Additive manufacturing (AM) is rapidly expanding towards the production of structural and functional components. This evolution inevitably requires a broader range of materials than those currently available. The materials currently used for these techniques are mainly metallic systems and alloys, including titanium alloys, nickel superalloys, cobalt-chromium alloys, aluminium alloys, stainless steels, and tool steels. These alloys were originally developed for conventional manufacturing processes and are not always suitable for AM processes. In fact, the processability of certain metal powder grades, such as nickel superalloys and aluminium alloys, can pose significant challenges due to their susceptibility to solidification and liquation cracking. Other alloys, such as copper and silver, are difficult to process because of their high laser reflectivity. As a result, a key challenge is the development of tailored alloys specifically designed to further advance additive manufacturing technologies. Modifying the original chemical composition of the powder is often necessary to produce dense, crack free structural parts, but this is not always feasible. Recent studies have shown that modifying only the surface of the powder particles can significantly enhance powder processability. One such approach is powder functionalization, which involves decorating the surface of powder particles with thin coatings or discrete particles. This strategy can improve powder processability by promoting the formation of fine microstructures in the deposited material, enhancing their flowability, or improving laser absorptivity. Moreover, surface functionalization offers an effective means to tailor material behaviour, enabling control over properties such as strength, ductility, stiffness, thermal conductivity, and coefficient of thermal expansion. This research introduces innovative powder modification strategies, demonstrating how surface and compositional modifications of atomized powders can expand the processing window and enable the design of alloys with tailored properties. Two representative alloy systems (Cu-based and Al-based alloy) were investigated to illustrate the versatility of the proposed approach. Regarding commercially pure copper, its processability in powder bed fusion - laser beam (PBF-LB/M) encounters significant challenges due to its high near-infrared (IR) laser reflectivity and thermal conductivity. Manufacturing fully dense Cu parts with conventional IR PBF-LB/M systems is challenging, often resulting in lack of fusion defects. In this work, two novel functionalization approaches were explored to address the poor processability of Cu, including (i) the electrodeposition of a thin Ag coating and (ii) the electrostatic self-assembly of TiC nanoparticles. Coating copper powders with silver enabled the formation of a Cu-Ag eutectic network during solidification, which effectively healed pores and improved densification. The decoration of copper powders with TiC nanoparticles increased laser absorptivity up to 80% and improved processability, leading to significantly denser components. Regarding aluminium alloys, the focus was on developing advanced Al2618/TiB2 metal matrix composites (MMCs) with improved processability and high stiffness and strength. The Al2618 is a widely used high-strength aluminium alloy for high temperature applications, and the addition of TiB2 particles was proved to increase its mechanical properties and PBF-LB/M processability. TiB2 was observed to be an effective grain refiner, capable of reducing the alloy's susceptibility to hot cracking. Different routes for feedstock preparation were systematically compared, including pre-alloying by gas atomization, plasma coating, and mechanical mixing. Building on their respective advantages, a hybrid approach relying on in-situ and ex-situ particles was proposed, enabling high ceramic fractions while maintaining good processability. In addition, a novel surface decoration of Ti-rich Al powders with B nanoparticles was investigated, promoting the in-situ formation of reinforcing phases during PBF-LB/M process. The methodology is versatile and can be extended to other Ti-containing Al systems, as the amount of B introduced onto the powder surface can be precisely adjusted to achieve the desired balance between grain size, strength, and ductility. This flexibility makes the approach particularly promising for optimizing both processability and performance in AM Al components, while mitigating the challenges associated with gas atomization of B-rich feedstocks. Finally, powder blending was explored as a strategy to target precise properties for specific applications, and it was employed to tune the thermal behaviour of Al-Si alloys. In the space sectors, Al mirrors are typically coated with a Ni-P layer to enhance polishability and surface quality. However, differences in the coefficient of thermal expansion (CTE) between the aluminium substrate and the nickel coating can lead to optical distortions and coating delamination during service. The last part of this thesis therefore examines the thermal and mechanical properties of Al-Si alloys with different silicon contents processed via PBF/LB-M. By blending AlSi10 and AlSi45 powders, intermediate hypereutectic compositions were produced and manufactured using PBF/LB-M. The microstructure, compressive strength and CTE of the printed alloys was systematically assessed. Among the compositions studied, AlSi40 emerged as a promising balance, offering a low CTE alongside excellent printability, positioning it as a strong candidate for nickel-coated space mirror applications.
La manifattura additiva (Additive Manufacturing, AM) sta rapidamente evolvendo verso la produzione di componenti strutturali e funzionali, richiedendo un ampliamento della gamma di materiali disponibili. Tuttavia, molte leghe attualmente utilizzate derivano da processi produttivi convenzionali e non risultano pienamente idonee ai processi AM, presentando problematiche quali criccabilità a caldo, liquation cracking o scarsa assorbività laser. Di conseguenza, lo sviluppo di materiali progettati specificamente per AM rappresenta una sfida cruciale. Questo lavoro esplora strategie innovative di modifica delle polveri metalliche volte a migliorare processabilità e proprietà finali dei componenti prodotti mediante Powder Bed Fusion – Laser Beam (PBF-LB/M). In particolare, vengono investigate tecniche di funzionalizzazione superficiale delle polveri, basate sull’applicazione di rivestimenti sottili o sulla decorazione con particelle, in grado di migliorare scorrevolezza, assorbimento laser e controllo microstrutturale. Due sistemi rappresentativi, a base rame e alluminio, sono stati studiati per dimostrare la versatilità dell’approccio. Per il rame commercialmente puro, sono state sviluppate due strategie di funzionalizzazione: rivestimento galvanico in argento e auto-assemblaggio elettrostatico di nanoparticelle di TiC. Il rivestimento in Ag favorisce la formazione di una rete eutettica Cu-Ag in grado di ridurre la porosità, mentre le nanoparticelle di TiC aumentano significativamente l’assorbività laser, migliorando la densificazione. Per le leghe di alluminio, lo studio si è concentrato su compositi a matrice Al2618 rinforzati con TiB₂, evidenziando il ruolo delle particelle come raffinatori di grano e riduttori della suscettibilità al cracking a caldo. Sono state confrontate diverse vie di preparazione delle polveri (prelega, rivestimento al plasma, miscelazione meccanica) e proposta una strategia ibrida che combina particelle in-situ ed ex-situ. Inoltre, la decorazione superficiale con nanoparticelle di B su polveri ricche in Ti ha consentito la formazione in-situ di fasi rinforzanti durante il processo PBF-LB/M. Infine, la miscelazione di polveri è stata impiegata per modulare il coefficiente di espansione termica (CTE) di leghe Al-Si destinate ad applicazioni spaziali. Miscelando polveri AlSi10 e AlSi45 sono state ottenute composizioni ipereutettiche intermedie. Tra queste, la lega AlSi40 ha mostrato un compromesso ottimale tra bassa espansione termica ed elevata stampabilità, risultando promettente per specchi spaziali rivestiti in Ni-P. Nel complesso, il lavoro dimostra come la modifica superficiale e composizionale delle polveri rappresenti una via efficace per ampliare la finestra di processo e progettare leghe ottimizzate per la manifattura additiva.
Design and functionalization of alloy and composite powders for PBF-LB/M
Lupi, Giorgia
2025/2026
Abstract
Additive manufacturing (AM) is rapidly expanding towards the production of structural and functional components. This evolution inevitably requires a broader range of materials than those currently available. The materials currently used for these techniques are mainly metallic systems and alloys, including titanium alloys, nickel superalloys, cobalt-chromium alloys, aluminium alloys, stainless steels, and tool steels. These alloys were originally developed for conventional manufacturing processes and are not always suitable for AM processes. In fact, the processability of certain metal powder grades, such as nickel superalloys and aluminium alloys, can pose significant challenges due to their susceptibility to solidification and liquation cracking. Other alloys, such as copper and silver, are difficult to process because of their high laser reflectivity. As a result, a key challenge is the development of tailored alloys specifically designed to further advance additive manufacturing technologies. Modifying the original chemical composition of the powder is often necessary to produce dense, crack free structural parts, but this is not always feasible. Recent studies have shown that modifying only the surface of the powder particles can significantly enhance powder processability. One such approach is powder functionalization, which involves decorating the surface of powder particles with thin coatings or discrete particles. This strategy can improve powder processability by promoting the formation of fine microstructures in the deposited material, enhancing their flowability, or improving laser absorptivity. Moreover, surface functionalization offers an effective means to tailor material behaviour, enabling control over properties such as strength, ductility, stiffness, thermal conductivity, and coefficient of thermal expansion. This research introduces innovative powder modification strategies, demonstrating how surface and compositional modifications of atomized powders can expand the processing window and enable the design of alloys with tailored properties. Two representative alloy systems (Cu-based and Al-based alloy) were investigated to illustrate the versatility of the proposed approach. Regarding commercially pure copper, its processability in powder bed fusion - laser beam (PBF-LB/M) encounters significant challenges due to its high near-infrared (IR) laser reflectivity and thermal conductivity. Manufacturing fully dense Cu parts with conventional IR PBF-LB/M systems is challenging, often resulting in lack of fusion defects. In this work, two novel functionalization approaches were explored to address the poor processability of Cu, including (i) the electrodeposition of a thin Ag coating and (ii) the electrostatic self-assembly of TiC nanoparticles. Coating copper powders with silver enabled the formation of a Cu-Ag eutectic network during solidification, which effectively healed pores and improved densification. The decoration of copper powders with TiC nanoparticles increased laser absorptivity up to 80% and improved processability, leading to significantly denser components. Regarding aluminium alloys, the focus was on developing advanced Al2618/TiB2 metal matrix composites (MMCs) with improved processability and high stiffness and strength. The Al2618 is a widely used high-strength aluminium alloy for high temperature applications, and the addition of TiB2 particles was proved to increase its mechanical properties and PBF-LB/M processability. TiB2 was observed to be an effective grain refiner, capable of reducing the alloy's susceptibility to hot cracking. Different routes for feedstock preparation were systematically compared, including pre-alloying by gas atomization, plasma coating, and mechanical mixing. Building on their respective advantages, a hybrid approach relying on in-situ and ex-situ particles was proposed, enabling high ceramic fractions while maintaining good processability. In addition, a novel surface decoration of Ti-rich Al powders with B nanoparticles was investigated, promoting the in-situ formation of reinforcing phases during PBF-LB/M process. The methodology is versatile and can be extended to other Ti-containing Al systems, as the amount of B introduced onto the powder surface can be precisely adjusted to achieve the desired balance between grain size, strength, and ductility. This flexibility makes the approach particularly promising for optimizing both processability and performance in AM Al components, while mitigating the challenges associated with gas atomization of B-rich feedstocks. Finally, powder blending was explored as a strategy to target precise properties for specific applications, and it was employed to tune the thermal behaviour of Al-Si alloys. In the space sectors, Al mirrors are typically coated with a Ni-P layer to enhance polishability and surface quality. However, differences in the coefficient of thermal expansion (CTE) between the aluminium substrate and the nickel coating can lead to optical distortions and coating delamination during service. The last part of this thesis therefore examines the thermal and mechanical properties of Al-Si alloys with different silicon contents processed via PBF/LB-M. By blending AlSi10 and AlSi45 powders, intermediate hypereutectic compositions were produced and manufactured using PBF/LB-M. The microstructure, compressive strength and CTE of the printed alloys was systematically assessed. Among the compositions studied, AlSi40 emerged as a promising balance, offering a low CTE alongside excellent printability, positioning it as a strong candidate for nickel-coated space mirror applications.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/249137