Laser Deep Melting TIG Composite Welding Arc Characteristics and Molten Pool Behavior

Literature Overview

The research by Li Wei, Zhu Jialei, Li Zhibo, Jiao Xiangdong, and Feng Cong (2019), published in Hot Working Technology, investigates the arc characteristics and molten pool behavior in laser deep melting TIG composite welding. This hybrid welding process combines the deep penetration capability of laser welding with the high deposition rate of TIG welding, creating a synergistic process that offers advantages of both methods. The study provides valuable insights into the fundamental physics of the hybrid process, which is essential for optimizing process parameters and achieving high-quality welds.

Hybrid Welding Process Configuration

The laser-TIG composite welding process configuration studied by the authors includes:

The process configuration involves the laser beam and TIG arc being combined at the workpiece surface, with the laser providing deep penetration and the TIG arc providing additional heat input and filler metal deposition. The relative positioning of the laser and TIG arc (lead or lag) significantly affects the welding outcome.

Arc Characteristics Analysis

The authors conducted detailed measurements of the TIG arc characteristics in the presence of the laser beam:

• Arc Voltage: The arc voltage was measured to be 18-22 V, slightly higher than in conventional TIG welding due to the interaction with the laser-induced plasma.
• Arc Stability: The TIG arc remained stable in the presence of the laser beam, but the arc shape was modified by the laser-induced gas flow and plasma.
• Arc Length: The effective arc length was shorter than in conventional TIG welding because the laser-induced plasma column compressed the arc.
• Current Distribution: The current distribution in the TIG arc was affected by the magnetic field generated by the laser-induced plasma, leading to a slight deflection of the arc.

The laser beam interaction with the TIG arc creates a complex plasma environment that modifies the arc behavior. The laser-induced plasma column acts as a conductive channel, reducing the arc resistance and increasing the arc temperature. This effect is beneficial for achieving deeper penetration but requires careful control to avoid excessive spatter or porosity.

Molten Pool Behavior and Penetration

The molten pool behavior in laser-TIG composite welding is characterized by:

• Penetration Depth: The penetration depth was found to be significantly greater than either laser welding alone or TIG welding alone. For 10 mm thick steel plates, the penetration depth reached 8-10 mm with a combined power of 4 kW laser and 180 A TIG current.
• Molten Pool Shape: The molten pool had a keyhole-shaped profile with a deep, narrow penetration channel formed by the laser and a wider deposition zone formed by the TIG arc.
• Molten Pool Temperature: The peak temperature in the molten pool exceeded 3000 °C, with the laser spot reaching temperatures above 10000 °C.
• Molten Pool Dynamics: The molten pool exhibited complex fluid dynamics, including Marangoni convection driven by surface tension gradients and electromagnetic stirring from the TIG arc current.

The keyhole welding mechanism was observed to be dominant in the laser region, with the TIG arc contributing to the overall heat input and filler metal melting. The interaction between the laser and TIG processes creates a synergistic effect that cannot be achieved by either process alone.

Process Parameter Optimization

The study identified optimal parameter combinations for different welding applications:

The optimal lead/lag angle was found to be 5-10 degrees, with the TIG arc leading the laser beam for butt joints and lagging for overlap joints. The lead configuration allows the TIG arc to pre-heat the workpiece and stabilize the keyhole, while the lag configuration allows the TIG arc to fill the weld gap and reduce porosity.

Defect Analysis and Control

Common defects in laser-TIG composite welding and their control measures include:

The study emphasized the importance of shielding gas coverage in preventing porosity, particularly in the deep penetration region where gas entrapment is more likely. The use of a dual-shielding gas nozzle design was recommended to ensure adequate protection of both the laser spot and the TIG arc.

Study Insights and Reflections

This research provides fundamental insights into the physics of laser-TIG composite welding, which is increasingly being adopted for thick plate welding applications where neither laser nor TIG welding alone can achieve the required penetration and deposition rate. The study's detailed analysis of arc characteristics and molten pool behavior provides a scientific basis for process optimization, moving beyond empirical parameter adjustment to a more rational approach. For engineers implementing laser-TIG composite welding in production, the key takeaways include the importance of proper laser-TIG arc positioning, adequate shielding gas coverage, and careful control of travel speed to balance penetration and deposition. The synergistic effect of the hybrid process, where the combined penetration exceeds the sum of individual process penetrations, makes it particularly attractive for welding thick plates of carbon steel, low-alloy steel, and stainless steel. The work by Li and colleagues demonstrates that a thorough understanding of the fundamental process physics is essential for achieving reliable and high-quality welds in hybrid welding applications.