CLADDING TECHNOLOGY SHANXI CO., LTDSingle Power Source Plasma-MIG Hybrid Welding Method
Literature Overview
This paper, published in 1990 in the Welding Journal of China by Zhou Dazhong, Sun Jun, and Huang Ziping from the Harbin Welding Research Institute (Ministry of Mechanical and Electrical Industry), presents a novel hybrid welding method that combines plasma arc welding (PAW) and gas metal arc welding (GMAW / MIG) using a single power source. The innovation lies in the integration of the two welding processes into a unified system, where the plasma arc serves as the primary heat source for deep penetration and the MIG arc provides a high deposition rate through wire feeding. This hybrid approach aims to combine the deep penetration capability of plasma welding with the high productivity of MIG welding, while simplifying the equipment configuration by eliminating the need for separate power sources.
Core Technical Content
Process Configuration
The single power source plasma-MIG welding method operates on the principle that the plasma arc and the MIG arc share a common electrical circuit. The plasma arc is established between a consumable or non-consumable electrode and the workpiece, creating a high-energy-density arc with intense penetration. Simultaneously, a MIG wire is fed through the plasma torch or a nearby nozzle, and the MIG arc is established between the wire tip and the workpiece. The two arcs interact in the weld pool, creating a synergistic effect that enhances both penetration and deposition rate.
| Process Parameter | Plasma Arc Component | MIG Arc Component | Combined Effect |
|---|---|---|---|
| Current | 20–80 A | 100–250 A | Total 120–330 A |
| Voltage | 80–120 V | 20–30 V | Combined 100–150 V |
| Gas flow | 5–15 L/min (Ar) | 15–25 L/min (Ar/CO₂) | Enhanced shielding |
| Penetration | Deep (up to 3 mm/mm) | Shallow (0.5–1.5 mm/mm) | Deep with wide bead |
| Deposition rate | Low (0.5–2 kg/h) | High (5–15 kg/h) | High with good quality |
Arc Interaction Mechanism
The key technical challenge in this hybrid method is managing the interaction between the plasma arc and the MIG arc. The plasma arc, with its high current density and concentrated energy, creates a deep, narrow weld pool. The MIG arc, with its lower current density and broader energy distribution, provides additional heat input and metal deposition. The interaction between the two arcs affects:
- The weld pool geometry and penetration profile.
- The dilution ratio between the base metal and the filler metal.
- The cooling rate and resulting microstructure of the weld.
- The arc force and spatter generation.
The researchers demonstrate that the optimal separation distance between the plasma arc and the MIG arc is critical. If the arcs are too close, they interfere with each other, causing arc wandering and instability. If they are too far apart, the synergistic effect is lost, and the combined process offers no advantage over the individual processes.
Power Source Design Considerations
The single power source design requires careful consideration of the electrical characteristics of both arcs. The plasma arc operates at a higher voltage than the MIG arc, and the two arcs must be maintained simultaneously without one extinguishing the other. The power source must provide sufficient voltage and current capacity to sustain both arcs, while also allowing independent adjustment of the plasma arc current and the MIG wire feed rate. The control system must ensure that the plasma arc is established first, followed by the MIG arc, to avoid arc instability during the initial ignition phase.
Connection to Cladding and Overlay Welding
The plasma-MIG hybrid welding method has significant implications for cladding and overlay applications. The combination of deep penetration and high deposition rate makes this process particularly suitable for:
- Multi-pass overlay welding on thick carbon steel substrates, where deep penetration ensures good bonding between the base metal and the overlay layer.
- Building up wear-resistant or corrosion-resistant overlays on large surface areas, where the high deposition rate improves productivity.
- Applying nickel-based alloy overlays on thick sections, where the deep penetration of the plasma arc helps to reduce the dilution ratio by creating a deeper weld pool that can be controlled more precisely.
Application to Bimetal Pressure Vessels
For bimetal pressure vessel fabrication, the hybrid plasma-MIG process could be used for the overlay welding of corrosion-resistant materials on thick carbon steel or low-alloy steel substrates. The deep penetration capability of the plasma arc ensures that the overlay layer is well-bonded to the base metal, while the high deposition rate of the MIG arc allows for efficient multi-pass overlay buildup. The single power source configuration simplifies the welding equipment, reducing the complexity and cost of the welding setup, which is advantageous for field fabrication of large pressure vessels.
However, several challenges must be addressed when applying this process to pressure vessel fabrication:
- The heat input must be carefully controlled to avoid excessive grain growth in the base metal HAZ, which could compromise the mechanical properties of the pressure boundary.
- The dilution ratio must be monitored and controlled to ensure that the overlay layer maintains the required corrosion resistance. For nickel-based alloy overlays, a dilution ratio above 30% can significantly reduce the corrosion resistance.
- The welding procedure must be qualified per ASME Section IX or NB/T 47014, including mechanical property testing, metallographic examination, and corrosion testing of the overlay and the interface.
Key Questions and Reflections
The paper raises the question of whether the synergistic effect of the plasma-MIG hybrid process is sufficient to justify the additional complexity of the equipment. The single power source design is an elegant solution, but it requires sophisticated control electronics to manage the two arcs simultaneously. In modern welding technology, the availability of digital power sources with programmable waveforms and real-time control has made it easier to implement hybrid welding processes. The question of whether the hybrid approach offers a significant advantage over conventional multi-pass overlay welding using a single process (e.g., PTA or SAW) depends on the specific application requirements.
Another important consideration is the effect of the hybrid process on the microstructure of the overlay layer. The combination of the high heat input from the plasma arc and the rapid solidification from the MIG arc could result in a complex microstructure with varying grain sizes and phase distributions across the overlay thickness. This microstructural heterogeneity could affect the mechanical properties and corrosion resistance of the overlay, and must be carefully evaluated through metallographic analysis and property testing.
Study Insights and Implications
This paper represents an innovative approach to hybrid welding that has direct relevance to the development of advanced overlay welding processes. The key insight for cladding engineers is that combining different welding processes can create synergistic effects that improve both productivity and weld quality. The single power source design is particularly attractive for industrial applications where equipment simplicity and reliability are important. For bimetal pressure vessel fabrication, the hybrid plasma-MIG process could provide a viable alternative to conventional overlay methods, offering improved penetration and deposition rate while maintaining the control and precision required for critical pressure boundary applications. The challenge lies in qualifying the process per the relevant standards and demonstrating consistent performance across a range of materials and configurations.