High frequency beam oscillation keyhole dynamics in laser melting revealed by in-situ x-ray imaging

Abstract

The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique.

Laser welding monitoring with multisensory data fusion: A brief review

“…For in-situ observation of the laser welding process, the X-ray phase contrast method can be used for identifying keyhole and melt pool [6] [7]. The dynamic radiography, or the X-ray imaging with high frame rate, can reach 50 thousand frames per second, meaning the increment change in 0.02 millisecond can be analyzed [8]. The classical problem of laser-induced cavitation bubble dynamics can be investigated attributed to the highly resolved images [9] [10] [11].…”

Understanding Melt Pool Behavior of 316L Stainless Steel in Laser Powder Bed Fusion Additive Manufacturing

“…In situ technologies through high-speed imaging, e.g., using X-ray radiography and optical or thermal cameras, have been employed exclusively to investigate the melt pool dynamics and understand the laser-matter interaction [20][21][22][23][24][25][26]. Zhao et al [27] probed the dynamics of the Ti-6Al-4V in the LPBF process from the inside, as well as above the surface of, the powder bed using high-speed hard X-ray imaging and diffraction techniques, concluding that the competition between the Marangoni convection and the recoil pressure primarily determines the extent of the mass, the heat transfers, and the dynamics of the melting process.…”

In situ observation and reduction of hot-cracks in laser additive manufacturing

“…Another approach is to modify the laser energy application, e.g., Wu et al 28 introduced high-frequency laser oscillation as an alternative approach. By distributing laser energy over a wider area, laser oscillation effectively changed the thermal gradients, thus the solidification rates and the thermal stresses.…”