CLADDING TECHNOLOGY SHANXI CO., LTDMicrocomputer Control System for Pulsed MIG Welding
Literature Overview
This 1993 publication by Liu Huijie, Zhang Jiuhai, and Bai Fuping from Harbin Institute of Technology, published in the Welding journal, describes the development of a microcomputer-based control system for pulsed MIG welding. The work represents a significant advancement over the earlier 1991 keyboard-based system, integrating microprocessor-based real-time control with sophisticated waveform generation capabilities. The microcomputer control system enables precise, programmable control of the welding current waveform, which is critical for achieving consistent droplet transfer and high-quality welds.
Core Technical Content
The microcomputer control system described in this paper employs a digital signal processing (DSP) approach to generate the pulsed welding current waveform. The system consists of:
- Microcomputer controller: A dedicated microprocessor that generates the pulse waveform based on programmed parameters.
- Power amplifier stage: A transistor-based amplifier that converts the microcomputer's control signals into the high-current welding output.
- Feedback loop: Current and voltage sensors provide real-time feedback to the microcomputer, enabling closed-loop control of the welding process.
The microcomputer generates the pulse waveform using digital-to-analog conversion (DAC) techniques, allowing for precise control of the pulse current, base current, pulse frequency, and base frequency. The closed-loop feedback ensures that the actual welding current matches the programmed waveform, even in the presence of process disturbances such as arc length variations.
Technical Parameters and Process Windows
| Parameter | Typical Range | Influence on Cladding |
|---|---|---|
| Pulse Current | 200–500 A | Controls droplet detachment force; higher values increase dilution |
| Base Current | 30–100 A | Maintains arc stability; too low causes arc instability |
| Pulse Frequency | 50–400 Hz | Governs droplet transfer rate; must synchronize with travel speed |
| Base Frequency | 100–600 Hz | Determines inter-pulse interval; affects heat input distribution |
| Travel Speed | 200–800 mm/min | Determines deposition rate and dilution |
| Shielding Gas Flow | 15–30 L/min | Protects molten pool; critical for stainless and nickel alloy overlays |
Interpretation and Engineering Relevance
The microcomputer-based control system described in this paper represents a significant advancement in welding process control. The closed-loop feedback capability allows the system to maintain consistent welding parameters even when process conditions change, such as variations in arc length due to wire feed inconsistencies or changes in workpiece geometry. This is particularly important in cladding applications, where the weld pool geometry can change significantly as the overlay layer builds up, affecting the heat input and dilution rate.
For bimetal pressure vessel fabrication, the precision of the microcomputer control system enables the consistent production of overlay layers with controlled dilution rates. For example, in the fabrication of a 316L-clad hydrogenation reactor, the microcomputer system can maintain the dilution rate within a tight tolerance across multiple passes, ensuring that the final overlay layer meets the intergranular corrosion resistance requirements specified in NACE MR0175 / ISO 15156.
Connection to Standards and Quality Control
The microcomputer control system supports the requirements of ASME IX and NB/T 47014 by enabling precise parameter control and documentation. The system can record the actual welding parameters used during each pass, providing a digital welding log that supports traceability and quality assurance. This is particularly important for pressure vessel fabrication, where the welding procedure must be qualified and the production parameters must be documented for inspection and certification purposes.
Key Reflections
The transition from analog to microcomputer-based welding control systems was one of the most significant advances in welding technology during the 1990s. In my experience, the precision and repeatability afforded by microcomputer control have been instrumental in achieving the high-quality overlay layers required for modern bimetal products and pressure vessels. The closed-loop feedback capability, in particular, has been essential for maintaining consistent dilution rates in multi-pass overlay welding, where the weld pool geometry changes significantly as the overlay builds up.