Welding of small thickness (5 - 0.2 mm) is a difficult task for many other methods of fusion welding.
Plasma arc is formed by a special plasma torch with a tungsten electrode which has a conical shape with the apex facing the product. This form is provided by its design features of the plasma torch, plasma, and the correct choice of protective gas and flow rate. The most often used as a plasma gas argon. At the bottom outlet of the nozzle plasma arc exists only in a stream of argon. As we move towards the anode (product), plasma-forming and protective gases are mixed and the plasma arc is already in a mixture of these gases. The closer to the anode, the more protective gas mixture is contained. If the shielding gas has a higher thermal conductivity than the plasma-forming, the degree of compression of the arc as it approaches the anode increases, and it takes a conical shape (the shape of a spear).
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| 1 - Tungsten Electrode, 2 - channel for supplying the plasma gas, 3 - a channel for supplying a protective gas, 4 - ceramic nozzle, 5 - the nozzle of the plasma channel, 6 - filler wire, 7 - welded parts; P - plasma-forming gas, N - protective gas. |
When properly selected plasma gas flow rate specified form maloampernoy plasma arc is formed when used as a shielding gas mixture argonovodorodnoy (90% Ar + 10% H2), helium, nitrogen and carbon dioxide. If a protective argon gas, the cylindrical shape of an arc or slightly expanding toward the anode. The same form is preserved under the arc of its existence at currents greater than 20 A, since the effect of shielding gas on compression of the arc significantly weakened. The effect of compression of the arc decreases with increasing plasma gas flow rate more than the optimum.
Especially thin metal arc welded maloampernoy in a pulsed mode with the formation of unipolar or bipolar pulses. Heating and melting of the base metal takes place during the current pulse duration of the arc of a certain polarity. During the pause, when current labor supply circuit of the plasma arc is zero, the liquid metal crystallizes and forms welded point. Pulse time and pause, and welding speed are chosen in such a way as to protect a certain amount of overlap of crystallizing point.
Pulsed microplasma welding greatly facilitates the process of obtaining a quality weld, because it allows such a way to pick up the current amplitude, pulse duration and pause welding, weld metal that burn marks were practically excluded, even if the process or stop the uneven implementation.
A variety of methods microplasma welding in the pulsed mode is pulses of welding. During the current pulse polarity welded edges provide enough heat, which ensures their melting, the formation of the weld pool and, after crystallization, the formation of weld points. During the current pulse of reverse polarity in the welded edges introduced a smaller amount of heat, sufficient only for the destruction of the oxide film. During this time the weld pool is cooled to a full or partial crystallization. The next current pulse polarity creates welded at some point (depending on the welding speed) distance from the previous one. Overlapping points given their size and pitch.
For welding of Al, Mg and their alloys, developed the process of microplasma welding in reverse polarity. Feature of the method consists in the fact that there are two power supply. One (IP1) for continuous support combustion Simmer between the tungsten electrode and copper nozzle of the plasma torch. Other (IP2) to supply the main arc burning between the copper nozzle (anode) and the work piece (cathode). Using a water-cooled copper nozzle as the anode of the arc core allows to exclude the influence of the main current of reverse polarity arc on a tungsten electrode, thereby ensuring its high resistance. The small diameter of the electrode provides a stable burning Simmer on the currents of 2 ... 5 A. At low flow rates of plasma-forming gas (0.2 ... 0.8 l / min), the anode spot is located inside the main arc of the nozzle channel, and the compression of the arc column on the open its site and near the cathode is provided with protective gas, as well as welding on the line polarity. Cathode spot continuously wanders over the surface of the product and destroy the refractory oxide film. This method allows you to connect AI and its alloys in the thickness of tenths and hundredths of a millimeter. However, wandering spot results in a wider weld and a large heat-affected zone.
Currently progressing microplasma welding of aluminum using alternating current. With this method, between the nozzle and the electrode of the plasma torch is lit continuously on duty direct current arc. When applied to the tungsten electrode is positive with respect to the product half-voltage between the arc formed by compressed reverse polarity with a nonstationary cathode spot that Deplete the oxide film. Then, on the tungsten electrode is negative with respect to the plasma torch arrives products semiprime-one strain. This will generate a compressed arc with a high density of energy sufficient to carry out welding of metals, on the surface of refractory oxide films. In this case, because of the tungsten electrode is only the current straight polarity, the destruction did not occur. Ability to separate current supply line and reverse polarity on the tungsten electrode is an important technological advantage of this method of microplasma welding.
Microplasma welding of the most technologically performed butt, and face flanged connections. The quality of the weld depends on the correct edge preparation for welding, assembly precision and quality used equipment. Butt joints are recommended for thicknesses 5 = 0.3 ... 2.0 mm. When 5 <0.3 mm smarter butt flare. Mechanical connections are recommended for all thicknesses, since their performance is no need to use filler wire and the pads. Lap and T-joints are not recommended and should be replaced at the butt, face and butt flare.
The reverse side of the seam (from the root) is formed either smooth lining process, the groove having an oval, triangular or rectangular cross section. For butt joints are used, usually one-sided welds performed in a single pass full penetration of edges in the entire thickness of the base metal. In some cases, such as butt welding annular seams, apply a one-way multipass welding. The first pass is performed at less than the rated current and is used to tack the edges of connected components, and following the passage of penetration is carried out for the whole thickness of the edges.
Microplasma welding method can perform the connection of parts in all positions. Although technically the most simple stitches run in the bottom position, much difficulty in carrying out vertical, horizontal and ceiling joints does not occur, since the volume of the weld pool is small, and the molten metal is easily kept in it by surface tension forces.
To build parts for welding high requirements. Layout of parts is allowed only soft tool, the presence of ticks, scratches and tears on the edges of the weld is not allowed. Richt welded edges carefully with a wooden hammer and cleaned of contaminants. The assembly is carried out in precision mandrels. When welding long seams tack made a 3 ... 5 mm.
The stability of burning plasma arc and the weld quality depend on the shape of the working electrode and its position in the channel of the nozzle. Sharpening angle of the working part of tungsten electrode must be within 10 ... 15 degrees. The axis of the electrode must match exactly with the axis of the nozzle, and the electrode should be sunk in the channel of the nozzle is not more than 0.5 mm. Subject to these requirements, the length of the plasma torch by burning Simmer must be at least 1.5 ... 2.0 mm.
The main arc of flame is excited by touching the duty of the arc weld metal. After ignition of the main arc plasma torch does not move up until the edges are formed on the weld pool (with manual welding). After this torch moves across the junction in the direction of welding. The angle of the plasma torch is kept within 60 ... 80 ° C with manual and 80 ... 90 ° with an automatic (welding angle forward). The transverse oscillations of the plasma torch does not produce. The protective gas is fed back in a few seconds after the welding process and disable the main arc.
In developing the technology of welding all metals should take into account their original properties. For example, low-carbon steel poluspokoynye calm and successfully welded when used as a shielding gas of argon-hydrogen mixture. Boiling become prone to the formation of pores that result from the interaction of hydrogen with iron oxides during solidification of the melt puddle. Therefore, the hydrogen content in the shielding gas should not exceed 3%.
The most successful microplasma welding is used for connecting parts made of corrosion-resistant austenitic steels. Welding of these materials is performed in hard fixtures to provide heat from the welding zone and reduce warping the workpiece and welding products.
See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units
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