• additive manufacturing
• fatigue
• HCF
• high-temperature
• L-PBF
• notched specimens
• process parameters
• SED
• SLM
• surface roughness

La presente ricerca tratta delle proprietà meccaniche statiche e cicliche di componenti metallici prodotti mediante la tecnologia di Additive Manufacturing denominata Selective Laser Melting (SLM), concentrandosi sullo studio della lega Inconel 718.
La ricerca è partita dallo studio del processo SLM, studiando il rapporto tra le difettosità tipicamente causate dal processo ed i principali parametri di scansione. È stata quindi sperimentalmente misurata la geometria della pozza fusa prodotta da diverse combinazioni di parametri di processo, nonché l’assorbimento del laser.
È stato indagato l'effetto di variazioni della rugosità superficiale di provini as-built, con materiale in condizione as-built o invecchiato, sulla resistenza ad HCF. Allo scopo sono stati sviluppati due set di parametri di processo caratterizzati da un elevato rapporto tra lunghezza e larghezza della pozza fusa e quindi da una elevata produttività del processo SLM, e sono state studiate le proprietà sia statiche, meccaniche e microstrutturali, che cicliche di provini cilindrici con essi prodotti. I dati sperimentali sono stati razionalizzati mediante l’approccio √(area_R) sviluppato dal Prof. Murakami, adattato alle difettosità intrinseche del materiale prodotto mediante SLM.
La tesi affronta quindi il comportamento a fatica, nel campo dell’ HCF, di provini recanti intagli macrogeometrici introdotti allo scopo produrre concentrazione di tensione e multiassialità locale. A tale scopo, provini cilindrici, lisci o recanti un intaglio a V con raggio compreso tra 0.3 e 2 mm, sono stati testati in condizione as-built con carico monoassiale e R 0.05. Il comportamento a fatica è stato quindi modellata mediante l'approccio Average Strain Energy Density (ASED), riconducendo le diverse curve S-N ad un’unica curva master. Analisi frattografiche e microstrutturali hanno consentito di studiare le modalità di nucleazione e propagazione della cricca per le diverse geometrie di provino.
La tesi estende infine l’analisi del comportamento a fatica dell’Inconel 718 alla tipica temperatura di lavoro di tale materiale, ovvero di 650°C. Il comportamento fatica isoterma di provini liscio recanti intagli analoghi a quanto applicato a temperatura ambiente, nonché le proprietà meccaniche e microstrutturali, del materiale prodotto mediante SLM e successivamente trattato termicamente come da standard di letteratura sono stati studiati nel dettaglio e confrontati quanto ottenuto da un’analoga campagna sperimentale condotta sul materiale prodotto mediante processi convenzionali. L’approccio ASED ha consentito di anche in questo caso razionalizzare i dati sperimentali riconducendoli ad un’unica curva master. Analisi frattografiche sono state effettuate per studiare la regione di nucleazione della cricca e confrontare il comportamento del materiale convenzionale con quanto prodotto mediante SLM.


English version

The Ph.D. research was aimed at understanding the effects of the Laser Powder Bed Fusion (L-PBF), also named Selective Laser Melting (SLM), process on the material's microstructural and mechanical properties, investigating and modeling the static and fatigue behavior of the material. Particular attention was paid to the local multiaxial stress states introduced by macroscopic geometrical notches, similar to the ones necessarily present in a complex-shaped industrial component that could be manufactured by the SLM process.
An analytical model leading to a first-order approximation of the SLM meltpool dimensions was developed with the contribution of my colleagues Mattia Moda and Andrea Chiocca. The model was validated both by a wide-range literature review covering all the materials employed in the SLM process and by an experimental campaign aimed to predict the occurrence of the lack of fusion and humping phenomena for the Inconel 718 alloy. The devised model allowed me also to carry out an indirect measurement of the effective laser absorptivity for the Inconel 718 alloy.
The analytical SLM feasibility region formulation was thus employed to devise novel process parameters sets aimed to increase the process productivity while introducing a significant increase in the as-built surface roughness. The material produced with the developed process parameters was investigated in terms of microstructural, static mechanical, and High Cycle Fatigue (HCF) properties and compared with the baseline SLMed material. The fatigue strength and the surface roughness were successfully correlated through the √(area_R) approach developed by Murakami. The effective surface profile was acquired and introduced in a specific Finite Element (FE) model to explore the stress concentration effect caused by the small notches introduced by the as-built surface roughness.
The HCF behavior of smooth and notched SLMed specimens featuring a local multiaxial stress state introduced by a macroscopic geometrical notch was investigated along with material microstructural and static mechanical properties. Round cylindrical and V-notched specimens, whose geometry was defined through a FE model to present a theoretical stress concentration factor comprised between 1.6 and 3.3. HCF tests in the specimen longitudinal direction were carried out at room temperature with a load ratio of 0.05. The Average Strain Energy Density (ASED) approach was demonstrated to be suitable for modeling the fatigue behavior of V-notched SLMed specimens in Inconel 718 in the as-built condition. The actual geometry in proximity to the notch root was analyzed as well. The specimen profile was measured and employed to set up a specimen-specific FE model reproducing the fatigue test load case. The ASED value, defined employing the critical radius defined for the nominal profile, was found to effectively describe the fatigue behavior also when the actual specimen profile was considered. Fractographic analyses were performed to understand the effects of the notch geometries on fracture nucleation and propagation. The residual stresses produced by the SLM process on the smooth and V-notched specimens were evaluated by means of a FE model set up in Ansys Mechanical, previously experimentally validated utilizing the Blind Hole Drilling (BHD) technique.
Finally, it is presented the investigation of the static and HCF behavior of the SLMed and aged material in as-built surface conditions at the typical operating temperature of the Inconel 718 alloy, 650°C. Microstructural investigations aimed to understand the effects of the aging treatment on the SLMed material were carried out as well. HCF tests were performed on smooth and V-notched round specimens in an axial direction and with a load ratio of 0.1. The same specimen geometries and test conditions were employed for the characterization of the static and HCF behavior of the conventional Inconel 718, which were compared with the SLMed material both in terms of S-N curves and fracture surfaces.