CLADDING TECHNOLOGY SHANXI CO., LTDArc Behavior of Plasma-MIG Hybrid Welding of Aluminum Alloys
Literature Overview
Published in the Journal of Welding (焊接学报) in 2022, this paper by Han Jiao, Han Yongquan, Hong Haitao, and Wang Xuelong from the Ministry of Education Advanced Lightweight Metal Materials Development and Processing Protection Engineering Research Center at Inner Mongolia University investigates the arc behavior of plasma-MIG hybrid welding applied to aluminum alloys. The research is supported by the National Natural Science Foundation of China (51665044), the Inner Mongolia Autonomous Region Science and Technology Department (2020GG0313), and the Inner Mongolia Natural Science Foundation (2019LH05017).
Core Technical Content
Plasma-MIG hybrid welding combines a plasma arc with a MIG (Metal Inert Gas) arc in a single welding process. The plasma arc provides high energy density and deep penetration, while the MIG arc contributes filler metal deposition and arc stability. This hybrid approach is particularly attractive for aluminum alloys, which present unique welding challenges due to their high thermal conductivity, low melting point, and tendency to form porosity.
Arc Configuration and Geometry
The plasma-MIG hybrid configuration involves two distinct arcs operating in close proximity:
- Plasma arc: Generated by a compressed gas stream through a constricted nozzle, producing a narrow, high-energy-density arc (typically 200–400 A, 20–40 V).
- MIG arc: A conventional gas-shielded metal arc (typically 150–300 A, 18–28 V) with continuous wire feed.
The spatial relationship between the two arcs is critical. The plasma arc is typically positioned slightly ahead of the MIG arc in the travel direction, creating a synergistic interaction where the plasma arc preheats and melts the base metal, and the MIG arc deposits filler metal into the prepared weld pool.
| Parameter | Plasma Arc | MIG Arc | Combined Effect |
|---|---|---|---|
| Current (A) | 200–400 | 150–300 | 350–700 total |
| Voltage (V) | 20–40 | 18–28 | 30–50 combined |
| Energy Density (kW/cm²) | 10–30 | 1–3 | 15–35 |
| Penetration Depth (mm) | 2–5 | 1–3 | 3–7 |
| Weld Width (mm) | 3–5 | 4–8 | 5–10 |
Arc Stability and Interaction
The interaction between the plasma and MIG arcs is complex and multifaceted. Key phenomena include:
- Magnetic interaction: The plasma arc's constricted geometry creates a strong magnetic field that can deflect the MIG arc, requiring careful alignment of the two torches.
- Thermal interaction: The plasma arc preheats the weld pool, reducing the energy required from the MIG arc for penetration. This thermal synergy allows for higher travel speeds or reduced total current.
- Electrical interaction: When the two arcs are sufficiently close, they can interact electrically, potentially creating an unstable arc or even merging into a single arc. Maintaining a separation distance of 2–5 mm between the arc roots is essential for stable operation.
The authors report that arc stability is most sensitive to the travel speed and torch angle. At travel speeds below 200 mm/min, the plasma arc tends to dominate and the MIG arc becomes unstable. At speeds above 800 mm/min, the weld pool becomes too elongated and porosity formation increases.
Droplet Transfer Characteristics
In plasma-MIG hybrid welding of aluminum alloys, the droplet transfer mode is predominantly short-circuiting transfer for the MIG arc, with occasional globular transfer at higher currents. The plasma arc does not directly contribute to filler metal deposition but affects the MIG arc's droplet transfer through:
- Reduced surface tension: The plasma arc's heat input reduces the weld pool surface tension, facilitating droplet detachment.
- Electromagnetic forcing: The combined magnetic fields from both arcs increase the electromagnetic force on the MIG arc's droplet, promoting more frequent and smaller droplet detachment.
- Arc length control: The plasma arc's presence effectively shortens the MIG arc length, stabilizing the transfer mode.
Aluminum Alloy Welding Challenges
Aluminum alloys present several unique challenges that the plasma-MIG hybrid approach addresses:
- High thermal conductivity: Aluminum's thermal conductivity (approximately 200 W/m·K for pure aluminum) causes rapid heat dissipation from the weld pool. The plasma arc's high energy density compensates for this by concentrating heat in a smaller area.
- Oxide film formation: The aluminum oxide (Al₂O₃) layer has a melting point of approximately 2050°C, far above aluminum's melting point of 660°C. The plasma arc's high energy density effectively breaks through this oxide layer, ensuring proper fusion.
- Hydrogen porosity: Aluminum readily absorbs hydrogen from the atmosphere and flux. The plasma arc's narrow, high-velocity gas stream provides superior shielding, reducing hydrogen pickup.
- Hot cracking susceptibility: Many aluminum alloys (particularly 2xxx and 7xxx series) are susceptible to hot cracking. The plasma-MIG hybrid's ability to control the weld pool geometry and cooling rate helps mitigate this.
Process Parameters and Optimization
The optimal process parameters for plasma-MIG hybrid welding of aluminum alloys depend on the specific alloy, plate thickness, and joint configuration. Based on the literature and general practice:
| Plate Thickness (mm) | Plasma Current (A) | MIG Current (A) | Travel Speed (mm/min) | Shielding Gas |
|---|---|---|---|---|
| 1.5–3 | 150–200 | 100–150 | 400–600 | Ar (plasma) + Ar (MIG) |
| 3–6 | 250–350 | 150–200 | 300–500 | Ar (plasma) + Ar/He (MIG) |
| 6–10 | 350–400 | 200–250 | 200–400 | Ar/He (plasma) + Ar/He (MIG) |
The use of helium in the shielding gas mixture increases arc energy and penetration depth, which is beneficial for thicker plates. However, helium is more expensive than argon and has lower ionization potential, requiring higher arc voltage.
Engineering Applications
Plasma-MIG hybrid welding of aluminum alloys is particularly suited for:
- Automotive body-in-white welding: Where high production rates and consistent weld quality are required.
- Aerospace structural components: Where lightweight aluminum alloys must be joined with high integrity.
- Railway vehicle fabrication: Where aluminum car bodies require reliable, high-speed welding.
- Heat exchanger tube-to-header joints: Where the combination of deep penetration and good fusion is critical.
For the bimetal pressure vessel industry, the plasma-MIG hybrid technique could be applied to aluminum-clad steel pressure vessels, where the aluminum cladding layer must be joined to the steel base plate without excessive heat input that would compromise the cladding bond.
Key Questions and Reflections
A fundamental question in plasma-MIG hybrid welding is the optimal balance between the two arc energies. If the plasma arc dominates, the process reduces to plasma welding with filler wire, losing the benefits of the MIG arc. If the MIG arc dominates, the plasma arc becomes unnecessary. The synergy zone where both arcs contribute optimally is relatively narrow and requires careful parameter optimization.
Another concern is the equipment complexity and cost. Plasma-MIG hybrid welding requires two separate power sources, two gas supply systems, and a dual-torch configuration. This increases capital expenditure and maintenance requirements compared to single-process welding. For high-volume production, this cost may be justified by the improved productivity and weld quality. However, for small-batch or custom fabrication, the economics may not favor this approach.
The long-term reliability of plasma-MIG hybrid welds in aluminum alloys, particularly under fatigue loading and corrosion exposure, requires further investigation. The weld pool's rapid cooling rate may lead to microstructural heterogeneity, including unmelted particles or precipitate-free zones that could serve as crack initiation sites.
Study Insights and Implications
This research contributes valuable knowledge to the understanding of arc behavior in plasma-MIG hybrid welding of aluminum alloys. The findings on arc interaction, droplet transfer, and process parameter optimization provide a solid foundation for industrial implementation.
For the cladding and bimetal manufacturing industry, the plasma-MIG hybrid technique represents a potential advancement for joining aluminum cladding to steel substrates. The high energy density of the plasma arc could improve fusion with the steel base plate while the MIG arc deposits aluminum filler metal, potentially reducing the intermetallic layer thickness at the interface.
The technique's potential for high-speed welding also makes it attractive for automated production environments, where consistent quality and high throughput are essential. However, the investment in dual-source equipment and process development must be carefully evaluated against the benefits of improved weld quality and productivity.