Effects of ultrasonic vibration on the strain field during laser cladding process (invited)

Objective Laser cladding, with the advantages of a small heat-affected zone, dense microstructure, and high bonding strength, is widely used in aerospace, marine, and other fields. However, the intense heating and cooling processes result in uneven strain, which leads to a decline in the mechanical properties of the formed parts. Ultrasonic vibration has shown significant effects in enhancing melt pool flow, reducing temperature gradients, and promoting uniform microstructure distribution, offering potential to reduce and homogenize the strain. Therefore, this paper carried out an ultrasound-assisted laser cutting test on a 316L stainless steel substrate to study the effect of ultrasound on the melting process. Current research on ultrasonic-assisted laser cladding predominantly relies on simulations and post-processing analyses, which do not allow for real-time stress observations. To address this limitation, this paper introduces a method utilizing Digital Image Correlation (DIC) technology to generate speckle patterns appropriate for strain measurement near the melt pool during the ultrasonic-assisted laser cladding process. Corresponding criteria for evaluating speckle quality are established. This method enables real-time strain monitoring, and the effects of ultrasound on strain distribution are analyzed, providing valuable insights for controlling strain during laser cladding.

Methods The experimental setup for ultrasonic-assisted laser cladding (Fig.1) primarily comprises a fiber-coupled semiconductor laser, a motion control system, a wire feeder, a high-speed video system, and an ultrasonic vibration device. The substrate and wire utilized in these experiments consist of 316 L stainless steel plates. This study builds upon established DIC speckle quality evaluation methods by incorporating the Structural Similarity Index (SSIM) algorithm and the number of speckle autocorrelation peaks to propose an optimized metric for assessing speckle quality. This metric guided the selection of speckles used in the experiments (Fig.6). High-temperature speckles were engraved onto the substrate surface using a laser marking machine, and a high-speed camera was employed to capture images during the ultrasonic-assisted laser cladding process. The captured images were subsequently analyzed using DIC to generate strain contour maps (Fig.8). Additionally, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized to compare cladding layers with and without ultrasound, thereby analyzing the mechanisms by which ultrasound affects strain distribution.

Results and Discussions Based on the simulation results of speckle generation using various methods (Fig.5), the optimized speckle quality evaluation method proposed in this paper demonstrates an advantage in identifying speckle periodicity. This advancement facilitates the preparation of speckles suitable for DIC analysis under high-temperature and high-light-intensity conditions in laser cladding (Fig.6). DIC strain analysis results indicate that, compared to cladding without ultrasound, the strain distribution around the melt pool becomes significantly more uniform with ultrasonic assistance (Fig.8). The standard deviation of strain distribution along a specified line in the cladding direction decreases by up to 82.93%, while in the vertical direction, it decreases by up to 67.47% (Fig.9). Under ultrasonic action, the strain peak at point P₂, located 1.5 mm from the cladding layer in a direction perpendicular to the layer, is reduced by 23.91% (Fig.11). When ultrasonic vibration is applied, ripples form on the surface of the molten pool, intensifying the flow within the pool. This phenomenon promotes a more uniform temperature and structural distribution, thereby reducing the occurrence of stress concentrations (Fig.12). SEM and EDS analyses of the precipitated phases in the cladding layer reveal that, following the application of ultrasound, the number of hard granular precipitates rich in Si and Mn is significantly reduced, along with a slight decrease in the corresponding elemental concentrations within the precipitates (Fig.13 and Fig.14). The primary effect of ultrasound is observed in the fragmentation of primary dendrites and the reduction of precipitated phases.

Conclusions Based on the MIG evaluation system, we propose a speckle quality evaluation metric tailored for strain observation during the laser cladding process. This metric considers the structural similarity of patterns and the number of autocorrelation peaks, and it can effectively guide speckle preparation for real-time strain observation using DIC on the laser cladding surface. The application of ultrasonic vibration results in a more uniform strain distribution in the area near the melt pool. Specifically, the standard deviation of strain along the cladding direction decreases by up to 82.93%, while in the vertical direction, it decreases by up to 67.47%. Under the influence of 2500 W ultrasonic vibration, the positive strain amplitude in the analysis area along the cladding direction decreases from 0.0100 to 0.0047, and the negative strain amplitude decreases from −0.0050 to −0.0041. In the vertical direction, the positive strain amplitude decreases from 0.0169 to 0.0094, while the negative strain amplitude decreases from −0.0079 to −0.0012. With the application of 2500 W ultrasonic vibration, the maximum reduction in strain peaks in the x and y directions during the entire cladding process can reach 30.15%. The effect of ultrasonic vibration on reducing strain during laser cladding is significant. Following the application of ultrasound, the number of granular precipitates in the cladding layer is markedly reduced, and the local network-like precipitates are disrupted, which facilitates stress relief and contributes to a more uniform strain distribution. Additionally, noticeable fluctuations occur on the surface of the molten pool, intensifying the flow within the pool. The suppression of granular phase precipitation and disruption of mesh-like structures contribute to stress relief and a more uniform strain distribution.