Zhuojin Pipe Fitting Co., LtdMechanical Characteristic Analysis of FSW and VP-TIG Welded Propellant Tank Bottom
Literature Overview
This research by Yan Dongyang, Wang Aimin, Pan Zhen, Wang Xiaobo, and Liang Xiaoguang from the Beijing Institute of Aerospace Systems Engineering, published in "Aerospace Materials and Technology" (2017, Vol. 47, No. 3, pp. 71-74), addresses a critical structural issue in liquid propellant tank design. The study focuses on tank bottom assemblies where the longitudinal seam is joined by Friction Stir Welding (FSW) and the circumferential seam is joined by Vacuum Plasma TIG (VP-TIG) welding. Hydraulic pressure testing was conducted with synchronized strain monitoring at typical weld locations to identify the weakest structural regions under operational loading conditions.
Hybrid Welding Configuration and Test Methodology
The propellant tank bottom represents a complex structural component in launch vehicles, combining two fundamentally different welding processes to form a load-bearing assembly. The FSW longitudinal seam provides a solid-state weld with no melting, while the VP-TIG circumferential seam is a fusion weld performed in a vacuum environment to minimize porosity and oxidation. The intersection of these two weld types creates a "T"-joint configuration that represents a geometric discontinuity and a potential stress concentration zone.
The hydraulic testing methodology involved pressurizing the tank bottom assembly while monitoring strain responses at multiple predefined locations along both weld seams and their intersection. This approach allows identification of the yielding sequence under increasing pressure, revealing the load-bearing hierarchy of different structural elements.
Key Findings and Structural Weakness Identification
The experimental results clearly demonstrate that the "T"-joint region formed by the intersection of FSW and VP-TIG welds yields first during hydraulic testing. This finding has significant implications for structural design and quality assurance of propellant tank assemblies.
| Weld Type | Welding Process | Yielding Sequence | Structural Role |
|---|---|---|---|
| Longitudinal Seam | FSW | Later yielding | Primary load-bearing path |
| Circumferential Seam | VP-TIG | Later yielding | Pressure containment |
| "T"-Joint Intersection | FSW/VP-TIG overlap | First to yield | Structural weak point |
The early yielding of the "T"-joint region can be attributed to several factors. First, the geometric discontinuity at the intersection creates a stress concentration that amplifies local stresses beyond the material's yield strength. Second, the thermal history at this intersection is complex, with the FSW weld zone potentially being reheated during subsequent VP-TIG welding, or vice versa, depending on the manufacturing sequence. This re-heating can alter the microstructure and reduce local strength. Third, the residual stress state at the intersection may be unfavorable, with superimposed residual stresses from both welding processes creating a net tensile stress state that promotes early yielding.
Engineering Practice and Design Recommendations
For aerospace engineers designing propellant tank bottom assemblies, this study provides actionable insights. The identification of the "T"-joint as the critical weak point necessitates design modifications to redistribute loads away from this region. Potential countermeasures include:
- Introducing a fillet transition or reinforcement pad at the "T"-joint intersection to reduce stress concentration.
- Optimizing the welding sequence to minimize thermal interaction between the two weld types.
- Implementing post-weld stress relief treatment specifically targeted at the intersection region.
- Incorporating additional strain monitoring points at "T"-joint locations during qualification testing.
From a quality control perspective, this study underscores the importance of non-destructive testing at weld intersections. Standard NDT procedures that focus on individual weld seams may miss critical defects at the intersection zone. Enhanced inspection protocols, including phased array ultrasonic testing (PAUT) and digital radiography (DR), should be applied to "T"-joint regions to detect potential lack of fusion, cracking, or microstructural degradation.
Study Insights and Implications
The finding that a hybrid weld configuration can create a new structural weakness at the intersection of two individually sound welds is a valuable lesson for aerospace structural engineering. It demonstrates that the system-level performance of a welded assembly cannot be predicted solely from the performance of individual welds. Engineers must consider the interaction effects between adjacent welds, particularly when different welding processes are used in close proximity. This study serves as a reminder that welding process selection must be evaluated not only for the weld itself but also for its interaction with neighboring joints under service loading conditions.