The irregular shape, sharp corners and edges of tungsten clips have a cutting effect on the welded joint, causing stress concentration and making it one of the basic causes of cracking.
Therefore, tungsten is a relatively serious defect.
Tungsten clamps on the welded joint are a new defect that has arisen with the adoption of tungsten electrode argon arc welding, a new and advanced welding method.
When the welding current is large and the welding time is long, the tungsten electrode melts into the molten pool because tungsten has a much higher melting point than the melting point of the weld metal.
Therefore, the tungsten solidifies before the weld metal.
Because tungsten solidifies into a spherical or ellipsoidal shape, not only does it not cause stress concentration like pores, but the tungsten staples in the welded joint can also withstand a small amount of stress.
Therefore, the damage of tungsten tweezers is less than that of pores and slag inclusions.
However, because the solidification point of tungsten is much higher than that of the weld metal, the tungsten solidifies before the weld metal and cannot fuse into an organic whole with the weld metal.
As a result, the strength of the joint welded with tungsten staples is slightly reduced, so tungsten is still considered a welding defect.
Cause of Tungsten Clamp Defect in Welded Joints
1. Operational factors
Poor welder proficiency and unskilled techniques may cause the tungsten electrode to frequently touch the workpiece or filler wire.
When the position of the welding seam is affected by the spatial position or the welding gun is obstructed, the difficulty of the operation will increase, which will increase the probability of the tungsten electrode touching the molten pool or filler wire.
2. Process factors
Under the circumstances of fixed tungsten electrode type and diameter, the tungsten electrode has a certain limit to its current carrying capacity. Excessive welding current will cause burning of the tungsten electrode and ultimately lead to the failure of the tungsten clamps.
During the shielded inert gas welding process, gas impurity or the selection of active mixed gas as shielding gas will reduce the shielding effect, resulting in lack of protection for the tungsten electrode and eventually oxidation.
When welding with direct current polarity or TIG welding in the negative half cycle of alternating current, the temperature of the tungsten electrode will increase, the burning will become severe, and the tungsten particles will enter the molten pool.
3. Material factors
Thoriated tungsten electrode: a traditional electrode with superior welding performance. It does not break the arc, does not spread, does not float, does not splash, has a stable arc and resistance to high temperatures. The tungsten electrode head is not easily rounded and its service life is long.
Zirconium tungsten electrode: It has excellent welding performance under AC conditions, especially under high load current, which cannot be replaced by other electrodes.
Yttrium and tungsten electrode: has a thin arc, high compression and maximum melting depth at medium and large currents. It is mainly used in the military and aerospace industries.
Ceriated tungsten electrode: has excellent arc starting performance under low current conditions and has relatively less holding currents.
Lanthanum tungsten electrode: can be used in AC and DC conditions, has high tolerance currents and the lowest wear rate.
Pure tungsten electrode: does not add rare earth oxides, has minimal electron emission capacity and is only suitable for welding under high AC loads.
Due to the alternation of cold and hot temperature during welding, the end angle of the tungsten electrode is small, making it prone to stress concentration and stress fracture of the tungsten electrode tip, resulting in tungsten clamps in the molten pool .
If the surface of the tungsten electrode is rough during welding, the oxide generated at high temperature may accumulate locally during movement towards the tip, leading to the accumulation of local oxides and the possibility of cracking, resulting in tungsten clamps.
Measures to control defects of tungsten clamps in welded joints
(1) Strictly select the welding current based on the type and diameter of the tungsten electrode used.
(2)Strictly control the purity of inert shielding gas (≥99.9%). Ar+O2 or Ar+CO2 gas cannot be used in TIG welding.
(3) Use short arc welding as much as possible during welding to increase the protective effect.
(4) Strengthen the welder's basic skills and strictly control the distance between the tungsten electrode and the workpiece to avoid tungsten clamps caused by contact with the workpiece during welding.
(5) Use positive polarity of direct current for DC TIG welding.
(6) Choose a reasonable gas flow rate and use a windshield plate in outdoor locations with strong winds during welding.
(7)Control the extension length of the tungsten electrode during welding. For butt welds, the tungsten electrode extension length is generally kept at 5-6mm, and for fillet welds, the tungsten electrode extension length is best at 7-8mm.
(8) Use dedicated grinding equipment to grind the tungsten electrode. For small current welding, grind a small clamping angle and an acute angle on the tungsten electrode, while for large current welding, grind an appropriate clamping angle and grind the end into a large angle and round corner.
