Metal additive manufacturing (AM), particularly laser powder bed fusion (L-PBF), aims to transition from rapid prototyping to large-scale production. Increasing productivity is crucial for competitiveness with conventional methods. Advances such as multi-laser systems, kW-class fiber lasers, beam shaping technologies, and scan path optimization are driving productivity gains. The standard linear scan with a Gaussian beam, while offering a process window at lower power and speed, has limitations at higher levels, often failing to control porosity or bead-up due to high peak fluence. High-speed beam oscillation offers a promising approach to achieve both increased productivity and improved microstructure control by providing hierarchical energy delivery, spatially and temporally. Laser beam oscillation has proven useful in laser welding, mitigating defects and enabling joining of dissimilar metals. Studies have shown its benefits in grain refinement and improved penetration depth, but these focused on lower frequencies relevant to laser welding's slower scan speeds. This study is the first to investigate the spatiotemporal evolution of keyholes under kHz laser oscillation in an emulated L-PBF setup, utilizing synchrotron-based in-situ X-ray imaging and a validated multiphysics model to understand keyhole dynamics and melt pool shape control.

Previous research on laser beam oscillation has primarily focused on laser welding, demonstrating its effectiveness in mitigating defects, improving penetration depth, and refining grain structures in various metal joints. Studies using different oscillation frequencies and patterns have revealed changes in melt pool shape and microstructure, leading to enhanced mechanical properties. However, these studies primarily utilized lower oscillation frequencies due to the limitations of the technology in laser welding applications. The application of beam oscillation in L-PBF requires significantly higher frequencies to compensate for the much faster linear scan speeds used in this process. While beam shaping techniques like the use of top-hat beam profiles have been investigated in L-PBF, the dynamic control offered by high-frequency beam oscillation opens new avenues for process control and part quality enhancement.

This research employed a synchrotron-based dynamic X-ray radiography (DXR) technique with a miniature powder bed fusion setup to image the dynamic keyhole oscillation in Ti-6Al-4V samples. The high-energy X-rays from the Advanced Photon Source (APS) at Argonne National Laboratory enabled visualization of the process within optically opaque materials with high temporal and spatial resolution. A ytterbium fiber laser, coupled to a fast scanning mirror (FSM) module, enabled the superposition of a circular oscillation onto a linear scan, with the FSM operating at 5.5 kHz. The experimental parameters included variations in laser power (300, 400, and 500 W), linear scan speed (0.2, 0.4, 0.6, 0.8, 1, and 1.2 m s⁻¹), and circular oscillation diameter (0, 0.09, 0.12, 0.18, and 0.24 mm). A total of 90 parameter sets were tested. The DXR system captured images at a frame rate of 50 kHz. The experimental data were complemented by simulations performed with the FaSTLAB (FAST Lattice Boltzmann) metal powder bed fusion code. This multiphysics model includes ray tracing, phase transitions, and detailed liquid dynamics, allowing for a detailed comparison between experimental and simulated keyhole behavior. The keyhole depth was measured from the substrate top to the keyhole bottom.

The study revealed distinct keyhole dynamics under oscillating laser processing compared to conventional linear scans. The oscillating keyhole exhibited continuous and periodic fluctuations influenced by the local pre-heat conditions and transient laser speed. The keyhole depth varied significantly within a single oscillation unit, with the deepest penetration occurring when the laser traversed previously molten material. Simulations accurately captured the keyhole fluctuations, with good agreement between experimental and simulated average keyhole depth. The formation of a characteristic chevron pattern on the solidified melt track surface was directly linked to the laser oscillation movement, with the number of chevron units matching the number of oscillation cycles. Laser oscillation effectively reduced the keyhole depth by distributing energy over a wider area, similar to beam defocusing or annular beam shaping but dynamically. Analysis of keyhole depth standard deviations indicated that larger oscillation diameters generally decreased keyhole fluctuations, potentially surpassing the stability of linear scans in specific parameter ranges. Frequency spectrum analysis of keyhole depth profiles revealed a characteristic peak corresponding to the oscillation frequency, further highlighting the influence of oscillation on keyhole behavior. Importantly, the study also demonstrated that laser oscillation can significantly increase the threshold for keyhole porosity formation. Even at higher energy densities, the oscillating beam produced fewer pores than the linear scan, suggesting that keyhole shape and stability play crucial roles in porosity suppression.

The findings directly address the research question of understanding high-frequency beam oscillation dynamics in laser melting. The observed continuous and periodic keyhole fluctuations, linked to both pre-heat conditions and transient speed, provide valuable insights into the complex interplay of thermal and fluid dynamics during the process. The successful validation of the experimental results against the multiphysics model strengthens the understanding of the underlying physical mechanisms. The significant reduction in keyhole depth and fluctuations achieved through laser oscillation offers a promising avenue for controlling melt pool morphology and minimizing defects. The direct correlation between oscillation cycles and the formation of the chevron pattern opens up new possibilities for precisely controlling surface texture and microstructure. Furthermore, the enhanced threshold for keyhole porosity formation demonstrates the potential of this technique for improving the overall quality of additively manufactured parts. This work contributes significantly to advancing the fundamental understanding and practical application of laser beam oscillation in metal additive manufacturing.

This study successfully characterized the keyhole dynamics of high-frequency beam oscillation in laser melting using a combination of synchrotron-based X-ray imaging and multiphysics modeling. The findings demonstrate the unique fluctuations of the oscillating keyhole and its impact on keyhole depth, stability, surface pattern formation, and porosity reduction. This technique shows promise for enhancing productivity and controlling melt pool shape in L-PBF. Future research should explore the detailed relationship between oscillation parameters, fluid dynamics, and resulting microstructure, and investigate the optimization of oscillation strategies for specific materials and applications.

The study was conducted using thin Ti-6Al-4V plates instead of powder beds, potentially influencing the results due to edge effects and the absence of powder interactions. The multiphysics model did not include metal vapor and cover gas dynamics, which could play a role in the actual process. Further research with thicker samples and the inclusion of these effects in the model will enhance the accuracy of the simulations and deepen the understanding of the process. The number of simulations performed was limited; therefore, a more extensive study with more simulations is needed to further validate the model and explore the impact of oscillation parameters and material properties more comprehensively.