcelona. Facultat de Química

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[eng] Additive manufacturing (AM) is a process of creating a physical object by adding material layer by layer rather than removing material through machining. It is also known as 3D printing, as it typically involves using a digital model to create a physical object by adding material in a series of layers. AM has been studied for many materials, such as polymers, ceramics, composites, biomaterials, and metallic alloys. It also has been employed for many applications and purposes, such as customization, complex geometries, lightweight structures, and sustainable manufacturing.
For AM of metals, different processes have been presented and studied, highlighting the laser- and welding-based ones presented in the literature, which are more industrially mature. Cold Spray Additive Manufacturing (CSAM) is an alternative to produce freeform parts by accelerating powder particles at supersonic speed, which impacting against a substrate material trigger a process to consolidate the CSAM-ed part by bonding mechanisms. As with any other AM processing, CSAM has pros and cons. Its advantages include its high deposition rate and efficiency, low thermal input, wide range of spraying materials, and low part distortion. However, low geometric accuracy, high equipment costs, and low mechanical properties are some CSAM disadvantages.
Scholars have contributed to improve the CSAM industrial maturity by solving some of the technique limitations. This thesis presents alternatives developed and evaluated to push this frontier of knowledge even further and make CSAM an even more attractive AM process. First, understanding the state-of-art by a critical review in the introduction section; second, presenting the solutions already published by scholars, which were the base for developing new and innovative solving alternatives. And then, evaluating the effectiveness of these new strategies
For the CSAM geometric accuracy limitation, this thesis presents different robot manipulation strategies, changing the CS gun linear velocity and deposition angles. It makes a constant part sidewall inclination that demands an angle rectifying layer. An innovative new CSAM strategy named Metal Knitting has been designed, developed and tested. In this strategy, the CS gun does not follow a linear path but a virtual cone-like one, which results in a high control of the part shape, e.g., vertical bulk sidewalls or thin and high walls.
To improve the CSAM-ed material mechanical properties, the CS process parameters are optimized to reach the most appropriate particle velocity, resulting in lower porosity and higher bonding and cohesion of particles. Besides that, deposition post-treatments are evaluated. For 316L, heat treatments improve the cohesion of particles and material isotropy but reduce its hardness. For CSAM-ed Maraging, the deposition of carbides by HVOF improved the
 
material wear performance, presenting a hybrid thermal spraying system with a CSAM-ed bulk strengthened by a hard coating layer.
To improve the economic feasibility of CSAM, the use of less sophisticated, and therefore less costly, feedstock powders has been deposited. They can be deposited obtaining parts with similar properties of those obtained using more sophisticated and pricey powders. Water-atomized 316L has the same performance than the gas-atomized powders, which are in a higher-level price. Irregular particles are also good choices for CSAM Al and Ti deposits. Regarding this reduction of CSAM deposition costs, the Metal Knitting strategy has an advantage over the traditional ones because of its shorter spraying time and higher layer thickness.

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[spa] La fabricación aditiva (AM) es un proceso de creación de piezas mediante la adición de material por superposición de capas en lugar de eliminar material a través del mecanizado. Se estudiada muchos materiales para AM, como polímeros, cerámicas, compuestos, biomateriales y aleaciones metálicas. Para AM de metales la literatura presenta diferentes procesos, con destaque para los basados en láser. AM por proyección fría (CSAM) es una alternativa para producir piezas mediante la aceleración de partículas de polvo en estado sólido a una velocidad supersónica, que al impactar contra el material de sustrato desencadena un proceso para consolidar la pieza. Como cualquier otro procesamiento AM, CSAM tiene ventajas y desventajas. Sus ventajas incluyen su alta tasa de deposición y eficiencia, bajo aporte térmico, amplia gama de materiales y baja distorsión de piezas. Sin embargo, la baja precisión geométrica, los altos costos de equipo y las bajas propiedades mecánicas son algunas de las desventajas de CSAM.
Esta tesis presenta alternativas desarrolladas y evaluadas para ampliar el conocimiento acerca de la CSAM y hacer de CSAM un proceso de AM aún más industrialmente maduro. Inicialmente, se presenta el estado del arte; seguido de las soluciones ya publicadas por académicos; proponiendo y desarrollando nuevas estrategias; y luego, evaluando la efectividad de estas nuevas estrategias.
Para la limitación de precisión geométrica de la CSAM, esta tesis presenta diferentes estrategias de manipulación de robot y una nueva e innovadora estrategia de CSAM llamada Metal Knitting. Para mejorar las propiedades mecánicas de las piezas hechas por CSAM, se presenta una optimización de parámetros de proceso y se evalúan diferentes postratamientos. Para el 316L, los tratamientos térmicos mejoran la cohesión de las partículas y la isotropía del material, pero reducen su dureza. Para piezas hechas en acero Maraging por CSAM, la deposición de WC por HVOF mejora su resistencia al desgaste. Para mejorar la viabilidad económica de la CSAM, estudios comparativos de polvos irregulares y esféricos muestran que CSAM es capaz de producir depósitos y piezas de igual calidad y propiedades proyectando polvos menos costosos, lo que no es posible con otras técnicas de AM.

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Forefront cold spray additive manufacturing development by leveraging new spraying strategies