What Is Variable Polarity Plasma Arc Welding?

Principle and development

Variable polarity plasma arc welding (VPPAW), namely asymmetric square wave AC plasma arc welding, is a new efficient welding process developed for aluminum and its alloys.

It combines the advantages of variable polarity TIG welding and plasma arc welding.

On the one hand, its characteristic parameters, current frequency, current amplitude and the ratio of positive and negative half waveguide passing time can be adjusted independently according to the process requirements, and the arc heat can be reasonably distributed to minimize the tungsten electrode burning loss while meeting the requirements of weldment melting and automatically removing the oxide film on the surface of weldment;

On the other hand, the characteristics of plasma beam, such as high energy density, high jet velocity and strong electric arc force, can be effectively used to form a piercing molten pool in the welding process to realize the single-sided welding and double-sided forming of aluminum alloy plates.

The variable polarity plasma arc welding technology is mainly used for the welding of various aluminum alloys.

The thickness of aluminum alloy in a single pass welding can reach 25.4mm.

VPPAW is characterized by that in the welding process, the amplitude of positive current (DCEN), the amplitude of reverse polarity current (DCEP), and the proportion of the duration of positive and reverse polarity current in a cycle can be independently adjusted, which is not only conducive to weld penetration, but also conducive to cleaning aluminum alloy oxide film.

VPPAW uses the small hole upward vertical welding process in aluminum alloy welding, which is not only conducive to the front formation of the weld, but also conducive to the escape of hydrogen in the molten pool, reducing the porosity defects of aluminum alloy, so it is called “zero defect welding” method.

Fig. 1a: Welding diagram

  • 1: Base metal;
  • 2: Fusion weld metal;
  • 3: Restraining arc nozzle
  • 4: Shielding gas;
  • 5: Coolant;
  • 6: Isoionic gas
  • 7: Welding wire;
  • 8: Plasma arc perforation

Fig. 1b: Variable polarity waveform

Fig. 1 shows VPPA perforation vertical welding and common current waveform.

In order to reduce the burning loss of the tungsten electrode, the amplitude of the reverse polarity current is higher than that of positive polarity, and the pulse width ratio of positive and negative polarity is about 19:4.

Foreign experience shows that for different aluminum alloys, the positive and negative polarity amplitude and pulse width parameters are slightly different, as shown in Table 1.

Table 1: Welding parameters of various aluminum alloys (5mm plate thickness)

Aluminum alloy materialsDCEN time/msDCEP time/msDCEN current/ADCEP current/A

Advantages of VPPAW keyhole welding

There are two main plasma arc welding process methods, namely, penetration method and keyhole method.

When the weldment is thin, the fusion method is often used for welding; When the weldment is thick, the small hole method is often used for welding.

When the keyhole method is used for welding, the plasma arc will completely melt the weldment and form a keyhole penetrating the weldment under the action of plasma flow force.

The molten metal is squeezed around the keyhole.

With the movement of the plasma arc in the welding direction, the molten metal moves along the weld pool wall around the arc to the rear of the weld pool, and crystallizes on the front and back, realizing single-sided welding and double-sided forming.

The research work of Boeing Company, Hobart Company and NASA Marshall space flight center in this area shows that keyhole plasma arc welding is the most ideal aluminum alloy welding method in aluminum alloy welding.

Compared with the general non compression TIG welding, the small hole VPPA welding method has many outstanding characteristics in the process.

1. There are few defects in the weld, such as air holes, slag inclusion, etc.

In the process of keyhole plasma arc welding, the plasma arc and the ion gas flow through the keyhole play a certain role in flushing.

In other welding methods, the gas left in the molten metal to generate pores will be taken away by the plasma arc and the ion gas flow through the keyhole, and the slag inclusion will also be washed away.

Reiner knock found that when conducting reverse polarity keyhole plasma arc welding of aluminum alloy, compared with TIG welding, the porosity was significantly reduced, and the welding effect for pure aluminum was more significant, basically without porosity.

Related reading: MIG vs TIG Welding

2. Wide range of weldable thickness.

The plasma arc has a strong penetration ability, and it can realize welding at various positions for aluminum alloy with a thickness of 6mm.

The research results show that if the welding wire is not filled and the flat plate is butt welded, the maximum thickness of single pass welding is 8mm.

If thicker materials are welded, the vertical welding method must be used.

For aluminum alloy below 15.9mm, one-time penetration welding is allowed;

For aluminum alloys above 15.9mm, it is usually necessary to prepare more complex welding joints.

Now, 25.4mm thick aluminum alloy has been used for one-time penetration welding.

At present, the maximum weldable thickness of single pass welding seems not to be limited by the welding method itself, but by the power of the welding power source.

If the rated power of the welding power source is increased, it is possible to weld thicker materials.

3. The weldment is small after welding.

Due to the strong penetration ability of plasma arc, concentrated heating, small melting area, and uniform heating of the front and back sides of the weldment by keyhole welding, the bending deformation of the weldment after welding is reduced.

Compared with welding, the bending deformation of the weldment is significantly reduced.

4. The mechanical properties of welds have been improved.

Compared with TIG welding seam, the yield strength of keyhole plasma arc welding seam is not much different in the post welding state.

However, the yield strength of keyhole plasma arc welding seam is higher than that of TIG welding seam when the root seam is scraped off and thickened.

This shows that the quality of plasma arc welding is higher than the weld quality of other arc welding methods to a certain extent, with good mechanical properties and small weld deformation.

5. High efficiency and low cost.

Due to the high energy density and strong penetrability of plasma arc, the weldable thickness of keyhole plasma arc welding is large.

Especially for thick plate welding, the number of weld passes is greatly reduced, and there are few defects such as pores and slag inclusions in the weld.

The deformation of welded joints is small, which reduces the workload of post welding inspection and repair.

The groove can be used for the joints, which is less sensitive to oil contamination and requires less preparation before welding.

It is obviously less than TIG welding and MIG welding in terms of time and cost.

It is a high efficiency and low-cost welding method.

Deficiencies of VPPAW welding process for aluminum alloy

1. There are many variable welding parameters and narrow specification range;

2. The vertical upward vertical welding process is adopted, which can only be automatically welded;

3. The welding gun has a great impact on the weld quality, and the nozzle life is short.

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