Effect of laser cleaning modification on mechanical properties of Kovar/CFRP lap welded joint

Objective The effective connection between Kovar alloy and CFRP is one of the important means to solve the weight problem of aerospace components. Methods In this paper, a nanosecond pulse laser was employed to modify the surface of Kovar alloy, and then welding heat conduction technology was used to achieve the overlap between Kovar alloy and CFRP. The surface of the modified material, the microstructure of the overlap welding interface, and the fracture surface of the alloy side were characterized and analyzed using a laser confocal microscopy, a metallographic microscopy and a scanning electron microscopy to elucidate effect of laser modification on the shear tensile force of the overlap welding joint. Results When the pulse laser scanning speed was relatively lower at 500 mm/s, the surface roughness of Kovar alloy could reach a maximum of 14.2 μm, which was significantly higher than that of 8.8 μm without modification. When further increasing the laser scanning speed to 1 000 mm/s and 2 000 mm/s, the surface roughness of Kovar alloy decreased to 12.6 μm and 10.2 μm, respectively. The surface of the material exhibited significant erosion after modification, and a large number of gully shaped erosion pits were distributed in the modified surface. When unmodified, the average shear tensile force of the Kovar alloy and CFRP lap welded joint was only about 450 N. When the laser scanning speed was 500 mm/s or 1 000 mm/s, the maximum tensile shear force of the joint could reach 1 100 N, which is nearly 2.5 times higher than that without laser cleaning modification. When the scanning rate was 2 000 mm/s, the shear tensile force of the joint decreased, but it was still significantly higher than the performance of the unmodified welded joint. The shear tensile force of the Kovar alloy and CFRP lap welded joint significantly increased after laser modification. Thereby, the shear tensile strength of Kovar/CFRP overlap welding joint could be improved significantly. Moreover, the fracture results also showed that a large amount of filled resin and some carbon fibers were distributed in the pits. Conclusion The deep and wide erosion pits not only facilitated the filling of molten resin at CFRP side, but also facilitated the filling of carbon fibers, promoting the formation of mechanical interlocking structures in the overlap welding interface.