CLADDING TECHNOLOGY SHANXI CO., LTDMechanical Characteristics Analysis of FSW and VP-TIG Welded Box Bottoms
Literature Overview
This research by Yan Dongyang, Wang Aimin, Pan Zhen, Wang Xiaobo, and Liang Xiaoguang from the Beijing Institute of Aerospace Systems Engineering compares the mechanical characteristics of box bottom structures welded by friction stir welding (FSW) and vibration-assisted plasma transfer arc welding (VP-TIG). Published in the Journal of Aerospace Materials and Technology in 2017, this work addresses the challenges of joining thin-walled aerospace box structures where both solid-state and arc welding processes are evaluated for their mechanical performance.
Core Technical Points
Aerospace box structures are critical load-bearing components that must exhibit high strength, fatigue resistance, and dimensional stability. The bottom plate of such structures is typically subjected to complex multiaxial loading conditions, making the selection of welding process a critical design decision. FSW is a solid-state joining process that avoids melting and associated defects, while VP-TIG employs mechanical vibration to enhance arc welding performance in thin-section applications.
Mechanical Property Comparison
The following table presents the comparative mechanical characteristics of the two welding processes:
| Property | Base Metal | FSW Joint | VP-TIG Joint |
|---|---|---|---|
| Tensile strength (MPa) | 420–450 | 380–410 | 360–400 |
| Yield strength (MPa) | 350–380 | 330–360 | 310–350 |
| Elongation (%) | 18–22 | 16–20 | 14–18 |
| Fatigue strength (MPa, 10⁷ cycles) | 250–280 | 220–250 | 190–220 |
| Microhardness (HV) | 120–130 | 100–120 (WZ) | 90–110 (WZ) |
| Residual stress (MPa) | 0 | ±30–50 | ±80–120 |
The FSW joint demonstrates superior fatigue performance due to the absence of residual tensile stresses, the lack of weld defects such as porosity and microcracks, and the refined grain structure in the stir zone. The VP-TIG joint, while showing acceptable tensile properties, exhibits higher residual stresses and more significant property degradation in the heat-affected zone due to the thermal cycling inherent in arc welding.
Stress Distribution and Fatigue Behavior
The residual stress distribution is a critical differentiator between the two processes. FSW produces predominantly compressive residual stresses in the stir zone and near the heat-affected zone, which is beneficial for fatigue resistance as compressive stresses inhibit crack initiation and propagation. In contrast, VP-TIG produces tensile residual stresses at the weld root and near-surface regions, creating favorable conditions for fatigue crack initiation. The vibration assistance in VP-TIG reduces residual stresses compared to conventional TIG but cannot eliminate them entirely.
The fatigue crack initiation sites differ significantly between the two processes. In FSW joints, cracks typically initiate from the nugget boundary where microstructural heterogeneity creates stress concentrations. In VP-TIG joints, cracks preferentially initiate from weld surface defects, porosity, or stress concentration points at the weld toe geometry.
Engineering Practice Considerations
For aerospace box structure fabrication, the selection between FSW and VP-TIG depends on the following factors:
- Service environment: Components subjected to high-cycle fatigue loading favor FSW due to superior fatigue life. Components in static or low-cycle loading may accept VP-TIG performance.
- Component thickness: FSW is most effective for thicknesses of 2–10 mm. Beyond this range, VP-TIG or other arc processes may be more practical.
- Production flexibility: VP-TIG can be adapted to various geometries and positions more easily than FSW, which requires specific fixture arrangements and tooling.
- Repair capability: FSW welds are generally not repairable by conventional welding methods, whereas VP-TIG welds can be ground and rewelded if necessary.
Key Questions and Reflections
The study raises an important question regarding the long-term reliability of FSW joints under sustained loading conditions. While FSW eliminates melt-related defects, the stir zone contains a complex microstructural evolution with multiple sub-regions of varying grain size and texture. Under sustained creep loading at elevated temperatures, the microstructural heterogeneity in the FSW joint may lead to differential deformation behavior that could affect long-term dimensional stability. This consideration is particularly relevant for aerospace applications where box structures may be exposed to elevated temperatures during flight.
Study Insights and Implications
This comparative study provides valuable guidance for process selection in aerospace box structure fabrication. The key finding is that FSW offers superior fatigue performance and lower residual stresses, making it the preferred process for fatigue-critical applications. However, VP-TIG remains a viable alternative for applications where fatigue life is less critical, production flexibility is required, or component thickness exceeds the practical range of FSW. For cladding applications on aerospace structures, where a dissimilar overlay layer must be deposited on a structural substrate, neither FSW nor VP-TIG directly applies as cladding processes, but the mechanical property data provides a benchmark for evaluating the performance of cladding welds relative to structural welds in the same component. Engineers should integrate the findings of this study into their process selection matrices, considering the full spectrum of mechanical requirements including static strength, fatigue resistance, and residual stress tolerance.