CLADDING TECHNOLOGY SHANXI CO., LTDResearch Progress on TIG Overlay Welding Technology
Literature Overview
This review article by Li Yulong and Yu Yexiao from the Key Laboratory of Robotics and Welding Automation at Nanchang University provides a comprehensive survey of the state of the art in gas tungsten arc welding (GTAW/TIG) overlay technology as of 2012. Funded by the Jiangxi Province Youth Scientist (Jinggang Star) Cultivation Program (Grant No. 2010DQ01000), the paper was published in the journal "Electric Welding Machine" and covers the fundamental principles, process parameters, materials, applications, and emerging trends in TIG overlay welding.
Core Technical Principles of TIG Overlay Welding
TIG overlay welding, also known as TIG surfacing or TIG cladding, uses a non-consumable tungsten electrode and a separate filler wire to deposit a layer of material onto a base metal substrate. The process is characterized by its high precision, low dilution rate (typically 10–30% for single-pass overlay), excellent surface quality, and the ability to deposit a wide range of materials including stainless steels, nickel-based alloys, copper alloys, and titanium alloys.
The fundamental process parameters include:
| Parameter | Typical Range | Effect on Overlay |
|---|---|---|
| Arc current | 40–200 A | Higher current increases dilution and penetration |
| Arc voltage | 12–20 V | Influences arc width and bead geometry |
| Travel speed | 20–200 mm/min | Controls deposition rate and heat input |
| Wire feed speed | 100–500 mm/min | Determines deposition thickness per pass |
| Shielding gas flow | 8–20 L/min | Protects molten pool and tungsten from oxidation |
| Tungsten electrode diameter | 1.6–4.0 mm | Controls arc stability and current density |
| Wire diameter | 1.0–3.2 mm | Affects deposition rate and bead width |
Process Variants and Advanced Configurations
The review highlights several advanced configurations that extend the capabilities of conventional TIG overlay welding:
- Pulsed TIG overlay: By modulating the arc current between a peak and a background value, pulsed TIG achieves lower average heat input, reduced dilution, and finer microstructure. The pulse frequency typically ranges from 10–100 Hz, with a peak-to-background current ratio of 2:1 to 5:1. This variant is particularly effective for depositing nickel-based alloys onto dissimilar substrates where dilution control is critical.
- Hot-wire TIG (HWT): An additional electrical circuit heats the filler wire before it enters the arc, increasing the deposition rate by 30–50% without proportionally increasing the arc heat input. This makes HWT particularly attractive for building up thick overlay layers economically.
- Plasma arc-assisted TIG: Combining a plasma arc with a TIG arc allows independent control of the heat source and the filler wire, enabling tailored dilution rates and improved process stability.
- Cold wire TIG: The filler wire is fed into the arc without electrical contact, resulting in minimal dilution (as low as 5%) and very high deposition rates. This variant is well-suited for depositing reactive metals and precious alloys.
- Automated TIG with wire oscillation: Mechanical oscillation of the tungsten electrode or wire creates a wider, flatter bead with improved wetting and reduced porosity.
Materials and Applications
TIG overlay welding is extensively used in the following application areas:
| Application | Substrate | Overlay Material | Purpose |
|---|---|---|---|
| Nuclear reactor components | Carbon steel | 304L/316L stainless steel | Corrosion resistance |
| Chemical processing equipment | Carbon steel | Hastelloy C-276 | Acid resistance |
| Aerospace engine components | Titanium alloy | Ti-6Al-4V | Wear resistance |
| Medical implants | Titanium alloy | Ti-6Al-4V | Dimensional repair |
| Power plant piping | Carbon steel | Stellite 6 | Erosion resistance |
| Heat exchanger tubes | Copper | Nickel alloy | Fouling resistance |
| Hydraulic cylinder bores | Carbon steel | Hardened alloy steel | Wear resistance |
The review emphasizes that TIG overlay is particularly advantageous for small to medium-sized components where high precision and low thermal distortion are required. For large-scale cladding of pressure vessels or thick-walled equipment, submerged arc welding (SAW) or electroslag welding (ESW) are typically preferred due to their higher deposition rates.
Quality Control and Defect Prevention
A critical aspect of TIG overlay welding is the prevention of defects that can compromise the overlay's performance:
- Porosity: Caused by inadequate shielding gas coverage, wire contamination, or moisture in the base metal. Countermeasures include increasing gas flow rate, using a trailing shield, and preheating the substrate to remove surface moisture.
- Cracking: Hot cracking in the overlay weld metal is common when depositing high-carbon or high-sulfur alloys. Preheating the substrate to 150–300 °C, reducing travel speed to increase cooling rate, and using low-sulfur filler materials are effective countermeasures.
- Insufficient fusion: Occurs when the arc current is too low or the travel speed is too high. This defect is particularly detrimental in overlay welding because it creates a weak interface that can lead to spallation during service.
- Tungsten inclusion: Caused by tungsten electrode erosion entering the molten pool. Regular electrode sharpening and maintaining proper electrode protrusion (3–5 mm) prevent this defect.
Critical Reflections and Engineering Practice
The review provides a solid foundation for understanding TIG overlay welding, but several practical aspects deserve emphasis from an engineering perspective. First, the dilution rate is the single most important parameter governing the final composition and properties of the overlay. For corrosion-resistant overlays, dilution must be controlled below 30% to ensure adequate alloying element content in the weld metal. For wear-resistant overlays, higher dilution (up to 40%) may be acceptable and even beneficial, as it can promote a harder martensitic structure.
Second, the review underemphasizes the importance of interpass temperature control in multi-pass overlay welding. For nickel-based alloys deposited on carbon steel substrates, interpass temperatures should be maintained below 150 °C to minimize the risk of hydrogen-induced cracking in the heat-affected zone of the base metal. For stainless steel overlays, interpass temperatures above 250 °C risk sensitization and intergranular corrosion of the weld metal.
Third, the review does not adequately address the inspection and qualification requirements for TIG overlay welding. According to NB/T 47014 and ASME IX, welder qualification for overlay welding requires demonstration of both bond strength and weld metal properties. The bond strength test, typically a tensile test on a coupon with a butt joint between the overlay and the substrate, must demonstrate a minimum shear strength of 150 MPa for carbon steel substrates.
Summary
This review article serves as an excellent reference for engineers entering the field of TIG overlay welding. It comprehensively covers process fundamentals, variants, materials, and applications. The practical takeaway is that TIG overlay welding, when properly executed with controlled dilution, appropriate shielding, and rigorous quality control, remains one of the most versatile and reliable methods for depositing protective and functional surface layers on engineering components.