【Abstract】Tungsten inert gas welding is a very important welding method in modern industrial manufacturing. This paper analyzes the stress of the stainless steel sheet welding pool and the welding deformation of the thin plate, and introduces the welding process essentials and practical application of manual tungsten inert gas welding of stainless steel thin plates.
Introduction
With the continuous development of modern manufacturing industry, stainless steel thin plates are widely used in defense, aviation, chemical industry, electronics and other industries, and the welding of 1-3mm stainless steel thin plates is also increasing. Therefore, it is very necessary to master the process essentials of stainless steel thin plate welding.
Tungsten inert gas welding (TIG) uses pulsed arc, which has the characteristics of low heat input, concentrated heat, small heat affected zone, small welding deformation, uniform heat input, and better control of line energy; the protective airflow has a cooling effect during welding, which can reduce the surface temperature of the molten pool and increase the surface tension of the molten pool; TIG is easy to operate, easy to observe the state of the molten pool, dense welds, good mechanical properties, and beautiful surface forming. At present, TIG is widely used in various industries, especially in the welding of stainless steel thin plates.
1. Technical essentials of tungsten inert gas welding
1.1 Selection of tungsten inert gas welding machine and power polarity
TIG can be divided into DC and AC pulses. DC pulse TIG is mainly used for welding steel, mild steel, heat-resistant steel, etc., and AC pulse TIG is mainly used for welding light metals such as aluminum, magnesium, copper and its alloys. Both AC and DC pulses use steep drop characteristic power supplies. TIG welding of stainless steel thin plates usually uses DC positive connection.
1.2 Technical essentials of manual tungsten inert gas welding
1.2.1 Arc starting
Arc starting has two forms: non-contact and contact short-circuit arc starting. The former has no contact between the electrode and the workpiece, which is suitable for both DC and AC welding, while the latter is only suitable for DC welding. If the short-circuit method is used to start the arc, the arc should not be started directly on the weldment, because it is easy to produce tungsten clamping or adhesion with the workpiece, the arc cannot be immediately stable, and the arc is easy to break through the parent material. Therefore, an arc starting plate should be used. A copper plate should be placed next to the arc starting point. The arc should be started on it first, and then the tungsten electrode head should be heated to a certain temperature before moving to the part to be welded. In actual production, TIG often uses an arc starter to start the arc. Under the action of high-frequency current or high-voltage pulse current, the argon gas is ionized and the arc is started.
1.2.2 Positioning welding
During positioning welding, the welding wire should be thinner than the commonly used welding wire. Because the temperature is low and the cooling is fast during spot welding, the arc stays for a long time, so it is easy to burn through. When performing spot fixed position welding, the welding wire should be placed at the spot welding part, and the arc should be moved to the welding wire after it is stable. After the welding wire melts and fuses with the parent materials on both sides, the arc is quickly stopped.
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1.2.3 Normal welding
When ordinary TIG is used for stainless steel sheet welding, the current is taken as a small value. However, when the current is less than 20A, arc drift is easy to occur, and the cathode spot temperature is very high, which will cause heating and burning in the welding area and deteriorate the electron emission conditions, causing the cathode spot to jump continuously, making it difficult to maintain normal welding. When pulse TIG is used, the peak current can make the arc stable and have good directivity, making it easy to melt the parent material and form it, and cyclically alternate to ensure the smooth progress of the welding process, so as to obtain a weld with good performance, beautiful appearance, and overlapping molten pools.
2. Weldability analysis of stainless steel sheet
The physical properties and plate shape of stainless steel sheet directly affect the quality of the weld. Stainless steel sheet has a small thermal conductivity and a large linear expansion coefficient. When the welding temperature changes rapidly, the thermal stress generated is large, and it is easy to burn through, undercut and wave deformation. Stainless steel sheet welding mostly adopts flat plate butt welding. The molten pool is mainly affected by the arc force, the gravity of the molten pool metal and the surface tension of the molten pool metal. When the volume, mass and molten width of the molten pool metal are constant, the depth of the molten pool depends on the size of the arc. The molten depth and arc force are related to the welding current, and the molten width is determined by the arc voltage.
The larger the molten pool volume, the greater the surface tension. When the surface tension cannot balance the arc force and the gravity of the molten pool metal, it will cause the molten pool to burn through. In addition, the weldment will be locally heated and cooled during the welding process, causing uneven stress and strain. When the longitudinal shortening of the weld produces stress on the edge of the thin plate that exceeds a certain value, it will produce more serious wave deformation, affecting the appearance quality of the workpiece. Under the same welding method and process parameters, using tungsten electrodes of different shapes to reduce the heat input on the welding joint can solve problems such as weld burn-through and workpiece deformation.
3. Application of manual tungsten inert gas welding in stainless steel sheet welding
3.1 Welding principle
Tungsten inert gas welding is an open arc welding with stable arc and concentrated heat. Under the protection of inert gas (argon), the welding pool is pure and the weld quality is good. However, when welding stainless steel, especially austenitic stainless steel, the back of the weld also needs to be protected, otherwise it will cause serious oxidation, affecting the weld formation and welding performance.
3.2 Welding characteristics
The welding of stainless steel sheet has the following characteristics:
1) The thermal conductivity of stainless steel sheet is poor and it is easy to burn through directly.
2) No welding wire is required during welding, and the parent material is directly fused.
Therefore, the quality of stainless steel sheet welding is closely related to factors such as operators, equipment, materials, construction methods, external environment during welding and detection.
In the welding process of stainless steel sheet, welding materials are not required, but the following materials are required to be relatively high: First, the purity, flow rate and argon flow time of argon gas, and second, tungsten electrode.
1) Argon
Argon is an inert gas and is not easy to react with other metal materials and gases. Because its gas flow has a cooling effect, the heat affected zone of the weld is small, and the deformation of the weld is small. It is the most ideal shielding gas for tungsten inert gas arc welding. The purity of argon must be greater than 99.99%. Argon is mainly used to effectively protect the molten pool, prevent air from eroding the molten pool and causing oxidation during welding, and effectively isolate the weld area from air, so that the weld area is protected and the welding performance is improved.
2) Tungsten electrode
The surface of the tungsten electrode should be smooth, the end must be sharpened, and the concentricity is good. In this way, the high-frequency arc is good, the arc stability is good, the melting depth is deep, the molten pool can remain stable, the weld is well formed, and the welding quality is good. If the surface of the tungsten electrode is burned or there are defects such as pollutants, cracks, shrinkage holes, etc. on the surface, high-frequency arc is difficult to start during welding, the arc is unstable, the arc has drift, the molten pool is dispersed, the surface is expanded, the melting depth is shallow, the weld is poorly formed, and the welding quality is poor.
4. Conclusion
1) Tungsten inert gas arc welding has good stability, and different tungsten electrode shapes have a great influence on the welding quality of stainless steel thin plates.
2) Flat-top cone-end tungsten inert electrode welding can improve the double-sided forming rate of single-sided welding, reduce the welding heat affected zone, make the weld beautiful, and have good comprehensive mechanical properties.
3) Using the correct welding method can effectively prevent welding defects.
Post time: Aug-21-2024
