Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of 2219 Aluminum Alloy Double-Shank FSW and Fusion Weld Cross Joints
Literature Overview
This paper published in Aerospace Materials and Technology (Vol. 48, No. 3, 2018, pp. 61–66) by Wei Ruigang and colleagues from Capital Aerospace Machinery Company investigates the microstructure and mechanical properties of cross joints formed by combining double-shank friction stir welding (SR-FSW) with variable-polarity TIG welding in 2219C10S aluminum alloy. Funded by a "Thirteenth Five-Year Plan" equipment pre-research project (No. 41423050128), this work addresses a practical challenge in aerospace manufacturing: the joining of complex geometries where multiple weld joints intersect.
Experimental Configuration
The experimental approach involves a two-step process:
- Step 1: Double-shank friction stir welding (SR-FSW) to create a baseline weld.
- Step 2: Variable-polarity TIG welding perpendicular to the SR-FSW weld direction to create a cross joint.
The comparison group consists of a single TIG weld joint for the same material, allowing direct comparison of microstructural and mechanical differences.
Microstructural Analysis
Common Features Between Cross Joint and Single TIG Joint
| Feature | Description |
|---|---|
| Weld zone composition | Columnar region (DC helium arc) + bowl-shaped region (AC pulse argon arc) |
| HAZ behavior | Grain coarsening on base metal grains |
| Fracture location | HAZ in both cases |
| Hardness profile | W-shaped distribution |
Distinctive Features of the Cross Joint
| Feature | Cross Joint | Single TIG Joint |
|---|---|---|
| Base metal microstructure | Fine equiaxed grains | Rolled band grains |
| HAZ hardness gradient | Lower | Higher |
| Softened zone width | Narrower | Wider |
| Tensile strength coefficient | 66.14% | Slightly higher |
| Elongation | ~30% higher | Baseline |
| Fracture mode | Fully ductile | Mixed ductile-brittle |
Interpretation of Microstructural Differences
The presence of fine equiaxed grains in the base metal adjacent to the cross joint (as opposed to rolled band grains in the single TIG joint) is a significant finding. This suggests that the SR-FSW process, even when performed perpendicular to the subsequent TIG weld, influences the microstructure in the vicinity of the cross intersection. The FSW process produces a refined, dynamically recrystallized microstructure that persists into the region subsequently affected by the TIG weld.
The narrower softened zone in the cross joint compared to the single TIG joint is particularly interesting. This may be attributed to the pre-existing fine equiaxed grains from the FSW process, which provide a higher density of grain boundaries that can act as nucleation sites during the TIG welding thermal cycle, thereby limiting grain coarsening in the HAZ.
Mechanical Property Analysis
The cross joint exhibits a tensile strength coefficient of 66.14%, which is slightly lower than the single TIG joint. However, the cross joint demonstrates approximately 30% higher elongation, indicating superior ductility. The fracture mode analysis reveals that the cross joint fractures in a fully ductile manner, while the single TIG joint shows mixed ductile-brittle characteristics.
| Property | Cross Joint | Single TIG Joint | Engineering Implication |
|---|---|---|---|
| Tensile strength coefficient | 66.14% | ~68-70% (estimated) | Acceptable for non-critical applications |
| Elongation | ~30% higher | Baseline | Better energy absorption |
| Fracture mode | Fully ductile | Mixed | More predictable failure |
| Softened zone width | Narrower | Wider | Less sensitive to stress concentration |
Engineering Practice Implications
This study has direct relevance to aerospace manufacturing where complex joint geometries are common. The findings suggest that:
- Hybrid joining is viable: Combining FSW with fusion welding for cross joints is technically feasible and produces acceptable mechanical properties.
- Ductility advantage: The cross joint's superior ductility and fully ductile fracture mode may be advantageous for applications subject to impact loading or where crack arrest is important.
- Strength trade-off: The slight reduction in tensile strength (approximately 2-4 percentage points) may be acceptable for non-primary structural components but would require further evaluation for primary load-bearing structures.
- HAZ softening mitigation: The narrower softened zone in the cross joint suggests that the FSW pre-treatment partially mitigates HAZ softening, which could be exploited in process design.
Key Questions and Reflections
A fundamental question remains: why does the cross joint exhibit lower tensile strength despite having a narrower softened zone and finer base metal grains? The answer may lie in the complex residual stress state at the cross intersection, where the interaction of FSW and TIG thermal cycles creates a non-uniform stress field that can reduce the effective load-bearing capacity. Additionally, the microstructural heterogeneity at the intersection zone—where FSW-affected material meets TIG-affected material—may create weak interfaces that reduce strength without necessarily reducing ductility.
Another practical consideration is the process complexity. Creating a cross joint requires two distinct welding operations with different equipment, parameters, and quality control procedures. The cost and time implications of this hybrid approach must be weighed against the mechanical property benefits.
Study Insights and Outlook
This work demonstrates that hybrid joining strategies can produce joints with favorable ductility characteristics, even at the expense of some tensile strength. For aerospace applications where damage tolerance and energy absorption are critical design criteria, the cross joint's fully ductile fracture mode and higher elongation may be more valuable than maximum tensile strength. Future research should focus on fatigue performance, residual stress characterization, and the optimization of the FSW-TIG parameter combination to maximize both strength and ductility simultaneously.