Ning Guo

2A12 aluminum alloy had the advantages of light weight and high strength. It could be used to manufacture the skin of the hypersonic aircraft. Due to the thermal deformation of fuselage and wing under long-term thermal and mechanical load, the accuracy of flight control was reduced. The shape memory effect of NiTi shape memory alloy (SMA) could be used to reduce the thermal deformation by realizing the laser welding of NiTi SMA and 2A12 aluminum alloy. According to previous studies on laser welding of NiTi SMA and TC4, the tendency to crack for the welded joints could be reduced by placing the laser beam on the side of TC4. Therefore, TC4 wire was used as the filling material. As the TC4 wire was constantly sent into the molten pool to absorb laser energy, the melting amount of NiTi SMA and 2A12 aluminum alloy were reduced. It was beneficial to reduce the formation of brittle intermetallic compounds. There were mainly the fusion zone (FZ), NiTi SMA/FZ interface, and the 2A12 aluminum alloy/FZ interface in the welded joints. With the increase of laser power, the growing distance of Ti ₂ Ni phase also increased. In addition, the Ti-Al intermetallic compounds and the fracture load of joints firstly increased and then decreased. When the laser power was high, Ni-Al intermetallic compounds increased. This caused the decrease of fracture load of welded joints. Besides, pores caused by the burning of elements in the FZ would also weaken the fracture load of welded joints. When the laser power was 2.4 kW, more Ti-Al intermetallic compounds appeared at the interface and the maximum fracture load of welded joint was 211 N mm ⁻¹ . The fracture mode was intergranular brittle fracture. The heat affected zone (HAZ) with optimal mechanical properties basically retained the shape memory effect of NiTi SMA.

The first study of thin-walled aluminum-alloy tubes with underwater-laser-nozzle in situ melting technology was carried out. The study mainly covered the influence of the water environment on the laser melting process, melting appearance, geometric characteristics, microstructure, regional segregation and microhardness. During the transfer of the cladding environment from air to water, the uniformity of the cladding layer became poor, but excellent metallurgical bonding was still obtained. The dilution rate (D) decreased from 0.46 to 0.33, while the shape factor (S) increased from 4.38 to 5.98. For the in-air and underwater samples, the microstructure of the melting zone (MZ) and the cladding zone (CZ) were columnar dendrites and equiaxed grains, respectively. In addition, the microstructure of the overlapping zone (OZ) was composed of columnar dendrites and equiaxed grains. The underwater average grain size was smaller than that of in-air. In addition, the water environment was beneficial for reducing the positive segregation in the columnar dendrite region. Compared with the in-air cladding sample, the precipitated phases in the OZ of the underwater cladding sample reduced. Under the combined action of grain refinement and precipitated phase reduction, the microhardness value of the underwater OZ was higher than that of the in-air OZ.

The aquatic environment has a significant impact on the efficiency of heat input to the base metal, significantly reducing the efficiency of the arc. As a result, in wet underwater welding, the penetration of the base metal is reduced compared to welding in the air with the same mode parameters. The purpose of the research was to determine the effectiveness of the influence of the parameters of the process of underwater wet welding with rutile-type flux-cored wire on the geometric parameters of the weld metal – its width and penetration depth of the base metal. In this case, the shape coefficient of the weld was determined - the ratio of width to penetration depth (W/P). It was found that the welding speed, wire feed speed and the amplitude of the welding torch oscillations have a significant impact on the ratio of the width to the penetration depth of the welds. The W/P ratio of welds decreases significantly with increasing welding speed without oscillation of the welding torch. So, when welding with transverse oscillations of the welding torch with an increase in the wire feed speed, it decreased from 7.14 to 3.85. And it linearly increased with increasing amplitude of the torch oscillations. It was found that the oscillation velocity, elongation of the flux-cored wire, and arc voltage have an insignificant effect on the W/P ratio.

Underwater wet welding technology is widely used. Because the stability of droplet transfer in underwater wet welding is poor, the feasibility of improving the droplet transfer mode has been discussed from various technical directions. In this work, the characteristics of pulsating wire feeding were studied in the pulsating wire feeding mode by investigating the effects of changing the pulsating frequency, the wire withdrawal speed, and the wire withdrawal quantity on the droplet transfer process and the welding quality. With the aim of improving weld forming and welding stability, the authors selected the coefficient of variation and the ratio of unstable droplet transfer as the indexes to evaluate the effect of droplet transfer control. The pulsating wire feeding process of underwater wet flux-cored wire was analyzed in depth, and the following conclusions were drawn: using the pulsating wire feeding mode and after comparing and analyzing the pulsed wire feeding process under the same frequency condition, the authors found that the forming and stability were better under the conditions of slower withdrawal speed and smaller withdrawal quantity. The short-circuit transition ratio decreased steadily with the increase of pulsating wire feeding frequency, the rejection transition ratio first rose and then decreased, and the splash ratio first decreased and then rose.