CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Separated Arc AA-TIG Welded Joints
Literature Overview
This 2013 study by Fan Ding and colleagues at Lanzhou University of Technology, supported by the National Natural Science Foundation of China (Grant 51074084) and the Gansu Provincial Natural Science Foundation (Grant 1010RJZA037), investigates the microstructural evolution and mechanical properties of welded joints produced using the Separated Arc Aluminum Alloy TIG (AA-TIG) welding process. The AA-TIG process represents an innovative approach to aluminum alloy welding that separates the arc into multiple smaller arcs, each with reduced heat input, to achieve improved weld quality and reduced distortion. This work provides valuable insights into advanced TIG welding techniques that have potential applications in overlay and cladding operations.
Process Description and Configuration
The AA-TIG process employs a specialized electrode configuration that divides the primary TIG arc into two or more separated sub-arcs. Each sub-arc operates at a lower current than the equivalent single-arc process, resulting in reduced peak temperatures and more controlled thermal input. The separated arcs are positioned symmetrically about the weld centerline, with the inter-arc spacing carefully controlled to ensure uniform heat distribution.
Process Parameters
| Parameter | Single Arc TIG | AA-TIG (2 arcs) | AA-TIG (3 arcs) |
|---|---|---|---|
| Total current (A) | 150–200 | 150–200 | 150–200 |
| Current per arc (A) | 150–200 | 75–100 | 50–67 |
| Peak temperature (°C) | 6500–7000 | 5500–6000 | 5000–5500 |
| Heat input (J/mm) | 300–500 | 200–350 | 150–250 |
| Weld width (mm) | 8–12 | 6–10 | 5–8 |
| Penetration depth (mm) | 3–5 | 2.5–4 | 2–3.5 |
Microstructural Characteristics
Weld Metal Microstructure
The separated arc configuration produces a weld metal microstructure that differs significantly from conventional single-arc TIG welds. The reduced heat input per arc results in finer grain structures with more uniform grain orientation:
- Grain size: 30–80 μm (compared to 50–150 μm in single-arc TIG)
- Grain morphology: Equiaxed rather than columnar
- Grain boundary character: Higher fraction of special boundaries (Σ3 twin boundaries)
- Precipitate distribution: More uniform with reduced centerline segregation
The finer grain structure is attributed to the reduced thermal gradient and slower cooling rate resulting from the distributed heat input of multiple arcs. The more equiaxed grain morphology is a consequence of the reduced directional thermal gradient, which promotes heterogeneous nucleation throughout the pool rather than preferential growth along the thermal gradient direction.
Heat-Affected Zone Microstructure
The HAZ in AA-TIG welds exhibits a narrower affected zone with less severe microstructural changes compared to single-arc TIG welds:
| HAZ Zone | Single Arc TIG | AA-TIG | Property Impact |
|---|---|---|---|
| Over-aged zone width (mm) | 2–4 | 1–2 | Reduced softening zone |
| Peak temperature (°C) | 480–520 | 420–460 | Less precipitate dissolution |
| Hardness reduction (%) | 50–60 | 30–40 | Better property retention |
| Recrystallization extent | Complete | Partial | Better strength retention |
The reduced HAZ width and severity are direct consequences of the lower peak temperatures achieved with the separated arc configuration. This is particularly beneficial for precipitation-hardened aluminum alloys where HAZ softening is the primary limiting factor on joint strength.
Mechanical Properties
Tensile Properties
The AA-TIG process produces joints with significantly improved mechanical properties compared to conventional single-arc TIG welding:
- Weld joint tensile strength: 250–300 MPa (vs. 180–220 MPa for single-arc)
- Yield strength: 200–240 MPa (vs. 150–180 MPa for single-arc)
- Elongation: 18–22% (vs. 15–20% for single-arc)
- Strength reduction factor: 0.55–0.60 (vs. 0.40–0.45 for single-arc)
The improved properties are attributed to the finer grain structure, reduced HAZ softening, and more uniform precipitate distribution. The reduction in strength loss relative to base metal is particularly significant for structural applications where joint strength is a critical design parameter.
Fatigue Properties
The AA-TIG process also improves fatigue performance through reduced residual stresses and improved microstructural uniformity:
- Fatigue strength (10⁷ cycles): 120–150 MPa (vs. 90–120 MPa for single-arc)
- Fatigue crack initiation life: 2–3× improvement
- Crack propagation resistance: 1.5–2× improvement
Application to Overlay Welding
The separated arc principle has several potential applications in overlay welding:
Multi-Arc Overlay Configuration
For thick overlay deposits, the separated arc approach can be adapted by positioning multiple arcs along the travel direction rather than transversely. This configuration provides:
- Reduced peak temperatures in each arc zone
- More uniform heat distribution along the overlay length
- Reduced thermal distortion of the substrate
- Improved bonding quality at the overlay-substrate interface
Interface Quality Enhancement
The reduced peak temperatures achieved with separated arcs can improve the bonding quality at the overlay-substrate interface by:
- Reducing excessive substrate melting that can dilute the overlay composition
- Minimizing intermetallic compound formation in dissimilar material overlays
- Reducing residual stresses at the interface
- Improving the metallurgical compatibility of the bond
Process Parameter Recommendations for Overlay Applications
| Overlay Requirement | Recommended Configuration | Key Benefit |
|---|---|---|
| Thin overlay (< 1 mm) | 2 arcs, 20–30 mm spacing | Controlled heat input, good bonding |
| Medium overlay (1–3 mm) | 2–3 arcs, 15–25 mm spacing | Uniform deposit, reduced cracking |
| Thick overlay (> 3 mm) | 3 arcs, 10–20 mm spacing | Reduced distortion, improved mixing |
| Dissimilar material overlay | 2 arcs, 25–35 mm spacing | Reduced intermetallic formation |
Defect Analysis
The AA-TIG process reduces several common weld defects through its distributed heat input approach:
| Defect | Single Arc TIG | AA-TIG | Mechanism |
|---|---|---|---|
| Porosity | 1–2% volume fraction | 0.3–0.5% | Reduced gas entrapment |
| Hot cracking | Occasional | Rare | Reduced centerline segregation |
| Distortion | 2–3 mm/100 mm | 0.5–1.5 mm/100 mm | Lower peak temperatures |
| Lack of fusion | Possible at edges | Unlikely | Better wetting from multiple arcs |
Study Insights and Reflections
This research demonstrates that the separated arc approach offers significant advantages over conventional single-arc TIG welding for aluminum alloy applications. The improvements in microstructure, mechanical properties, and defect reduction are achieved through a fundamentally different thermal input strategy that reduces peak temperatures while maintaining adequate heat input for weld formation.
For engineers involved in overlay welding, the separated arc concept provides a pathway to improving overlay quality, particularly for thick deposits and dissimilar material combinations. The reduced peak temperatures are particularly beneficial for overlays where substrate dilution or intermetallic compound formation is a concern. However, the implementation of separated arc technology requires specialized electrode configurations and power supply arrangements that may not be readily available in all production environments.
The key insight from this research is that welding process quality is not solely determined by total heat input but is significantly influenced by the spatial and temporal distribution of that heat input. Engineers should consider process innovations that modify heat distribution rather than simply adjusting conventional parameters. The separated arc approach exemplifies this philosophy, demonstrating that intelligent process design can achieve quality improvements that are unattainable through parameter optimization alone. This perspective should guide future process development efforts in the overlay and cladding welding field.