CLADDING TECHNOLOGY SHANXI CO., LTDResearch and Application of TIG Welding Lift-Arc Method
Literature Overview
This 1997 publication by Du Dong, Han Zandong, Zhang Renhao, Gong Huasong, Zhao Fengliang, and Ge Wanjun from Tsinghua University and Tianjin Electric Welding Machine General Factory investigates the development and application of a lift-arc (also known as scratch-free or contactless) method for initiating the TIG welding arc. The lift-arc method eliminates the need for the traditional scratch-start technique, which can introduce tungsten contamination, tungsten inclusions, and crater defects at the weld start point.
Technical Background and Problem Statement
The traditional methods of initiating a TIG arc include:
- Scratch method (contact start): The tungsten electrode is touched to the workpiece and quickly lifted to create an arc. This method risks tungsten contamination of the electrode tip and the weld pool, leading to tungsten inclusions in the weld metal.
- High-frequency (HF) start: An HF signal is applied between the electrode and workpiece to ionize the gas gap and initiate the arc without contact. While effective, HF generation equipment is complex, expensive, and can cause electromagnetic interference (EMI) with nearby electronic equipment.
- Pilot arc method: A small pilot arc is established between the electrode and a separate electrode or the torch body, and this pilot arc is then transferred to the workpiece.
The lift-arc method offers a compromise between these approaches: it uses a controlled lift of the electrode from a low starting position to initiate the arc without direct contact, combining the simplicity of the scratch method with the cleanliness of the HF method.
Lift-Arc Method Principle and Implementation
The lift-arc method operates on the following principle:
- The tungsten electrode is positioned at a very close distance (0.5–2.0 mm) from the workpiece surface.
- A controlled mechanical lift mechanism raises the electrode at a precise rate (typically 10–50 mm/s).
- As the electrode is lifted, the electric field between the electrode and workpiece increases, eventually ionizing the gas gap and establishing a stable arc.
- The welding current is then applied at the moment of arc establishment.
| Parameter | Typical Value | Range |
|---|---|---|
| Initial electrode-to-work distance | 1.0 mm | 0.5–2.0 mm |
| Lift speed | 20 mm/s | 10–50 mm/s |
| Lift distance | 3.0–5.0 mm | 2.0–6.0 mm |
| Arc establishment time | 0.05–0.2 s | 0.03–0.3 s |
| Welding current ramp-up time | 0.1–0.5 s | 0.05–1.0 s |
Comparison of Arc Initiation Methods
| Method | Tungsten Contamination | Equipment Complexity | Cost | EMI Risk | Reliability |
|---|---|---|---|---|---|
| Scratch | High | Low | Low | None | Moderate |
| HF start | None | High | High | High | High |
| Pilot arc | Low | Moderate | Moderate | Low | Moderate |
| Lift-arc | Very low | Moderate | Moderate | Low | High |
Application in Cladding and Pressure Vessel Welding
The lift-arc method has particular significance for cladding and pressure vessel welding applications:
Cladding Applications
- Weld-overlay cladding of pressure vessels: The weld start and stop points are critical locations for defect initiation. The lift-arc method eliminates tungsten contamination at the start point, which is particularly important for corrosion-resistant cladding layers where even small inclusions can compromise corrosion resistance.
- Multi-pass cladding: Each pass requires a new arc start; the lift-arc method ensures consistent, clean starts for every pass, maintaining uniform cladding layer quality.
- Bond line integrity: Clean arc initiation reduces the risk of oxide inclusions at the bond line between the cladding layer and the base metal.
Pressure Vessel Applications
- Nozzle-to-shell welds: The start and stop points of nozzle welds are subject to high stress concentrations; eliminating tungsten contamination at these locations is critical for fatigue life.
- Circumferential welds: Long circumferential welds require multiple arc starts; the lift-arc method ensures consistent quality throughout the weld length.
- Code compliance: The lift-arc method supports compliance with code requirements for weld quality, particularly in nuclear and high-pressure applications where tungsten inclusions are unacceptable.
Quality Control and Verification
The effectiveness of the lift-arc method must be verified through the following quality control measures:
- Visual inspection: The weld start point must be free of tungsten inclusions, excessive reinforcement, and crater defects.
- Radiographic testing (RT): The weld start region must be examined for tungsten inclusions (classified as Type 5 defects per ISO 17636-1).
- Ultrasonic testing (UT): The weld start region must be examined for lack of fusion and incomplete penetration, which can be exacerbated by improper arc initiation.
- Metallographic examination: Cross-sections through the weld start point should be examined for tungsten inclusions, micro-porosity, and oxide films.
- Mechanical testing: Weld start and stop regions should be included in mechanical test specimens to verify that properties are representative of the weld as a whole.
Key Reflections
The lift-arc method represents a practical engineering solution to a fundamental problem in TIG welding: the reliable and clean initiation of the welding arc. The key insight is that arc initiation quality has a direct and measurable impact on weld quality, particularly at the weld start point where defects are most likely to occur.
From a process development perspective, the lift-arc method requires careful integration of mechanical lift control, electrical timing, and welding parameter control. The timing of the current ramp-up relative to the lift motion is critical: if the current is applied too early, the electrode may contact the workpiece; if applied too late, the arc may not establish reliably. This timing relationship must be optimized for each specific torch-electrode-workpiece combination and documented in the WPS.
A particularly important consideration for cladding applications is the interaction between the lift-arc method and the cladding process parameters. In multi-pass cladding, the arc start point of each subsequent pass overlaps with the previous pass, and any defects at the start point can propagate through the cladding layer. The lift-arc method's ability to provide clean, consistent arc starts is therefore not merely a convenience but a critical quality control measure.
The study also highlights the importance of operator training and procedural discipline. Even with an automated lift-arc system, the operator must ensure proper torch positioning, electrode preparation, and joint preparation to achieve optimal results. The lift-arc method should be considered as one element of a comprehensive welding quality system, not as a standalone solution.
The long-term significance of this work lies in demonstrating that relatively simple mechanical and electrical modifications to standard TIG equipment can yield significant improvements in weld quality, particularly for critical applications in pressure vessel and cladding fabrication where weld start quality directly impacts structural integrity and service life.