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Gas tungsten arc welding process is mainly refers to arc weld groove form and the selection of welding parameters, the thickness of the delta 3 mm or less of carbon steel, low alloy steel, stainless steel, aluminum and its alloy butt joint, delta and the thickness of 2.5 mm or less high alloy, generally open I groove in 3 ~ 12 mm thickness of the material, can open V and Y groove Angle requirements for V groove As follows:
The groove Angle of carbon steel, low alloy and stainless steel is 60°, and that of high nickel alloy is 80°. When welding aluminum and its alloys with alternating current, it is usually 90°
Gas Tungsten Arc welding process has a great influence on weld forming. The main technological parameters of manual ARGON TUNGSTEN arc welding include tungsten electrode diameter, welding current, arc voltage, welding speed, type and polarity of power supply, tungsten extension length, nozzle diameter, distance between nozzle and workpiece and argon flow rate, etc
The welding current is usually selected according to the material, thickness and space position of the joint. Weld width and residual height increase slightly, but the increase is very little.
The diameter of tungsten used in gas tungsten Arc welding is an important parameter, because the diameter of tungsten determines the structure size, weight and cooling form of welding gun, which will directly affect the welder’s working conditions and welding quality. Therefore, the appropriate tungsten electrode diameter must be selected according to the welding current. If the tungsten electrode is thick, the welding current is very small, due to the low current density, tungsten extreme temperature is not enough, the arc will drift irregularly in the tungsten extreme, the arc is very unstable, destroy the protection zone, the molten pool is oxidized, the weld is not good, and easy to produce porosity.
The arc is stable when the welding current is suitable. Table Gas tungsten arc welding process-1 lists the allowable current ranges for different diameters and grades of tungsten electrodes.
It can be seen from the table that the same diameter of tungsten electrode, under different power supply and polarity conditions, the allowable range of current is different. The same diameter tungsten electrode, dc direct connection of the maximum allowable current; Minimum allowable current for DC reverse connection; The allowable current for ac is somewhere in between.
When the type and size of the current change, the tungsten extremes should be ground into different shapes to keep the arc stable, as shown in Figure gas tungsten arc welding-1 and Figure gas tungsten arc welding-2. Practice shows that the shape of tungsten extreme has an effect on the allowable current and weld forming. Generally, when welding sheet metal and welding current is small, small diameter tungsten poles can be used and their ends can be ground to a sharp cone Angle, about 30°, so that the arc is easy to arc and stable. However, when the welding current is large, the sharp Angle is still used, which will cause the end to overheat and melt and increase the burning loss due to the excessive current density, so that the arc column will obviously diffuse and drift unstable and affect the weld forming. Therefore, in high current welding, the tungsten extreme is required to be ground into a blunt taper Angle (generally greater than 90°) or a cone with a flat top.
Arc voltage of Gas Tungsten Arc welding is mainly determined by Arc length. As the Arc length increases, the weld width increases and the weld depth decreases slightly. If the arc is too long, it is easy to cause incomplete welding and edge biting, and the protection effect is not good; If the arc is too short to see the molten pool, and when the wire is easy to encounter tungsten caused by short circuit, so that the tungsten is polluted, increase the tungsten burning loss, but also easy to cause tungsten clamping. The arc length is usually approximately equal to the tungsten pole diameter.
When the speed of Gas Tungsten Arc welding increases, the weld depth and weld width decrease. When Gas Tungsten Arc welding speed is too fast, it is easy to produce incomplete welding, high and narrow welding seam and poor fusion on both sides; When Gas Tungsten Arc welding speed is too slow, the weld is very wide, and defects such as welding leakage and burning through may occur. In manual TIG welding, the welder usually adjusts the welding speed of Gas Tungsten Arc according to the size and shape of the pool and the fusion condition of both sides.
When selecting Gas Tungsten Arc welding speed, the following factors should be considered:
The current type and polarity of Gas Tungsten Arc welding are related to the metal and alloy to be welded. Some metals can only use dc positive polarity or reverse polarity, some ac, DC current can be used. Therefore, power supply and polarity should be selected according to different materials, as shown in Table Gas Tungsten Arc welding-2.
When dc polarity is positive, weldment is connected to positive pole, high temperature, suitable for welding thick weldment and metal with fast heat dissipation.
When using alternating current welding, it has the effect of cathode breakage, that is, when the welding piece is negative, the oxide film on the surface of the welding piece is broken due to the bombardment of positive ions, so that the liquid metal is easy to fuse together, which is usually used for welding aluminum, magnesium and their alloys.
The larger the nozzle diameter (refers to the inner diameter), the larger the scope of protection, the larger the flow of gas protection is required. Press down to select nozzle inner diameter:
D — Nozzle diameter or inner diameter (mm);
dw — Tungsten pole diameter (mm).
Generally, the diameter of the nozzle can rarely be changed after the welding torch is selected, so it is not considered as an independent welding parameter in actual production. When the nozzle diameter is determined, it is the air flow rate that determines the protection effect. Argon flow is too small, the protection airflow is weak, the protection effect is not good. Argon flow is too large, easy to produce flow, the protection effect is not good. When the flow of protective gas is appropriate, the ejected air flow is laminar flow, and the protection effect is good. The flow rate of argon gas can be calculated as follows:
Q — Argon flow rate (L/min)
D — Nozzle diameter (mm)
When D is small Q lower limit;
When D is big Q high limit.
In actual work, the flow rate can be selected according to the test welding situation. When the flow rate is appropriate, the protection effect is good, the molten pool is stable, the surface is bright and there is no slag, the weld is beautiful in appearance, and the surface has no oxidation trace. If the flow is not appropriate, the protection effect is not good, the molten pool surface slag, weld surface black or oxide skin.
The following factors should also be considered when selecting the argon flow rate:
1) Influence of external air flow and welding speed
The higher the welding speed is, the greater the resistance of the air flow, which makes the air flow deviate to the opposite direction of the movement. If the welding speed is too high, the protection will be lost. Therefore, increasing the welding speed should increase the gas flow rate accordingly.
When welding in windy places, argon flow should be appropriately increased. Generally, it is best to weld in a sheltered place, or take windward measures.
2) Influence of welding joint form
The butt joint and t-shaped joint provide good protection, as shown in Figure Gas Tungsten Arc welding-3a. When welding this kind of workpiece, there is no need to take other technological measures; The protection effect is the worst in end welding and end fillet welding, as shown in Figure Gas Tungsten Arc welding-3b. When welding such joints, besides increasing the flow of argon gas, baffle should also be added, as shown in Figure Gas Tungsten Arc welding-4.
In addition, welding current, voltage, welding gun tilt Angle, filling wire feeding condition also have a certain influence on the protective gas layer. In order to obtain satisfactory protection effect, the comprehensive influence of various factors must be considered in production practice. In order to evaluate the protection effect, the following methods can be used for testing:
When argon arc welding is used for oxidation and nitriding sensitive metals and their alloys (such as titanium and its alloys), better protection effect is required. Specific measures to improve the protection effect include: increasing the diameter of the nozzle, adding a drag cover (to increase the protection area) and back protection when welding titanium alloy in a factory, the drag cover and back protection device are shown in Figure Gas tungsten arc welding-6 and Figure Gas tungsten arc welding-7.
Argon gas shall be separately pumped into the drag cover and back protection device. During welding, in order to prevent oxidation, nitriding and hydrogen absorption of titanium alloy at 400-500℃, the drag cover is required to be close to the workpiece, the drag cover is made high and narrow, and the copper net is added to increase the stability of argon flow. In order to prevent the grain growth of titanium alloy weld, multi-layer welding with small parameters should be adopted.
In order to prevent arc heat from burning the nozzle, the tungsten extreme should protrude out of the nozzle. The distance from the tungsten extreme head to the nozzle end is called the tungsten extension length. The smaller the tungsten extension length, the closer the distance between nozzle and weldment, the better the protection effect, but too close will hinder the observation of molten pool. Usually when welding the joint, the tungsten pole extension length of 5-6mm is better; When fillet welding, tungsten pole extension length of 7~8mm is better.
This refers to the distance between the end face of the nozzle and the weldment. The smaller the distance, the better the protection effect, but the scope of observation and protection are small. The greater the distance, the less effective the protection.
Select the wire diameter according to the welding current. Table Gas tungsten arc welding-4 shows the relationship between them.
Figure Gas tungsten arc welding-8 shows the forehand welding and backhand welding methods. During the welding process, the welding wire and the welding gun move from right to left, the welding arc points to the unwelded part, and the welding wire is located in front of the arc movement, known as the forehand welding method. If, during the welding process, the welding wire and welding gun are welded from the left end to the right, the welding arc points to the welded part, and the filler wire is behind the arc movement, it is called the backhand welding method.