The welding process of manual argon tungsten arc welding mainly refers to the bevel form of the argon arc welding seam and the selection of welding parameters. Butt joints of carbon steel, low alloy steel, stainless steel, aluminum and their alloys with a thickness σ≤3mm, and high-nickel alloys with a thickness σ≤2.5mm, generally have I-shaped grooves. V-shaped and Y-shaped grooves can be made for the above-mentioned materials with a thickness of 3 to 12mm. The angle requirements of the V-shaped groove are as follows: the groove angle of carbon steel, low alloy steel and stainless steel is 60°, and the high nickel alloy is 80°; when welding aluminum and its alloys with alternating current, it is usually 90°.
The welding parameters of manual argon tungsten arc welding have a great influence on the shape of the weld seam. The main process parameters of manual argon tungsten arc welding are diameter, welding current, arc voltage, welding speed, power supply type and polarity, tungsten electrode extension length, nozzle diameter, distance between nozzle and workpiece, and argon gas flow rate.
The welding current is usually selected according to the material thickness of the weldment and the spatial position of the joint. When the welding current increases, the depth of penetration increases; the width and reinforcement of the weld increase slightly, but the increase is very small.
The diameter of the tungsten electrode used for manual argon tungsten arc welding is a relatively important parameter, because the diameter of the tungsten electrode determines the structural size, weight and cooling form of the welding torch, which will directly affect the working conditions and welding quality of the welder. Therefore, the appropriate tungsten electrode diameter must be selected according to the welding current. If the tungsten pole is thick and the welding current is small, due to the low current density and insufficient temperature at the tip of the tungsten pole, the arc will drift irregularly at the tip of the tungsten pole. The shape is not good, and it is easy to produce air holes.
When the welding current exceeds the allowable current of the corresponding diameter, due to the high current density, the temperature at the tip of the tungsten pole reaches or exceeds the melting point of the tungsten pole, and you can see signs of melting at the tip of the tungsten pole, and the end is very bright. When the current continues When it increases, the melted tungsten electrode forms a small pointed protrusion at the end, which gradually becomes larger to form a droplet, and the arc drifts with the tip of the droplet, which is very unstable, which not only destroys the argon protection zone, but also makes the molten pool Oxidized, the weld is not well formed, and the molten tungsten drips into the molten pool to create clamping Tungsten defects.
When the welding current is suitable, the arc is very stable. Table When the welding current exceeds the allowable current of the corresponding diameter, due to the high current density, the temperature at the tip of the tungsten pole reaches or exceeds the melting point of the tungsten pole, and you can see signs of melting at the tip of the tungsten pole, and the end is very bright. When the current continues When it increases, the melted tungsten electrode forms a small pointed protrusion at the end, which gradually becomes larger to form a droplet, and the arc drifts with the tip of the droplet, which is very unstable, which not only destroys the argon protection zone, but also makes the molten pool Oxidized, the weld is not well formed, and the molten tungsten drips into the molten pool to create clamping
Tungsten defects. lists the allowable current ranges for tungsten electrodes with different diameters and brands.
It can be seen from the table that the tungsten electrode with the same diameter has different allowable current ranges under different power supply and polarity conditions. For tungsten poles with the same diameter, the allowable current is the largest when the direct current is connected; the allowable current is the smallest when the direct current is reversed; the allowable current is between the two when the alternating current is used.
When the type and size of the current change, in order to keep the arc stable, the tip of the tungsten pole should be ground into different shapes, as shown in Figure Commonly used tungsten tip shape and Figure Current and Tungsten Tip Shape. Practice has shown that the shape of the tip of the tungsten pole has an influence on the allowable welding current and the shape of the weld. Generally, when welding thin plates and welding current is small, a small-diameter tungsten electrode can be used and its end be ground into a sharp cone angle, about 30°, so that the arc is easy to start and stable. However, when the welding current is high, sharp corners are still used, because the current density is too high, the end will be overheated and melted, and the burning loss will be increased, so that the arc column will obviously spread and flutter, which will affect the weld formation. Therefore, during high-current welding, the tip of the tungsten pole is required to be ground into a blunt cone angle (generally greater than 90°) or a cone with a flat top.
The arc voltage is mainly determined by the arc length, the arc length increases, the weld width increases, and the penetration depth decreases slightly. If the arc is too long, it is easy to cause incomplete penetration and undercut, and the protection effect is not good; if the arc is too short, it is difficult to see the molten pool, and it is easy to touch the tungsten electrode when feeding the wire and cause a short circuit, which will pollute the tungsten electrode. Increase the burning loss of tungsten electrode, and it is easy to cause tungsten inclusion. Usually the arc length is approximately equal to the diameter of the tungsten pole.
When the welding speed increases, the penetration depth and penetration width decrease. When the welding speed is too fast, it is easy to produce incomplete penetration, high and narrow weld seam, and poor fusion on both sides; when the welding speed is too slow, the weld seam is very wide, and defects such as weld leakage and burn-through may also occur. During manual argon tungsten arc welding, the welder usually adjusts the welding speed at any time according to the size of the molten pool, the shape of the molten pool and the fusion of both sides.
When selecting the welding speed, the following factors should be considered:
The current type and polarity selection used in argon arc welding are related to the type of metal and its alloy to be welded. Some metals can only be used with DC positive or reverse polarity, and some AC and DC currents can be used. Therefore, power supply and polarity need to be selected according to different materials, see Table Selection of welding power source type and polarity.
When the DC is positive, the weldment is connected to the positive pole, and the temperature is higher, which is suitable for welding thick weldments and metals with fast heat dissipation.
When using alternating current welding, it has a cathodic crushing effect, that is, when the weldment is a negative electrode, due to the bombardment of positive ions, the oxide film on the surface of the weldment is broken, so that the liquid metal is easily fused together. It is usually used to weld aluminum, magnesium and others. alloy.
The larger the diameter of the nozzle (referring to the inner diameter), the larger the range of the protection zone, and the greater the flow rate of the protection gas required. The inner diameter of the nozzle can be selected as follows:
D ——Nozzle Diameter or Inner Diameter (mm)
dw——Tungsten pole diameter (mm)
Usually after the welding torch is selected, the diameter of the nozzle can rarely be changed, so it is not selected as an independent welding parameter in actual production. When the nozzle diameter is determined, the argon flow rate determines the protection effect. If the argon gas flow rate is too small, the protective airflow will be weak and the protective effect will not be good. If the flow rate of argon gas is too large, it is easy to generate turbulent flow and the protection effect is not good. When the protective gas flow rate is appropriate, the ejected air flow is laminar, and the protection effect is good. The flow rate of argon can be calculated as follows:
D——Nozzle diameter (mm)
When D is small, Q takes the lower limit; when D is large, Q takes the upper limit.
In actual work, the flow rate can usually 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 without slag, the weld seam has a beautiful appearance, and there is no oxidation trace on the surface; if the flow rate is not suitable, the protection effect is not good. There is slag on the surface of the molten pool, and the surface of the weld is black or scaled.
The following factors should also be considered when selecting the argon flow rate:
The influence of external airflow and welding speed The greater the welding speed, the greater the air resistance encountered by the shielding airflow, which makes the shielding gas deflect to the opposite direction of movement; if the welding speed is too high, the protection will be lost. Therefore, while increasing the welding speed, the gas flow should be increased accordingly. When welding in a windy place, the argon flow should be increased appropriately. Generally, it is best to weld in a sheltered place, or take measures to keep out the wind.
Influence of the form of welded joints When welding butt joints and T-shaped joints, they have a good protective effect, as shown in Figure Protective effect of argon-a. When welding this kind of workpiece, it is not necessary to take other technological measures; and the protection effect is the worst when performing end welding and end fillet welding, as shown in Figure Protective effect of argon-b. When welding such joints, in addition to increasing the argon flow, A baffle should also be added, as shown in Figure Add baffle.
In addition, the feeding conditions of welding current and voltage welding torch inclination angle also have certain influence on the shielding gas layer. In order to obtain a satisfactory protection effect, in production practice, the comprehensive influence of various factors must be considered. In order to evaluate the protective effect, the following methods can generally be used for testing:
Use an aluminum plate as the workpiece, use an AC power source, and select certain welding parameters. After arcing, the welding torch is fixed, and after burning for 5~10s, cut off the power to extinguish the arc. At this time, the graphics shown in Figure Effective protection area are left on the aluminum plate. If the protection is good, an obvious bright circle can be distinguished on the aluminum plate, which is the result of good argon protection and cathode fragmentation. If the protection is not good, the bright surface can hardly be seen. This bright circle is the effective protection area, and the diameter of the effective protection area can be used as a scale to measure the protection effect.
When conducting the test, stainless steel can also be used as the test workpiece, and a DC power supply is used. In this case, the unoxidized areas are bright silvery white, while the oxidized areas are dark black. In actual production, the gas shielding effect can also be judged by the discoloration of the weld surface, see TableWeld color and protection effect (stainless steel) and Table Weld color and protection effect (titanium alloy）.
When argon arc welding is used for metals and their alloys that are very sensitive to oxidation and nitriding (such as titanium and its alloys), better protection is required. The specific measures to improve the protection effect include: increasing the diameter of the nozzle and adding a drag cover (to increase the protection area) and back protection. When a factory welds titanium alloys, the drag cover and back protection device used are shown in Figure Titanium plate butt joint protection fixture and Figure Front side weld protection welding for titanium alloy welding by manual argon arc welding.
The tow cover and back protection device should be filled with argon gas separately. During welding, in order to prevent titanium alloy from oxidizing, nitriding and absorbing hydrogen at 400~500C, the drag cover is required to be close to the workpiece. The drag cover is made high and narrow, and a copper mesh is added inside to increase the stability of the argon flow. In order to prevent the grain growth of titanium alloy welds, multi-layer welding with small parameters should be adopted.
In order to prevent the arc heat from burning the nozzle, the tip of the tungsten pole should protrude beyond the nozzle. The distance from the tip of the tungsten pole to the tip of the nozzle is called the extension length of the tungsten pole. The smaller the protruding length of the tungsten electrode, the closer the distance between the nozzle and the weldment, and the better the protection effect, but too close will hinder the observation of the molten pool. Generally, when welding butt joints, the extension length of the tungsten electrode is 5~6mm; when welding fillet welds, the extension length of the tungsten electrode is preferably 7~8mm.
This refers to the distance between the nozzle end face and the weldment. The smaller the distance, the better the protection effect, but the observation range and protection area are small; the larger the distance, the worse the protection effect.
Select the wire diameter according to the size of the welding current. Table The matching relationship between welding current and welding wire diameter shows the relationship between them.