The main purpose of this thesis is the thermo-electric characterization of components made of a refractory metal alloy, specifically Tantalum-Tungsten 10% (TaW10), produced by means of Additive Manufacturing (AM) technologies. This research is carried out within the framework of the HISOL_NEXT experiment which focuses on the development of high performance components for the SPES accelerator facility at the Legnaro National Laboratories (LNL-INFN). In particular, refractory metals are widely utilized for ion source parts due to their ability to withstand extreme operating conditions, such as ultra-high temperatures (>2000°C) in high vacuum. However, these materials are extremely difficult to machine and shape due to their high melting points, stiffness and hardness. To overcome these challenges, additive manufacturing techniques are employed, specifically Laser Powder Bed Fusion (LPBF). Previous studies have shown that applying the LPBF technique to pure refractory metals presents intrinsic process-related limitations: pure tungsten (W) suffers from severe cracking due to laser-induced thermal shocks, drastically degrading its excellent thermoelectric properties, whereas pure tantalum (Ta) exhibits excellent printability thanks to its ductility, but lacks sufficient thermomechanical properties to resist creep and deformation at ultra-high operating temperatures. The focus of this work is the study of the TaW10 alloy processed via LPBF, specifically designed to exploit the ductility of the tantalum matrix to absorb printing stresses, while simultaneously using tungsten as a structural strengthener. TaW10 samples were produced in collaboration with the DIAM laboratory of the INFN Padova section, where R&D activities on LPBF are performed. The manufactured samples underwent an extensive characterization campaign at INFN-LNL, which involved a preliminary step of morphological characterization, including density measurements and microstructural observations via SEM/EDX, and the subsequent estimation of the thermal and electric properties of major interest, in particular emissivity, thermal conductivity and electrical resistivity. The results show alignment with the theoretical values calculated with a weighted average of the properties of pure additively manufactured Ta and W, following the solid solution hypothesis. The characterized alloy was also employed to fabricate prototype ion-source components, specifically cathodes for the SPES-FEBIAD ion source, whose operating principle relies on thermionic electron emission from a high-temperature cathode towards an anode. To validate the reliability of the estimated thermal and electrical properties, a finite element method (FEM) model of a FEBIAD ion source equipped with an LPBF-manufactured TaW10 cathode was developed in ANSYS. The numerical results showed good agreement with the experimental data, confirming the validity of the estimated material properties. To further assess the suitability of TaW10 for the fabrication of operational ion-source components, the manufactured cathode was subjected to a custom experimental procedure aimed at measuring thermionic electron emission. The results demonstrated that the TaW10 cathode is capable of providing the electron emission required for FEBIAD source operation, thereby confirming the feasibility of employing this alloy for the fabrication of functional FEBIAD ion sources. Nevertheless, post-test inspections revealed the presence of several cracks and manufacturing defects within the material, highlighting potential reliability concerns that may affect long-term operation and warrant further investigation and process optimization.
L’obiettivo principale di questa tesi è la caratterizzazione termoelettrica di componenti in lega refrattaria Tantalio-Tungsteno 10% (TaW10), prodotti tramite Additive Manufacturing (AM). La ricerca si inserisce nell'ambito dell'esperimento HISOL_NEXT, finalizzato allo sviluppo di elementi ad alte prestazioni per la facility dell'acceleratore SPES presso i Laboratori Nazionali di Legnaro (LNL-INFN). I metalli refrattari sono impiegati nelle sorgenti ioniche per la capacità di operare in condizioni estreme, a temperature superiori a 2000 °C in alto vuoto; tuttavia, l'elevato punto di fusione, la durezza e la rigidezza ne rendono critica la lavorazione con machining tradizionale. Per superare questi vincoli è stata adottata la tecnologia Laser Powder Bed Fusion (LPBF). L'uso della LPBF su metalli refrattari puri presenta limiti intrinseci: il tungsteno puro (W) è soggetto a esteso cracking indotto da shock termico laser, degradandone le proprietà termoelettriche; il tantalio puro (Ta) garantisce un'eccellente printability grazie alla duttilità, ma mostra ridotta resistenza a creep e deformazione ad altissima temperatura. La lega TaW10 è stata quindi studiata per combinare la duttilità della matrice di tantalio nell'assorbire gli stress termomeccanici di processo con l'effetto rinforzante del tungsteno. I campioni in TaW10, realizzati in collaborazione con il laboratorio DIAM dell'INFN di Padova, sono stati caratterizzati presso i LNL-INFN attraverso rilievi morfologici (misure di densità e analisi microstrutturali SEM/EDX) e la stima di emissività, conducibilità termica e resistività elettrica. I risultati sperimentali concordano con i valori teorici calcolati come media ponderata delle proprietà di Ta e W puri via AM, validando la solid solution hypothesis. La lega è stata poi impiegata per fabbricare catodi prototipali per la sorgente ionica SPES-FEBIAD, il cui funzionamento si basa sull'emissione termoionica verso l'anodo. Per validare l'affidabilità delle proprietà stimate, è stato implementato in ANSYS un modello a elementi finiti (FEM) della sorgente equipaggiata con il catodo in TaW10, ottenendo un'ottima concordanza con i dati sperimentali. Infine, il catodo è stato sottoposto a test sperimentali custom per misurare l'emissione termoionica, confermando la capacità del componente di erogare la corrente elettronica richiesta dal funzionamento della sorgente FEBIAD e validando la fattibilità tecnologica della soluzione. Nondimeno, le indagini post-test hanno evidenziato cricche e difetti di fabbricazione residui, evidenziando criticità di affidabilità sul lungo termine che richiedono un'ulteriore ottimizzazione del processo LPBF.
Electrical and Thermal Characterization of Tantalum–Tungsten Alloy Components Produced by Laser Powder Bed Fusion
CARUSO, FRANCESCO
2025/2026
Abstract
The main purpose of this thesis is the thermo-electric characterization of components made of a refractory metal alloy, specifically Tantalum-Tungsten 10% (TaW10), produced by means of Additive Manufacturing (AM) technologies. This research is carried out within the framework of the HISOL_NEXT experiment which focuses on the development of high performance components for the SPES accelerator facility at the Legnaro National Laboratories (LNL-INFN). In particular, refractory metals are widely utilized for ion source parts due to their ability to withstand extreme operating conditions, such as ultra-high temperatures (>2000°C) in high vacuum. However, these materials are extremely difficult to machine and shape due to their high melting points, stiffness and hardness. To overcome these challenges, additive manufacturing techniques are employed, specifically Laser Powder Bed Fusion (LPBF). Previous studies have shown that applying the LPBF technique to pure refractory metals presents intrinsic process-related limitations: pure tungsten (W) suffers from severe cracking due to laser-induced thermal shocks, drastically degrading its excellent thermoelectric properties, whereas pure tantalum (Ta) exhibits excellent printability thanks to its ductility, but lacks sufficient thermomechanical properties to resist creep and deformation at ultra-high operating temperatures. The focus of this work is the study of the TaW10 alloy processed via LPBF, specifically designed to exploit the ductility of the tantalum matrix to absorb printing stresses, while simultaneously using tungsten as a structural strengthener. TaW10 samples were produced in collaboration with the DIAM laboratory of the INFN Padova section, where R&D activities on LPBF are performed. The manufactured samples underwent an extensive characterization campaign at INFN-LNL, which involved a preliminary step of morphological characterization, including density measurements and microstructural observations via SEM/EDX, and the subsequent estimation of the thermal and electric properties of major interest, in particular emissivity, thermal conductivity and electrical resistivity. The results show alignment with the theoretical values calculated with a weighted average of the properties of pure additively manufactured Ta and W, following the solid solution hypothesis. The characterized alloy was also employed to fabricate prototype ion-source components, specifically cathodes for the SPES-FEBIAD ion source, whose operating principle relies on thermionic electron emission from a high-temperature cathode towards an anode. To validate the reliability of the estimated thermal and electrical properties, a finite element method (FEM) model of a FEBIAD ion source equipped with an LPBF-manufactured TaW10 cathode was developed in ANSYS. The numerical results showed good agreement with the experimental data, confirming the validity of the estimated material properties. To further assess the suitability of TaW10 for the fabrication of operational ion-source components, the manufactured cathode was subjected to a custom experimental procedure aimed at measuring thermionic electron emission. The results demonstrated that the TaW10 cathode is capable of providing the electron emission required for FEBIAD source operation, thereby confirming the feasibility of employing this alloy for the fabrication of functional FEBIAD ion sources. Nevertheless, post-test inspections revealed the presence of several cracks and manufacturing defects within the material, highlighting potential reliability concerns that may affect long-term operation and warrant further investigation and process optimization.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/118120