Zhuojin Pipe Fitting Co., LtdDevelopment of Square-Wave AC Pulsed TIG Welding Machine
Literature Overview
The paper by Hang Zhengxiang and Liu Guiqiu from Shenyang University of Technology, published in Welding (No. 10, 1995), describes the design and development of a square-wave alternating current pulsed TIG welding machine. The machine employs thyristor-based power regulation with pulse modulation technology, enabling both AC and DC TIG welding capabilities. The control system is entirely electronic, representing a significant advancement over earlier contactor-based AC TIG machines.
Technical Context and Historical Significance
In the mid-1990s, AC TIG welding was primarily used for welding aluminum and magnesium alloys, where the alternating current provides two key benefits:
- Cathodic Cleaning Effect: During the negative half-cycle (electrode negative), high current density at the cathode causes mechanical removal of the Al₂O₃ oxide film from the workpiece surface. This is essential for achieving clean, oxide-free welds in aluminum alloys.
- Heat Balance: The positive half-cycle (electrode positive) deposits more heat into the workpiece, compensating for the lower heat input during the cleaning half-cycle and preventing excessive tungsten overheating.
However, traditional AC TIG machines used contactor-based switching (electromechanical choppers) to interrupt and reverse the current, which resulted in:
- Slow switching speeds (typically 50–100 Hz)
- Arc instability and noise
- Limited control over the positive-to-negative current ratio
- Mechanical wear and maintenance requirements
The square-wave AC pulsed TIG machine described in this paper addresses these limitations through thyristor-based electronic control, enabling faster switching, more precise current shaping, and improved arc stability.
Main Circuit Design
The main circuit of the welding machine employs a thyristor (SCR) bridge configuration for AC/DC conversion and current regulation. The key design features include:
| Component | Function | Specification |
|---|---|---|
| Thyristor Bridge | AC-DC conversion and current regulation | Phase-controlled firing |
| Pulse Modulator | Generates square-wave AC output | Electronic pulse generator |
| Current Transformer | Feedback for current regulation | Proportional to welding current |
| Snubber Circuits | Voltage spike suppression | RC networks across thyristors |
| Transformer | Voltage step-up/down and isolation | Custom design for welding application |
The square-wave AC output is generated by modulating the thyristor firing angles in a specific sequence. During the positive half-cycle, the thyristors conduct with a controlled delay to produce a square-wave current with adjustable magnitude. During the negative half-cycle, the thyristors are triggered to conduct in the opposite direction, producing the negative square-wave current. The ratio of positive to negative current (typically 60:40 or 70:30 for aluminum welding) is controlled by adjusting the firing angles.
Control System Design
The control system is entirely electronic, eliminating the need for electromechanical switching components. The system includes:
- Pulse Generator: Generates the trigger pulses for the thyristors at the desired switching frequency. The frequency is typically adjustable in the range of 50–200 Hz.
- Current Regulation Loop: Uses a current transformer to sense the welding current and adjusts the thyristor firing angle to maintain the set current value. This closed-loop control ensures stable arc current despite variations in arc voltage.
- Mode Selection: The control system allows switching between AC and DC modes. In DC mode, the thyristors are configured to conduct only in one direction, producing DC output for steel and stainless steel welding.
- Parameter Setting: The control panel provides setpoints for welding current, AC frequency, positive-to-negative current ratio, and pulse parameters.
Technical Advantages of Square-Wave AC
The square-wave AC output offers several advantages over traditional sine-wave AC:
| Characteristic | Sine-Wave AC | Square-Wave AC |
|---|---|---|
| Current Waveform | Sinusoidal | Rectangular (square) |
| Switching Speed | 50–100 Hz (mechanical) | 50–200 Hz (electronic) |
| Current Ratio Control | Limited | Precise (adjustable) |
| Arc Stability | Moderate | Excellent |
| Cathodic Cleaning | Effective | Enhanced (higher peak current) |
| Tungsten Life | Limited | Extended (lower average current) |
| Weld Quality | Acceptable | Superior |
The square-wave waveform allows the peak current to be significantly higher than the average current, enhancing the cathodic cleaning effect during the negative half-cycle. The rapid switching between positive and negative polarity also reduces arc drift and improves arc stability, resulting in narrower, more uniform weld beads.
Engineering Applications
The square-wave AC pulsed TIG welding machine is applicable to a wide range of aluminum and magnesium alloy welding applications:
| Application | Material | Typical Parameters |
|---|---|---|
| Aerospace Structures | 2xxx, 5xxx, 6xxx, 7xxx Al alloys | 100–300 A, 50–100 Hz |
| Automotive Components | 6061, 7075, 2024 Al alloys | 80–250 A, 50–100 Hz |
| Marine Applications | 5083, 5086 Al alloys | 100–300 A, 50–100 Hz |
| Heat Exchangers | Al and Al alloys | 50–200 A, 50–150 Hz |
| Repair Welding | Various Al alloys | 50–200 A, adjustable |
The machine's dual AC/DC capability also makes it suitable for welding steel and stainless steel in DC mode, providing a versatile welding solution for multi-material workshops.
Key Technical Insights
The development of this welding machine represents a significant step in the evolution of TIG welding technology. The transition from mechanical to electronic control of AC TIG welding enabled:
- Higher switching frequencies: Electronic thyristor switching operates at frequencies well above the mechanical limits of contactors, improving arc stability and cleaning efficiency.
- Precise current control: The thyristor-based current regulation provides tight control over the welding current, reducing arc fluctuations and improving weld quality.
- Adjustable polarity ratio: The ability to independently adjust the positive and negative half-cycle currents allows optimization of the heat input and cleaning effect for different aluminum alloys and thicknesses.
- Pulse welding capability: The pulse modulation technology enables pulsed TIG welding, which is particularly useful for thin-gauge aluminum welding where precise heat input control is critical.
From a design perspective, the all-electronic control system eliminates the maintenance issues associated with mechanical contactors and offers faster response times for current regulation. The thyristor-based power stage provides efficient conversion with minimal power loss, resulting in a compact and energy-efficient welding machine.
Reflections and Legacy
This 1995 paper represents an important milestone in Chinese welding technology development. The square-wave AC pulsed TIG welding machine described here was among the early Chinese-developed electronic AC TIG power sources, demonstrating the capability of domestic research institutions to design and implement advanced welding equipment.
The design principles described in this paper—thyristor-based power regulation, electronic pulse modulation, and closed-loop current control—are still fundamental to modern AC TIG welding machines. Contemporary machines use IGBT (Insulated Gate Bipolar Transistor) inverters instead of thyristors, offering even higher switching frequencies and more advanced control capabilities, but the underlying design philosophy remains unchanged.
For practitioners working with aluminum and magnesium alloys, the square-wave AC pulsed TIG process remains the preferred welding method for achieving high-quality, oxide-free welds with excellent mechanical properties. The machine described in this paper, while technologically dated, represents the foundational work that enabled the widespread adoption of electronic AC TIG welding in Chinese manufacturing.