CLADDING TECHNOLOGY SHANXI CO., LTDArc Initiation Mechanism in Plasma-MIG Hybrid Welding Process
Literature Overview
This study by Li Deyuan, Zhang Yishun, and Dong Xiaoqiang from the School of Materials Science and Engineering, Shenyang University of Technology, published in the Welding Journal in 2007, addresses a fundamental yet practically critical aspect of hybrid plasma-MIG welding: the arc initiation process. The plasma-MIG hybrid welding technique combines the deep penetration characteristics of plasma arc with the high deposition rate of MIG (Gas Metal Arc) welding, making it particularly attractive for thick-section cladding applications and heavy-walled vessel fabrication. However, the successful and stable initiation of the plasma arc in a hybrid configuration with a simultaneous MIG arc presents unique challenges that directly affect process reliability and weld quality.
Core Technical Content
The authors investigated the arc starting characteristics of the plasma-MIG hybrid process, focusing on the interaction between the plasma arc and the MIG arc during initiation. The key findings revolve around several critical phenomena:
- The plasma arc, being a constricted arc with higher current density (typically 200–400 A in a 6–10 mm transfer nozzle), requires a distinct ignition sequence compared to conventional MIG welding.
- The presence of the MIG arc during plasma arc initiation creates electromagnetic interference that can destabilize the plasma arc formation.
- The arc transfer characteristics differ significantly when both arcs are established simultaneously versus sequentially.
Process Parameters and Arc Initiation Characteristics
| Parameter | Typical Range | Effect on Arc Initiation |
|---|---|---|
| Plasma arc current | 100–400 A | Higher current facilitates easier arc strike but increases spatter |
| MIG arc current | 150–350 A | Simultaneous initiation with plasma can cause arc instability |
| Shielding gas flow rate | 15–25 L/min (plasma) + 10–18 L/min (MIG) | Insufficient gas causes arc wandering; excess causes turbulence |
| Transfer nozzle diameter | 6–10 mm | Determines arc constriction and stability |
| Arc length (plasma) | 2–4 mm | Must be maintained for stable plasma arc |
| Arc length (MIG) | 8–15 mm | Standard short-circuit or spray transfer range |
Technical Analysis and Interpretation
The study reveals that the arc initiation sequence is a decisive factor in hybrid plasma-MIG welding stability. When the plasma arc is initiated first and the MIG arc is struck subsequently, the plasma arc tends to be more stable because its transfer nozzle geometry provides a defined arc path. Conversely, simultaneous arc initiation leads to electromagnetic interference where the MIG arc's electromagnetic field distorts the plasma arc's constricted path, potentially causing arc blow or arc wandering.
The electromagnetic interaction between the two arcs is governed by the Lorentz force acting on the plasma arc column. The MIG arc, carrying substantial current through the wire electrode, generates a magnetic field that exerts force on the plasma arc plasma. This force is proportional to the product of the MIG current and the plasma current, and inversely proportional to the distance between the two arcs. In practice, this means that the geometric arrangement of the twin torches—typically offset by 30–60 mm with an included angle of 30°–90°—must be carefully optimized to minimize adverse electromagnetic interaction during arc initiation.
Engineering Practice Implications
From a practical standpoint, this research has direct implications for:
- Cladding applications on thick-section vessels: When applying nickel-based alloy cladding (e.g., Inconel 625 or Hastelloy C276) to carbon steel or low-alloy steel pressure vessels, the plasma-MIG hybrid process offers superior dilution control compared to conventional MIG overlay. The deep plasma arc penetration ensures good metallurgical bonding at the interface, while the MIG arc provides high deposition rate. However, reliable arc initiation is essential for long production runs where hundreds of weld passes may be required.
- Weld overlay qualification procedures: According to NB/T 47014 and ASME IX, the welding procedure specification (WPS) must define the arc initiation method. The findings of this study suggest that sequential arc initiation (plasma first, then MIG) should be specified in the WPS for hybrid plasma-MIG overlay processes to ensure repeatability and qualification success.
- Equipment design considerations: The study highlights the need for independent arc initiation circuits with sufficient inductance to prevent current interference during the strike sequence. Modern hybrid welding power sources with independent plasma and MIG channels, featuring programmable arc start sequences, are now available and represent the engineering solution to the problems identified in this 2007 publication.
Key Questions and Reflections
Several questions arise from this study that remain relevant in contemporary practice:
- How does the arc initiation sequence affect the dilution rate at the cladding base metal interface? This is critical for hydrogenation reactors where the dilution of the stainless steel or nickel alloy overlay layer must be controlled below specified limits (typically less than 30% for 300-series stainless steel cladding per GB/T 150 requirements).
- What is the effect of arc initiation transients on the microstructure of the first weld pass? The thermal cycle during arc initiation may produce a different microstructure compared to steady-state welding, potentially affecting the intergranular corrosion resistance of the overlay layer.
- Can the findings be extended to other hybrid configurations, such as plasma-GTAW or laser-MIG? The electromagnetic interaction principles are universal, but the specific parameters and geometries differ significantly.
Study Insights and Implications
This relatively concise study addresses a practical problem that, while seemingly minor, has profound implications for production reliability in hybrid welding applications. In my experience with cladding fabrication, arc instability during initiation is one of the most common causes of rework in multi-pass overlay welding. A single unstable arc strike can produce a weld start defect that requires grinding and re-welding, potentially compromising the bond strength of the cladding layer. The systematic investigation of arc initiation in hybrid configurations provides a scientific basis for optimizing torch sequencing and power source programming.
The study also underscores the importance of understanding electromagnetic interactions in multi-arc processes—a principle that has since been extended to laser-MIG hybrid welding, where the laser's electromagnetic field also interacts with the MIG arc. For engineers involved in the fabrication of bimetallic pressure vessels using hybrid welding processes, understanding these fundamental arc physics is essential for developing robust welding procedures that minimize defects and ensure consistent overlay quality throughout long production runs.