1. Technical essentials of argon tungsten arc welding
1.1 Selection of tungsten argon arc 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 their alloys. Both AC and DC pulses use a power supply with steep drop characteristics, and TIG welding of stainless steel sheets usually uses DC positive connection.
1.2 Technical essentials of manual argon tungsten arc welding
1.2.1 Arc striking
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 short-circuit method is used to strike the arc, the arc should not be started directly on the weldment, because it is easy to cause tungsten inclusion or bonding with the workpiece, the arc cannot be stabilized immediately, and the arc is easy to penetrate the base material, so the arc strike plate should be used. Put a red copper plate next to the arc point, start the arc on it first, and then move to the part to be welded after the tungsten tip is heated to a certain temperature. In actual production, TIG usually uses an arc starter to start the arc. Under the action of the pulse current, the argon gas is ionized to initiate the arc.
1.2.2 Tack welding
During tack welding, the welding wire should be thinner than common welding wire. Because of the low temperature and fast cooling during spot welding, the arc stays for a long time, so it is easy to burn through. When performing spot welding, the welding wire should be placed on the spot welding position, and the arc is stable Then move to the welding wire, and stop the arc quickly after the welding wire melts and fuses with the base metal on both sides.
1.2.3 Normal welding
When ordinary TIG is used for welding stainless steel sheets, the current takes a small value, but when the current is less than 20A, arc drift is easy to occur, and the temperature of the cathode spot is very high, which will cause heat loss in the welding area and poor electron emission conditions, resulting in The cathode spot is constantly jumping and it is difficult to maintain a normal soldering. When pulsed TIG is used, the peak current can make the arc stable, the directivity is good, and the base metal is easy to melt and form, and the cycles are alternated to ensure the smooth progress of the welding process. welds.
2. Weldability analysis of stainless steel sheet
The physical properties and shape of the 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 cause burn-through, undercut and wave deformation. The welding of stainless steel sheets mostly adopts 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, quality and molten width of the molten pool metal are constant, the depth of the molten pool depends on the arc. The size, penetration depth and arc force are related to the welding current, and the fusion 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 gravity of the molten pool metal, it will cause the molten pool to burn through, and it will be heated and cooled locally during the welding process, causing the weldment to Inhomogeneous stress and strain, when the longitudinal shortening of the weld seam causes the stress on the edge of the thin plate to exceed a certain value, it will produce more serious wave deformation and affect 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 welding 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
Argon tungsten arc welding is a kind of open arc welding with stable arc and relatively concentrated heat. Under the protection of inert gas (argon gas), the welding pool is pure and the quality of the weld seam 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 sheets 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 sheets, welding consumables are not required, but the requirements for the following materials are relatively high: one is the purity of argon gas, the flow rate and the time of argon flow, and the other is the tungsten electrode.
1) Argon
Argon is an inert gas, and it is not easy to react with other metal materials and gases. Due to the cooling effect of its airflow, the heat-affected zone of the weld is small, and the deformation of the weldment is small. It is the most ideal shielding 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 the 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, and the end must be sharpened with good concentricity. In this way, the high-frequency arc ignition is good, the arc stability is good, the welding depth is deep, the molten pool can be kept stable, the weld 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 cavities on the surface, it will be difficult to start the high-frequency arc during welding, the arc will be unstable, the arc will drift, the molten pool will disperse, the surface will expand, the penetration depth will be shallow, and the weld seam will be damaged. Poor forming, poor welding quality.
4 Conclusion
1) The stability of argon tungsten arc welding is good, and different tungsten electrode shapes have a great influence on the welding quality of stainless steel sheets.
2) Tungsten electrode welding with flat top and conical tip can improve the formation rate of single-sided welding and double-sided welding, reduce the heat-affected zone of welding, the weld shape is beautiful, and the comprehensive mechanical properties are better.
3) Using the correct welding method can effectively prevent welding defects.
Post time: Jul-18-2023