TIG welding is a very important welding method in modern industrial manufacturing. This paper analyzes the stress of the welding pool of stainless steel sheet and the welding deformation of the sheet, and introduces the welding process of manual TIG welding of stainless steel sheet. Essentials and practical application.
With the continuous development of modern manufacturing, stainless steel sheets are widely used in defense, aviation, chemical industry, electronics and other industries, and more and more 1-3mm stainless steel sheets are welded. Therefore, it is necessary to master the process of stainless steel sheet welding. .
TIG welding (TIG) uses pulsed arc, which has the characteristics of low heat input, heat concentration, small heat affected zone, small welding deformation, uniform heat input, and better control of line energy; The cooling effect 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, the weld is dense, the mechanical properties are good, and the surface is beautiful. At present, TIG is widely used in various industries, especially in the welding of stainless steel sheets.
1. Technical essentials of tungsten argon arc welding
1.1 Selection of TIG welding machine and power supply 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 the power supply with steep drop characteristics, and the TIG welding stainless steel sheet usually adopts the DC positive connection method.
1.2 Technical essentials of manual tungsten argon arc welding
1.2.1 Arc ignition
There are two types of arc ignition: non-contact and contact short-circuit arc ignition. The former electrode is not in contact with the workpiece and 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 cause tungsten clipping or bonding with the workpiece, the arc cannot be stabilized immediately, and the arc is easy to break down the base metal. Therefore, an arc-starting plate should be used. Place a red copper plate next to the arc point, first start the arc on it, wait for the tungsten tip to heat to a certain temperature, and then move it to the part to be welded. Under the action of the pulse current, the argon gas is ionized and the arc is induced.
1.2.2 Tack welding
During tack welding, the welding wire should be thinner than the commonly used welding wire. Because of the low temperature, fast cooling and long arc dwell time during spot welding, it is easy to burn through. When spot welding is performed, the welding wire should be placed at the spot welding position, and the arc is stable. Then move it to the welding wire, and stop the arc quickly after the welding wire is melted and fused with the base metal on both sides.
1.2.3 Normal welding
When using ordinary TIG to weld stainless steel sheets, the current takes a small value, but when the current is less than 20A, arc drift is likely to occur, and the cathode spot temperature is very high, which will cause heating and burning in the welding area and deterioration of electron emission conditions, resulting in Cathode spots are constantly beating, making it difficult to maintain proper welding. When pulsed TIG is used, the peak current can make the arc stable, the directivity is good, and the base metal can be easily melted and formed, and the cycle is alternated, ensuring the smooth progress of the welding process, and can get good performance, beautiful appearance, and the formation of molten pools overlapping each other. Welds.
2. Weldability analysis of stainless steel sheet
The physical properties and shape of the stainless steel sheet directly affect the weld quality. 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. The welding of stainless steel sheets is mostly flat 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 width of the molten pool metal are constant, the molten pool depth depends on the arc. The size, penetration and arc force are in turn related to the welding current, and the penetration width is determined by the arc voltage.
The larger the volume of the molten pool, the greater the surface tension. When the surface tension cannot balance the arc force and the metal gravity of the molten pool, it will cause the molten pool to burn through, and it will be heated and cooled locally during the welding process. Uneven stress and strain, when the stress generated by the longitudinal shortening of the weld on the edge of the thin plate exceeds a certain value, serious wave deformation will occur, affecting the shape quality of the workpiece. Under the same welding method and process parameters, different shapes of tungsten electrodes are used to reduce the heat input on the welded joint, which can solve the problems of weld burn-through and workpiece deformation.
3. Application of manual tungsten argon arc welding in stainless steel sheet welding
3.1 Welding principle
TIG welding is a type of open arc welding with stable arc and relatively 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 serious oxidation will occur, which will affect 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 the stainless steel sheet is poor, and it is easy to burn through directly.
2) No welding wire is required during welding, and the base metal 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 and testing during welding.
In the welding process of stainless steel sheet, no welding consumables are required, but the following materials are required relatively high: First, the purity of argon gas, flow rate and argon passage time, and second, tungsten electrode.
1) Argon
Argon is an inert gas and is not easy to react with other metal materials and gases. Due to the cooling effect of its air flow, the heat affected zone of the weld is small, and the deformation of the weldment is small. It is the most ideal protective gas for argon tungsten arc welding. The purity of argon must be greater than 99.99%. Argon is mainly used to effectively protect the molten pool, prevent the air from eroding the molten pool and cause oxidation during the welding process, and at the same time 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 should be good. In this way, the high-frequency arc ignition is good, the arc stability is good, the penetration depth is deep, the molten pool can be kept stable, the welding seam is well formed, and the welding quality is good. If the surface of the tungsten electrode is burnt out or there are defects such as pollutants, cracks and shrinkage holes on the surface, it will be difficult to start high-frequency arcing during welding, the arc will be unstable, the arc will drift, the molten pool will be scattered, the surface will expand, the penetration depth will be shallow, and the weld seam will be shallow. Poor forming and poor welding quality.
