CLADDING TECHNOLOGY SHANXI CO., LTDArc Morphology and Characteristic Temperature Evolution in Pulsed TIG Welding Based on Arc Imaging
Literature Overview
The study by Cheng Shijia, Zhu Zhiming, and Fu Pingpo, published in Journal of Tsinghua University (Science and Technology) (2021), investigates the arc morphology and characteristic temperature evolution in pulsed TIG welding using arc imaging techniques. Conducted at the Department of Mechanical Engineering, Tsinghua University, within the Key Laboratory of Advanced Formulation and Manufacturing (Ministry of Education), and supported by the National Natural Science Foundation of China (51775301), this research provides direct experimental evidence of arc behavior under pulsed current conditions.
Core Technical Content
Arc Imaging Methodology
The researchers employed high-speed imaging and spectral analysis techniques to capture the dynamic behavior of the TIG arc under pulsed current. Key measurement parameters include:
| Parameter | Specification |
|---|---|
| Camera frame rate | 10,000–100,000 fps |
| Spectral range | 200–1000 nm |
| Spatial resolution | 0.1–0.5 mm/pixel |
| Exposure time | 1–10 μs |
| Arc current range | 5–200 A peak |
| Pulse frequency | 1–100 Hz |
| Background current | 5–50 A |
The arc imaging technique allows direct observation of the arc column shape, arc root behavior, and plasma plume dynamics during each phase of the pulse cycle. This is a significant advancement over conventional arc measurement techniques that rely on indirect measurements such as arc voltage and current.
Arc Morphology Evolution
The arc morphology undergoes distinct changes during each phase of the pulse cycle:
Peak Current Phase:
- The arc column contracts due to increased Lorentz forces (pinch effect).
- Arc diameter decreases by 30–50% compared to the background phase.
- Arc pressure increases significantly, reaching values of 0.05–0.2 MPa.
- The arc root becomes more concentrated, increasing current density at the electrode-workpiece interface.
- The plasma plume becomes more turbulent and elongated due to increased gas flow velocity.
Background Current Phase:
- The arc column expands as Lorentz forces decrease.
- Arc diameter increases by 50–100% compared to the peak phase.
- Arc pressure decreases to near-ambient levels.
- The arc root becomes more diffuse, reducing current density.
- The plasma plume becomes more stable and less turbulent.
Transition Phases:
- Rapid arc contraction during the rising edge of the pulse.
- Gradual arc expansion during the falling edge.
- Potential arc instability during rapid transitions at high frequencies.
Characteristic Temperature Evolution
The arc imaging data, combined with spectral analysis, allows determination of the arc temperature at different locations and pulse phases:
| Location / Phase | Peak Current | Background Current |
|---|---|---|
| Arc root (cathode) | 8000–12000 K | 5000–7000 K |
| Arc column center | 10000–15000 K | 6000–9000 K |
| Arc column edge | 8000–10000 K | 5000–7000 K |
| Plasma plume | 6000–8000 K | 4000–6000 K |
| Workpiece surface | 2000–3000 K | 1000–1500 K |
The temperature evolution during the pulse cycle has direct implications for the welding process:
- Peak temperature during peak current determines the maximum penetration depth and dilution.
- Temperature during background current determines the degree of solidification between pulses, affecting bead geometry and microstructure.
- Temperature gradient between the arc root and the workpiece surface drives the molten pool flow patterns.
Quantitative Arc Parameters
The arc imaging technique allows quantitative measurement of the following arc parameters:
| Parameter | Measurement Method | Typical Range |
|---|---|---|
| Arc diameter | Image analysis | 2–10 mm |
| Arc length | Image analysis | 1–5 mm |
| Arc pressure | Momentum balance | 0.01–0.2 MPa |
| Arc voltage | Electrical measurement | 10–25 V |
| Arc current density | Current / area | 10³–10⁵ A/mm² |
| Electron temperature | Spectral analysis | 1–3 eV |
| Ion temperature | Spectral analysis | 0.5–1.5 eV |
Key Technical Points and Reflections
Process Optimization Based on Arc Morphology
The understanding of arc morphology evolution provides a basis for optimizing pulsed TIG welding parameters for specific applications:
- Dilution control: By adjusting the peak current and background current ratio, the arc diameter and penetration can be controlled to achieve the desired dilution level. A higher peak-to-background ratio produces deeper penetration and potentially higher dilution.
- Bead geometry control: The arc diameter during the background phase determines the bead width, while the arc diameter during the peak phase determines the penetration depth. By adjusting the pulse parameters, the weld geometry can be tailored to the specific application.
- Microstructural control: The cooling rate between pulses is determined by the background current level and duration. A lower background current or longer background time increases the cooling rate, promoting finer grain structures.
- Defect prevention: Understanding the arc morphology helps identify conditions that lead to defects such as porosity (excessive arc instability), lack of fusion (insufficient arc diameter), and undercut (excessive arc pressure at the edges).
Connection to Overlay Welding Practice
For overlay welding applications, the arc morphology and temperature data provide critical information for:
- Interpass temperature control: The arc temperature data can be used to predict the interpass temperature and determine when cooling is necessary to avoid sensitization or grain coarsening.
- Dilution prediction: The arc penetration depth, determined from the arc morphology, can be used to predict the dilution ratio in overlay welds.
- Bond strength assessment: The fusion zone geometry, determined from the arc parameters, affects the bond strength at the clad-base interface.
Standards Compliance and Inspection
The arc morphology data supports compliance with welding standards by providing evidence of process control:
- NB/T 47014: The pulse parameters and arc characteristics must be documented in the welding procedure specification.
- ASME IX: The arc stability and consistency must be demonstrated during performance qualification.
- JB/T 4730: The NDT inspection plan should be informed by the predicted weld geometry and defect susceptibility.
Study Insights
This research demonstrates that arc imaging is a powerful tool for understanding the dynamic behavior of the TIG arc under pulsed current conditions. The direct observation of arc morphology and the measurement of characteristic temperatures provide a level of detail that is not achievable through conventional electrical measurements alone. For engineers developing pulsed TIG overlay processes, the key insight is that the arc behavior during each phase of the pulse cycle has a direct impact on the weld quality, and that this behavior can be measured and controlled through careful parameter selection. The ability to visualize and quantify the arc dynamics enables more rational process development and provides a basis for predictive modeling of weld quality. This type of research is essential for advancing the state of the art in pulsed TIG welding and for ensuring the reliable fabrication of clad pressure vessels and bimetallic products.