Laser milling: surface integrity, removal strategies and process accuracy

Laser milling is capable of processing a large range of materials which are not machinable with conventional manufacturing processes. Engineering materials such as glass, metals and ceramics can be machined without requiring expensive special tools and without any limitations on the 3D complexity of the component. Laser milling is still in its infancy. Laser material interactions are not yet fully understood. Much effort in research and development of the available laser sources is still needed. Ultrafast lasers are beginning to be applied. They can offer more precise machining without the thermal damage that accompanies long-pulse laser manufacturing. Laser pulse duration and its effect on resulting surface integrity has been studied as well as material removal strategy and process accuracy. In order to characterise the resulting surface after laser ablation, the heat affected zone is usually specified. In most cases, visual inspection would be performed without further analysis, resulting in variance of the findings attributed to the operator. A new methodology was required to accurately and impartially assess the heat penetration and quantify the findings. Based on material grain refinement, a comprehensive new methodology was created. By monitoring the changes in grain sizes, a chart of the heat penetration could be created accurately with automated routines. Surface integrity is a critical factor for many applications and a methodology based on analysis of grain refinement in the vicinity of the processed area would create a full map of - iii - the changes happening after laser ablation. Furthermore, the impact of the laser pulse duration is studied utilising the above mentioned development. Further to the surface roughness and heat affected zone, an in-depth analysis was completed on the micro hardness of the material in order to create a comprehensive chart of the changes induced by the laser milling process. Material removal is based on the overlapping of single craters, and the way the craters overlap is referred to as material removal strategy. Generally there are many strategies formulated for material removal but none of them takes into account the specifics of laser milling. Based on surface orientation, dimensions and feature importance, an assessment of material removal strategies is presented. Although ‘laser milling’ is a term used for a number of material removal processes, there are significant differences between them. New strategies for material removal are formulated and reported based on surface topography and orientation. Advanced programming is realised using a commercially available generic CAM package but taking into account the specifics of the laser milling process. The accuracy of the laser milling process depends on the laser-material interaction, and also on the machine hardware, control system and software. Most of the factors affecting accuracy cannot be changed once the machine is built, but there are some that can be optimised to improve process accuracy. The laser source with its characteristics is as important as the material being processed. The relationship between pulse duration, pulse shape and accuracy of the process was demonstrated through a series of experiments designed to expose the correlation between, and impact of, these parameters