Tao Liu

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.