What is the difference between weld metal and deposited metal, and how does the fusion ratio affect the structure and performance of the welded joint?
The weld metal consists of two parts: one part is the molten electrode or wire; the other part is the molten base metal. The deposited metal refers to the part of the electrode or wire that melts in the weld. The deposited metal is welded in the butt groove with a large gap (13mm) according to the standard. The weld metal is welded in a K-shaped groove with no gaps and has a large fusion ratio. All metal welding material manufacturers and the relevant standards for welding materials have been evaluated and accepted according to the deposited metal, and the influence of the base metal composition on the weld seam is completely omitted. Therefore, the performance data listed in the quality assurance letter of welding rod or welding wire is not the actual weld performance formed by actually welding a certain metal material. The two may be close, but they may also be very different. Welded joints using filler materials often have areas rich in filler material.
During the fusion welding process, both the base metal and the welding material are melted, and the weld metal is inevitably diluted by the base material. The penetration of the weld cross-sectional area of
θ = Am / （Ah + Am）
In the formula： Am——the cross-sectional area occupied by the base metal on the weld cross-section, mm2;
Ah——The cross-sectional area occupied by the filler metal on the weld cross-section, mm2.
The size of the fusion ratio is related to the welding method, joint form, welding parameters and thermal physical properties of the material. Opening the groove can reduce the fusion ratio. In multi-layer multi-pass welding, the fusion ratio of each welding layer is different, so the chemical composition and performance of the weld metal are also different.
In butt welds, as the groove angle increases, the fusion ratio decreases. When the shape of the groove and the weld pool changes, the fusion ratio will change. For example, after the weldment is opened, the fusion ratio will be significantly reduced compared with the I-shaped groove. When arc welding medium carbon steel, alloy steel and non-ferrous metals, the chemical composition of the weld is often adjusted by changing the fusion ratio to reduce the sensitivity of welding cracks and improve the mechanical properties of the weld metal. The effect of fusion ratio on the weld performance is related to the chemical composition of the welding material and the base material.
When the chemical composition of the welding material and the base metal are basically similar and the molten pool is well protected, the fusion ratio has no significant effect on the performance of the fusion zone of the weld. When the chemical composition of the welding material and the base metal are different, such as the content of carbon, alloying elements and S, P impurity elements are greatly different, then the chemical composition of the welding zone in the immediate vicinity of the fusion zone changes. The greater the fusion ratio, the greater the change in the chemical composition of the weld metal caused by the melting transition of the welding material into the weld. This will cause the microstructure of the fusion zone to be complicated, and under certain conditions, unfavorable tissue bands may also appear, resulting in greatly reduced performance of the welded joint.
