Liquid ramjet is the optimum power device for high-dynamic near-space vehicles because of its high thrust-weight ratio, simple structure, and lightweight. Ramjet technology is being extensively developed by the military around the world. However, during a rigorous test, the welding spots between the flame tube and reinforcement ring of a ramjet, which was identified as the weak link, were damaged. The flame tube and strengthening ring are both made of GH3230 superalloy. First, resistance spot welding is used to weld the two parts. Moreover, laser welding was chosen as the welding method because of its high energy density, fast welding speed, and excellent welding quality. However, there is still the possibility of weld tearing between the two parts. Currently, the shape of the laser welding spot is a popular study area, and optimizing it can significantly increase welding strength and quality. Thus far, several researchers have proposed various welding shapes, such as ring- and C-welding spots. It was found that the shear performance of welding spots has a positive connection with the area of fusion surfaces. Furthermore, the ring-welding spot welding path is considered a closed curve, which is unfavorable to stress release. The C-welding spot welding path is open and can release stress; thus, it is superior to the ring-welding spot. However, the existing weld shapes still have insufficient welding strength, which reduces the reliability and assembly precision of the products. Therefore, four weld shapes, such as ring-, C-, oval-, and S-welding spots, were designed based on the real structure of the product. The differences in performance between the welding spots were compared to determine the best welding scheme. This study can be used as a reference for welding ramjet products, and it has both innovative and practical values.
Laser welding experimental research was conducted for lap welding of GH3230 superalloy sheets. The differences in the weld formation, mechanical properties, fracture behavior, and microstructure of the welding spots were compared. The thickness of plate Ⅰ, the thickness of plate Ⅱ, and the width of the overlap region were 1, 0.6, and 12 mm, respectively. The welding equipment is a UPRB4600 laser welding machine, which is equipped with a 2 kW fiber laser system. During welding, a single pass was performed; the defocusing distance was set to 0 mm and three specimens were welded for every process parameter. After welding, the microstructure of the weld joint was observed using a digital optical microscope; the welding strength was tested using a universal testing machine, and the fracture morphologies was analyzed using a scanning electron microscope.
Four weld shapes were designed when the overlap zone was certain: ring-, C-, oval-, and S-welding spots (Fig. 1). The differences in the weld formation, mechanical properties, fracture behavior, and microstructure of the welding spots were compared, and the following results were obtained: 1) four weld shapes showed significant adaptability in the weld formation and microstructure (Fig. 3 and Fig. 12 ). The weld width increased as the heat input increased, and the increase in the back weld width was greater than that of the top weld width (Fig. 4). 2) The shear performance of the welding spots was related to the area of the fusion surface, which increased as the fusion area increased. The S- and oval- welding spots had higher welding strength followed by the C- and ring-welding spots (Fig. 6). In the ring- and C-welding spots fracture modes, the weld departed from the interface, and the weld broke along the cross-section of the oval- and S-welding spots (Fig. 11). 3) The S-welding spot showed the best performance among four welding spots when the overlap zone was certain because of its geometric shape. The product of the flame tube was welded using the S-welding spot, resulting in a good welding quality. The welding strength of the S-welding spot was 1.8 times stronger than that of the ring-welding spot. Other thin-wall products that require high welding strength can benefit from the S-welding spot.
Herein, the effects of four welding spots shapes on welding properties were studied and the optimal welding scheme was determined. The results show that the S-welding spot has better tensile-shear performance and weld formation when the overlap zone is certain because it has a large welding area, and the welding path is open, thus releasing stress. The tensile-shear strength of the improved weld is 1.8 times stronger than that of the original welding spot (C-welding spot). The S-welding spot is suitable for welding thin-wall products, such as flame tubes.
