DSP-Controlled Square-Wave AC TIG Welding Current Waveform Realization

Literature Overview

Published in 2009 by Chen Jing, Chen Kexuan, Li Heqi, Li Chunxu, and Bai Xuemei from the Key Laboratory of Nonferrous Alloy Materials at Lanzhou University of Technology, this paper presents the design and implementation of a digital signal processor (DSP)-controlled power supply for generating square-wave AC TIG welding currents. The work addresses a fundamental limitation of conventional AC TIG welding — the asymmetric and non-rectangular current waveform produced by traditional thyristor-based inverters — and demonstrates how DSP-based control algorithms can achieve near-perfect square-wave current profiles with independent control of positive and negative half-cycle parameters.

Core Technical Principles

AC TIG welding is indispensable for welding aluminum and magnesium alloys, where the cathodic (negative) half-cycle provides arc cleaning action that removes the refractory aluminum oxide film from the weld surface, while the anodic (positive) half-cycle provides the deeper penetration required for joint fusion. The balance between these two half-cycles is critical to achieving sound welds.

Conventional AC TIG inverters produce trapezoidal or triangular waveforms due to the inherent switching characteristics of IGBT modules and the limited control resolution of analog or microcontroller-based controllers. A true square-wave waveform offers several advantages:

• Constant current during each half-cycle, producing uniform energy delivery
• Sharp current transitions that enhance arc cleaning effectiveness
• Independent control of positive and negative current amplitudes, frequencies, and duty ratios
• Reduced arc drift and improved weld bead geometry

DSP Control Architecture

The system described in this paper employs a TMS320F2812 DSP as the central controller, with the following architecture:

The DSP generates the square-wave current by modulating the PWM duty cycle of the inverter bridge in real time. During the positive half-cycle, the IGBTs conduct in the forward direction, and the PWM duty cycle is adjusted to maintain the set current amplitude. During the negative half-cycle, the current direction reverses through the full-bridge topology, and the PWM duty cycle is independently controlled to achieve the desired negative current amplitude.

Technical Parameters and Performance

The achieved waveform characteristics include:

Welding Performance on Aluminum Alloys

The square-wave AC TIG process was validated on AA6061-T6 aluminum alloy plates with the following results:

For a 3 mm thick plate welded with 150 A positive current, 200 A negative current, 100 Hz frequency, and 60:40 positive-to-negative duty ratio, the weld achieved full penetration with a smooth bead profile and no porosity. The arc cleaning action was visibly superior to that of a conventional AC TIG process, as evidenced by the absence of oxide inclusions in metallographic examination.

The independent control of positive and negative current amplitudes allows the operator to optimize the balance between cleaning and penetration for different material thicknesses. For thin sheets (1–2 mm), a higher negative current amplitude with a shorter negative half-cycle provides sufficient cleaning while minimizing heat input. For thick sections (6–12 mm), a higher positive current amplitude with a longer positive half-cycle ensures adequate penetration depth.

Engineering Significance

For aluminum and magnesium alloy welding in aerospace, automotive, and marine applications, the ability to precisely control the AC waveform has direct implications for weld quality and process repeatability. The DSP-based approach eliminates the need for analog waveform shaping networks, which are prone to drift and component aging. The digital implementation ensures consistent performance over the equipment's operational lifetime.

The research also demonstrates that the square-wave waveform produces a more stable arc with reduced arc force fluctuation, which is particularly beneficial for automated welding applications where consistent bead geometry is required. The reduced arc drift also improves the accuracy of weld tracking systems.

Key Reflections

This research represents an important step in the digitalization of welding power sources. The DSP-based approach not only achieves superior waveform quality but also provides a platform for implementing advanced control algorithms such as adaptive current regulation, arc voltage feedback, and welding defect detection. The modular architecture described in the paper can be extended to support other advanced welding processes, including pulsed DC TIG, L-TIG, and hybrid welding configurations.

For engineers involved in welding process development, the key takeaway is that waveform quality directly influences weld quality, and that modern digital power electronics can achieve waveform precision that was previously unattainable with analog technology. This has implications for welding procedure qualification, where the waveform parameters must be specified and controlled with the same rigor as current, voltage, and travel speed.

Summary

This work demonstrates that DSP-based control can produce near-ideal square-wave AC TIG current waveforms with independent control of positive and negative half-cycle parameters, resulting in superior arc cleaning, improved penetration control, and enhanced weld quality on aluminum alloys. The digital implementation provides long-term reliability and extensibility that analog systems cannot match, making it a preferred approach for advanced welding applications requiring precise thermal input control.