Fatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V
dc.contributor.advisor | Becker, T. H. | en_ZA |
dc.contributor.author | Dhansay, N. M. | en_ZA |
dc.contributor.other | Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. | en_ZA |
dc.date.accessioned | 2021-11-30T20:17:14Z | |
dc.date.accessioned | 2021-12-22T14:25:47Z | |
dc.date.available | 2021-11-30T20:17:14Z | |
dc.date.available | 2021-12-22T14:25:47Z | |
dc.date.issued | 2021-12 | |
dc.description | Thesis (PhD)--Stellenbosch University, 2021. | en_ZA |
dc.description.abstract | ENGLISH ABSTRACT: Typically, producing Ti-6Al-4V through the laser powder bed fusion (LPBF) technique, results in the material having large residual stresses and martensitic microstructure. These stresses and microstructure have been shown to result in Ti-6Al-4V having poor fatigue properties. However, the insight into the fatigue failure mechanisms caused by the residual stress and microstructure has been limited. LPBF is one of many additive manufacturing (AM) techniques in which parts are built in a layer- wise manner with the use of powdered material and consolidated through high power laser melting. This allows for complex geometries and previously impossible geometries to be manufactured with minimal material wastage. Many industries are aware of the potential in this manufacturing technique and have shown interest in it becoming a viable option for the manufacturing of some of their components. In particular, the use of LPBF produced Ti-6Al-4V is of interest to the aerospace and biomedical industries. This is because Ti-6Al-4V is already well established in many existing industries. For LPBF produced Ti-6Al-4V to be a viable option in industry, researchers need to have an adequate understanding of the material and the implications for its mechanical properties. Fatigue property investigations have largely focused on the fatigue life approach (crack initiation) and Paris regime (region II, crack propagation). However, in recent years, the near-threshold regime (region I, crack propagation) has become of interest, albeit limited in approach. The consensus within literature shows that the large tensile residual stresses, martensitic microstructure and porosity results in poor fatigue properties. Unfortunately, the insight into the fatigue fracture mechanisms brought about by the residual stress and microstructure is not yet well established. Furthermore, the near-threshold fatigue crack growth rate regime (FCGR) experiences crack closure mechanisms which result in premature near-threshold values. The majority of the near-threshold investigations on LPBF produced Ti-6Al-4V do not account for the crack closure mechanisms and therefore produce premature results. As a result of the low crack growth rates achieved in the near-threshold regime, a window into observing the fatigue crack initiation mechanisms is obtained. More specifically, how the fatigue crack initiation mechanisms are influenced by residual stress, martensitic microstructure and changing microstructural morphology. Literature has shown that porosity acts as crack initiation sites and reduce the fatigue life of a component. Furthermore, surface and near-surface porosity have been shown to have a more severe impact on fatigue life than internal porosity. It is through using the near-threshold FCGR approach in which one can calculate the allowable pore size under operational loads. Using the load-shedding technique to obtain near-threshold FCGRs, the results showed anisotropic behaviour dependent on residual stress levels and R-ratios. The fatigue fracture mechanisms were predominantly governed by transgranular quasi-cleavage mechanisms. Furthermore, the fracture is directed by PBG morphology which results in anisotropic crack- closure effects. In addition, primary α lath orientation is governed by the crystallographic texture of the PBG which influences the mechanisms of crack propagation. With the increase in grain size, the presence of β improves the near-threshold FCGRs in the duplex anneal (DA) condition due to the superior plastic flow abilities more than in the as-fabricated and stress relief conditions. This study investigates the near-threshold FCGRs of LPBG produced Ti-6Al-4V in the as- fabricated, stress relief and bi-modal conditions in three build orientations. In addition, crack closure mechanisms are accounted for by implementing variable R-ratio testing. This research presents the influencing mechanisms of residual stress and microstructure on fatigue behaviour. | en_ZA |
dc.description.abstract | AFRIKAANSE OPSOMMING: Gewoonlik veroorsaak die vervaardiging van Ti-6Al-4V deur middel van die laser poeierbed fusie (LPBF) tegniek dat die materiaal groot respannings en ‘n martensitiese mikrostruktuur het. Daar is getoon dat hierdie spanning en mikrostruktuur veroorsaak dat Ti-6Al-4V swak vermoeidheideienskappe het. Insig in die meganismes van vermoeidheid faling, wat veroorsaak word deur die resspanning en mikrostruktuur, is egter beperk. LPBF is een van vele tegnieke van additiewe vervaardiging waar parte laagvormig gebou word uit metaalpoeier deur konsolidasie met ‘n hoë-drywing laser. Dit maak voorsiening vir die vervaardiging van komplekse vorms, voorheen onmoontlike vorms en minimale vermorsing van die materiaal. Baie nywerhede is bewus van die potensiaal van hierdie vervaardigingstegniek en toon ook belangstelling in hierdie lewensvatbare alternatief vir die vervaardiging van sommige van hul komponente. Die gebruik van LPBF-geproduseerde Ti-6Al-4V is van belang vir die lugvaart- en biomediese industrieë omdat hierdie metaal gevestig is in hierdie industrieё.Vir LPBF-geproduseerde Ti-6Al-4V om 'n lewensvatbare opsie in die industrie te wees, moet navorsers voldoende kennis dra oor die materiaal en die proses-afhanklike meganiese eienskappe. Navorsing van vermoeidheidseienskappe het voorheen grootliks gefokus op die benadering tot vermoeidheidslewe (dws. Die ontstaan van krake) en Parys-regime (dws. deel II van die kraakgroeiekurwe). In onlangse jare het die nabye drempelregime (dws. deel I van die kraakgroeiekurwe) egter van belang geword, al is dié benadering beperk. Die literatuur stem ooreen dat die groot resspannings en martensitiese mikrostruktuur swak vermoeidheidseienskappe tot gevolg het. Ongelukkig is die meganismes van vermoeidheidfaling wat deur die resspanning en mikrostruktuur veroorsaak word, nog nie goed vasgestel nie. Die regime van naby-drempel vermoeidheidskraak groeikoers (VKGK) ondervind verder dat kraak- sluitingsmeganismes tot voortydige naby-drempelwaardes lei. Die meerderheid van die studies waarin naby-drempelwaardes van LPBF-geproduseerde Ti-6Al-4V ondersoek is, neem nie krakingsmeganismes in ag nie en dus is die resultate onvoldoende. As gevolg van die lae kraak groeitempo wat in die nabye drempelregime behaal word, word 'n ingangspunt gevind om die meganismes vir die ontstaan van vermoeidheidskrake waar te neem. Meer spesifiek - hoe die meganisme vir die onstaan van krake beïnvloed word deur resspannings, martensitiese mikrostruktuur en veranderende mikrostrukturele morfologieё. Literatuur toon dat porositeit dien as skeurinisiëring en dus lei tot die vermindering van die lewensduur van 'n komponent. Verder is aangetoon dat oppervlak- en naby-oppervlakporositeit 'n ernstiger invloed op die lewensvermoeidheid het as die interne porositeit. Dit is deur middel van die naby-drempel VKGK-benadering waarin 'n mens die toegelate poriegrootte kan bereken onder operasionele vragte. Nabye-drempel-VKGK, verkry deur die beurtkragtegniek, toon aan dat anisotropie in die materiaal van resspanningsvlakke en R-verhoudings afhang. Die vermoeidheidsfaling meganismes is hoofsaaklik beheer deur transgranulêre, kwasi-splytsingsmeganismes. Breuk rigting word deur voorafgaande-beta korrelgrense beїnvloed wat anisotropiese kraak- sluitingseffekte tot gevolg het. Verder word primêre alfa-korreloriëntasie bepaal deur die orientasie van die voorafgaande-beta korrel waaruit dit vorm en dus word die meganismes van kraakverspreiding beïnvloed. Die toename in korrelgroottte na ‘n uitgloei hittebehandeling, asook die teenwoordigheid van beta fase, verbeter die nabye-drempel-VKGK’s van die bimodale mikrostruktuur as gevolg van die beter plastiese vloei-vermoëns . Hierdie studie ondersoek die naby-drempel-VKGK's van LPBG geproduseerde Ti-6Al-4V in die onverwerkte-, spanningverligde- en bimodale mikrostruktuur in drie bou-oriëntasies. Daarbenewens word die sluitingsmeganismes in ag geneem deur die implementering van veranderlike R-verhoudingstoetse. Hierdie navorsing verduidelik die meganisme waarmeё die resspanning en mikrostruktuur die drempelgedrag beїnvloed. | af_ZA |
dc.description.version | Doctoral | en_ZA |
dc.format.extent | 174 pages | en_ZA |
dc.identifier.uri | http://hdl.handle.net/10019.1/123863 | |
dc.language.iso | en_ZA | en_ZA |
dc.publisher | Stellenbosch University | en_ZA |
dc.rights.holder | Stellenbosch : Stellenbosch University | en_ZA |
dc.subject | Additive manufacturing | en_ZA |
dc.subject | Titanium | en_ZA |
dc.subject | Materials -- Fatigue | en_ZA |
dc.subject | UCTD | en_ZA |
dc.subject | Microstructure -- Cracking | en_ZA |
dc.title | Fatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V | en_ZA |
dc.type | Thesis | en_ZA |