CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Arc Hot-Wire TIG Welding Process Characteristics
Literature Overview
This paper, published in Welding in 2007 by Lv Shixiong, Sun Qingjie, Fan Yangyang, and Yang Shiqin from the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology, provides a systematic analysis of the process characteristics of arc hot-wire TIG welding. This technique combines the precision of conventional TIG welding with the high deposition rate of solid wire feeding, and is of considerable importance in the overlay welding and cladding industry where productivity and weld quality must be balanced.
Core Technical Points
Process Description
Arc hot-wire TIG welding (also referred to as hot-wire GTAW or HGW-TIG) involves feeding a solid filler wire into the arc zone at a controlled rate while simultaneously applying a TIG arc. The wire is heated by the arc and melts into the weld pool, providing additional material that is not dependent on the arc current for melting. This decouples the deposition rate from the heat input, allowing independent control of pool geometry and weld metal volume.
The key process parameters include:
| Parameter | Typical Range | Function |
|---|---|---|
| Arc current | 80–200 A | Controls pool size and base metal penetration |
| Wire feed rate | 2–10 m/min | Controls deposition rate and weld reinforcement |
| Wire diameter | 1.0–3.0 mm | Affects deposition volume and arc stability |
| Travel speed | 100–500 mm/min | Controls heat input per unit length |
| Wire lead angle | 5°–45° from horizontal | Controls wire penetration and bead profile |
| Wire stick-out length | 5–15 mm | Affects arc stability and deposition efficiency |
Heat Input and Deposition Rate Relationship
The fundamental advantage of hot-wire TIG welding is the ability to increase the deposition rate without proportionally increasing the heat input. In conventional TIG welding, the deposition rate is directly proportional to the arc current, which also determines the heat input. This creates a constraint: increasing deposition rate inevitably increases heat input, leading to wider weld beads, deeper penetration, and greater dilution.
In hot-wire TIG welding, the arc current can be maintained at a level that produces the desired pool geometry and penetration, while the wire feed rate is adjusted independently to achieve the required deposition volume. This is particularly advantageous in overlay welding, where the goal is to deposit a corrosion-resistant layer with minimal dilution of the base metal.
Weld Pool Dynamics
The introduction of the solid wire into the arc zone modifies the pool dynamics in several ways:
- Electromagnetic stirring: The current flowing through the wire generates a magnetic field that interacts with the arc current, producing additional electromagnetic stirring within the pool.
- Mechanical stirring: The physical presence of the wire and its motion within the pool provides mechanical agitation that promotes mixing and homogenization.
- Heat concentration: The wire acts as a heat sink, concentrating the arc energy and producing a deeper, narrower pool than would be expected from the arc current alone.
Bead Profile and Penetration
The bead profile in hot-wire TIG welding is characterized by a wider reinforcement than conventional TIG welding, with the penetration depth remaining relatively constant for a given arc current. The wire lead angle significantly affects the bead shape:
- Low lead angles (5°–15°) produce a flat, wide bead with minimal penetration
- Medium lead angles (15°–30°) produce a slightly convex bead with moderate penetration
- High lead angles (30°–45°) produce a more pronounced reinforcement with deeper penetration
Application to Overlay Welding and Cladding
Single-Pass Overlay
For single-pass overlay applications where a thin layer of corrosion-resistant material must be deposited on a base metal, hot-wire TIG welding offers significant advantages. The arc current can be set to a level that produces just sufficient penetration into the base metal for a strong metallurgical bond, while the wire feed rate deposits the required overlay volume without excessive heat input.
Multi-Pass Overlay Buildup
In multi-pass overlay welding, where several layers of overlay material are deposited to build up a thick corrosion-resistant layer, hot-wire TIG welding provides the following benefits:
- Reduced interpass temperature: The lower heat input per pass allows better cooling between passes, reducing the risk of microstructural degradation in previously deposited layers.
- Improved layer homogeneity: The electromagnetic and mechanical stirring produced by the wire feed promotes mixing between adjacent layers, reducing compositional banding.
- Higher productivity: The increased deposition rate reduces the number of passes required to achieve the target overlay thickness.
Bond Line Quality
The bond line between the overlay material and the base metal is the most critical feature in any cladding application. In hot-wire TIG overlay welding, the bond line quality can be optimized by:
- Using a low arc current to minimize base metal penetration and dilution
- Selecting a wire composition that is compatible with the base metal to reduce intermetallic compound formation
- Controlling the interpass temperature to prevent excessive grain growth at the bond line
- Using a wire lead angle that directs the molten wire toward the bond line for improved fusion
Process Comparison with Conventional TIG and PTA
| Feature | Conventional TIG | Hot-Wire TIG | PTA Powder Cladding |
|---|---|---|---|
| Deposition rate | Low (100–300 g/h) | Medium (300–800 g/h) | High (800–2000 g/h) |
| Dilution control | Poor | Good | Excellent |
| Equipment cost | Low | Medium | High |
| Flexibility | High | Medium | Low |
| Overlay thickness | Limited | Moderate | Unlimited |
| Microstructure control | Moderate | Good | Excellent |
Defect Analysis and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Lack of fusion at bond line | Insufficient arc current or excessive travel speed | Increase arc current or reduce travel speed |
| Excessive dilution | Excessive arc current or wire composition mismatch | Reduce arc current or select compatible wire |
| Wire burning | Wire feed rate too low relative to arc current | Increase wire feed rate or reduce arc current |
| Wire spatter | Wire stick-out length too long or lead angle too high | Reduce stick-out length or lower lead angle |
| Porosity | Inadequate gas shielding or wire contamination | Increase gas flow or clean wire surface |
Engineering Practice Considerations
For pressure vessel fabricators, the adoption of hot-wire TIG welding for overlay applications requires careful consideration of the following factors:
- Procedure qualification: The hot-wire TIG process must be qualified under NB/T 47014 or ASME IX, with specific attention to the wire feed rate and lead angle parameters.
- Equipment capability: The welding equipment must provide independent control of arc current and wire feed rate, with precise synchronization between the two systems.
- Operator training: The process requires a higher level of operator skill than conventional TIG welding, as the wire feed parameters must be adjusted continuously to maintain consistent bead quality.
- Quality control: Non-destructive testing procedures must be adapted to account for the different weld geometry and microstructure produced by hot-wire TIG welding.
Summary
This study provides a thorough analysis of the arc hot-wire TIG welding process, highlighting its unique capability to decouple deposition rate from heat input. For engineers involved in overlay welding and cladding, this process offers a practical solution to the fundamental challenge of depositing corrosion-resistant materials with minimal dilution while maintaining acceptable productivity. The process should be evaluated as a viable alternative to conventional TIG overlay welding and as a complement to PTA powder cladding in applications where equipment cost and process flexibility are important considerations.