Laser cladding, a new type of surface strengthening and repair technology, has been widely used in the automobile and aerospace fields for parts repair. Owing to the limitations of the laser spot size and robot travel range, meeting the industrial production needs of large-area surface modification and remanufacturing is challenging for single-pass cladding. During multipass cladding, a high-energy-density laser beam induces the secondary melting and solidification of alloy powder and the substrate surface coating. This process alters heat conduction and the melt flow mode inside a molten pool, resulting in an instantaneous and complex dynamic evolution of the molten pool. Numerical simulation can deeply analyze such complex phenomena inside a molten pool, offering better insights into the influence mechanism of the melt flow and temperature distribution on the molding and quality of the cladding layer. In this study, we simulated the multipass cladding process, revealed the evolution of the temperature and flow fields, studied the influence of the overlap ratio on cladding layer flatness, and observed the morphology and microstructure of the cladding layer under the optimal overlap ratio through experimental methods. The results of this study will provide a valuable reference for analyzing the evolution law of the temperature and flow fields of a multipass molten pool through numerical simulation and determining the appropriate multipass cladding process parameters.

Considering the temperature and laser attenuation coefficient of the cladding powder, and the Gaussian distribution powder equation, a multiphase flow model of multipass laser cladding was established via numerical simulation. First, the evolution of the internal temperature and flow fields within a molten pool during the multipass cladding process was analyzed. Second, numerical models for multipass cladding under different overlap ratios of 25%‒60% were established to study the effects of the overlap ratio on the internal flow field of multipass molten pools, and the surface flatness values of the cladding layer under different overlap ratios were obtained; moreover, the overlap ratio yielding a highly flat cladding layer surface was determined. Finally, the laser cladding experiment on a 42CrMo substrate was performed under the optimal overlap ratio, and the microstructure of the cladding layer was observed.

In the first cladding pass, the temperature gradient distribution of a molten pool exhibits characteristics of high in the middle and low on both sides. During subsequent laser cladding, this gradient gradually flattens as the number of cladding passes increases (Fig. 6). Owing to the preheating effect from previous cladding passes, initial temperatures at three observation points gradually increase with each additional cladding sequence. In addition, the secondary peaks in the temperature curves at observation points D and E align with the peak temperature of subsequent measurement points. These peaks exceed the liquidus temperature of the cladding powder, indicating remelting at these points. This remelting causes the previous cladding layer to re-form into a molten pool (Fig. 9). During the first cladding pass, two stable and symmetrical counterclockwise swirls form within a molten pool under the combined action of Marangoni forces and buoyancy. In subsequent cladding passes, the lapped section of the cladding layers remelts into a molten pool. Such molten pools take on an obliquely elliptical shape, generating a gravitational component to the unlapped side, breaking the original force balance, and affecting the flow field motion law dominated by Marangoni forces. And thus the two internal swirls gradually become asymmetrical. This phenomenon becomes more pronounced as the number of cladding passes increases, resulting in changes to the microstructure and uneven topography of the cladding layer (Fig. 11). The flatness of the cladding layer first increases and then decreases as the overlap ratio increases. Flatness reaches its maximum value at a 40% overlap ratio (Table 4). At an overlap ratio of 35%, two swirls of different sizes form within a molten pool. At an overlap ratio of 45%, the overall height of the cladding layer increases, and surface tension on the overlap side is almost balanced by gravity. Consequently, only one swirl remains in the molten pool; furthermore, maintaining uniform melt flow is difficult and thus considerably decreases cladding layer flatness (Fig. 13). At an overlap ratio of 40%, the microstructure of the cladding layer transitions from planar crystals at the bottom to columnar, cellular, and equiaxed crystals toward the top. The crystal distribution is relatively uniform, and the distribution of dendrite is consistent with the simulation results (Fig. 15).

The temperature gradient of single-pass cladding shows a pattern of high in the middle and low on both sides under the Marangoni flow effect. In multipass cladding, this gradient gradually flattens with increasing the number of cladding passes. The initial and peak temperatures of the cladding layer rise with each successive pass, and when the temperature in the subsequent cladding process exceeds the liquidus temperature, remelting of the previous cladding layer occurs. In single-pass cladding, two symmetrical swirls form within a molten pool flow field under the action of Marangoni forces, which guide a melt to exhibit a cyclic flow. As the overlap ratio increases, the influence of gravity on the flow field of the molten pool increases, eventually causing the swirl on the lap side to disappear. This phenomenon changes the microstructure and causes the uneven surface morphology of the cladding layer. In addition, with the increase in the overlap ratio, the flatness of the cladding layer shows a trend of first increasing and then decreasing, and the optimal flatness value is observed at a 40% overlap ratio. Experimental results confirm that at this overlap ratio, the cladding layer exhibits a relatively flat surface with relatively uniform microstructure distribution.