Jianglin Zou

High-power laser manufacturing relies on the optical-thermal effect for material machining, which represents its core working mechanism. In this paper, through in situ optical observation and simulation experiments, the conversion of optical to thermal energy and coupling characteristics inside the keyhole were studied. During the evolution from molten pool formation to keyhole initiation, melting efficiency rises gradually from zero and experiences an obvious jump at the moment of keyhole formation, with its maximum increment reaching 23.5%. As the keyhole continues to deepen, the melting efficiency eventually stabilizes, reaching a balance between the energy dissipated through heat conduction and the energy absorbed by the keyhole. Inside the keyhole, laser beams primarily interact with the front keyhole wall (FKW). Repeated multireflection and energy absorption of laser beams on keyhole inner walls constitute the primary cause of the sharp increase in overall energy absorptivity and melting efficiency. The laser energy reflected from FKW to the rear keyhole wall causes only 17.9% of the melting induced by FKW's primary absorption, having a limited impact on keyhole depth. Consequently, the direct absorption of laser energy on the FKW surface is the key factor determining the keyhole depth.

Laser-arc interaction is the key to producing the “1 + 1 > 2” synergistic enhancement effect in laser-arc hybrid welding. This study explores the interaction process between the laser and arc in hybrid welding by collecting the characteristic line-spectrum optical signals from plasma radiation during laser-TIG hybrid welding of 5083 aluminum alloy, combined with signal analysis methods such as empirical mode decomposition (EMD). EMD can decompose the original optical signal into 10 intrinsic mode function (IMF) components, where the high-frequency component IMF2 corresponds to the ionization effect of the arc and the medium-low frequency components IMF6 and IMF7 characterize the influence of laser on plasma characteristic behavior. Increasing laser power, decreasing welding speed, and reducing heat source spacing, all of which enhance the proportion of laser energy in the hybrid heat source, significantly improve the correlation between IMF6 and IMF7 components and the original signal. Increasing arc current significantly raises the correlation of IMF2, suggesting that the arc ionization effect strengthens with the increase in arc energy proportion. The method proposed herein overcomes the generalized analysis mode of traditional broad-spectrum signals, enabling high-sensitivity capture of the interaction intensity and fluctuations between arc plasma and metallic plasma in the keyhole opening region. This approach is expected to advance the development of process signal acquisition in intelligent hybrid welding processes.

The dynamic behavior of the molten pool surface during laser spot welding significantly impacts forming quality. This study combines numerical simulations and experiments to comprehensively investigate the effect of surface oxidation in an air atmosphere on the molten pool flow behavior and solidification process during laser spot welding. Throughout the laser processing, the metal surface undergoes three stages: solid-phase heating, molten pool expansion, and molten pool solidification. The durations of solid-phase heating and molten pool expansion are similar in both argon and air, but the solidification time in air is 63.7% longer than in argon. During the early stages of processing in air, the expansion rate of the molten pool increases rapidly, with the expansion rate in air reaching twice that in argon. During the solidification process in air, within 0.3–1.2 ms after the laser is turned off, strong oscillations appear on the surface of the molten pool, and it is observed that the contraction rate of the molten pool during solidification slows down. This oscillatory behavior is the main cause of the ripple-like texture on the solidified surface in air and leads to a significant increase in roughness. During solidification, rapid oxidation and the subsequent drop in surface temperature change the temperature dependence of surface tension. The surface tension's temperature coefficient shifts from negative to positive, reversing the Marangoni flow direction from outward to inward. With extended solidification time and gravity's influence, this reduces the final residual height.

The physical essence of laser welding lies in the utilization of the photothermal effect of lasers. This paper systematically investigates the influence of laser power variations on weld morphology, molten pool characteristics, melting efficiency, and laser absorption rate by fixing the welding speed and plate thickness while adjusting the laser power to alter the plate's penetration state. It also analyses the photothermal energy conversion characteristics under different penetration states. The surface dimensions of the molten pool, the total absorption rate of the material, and the melting efficiency all first increase and then decrease with the increase in fiber laser power. As the fiber laser power increases, the weld seam successively exhibits the typical states of incomplete penetration, moderate penetration, and overpenetration. The cross section of the weld seam transitions from a “Y” shape in the incomplete penetration state to an “X” shape in the moderate penetration state, with further changes in the overpenetration state. Among these, the moderate penetration stage features the largest molten pool size, the most stable weld surface formation, and the highest melting efficiency and total absorption rate. In the incomplete penetration state, laser energy struggles to penetrate deeply into the material, resulting in low melting efficiency. In the overpenetrated state, the front wall of the keyhole tends to become vertical, and the rear portion of the laser spot directly penetrates the material, causing the absorption rate and melting efficiency to decrease.

The behavior of laser-TIG hybrid welding plasma is closely related to the welding process. In order to obtain the behavior of plasma more quickly, this paper proposes a new method for in situ monitoring of plasma by acquiring the line spectrum of plasma radiation. By applying narrow-band filtering, the time-varying intensity profiles of specific Mg-I spectral lines (470, 518, and 552 nm) in the plasma radiation generated during the hybrid welding of 5083 aluminum alloy are selectively obtained. These optical signals are then denoised using wavelet packet transform and Savitzky–Golay smoothing filters. Further analysis is performed using empirical mode decomposition, fast Fourier transform, and Hilbert transform to extract comprehensive time, frequency, and time–frequency features. The results demonstrate that, compared to full-spectrum acquisition, the selectively captured line spectra exhibit more pronounced variations in signal intensity and frequency, making them more representative of plasma behavior. Among them, the Mg-I 518 nm line—with the highest intensity—exhibits the strongest sensitivity to changes in plasma dynamics. This targeted spectral approach significantly enhances the accuracy and responsiveness of real-time monitoring. The findings provide a promising strategy for the intelligent in situ monitoring of laser-TIG hybrid welding processes, contributing to improved weld quality and process stability.

The establishment of an accurate prediction model of keyhole depth during laser welding is of great significance for predicting the weld depth or pre-selecting suitable welding process parameters. In this paper, based on the primary absorption of the incident laser by the front keyhole wall, the tilt angle of the keyhole wall was calculated point by point according to the equilibrium relationship between the laser energy absorbed by the material and the energy required for evaporation at any position of the front keyhole wall. A prediction model for the keyhole depth in laser deep penetration welding has been established by taking into account the laser optical parameters, the focusing system parameters, the welding process parameters, and the material properties. The transformation law of the front keyhole wall profile, the Fresnel absorption coefficient, and the keyhole depth with laser power, welding speed, focusing focal length, and other parameters was calculated in the model, which is basically in line with the consensus. At the same time, the keyhole depth results of calculations and measurements under some parameter conditions were compared. It can be found that the prediction model of keyhole depth during laser deep penetration welding can be used to approximate the calculation of weld depth, and the feasibility of the model was preliminarily verified. Moreover, combined with previous experimental results, the conclusion that the primary absorption of front keyhole wall is the key factor determining the keyhole depth can be re-verified by using mathematical calculations. It also shows that the complex energy coupling law in keyhole has a noticeable difference in the influence of different physical phenomena during laser deep penetration welding.