Principle and development
Variable polarity plasma arc welding (VPPAW), i.e. asymmetric square wave AC plasma arc welding, is a new efficient welding process developed for aluminum and its alloys.
This welding technique combines the benefits of variable polarity TIG welding and plasma arc welding. It allows independent adjustment of key parameters such as current frequency, amplitude and passage time ratio of the positive and negative waveguide medium to meet the specific requirements of the welding process.
This results in a more reasonable distribution of arc heat, minimizing tungsten electrode loss and ensuring that the welding automatically melts and removes any oxide film on the surface. Furthermore, the high energy density, speed and strong electric arc force of the plasma beam can effectively create a piercing molten pool, enabling single-sided and double-sided welding of aluminum alloy plates.
Variable polarity plasma arc welding technology is mainly used for welding various aluminum alloys.
The thickness of aluminum alloy that can be achieved in single pass welding is 25.4 mm.
VPPAW, or Variable Polarity Plasma Arc Welding, is known for its ability to independently adjust the positive current amplitude (DCEN), the reverse polarity current amplitude (DCEP), and the positive to reverse polarity current duration ratio in each cycle. This feature contributes to both improving weld penetration and removing the aluminum alloy oxide film.
When welding aluminum alloys, VPPAW employs the vertical welding process with small holes upwards, which not only assists in the formation of the weld at the front, but also facilitates the release of hydrogen from the molten pool, thus reducing porosity defects in the aluminum alloy. As a result, this method is known as the “zero defect welding” technique.
Fig. 1a: Welding diagram
- 1: Base metal;
- 2: Metal fusion welding;
- 3: Restriction arc nozzle
- 4: Shielding gas;
- 5: Coolant;
- 6: Isoionic gas
- 7: Welding wire;
- 8: Plasma Arc Drilling
Fig. 1b: Variable polarity waveform
Figure 1 shows vertical welding with VPPA perforation and common current waveform.
To reduce tungsten electrode waste, the amplitude of the reverse polarity current is greater than that of the positive polarity, with a positive to negative polarity pulse width ratio of approximately 19:4.
According to foreign studies, the amplitude and pulse width parameters for positive and negative polarities may vary slightly for different aluminum alloys, as shown in Table 1.
Table 1: Welding parameters of various aluminum alloys (plate thickness 5 mm)
| Aluminum Alloy Materials | DCEN time/ms | DCEP time/ms | DCEN/A current | DCEP/A current |
| 5456 | 19 | 3 | 130 | 185 |
| 2219 | 19 | 3 | 140 | 180 |
| 5086 | 19 | 4 | 145 | 180 |
Advantages of VPPAW Keyhole Welding
There are two main plasma arc welding techniques: the penetration method and the keyhole method.
For thin weldments, the fusion method is commonly used, while for thick weldments, the small hole method is more often used.
When using the keyhole method, the plasma arc completely melts the welding and creates a keyhole that penetrates the material under the force of the plasma flow.
The molten metal is then forced around the keyhole.
As the plasma arc moves in the welding direction, the molten metal flows along the weld pool wall and solidifies both at the front and back, resulting in one-sided and double-sided welding.
Research conducted by the Boeing Company, Hobart Company, and NASA Marshall Space Flight Center has shown that keyhole plasma arc welding is the most effective method for welding aluminum alloys.
Compared with traditional TIG welding, the small hole VPPA welding technique has many advantageous qualities in the welding process.
1. There are few defects in the weld, such as air holes, slag inclusion, etc.
In the keyhole plasma arc welding process, the plasma arc and ionic gas flow through the keyhole play an important role in discharge.
Compared to other welding methods, the plasma arc and ionic gas flow through the keyhole removes any gas that would otherwise create pores in the molten metal and removes slag inclusions.
Reiner Knock discovered that when reverse polarity plasma arc welding of aluminum alloys was performed, porosity was significantly reduced compared to TIG welding. The effect of welding on pure aluminum was even more pronounced, resulting in virtually no porosity.
Related Reading: MIG vs TIG Welding
2. Wide range of weldable thicknesses.
The plasma arc has a powerful penetration capacity and is capable of welding 6 mm thick aluminum alloys in different positions.
The research results indicate that for flat plates butt-welded with unfilled welding wire, the maximum single-pass welding thickness is 8 mm.
When welding thicker materials, the vertical welding method should be employed.
Single pass penetration welding is permitted for aluminum alloys with a thickness of less than 15.9 mm.
Complex welding joints are often required for aluminum alloys thicker than 15.9 mm.
Currently, aluminum alloys 25.4 mm thick can be subjected to single-pass penetration welding.
Currently, the maximum thickness of single-pass welding does not appear to be limited by the welding method itself, but rather by the power of the welding power source.
By increasing the rated power of the welding power source, it becomes possible to weld thicker materials.
3. The welding is small after welding.
The welding bending deformation after welding is reduced due to the strong penetration ability of plasma arc welding, which results in concentrated heating and a small melting area, as well as uniform heating of both sides of the welding through welding keyhole type.
Compared with traditional welding methods, plasma arc welding significantly reduces the bending deformation of welding.
4. The mechanical properties of welds have been improved.
The yield strength of plasma arc welding seam is not significantly different from that of TIG welding seam after welding. However, if the root seam is removed and thickened, the yield strength of the keyhole plasma arc welding seam is higher than that of the TIG welding seam. This indicates that plasma arc welding is of superior quality to other arc welding methods, with improved mechanical properties and minimal deformation of the weld.
5. High efficiency and low cost.
Due to its high energy density and strong penetrability, plasma arc welding has a large weldable thickness. This makes it ideal for welding thick sheets, as it reduces the number of welding passes and minimizes defects such as porosity and slag inclusions.
The resulting welded joints exhibit minimal deformation, reducing the need for post-weld inspection and repair. Additionally, the groove can be used for joints and is less sensitive to oil contamination, requiring less preparation before welding.
Compared to TIG and MIG welding, plasma arc welding is more efficient and economical. It is a high-efficiency and low-cost welding method.
Shortcomings of VPPAW welding process for aluminum alloy
1. There are countless variables in welding parameters with a narrow range of specifications.
2. The welding process used is the vertical upward welding method, which can only be performed automatically.
3. The welding quality is greatly affected by the welding gun and the nozzle has a limited service life.
