Browsing by Author "Malcolm, J. S."

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Chemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V.
    (Stellenbosch : Stellenbosch University, 2021-12) Malcolm, J. S.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    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.