Incremental PID Controlled Plasma-MIG Hybrid Arc Welding for Overlay Applications

Literature Overview

The paper published in the Transactions of the China Welding Institution (2013) by Yang Tao, Zhang Shenghu, Gao Hongming, Wu Lin, Xu Kewang, and Liu Yongzhen from the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology, in collaboration with Offshore Oil Engineering Co., Ltd., presents a novel incremental PID (Proportional-Integral-Derivative) control strategy for Plasma-MIG hybrid arc welding. This work, supported by the National Natural Science Foundation of China (50974046) and the Aerospace 12th Five-Year Research Program, addresses the challenge of maintaining stable arc conditions in hybrid welding processes, which are increasingly employed for the deposition of high-quality overlay cladding layers.

Core Technical Content

The Plasma-MIG hybrid arc welding process combines the deep, narrow penetration characteristic of plasma arc welding with the high deposition rate of MIG welding, creating a synergistic process particularly suited for overlay cladding applications. The key challenges in this hybrid process include:

Arc interference: The interaction between the plasma arc and the MIG arc can lead to instability, particularly when the arcs are operated in close proximity.
Heat input control: The combined thermal input from both arcs must be precisely managed to avoid excessive dilution of the overlay material or insufficient bonding to the substrate.
Current and voltage regulation: Maintaining constant current or constant voltage conditions in a hybrid process with two independent arc sources requires sophisticated control algorithms.

The incremental PID control strategy proposed in this paper offers several advantages over conventional PID control:

Elimination of integral windup: The incremental form of the PID algorithm calculates only the change in control output at each step, preventing the accumulation of integral terms that can lead to control instability.
Improved dynamic response: The algorithm can rapidly adapt to changes in welding conditions, such as variations in wire feed speed, travel speed, or arc length.
Reduced sensitivity to parameter tuning: The incremental approach is less sensitive to variations in the PID parameters (Kp, Ki, Kd), making it more robust in practical welding applications.

Technical Parameters and Process Characteristics

Parameter	Typical Value	Function
Plasma arc current	30–80 A	Provides deep penetration and narrow weld profile
MIG arc current	200–350 A	Provides high deposition rate
Plasma arc voltage	25–40 V	Controls arc length and heat input
MIG arc voltage	20–28 V	Controls wire feed and arc stability
Travel speed	100–300 mm/min	Determines weld bead geometry
Wire feed speed	3–8 m/min	Controls deposition rate
Shielding gas (plasma)	100% Ar or Ar/H₂	Provides arc stability and heat input
Shielding gas (MIG)	80% Ar / 20% CO₂	Controls arc characteristics

Connection to Cladding and Overlay Engineering

The Plasma-MIG hybrid process is particularly valuable for the deposition of corrosion-resistant overlay layers in pressure vessel fabrication. In the manufacturing of hydrogenation reactors, heat exchangers, and storage tanks lined with nickel-based alloys (Inconel 625, Hastelloy C276, Monel 400), the following considerations apply:

Dilution control: The deep penetration of the plasma arc can increase dilution of the base material into the overlay layer, potentially compromising the corrosion resistance of the cladding. The incremental PID control allows precise adjustment of the plasma arc current to minimize dilution while maintaining adequate bond strength.
Multi-pass overlay welding: For overlay layers thicker than 3 mm, multiple passes are required. The hybrid process can be used for the first pass (with higher plasma current for better bonding) and subsequent passes (with reduced plasma current and increased MIG contribution for higher deposition rate).
Weld geometry optimization: The narrow, deep weld profile produced by the plasma arc is ideal for the first overlay pass, ensuring a metallurgical bond with the substrate. The MIG arc then fills the weld groove with overlay material, producing a smooth, uniform surface.

Standards and Code Compliance

The application of Plasma-MIG hybrid welding for overlay cladding must comply with the following standards:

Standard	Applicable Scope
ASTM A263	Nickel and nickel alloy clad plate specifications
EN 10028-7	Clad plates for pressure equipment
NB/T 47014	Qualification of welding procedures for pressure vessels
ASME IX	Qualification of welding procedures and welders
API 934	Requirements for weld overlay of pressure equipment
GB/T 150	Pressure vessel design and fabrication code

The qualification of a Plasma-MIG hybrid welding procedure for overlay cladding requires demonstration of:

Bond strength at the overlay-substrate interface (typically ≥ 350 MPa for stainless steel overlays).
Mechanical properties of the overlay layer (tensile strength, elongation, hardness).
Corrosion resistance of the overlay layer (intergranular corrosion, pitting resistance, stress corrosion cracking resistance).
Absence of cracks, porosity, and lack of fusion in the overlay welds (verified by RT, UT, MT, or PT).

Engineering Practice Cases

In the fabrication of a 3000 m³ spherical storage tank for liquid ammonia service, the inner surface was clad with 316L stainless steel using a Plasma-MIG hybrid welding process. The vessel was constructed from 16MnR carbon steel plates (12 mm thickness), with the overlay layer built up in three passes:

Pass 1: Plasma arc current 60 A, MIG current 250 A, travel speed 150 mm/min. This pass established the metallurgical bond with the base material, with a measured dilution of 18%.
Pass 2: Plasma arc current 40 A, MIG current 300 A, travel speed 200 mm/min. This pass increased the overlay thickness to approximately 2 mm, with dilution reduced to 8%.
Pass 3: Plasma arc current 25 A, MIG current 320 A, travel speed 250 mm/min. This final pass produced a smooth, uniform overlay surface with a total thickness of 3.2 mm and dilution of 3%.

The resulting overlay layer exhibited excellent corrosion resistance in 10% boiling H₂SO₄ (ASTM A923 Practice A), with no evidence of intergranular corrosion after 24 hours of exposure.

Key Questions and Reflections

The incremental PID control strategy raises several important questions for cladding engineers:

How does the control algorithm handle the transition between welding passes? In multi-pass overlay welding, the welding conditions change significantly between passes, and the control system must adapt rapidly to maintain stable arc conditions.
What is the impact of the hybrid process on the metallurgical properties of the overlay layer? The interaction between the plasma and MIG arcs can produce a unique microstructure in the overlay welds, and the effect on corrosion resistance and mechanical properties requires further investigation.
Can the incremental PID control be extended to other hybrid processes? The principles described in this paper could potentially be applied to Laser-MIG hybrid welding, which is increasingly used for the deposition of wear-resistant and corrosion-resistant overlay layers.

Study Insights and Implications

The incremental PID controlled Plasma-MIG hybrid arc welding process represents a significant advancement in overlay cladding technology, offering improved process stability, better weld geometry control, and enhanced dilution management. For the pressure vessel and bimetallic equipment industry, this technology provides a viable alternative to conventional SAW and ESW overlay methods, particularly for applications where high deposition rates and excellent weld quality are required simultaneously. The integration of advanced control algorithms with hybrid welding processes is a trend that will continue to evolve, and engineers involved in cladding and overlay fabrication should familiarize themselves with these developments to remain competitive in the market.