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Title: Laser cleaning prior to diffusion bonding of the Ti-6Al-4V for aerospace applications
Author: Mhich, Abdeslam
ISNI:       0000 0004 9353 8140
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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Established cleaning processes for titanium alloy components in aerospace applications prior to joining process normally use aggressive and hazardous chemicals, which lead to environmental concerns. Laser cleaning is a non-contact, highly controllable process for the removal of contaminants from a surface with minimum or no damage to the substrate material and environmentally friendly technique. However when applying laser cleaning processes to Ti alloys, some process challenges have been identified which can perhaps affect subsequent joining processes such as the diffusion bonding process. These challenges are at first during laser cleaning processes, titanium might be oxidised or can generate hard and brittle layer (alpha-case). Secondly, the laser beam interaction with organic contaminated surfaces could result in rapid evaporation of contaminates to release reactive carbon fragments, that may subsequently react with titanium to form carbides. The effects of the oxide layer and the generation of carbides might be considered as impurities during diffusion bonding process to form microstructural defects, leading to reduce the material performances. In this PhD project the feasibility of laser cleaning of the as-manufactured surface and the precontaminated surfaces of Ti-6Al-4V alloy, by organic and inorganic contaminants, was explored to establish an optimum operating-window and to understand the fundamental phenomena involved in the process. Two different types of lasers have been applied, a KrF excimer laser and a pulsed TEA-CO laser. The materials' characteristics were examined in terms of surface morphology, phase changes, oxide film formation and surface chemistry. The surface analysis was undertaken before and after laser application, using optical microscopy, SEM/EDX, TEM, rf-GDOES and XPS. Surface cleanliness was evaluated based on the measurement of the carbon content and other contaminants typically containing yttria (yttrium oxide) and magnesium. Then an investigation was made on the properties of diffusion bonding process for the laser-cleaned alloys. In addition to this, the water contact angle technique was also used to evaluate the degree of cleanliness. The integrity of the laser-cleaned and diffusion-bonded alloys and the degree of bondability were determined. An analysis of the fume generated during the laser cleaning process was carried out in order to provide a guideline on the selection of a fume extraction system. A key contribution of the research is the identification of an optimum operating window for the laser surface cleaning of Ti-6Al-4V suitable for diffusion bonding without causing damages to the substrate surfaces. Another key contribution is the understanding of laser beam interaction with various contaminants and the Ti alloy substrates in the laser cleaning process. The project has demonstrated, for the first time, the successful use of laser cleaning without any specific atmospheric protection for diffusion bonding of aerospace components. Appropriate laser energy per pulse and number of laser pulse required to perform a suitable cleaning by laser radiation, delivered by KrF excimer laser and pulsed TEA-CO laser, have been optimized to clean the as manufactured and the pre-contaminated Ti-6Al-4V surfaces. The laser fluence and the number of laser pulses were found to be the principal parameters that could affect the degree of cleanliness. The pulse frequency and the glancing angle had less effect on the cleaning process. Besides, it has been found that under the established laser cleaning condition no surface hardening effect was observed. The measurement of cleanliness by the water contact angle cannot guarantee a fully cleaned surface from carbon contaminant seven if the water contact angle is small. The laser cleaned and diffusion bonded alloys presented a high degree of bondability. The fume generated during laser cleaning showed the presence of a high concentration for particles sized between 300 nm and 2 m representing more than 85% of all the particles generated.
Supervisor: Liu, Zhu Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Laser cleaning ; Ti-6Al-4V ; wetability ; diffusion bonding