Design and Optimization of a Blue Laser Processing Head for Additive Manufacturing Applications

Recent applications of Additive Manufacturing (AM) pose new challenges that are difficult to meet with the commonly used Infrared (IR) laser, but instead could be well addressed by blue lasers. Indeed, while high-power IR lasers have found extensive usage in materials processing, switching to blue lasers would improve the processing quality and efficiency of some materials that are finding increasing importance in industrial applications, such as copper, gold and aluminum, because of their higher absorption in the blue wavelengths than in the IR region. This also improves the energy consumption since blue laser diodes show an higher Wall Plug Efficiency (WPE) than IR fiber lasers. The objective of the thesis is the design, fabrication and preliminary characterization of a processing head for blue light, specifically conceived to be installed on the prototype of an innovative, low-cost AM machine. The laser source can emit some hundreds of watts, obtained from the combination of multi-emitter diode modules. The new machine is intended for the manufacturing of small-volume parts utilizing both high-performance semi-crystalline engineering thermoplastics and metals that are currently challenging to process using lasers. The processing head consists of an optical system based on spherical lenses and does not make use of galvo mirrors: the selection of this type of lenses as the preferred optics to be installed in the processing head was motivated by the need to maintain low construction costs. As a preliminary design step a custom code for the analysis of a lens system using Gaussian beams, ray tracing and the ray transfer matrix approach was developed and validated against a commercial software. This has allowed to determine the optimal sequence of lenses and focal lengths required to manipulate the beam, so as to yield the desired characteristics in terms of focused spot shape and power density. Then, thermal simulations were performed using a finite element analysis software (COMSOL) to evaluate the heating of the optics and to determine the optimal lens material. A Gaussian beam with power characteristics similar to those of the employed laser source was simulated to propagate through the optical system devised in the initial design phase. The predictions were validated with some lab experiments. As a final phase of the design process, advanced ray tracing simulations were conducted using Zemax OpticStudio software to model a source comprised of a combination of multi-emitters. This source possessed beam characteristics that were closely akin to those of the commercial modules that will be employed in the construction of the final product. The use of OpticStudio facilitated the simulation of laser beams that exhibited more intricate field distributions than Gaussian ones. It also allowed for a more precise prediction of the intensity profile of the focused spot in the end product. A lab prototype of the processing head was built and fully characterized, obtaining results in good agreement with the theoretical predictions. These findings provide the basis for future developments, with the potential for achieving a modular and scalable in power system.