Surface cleaning is a standard manufacturing procedure in diffusion bonding of titanium alloys for aerospace component and structures. Conventionally such cleaning procedures are based on chemical etching with hydrofluoric and nitric acids. Although highly effective, the fluids are hazardous and not environmental friendly. In recent years, laser cleaning, as a safe, environmentally friendly technique, has been demonstrated as an important alternative to the established chemical-based processes. However, for a successful application of laser cleaning of titanium alloys, new challenges have been met. Over-exposure or under-exposure to the laser beam as well as the effect of surface geometry may result in undesirable cleaning and may subsequently lead to microstructural defects within diffusion regions. Therefore, in-process monitoring of the cleaning process is required not only to understand the status of surface cleaning, but also enable quality control of the process. This project was initiated to develop an on-line monitoring system to infer the status of surface cleanliness so as to avoid surface damages and insufficient contamination removal during excimer laser cleaning of Ti-6Al-4V alloys. The thesis describes the investigation into three monitoring techniques: acoustic wave emission, probe beam reflection and fiber optics total reflection. The on-line acoustic monitoring process using a microphone was investigated with an aim of achieving a reliable and cost effective monitoring system for practical applications. The acoustic sensing has been shown to be able to differentiate between cleaned and un-cleaned surfaces. Furthermore, degree of cleanliness was assessed by comparing the acoustic signals with other direct measurement techniques. The changes in the acoustic emission signal parameters are directly related to the progress of the laser cleaning and/or the surface condition. The surface cleanliness and changes of surface morphology before and after laser cleaning were confirmed by the use of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The acoustic emission relies on the cleaning laser to generate a shock wave as a result of rapid material decomposition, vaporisation and plasma generation. Therefore, for in-process monitoring, calibration is needed to correlate the surface conditions with the acoustic emission signals. Post cleaning i.e. running of the cleaning laser at lower energy input is used to examine the surface cleanliness. The optical monitoring of laser cleaning process was based on detection of the significant difference in reflectivity before and after laser cleaning. For the probe beam reflection technique, the variation in the intensity of the reflected beam from the surface with different cleaning conditions such as laser fluencies and number of pulses, as well as type of surface contaminants applied were investigated and their relationships established. Although this method can differentiate between the cleaned and un-cleaned surfaces, it can only be applied to flat surfaces. To enable process monitoring for wider component geometry, fiber optics total reflection technique was investigated. The reflection spectra of the surface with different surface cleanliness conditions were examined. An effort was made to maximise the sensitivity of the monitoring system by selecting the correct optical wavelength. The problem of geometry limitation has been overcome. A discussion is made on the understanding the mechanisms of the monitoring system and comparison of three different monitoring techniques.
|Date of Award||31 Dec 2011|
- The University of Manchester
|Supervisor||Zhu Liu (Supervisor)|