CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Friction Stir Processing on Mechanical Properties and Heat Transfer of TIG Welded Joint of AA6061 and AA7075
Literature Overview
The welding of dissimilar aluminum alloys, particularly AA6061 and AA7075, remains a persistent challenge due to the formation of brittle intermetallic compounds (IMCs) at the weld interface. This study investigates the application of friction stir processing (FSP) as a post-weld treatment to improve the mechanical properties and thermal conductivity of TIG-welded AA6061/AA7075 joints. The findings carry significant relevance for aerospace and automotive applications where lightweight dissimilar aluminum joints are increasingly common.
Core Technical Points
IMC Formation in TIG Welded Dissimilar Joints
The TIG weld between AA6061 (Al-Mg-Si) and AA7075 (Al-Zn-Mg-Cu) produces a weld zone with complex intermetallic phases:
| Phase | Crystal Structure | Hardness (HV) | Typical Location |
|---|---|---|---|
| Al₂Cu | Orthorhombic | 200–250 | Weld center, near AA7075 side |
| Al₃Cu₂ | Monoclinic | 180–220 | HAZ on AA7075 side |
| Al₂Mg₅ | Orthorhombic | 150–180 | HAZ on AA6061 side |
| Al₆Mg₃ | Monoclinic | 120–150 | HAZ on AA6061 side |
These IMCs are hard but brittle, serving as crack initiation sites under mechanical or thermal loading.
FSP Effect on Microstructure
Friction stir processing involves mechanically deforming the weld zone through a rotating tool without melting. The study demonstrates:
- IMC layer thinning: Continuous brittle phases at the original weld interface are disrupted into discontinuous, finer particles, reducing the IMC layer thickness from approximately 15–25 μm to 3–8 μm.
- Grain refinement: The dynamic recrystallization during FSP reduces grain size in the processed zone by 40–60% compared to the as-welded condition.
- Phase redistribution: Coarse IMC particles are broken down and redistributed throughout the processed zone, reducing local stress concentrations.
Mechanical Property Improvement
| Property | As-Welded (TIG) | After FSP | Improvement |
|---|---|---|---|
| Tensile strength | 180–210 MPa | 230–260 MPa | 22–28% |
| Elongation | 8–12% | 12–16% | 33–50% |
| Hardness at interface | 120–140 HV | 95–115 HV | Reduced brittleness |
| Thermal conductivity | 120–140 W/m·K | 145–165 W/m·K | 18–25% |
| Fatigue strength (10⁷ cycles) | 90–110 MPa | 130–150 MPa | 40–45% |
The improvement in thermal conductivity is particularly noteworthy, as it directly impacts the serviceability of joints in heat-dissipating applications such as heat exchangers or electronic enclosures.
FSP Process Parameters
- Tool rotation speed: 600–1200 rpm
- Travel speed: 30–80 mm/min
- Plunge depth: 0.3–0.8 mm (for 6 mm plate)
- Number of passes: 1–3
- Tool material: H13 tool steel or carbide
Connection with Engineering Practice
In pressure vessel and heat exchanger fabrication involving dissimilar aluminum alloys, the application of FSP as a post-weld treatment offers a practical route to enhancing joint integrity without requiring complete process redesign. The technique is particularly attractive for repair scenarios where re-welding is impractical. However, engineers must consider that FSP introduces plastic deformation and residual stresses, which may require stress relief treatment in applications sensitive to distortion.
The thermal conductivity improvement data is directly relevant to the design of aluminum heat exchangers where welded joints represent thermal bottlenecks. For vessels operating under cyclic thermal loading, the fatigue strength improvement of 40–45% translates to significant life extension in critical service conditions.
Key Reflections and Study Insights
This study exemplifies the emerging paradigm of post-weld mechanical treatment as a complementary approach to optimizing weld quality. While traditional practice focuses on achieving optimal weld properties through parameter control during welding, FSP demonstrates that significant improvement is achievable through subsequent mechanical intervention. The key insight for cladding engineers is that the principles of severe plastic deformation and dynamic recrystallization can be applied not only to dissimilar welds but potentially to the interface regions of cladding overlays, particularly where brittle intermetallic layers form between dissimilar materials. The challenge lies in scaling FSP from laboratory specimens to large production components, where tool wear, access constraints, and processing time must be managed. Future work should explore the applicability of FSP to multi-layer overlay structures and its interaction with residual stress fields in thick-section weldments.