CLADDING TECHNOLOGY SHANXI CO., LTDArc-Weld Pool Characteristics in DC Superimposed Pulsed TIG Welding
Research Context and Innovation
This 2022 study by Zhang Gang, Xu Zilong, Wang Kaifei, Zhu Ming, and Shi Yu from the State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials and Processing at Lanzhou University of Technology presents a novel welding process concept: DC superimposed pulsed TIG welding. Published in the Transactions of the China Welding Institute and supported by multiple funding sources including the National Natural Science Foundation, this research represents a significant advancement in arc welding technology.
The DC superimposed pulsed approach combines the advantages of continuous DC TIG welding (stable arc, consistent heat input) with the benefits of pulsed welding (controlled penetration, reduced heat input). This hybrid waveform creates a unique arc and weld pool behavior that offers new possibilities for cladding and overlay welding applications.
Process Principle and Configuration
The DC superimposed pulsed TIG welding process operates by superimposing a pulsed current waveform on a DC base current. The resulting waveform consists of:
- DC base current: Provides a continuous minimum current level that maintains arc stability and provides baseline heat input.
- Pulsed component: Superimposed on the DC base, this component provides periodic high-current pulses that drive penetration and electromagnetic stirring.
The key innovation is that the minimum current never drops to zero (as in conventional pulsed TIG), maintaining continuous arc stability while still providing the benefits of pulsed operation.
Waveform Parameters
| Parameter | Symbol | Typical Range | Function |
|---|---|---|---|
| DC base current | I_dc | 50 - 150 A | Arc stability, baseline heat |
| Pulse amplitude | ΔI | 100 - 200 A | Penetration control |
| Pulse frequency | f | 5 - 50 Hz | Thermal cycling rate |
| Pulse duty cycle | D | 20 - 80% | Energy distribution |
| Total peak current | I_peak | 150 - 350 A | Maximum energy input |
| Travel speed | v | 100 - 500 mm/min | Heat input control |
Arc Characteristics Analysis
The study provides detailed analysis of the arc behavior under DC superimposed pulsed conditions:
Arc voltage characteristics: The arc voltage exhibits periodic variations synchronized with the pulse waveform. During peak current periods, the arc voltage increases slightly due to increased arc column length and pressure. During the DC base periods, the voltage stabilizes at a lower level.
Arc force dynamics: The electromagnetic force on the weld pool surface varies periodically with the current waveform. The force during peak periods is 2-3 times greater than during base periods, creating strong periodic stirring that enhances weld pool mixing and homogeneity.
Arc stability: The DC base current ensures continuous arc ignition and prevents arc extinction during low-current periods. This results in significantly improved arc stability compared to conventional pulsed TIG welding, particularly at low frequencies where arc extinction is a common problem.
Arc-Weld Pool Interaction Parameters
| Parameter | DC TIG | Pulsed TIG | DC Superimposed Pulsed |
|---|---|---|---|
| Arc stability | High | Moderate | High |
| Arc force variation | None | Large | Moderate |
| Penetration control | Limited | Excellent | Excellent |
| Heat input control | Moderate | Excellent | Excellent |
| Arc lifetime | Long | Moderate | Long |
| Weld pool oscillation | None | Strong | Moderate |
Weld Pool Behavior and Solidification Characteristics
The numerical and experimental results reveal distinctive weld pool behavior under DC superimposed pulsed conditions:
Hybrid flow patterns: The weld pool exhibits a combination of steady-state flow (from the DC component) and periodic oscillation (from the pulse component). This hybrid flow pattern promotes better mixing of base metal and filler material, resulting in more homogeneous weld compositions.
Controlled solidification: The DC base current maintains a minimum temperature level in the weld pool, preventing complete solidification between pulses. This creates a "persistent liquid zone" that allows for continuous solidification front advancement, resulting in more uniform grain structure and reduced segregation.
Reduced defect susceptibility: The combination of continuous arc stability and periodic stirring reduces the susceptibility to common welding defects. Hot cracking is reduced due to more uniform temperature gradients, while porosity is reduced due to enhanced gas escape during peak periods.
Engineering Applications and Implications
For cladding and overlay welding operations, the DC superimposed pulsed TIG process offers several advantages:
Improved overlay quality: The enhanced mixing and more uniform solidification patterns result in overlay layers with more homogeneous composition and microstructure. This is particularly important for nickel-based alloy overlays where compositional uniformity directly affects corrosion resistance.
Enhanced bond strength: The persistent liquid zone and continuous arc stability promote better metallurgical bonding between the overlay layer and base metal. The periodic stirring during peak periods enhances mechanical interlocking at the interface.
Reduced rework: The improved arc stability and reduced defect susceptibility lead to lower rework rates, improving productivity and cost-effectiveness in production environments.
Study Insights and Practical Recommendations
The DC superimposed pulsed TIG welding process represents a significant advancement in welding technology that addresses several limitations of both DC and conventional pulsed TIG welding. The key insight is that by maintaining a DC base current, the process achieves arc stability while still providing the penetration and heat input control benefits of pulsed welding.
For practical implementation, the following recommendations are offered:
- Equipment requirements: The process requires a welding power source capable of generating a DC base current with superimposed pulse waveform. Not all conventional pulsed TIG power sources support this configuration, so equipment verification is essential.
- Parameter optimization: The optimal DC base current level depends on the material combination and welding position. A starting point of 50-70% of the conventional DC TIG current is recommended, with the pulse amplitude adjusted to achieve the desired penetration depth.
- Quality monitoring: In-situ monitoring of arc voltage and current waveforms is essential for maintaining process consistency. Deviations from the expected waveform pattern indicate potential process issues that require immediate attention.
The study demonstrates that DC superimposed pulsed TIG welding is a versatile process that can be adapted to a wide range of cladding and overlay applications, from thin-section stainless steel overlay to thick-section nickel-based alloy cladding.