Statistical assessment of keyhole stability in laser beam welding assisted by oscillating magnetic fields

Keyhole instability is a critical challenge in high-power laser beam welding as it can induce defects such as porosity, spatter, and spiking. However, conventional methods for evaluating keyhole stability, based on the transient keyhole geometry or keyhole depth variation, are limited in accuracy and statistical significance. To address this, a novel evaluation framework from a statistical perspective is proposed in this paper. Oscillating magnetic fields were employed as an active control strategy to generate different levels of keyhole stability, thereby validating the applicability and effectiveness of the proposed framework under different conditions. This method is developed based on a transient three-dimensional multi-physics coupled model incorporating with oscillating magnetic fields. By calculating the equivalent keyhole diameter based on the gas phase area at each discrete layer, the two dimensional keyhole morphology on each layer is reduced to a one dimensional diameter, which is then used to quantify keyhole stability. The spatial average of the keyhole diameter standard deviation is proposed as a metric to quantify keyhole stability, providing a multi-dimensional and statistically robust assessment. Using this novel approach, it is demonstrated that the application of oscillating magnetic fields can significantly enhance keyhole stability, with an improvement of up to 17.5% at 280 mT compared to the reference case. This provides direct statistical evidence that magnetic fields can stabilize the keyhole. Furthermore, a clear and statistically meaningful time-averaged keyhole morphology has not yet been obtained. A time-averaged keyhole reconstruction method is proposed to investigate the time-averaged keyhole morphology. The reconstructed profiles, over a 250 ms time span, exhibit banana-like shapes and capture trailing tails at the keyhole bottom.