Numerical Simulation of Laser Effect on TIG Arc Temperature Distribution

Literature Overview and Technical Motivation

Published in 2013 in the journal Hot Working Technology by Zhang Wenchao, Li Zhiyong, Zhang Qiang, and Xing Lei from the School of Materials Science and Engineering at North University of China, this study presents a numerical simulation investigation of the effect of laser irradiation on the temperature distribution within a TIG welding arc. Funded by the Shanxi Provincial Natural Science Foundation (2010011031-3) and the Shanxi Provincial Overseas Returnees Research Fund (2012-6974), this research explores the hybrid laser-TIG welding process, which combines the deep penetration capability of laser welding with the wide fusion zone and low spatter characteristics of TIG welding.

Core Technical Findings

The study develops a three-dimensional numerical model of the TIG arc temperature field and investigates how the introduction of a laser beam modifies the arc temperature distribution, plasma flow patterns, and energy density profile. The model accounts for the electromagnetic force, buoyancy force, and surface tension force acting on the plasma, as well as the energy coupling between the laser and the plasma.

Laser-Plasma Interaction Mechanisms

The simulation reveals three primary mechanisms by which the laser affects the TIG arc temperature distribution:

1. Direct energy addition: The laser beam deposits energy directly into the plasma, increasing the local temperature and creating a high-temperature zone along the laser axis. This effect is most pronounced in the region between the arc cathode and the workpiece surface.
2. Plasma compression: The high-temperature zone created by the laser induces a pressure gradient that compresses the surrounding plasma, reducing the effective arc radius and increasing the energy density at the arc center. This compression effect is analogous to the electromagnetic compression of the TIG arc but is driven by thermal rather than electromagnetic forces.
3. Plasma flow modification: The laser-induced temperature gradient creates a Marangoni-type flow in the plasma, with high-temperature plasma flowing outward from the laser axis and cooler plasma flowing inward at the periphery. This modified flow pattern enhances heat transfer to the workpiece and increases penetration depth.

Temperature Distribution Analysis

The simulation results show that the laser introduction creates a pronounced asymmetry in the arc temperature distribution. The peak temperature increases from 18,500 K (TIG only) to 28,500 K (TIG + 5 kW laser), with the highest temperature occurring at the intersection of the laser beam and the TIG arc. The temperature gradient perpendicular to the arc axis increases significantly with laser power, from approximately 500 K/mm (TIG only) to over 2,000 K/mm (TIG + 5 kW laser).

The simulation also predicts that the laser creates a high-temperature channel along its axis that extends from the arc source to the workpiece surface. This channel acts as a preferential path for energy transfer, concentrating the welding energy in a narrow region and producing a narrow, deep weld. The model predicts that the weld width decreases by 20–30% while the penetration depth increases by 50–150% compared to TIG-only welding at the same current level.

Engineering Practice Integration

The findings of this study have direct relevance to the development of hybrid laser-TIG welding processes for cladding and overlay applications. The ability to control the arc temperature distribution through laser power adjustment provides a powerful tool for optimizing the welding process for specific applications.

Application to Overlay Welding

For overlay welding of corrosion-resistant alloys on carbon steel, the hybrid laser-TIG process offers several advantages:

1. Reduced dilution: The concentrated energy input produces a narrower weld with less base metal entrainment, reducing the dilution of the overlay material. This is critical for maintaining the corrosion resistance of the overlay layer.
2. Improved bonding: The high energy density at the weld center promotes complete melting of the base metal surface, ensuring good metallurgical bonding between the overlay and base metal.
3. Reduced heat-affected zone: The narrow heat-affected zone in hybrid laser-TIG welding minimizes the thermal damage to the base metal, which is particularly important for base metals with limited weldability such as high-strength steels and precipitation-hardened alloys.
4. Higher deposition rates: The combination of the laser's deep penetration and the TIG's wide fusion zone allows for higher deposition rates while maintaining good weld quality.

Process Parameter Optimization

The simulation provides a basis for optimizing the laser power and TIG current combination for specific overlay applications. The study recommends the following parameter ranges for overlay welding of nickel-based alloys on carbon steel:

• TIG current: 150–250 A
• Laser power: 1–3 kW
• Travel speed: 200–500 mm/min
• Laser-TIG standoff distance: 5–10 mm
• Laser lead angle: 5–15° (laser leading the arc)

The simulation predicts that these parameters produce a weld with a width of 4–6 mm, a penetration depth of 3–5 mm, and a dilution ratio of 15–25%, which are all within the acceptable range for overlay welding applications.

Key Questions and Reflections

A significant question raised by this study is the accuracy of the numerical model in predicting the actual arc temperature distribution. The model assumes idealized boundary conditions and simplified material properties that may not fully capture the complex physics of the hybrid laser-TIG arc. In particular, the model does not account for the interaction between the laser and the shielding gas, which can significantly affect the arc stability and temperature distribution. Experimental validation of the model predictions is essential before the process can be confidently applied to production welding.

Another important consideration is the effect of the laser on the arc stability. The simulation assumes a steady-state arc, but in practice, the laser can induce arc instability through modulation of the plasma flow and electromagnetic force. The study does not address this issue, but experimental observations of hybrid laser-TIG welding often report arc oscillation and instability at high laser powers. Understanding and controlling this instability is critical for achieving consistent weld quality in production applications.

Study Insights and Implications for Welding Process Development

This study provides valuable theoretical insights into the physics of hybrid laser-TIG welding and offers a basis for process optimization. The most significant practical insight is that the laser power can be used as a process variable to control the arc temperature distribution and energy density profile, providing a degree of process flexibility that is not available with conventional TIG welding alone.

For cladding and overlay applications, the hybrid laser-TIG process represents a promising technology that combines the advantages of both laser and arc welding. The reduced dilution, improved bonding, and higher deposition rates make it particularly attractive for overlay welding of expensive corrosion-resistant alloys on structural steels. However, the process requires careful development and validation before it can be confidently applied to critical applications such as pressure vessel fabrication. Engineers should approach the adoption of this technology with a systematic qualification program that includes both laboratory testing and production trials to ensure that the process meets the required quality standards.