Analysis on exceptional cryogenic mechanical properties of AA2219 alloy FSW joints in multi-scale

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2022-08, Vol.850, p.143489, Article 143489
• Main Authors: Hao, Yong-gang, Liu, Wei
• Format: Article
• Language: English
• Subjects: Aluminum base alloys; Base metal; Cryogenic deformation; Cryogenic temperature; Cryogenics; Deformation; Edge dislocations; Friction stir welding; FSW joints; Heterogeneity; Localization; Low temperature; Mechanical properties; Multi-scale; Multi-slips; Plastic deformation; Strain concentration; Strain distribution; Strain heterogeneity; Strain localization; Welded joints
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2022.143489

In this paper, the multi-scale strain characteristics of AA2219 alloy friction stir welded joints deformation at cryogenic temperature were analyzed quantitatively. The macro strain distribution of the joints was analyzed using the quasi in-situ DIC technology during uniaxial tension at room and cryogenic temperature −196°C. The mesoscopic/microscopic strain distribution were studied by SEM-DIC, EBSD and geometric phase analysis. The results show that ultimate fracture strain of the FSW joints increased from 0.16 to 0.28 with an increase of 75% and the strain heterogeneity of the joints was significantly reduced at −196°C. The major strain of the weld zone increased by 128.2%. The mesoscale local strain decreased by 32.4% and 25% in the base metal and weld zone, respectively. The microscale deformation localization between grains was improved, and the dislocation distribution within the grains was more uniform in nanoscale level. The improved strain localization in multi-scale levels at cryogenic temperature contributed to the superior mechanical properties of the joints. At −196°C, the decreased strain heterogeneity was related to the decreased strength differences between the weld and BM zone directly. The intergranular deformation uniformity of the fine-grained weld zone were improved at cryogenic temperature. The ductile micro-voids coalescence instead of connecting along the slip bands rapidly contributed to the increased plasticity of weld zone at −196°C, and the cryogenic plastic deformation was dominated by the activated cross and multi-slips. The ability to resist strain concentration caused by local slip was improved at ultra-low temperature. •A multi-scale method was proposed to investigate the cryogenic strain delocalization behavior of FSW joints.•The macro/meso strain characteristics of FSW joints were analyzed using the quasi in-situ tension with DIC at −196°C.•The micro strain delocalization mechanism of the base metal and weld zone was revealed respectively.•The improved macro strain localization, fracture mode transition and increased multiple slips were discussed.