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Plasma welding
4 Plasma welding 4.1
A description of the method Electrode Plasma gas
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Cooling water Shielding gas Resistance Gas nozzle Plasma nozzle Arc (plasma jet)
Figure 4.1 Schematic diagram ofplasma welding. Resistor R limits the current in the pilot arc which can be ignited also when the torch is apart from the workpiece. The plasma welding method employs an inner plasma gas and outer shielding gas, as shown in Figure 4.1. The plasma gas flows around a retracted centred electrode, which is usually made of tungsten. The shielding gas flows through the outer jet, serving the same purpose as in TIG welding. A plasma arc is considerably straighter and more concentrated than, for example, a TIG arc, which means that the method is less sensitive to arc length variations: see Figure 4.2. -
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Plasma
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Figure 4.2 The plasma arc is not as conical as the TIG are, which means that it is much less sensitive to arc length variations. The plasma welding process can accept variations of 2-3 rom in the arc length without significantly altering the heat input to the workpiece. This this is approximately ten times better than the corresponding value for TIG welding. However, because the arc
WELDING PROCESSES HANDBOOK
is narrower, more accurate transverse control is important, which means that the method is normally used in mechanised welding. Characteristic features of the method include: the concentrated stable arc high welding speed insignificant deformation of the workpiece reliable arc ignition. With the exception of magnesium, the method is suitable for welding the same materials as those that can be welded by TIG welding. Automated welding of stainless steel pipes is a major application area.
Classification of plasma welding methods There are three different classes of plasma welding, depending on the current range:
Micro plasma (0.1-15 A). The concentrated arc enables it to remain stable down to a current of about 0.1 A, which means that the process can be used for welding metal thicknesses down to about 0.1 rum. This makes the process attractive to, for example, the space industry. Medium plasma welding (15-100 A). In this range, the method competes more directly with TIG welding. It is suitable for manual or mechanised welding and is used in applications such as the automotive industry for welding thin sheet materials without introducing distortion or unacceptable welded joints, as are produced by MIG welding, or for the welding of pipes in breweries or dairies. Keyhole plasma welding (> 100 A). The third type of plasma welding is referred to as keyhole plasma welding, taking its name from the 'keyhole' that is produced when the joint edges in a butt weld are melted as the plasma jet cuts through them. As the jet is moved forward, the molten metal is pressed backwards, filling up the joint behind the jet.
Figure 4.3 Keyhole welding. The main benefits of plasma welding are to be found in the fact that the keyhole welding method can be used for butt welds from about 3 rum up to 7-& mm. The keyhole
38
PLASMA WELDING
provides a guarantee of full penetration; by comparison the TIO method is only suitable for butt welds up to about 3-4mm thick. Joints with thicker materials have to be prepared with a V or U joint and then filled with filler material. Keyhole welding is not suitable for thinner materials below 3 mm: in these circumstances, the process becomes much more like TIO welding. Reducing the plasma gas flow to a low level can make the plasma torch work in the same way as a conventional TIO torch, which can be useful when making tack welds or cladding welds. The main advantage over conventional TIO welding is primarily the excellent arc stability. There are two types of working systems employed: with transferred and non-transferred arcs, as shown in Figure 4.4.
Electrode
Plasma nozzle
Transferred
Non-transferred
Figure 4.4 Transferred and non-transferred plasma welding arcs.
4.2
Equipment
The following equipment is required for plasma welding:
Welding torch The same basic requirements apply here as for TIO welding. Plasma welding torches are generally water-cooled. Power source Plasma welding employs DC, with a drooping characteristic, as for TIG welding. Open circuit voltage should be at least 80 V. HF generator In principle, the purpose of the HF generator is the same as in TIG welding. However, when used in plasma welding, the HF generator does not normally strike the main arc: instead, it strikes a pilot arc as a non-transferred arc, with the current flowing between the electrode and the inner gas nozzle. The pilot arc, in other words, can be maintained in air: as the torch approaches the workpiece, the main arc strikes and the pilot arc is extinguished. Control equipment The necessary control equipment depends on to what extent the welding process is mechanised. However, it is usual for the pre-flow and post-flow of the shielding gas, the
39
WELDING PROCESSES HANDBOOK
HF generator and the pilot are, to be automatically controlled. There is often automatic control to ensure that the arc is struck in pure argon, after which the gas supply changes over to the particular gas that is being used.
4.3
Gases for plasma welding
Normally, the same gas is used for both the plasma and the shielding gas. This avoids variations in the plasma jet, which would otherwise be the case if two different gases or gas mixtures were being used. An argonlhydrogen mixture is generally used as the plasma and shielding gas. However, hydrogen cannot be used as a constituent when welding mild steel or reactive metals such as zirconium or titanium. Mixtures of argon/helium/nitrogen are used when welding duplex stainless steels, as these contain nitrogen in their alloying. Pure helium is not suitable, as the resulting high heat losses in the plasma gas will substantially reduce the life of the plasma torch. Argonlhelium mixtures result in a higher energy in the plasma jet at constant current. However, the mixture must contain at least 50 % helium if any significant difference is to be noted. On the other hand, mixtures containing more than 75 % helium have the same characteristics as pure helium. Pure argon, or argonlhelium mixtures, are well suited to the welding of mild steel and reactive metals (titanium, aluminium, zirconium etc.), for which hydrogen or nitrogen cannot be used.
4.4
The advantages of the plasma method
Plasma welding has the following advantages, relative to those of other methods: 1. Reliable penetration with the keyhole method. 2. Very high speed: often 400 % higher than that of conventional TIG welding. 3. Butt welds possible in thick materials (8 mm) without the use of fillers. 4. Fusion welding possible even in very thin materials (0.03 mm). 5. Low weld convexity and root bead. This is particularly beneficial when welding structures that will be subjected to fatigue loading, in addition to reducing the work required in other welds where the weld convexity and root bead would otherwise have to be ground away. Plasma welding of 5 mm austenitic stainless steel produces a weld convexity of about 0.3 mm and a root bead of about 0.2 mm. Increasing the steel thickness to 8 mm results in corresponding respective thicknesses of 0.7 mm andO.6mm. 6. Low heat-affected zone and little distortion. 7. High arc stability at low arc currents. 8. Little sensitivity to arc length variations as a result of the concentrated arc. 9. Assessment of the weld quality possible while welding is in progress. 10. High metallurgical quality in comparison with that of conventional TIG welded materials.
11. Flexibility, due to the ability to perform keyhole welding and melt-in welds using the same equipment.
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A description of the method Electrode Plasma gas
+
Cooling water Shielding gas Resistance Gas nozzle Plasma nozzle Arc (plasma jet)
Figure 4.1 Schematic diagram ofplasma welding. Resistor R limits the current in the pilot arc which can be ignited also when the torch is apart from the workpiece. The plasma welding method employs an inner plasma gas and outer shielding gas, as shown in Figure 4.1. The plasma gas flows around a retracted centred electrode, which is usually made of tungsten. The shielding gas flows through the outer jet, serving the same purpose as in TIG welding. A plasma arc is considerably straighter and more concentrated than, for example, a TIG arc, which means that the method is less sensitive to arc length variations: see Figure 4.2. -
I
I
-I
Plasma
I
I
Figure 4.2 The plasma arc is not as conical as the TIG are, which means that it is much less sensitive to arc length variations. The plasma welding process can accept variations of 2-3 rom in the arc length without significantly altering the heat input to the workpiece. This this is approximately ten times better than the corresponding value for TIG welding. However, because the arc
WELDING PROCESSES HANDBOOK
is narrower, more accurate transverse control is important, which means that the method is normally used in mechanised welding. Characteristic features of the method include: the concentrated stable arc high welding speed insignificant deformation of the workpiece reliable arc ignition. With the exception of magnesium, the method is suitable for welding the same materials as those that can be welded by TIG welding. Automated welding of stainless steel pipes is a major application area.
Classification of plasma welding methods There are three different classes of plasma welding, depending on the current range:
Micro plasma (0.1-15 A). The concentrated arc enables it to remain stable down to a current of about 0.1 A, which means that the process can be used for welding metal thicknesses down to about 0.1 rum. This makes the process attractive to, for example, the space industry. Medium plasma welding (15-100 A). In this range, the method competes more directly with TIG welding. It is suitable for manual or mechanised welding and is used in applications such as the automotive industry for welding thin sheet materials without introducing distortion or unacceptable welded joints, as are produced by MIG welding, or for the welding of pipes in breweries or dairies. Keyhole plasma welding (> 100 A). The third type of plasma welding is referred to as keyhole plasma welding, taking its name from the 'keyhole' that is produced when the joint edges in a butt weld are melted as the plasma jet cuts through them. As the jet is moved forward, the molten metal is pressed backwards, filling up the joint behind the jet.
Figure 4.3 Keyhole welding. The main benefits of plasma welding are to be found in the fact that the keyhole welding method can be used for butt welds from about 3 rum up to 7-& mm. The keyhole
38
PLASMA WELDING
provides a guarantee of full penetration; by comparison the TIO method is only suitable for butt welds up to about 3-4mm thick. Joints with thicker materials have to be prepared with a V or U joint and then filled with filler material. Keyhole welding is not suitable for thinner materials below 3 mm: in these circumstances, the process becomes much more like TIO welding. Reducing the plasma gas flow to a low level can make the plasma torch work in the same way as a conventional TIO torch, which can be useful when making tack welds or cladding welds. The main advantage over conventional TIO welding is primarily the excellent arc stability. There are two types of working systems employed: with transferred and non-transferred arcs, as shown in Figure 4.4.
Electrode
Plasma nozzle
Transferred
Non-transferred
Figure 4.4 Transferred and non-transferred plasma welding arcs.
4.2
Equipment
The following equipment is required for plasma welding:
Welding torch The same basic requirements apply here as for TIO welding. Plasma welding torches are generally water-cooled. Power source Plasma welding employs DC, with a drooping characteristic, as for TIG welding. Open circuit voltage should be at least 80 V. HF generator In principle, the purpose of the HF generator is the same as in TIG welding. However, when used in plasma welding, the HF generator does not normally strike the main arc: instead, it strikes a pilot arc as a non-transferred arc, with the current flowing between the electrode and the inner gas nozzle. The pilot arc, in other words, can be maintained in air: as the torch approaches the workpiece, the main arc strikes and the pilot arc is extinguished. Control equipment The necessary control equipment depends on to what extent the welding process is mechanised. However, it is usual for the pre-flow and post-flow of the shielding gas, the
39
WELDING PROCESSES HANDBOOK
HF generator and the pilot are, to be automatically controlled. There is often automatic control to ensure that the arc is struck in pure argon, after which the gas supply changes over to the particular gas that is being used.
4.3
Gases for plasma welding
Normally, the same gas is used for both the plasma and the shielding gas. This avoids variations in the plasma jet, which would otherwise be the case if two different gases or gas mixtures were being used. An argonlhydrogen mixture is generally used as the plasma and shielding gas. However, hydrogen cannot be used as a constituent when welding mild steel or reactive metals such as zirconium or titanium. Mixtures of argon/helium/nitrogen are used when welding duplex stainless steels, as these contain nitrogen in their alloying. Pure helium is not suitable, as the resulting high heat losses in the plasma gas will substantially reduce the life of the plasma torch. Argonlhelium mixtures result in a higher energy in the plasma jet at constant current. However, the mixture must contain at least 50 % helium if any significant difference is to be noted. On the other hand, mixtures containing more than 75 % helium have the same characteristics as pure helium. Pure argon, or argonlhelium mixtures, are well suited to the welding of mild steel and reactive metals (titanium, aluminium, zirconium etc.), for which hydrogen or nitrogen cannot be used.
4.4
The advantages of the plasma method
Plasma welding has the following advantages, relative to those of other methods: 1. Reliable penetration with the keyhole method. 2. Very high speed: often 400 % higher than that of conventional TIG welding. 3. Butt welds possible in thick materials (8 mm) without the use of fillers. 4. Fusion welding possible even in very thin materials (0.03 mm). 5. Low weld convexity and root bead. This is particularly beneficial when welding structures that will be subjected to fatigue loading, in addition to reducing the work required in other welds where the weld convexity and root bead would otherwise have to be ground away. Plasma welding of 5 mm austenitic stainless steel produces a weld convexity of about 0.3 mm and a root bead of about 0.2 mm. Increasing the steel thickness to 8 mm results in corresponding respective thicknesses of 0.7 mm andO.6mm. 6. Low heat-affected zone and little distortion. 7. High arc stability at low arc currents. 8. Little sensitivity to arc length variations as a result of the concentrated arc. 9. Assessment of the weld quality possible while welding is in progress. 10. High metallurgical quality in comparison with that of conventional TIG welded materials.
11. Flexibility, due to the ability to perform keyhole welding and melt-in welds using the same equipment.
40