0Cr18Ni9 不锈钢高频感应辅助激光钎焊工艺及组织性能研究

Objective To address the insufficient filling depth of Cu-Ni brazing material in single-laser brazing of 0Cr18Ni9 stainless steel lap joints and the difficulty in regulating heat input in traditional brazing processes, this study proposes a high-frequency induction-assisted laser brazing technique. The effects of preheating temperature, laser heat input, and laser spot position on the wetting behavior of the filler metal, the filling depth, and the joint performance are systematically investigated. The diffusion mechanism of interfacial elements and fracture behavior are also revealed. The study aims to provide a novel approach for improving the brazing quality and the joint performance of 0Cr18Ni9 stainless steel, which is widely used in aerospace applications due to its excellent corrosion resistance and mechanical properties. Methods Before the experiments, the 0Cr18Ni9 stainless steel plates and the HBCu68NiSiB filler wires are polished with sandpaper and cleaned with anhydrous ethanol to ensure surface cleanliness and enhance brazing performance. To improve the wetting of the filler metal into the gap between the plates, a 201 flux is uniformly applied to the welding area. The gap between the plates is precisely controlled at 0.15 mm using a feeler gauge. The brazing system integrates a six-axis robot, a fiber laser, an automatic wire feeder, and a high-frequency induction heating device. During laser brazing, the process parameters are optimized as follows: 15 L/min Ar shielding gas flow, 8 mm/s welding speed, 0.5 m/min wire feed speed, 600 ‒ 800 W laser power, 400 ‒ 800 ℃ preheating temperature, +5 mm defocus distance, and 2.5 mm laser spot diameter. Temperature distribution during the process is monitored using an infrared thermal imaging camera to ensure precise control of the thermal field. The wetting depth of the brazed joints is analyzed using a metallographic microscope, and the microstructure of the brazed seams is characterized via scanning electron microscope (SEM). The mechanical properties of the joints are evaluated using an electronic universal testing machine for room-temperature and 200 ℃ high-temperature tensile-shear tests, with three replicates conducted to ensure data reliability. Results and Discussions The experimental results indicate that preheating the stainless steel substrate is effective in elevating its surface temperature, which facilitates the molten filler metal to fill the gap between the plates. Compared to single-laser brazing, the high-frequency induction-assisted process results in a smaller wetting angle (Fig. 3). As the preheating temperature increases, there is a continuous increase in the filling depth. At a preheating temperature of 800 ℃, the filling depth reaches 1300 µm, and the wetting angle decreases to 43° (Fig. 5). Increasing the laser heat input effectively reduces the wetting angle, but has limited impact on the filling depth. Excessive heat input causes substrate burning. Moving the laser spot 0.5 mm to the left increases the filling depth to 2000 µm (Fig. 9). Observations from the induction-assisted brazed joints reveal island-like precipitates, primarily composed of CuNi solid solution (Fig. 10). Energy-dispersive spectroscopy (EDS) and X-ray diffractometer (XRD) analyses (Fig. 11) indicate that these precipitates gradually accumulate at the substrate interface as the preheating temperature increases, forming a distinct interfacial layer at 800 ℃. The diffusion of Fe, Cr, and Cu elements at the interface is confirmed by line scan analysis (Fig. 12). The average tensile-shear strength of the joints exhibits a linear growth trend with increasing preheating temperature (Fig. 14). At 800 ℃, the room-temperature tensile-shear strength reaches 478 MPa, equivalent to 92.1% of the base material tensile strength. Even at 200 ℃, the high-temperature tensile-shear strength remains as high as 445 MPa, corresponding to 85.5% of the base material strength (Fig. 15). The fracture surfaces displays typical ductile fracture characteristics, with fibrous morphology and dimples observed under scanning electron microscope (SEM) (Fig. 16). Conclusions In this study, the high-frequency induction-assisted laser brazing process achieves effective gap filling between the stainless steel plates. Increasing the induction preheating temperature enhances the filling depth and reduces the wetting angle. During the process, laser energy primarily melts the filler metal, while adjusting the laser spot position allows partial preheating of the substrate, promoting filler wetting and penetration. Elevated preheating temperatures facilitate elemental diffusion between the filler and the substrate. The CuNi solid solution in the brazed joints gradually deposits at the substrate interface as the preheating temperature increases, forming a distinct interfacial layer at 800 ℃ . This interfacial layer, enriched with Cu and Ni elements, significantly enhances the metallurgical bonding between the filler and the substrate. The synergistic mechanism of increased filling depth and elemental diffusion effectively improves the interfacial bonding quality, thereby enhancing the tensile-shear load-bearing capacity of the joints. The optimization of process parameters not only addresses the challenge of insufficient filling depth but also demonstrates the potential of this technique for high-precision joining applications in demanding industrial environments.