Laser cleaning prior to diffusion bonding of the Ti-6Al-4V for aerospace applications

Abstract

Established cleaning processes for titanium alloy components in aerospace applications priorto joining process normally use aggressive and hazardous chemicals, which lead toenvironmental concerns. Laser cleaning is a non-contact, highly controllable process for theremoval of contaminants from a surface with minimum or no damage to the substrate materialand environmentally friendly technique. However when applying laser cleaning processes toTi alloys, some process challenges have been identified which can perhaps affect subsequentjoining processes such as the diffusion bonding process. These challenges are at first duringlaser cleaning processes, titanium might be oxidised or can generates hard and brittle layer(alpha-case). Secondly, the laser beam interaction with organic contaminated surfaces couldresult in rapid evaporation of contaminates to release reactive carbon fragments, that maysubsequently react with titanium to form carbides. The effects of the oxide layer and thegeneration of carbides might be considered as impurities during diffusion bonding process toform microstructural defects, leading to reduce the material performances. In this PhD project the feasibility of laser cleaning of the as-manufactured surface and the precontaminatedsurfaces of Ti-6Al-4V alloy, by organic and inorganic contaminants, wasexplored to establish an optimum operating-window and to understand the fundamentalphenomena involved in the process. Two different types of lasers have been applied, a KrFexcimer laser and a pulsed TEA-CO laser. The materials' characteristics were examined interms of surface morphology, phase changes, oxide film formation and surface chemistry. Thesurface analysis was undertaken before and after laser application, using optical microscopy, ,SEM/EDX, TEM, rf-GDOES and XPS. Surface cleanliness was evaluated based on themeasurement of the carbon content and other contaminants typically containing yttria (yttriumoxide) and magnesium. Then an investigation was made on the properties of diffusion bondingprocess for the laser-cleaned alloys. In addition to this, the water contact angle technique wasalso used to evaluate the degree of cleanliness. The integrity of the laser-cleaned anddiffusion-bonded alloys and the degree of bondability were determined. An analysis of thefume generated during the laser cleaning process was carried out in order to provide aguideline on the selection of a fume extraction system. A key contribution of the research is the identification of an optimum operating window forthe laser surface cleaning of Ti-6Al-4V suitable for diffusion bonding without causingdamages to the substrate surfaces. Another key contribution is the understanding of laser beaminteraction 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 withoutany specific atmospheric protection for diffusion bonding of aerospace components.Appropriate laser energy per pulse and number of laser pulse required to perform a suitablecleaning by laser radiation, delivered by KrF excimer laser and pulsed TEA-CO laser, havebeen 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 thatcould affect the degree of cleanliness. The pulse frequency and the glancing angle had lesseffect on the cleaning process. Besides, it has been found that under the established lasercleaning condition no surface hardening effect was observed. The measurement of cleanlinessby the water contact angle cannot guaranties a fully cleaned surface from carbon contaminantseven if the water contact angle is small. The laser cleaned and diffusion bonded alloyspresented 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 representingmo

Date of Award	31 Dec 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhu Liu (Supervisor)