Cross-contamination quantification in powders for additive manufacturing: A study on Ti-6Al-4V and maraging steel

Metal additive manufacturing is now taking the lead over traditional manufacturing techniques in applications such as aerospace and biomedicine, which are characterized by low production volumes and high levels of customization. While fulfilling these requirements is the strength of metal additive manufacturing, respecting the tight tolerances typical of the mentioned applications is a harder task to accomplish. Powder bed fusion (PBF) is a class of additive manufacturing in which layers of metal powder are fused on top of each other by a high-energy beam (laser or electron beam) according to a computer-aided design (CAD) model. The quality of raw powders for PBF affects the mechanical properties of additively manufactured parts strongly, and therefore it is crucial to avoid the presence of any source of contamination, particularly cross-contamination. In this study, the identification and quantification of cross-contamination in powders of Ti-6Al-4V and maraging steel was performed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) techniques. Experimental results showed an overall good reliability of the developed method, opening the way for applications in machine learning environments

Studio di materiali nanostrutturati mediante tecniche di microscopia elettronica e diffrazione di raggi X

Il lavoro svolto durante il Dottorato di ricerca riguarda l’applicazione di tecniche di microscopia elettronica e diffrazione di raggi X a materiali nanostrutturati. La ricerca è stata articolata su tre macroaree tematiche: materiali per l’immagazzinamento dell’idrogeno allo stato solido (film sottili e compositi), materiali per uso biomedico (biomateriali e detector) e leghe metalliche leggere. Per la prima area, film sottili di Mg puro e Mg drogato con Nb e campioni compositi di particelle di Pd e silicone, sono stati sottoposti a cicli di assorbimento/desorbimento di H2. La caratterizzazione ha mostrato che il Nb nei film sottili forma dei clusters, creando percorsi percolativi che velocizzano la reazione con H2. I compositi hanno mostrato, invece, un comportamento peculiare rispetto all’idrogeno e una propria identità scientifica, aprendo nuove prospettive applicative. Nel campo dei biomateriali è stata caratterizzata la lega Co-Cr-Mo prodotta tramite Direct Metal Laser Sintering (DMLS), al fine di ottimizzare i parametri produttivi e far luce sui fenomeni che hanno luogo nella microstruttura durante la produzione. I risultati hanno indicato che il laser induce una trasformazione martensitica atermica dalla fase da γ (fcc) alla fase ε (hcp) nella polvere metallica, producendo un intricato network di lamelle ε nella fase γ. E’ la prima volta in assoluto che viene osservato un fenomeno di questo genere in simili condizioni. Nel campo dei detector per applicazioni mediche è stata caratterizzata una serie di cristalli scintillatori LYSO (detector per la PET). Questo lavoro ha permesso di chiarire la microstruttura e legare la produzione di luce alle caratteristiche strutturali di tali cristalli. Nel campo delle leghe metalliche leggere per impieghi aereonautici, è stata affrontata la caratterizzazione strutturale della lega AZ31B saldata per Friction Stir Welding (FSW) e della lega di allumino 2219, sottoposta ad Equal Channel Angular Pressing (ECAP).This Ph.D. Thesis is about the application of electron microscopy and X-ray diffraction to several classes of materials. Three different macro areas have been addressed to: materials for solid state hydrogen storage (thin films and composites), materials for biomedical use (biomaterials and detectors) and light metal alloys. In the first thematic area, thin films of pure Mg and Mg doped with 5at.% Nb and composite samples of particles of Pd and silicon, have all been subjected to cycles of absorption/desorption of hydrogen. The characterization showed that Nb in thin films forms clusters, creating percolative paths that speed up the reaction with H2. The composites showed a behavior with hydrogen which makes them materials having their own scientific identity, opening new application perspectives . In the field of biomaterials a Co-Cr-Mo alloy produced via Direct Metal Laser Sintering (DMLS) has been characterized, in order to optimize the production parameters and clarify the phenomena occurring in the microstructure during production. The results showed that the laser induces, in the powder, a martensitic athermal transformation from the γ (fcc) phase to the ε phase (hcp), producing an intricate network of ε lamellae in the γ phase. This is the first time ever that this phenomenon was observed in similar conditions. In the field of detectors for medical applications a series of LYSO crystal scintillators (detectors for PET) has been characterized. This work allowed to clarify the microstructure and to correlate the light yield properties to the microstructure of the crystals. In the field of light metal alloys for aeronautical applications, the research was focused on the structural characterization of the AZ31B alloy welded with Friction Stir Welding (FSW) and on the aluminum alloy 2219 subjected to Equal Channel Angular Pressing (ECAP)

Studio di materiali nanostrutturati mediante tecniche di microscopia elettronica e diffrazione di raggi X

Il lavoro svolto durante il Dottorato di ricerca riguarda l’applicazione di tecniche di microscopia elettronica e diffrazione di raggi X a materiali nanostrutturati. La ricerca è stata articolata su tre macroaree tematiche: materiali per l’immagazzinamento dell’idrogeno allo stato solido (film sottili e compositi), materiali per uso biomedico (biomateriali e detector) e leghe metalliche leggere. Per la prima area, film sottili di Mg puro e Mg drogato con Nb e campioni compositi di particelle di Pd e silicone, sono stati sottoposti a cicli di assorbimento/desorbimento di H2. La caratterizzazione ha mostrato che il Nb nei film sottili forma dei clusters, creando percorsi percolativi che velocizzano la reazione con H2. I compositi hanno mostrato, invece, un comportamento peculiare rispetto all’idrogeno e una propria identità scientifica, aprendo nuove prospettive applicative. Nel campo dei biomateriali è stata caratterizzata la lega Co-Cr-Mo prodotta tramite Direct Metal Laser Sintering (DMLS), al fine di ottimizzare i parametri produttivi e far luce sui fenomeni che hanno luogo nella microstruttura durante la produzione. I risultati hanno indicato che il laser induce una trasformazione martensitica atermica dalla fase da γ (fcc) alla fase ε (hcp) nella polvere metallica, producendo un intricato network di lamelle ε nella fase γ. E’ la prima volta in assoluto che viene osservato un fenomeno di questo genere in simili condizioni. Nel campo dei detector per applicazioni mediche è stata caratterizzata una serie di cristalli scintillatori LYSO (detector per la PET). Questo lavoro ha permesso di chiarire la microstruttura e legare la produzione di luce alle caratteristiche strutturali di tali cristalli. Nel campo delle leghe metalliche leggere per impieghi aereonautici, è stata affrontata la caratterizzazione strutturale della lega AZ31B saldata per Friction Stir Welding (FSW) e della lega di allumino 2219, sottoposta ad Equal Channel Angular Pressing (ECAP)

Material reuse in laser powder bed fusion: Side effects of the laser—metal powder interaction

Metal additive manufacturing is changing the way in which engineers and designers model the production of three-dimensional (3D) objects, with rapid growth seen in recent years. Laser powder bed fusion (LPBF) is the most used metal additive manufacturing technique, and it is based on the ecient interaction between a high-energy laser and a metal powder feedstock. To make LPBF more cost-ecient and environmentally friendly, it is of paramount importance to recycle (reuse) the unfused powder from a build job. However, since the laser–powder interaction involves complex physics phenomena and generates by-products which might aect the integrity of the feedstock and the final build part, a better understanding of the overall process should be attained. The present review paper is focused on the clarification of the interaction between laser and metal powder, with a strong focus on its side eects

On the Short-Term Creep Response at 482 °C (900 °F) of the 17-4PH Steel Produced by Bound Metal Deposition

The creep response of the 17-4PH precipitation hardening steel produced by a new additive manufacturing technology (Bound Metal Deposition) was investigated at 482 °C (900 °F), under stresses ranging from 350 to 600 MPa. Two different sets of samples produced with different deposition parameters were considered. Prior heat treatment consisted of ageing either at 482 °C (state H900) or at 621 °C (H1150). The minimum creep rate and time to rupture dependencies on applied stress were obtained. The creep response in terms of time to rupture under a given stress, in particular, was compared with the only other available literature dataset on a similar steel processed by traditional technologies. The analysis of the experiments demonstrated that the presence of dispersed defects causes, in the Bound Metal Deposited steel, a substantial reduction (35–40%) of the creep strength

Effect of Friction Stir Welding on Short-Term Creep Response of Pure Titanium

Friction Stir Welding (FSW) is a recent joining technique that has received considerable attention. FSW causes significant variations in the material microstructure commonly associated with changes in the mechanical properties. The present study deals with the creep response of pure titanium (CP-Ti grade 2) after FSW. Dog-bone creep samples, obtained by machining, which show the longitudinal axis of each sample being perpendicular to the welding direction, were tested in constant load machines at 550 and 600 °C. The creep response of the FSW samples was analyzed and compared with that of the unwelded material. The shape of the creep curves was conventional, although the FSW samples went to rupture for strains lower than the base metal. The minimum creep rates for FSW samples were, in general, lower than for the unwelded metal tested in equivalent conditions. In addition, when the applied stress was high, deformation concentrated in the parent metal. The creep strain became more and more homogeneous along the gauge length as testing stress decreased. A constitutive model, recently developed for describing the creep response of the base metal, was then used to rationalize the observed reduction in the minimum strain rate in FSW samples