Doctoral Degrees (Industrial Engineering)

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Browsing Doctoral Degrees (Industrial Engineering) by browse.metadata.advisor "Akdogan, G."

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    Wear characterisation in milling of Ti6Al4V : a wear map approach
    (Stellenbosch : University of Stellenbosch, 2010-12) Oosthuizen, Gert Adriaan; Van der Merwe, A. F.; Akdogan, G.; University of Stellenbosch. Faculty of Engineering. Dept. of Industrial Engineering.
    ENGLISH ABSTRACT: Information on the milling of Ti6Al4V is limited; with most studies concluding that it is not possible to obtain a significant increase in the material removal rate (Qw). Tool wear maps can be a diagnostic instrument for failure analysis. Cutting speed (vc), maximum un-deformed chip thickness (heMax) and the radial immersion percentage (ae/Ø %) are the key variables in understanding the milling of titanium alloys. The objective of this research study was to construct tool wear maps for the milling of Ti6Al4V. This will form the foundation of understanding the cutting demands on the tool, in order to analyse the main wear mechanisms. Remedial actions, which are developed by tool suppliers, can be considered and integrated via this understanding of the failure modes and related mechanisms. Firstly, experimental data from background studies, literature and industry on wear rates and wear mechanisms pertaining to the milling conditions was gathered to construct the tool wear map. Mathematical models describing the wear behaviour for these conditions were also investigated. Secondly, work piece failure maps have been superimposed onto the tool wear maps constructed to understand the global failure boundaries. Experimentation was carried out to validate the constructed maps. The tool wear map could then be used to discuss the observed effects and consider remedial actions. Cutting speed corresponds to the magnitude of the thermal load and heMax represents the mechanical load. The ae/Ø % defines the duration of the exposure to the thermal load at the edge of the cutting tool. This investigation has shown the following issues to be of importance when considering tool performance via the tool wear map approach: 1. The key to designing tool wear maps is to identify the most economic Scheduled Replacement Time (SRT) for the specific components. Knowing the correct SRT makes it possible to optimize the milling conditions so that the cutting tool wears gradually under the cutting conditions, and lasts longer than the economic SRT. 2. Increased vc will decrease tool life (TL). However, in low transverse rupture strength tools there may be a minimum vc below which mechanical overload may occur. Similarly, a local maximum TL (a sweet spot) may exist if there is a phase change in the work piece material. 3. Increased heMax will decrease TL. However, heMax must be kept below a maximum critical value to avoid mechanical overload, but above a minimum critical value to avoid work hardening. 4. Increased ae/Ø % will decrease TL. The best balance of high Qw and economic TL is found with ae/Ø between 30-40% for rough milling. In finish milling the radial cut is limited to 1 mm finishing stock of the work piece. This study revealed the following important factors when considering work piece failure in the milling of Ti6Al4V: 1. Increased vc will reduce the cutting resistance of the work piece and increase Qw. However, vc must be kept below a maximum critical value to avoid work piece material burn, but above a minimum critical value to avoid burring and poor surface finish, due to tool build-up and chip jamming. 2. Increased heMax will increase the cutting resistance of the work piece and increase Qw. The heMax must be kept below a maximum critical value to avoid poor surface finish, poor flatness and parallelism (due to work piece bending). Likewise, heMax must be kept above a minimum critical value to avoid work hardening and burring. The constructed tool wear maps are validated with experimental work. This research work identified safe zones to productively mill Ti6Al4V, while producing components with a sufficient surface integrity.