Experimental SectionExperimental materials and equipment The experimental material was ZA alloy, processed into a test plate with dimensions of 200mm × 140mm × 20mm using metal mold casting. The filler material was a welding rod made of ZA alloy of the same material as the base material through metal mold casting. By analyzing the effects of the composition of aluminum and its alloy flux, combined with the characteristics of ZA alloy itself, fluorides and chlorides were selected as the flux [2]. The flux used in this study was a mixture of KCl, KF, LiCl, and ZnCl2. Among them, KF has the characteristic of dissolving metal oxides and is added as a flux. Before the experiment, the flux was prepared into an aqueous solution, and then evenly applied to the surface of the welding rod and the test plate to be welded using a brush. The experimental equipment was an NSA-200 AC tungsten inert gas arc welding machine.Experimental MethodBefore welding, samples were taken from the test plate and measured using an X-ray diffractometer to determine the composition of the surface oxide film. First, surface cladding was performed without adding flux, and then flux was added to clad the test plate. During the welding process, the stability of the arc and the molten pool were observed, and the effect of the flux on the weldability was preliminarily judged. Weld seam samples were taken and metallographic samples were prepared, and the microstructure of the base material, weld seam, and fusion zone were observed under a metallographic microscope.

Experimental Results and DiscussionX-ray diffraction curve of the base material surface. From the X-ray diffraction curve of the base material surface, it can be seen that the main components of the surface oxide film are ZnO, Al2O3, MgAl2O4, and ZnAl2O4. The melting points of these oxide films are relatively high, such as ZnO 1975℃, Al2O3 2050℃, MgAl2O4 2130℃, and they are not easily decomposed at high temperatures. Macroscopic observations during the experiment found that less spattering occurred when flux was added, and the welding process was stable. This can be explained by the fact that the added flux removes the oxide film and ensures stable arc burning. The residue on the weld seam surface was collected after welding and subjected to X-ray diffraction analysis, and it was found that the phase composition of the surface residue was mainly a large number of oxides. This indicates that serious oxidation occurred during the welding process.Metallographic microstructure of ZA alloy base material (a), (b). (c) is the microstructure of the weld seam without flux. (d) is the microstructure of the weld seam with mixed flux added.

X-ray diffraction curve of the post-weld residue of the mixed fluxIn order to study its film removal mechanism, LiCl was used alone as the flux for welding. X-ray diffraction curve of the post-weld residue sample. Li2O, Li2O2, and ZnCl2 were found in the post-weld residue sample using LiCl flux, and it was observed that LiCl has a certain effect of removing the oxide film. This is because LiCl undergoes the following metallurgical reactions under welding conditions: The generated AlCl3 gas escapes from under the oxide film, breaking the oxide film and forming white welding smoke, while Li2O floats on the surface of the welding molten pool in a slag-like state, and may continue to be oxidized, undergoing the following reactionRegardless of whether flux is used or not, there is a large amount of oxide film on the weld seam surface. The main function of adding flux is to break the oxide film on the surface of the molten pool during welding and ensure stable arc burning. Among the added fluxes, only ZnCl2 can transfer alloying elements to the weld seam through metallurgical reactions to supplement the Zn evaporated and oxidized during welding.ConclusionThe main components of the oxide film formed on the surface of the base material are ZnO, Al2O3, MgAl2O4, and ZnAl2O4. The melting points of these oxides are relatively high, and they are not easily decomposed at high temperatures. Due to the faster cooling rate of the weld seam, its lamellar eutectic structure is smaller than that of the base material. The number of α′ dendritic crystals in the weld seam is more than that in the base material, and the relative amount of α′ + β binary eutectic structure is reduced. When flux is added, the number of daisy-like structures in the weld seam is more than when no flux is added, indicating that the flux has the function of transferring the alloying element Zn to the weld seam. The main function of adding flux is to break the refractory oxide film during the welding process, ensuring that the molten metal in the molten pool and the base material are in good contact and fusion; the flux can transfer Zn to the weld seam through metallurgical reactions to compensate for the evaporation and oxidation loss of Zn during welding.