Key Takeaways & Executive Findings
- •• RFSSW of dissimilar 2B06/7B04 Al alloys produces distinct microstructural zones with grain size order HAZ > TMAZ > SZ. • Larger secondary-phase particles are present in TMAZ and HAZ, with HAZ particle size increasing at higher rotational speeds. • Corrosion susceptibility follows HAZ > TMAZ > SZ > BM, and increases with higher rotational speeds. • Post-corrosion mechanical degradation is mainly due to crevice corrosion at joint overlaps, not microstructural changes.
Abstract
The dissimilar 2B06 and 7B04 Al alloy joints were prepared by refill friction stir spot welding (RFSSW), and the microstructural evolution and corrosion behavior of the joints were investigated. Based on microstructural analysis, the welded joints exhibit distinct microstructural zones, including the stir zone (SZ), thermomechanically affected zone (TMAZ), and heat-affected zone (HAZ). The grain size of each zone is in the order of HAZ > TMAZ > SZ. Notably, the TMAZ and HAZ contain significantly larger secondary-phase particles compared to the SZ, with particle size in the HAZ increasing at higher rotational speeds. Electrochemical tests indicate that corrosion susceptibility follows the sequence of HAZ > TMAZ > SZ > BM, with greater sensitivity observed at increased rotational speeds. Post-corrosion mechanical performance degradation primarily arises from crevice corrosion at joint overlaps, but not from the changes in the microstructure.
1. Introduction
High-strength 2xxx and 7xxx series Al alloys are widely used in aircraft structure manufacturing due to their high specific strength [1−3]. However, physical property differences between these alloys often lead to metallurgical defects [4], such as pores, inclusions, and thermal cracks, during fusion welding [5−12], compromising weld quality and the reliability of dissimilar Al alloy joints. Additionally, joining Al alloys via riveting and bolting is labor-intensive and costly, making it impractical for many production environments [13].
Refill friction stir spot welding (RFSSW), an innovative solid-state spot joining technique derived from friction stir welding (FSW), has gained traction for effectively joining similar and dissimilar Al alloys and Al−Mg alloys [14−23]. Unlike fusion welding, RFSSW avoids metallurgical issues [24−28] and offers pollutant-free, energy-efficient, and high-strength bonding, making it suitable for the high-performance requirements in the aerospace industry [29].
The corrosion behavior of Al alloy RFSSW joints is closely related to the microstructural characteristics of the welded joint region. ZHANG et al [30,31] conducted separate studies on RFSSW joints involving 2524 and 7050 Al alloys. The experimental results revealed a clear correlation between the microstructural features of the joints and their macroscopic corrosion behavior. The continuity of the precipitated phase was identified as a crucial factor. The highest degree of continuous distribution of intergranular precipitated phases is found in the heat-affected zone (HAZ), where there is a high susceptibility to intergranular corrosion and exfoliation corrosion. However, there is currently limited research on the corrosion behavior of RFSSW joints involving dissimilar Al alloys, with most studies focusing on FSW. In an investigation of AA2024/7075 FSW joints, ZHANG et al [32−34] categorized micro-regions based on material mixing in the stir zone (SZ) and conducted extensive tests to assess corrosion resistance across regions. These findings suggest that sufficient material mixing in FSW can effectively reduce corrosion sensitivity. XU et al [35] investigated the impact of process parameters in single and double-side friction stir welding of AA7085−T7452 on stress corrosion cracking (SCC) sensitivity. The results revealed that a reduction in Cu content in the grain boundary precipitated phase of the FSW joint increases SCC sensitivity, th
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Fang-yuan JIANG, Da ZHANG, Yan-kun MA, Jiang-tao XIONG, Wei GUO, Jing-long LI (2025). Microstructure evolution and corrosion behavior of refill friction stir spot welding joint for dissimilar Al alloys. SinoTechIntel Verified Research. https://doi.org/10.1016/S1003-6326(25)66950-0
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Frequently Asked Questions
What is refill friction stir spot welding (RFSSW)?
RFSSW is a solid-state spot joining technique derived from friction stir welding (FSW). It avoids metallurgical issues common in fusion welding and offers pollutant-free, energy-efficient, and high-strength bonding, making it suitable for aerospace applications.
What are the distinct microstructural zones in RFSSW joints of dissimilar Al alloys?
The welded joints exhibit distinct microstructural zones: the stir zone (SZ), thermomechanically affected zone (TMAZ), and heat-affected zone (HAZ). Grain size follows the order HAZ > TMAZ > SZ.
How does rotational speed affect the microstructure and corrosion behavior?
Higher rotational speeds increase the size of secondary-phase particles in the HAZ and increase corrosion susceptibility. The corrosion susceptibility sequence is HAZ > TMAZ > SZ > BM, with greater sensitivity at higher rotational speeds.
What is the main cause of post-corrosion mechanical performance degradation in these joints?
Post-corrosion mechanical performance degradation primarily arises from crevice corrosion at joint overlaps, not from changes in the microstructure.
Why is corrosion behavior important for dissimilar Al alloy RFSSW joints?
Corrosion behavior is closely related to microstructural characteristics, and understanding it is crucial for ensuring the reliability and durability of dissimilar Al alloy joints in aerospace applications.
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