Solid phase by-products of laser material processing.
thesisposted on 19.08.2013, 13:20 by Leon M. Lobo
The analysis of the particulate generated by laser material removal processes is important not only from a health aspect but is also useful to understand the underlying process. The mechanisms of particle formation during CO2 laser cutting of mild steel have been established and the influence of the laser processing parameters on the particles is discussed. Hydrodynamic instabilities in the melt zone produce airborne particulate during cutting. Larger particulate tend to be formed by shearing of the melt film (50-1OOum) and by pressure gradients (>1OOum ) within the melt. Additionally the causes of secondary droplet break-up have been addressed. The investigation of these mechanisms enables an enhanced understanding of the cutting process. Investigation of the particle characteristics yields information regarding the health of the laser process. The size distribution of the particulate has been analysed using laser diffraction particle sizing and is shown to relate to the process parameters (cutting velocity, laser power) as well as the product quality parameters (cut surface roughness, kerf width, striation frequency, etc.). Additionally, analysis of the particle morphology using scanning electron microscopy establishes that the particulate generated are often thin-walled hollow spheres. The information provided by the experimental data and the knowledge of the mechanisms of particle formation allows a process control methodology to be applied. In order to implement the system, a particle-sizing instrument has been designed. This instrument is based around a linescan CM OS sensor and is designed to tailor for the needs of an online, real-time control system. The system is entirely optical in nature and works on the principle of laser diffraction. Much of the mathematical processing is done offline enabling it to display size distributions in real-time. Much of the literature, available to date, regarding the solid byproducts of laser cutting use techniques of particle capture and analysis that are not ideal nor informative enough. In order for a realtime particle analysis system to be successfully implemented, the method of capture and the selection of the optimum size band have been thoroughly investigated. Additionally, the particle formation during concrete scabbling was investigated. Concrete was scabbled using a 1 kW C02 laser and the particles ejected from the surface were investigated for particle size and morphology. Scabbling produces particles which are airborne and hence potential health hazards. The morphology of the airborne particulate showed that the components of the concrete are released from the parent matrix. Some components have particle morphologies that may pose health risks if inhaled (dendritic and needle shaped structures).
- Mechanical, Electrical and Manufacturing Engineering