Laser powder bed fusion produced Ti-6AI-4V: microstructural transformations and changes in deformation behaviour through thermal treatments.
| dc.contributor.advisor | Becker, T. H. | en_ZA |
| dc.contributor.author | Ter Haar, G. | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. | en_ZA |
| dc.date.accessioned | 2021-06-23T14:27:12Z | |
| dc.date.accessioned | 2021-12-22T14:11:38Z | |
| dc.date.available | 2021-06-23T14:27:12Z | |
| dc.date.available | 2021-12-22T14:11:38Z | |
| dc.date.issued | 2021-12 | |
| dc.description | Thesis (PhD)--Stellenbosch University, 2021. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: The current 4th Industrial Revolution has brought about enormous potential for social-economic development in South Africa. Additive manufacturing, a key technology of the 4th Industrial Revolution, and titanium, an abundantly mined but under-developed mineral commodity, collectively inspired industrial embracing of titanium-powder additive manufacturing, and specifically, laser powder bed fusion produced Ti-6Al-4V. Manufactured part quality concerns and the limited understanding of the innovative process-structure-property interactions impede its industry acceptance. Through developing fundamental insights into interactions between the microstructure and thermal treatments, mechanical performance can be improved, tailored, and optimised. Since conventional process routes cannot be applied to additively manufactured components, novel thermal treatments need to be developed. This dissertation documents experimental research into thermal treatments of laser powder bed fusion produced Ti-6Al-4V. By developing novel insights into the material’s unique metallurgical response to thermal treatments, tensile behaviour of the material is improved. The research investigates three temperature regions for thermal treatments post-fabrication and one in-situ approach. Thermal treatments between 750 – 960 °C are used to develop insight into the microstructure’s large-scale morphological transformation. Thermal treatments at 960 °C achieve fragmentation and grain globularisation. This is followed by quenching to attain a superior bi-modal microstructure. Thermal treatments at temperatures below 650 °C are used to develop insight into the initial stages of martensite decomposition phase transformation and material stress relief. Microhardness and tensile properties revealed material embrittlement. Fine precipitates in the microstructure were identified using high-resolution transmission electron microscopy. Based on these findings, two theories for the cause of material embrittlement based on two possible transformation routes of the initial stages of martensite decomposition are proposed. Mechanical properties of laser powder bed fusion produced Ti-6Al-4V depend on build orientation. Although the unique columnar-shaped and textured prior-β microstructure is identified as a probable cause of deformation anisotropy, limited insight into the cause of anisotropic deformation exists. Thermal treatments above 975 °C are used to globularise the columnar prior-β grain morphology. Microstructural anisotropy between two orientations is quantified using electron backscatter diffraction maps, and the influence of microstructure on deformation and crack initiation is studied. Insight into the deformation behaviour and the identified relation between prior-β crystallographic texture and α-lath morphological texture is used to formulate a theory of the probable cause of material deformation anisotropy. The study lastly investigates a novel approach to thermal treatments by using high-energy process parameters to achieve in-situ heating. An iterative approach for part optimisation using non-default process parameters is undertaken. While findings indicate that an improved microstructural and residual stress state can be achieved, detrimental effects of part oxidation and part-edge bulging are also observed. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Die huidige 4de Industriële Rewolusie het enorme potensiaal vir sosiale-ekonomiese ontwikkeling in Suid-Afrika teweeggebring. Additiewe vervaardiging, 'n belangrike tegnologie van die 4de Industriële Revolusie, en titaan, 'n oorvloed ontginde, maar onderontwikkelde minerale kommoditeit, het gesamentlik die aanvaarding van titanium-poeier additiewe vervaardiging, of spesifiek laser-poeierbedfusie geproduseerde Ti-6Al-4V, geïnspireer. Bekommernisse oor die kwaliteit van vervaardigde onderdele en die beperkte begrip van die interaksies tussen die proses-struktuur-eienskap belemmer die aanvaarding daarvan deur industrie. Deur die ontwikkeling van fundamentele insigte in interaksies tussen die mikrostruktuur en termiese behandelings, kan meganiese eienskappe verbeter, aangepas en geoptimaliseer word. Aangesien konvensionele verwerkings roetes nie op additiewe vervaardigde komponente toegepas kan word nie, moet nuwe termiese behandelings ontwikkel word. Hierdie proefskrif dokumenteer eksperimentele navorsing oor termiese behandelings van laser-poeierbedfusie vervaardigde Ti-6Al-4V komponente. Deur die ontwikkeling van nuwe insigte in die unieke metallurgiese reaksie van die materiaal op termiese behandelings, word die trekgedrag van die materiaal verbeter. Die navorsing ondersoek drie temperatuurstreke vir termiese behandeling na-vervaardiging en een in-situ benadering. Termiese behandelings in die temperatuurstreek 750 – 960 °C word gebruik om insig te ontwikkel in die grootskaalse morfologiese transformasie van die mikrostruktuur. Termiese behandelings by 960 °C bewerk fragmentering en bolvorming van korrels. Dit word gevolg deur te blus om 'n beter tweemodale mikrostruktuur te verkry. Termiese behandelings onder 650 °C word gebruik om insig te ontwikkel in die aanvanklike fase van transformasie van martensietontbinding en materiële spanningsverligting. Mikrohardheid en trekkeienskappe het materiële broosheid aan die lig gebring. Fyn korrels in die mikrostruktuur is geïdentifiseer met behulp van hoë resolusie transmissie- elektronmikroskopie. Op grond van hierdie bevindinge word twee teorieë oor die oorsaak van materiële broosheid voorgestel, gebaseer op twee moontlike transformasie-roetes van die aanvanklike stadiums van martensietontbinding. Laser-poeierbed-versmelting vervaardigde Ti-6Al-4V meganiese eienskappe hang af van die bou-oriëntasie. Alhoewel die unieke kolomvormige en tekstuur voorafgaande-β mikrostruktuur geïdentifiseer is as 'n waarskynlike oorsaak van vervormingsanisotropie, bestaan daar beperkte insig in die oorsaak van anisotropiese vervorming. Termiese behandelings bo 975 °C word gebruik om die kolomvormige morfologie van voorafgaande-β korrels te laat bolvorm. Mikrostrukturele anisotropie tussen twee oriëntasies word gekwantifiseer aan die hand van elektron-terugspreid-diffraksiekaarte, en die invloed van mikrostruktuur op vervorming en ineenstorting van die krake word bestudeer. Insig in die vervormingsgedrag en die geïdentifiseerde verband tussen voorafgaande-β kristallografiese tekstuur en α-korrel morfologiese tekstuur word gebruik om 'n teorie te formuleer oor die waarskynlike oorsaak van materiële vervorming anisotropie. Die studie ondersoek laastens 'n nuwe benadering tot termiese behandelings deur gebruik te maak van hoë-energie prosesparameters om in-situ verwarming te bewerkstellig. 'n Iteratiewe benadering vir onderdeeloptimalisering met nie-standaard prosesparameters word onderneem. Alhoewel bevindinge aandui dat 'n verbeterde mikrostruktuur- en residuele spanningstoestand bereik kan word, word nadelige gevolge van oksidasie en gedeeltelike bulting ook waargeneem. | af_ZA |
| dc.description.version | Doctoral | en_ZA |
| dc.format.extent | 219 pages | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/123601 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Laser powder bed fusion | en_ZA |
| dc.subject | Titanium alloys | en_ZA |
| dc.subject | Microstructure | en_ZA |
| dc.subject | Thermal treatments | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.subject | Deformations (Mechanics) | en_ZA |
| dc.title | Laser powder bed fusion produced Ti-6AI-4V: microstructural transformations and changes in deformation behaviour through thermal treatments. | en_ZA |
| dc.type | Thesis | en_ZA |