(2) The stability of the printing process is affected significantly by the process parameters owing to the low melting and boiling points of Zn. Therefore, LPBF machines equipped with the appropriate gas flow field can prevent Zn vapor from destroying laser propagation.

(3) When a laser source with Gaussian-distribution characteristics is used, the temperature in the central region of the molten pool exceeds the boiling point, even when a laser power as low as 30 W is used, which is not conducive to the stable formation of Zn. Laser-beam shaping or positive defocusing can be considered to weaken the high energy density in the central region of the laser to reduce evaporation, thus ultimately improving the forming quality of LPBF-printed Zn.

Laser powder bed fusion (LPBF) additive manufacturing technology has been widely utilized to fabricate degradable zinc (Zn) implants and is a novel approach for creating complex structures with controllable shape and exceptional performance. However, printing Zn is challenging owing to its evaporative nature and narrow fabricating window arising from its low melting and boiling points. Therefore, a comprehensive investigation must be conducted to reveal the mechanisms of heat and mass transfer in molten pool during LPBF, which can provide theoretical guidance for the optimization of printing-process parameters.

A mesoscopic-scale heat transfer and flow coupling model of molten pool during the LPBF of pure Zn was established using discrete-element and computational fluid dynamics methods. Single molten-track experiments were designed to verify the numerical model. The mechanisms by which the process parameters affect the temperature field, flow field evolution, and morphology of the molten track were discussed.

Pure Zn is sensitive to changes in transient heat input owing to its low melting and boiling points. Increasing the laser power significantly alters the molten-track size, peak temperature, and cooling rate. Specifically, when the laser power is increased from 30 W to 60 W and 90 W, the real-time volume of the molten pool increases nonlinearly by 510% and 1730%, respectively (Fig. 9). At higher scanning rates, more laser energy is absorbed by the surface of Zn powder, the length-width ratio of the molten pool changes gradually from 1.28 to 1.98, and the length-depth ratio changes from 1.61 to 3.45 (Figs. 4 and 5). Consequently, the molten pool is longer, shallower, and more narrow, thus resulting in larger temperature gradients along the direction of the molten-pool depth, with the maximum cooling rate increasing from 3.6×10⁶ K·s^-1 to 1.3×10⁷ K·s^-1 (Figs. 6 and 7). Furthermore, the real-time volume fluctuated considerably and erratically during molten-track formation. As the laser energy density within the molten pool increases further, the internal flow accelerates and the evaporation of Zn at the center becomes evident, thus changing the Marangoni convection caused by temperature gradient into evaporative recoil pressure as the dominant driving force for flow within the molten pool. The morphology of the printed molten tracks transformed from central point-like pits into continuous slit-like shapes (Fig. 11). The findings of this study can provide theoretical guidance for the evolution of the molten pool and for optimizing the LPBF processing of metals with low melting and boiling points.

(1) Significant evaporation is observed under high laser power during the LPBF printing of pure Zn, whereas the molten tracks indicate low stability at high scanning rates. Under laser power levels and laser scanning rates of 45?60 W and 300?600 mm·s^-1, respectively, the simulation results indicate strong metallurgical bonding between Zn powders and Zn substrate, thus implying the high stability of the molten tracks.