Nowadays, in a market where the demand of highly customisable and complex-shaped components is constantly growing, additive manufacturing (AM) is playing an increasingly relevant role due to its greater flexibility with respect to the traditional subtractive processes. In fact, no specific tools are present, rather the part is obtained by progressively adding thin layers of material. In these terms, laser powder bed fusion (L-PBF) is one of the most common metal AM methods. Despite offering several advantages, the technique introduces features such as high surface roughness, internal and superficial defects, anisotropy and residual stresses that result to be detrimental for the component’s fatigue life. Although the optimisation of process parameters allows to mitigate the harmful effects, post-processes are usually needed to ensure specific mechanical properties and geometrical design requirements. The intricate geometries generally characterising AMed components and the complex thermal history experienced by the material during the production, lead to an intrinsic difficulty of predicting the magnitude and distribution of residual stresses. Commonly, L-PBF induces tensile residual stresses in the external material layers, resulting in an increased vulnerability of such location from the fatigue cracks initiation and propagation perspective. The present thesis deals with an experimental campaign aimed at studying the role played by process-induced residual stresses on the fatigue life of parts produced in AlSi10Mg by L-PBF. Specifically, T5-heat treated hourglass-shaped components have been subjected to a sandblasting process, and the residual state of stress characterising the superficial material layers was investigated through X-ray diffraction (XRD) method. Fatigue properties were assessed by carrying out uniaxial high cycle fatigue tests in fully reversed loading conditions, showing a relevant fatigue strength enhancement with respect to analogous parts in net-shape conditions (lacking the sandblasting process). Defects at fracture origin were inspected and included in a statistical analysis which enabled to compare samples’ defect size distributions. The information were implemented in a fracture mechanics-based approach which gave the possibility to successfully correlate the fatigue strength to the defect size, including the residual stresses’ contribution. In addition, the latter allowed to broaden the understanding of residual stresses relaxation under cyclic loading. Being the geometry a fundamental factor influencing magnitude and distribution of residual stresses, analyses involved specimens both in T5-heat treated and as-built conditions, characterised by a design resembling the one of a functional parts. XRD and hole-drilling techniques have been employed to extensively investigate the residual state of stress, in correspondence of critical components' locations, highlighting the differences caused by the post-process. Some of the samples have been subjected to a tensile loading that gave the possibility to study the residual stresses relief phenomenon. Results showed the presence of a significant residual stress pattern of tensile stresses able to influence the fatigue response of the components (as it was demonstrated for net-shape and sandblasted specimens).
Al giorno d’oggi, in un mercato dove la domanda di prodotti altamente personalizzati e caratterizzati da geometrie complesse è in continua crescita, l’additive manufacturing (AM) ricopre un ruolo sempre più rilevante grazie alla sua maggior flessibilità rispetto ai tradizionali processi sottrattivi. Infatti, non sono usati utensili specifici, ma piuttosto la parte è ottenuta aggiungendo progressivamente sottili strati di materiale. In questi termini, laser powder bed fusion (L-PBF) è uno dei più comuni metodi di AM per metalli. Nonostante i numerosi vantaggi offerti, la tecnica introduce caratteristiche come alta rugosità superficiale, difetti interni e superficiali, anisotropia e stress residui che risultano deleteri per la fatica del componente. Sebbene l’ottimizzazione dei parametri di processo permetta di mitigare gli effetti dannosi, processi successivi (post-processes) sono frequentemente necessari al fine di assicurare specifiche proprietà meccaniche e geometriche richieste. Le intricate geometrie che generalmente caratterizzano i componenti prodotti attraverso AM e la complessa storia termica a cui è soggetto il materiale durante la produzione, portano a difficoltà intrinseche nel predire l’entità e la distribuzione degli stress residui. Comunemente, L-PBF induce stress residui di trazione negli strati esterni di materiale, incrementando la vulnerabilità di quest’area dal punto di vista della nucleazione e propagazione di cricche a fatica. La presente tesi si occupa di una campagna sperimentale avente lo scopo di studiare il ruolo ricoperto dagli stress residui indotti dal processo, sulla vita a fatica di parti prodotte in AlSi10Mg attraverso L-PBF. Specificatamente, dei provini a forma di clessidra soggetti ad un trattamento termico T5 sono stati sottoposti ad un processo di sabbiatura, e lo stato di stress residuo caratterizzante gli strati superficiali di materiale è stato investigato attraverso il metodo di diffrazione a raggi X (XRD). Le proprietà a fatica sono state verificate svolgendo prove a fatica uniassiale ad alto numero di cicli in fully reversed loading conditions, che hanno mostrato un incremento rilevante della resistenza a fatica rispetto a componenti analoghi in condizioni net-shape (non soggetti al processo di sabbiatura). I difetti all’origine del cedimento sono stati ispezionati ed inclusi all’interno di un’analisi statistica che ha permesso di confrontare le distribuzioni relative alle dimensioni del difetto, includendo il contributo relativo agli stress residui. In aggiunta, quest’ultima ha permesso di allargare la comprensione di fenomeni relativi al rilassamento di stress residui a seguito del caricamento ciclico. Essendo la geometria un fattore fondamentale influenzante l’entità e la distribuzione degli stress residui, le analisi hanno compreso provini sia soggetti a trattamento T5 che in condizioni as-built, riproducenti parti funzionali. La tecnica XRD ed il metodo hole-drilling sono stati largamente impiegati per investigare lo stato di stress residuo in corrispondenza delle posizioni critiche del componente, evidenziando le differenze causate dal post-process. Alcuni dei provini sono stati soggetti ad un caricamento in trazione che ha dato la possibilità di studiare il fenomeno di rilassamento degli stress residui. I risultati hanno mostrato la presenza di significativi stress residui di trazione capaci di influenzare il comportamento a fatica dei componenti (come era stato dimostrato per i provini net-shape e sabbiati).
Fatigue of components manufactured in AlSi10Mg by L-PBF : the effect of process-induced residual stresses
Tezzele, Christian
2020/2021
Abstract
Nowadays, in a market where the demand of highly customisable and complex-shaped components is constantly growing, additive manufacturing (AM) is playing an increasingly relevant role due to its greater flexibility with respect to the traditional subtractive processes. In fact, no specific tools are present, rather the part is obtained by progressively adding thin layers of material. In these terms, laser powder bed fusion (L-PBF) is one of the most common metal AM methods. Despite offering several advantages, the technique introduces features such as high surface roughness, internal and superficial defects, anisotropy and residual stresses that result to be detrimental for the component’s fatigue life. Although the optimisation of process parameters allows to mitigate the harmful effects, post-processes are usually needed to ensure specific mechanical properties and geometrical design requirements. The intricate geometries generally characterising AMed components and the complex thermal history experienced by the material during the production, lead to an intrinsic difficulty of predicting the magnitude and distribution of residual stresses. Commonly, L-PBF induces tensile residual stresses in the external material layers, resulting in an increased vulnerability of such location from the fatigue cracks initiation and propagation perspective. The present thesis deals with an experimental campaign aimed at studying the role played by process-induced residual stresses on the fatigue life of parts produced in AlSi10Mg by L-PBF. Specifically, T5-heat treated hourglass-shaped components have been subjected to a sandblasting process, and the residual state of stress characterising the superficial material layers was investigated through X-ray diffraction (XRD) method. Fatigue properties were assessed by carrying out uniaxial high cycle fatigue tests in fully reversed loading conditions, showing a relevant fatigue strength enhancement with respect to analogous parts in net-shape conditions (lacking the sandblasting process). Defects at fracture origin were inspected and included in a statistical analysis which enabled to compare samples’ defect size distributions. The information were implemented in a fracture mechanics-based approach which gave the possibility to successfully correlate the fatigue strength to the defect size, including the residual stresses’ contribution. In addition, the latter allowed to broaden the understanding of residual stresses relaxation under cyclic loading. Being the geometry a fundamental factor influencing magnitude and distribution of residual stresses, analyses involved specimens both in T5-heat treated and as-built conditions, characterised by a design resembling the one of a functional parts. XRD and hole-drilling techniques have been employed to extensively investigate the residual state of stress, in correspondence of critical components' locations, highlighting the differences caused by the post-process. Some of the samples have been subjected to a tensile loading that gave the possibility to study the residual stresses relief phenomenon. Results showed the presence of a significant residual stress pattern of tensile stresses able to influence the fatigue response of the components (as it was demonstrated for net-shape and sandblasted specimens).| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/183638