The Laser Powder Bed Fusion (LPBF) process provides significant opportunities to design novel geometries, complex internal structures, lightweight components, and customized parts. Currently, the most common metallic materials manufactured by the LPBF are iron-based, titanium-based, aluminium-based, nickel-based, cobalt-based, copper-based alloys, and some pure metals such as titanium, gold, silver, and some others. However, the currently in use cutting-edge alloys do not exploit the enormous opportunities inherent in the technique at all. The development of alloys specifically designed for the LPBF process is fundamental in order to exploit the full potential of AM technology. This thesis aims to develop and characterize materials that are not yet commercially available with license tools or well defined and characterized by state of the art for the LPBF process. The selection of the materials is related to the National Institute for Nuclear Physics experiments and projects. This study is focused on additively manufactured refractory metals, especially molybdenum. As a refractory material, molybdenum is regarded with high interest for high-temperature applications. The concurrent use of powder bed Additive Manufacturing (AM) technology can provide significant design and production advantages. Initially, the pure molybdenum was additively manufactured using an AISI 304 building-plate. Adhesion issues were found at the interface between the two materials from the early stage of the production process. Interface analysis was performed to investigate the possible origins of the failure. The investigation showed that large cracks could propagate and lead to the separation of the printed part from the substrate when the dilution of Mo on the AISI 304 platform was approximately in the range of 40÷50. The brittle intermetallic sigma phase was extracted and analysed by XRD on Mo-AISI 304 interface specimens. It is assumed that the sigma phase impaired the interface strength, providing a preferential route for brittle crack propagation. Once the adhesion issue was solved using a pure copper substrate, the process parameters tuning led to produce almost fully-dense AM Mo blocks (density of 99.5±0.5 %). Fine-tuning of the parameters also involved the Single Scan Tracks analysis, which is aimed at continuous and homogeneous melt-pools production. High-density Mo specimens were characterized at room- and high-temperature in terms of thermal and mechanical properties, then compared with conventionally manufactured Mo samples. The thermal diffusivity measurement at room temperature confirmed that AM Mo has a thermal conductivity value that is roughly half that of standard Mo. Stress relieving heat treatment improves the thermal conductivity by approximately 13%. The estimation of emissivity and thermal conductivity carried out in the 600÷1600 °C temperature range led to a similar result. The Vickers microhardness measured on fully dense specimens (212 ± 18 HV0.15) is similar to that of commercially available Mo. Tensile tests were performed at both room temperature and 600°C. The effect of building direction and post-processing machining of AM specimens was also investigated for tests at room temperature. Although the AM samples exhibited a very similar density to standard Mo, the AM Mo mechanical properties resulted generally lower. Finally, the process parameters tuning was performed for secondary parameters, related to geometrical performances, such as the evaluation of geometrical integrity as the thickness changes, the production of complex geometry, and the overhang angle study. As a result of this characterization step, the first AM molybdenum component was successfully produced. This component is the anode of the FEBIAD (Forced Electron Beam-Induced Arc Discharge) Ion Source of the Selective Production of Exotic Species (SPES) project. The physical performance of the AM anode was evaluated thanks to the proof-of-concept test carried out at the ISOLDE project’s off-line system at CERN. The comparison between the ionization efficiency estimated with the totally conventional FEBIAD ion source and the one evaluated with the traditional ion source with the AM Mo anode confirms that the LPBF technology is compatible with the production of devices of such kind, thus opening up the possibility of fully exploiting its technological advantages, for instance, exploring new design solutions for the entire ion source assembly.
Il processo Laser Powder Bed Fusion (LPBF) offre opportunità significative per progettare nuove geometrie, strutture interne complesse, componenti leggeri e componenti personalizzati. Attualmente, i materiali metallici comunemente prodotti via LPBF sono leghe a base di ferro, titanio, alluminio, nichel, cobalto, rame e alcuni metalli puri come titanio, oro, argento ecc. Tuttavia, le leghe all'avanguardia attualmente in uso non sfruttano le enormi opportunità insite nella tecnologia. Lo sviluppo di leghe appositamente progettate per il processo LPBF è fondamentale per sfruttare tutte le potenzialità della tecnologia AM. Questa tesi si propone di sviluppare e caratterizzare materiali non ancora disponibili in commercio con strumenti di licenza o ben definiti e caratterizzati dallo stato dell'arte per il processo LPBF. La selezione dei materiali è correlata agli esperimenti e ai progetti dell'Istituto Nazionale di Fisica Nucleare. Questo studio si concentra sui metalli refrattari prodotti in modo additivo, in particolare il molibdeno. Come materiale refrattario, il molibdeno è considerato con grande interesse per le applicazioni ad alta temperatura. L'uso simultaneo della tecnologia di produzione additiva (AM) a letto di polvere può fornire vantaggi significativi in termini di progettazione e produzione. Inizialmente, il molibdeno puro è stato prodotto additivamente utilizzando una piattaforma di stampa in AISI 304. Sono stati riscontrati problemi di adesione all'interfaccia tra i due materiali sin dalle prime fasi del processo produttivo. L'analisi dell'interfaccia è stata eseguita per indagare sulle possibili origini della problematica. Tale studio ha mostrato che cricche di grandi dimensioni potrebbero propagarsi e portare alla separazione della parte stampata dal substrato quando la diluizione del Mo sulla piattaforma in AISI 304 risulta compresa nel range 40÷50 %. La fase intermetallica sigma è stata estratta e analizzata all'interfaccia Mo-AISI 304 mediante XRD. Si presume che la fase infragilente sigma abbia compromesso la resistenza dell'interfaccia, fornendo una via preferenziale per la propagazione delle cricche. Una volta risolto il problema di adesione utilizzando un substrato di rame puro, l'ottimizzazione dei parametri di processo ha portato a produrre blocchi di Mo AM quasi completamente densi (densità = 99,5±0,5%). La messa a punto dei parametri è stata svolta anche con l'analisi delle singole linee di scansione (Single Scan Tracks – SSTs), finalizzata alla produzione continua ed omogenea delle pozze di fusione. I campioni di Mo con i più elevati valori di densità sono stati caratterizzati a temperatura ambiente e ad alta temperatura, sia in termini di proprietà termiche che meccaniche, e sono stati confrontati con campioni di Mo fabbricati in modo convenzionale. La misura della diffusività termica a temperatura ambiente ha confermato che il Mo prodotto additivamente ha un valore di conducibilità termica che è circa la metà rispetto al Mo standard. Il trattamento termico di distensione migliora la conducibilità termica di circa il 13%. La stima dell'emissività e della conducibilità termica effettuata nell'intervallo di temperatura 600÷1600 °C ha portato a un risultato simile. La microdurezza Vickers misurata su provini completamente densi (212 ± 18 HV0,15) è simile a quella del Mo disponibile in commercio. Le prove di trazione sono state eseguite sia a temperatura ambiente che a 600°C. Per i test a temperatura ambiente è stato anche studiato l'effetto della direzione di costruzione e della lavorazione di post-processo (tornitura) di provini AM. Sebbene i campioni AM abbiano mostrato una densità molto simile al Mo standard, le proprietà meccaniche del Mo AM sono risultate generalmente inferiori. Infine, è stata eseguita la messa a punto dei parametri di processo relativi alle prestazioni geometriche, come la valutazione dell'integrità geometrica al variare dello spessore, la produzione di geometrie complesse e lo studio dell'angolo di sbalzo. Come risultato di questa fase di caratterizzazione, è stato prodotto con successo il primo componente di molibdeno prodotto con tecnologia AM. Questo componente è l'anodo della sorgente di ionizzazione tipo FEBIAD (Forced Electron Beam-Induced Arc Discharge) del progetto SPES (Selective Production of Exotic Species). Le prestazioni fisiche dell'anodo AM sono state valutate grazie al proof-of-concept effettuato presso il sistema off-line della facility ISOLDE presso il CERN. Il confronto tra l'efficienza di ionizzazione stimata con la sorgente di ionizzazione tipo FEBIAD prodotta con tecniche totalmente tradizionali e quella stimata utilizzando la sorgente tradizionale in cui l’anodo di Mo è stato prodotto additivamente conferma che la tecnologia LPBF è compatibile con la produzione di dispositivi di questo tipo, aprendo così la possibilità di sfruttare maggiormente i vantaggi tecnologici dell’AM, ad esempio esplorando nuove soluzioni di design per l'intera sorgente di ionizzazione.
Characterization of Molybdenum produced by Laser Powder Bed Fusion for the high-temperature Ion sources of the INFN SPES facility
Rebesan, Pietro
2021/2022
Abstract
The Laser Powder Bed Fusion (LPBF) process provides significant opportunities to design novel geometries, complex internal structures, lightweight components, and customized parts. Currently, the most common metallic materials manufactured by the LPBF are iron-based, titanium-based, aluminium-based, nickel-based, cobalt-based, copper-based alloys, and some pure metals such as titanium, gold, silver, and some others. However, the currently in use cutting-edge alloys do not exploit the enormous opportunities inherent in the technique at all. The development of alloys specifically designed for the LPBF process is fundamental in order to exploit the full potential of AM technology. This thesis aims to develop and characterize materials that are not yet commercially available with license tools or well defined and characterized by state of the art for the LPBF process. The selection of the materials is related to the National Institute for Nuclear Physics experiments and projects. This study is focused on additively manufactured refractory metals, especially molybdenum. As a refractory material, molybdenum is regarded with high interest for high-temperature applications. The concurrent use of powder bed Additive Manufacturing (AM) technology can provide significant design and production advantages. Initially, the pure molybdenum was additively manufactured using an AISI 304 building-plate. Adhesion issues were found at the interface between the two materials from the early stage of the production process. Interface analysis was performed to investigate the possible origins of the failure. The investigation showed that large cracks could propagate and lead to the separation of the printed part from the substrate when the dilution of Mo on the AISI 304 platform was approximately in the range of 40÷50. The brittle intermetallic sigma phase was extracted and analysed by XRD on Mo-AISI 304 interface specimens. It is assumed that the sigma phase impaired the interface strength, providing a preferential route for brittle crack propagation. Once the adhesion issue was solved using a pure copper substrate, the process parameters tuning led to produce almost fully-dense AM Mo blocks (density of 99.5±0.5 %). Fine-tuning of the parameters also involved the Single Scan Tracks analysis, which is aimed at continuous and homogeneous melt-pools production. High-density Mo specimens were characterized at room- and high-temperature in terms of thermal and mechanical properties, then compared with conventionally manufactured Mo samples. The thermal diffusivity measurement at room temperature confirmed that AM Mo has a thermal conductivity value that is roughly half that of standard Mo. Stress relieving heat treatment improves the thermal conductivity by approximately 13%. The estimation of emissivity and thermal conductivity carried out in the 600÷1600 °C temperature range led to a similar result. The Vickers microhardness measured on fully dense specimens (212 ± 18 HV0.15) is similar to that of commercially available Mo. Tensile tests were performed at both room temperature and 600°C. The effect of building direction and post-processing machining of AM specimens was also investigated for tests at room temperature. Although the AM samples exhibited a very similar density to standard Mo, the AM Mo mechanical properties resulted generally lower. Finally, the process parameters tuning was performed for secondary parameters, related to geometrical performances, such as the evaluation of geometrical integrity as the thickness changes, the production of complex geometry, and the overhang angle study. As a result of this characterization step, the first AM molybdenum component was successfully produced. This component is the anode of the FEBIAD (Forced Electron Beam-Induced Arc Discharge) Ion Source of the Selective Production of Exotic Species (SPES) project. The physical performance of the AM anode was evaluated thanks to the proof-of-concept test carried out at the ISOLDE project’s off-line system at CERN. The comparison between the ionization efficiency estimated with the totally conventional FEBIAD ion source and the one evaluated with the traditional ion source with the AM Mo anode confirms that the LPBF technology is compatible with the production of devices of such kind, thus opening up the possibility of fully exploiting its technological advantages, for instance, exploring new design solutions for the entire ion source assembly.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/189834