Grain boundaries with high Σ value and strain in grain matrix induce crack initiation in extruded 6000 series aluminium alloys

Significance 

Among the known age-hardening aluminum (Al) alloys, 6000 series Al alloys stand out. Some types of 6000 series Al alloys, such as A6005C and A6061, are widely are commonly used in structural engineering to meet the strength, corrosion resistance and deformation requirements. Additionally, some elements can be added to the alloy to improve its properties. For example, adding copper can improve strength, while some transitional elements like manganese can induce grain refinement effects. Bending test is one of the commonly used tests to evaluate their properties, especially the shock absorption under continuous deformation. However, the trade-off existing between the bendability and tensile strength of these alloys affect their shock-absorption performance, especially for those with high yield strength.

The mechanical properties of 6000 series Al alloys, including bending, are mainly affected by the morphology and distribution of the precipitates, distribution of the residual stress, preferred orientation, secondary phase particles and shear band generation. Recrystallized textures of these alloys are normally influenced by the geometry of the extruded profiles that are usually initiated at depths of 100-200 µm and the process conditions like cooling rate and temperature. In addition, the recrystallization structure between the bulk and the surfaces is primarily caused by the friction-induced shear at the Al flow and molding area. Based on these considerations, it is important to clarify the texture formation mechanism and evaluate the grain matrix and boundaries at the micro level to improve the mechanical properties of these alloys through texture optimization. Despite the research efforts, this has not been fully achieved.

Herein, Dr. Shogo Oda from YKKAP Inc. and Professor Shun-Ichiro Tanaka from Tohoku University sought to clarify and provide more insights into the crack initiation mechanism during the bending process of 6000 series Al alloys at the micro-nano scale level based on kernel average misorientation (KAM) and sigma (Σ) value at the crack initiation point. To achieve this, the precipitation distribution strain affecting crack generation as well as the grain boundary energetic stability were investigated. Electron backscatter diffraction measurements were carried out to obtain the Σ value and KAM map. Moreover, X-ray diffraction stress analysis was performed to clarify the relationship between the texture distribution and residual stress. Their work is currently published in the journal, Materials Science and Engineering A.

The authors showed that for cross-sections from the surface region to 400 µm depth, A6061 alloy exhibited 15% more low-order Σ values of 3, 5 and 7 compared to the Σ values of the A6005C alloy. This suggested that the A6061 alloy is more energetically stable with a strong interface. Upon applying the tensile force, the KAM map of the initial crack position surroundings revealed that the initial crack mainly occurred at the grain boundary with high Σ and strain concentration values. Furthermore, it was observed that the number density and precipitate-free zone width of the grain matrix precipitate of the AlMgSiCu-based Q and Q’ compounds were almost similar for both A6005C and A6061 alloys, though they had a small impact on crack initiating. Nevertheless, compared with the A6005C, the morphology of the A6061 precipitates was quite smaller and could affect the strain distribution in the grain matrix.

In summary, this is the first study to assess the origin of cracks of 6000 series Al alloys during the bending process based on strain in the grain boundary and grain matrix stability, considering the impacts of the precipitates. The potential effects of the precipitates on the strain distribution and crack initiation mechanism were clarified based on the energetic stability of the grain boundaries and matrix. In a statement to Advances in Engineering, Professor Shun-Ichiro Tanaka said their findings provide useful insights that would enable optimization of 6000 series Al alloys properties for various practical applications.

Grain boundaries with high Σ value and strain in grain matrix induce crack initiation in extruded 6000 series aluminium alloys - Advances in Engineering Grain boundaries with high Σ value and strain in grain matrix induce crack initiation in extruded 6000 series aluminium alloys - Advances in Engineering

Reference

Oda, S., & Tanaka, S-H. (2022). Grain boundaries with high Σ value and strain in grain matrix induce crack initiation in extruded 6000 series aluminium alloysMaterials Science and Engineering: A, 834, 142630.

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