CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Microcomputer-Controlled Pulsed MIG Welding Arc Ignition and Arc Termination
Literature Overview
This 1992 study by Gang Tie, Yin Shuyan, Wang Kehong, and Huang Zanzhi represents an early contribution to the field of computer-controlled welding process development in China. The authors, representing Harbin Institute of Technology, Huazhong Institute of Technology, and Mudanjiang Radio Factory No. 6, investigated the critical challenges of arc ignition and arc termination in microcomputer-controlled pulsed MIG welding. These two events—starting and stopping the welding arc—represent the most vulnerable phases of the welding cycle, where process instability is most likely to occur and where defects such as undercut, porosity, and incomplete fusion are most frequently observed.
Core Technical Viewpoints
The authors identify arc ignition and termination as the primary quality-critical events in pulsed MIG welding and develop microcomputer-controlled strategies to ensure stable and repeatable performance at both ends of the weld. Their research addresses several specific challenges:
- Arc ignition stability: In pulsed MIG welding, the arc must be established under conditions where the current waveform is transitioning from zero to the programmed pulsing parameters. The authors propose a staged ignition sequence where the current ramps up gradually over 100–300 ms to achieve stable arc initiation without excessive spatter or arc blow.
- Arc termination quality: The end of the weld is particularly susceptible to defects such as crater porosity, crater undercut, and incomplete fill. The authors develop a programmed termination sequence that gradually reduces the current and voltage over a controlled time interval, allowing the weld pool to solidify uniformly and preventing crater formation.
- Microcomputer control advantages: The authors demonstrate that microcomputer-based control offers superior repeatability compared to analog controllers, as the programmed sequences can be stored and reproduced exactly for each weld. This is particularly important for cladding operations where multiple identical welds must be deposited with consistent quality.
Control Strategy and Parameter Sequences
| Phase | Parameter | Control Strategy |
|---|---|---|
| Pre-ignition | Gas flow | Establish shielding gas flow 2–3 seconds before arc start |
| Ignition ramp | Current | Ramp from 0 to 60% of I_bg over 100–200 ms |
| Stabilization | Current | Ramp from 60% to 100% of I_bg over 100–200 ms |
| Steady pulsing | Current waveform | Maintain programmed I_peak and I_bg pulsing |
| Termination ramp | Current | Reduce I_peak by 50% over 200–500 ms |
| Final termination | Current | Ramp to 0 over 200–500 ms |
| Post-termination | Gas flow | Maintain gas flow 3–5 seconds after arc extinction |
Technical Implementation Details
The authors describe a hardware architecture consisting of a microcomputer (likely an 8031 or 8051 series microcontroller, typical of the era), a current control amplifier, and a welding power source interface. The microcomputer generates the pulsing waveform through digital-to-analog conversion and sends the control signal to the power source's current regulation loop. The arc ignition and termination sequences are implemented as timed routines within the microcomputer's control program.
The study also addresses the challenge of arc length control during ignition and termination. In conventional welding, the arc length is maintained by the balance between wire feed speed and arc voltage. During the ignition ramp, the arc length tends to be longer than steady-state due to the lower current. The authors propose using a slight increase in wire feed speed during ignition to compensate for this effect and achieve rapid arc length stabilization.
Connection with Cladding and Overlay Practice
From a cladding engineering perspective, the quality of arc ignition and termination at each weld start and stop is directly related to the integrity of the overlay layer. In multi-pass cladding builds, each pass begins and ends at a different location, and defects at these transition points can create stress concentration sites that compromise the structural integrity of the cladding. The authors' microcomputer-controlled approach ensures that each pass start and stop is executed with identical parameters, minimizing variability in weld quality.
For single-sided welding operations (common in cladding where access is limited), the quality of the weld termination is particularly important because there is no opportunity to repair defects from the opposite side. The programmed termination sequence described by the authors—gradual current reduction followed by extended gas purging—provides a reliable method for producing sound weld terminations without crater defects.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Crater porosity | Abrupt arc termination; insufficient gas coverage | Programmed current ramp-down; extended post-termination gas flow |
| Crater undercut | Excessive current at weld end; rapid solidification | Reduce I_peak during termination; increase travel speed slightly |
| Arc blow at ignition | Magnetic field instability; improper gas flow | Establish gas flow before ignition; use DC with correct polarity |
| Incomplete fill at termination | Insufficient weld pool volume at weld end | Programmed current reduction rate; controlled travel speed reduction |
| Excessive spatter at ignition | Rapid current ramp; unstable arc | Gradual current ramp-up over 200–300 ms |
Key Questions and Reflections
The study raises the question of how microcomputer control can be extended to real-time adaptive control of arc ignition and termination. The authors' approach relies on pre-programmed sequences with fixed timing parameters. In practice, the optimal ignition and termination sequences may vary depending on the base metal thickness, joint configuration, and ambient conditions. A more advanced control strategy would incorporate real-time feedback from arc voltage, arc current, and arc length sensors to dynamically adjust the ignition and termination parameters.
Another reflection concerns the scalability of the microcomputer control approach to modern welding applications. The 8-bit microcontrollers used in this study have limited processing capability and memory. Modern welding power sources employ 32-bit or 64-bit microprocessors with real-time operating systems, enabling far more sophisticated control algorithms. However, the fundamental principles established by the authors—staged ignition, programmed termination, and gas flow management—remain the foundation of modern welding process control.
Study Insights and Implications
This study represents a pioneering contribution to computer-controlled welding process development and provides a conceptual framework that continues to influence modern welding automation. The authors' systematic approach to analyzing the arc ignition and termination phases, combined with their development of microcomputer-controlled solutions, demonstrates the value of digital control in achieving consistent weld quality. For cladding engineers, the key takeaway is that the quality of each weld start and stop must be carefully controlled to ensure the integrity of the overlay layer, and that digital control offers the repeatability and precision required for high-quality cladding operations. The study's emphasis on gas flow management during arc ignition and termination is particularly relevant to cladding applications where atmospheric contamination of the overlay material can compromise corrosion resistance and mechanical properties.