Zhuojin Pipe Fitting Co., LtdComparative Analysis of FSW and MIG Welding for 3A21 Aluminum Alloy
Literature Overview
This paper by Wu Xinghuan, Tang Wenbin, and Chen Yuhua, published in Welding (2014, No. 5, pp. 67-70), presents a comparative study of friction stir welding (FSW) and MIG welding for 8 mm thick 3A21 (Al-Cu-Mg) aluminum alloy. The research was supported by the Hunan Provincial Science and Technology Department project (2011CK3056). The study examines microstructure, mechanical properties, and defect characteristics of both welding methods, providing valuable insights for process selection in aerospace and transportation applications.
Metallurgical Comparison
The fundamental difference between FSW and MIG welding lies in the joining mechanism: FSW is a solid-state process that does not involve melting, while MIG is a fusion process. This distinction has profound effects on weld microstructure and properties.
| Feature | FSW | MIG |
|---|---|---|
| Joining mechanism | Solid-state plastic deformation | Fusion and solidification |
| HAZ formation | No true HAZ; thermomechanically affected zone (TMAZ) | Distinct HAZ with precipitate dissolution |
| Microstructure | Dense concentric ring structure (onion-ring pattern) | Columnar dendritic structure with directional grain growth |
| Defect susceptibility | Very low; no gas porosity or hot cracking | Moderate; sensitive to porosity and hot cracking |
| Residual stress | Lower magnitude; compressive stress in stir zone | Higher magnitude; tensile stress in HAZ |
For 3A21 aluminum alloy (equivalent to 2A12 in Chinese standards, similar to 2024-T3/T4 in ASTM designations), the high Cu and Mg content provides excellent strength through age hardening but makes the alloy susceptible to hot cracking during fusion welding.
Mechanical Property Results
The study reports the following mechanical property comparisons:
| Property | FSW | MIG |
|---|---|---|
| Tensile strength (Rm) | 89.5 MPa (maximum) | Lower than FSW |
| Front-side bend test | Lower than MIG | Higher than FSW |
| Back-side bend test | Comparable to MIG | Comparable to FSW |
| Hardness profile | More uniform; lower softening in TMAZ | Significant softening in HAZ |
The higher tensile strength of FSW welds is attributed to the absence of a true HAZ. In MIG welding, the heat cycle dissolves the strengthening precipitates (S-phase, Al2CuMg) in the HAZ, creating a softened zone that becomes the weakest link in the joint. FSW, by contrast, produces a thermomechanically affected zone where precipitate dissolution is limited, and the severe plastic deformation refines the grain structure, contributing to higher strength retention.
The interesting finding regarding bend performance is that MIG welds show better front-side bend performance than FSW welds. This may be related to the compressive residual stress distribution in FSW welds and the different microstructural features at the weld toe. The back-side bend performance is comparable between the two methods, indicating similar toughness at the weld root.
Defect Analysis
The paper highlights that MIG welds of 3A21 alloy exhibited porosity defects under the same environmental conditions, attributed to:
- Directional grain growth: Columnar dendrites create channels for gas bubble migration
- High environmental humidity: Hydrogen pickup from moisture in the air
- Alloy chemistry: High Cu and Mg content increases hot cracking sensitivity
FSW welds, by contrast, produced dense, defect-free joints with concentric ring structures formed under full plastic deformation conditions. The solid-state nature of FSW eliminates the possibility of gas porosity and hot cracking, which are inherent risks in fusion welding.
Process Selection Guidance
Based on this study and broader engineering experience, the following process selection criteria can be established:
| Application Requirement | Preferred Process | Rationale |
|---|---|---|
| Maximum strength retention | FSW | No HAZ softening; superior tensile strength |
| High production speed (long joints) | MIG (with optimization) | Higher travel speed; simpler equipment |
| Thick plate (>25 mm) | MIG | FSW has thickness limitations (typically <25 mm) |
| Dissimilar joint configurations | MIG | FSW requires matched tool materials |
| Fatigue-critical applications | FSW | Lower residual stress; no porosity |
| Cost-sensitive production | MIG | Lower equipment cost; simpler setup |
Study Insights
This comparative study reinforces the principle that welding process selection must be driven by application requirements rather than default assumptions. For 3A21 aluminum alloy, where strength retention is critical (as in aerospace structural components), FSW offers clear advantages in terms of mechanical properties and defect-free welds. However, the practical limitations of FSW, including equipment cost, joint geometry constraints, and thickness limitations, mean that MIG welding remains the more versatile option for many industrial applications. Engineers should evaluate both processes during the design phase and make informed trade-offs between performance, cost, and manufacturability. The study also highlights the importance of environmental control in aluminum fusion welding, as even small variations in humidity can significantly impact weld quality.