Additive Manufacturing (AM) has exponentially grown during recent years in several industrial sectors. Aluminium alloys are widely used in AM with techniques such as Laser Powder Bed Fusion (LPBF) and Direct Energy Deposition (DED). One of the main obstacles in the production of aluminium alloys by AM is the aging of powder. In particular, oxidised powder can induce defects during the printing process and negatively affect mechanical properties of the final component. The oxygen pick up can occur due to the storing of the powders, during their use or due to powders recycling. In this work, AlSi10Mg powders were heat-treated at different times and temperatures to simulate the effect of aging of powder during recycling and storage. The effects of oxidised powder on microstructure, geometrical features and mechanical properties of the material were studied. Both for DED and LPBF processes, it was found that a higher amount of oxygen in the powder feedstock leads to a higher oxygen content in materials produced by AM. Moreover, materials printed with aged powders are more likely to form defects, such as lack-of-fusion flaws and spherical pores. Tensile test specimens were printed with LPBF and DED by using powders with different aging conditions. The results of tensile tests show that the quantity of oxygen has a strong influence on tensile properties: an increasing amount of oxygen in the powder leads to a higher number of pores and, as a consequence, to lower mechanical properties . Another aspect that was investigated in this work is the effect of building orientation on mechanical properties of the material. To this purpose, AlSi10Mg fracture toughness specimens were printed by DED and LPBF with cracks in three orthogonal orientations and heat-treated according to different schedules. The building strategy has a significant effect on mechanical properties and a considerable variation in toughness values was measured, indicating the existence of anisotropy within the specimens produced by AM techniques. In particular, when the crack can preferentially propagate along melt pool boundaries, an easy path for crack growth occurs and fracture toughness of the material tends to decrease.
Negli ultimi anni, la Manifattura Additiva (AM), grazie ai suoi innumerevoli vantaggi, si è diffusa facendosi strada in numero sempre più grande di industrie. Le leghe di alluminio sono ampiamente utilizzate nella stampa 3D di materiali metallici. In particolare, le tecniche maggiormente impiegate per la stampa sono la Fusione laser selettiva (LPBF) e la Deposizione ad energia diretta (DED). L’ossidazione è uno dei problemi principale nella Manifattura Additiva di leghe di alluminio, in particolare una polvere ossidata può causare difetti e influenzare le proprietà meccaniche del materiale. L’ossigeno nelle polveri può essere accumulato durante lo stoccaggio, la stampa o un processo di riciclo delle polveri. In questo progetto, polveri AlSi10Mg sono state trattate termicamente con diversi tempi e temperature per studiare gli effetti dell’invecchiamento delle polveri durante processi di riciclo e di stoccaggio. Sono stati valutati gli effetti dell’ossigeno sulla microstruttura, sulle caratteristiche geometriche e sulle proprietà meccaniche del materiale stampato. Alla luce di queste analisi, è possibile correlare la maggior quantità di ossigeno nelle polveri al maggior contenuto di ossigeno nel materiale stampato. Inoltre, i materiali stampati con polveri invecchiate sono risultati più inclini alla formazione di difetti. Provini per il test di trazione sono stati stampati tramite LPBF e DED con diverse condizioni di invecchiamento delle polveri. I risultati dei test di trazione indicano che la quantità di ossigeno ha una grande influenza sulle proprietà del materiale: all’aumentare del contenuto di ossigeno nelle polveri aumenta il numero e la dimensione dei pori nel materiale e, di conseguenza, le proprietà meccaniche peggiorano. Un altro aspetto studiato in questo lavoro è l’effetto delle diverse orientazioni di stampa sulle proprietà meccaniche del materiale. A questo scopo, campioni per il test di tenacità a frattura sono stati stampati con diverse orientazioni e trattati termicamente. Dai risultati si evince che la direzione di stampa ha un effetto significativo sulle proprietà meccaniche; pertanto, è evidente l’esistenza di anisotropia nella tenacità a frattura del materiale. In particolare, la tenacità a frattura diminuisce quando esiste un percorso preferenziale per la cricca, ovvero quando la cricca propaga attraverso i contorni delle pozze di fusione.
Effects of powder oxidation and different build orientations on microstructure and mechanical properties of additively manufactured AlSi10Mg alloy
Lupi, Giorgia
2020/2021
Abstract
Additive Manufacturing (AM) has exponentially grown during recent years in several industrial sectors. Aluminium alloys are widely used in AM with techniques such as Laser Powder Bed Fusion (LPBF) and Direct Energy Deposition (DED). One of the main obstacles in the production of aluminium alloys by AM is the aging of powder. In particular, oxidised powder can induce defects during the printing process and negatively affect mechanical properties of the final component. The oxygen pick up can occur due to the storing of the powders, during their use or due to powders recycling. In this work, AlSi10Mg powders were heat-treated at different times and temperatures to simulate the effect of aging of powder during recycling and storage. The effects of oxidised powder on microstructure, geometrical features and mechanical properties of the material were studied. Both for DED and LPBF processes, it was found that a higher amount of oxygen in the powder feedstock leads to a higher oxygen content in materials produced by AM. Moreover, materials printed with aged powders are more likely to form defects, such as lack-of-fusion flaws and spherical pores. Tensile test specimens were printed with LPBF and DED by using powders with different aging conditions. The results of tensile tests show that the quantity of oxygen has a strong influence on tensile properties: an increasing amount of oxygen in the powder leads to a higher number of pores and, as a consequence, to lower mechanical properties . Another aspect that was investigated in this work is the effect of building orientation on mechanical properties of the material. To this purpose, AlSi10Mg fracture toughness specimens were printed by DED and LPBF with cracks in three orthogonal orientations and heat-treated according to different schedules. The building strategy has a significant effect on mechanical properties and a considerable variation in toughness values was measured, indicating the existence of anisotropy within the specimens produced by AM techniques. In particular, when the crack can preferentially propagate along melt pool boundaries, an easy path for crack growth occurs and fracture toughness of the material tends to decrease.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/183572