Chemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V.
| dc.contributor.advisor | Becker, T. H. | en_ZA |
| dc.contributor.author | Malcolm, J. S. | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. | en_ZA |
| dc.date.accessioned | 2021-06-23T15:02:31Z | |
| dc.date.accessioned | 2021-12-22T14:11:59Z | |
| dc.date.available | 2021-06-23T15:02:31Z | |
| dc.date.available | 2021-12-22T14:11:59Z | |
| dc.date.issued | 2021-12 | |
| dc.description | Thesis (MEng)--Stellenbosch University, 2021. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: An experimental methodology which implemented the practice of chemical etching was developed with the aim of reducing the surface roughness of laser powder bed fusion (LPBF) produced Ti-6AI-4V and thereby, improving the high cycle fatigue (HCF) performance. Hydrofluoric-nitric acid solutions, HF-HNO3, were used in the chemical etching experiments. The concentration of the HNO3 acid was fixed at 20 % while the concentration of the HF acid was varied to alter the solution concentration. The surface roughness of the LPBF produced Ti-6AI-4V samples were measured using a skidded contact profiler instrument. The subsequent chemical etching investigations revealed that an equivalent reduction in surface roughness can be achieved using either a low 2 % HF acid, or a high 12 % HF acid etching solution concentration. The solution temperature investigation demonstrated that the mass removal rate of a chemical etching process can be increased with raising solution temperature, albeit at the expense of process control. From the insights obtained from the chemical etching investigations, the recommended solution to reduce the surface roughness is a 2 % - 4 % HF acid concentration in conjunction with a solution temperature that is below room temperature. Etching durations of 60 minutes and longer allow for the minimum surface roughness that chemical etching as-built (AB) LPBF samples can be achieved. The internal pores in LPBF produced samples become exposed at the surface during the chemical etching process which limits the average surface roughness, Ra, from reducing to values significantly lower than Ra ≈ 2 μm. The HCF testing conducted was tension-tension cyclic uniaxial testing with a stress ratio, R, of R = 0.1. The LPBF produced Ti-6Al-4V fatigue samples were separated into four groups: the AB samples and then the three remaining sample groups which were chemically etched in a 4%HF20%HNO3 solution for 5, 15 and 90 minutes, respectively. The fatigue samples which received the 90 minute etch had both the lowest average surface roughness, Ra = 1.84 ± 0.03 μm, and the largest improvement in fatigue performance with reference to the AB sample’s fatigue behaviour. All the fatigue failed samples had crack initiation sites at the surface of the sample. The fabrication process-induced surface roughness acted as crack initiation sites for the AB and 5 minute etch sample groups. Chemical etching caused the controlling defect responsible for fatigue crack initiation to change from surface roughness to internal defects which were brought to the surface by the chemical etching mechanism. The opened subsurface defects acted as stress raisers which caused the fatigue crack initiations for the 15 and 90 minute etch sample groups. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: n Proefondervindelike metode is ontwerp om deur middel van chemiese laser poeier oppervlakte (LPBF – Laser power bed fusion) n verbeterde oppervlakte Ti- 6AI-4V te bewerkstellig. Die doel hiervan is om die hoë siklus vermoeidheidsweerstand (HCF - High cycle fatigue) gedrag te verbeter. Waterstoffluoriet-. stikstofsuur (HF-HNO3) was gebruik. HF-HNO3 is gebruik. Die konsentrasie van die HNO3 was vasgestel op 20% terwyl die konsentrasie van die HF suur gevarieer het om die konsentrasie van die oplossing te varieer. Die grofheid van oppervlakte wat veroorsaak is deur die Ti-6al-4V monsters is gemeet deur gebruik te maak van glykontak profiel instrument. Die ondersoeke het getoon dat gelykstaande oppervlakte grofheid verkry kan word deur ‘n lae HF of ‘n hoë 12% HF ets oplossing te gebruik. Die beheer van die ets proses word verminder deur verhoging in die temperatuur. Die beste resultaat kan verkry word deur die gebruik van ‘n 2 - 4% oplossing van HF suur teen laer as in kamertemperatuur. Deur vir 60 minute of langer te ets word die minimum oppervlakte grofheid verkry deur die gebruik van Ti-6Al-4V. Die interne porieë wat veroorsaak word deur die gebruik van LPBF gedurende die ets proses word na die oppervlakte gebring wat die gemiddelde oppervlakte grofheid, Ra, verhoed om te daal na vlakke noemenswaardg laer as Ra ≈ 2 μm. Die HCF toets uitgevoer was spanning – spanning uni-aksiaal met ‘n spanning verhouding, R, waar R = 0.1. Die LPBF wat geproduseer is, is verdeel in 4 groepe monsters: die geproduseerde monsters en die monsters chemies geëts in ‘n 4% HF 20% HNO3 oplossing vir 5, 15 en 90 minute onderskeidelik. Die monsters wat vir 90 minute behandel is het beide die laagste gemiddelde opper vlakte grofheid, Ra = 1.84 ± 0.03 μm, en die grootste verbetering in die vermoeidheidsprestasie met verwysing na die geproduseerde monsters se vermoeidheidsprestasie. Al die monsters wat die vermoeidheidstoets nie geslaag het nie, het getoon dat krake begin vorm. Die produksie proses geinduseerde oppervlakte grofheid in die 5 minute etsgroep het gelei na kraak inisiasie ares. Die chemiese etsproses het veroorsaak dat die defek verantwoordelik vie vermoeidheidskrake verander van oppervlakgrofheid na interne defekte. | af_ZA |
| dc.description.version | Masters | en_ZA |
| dc.format.extent | 136 pages | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/123608 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Additive manufacturing | en_ZA |
| dc.subject | Ti-6Al-4V | en_ZA |
| dc.subject | Chemical -- Etching | en_ZA |
| dc.subject | High cycle fatigue testing | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.subject | Cycling -- Fatigue testing machines | en_ZA |
| dc.title | Chemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V. | en_ZA |
| dc.type | Thesis | en_ZA |