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
This study by Husain Mehdi and R.S. Mishra, published in Defence Technology in 2021, investigates the application of friction stir processing (FSP) to improve the mechanical properties and thermal performance of TIG-welded joints between AA6061 and AA7075 aluminum alloys. These two alloys are widely used in aerospace and defense applications—AA6061 for structural components requiring good formability and weldability, and AA7075 for high-strength applications. The dissimilar joint between these alloys presents significant challenges due to the formation of brittle intermetallic compounds (IMCs) at the weld interface, which severely degrade the mechanical properties and create preferential paths for corrosion.
Core Technical Content
Dissimilar Joint Challenges
The TIG welding of AA6061 and AA7075 produces a weld joint with the following characteristics:
| Property | AA6061 Base Metal | AA7075 Base Metal | TIG Weld Joint |
|---|---|---|---|
| Tensile strength (MPa) | 310 | 570 | 180-220 |
| Yield strength (MPa) | 275 | 505 | 120-160 |
| Elongation (%) | 12 | 11 | 5-8 |
| Hardness (HV) | 80-100 | 150-170 | 60-80 |
The significant reduction in mechanical properties is attributed to:
- Brittle intermetallic compounds: The formation of Al-Cu-Mg intermetallics (such as Al2CuMg5 and Al2Cu) at the weld interface, which are hard but brittle and prone to cracking.
- Microstructural inhomogeneity: The heterogeneous microstructure resulting from the different alloy compositions and solidification behaviors of the two alloys.
- Residual stresses: High residual tensile stresses at the weld interface due to differential thermal expansion and solidification shrinkage.
- Corrosion susceptibility: Galvanic coupling between the dissimilar alloys in the presence of corrosive environments.
Friction Stir Processing Parameters
FSP is a solid-state post-weld treatment that involves the insertion of a rotating tool into the weld joint, producing severe plastic deformation through frictional heating and mechanical stirring. The process parameters are critical to achieving optimal results:
| Parameter | Typical Range | Effect on Joint Properties |
|---|---|---|
| Tool rotation speed | 600-1500 rpm | Higher speed increases temperature and stirring intensity |
| Travel speed | 30-100 mm/min | Lower speed increases deformation and material mixing |
| Shoulder diameter | 15-25 mm | Larger shoulder increases heat input and material flow |
| Pin diameter | 3-6 mm | Larger pin increases deformation volume |
| Pin length | 6-10 mm | Must exceed weld thickness for complete processing |
| Tool tilt angle | 2-3 degrees | Affects material flow pattern and defect formation |
Mechanical Property Improvement
The FSP treatment produces significant improvements in the mechanical properties of the TIG-welded dissimilar joint:
- Tensile strength: Increased from approximately 180-220 MPa to 280-350 MPa, representing a 60-80% improvement.
- Hardness: Increased from 60-80 HV to 100-130 HV in the processed zone, approaching the hardness of the AA6061 base metal.
- Microstructural refinement: The grain size in the processed zone is reduced to 1-5 micrometers, compared to 50-200 micrometers in the unprocessed weld.
- IMC modification: The FSP treatment breaks up and redistributes the brittle intermetallic compounds, reducing their continuity and improving the ductility of the joint.
- Residual stress relief: The severe plastic deformation relieves a significant portion of the residual tensile stresses, reducing the risk of cracking and improving fatigue life.
Heat Transfer Performance
The study also investigates the thermal performance of the FSP-treated joint. The improved microstructural homogeneity and reduced porosity result in enhanced thermal conductivity across the joint. The thermal resistance of the joint is reduced by approximately 30-50% compared to the untreated TIG weld, which is significant for applications involving heat transfer, such as heat exchangers, radiators, and thermal management systems in aerospace and defense applications.
Engineering Practice Integration
Relevance to Cladding and Bimetal Manufacturing
The principles of friction stir processing are directly applicable to the improvement of clad plate and weld overlay joints in bimetal manufacturing. Several key parallels exist:
- Microstructural refinement: FSP can be used to refine the microstructure of the overlay layer in clad plates, improving the mechanical properties and reducing the sensitivity to intergranular corrosion.
- Residual stress relief: The residual stresses introduced during overlay welding can be partially relieved through FSP, reducing the risk of cracking and distortion in clad plates and pressure vessel components.
- Bond strength improvement: FSP can enhance the bond strength between the overlay layer and the base metal by promoting metallurgical bonding and reducing the thickness of the diffusion layer.
Application to Pressure Vessel Fabrication
In the fabrication of bimetal pressure vessels, FSP could be employed as a post-weld treatment to improve the quality of overlay welds. For example:
- Nickel alloy overlay on carbon steel: FSP could be used to refine the microstructure of the overlay layer, improving its corrosion resistance and mechanical properties.
- Stainless steel overlay on low-alloy steel: FSP could enhance the bond strength and reduce the susceptibility to intergranular corrosion at the overlay-base metal interface.
- Copper alloy overlay on steel: FSP could improve the thermal and mechanical properties of the joint, reducing the risk of cracking during service.
Key Questions and Reflections
The study raises several important questions for engineers working in cladding and bimetal manufacturing:
- Applicability to thick sections: The study focuses on relatively thin plates (typically 5-10 mm). The scalability of FSP to thick-section clad plates and pressure vessel components (20-100 mm) requires further investigation, as the process parameters and tool design must be adapted for thicker sections.
- Long-term stability: The long-term stability of the FSP-improved microstructure under elevated temperature service must be evaluated. Grain growth and IMC re-formation could degrade the mechanical properties over time, particularly in high-temperature applications.
- Process integration: The integration of FSP into the manufacturing workflow of clad plates and pressure vessels requires careful consideration of process sequence, equipment requirements, and quality control procedures. The process must be compatible with existing manufacturing practices and quality assurance systems.
Study Insights and Implications
This research demonstrates that friction stir processing is an effective post-weld treatment for improving the mechanical properties and thermal performance of dissimilar aluminum alloy joints. The significant improvements in tensile strength, hardness, and thermal conductivity make FSP a promising technology for enhancing the performance of clad plates and weld overlay joints in bimetal manufacturing. For engineers involved in pressure vessel fabrication, the key implication is that FSP can be used to improve the quality and reliability of overlay welds, particularly in applications where the mechanical properties and thermal performance of the joint are critical. The adoption of FSP requires careful process development, qualification, and quality control, but the potential benefits in terms of improved joint performance and extended service life make it a technology worth pursuing. The study also underscores the importance of post-weld treatments in achieving optimal joint properties, a principle that applies to all aspects of cladding and bimetal manufacturing.