Mohamed Sobih

The rapid growth in laser technology has made many previously untenable applications possible, replacing mechanical methods. A large body of research has reported investigations into the laser metal-cutting process, for obtaining fundamental understanding of basic phenomena and for process optimization. Although laser metal cutting is widely used, it is still dominantly confined to cutting of uniform thickness material even though many applications require the cutting of materials with variable thickness. These applications can be categorized into two classes, viz. cases where the quality of cut is not important, and where it is of critical importance. In the former class we typically find civil engineering and nuclear decommissioning and decontamination projects. The geometry of pipes and I-beams makes it, per definition, a work-piece of non-uniform thickness, and thus the cutting of any pipe is similarly a non-standard process. This thesis presents an investigation into the effect of thickness variation upon the cut quality and to establish the limits of thickness variation over which one can achieve a cut quality within given tolerances. Mild steel sheets of 2-3 mm thickness with steps of 0.25 mm were cut using CO2, Nd:YAG and fibre lasers in 4 different cutting arrangements. The quality of cut was examined in terms of dross attachment, surface roughness, perpendicularity, top kerf width, and striation depth. The work shows that variation in workpiece thickness affects the cut surface quality due to several factors related to irradiance and assist gas flow. In some situations these effects can be minimized within certain tolerances. Fibre laser showed an improved cut quality over the other studied lasers and thus an extended statistical study of the effects of process parameters and their interactions on the quality achieved during fibre laser cutting of mild steel was undertaken. The third part of the work concerns oxygen-assisted fibre laser cutting of EN43 mild steel sheets. This part starts with statistical analysis to find the most effect parameter and there. This study led to the conclusion that under certain circumstances, striation-free cutting, something never before reported in the literature, might be possible. This led to a directed search for optimal conditions and striation-free laser cuts were demonstrated for 1 and 2 mm thick mild steel sheets. The optimal operating windows are presented and a mathematical method is proposed to estimate the boundary of the cutting speed operating window over which striation free cut can be obtained. This work results in the publication of three journal papers and protected by two patents.