Comparative Study of Interaction Characteristics between High-Power Lasers and TIG Arc

Literature Overview

This 2010 study published in Chinese Journal of Lasers presents a comparative investigation of the interaction between high-power CO2 and Yb:YAG lasers with a DC TIG arc. The research team from Beijing University of Technology employed laser power meters, beam quality diagnostic instruments, and high-speed cameras to characterize the beam and arc characteristics during perpendicular interaction. Funded by the Beijing Municipal Education Commission Science and Technology Development Program, this work addresses a fundamental question in hybrid laser-arc welding: how does the type of laser source influence its interaction with the electric arc?

Experimental Setup and Measurement Methodology

The experimental configuration involves directing the laser beam perpendicularly through the TIG arc, allowing simultaneous measurement of the transmitted beam characteristics and the arc parameters. This perpendicular arrangement minimizes the path length through the arc plasma while still providing sufficient interaction for measurable effects.

Key Findings on Laser-Arc Interaction

The results reveal a dramatic difference in the interaction behavior between the two laser types:

CO2 Laser Interaction:

• Significant attenuation of laser power after passing through the TIG arc
• Beam defocusing and severe degradation of energy distribution
• Reduction in arc voltage
• Arc volume expansion
• Potential generation of laser-supported combustion waves

Yb:YAG Laser Interaction:

• Minimal absorption of laser energy by the arc
• Negligible impact on laser beam characteristics
• No significant change in arc voltage or arc morphology

The quantitative analysis provides the fundamental physical explanation for these differences:

Physical Mechanism Analysis

The fundamental reason for the dramatic difference in interaction behavior lies in the wavelength-dependent absorption characteristics of the arc plasma. The inverse bremsstrahlung (free-free) absorption coefficient is approximately one order of magnitude higher for CO2 laser radiation (10.6 μm) than for Yb:YAG laser radiation (1.064 μm). This is because the absorption cross-section for free-free transitions depends strongly on wavelength, with longer wavelengths experiencing significantly higher absorption in the plasma.

The refractive index measurements further confirm this difference. The CO2 laser experiences a refractive index change of approximately 0.005-0.008 when passing through the arc, leading to beam defocusing and energy redistribution. In contrast, the Yb:YAG laser experiences a refractive index change of only 0.00006-0.00008, which is negligible compared to the refractive index of air and results in virtually no beam distortion.

Engineering Practice Implications for Hybrid Welding

These findings have profound implications for the design and operation of hybrid laser-arc welding systems:

• Laser source selection: For hybrid welding applications, Yb:YAG or other near-infrared lasers (such as fiber lasers) are strongly preferred over CO2 lasers because they pass through the arc with minimal energy loss and beam degradation.
• Beam quality preservation: The preservation of beam quality when using Yb:YAG lasers ensures consistent deep penetration throughout the welding process, even when the arc is present.
• Process stability: The absence of arc voltage and morphology changes when using Yb:YAG lasers means that the TIG arc operates independently of the laser, simplifying process control and parameter optimization.
• Safety considerations: The potential generation of laser-supported combustion waves with CO2 lasers represents a safety hazard that must be considered in process design.

Study Insights and Reflections

This study provides a clear and quantitative demonstration of why fiber lasers and Yb:YAG lasers have largely displaced CO2 lasers in hybrid laser-arc welding applications. The one-order-of-magnitude difference in absorption coefficients and the negligible refractive index change for near-infrared lasers are the fundamental physical reasons for their superior performance in hybrid welding configurations.

From a practical standpoint, this research validates the industry trend toward fiber laser-based hybrid welding systems. The ability of fiber lasers to pass through the TIG arc without significant energy loss means that the full laser power is available for material interaction, resulting in deeper penetration, higher welding speeds, and better weld quality. This is particularly important for welding thick-section materials such as carbon steel plate used in structural applications and heavy-duty piping.

The work also highlights an important consideration for process development: the interaction between the laser and the arc is not merely a nuisance to be minimized but a phenomenon that can be exploited. For example, the slight beam defocusing of CO2 lasers through the arc could potentially be used to create a wider heat input zone, though this would come at the cost of reduced penetration depth. The key is to select the laser source that provides the desired interaction characteristics for the specific application.

These five studies collectively represent a comprehensive exploration of modern welding technology, spanning fundamental arc physics, numerical simulation, hybrid process development, and microstructural characterization. Together, they provide a rich foundation for engineers seeking to understand and optimize welding processes for critical applications in piping, fabrication, and additive manufacturing. The progression from fundamental laser-arc interaction physics through process modeling and into practical joint characterization reflects the multidisciplinary nature of modern welding engineering, where metallurgical knowledge, computational methods, and process physics converge to deliver reliable and high-quality welded joints.