The basic methods of plasma arc welding mainly include three types: penetration plasma arc welding, penetration plasma arc welding and microbeam plasma arc welding.
The arc penetrates to form a small hole in front of the molten pool, and the welding method that forms a weld bead behind the small hole as the heat source moves is called penetrating plasma arc welding. Due to the high energy density of the plasma arc and the large plasma flow force, welding The weldment penetrates and creates a small hole through the weldment (Figure Penetrating plasma arc welding). The molten metal is balanced under the interaction of arc blowing force, surface tension and metal gravity. When the welding torch is advanced, the small hole is locked behind the arc, forming a fully penetrated weld.
The pinhole effect can only be formed under the condition of sufficient energy density. When the plate thickness increases, the energy density required also increases. However, the improvement of the plasma arc energy density is limited, so the penetration plasma arc welding can only be realized within a certain plate thickness range.
Penetration plasma arc welding, that is, a welding method that penetrates the weldment during the welding process, is referred to as the penetration method. This welding method only penetrates the weldment without producing pinhole effect during the welding process. When the ion gas flow rate is small and the compression degree of the arc column is weak, the penetration ability of the plasma arc is also low. This method is mostly used for single-sided welding and double-sided forming of thin plates with a thickness of less than 3 mm and multi-layer welding of thick plates.
Plasma arc welding that uses a small current (usually below 30 A) for welding is usually called micro-beam plasma arc welding (Figure Microbeam plasma arc welding). It adopts φ0.6~1.2 mm small-aperture compression nozzle and combined arc. Microbeam plasma arc is also called needle plasma arc. When the welding current is less than 1 A, it still has good stability. It is characterized by being able to weld filaments and foils. The deformation of the weldment and the range of the heat-affected zone are relatively small.
When using transferred arc welding, sometimes in addition to the plasma arc burning between the tungsten electrode and the weldment, there will also be a series arc burning between the tungsten nozzle weldment, this phenomenon is called double arc, as shown in the figure double arc phenomenon shown. The double arc phenomenon reduces the main arc current, destroys the normal welding or cutting process, and causes the nozzle to burn out in severe cases.
Measures to prevent double arcs are:
(1) Select the current and ion gas flow correctly.
(2) Reduce the inrush current during arc turning.
(3) The nozzle hole should not be too long.
(4) The electrode and nozzle should be as centered as possible.
(5) The distance from the nozzle to the weldment should not be too close.
(6) The shrinkage of the electrode should not be too large.
(7) Strengthen the cooling of the nozzle and electrode.
The welding parameters of penetration plasma arc welding of commonly used metal materials such as carbon steel and low alloy steel, stainless steel titanium alloy, copper and brass are shown in Table Welding parameters for penetration plasma arc welding. The welding parameters of penetration plasma arc welding are shown in Table Welding parameters of penetration plasma arc and Table Welding parameters of thin plate end joints by plasma arc welding. Medium and small current (0.2 ~ 100 A) penetration plasma arc welding usually adopts combined arc. Due to the existence of the maintenance arc (non-transfer arc), the main arc burns stably at a small current (<1A). The maintenance arc current is generally 2~5 A, because the anode spot of the maintenance arc is located on the wall of the nozzle hole, if the maintenance arc current is too large, it is easy to burn the nozzle.
To be familiar with the basic steps of plasma arc welding, to use the plasma arc welding machine correctly, to master the basic skills of plasma arc welding, arc starting and closing methods and penetration type plasma arc welding operation.
Equipment: LH 30 plasma arc welding machine argon cylinder, QD- 1 single-stage reaction decompression gauge, 2 LZB rotameters.
Plasma arc welding torch.
Cerium tungsten electrode: diameter 1.0 mm.
Welding parts: length x width x thickness of stainless steel plate is 200mm x100mm x1.0mm.
Stainless steel welding wire: 1.0 mm in diameter.
Protective equipment: masks, overalls, rubber shoes, insulating gloves, etc.
Copper backing plate.
Check whether the joints of the gas circuit, water circuit and electrical system of the welding machine are in the correct position and whether they are firmly fixed.
Check the concentricity of electrode and nozzle. Connect the high-frequency oscillating circuit, let the high-frequency sparks be added between the electrode and the nozzle, if the high-frequency sparks are evenly distributed in a circle, the concentricity is the best; for plasma arc welding, the sparks cover more than 80% of the circle, The concentricity is considered qualified.
Clean the oil on the weldment.
Open the air circuit and water circuit, close the switch of the power switch, press the control button, send air in advance, connect the high-frequency oscillation circuit and the power circuit of the electrode and the nozzle, and ignite the non-transfer arc; then aim the welding torch at the weldment, and the transfer arc is established. The main arc current is formed, and the plasma arc welding can be performed on the weldment. At this time, the high-frequency circuit of the arc-maintaining circuit is automatically disconnected, and the arc-maintaining current disappears.
Another arc starting method is that the electrode is in contact with the nozzle, that is, after the gas circuit, water circuit, and circuit are all in the power-on state, press the control button, the no-load voltage of the arc maintenance circuit is added, and the electrode is adjusted downward so that it short-circuits with the nozzle first. , and then withdraw upwards, so that an arc is generated between the electrode and the nozzle, and the maintenance arc is established. The welding torch is aimed at the weldment, the plasma arc is established, and the maintenance arc circuit is disconnected, the arc starting process is completed, and the plasma arc welding process is entered.
When the penetration method is used for welding, both the longitudinal seam and the circular seam are directly ignited on the weldment.
For the arc ignition of the small hole type welding arc, the longitudinal seam and circular seam with a plate thickness less than 3 mm can directly strike the arc on the weldment. The place where small holes are established generally does not produce defects, but when the thickness of the weldment is large, due to the high current of plasma arc welding, defects such as air holes and depressions are prone to occur at the arc starting place. The arc strike plate can be used to solve the longitudinal seam, first dig a small hole on the arc strike plate, and then transition to the weldment. However, the circular seam must be directly ignited on the weldment. At this time, both the power supply and the ion gas flow are required to be controllable, that is, to have a slope incremental control means to complete the need for direct arc ignition on the weldment.
The plasma arc welding process, no matter manual or automatic, its operation method is similar to that of argon arc welding.
When the penetration method is used for welding, arc closing can be carried out on the weldment, but it is required that the ion gas flow and welding current should have attenuation devices to meet the defects of arc craters during welding arc closing. If the attenuation device can not meet the requirements of the arc crater, it is allowed to add the welding wire consistent with the base metal metal to make up the arc crater.
When the small hole method is used for welding, the longitudinal seam of the thick plate uses the lead-out plate to close the small hole on the lead-out plate. The circular seam of the thick plate is similar to the arc strike. The arc is closed on the weldment, but the slope descending control method is adopted to gradually reduce the current and ion gas flow to close the small hole.
For low-carbon steel, low-alloy steel and stainless steel, when the thickness of the weldment is greater than 1.6 mm and less than 8 mm, the bevel may not be opened, and the small hole type single-sided welding is used for one-time forming. Groove butt welding is required for thicker weldments. Compared with argon tungsten arc welding, a larger blunt edge and a smaller groove angle should be used (for example, for a weldment with a thickness of 10 mm, the thickness of the blunt edge is 5 mm, and the groove angle is 60°).
The thickness of the weldment is between 0.05 mm and 1.6 mm, and penetration welding is usually used, and the commonly used joint forms are I-shaped, curled butt joints, and end joints.
If the weldment is a stainless steel plate with a thickness of 1.0 mm, use penetration plasma arc welding, I-shaped butt joints, butt welding without leaving a gap, and control the root gap to not exceed 1/10 of the plate thickness, and there will be no misalignment. Before welding, place the welded parts cleaned of oil stains on the copper backing plate and clamp them, as shown in Figure Schematic diagram of flat butt welding.
When welding, the left welding method is adopted, the angle between the welding torch and the weldment is about 80°, and the angle between the welding wire and the welding torch is about 90°. The welding torch is always aimed at the weldment interface, and pay attention to observe the penetration of the weldment, and add welding wire in a timely and regular manner. The welding torch should move smoothly and the speed should be uniform, and the distance between the nozzle and the weldment should be kept between 4 and 5 mm. When welding to the end of the weld, add welding wire appropriately, release the button, and extinguish the arc with the current decay.