Abstract : The theory of fluid stability with free surface is applied to explain the mechanism of hump weld bead during high-speed welding. The theory is verified by real-time image acquisition of the molten pool formation process. Theoretical analysis and experimental results show that the welding is carried out in the form of free transitional droplet transfer. The workpiece is always under the heating of the welding arc. During high-speed welding, the molten pool cannot be cooled in time, the molten pool is obviously elongated, and the vibration caused by the disturbance The wavelength is large. When the elongation of the molten pool reaches a certain level, the instability and necking of the liquid metal may occur, resulting in a hump bead. On this basis, it is proposed that the short-circuit transition can be used instead of the free transition under high voltage. When the short-circuit transition is used, the periodic arc extinguishing can reduce the heating amount of the molten pool and prevent the molten pool from being too long and unstable. The test results show that the short-circuit transition welding can effectively prevent the occurrence of hump weld bead and improve the welding speed.
Key words: Stability of high-speed welding weld puddle
0 Preface
Welding is an important part of industrial production, and its processing speed has a significant impact on overall productivity. In order to improve production efficiency and enhance market competitiveness, production companies have put forward more and more urgent requirements for improving welding speed. However, an increase in the welding speed brings about some problems different from those in conventional speed welding. The most important of these is the poor weld formation, the phenomenon of beading of the weld bead, and the “humped†humping [1, 2] occurs when the speed is further increased. In the past work, the author has studied the mechanism of weld beading during high-speed CO 2 welding [3]. In this paper, the high-speed CO 2 welding is still the object of the theoretical analysis and experimental research on the mechanism of the "hump" weld bead, and on this basis, the method to prevent this defect is tried.
1 Morphological characteristics of hump defects
The so-called "hump" weld bead refers to a weld bead forming defect in which the weld metal is distributed unevenly along the welding direction. Figure 1 is a view of a typical hump bead, and Figure 2 is a cross-sectional photograph of the bead, wherein Figure 2a is the "peak" section of the hump and Figure 2b is the "bottom" section. It can be seen that although the weld penetration and the melt width change little, the weld metal cross section has undergone a great change. In addition, it is worth noting that the distribution of hump defects along the weld is relatively regular, so it can be said that this phenomenon is not caused by accidental changes in welding conditions and parameters, but is determined by the special force of the molten pool during high-speed welding. of.
Figure 1 Hump weld bead appearance 
Figure 2 Cross section of the hump weld
2 Relationship between weld formation and weld pool stability
2.1 Stable conditions of the weld pool
The shape of the weld pool and the final weld formation are the result of a combination of various external forces, such as arc force, gravity, surface tension, etc. The characteristics of the molten pool during high-speed welding are mainly the slender shape, and the free surface of the molten pool is large. At this time, the influence of surface tension on the formation of the weld is particularly prominent. The theory of fluid stability with free surface can be used to better understand the mechanism of the formation of hump bead. As a qualitative analysis, the weld pool can be simplified to a section of liquid column maintained only by surface tension, as shown in Figure 3.
Figure 3 shows the instability principle of the liquid column
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