TIG-MIG Indirect Arc Welding Process

Literature Overview

Published in Transactions of the China Welding Institution (Vol. 30, No. 2, 2009, pp. 145-148), this paper by Wang Jun, Feng Jicai, He Peng, and Zhang Hongtao from Harbin Institute of Technology introduces the TIG-MIG indirect arc welding process. By modifying a conventional TIG welding system so that the arc is established and maintained between the tungsten electrode and the welding wire (rather than between the tungsten and the workpiece), the authors create a novel welding process that combines the precision of TIG with the deposition efficiency of MIG. The study demonstrates the process on 30CrMnSi steel with CuSi3 filler wire for surfacing applications.

Process Principle and Configuration

The TIG-MIG indirect arc process fundamentally reconfigures the conventional TIG welding setup. In standard TIG welding, the arc is struck between the tungsten electrode and the workpiece, with no consumable filler wire. In the TIG-MIG indirect arc configuration, the arc is transferred to burn between the tungsten electrode and a continuously fed welding wire, with the molten wire metal being deposited onto the workpiece. This configuration is analogous to a plasma arc transfer process but is achieved through TIG equipment modification rather than plasma arc generation.

System Configuration

The modified TIG system includes:

• A tungsten electrode serving as the non-consumable arc cathode
• A continuously fed welding wire (CuSi3 in this study) serving as the consumable anode
• A wire feeding mechanism integrated into the torch assembly
• Shielding gas supply (typically argon or argon-helium mixture)
• The arc burns in the space between the tungsten tip and the wire end, with the molten droplets transferred to the workpiece

The key innovation is that the arc does not directly contact the workpiece. Instead, the workpiece is heated by the thermal radiation and plasma jet from the arc, while the filler metal is melted and transferred from the wire. This indirect heating approach reduces the heat input to the workpiece while maintaining high filler metal deposition rates.

Experimental Results

Welding Stability

The study systematically examines the effects of welding current and wire feed speed on welding stability. The arc stability in the TIG-MIG indirect arc process depends on maintaining a consistent arc length between the tungsten and the wire end. Key findings include:

Weld Formation

The study examines the effects of flux, welding current, welding speed, and torch-to-workpiece height on joint formation:

• Flux: The application of flux improves wetting and spreading of the deposited metal, particularly for surfacing applications where good substrate adhesion is required.
• Welding current: Higher currents increase the deposited metal volume and weld bead width but also increase heat input and dilution.
• Welding speed: Higher speeds reduce heat input and deposited metal volume per unit length, producing narrower, shallower beads.
• Torch height: Increasing the torch-to-workpiece distance reduces heat concentration and increases the heat input spread, affecting weld pool geometry and penetration.

Key Performance Characteristics

The TIG-MIG indirect arc process demonstrates several distinctive advantages:

1. High wire deposition rate: The process achieves significantly higher filler metal deposition rates than conventional TIG welding, approaching or exceeding MIG welding deposition rates.
2. Reduced heat input: The indirect arc configuration reduces the heat input to the workpiece compared to conventional arc welding processes where the arc directly contacts the substrate.
3. Reduced dilution: Lower heat input translates to reduced melting of the base metal, resulting in lower dilution rates and better retention of the filler metal's alloying composition.
4. Suppression of unalloyed iron phenomenon: In surfacing applications, the reduced dilution helps suppress the formation of unalloyed iron layers (泛铁现象) that can compromise the performance of overlay welds.

Application to Surfacing and Cladding

The study specifically demonstrates the TIG-MIG indirect arc process for surfacing 30CrMnSi steel with CuSi3 filler wire. This application is relevant to several engineering scenarios:

• Corrosion-resistant cladding: Depositing copper-alloy overlays on carbon steel substrates for corrosion protection in marine and chemical environments.
• Wear-resistant surfacing: Adding hard alloy layers to critical components such as pump shafts, valve seats, and bearing surfaces.
• Restoration welding: Building up worn or damaged surfaces on expensive components without excessive heat distortion.

The suppression of the unalloyed iron phenomenon is particularly significant for surfacing applications. In conventional arc surfacing, the high dilution between the base metal and the overlay material can create intermediate layers with poor corrosion or wear resistance. The TIG-MIG indirect arc process, by reducing dilution, produces overlay layers that more closely match the intended composition and properties of the filler material.

Comparison with Conventional Processes

Engineering Practice Considerations

For pipe and fitting manufacturing, the TIG-MIG indirect arc process offers potential applications in several areas:

1. Pipe repair: Localized repair of corrosion damage or mechanical defects on in-service pipes without excessive heat input that could affect the surrounding pipe material.
2. Cladding of critical sections: Depositing corrosion-resistant or wear-resistant overlays on pipe ends, flange faces, or internal surfaces.
3. Low-dilution surfacing: Applications where maintaining the composition of the overlay material is critical, such as depositing nickel-based alloys for high-temperature or corrosion-resistant service.

The process requires careful parameter control to maintain arc stability and consistent metal transfer. The synchronization between wire feed speed and welding current is critical, as any mismatch will cause arc length variation and unstable welding. Engineers implementing this process should invest in automated wire feed and current control systems to ensure process consistency.

Study Insights

The TIG-MIG indirect arc welding process represents a creative adaptation of existing welding equipment that achieves performance characteristics not readily available from conventional TIG or MIG processes. The fundamental insight is that by transferring the arc from the workpiece to the wire, the process decouples the arc energy from the substrate heat input, enabling high deposition rates with low dilution.

This approach is conceptually similar to plasma arc welding, where the arc is constricted and transferred through a nozzle. However, the TIG-MIG indirect arc process achieves a similar effect through a simpler modification of existing TIG equipment, making it more accessible to workshops without dedicated plasma arc welding systems.

The demonstration on 30CrMnSi steel with CuSi3 filler wire validates the process for practical surfacing applications. The suppression of the unalloyed iron phenomenon addresses a well-known challenge in overlay welding and demonstrates the practical value of the reduced dilution capability.

Summary

The TIG-MIG indirect arc welding process offers a novel approach to surfacing and cladding applications by establishing the arc between the tungsten electrode and the welding wire rather than between the electrode and the workpiece. This configuration achieves high wire deposition rates while reducing heat input and dilution, effectively suppressing the unalloyed iron phenomenon that commonly compromises overlay weld quality. For engineers in pipe and fitting manufacturing, this process provides a practical tool for localized repair, corrosion-resistant cladding, and wear-resistant surfacing applications where low dilution and controlled heat input are critical requirements.