Study Note on IGBT Dual-Inverter Square-Wave AC TIG Welding Machine

Literature Overview

This paper by Liu Guiqiu, Hang Zhengxiang, and Wang Hui, published in Electric Power Electronics, Volume 31, Issue 3, 1997, presents the design and operation of an IGBT dual-inverter square-wave AC TIG welding machine. The research is conducted at Shenyang University of Technology and Liaoyang Petrochemical Vocational College. The paper introduces the basic structure and working principles of the main circuit, proposes methods for constant current characteristics using primary inverter current negative feedback with threshold comprehensive control, and secondary inverter arc starting control. Experimental results demonstrate stable welding of aluminum alloys at an open-circuit voltage of 45V and welding current of 15A or higher.

Core Technical Points

The paper describes a welding machine that employs IGBT (Insulated Gate Bipolar Transistor) technology in a dual-inverter configuration to produce a square-wave AC output suitable for TIG welding of aluminum and other reactive metals. The square-wave AC waveform is particularly advantageous for AC TIG welding because it provides distinct cathodic and anodic phases with adjustable duty cycles, allowing independent control of cathode cleaning and anodic heating.

The dual-inverter architecture consists of a primary inverter that converts the input DC power to a high-frequency AC signal, and a secondary inverter that converts the high-frequency AC to a square-wave AC output at the welding frequency. This two-stage inverter approach provides superior waveform control compared to conventional transformer-based AC welding machines.

The constant current characteristic is achieved through a combination of primary inverter current negative feedback and threshold comprehensive control. This control strategy ensures that the welding current remains stable despite variations in arc voltage, electrode wear, and other process disturbances. The secondary inverter arc starting control method provides reliable arc initiation, which is particularly challenging for AC TIG welding due to the zero-crossing of the current waveform.

Technical Specifications

Interpretation of Technical Points

The use of IGBTs in the welding machine power supply represents a significant advancement over conventional thyristor-based inverters. IGBTs offer faster switching speeds, lower switching losses, and better controllability, which enable higher frequency operation and more precise waveform shaping. The dual-inverter configuration provides additional flexibility in waveform generation, allowing for independent control of the rising and falling edges of the square wave and the duty cycle of the positive and negative half-cycles.

The square-wave AC output is particularly beneficial for AC TIG welding of aluminum alloys. During the cathodic phase (electrode negative), the concentrated electron emission at the electrode tip provides intense heat input into the workpiece, promoting deep penetration and good fusion. During the anodic phase (electrode positive), the ion bombardment of the workpiece surface removes the tenacious aluminum oxide film, ensuring good fusion and preventing oxide inclusions in the weld. The adjustable duty cycle allows optimization of the balance between penetration and cleaning action based on the specific welding application.

The constant current control method proposed in the paper combines primary inverter current negative feedback with threshold comprehensive control. The primary inverter current negative feedback provides fast response to current deviations, while the threshold comprehensive control ensures that the current remains within a specified range even under large disturbances. This dual-control approach provides both dynamic stability and static accuracy, which are essential for consistent weld quality.

The secondary inverter arc starting control method addresses a critical challenge in AC TIG welding. At the zero-crossing of the AC current, the arc can extinguish, requiring a high-frequency or high-voltage pulse to re-ignite the arc. The proposed method uses the secondary inverter to generate a controlled pulse that ensures reliable arc re-ignition at each zero-crossing, maintaining arc stability throughout the welding process.

Relevance to Aluminum Alloy Welding

Aluminum alloys are widely used in aerospace, automotive, and construction industries due to their lightweight, high strength-to-weight ratio, and excellent corrosion resistance. AC TIG welding is the preferred process for aluminum alloy welding because the alternating polarity provides both deep penetration and oxide removal, which are essential for producing high-quality welds in aluminum alloys.

The IGBT dual-inverter square-wave AC TIG welding machine described in this paper offers several advantages for aluminum alloy welding:

1. Adjustable duty cycle: The ability to adjust the duty cycle of the square wave allows optimization of the balance between cathodic penetration and anodic cleaning, which is critical for achieving good weld quality in aluminum alloys.
2. High-frequency switching: The high switching frequency of IGBTs enables fast current rise and fall times, resulting in a well-defined square wave with minimal transition time. This reduces the time spent at low current levels where arc stability is compromised.
3. Precise current control: The constant current control method ensures stable welding current despite variations in arc voltage and other process parameters, leading to consistent weld geometry and metallurgical quality.
4. Reliable arc starting and maintenance: The secondary inverter arc starting control method ensures reliable arc initiation and re-ignition, which is essential for continuous welding without interruptions.

Engineering Practice Considerations

For engineers considering the application of IGBT dual-inverter square-wave AC TIG welding machines, the following practical considerations are important:

1. Parameter optimization: The duty cycle, frequency, and current magnitude should be optimized for the specific aluminum alloy and welding application. Thicker sections may require a higher cathodic duty cycle for deeper penetration, while thinner sections may require a higher anodic duty cycle for better oxide removal.
2. Electrode selection: The electrode material and geometry should be selected to provide stable arc characteristics and adequate current carrying capacity. Ceriated tungsten electrodes are commonly used for AC TIG welding of aluminum alloys due to their excellent arc stability and long service life.
3. Shielding gas selection: Helium or helium-argon mixtures are often preferred for AC TIG welding of aluminum alloys due to their higher ionization potential, which provides better arc stability and deeper penetration. Pure argon may be used for thinner sections where lower heat input is desired.
4. Travel speed control: The travel speed should be adjusted to maintain a consistent weld bead geometry and avoid defects such as lack of fusion or excessive undercut. Too slow a travel speed can lead to excessive heat input and distortion, while too fast a travel speed can lead to insufficient penetration.
5. Preheating considerations: For thicker sections or thicker-walled components, preheating may be necessary to reduce thermal stresses and prevent cracking. The preheat temperature should be carefully controlled to avoid adverse effects on the weld metal and heat-affected zone microstructure.

Study Insights and Reflections

This paper represents an important contribution to the development of IGBT-based welding power supplies, demonstrating the feasibility of using dual-inverter architecture for square-wave AC TIG welding. The proposed control methods for constant current characteristics and arc starting provide practical solutions to key challenges in AC TIG welding power supply design.

The experimental results demonstrating stable welding of aluminum alloys at a minimum current of 15A are significant, as they indicate that the machine is suitable for welding a wide range of aluminum alloy thicknesses, from thin sheet to moderately thick sections. The open-circuit voltage of 45V is sufficient for reliable arc starting and maintenance, while the square-wave output provides the flexibility needed for optimizing weld quality across different applications.

The work described in this paper was conducted in 1997, a period of rapid advancement in power electronics and welding technology. The concepts and techniques presented have since been refined and commercialized, with modern IGBT-based welding machines offering even greater flexibility and control. However, the fundamental principles and control strategies described in this paper remain relevant and continue to inform the design of advanced welding power supplies.

In summary, this research provides valuable insights into the design and control of IGBT dual-inverter square-wave AC TIG welding machines, demonstrating the advantages of this technology for aluminum alloy welding. The proposed control methods and experimental results offer practical guidance for engineers seeking to implement or optimize AC TIG welding processes for aluminum alloys.