Shedding Light on Laser-Metal Interactions: In situ Monitoring with Inline Coherent Imaging and Integrating Sphere Radiometry

Abstract

This thesis presents a comprehensive investigation into the complex phenomena underlying high-irradiance laser-metal interactions. The research contained herein comprises three distinct studies, each shedding light on critical aspects of this complicated problem. The first study employs state-of-the-art measurement techniques to capture the dynamic evolution of keyhole formation during laser-metal interactions in inert environments. By employing inline coherent imaging (ICI) and integrating sphere radiometry (ISR) at high imaging rates (200 kHz), the measurements reveal the time-dependent cavity-enhanced absorptance within these keyholes. The results show that processing in an argon-rich environment, as opposed to air, significantly reduces coupling efficiency in conduction mode, while having only a minor impact in keyhole mode. Furthermore, high imaging rates enable the observation of liquid surface oscillations and corresponding changes in absorptance, providing unprecedented insights into this complex phenomenon. The second study focuses on the challenging task of joining aluminum and steel. This research again utilizes simultaneous time-resolved measurements of keyhole depth (ICI) and absolute absorptance (ISR) during laser spot welds. It was found that welding through steel into aluminum offers the initial benefits of steel keyhole welding while surprisingly leading to more stable keyholes after punching into aluminum. The discovery of this unexpected stable regime highlights how little we know about these interactions on the time-resolved scale, and emphasizes the importance of understanding the underlying physics. The third study addresses the field of AM. By implementing ISR inside a commercial AM machine, the research illuminates the fundamental laser-powder interaction that governs AM techniques in an environment that matches real build conditions. This work reveals that laser energy coupling is a highly dynamic quantity and that materials in powder form do not inherit their bulk properties. Insights gained from this study have the potential to significantly enhance process optimization for diverse part designs and encourage the broader adoption of ISR and other high-speed, in situ measurement techniques as research tools. Collectively, these studies advance our understanding of laser-metal interactions across a range of applications, offering valuable insights that can inform industrial processes and facilitate the development of more accurate models for laser welding and laser-based metal AM.