Influence of laser surface treatment on residual stress distribution and dynamic properties in rotary friction welded ti-6al-4v components
- Authors: Tsikayi, Davies Shamiso
- Date: 2019
- Subjects: Lasers -- Industrial applications , Friction welding Pressure welding Metals -- Research
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/43823 , vital:37050
- Description: This manuscript details a study on laser surface treatment, a surface modification technique that is an easily flexible way of improving material surface properties of complex geometries. The research explored the potential of laser surface modification/treatment as a post welding surface processing technique for RFW Ti-6Al-4V ELI components by evaluating the microstructural effects, influence on fatigue life and the depth and magnitude of residual stresses induced. The outcome of this study reveals how post processing by laser surface modification affects crack initiation hence fatigue life and further explains mechanisms potentially contributing to enhanced joint properties. This study was accomplished by investigating the effect of laser surface treatment on surface properties of hourglass cylindrical rotary friction welded Ti-6Al-4V ELI specimens. Preliminary work was done in two stages. The first stage involved conducting laser surface treatment on 3 mm Ti-6Al-4V sheets. In this stage, an understanding of the process variables concerning the laser surface treatment process characteristics was established. Laser power and focus position were varied whilst scanning speed was kept constant. The observed macrographs were quantified in terms of laser penetration depth and width. A hardness and microstructural analysis was also conducted on selected specimens of the laser surface treated flat sheets trials. The second stage involved surface treatment of the hourglass fatigue specimen. This preliminary work allowed for the type and influence of treatment strategy to be analysed. The influence of treatment strategy on the depth of penetration was established with an emphasis on achieving homogeneity of the laser surface treated zone’s depth of penetration around the complete cylindrical specimen’s diameter. The final matrix involved varying laser power, scanning speed and focus position and the specimens were characterised by comparing hardness, residual stresses and microstructure. The results showed that laser surface treatment changed the hardness profile of the near surface of the specimen owing to the introduction of a homogenous microstructure at the surface as compared to a friction welded specimen. The microstructure was resolved using electron backscatter diffraction. A fully α-lamella microstructure was observed in the two specimens analysed at a position of 200 μm from the surface. The α-lamella had different width sizes with the low-power density specimen having a very fine microstructure as compared to that of the high-power density specimen. EBSD phase maps were also analysed for the parent, rotary friction welded only and friction welded laser surface treated specimens. The laser treated specimens showed virtually no β phase present as compared to the parent and rotary friction welded only specimens. LST processing improved the fatigue properties of the RFW specimens. The position of failure shifted from the HAZ to outside the RFW joint. This change in position was attributed to the surface modification by LST thereby introducing a more homogenous microstructure at the surface of the specimen. Additionally, it was also observed that the power density had an important role to play in the fatigue properties of the laser surface treated specimens. The high-power density LST specimens had a low fatigue limit compared to the low-power density specimens. The low fatigue limit at high- power density correlated with the residual stress results where the high-power density specimen had the highest attained surface tensile axial residual stresses. In conclusion, the main influences of laser surface treatment of small friction welded Ti-6Al-4V ELI components relate to an increase in fatigue properties by shifting crack initiation sites to less stressed areas. In this way, laser surface treatment could assist in the optimisation of manufacturing methodologies for small near net shape complex geometry components. The uniform and homogenous microstructure eliminates or reduces microstructural variations as observed in as welded components, reducing weld zone hardness variation. Additionally, the study showed that the introduction of a near surface refined microstructure inhibited crack initiation in the welded region.
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- Date Issued: 2019
Friction hydro pillar riveting process of Ti-6AI-4V titanium sheet
- Authors: Tsikayi, Davies Shamiso
- Date: 2015
- Subjects: Friction welding , Titanium alloys -- Welding , Sheet-metal
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: http://hdl.handle.net/10948/6357 , vital:21078
- Description: Mechanical fasteners are used extensively in the joining of two or more metal plates or sheets. Riveted joints have been the joints of choice mainly for the Aerospace Industry. However for this research, Friction Hydro Pillar Processing has been used to develop and characterise a new riveting technique termed Friction Hydro Pillar Riveting (FHPR). Two overlapping 3.17 mm Ti-6Al-4V sheets were joined together using Ø6 mm rivet which was friction processed. This research has focussed on the initial development of Friction Hydro Pillar Riveting thereby establishing a basic understanding of the influences of main process parameters, rotational speed and axial force - and also joint configurations. The results showed that with a decrease in the bottom hole chamfer angle, there was resulting overall increase in the rivet joint pull off strength. From the best performing joint configuration in pull off tests, shear tests were conducted whilst a blind hole FHPR joint was also done and tested in pull off and shear strength. The shear test fracture surfaces exhibited ductile failure. The microstructure of the joints was thus evaluated. From parent material, heat affected zone and to weld zone there was a variation in the microstructure analysed. The hardness profiles showed increased hardness in the weld zone which partly explained the shear results. The hardness increase was mainly due to grain refinement in the weld zone by the Friction Hydro Pillar Riveting process.
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- Date Issued: 2015