Characteristics and mechanical properties of retractable pin-tool friction stir welded joints of A356-alloy LPDC hollow hubs

• Published in: Materials characterization 2024-04, Vol.210, p.113707, Article 113707
• Main Authors: Lu, Jian-Qi, Yang, Zhao, Fan, Kang-Hsien, Wu, Jia, Liu, Jia-Rui, Yang, Xu-Yue
• Format: Article
• Language: English
• Subjects: A356 alloy; Friction stir welding; Mechanical property; Precipitation phenomena; Recrystallisation; Retraction pin tool
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2024.113707

To produce high-performance noise- and weight-reducing A356-alloy hollow hubs, hub-rims and hubcaps were prepared separately through low-pressure die casting and joined together through retractable pin-tool friction stir welding (RPT-FSW). The effects of the pin retraction speed (6–37.5 mm/min) on the microstructural and mechanical properties were analysed. The results showed that the stirred zone (SZ) could be divided into a double stirred zone (DSZ) and single stirred zone (SSZ) within the pin retraction region. During the pin retraction stage, the DSZ underwent severe deformation and had a longer high-temperature duration, had larger recrystallised grains after the second dynamic recrystallisation, and had a finer and denser β'′ phase after dissolution and re-precipitation. In contrast, the SSZ underwent static recovery due to the low processing temperature, resulting in less-coarsened original recrystallised grains accompanied by much coarsened original β'′ precipitates. The combination of severer deformation and higher temperature resulted in coarser nanoscale Si precipitations in the DSZ than in the SSZ. Statistical data showed that β'′ precipitates dominated the variation in strength in the RPT-FSW region. As slow pin retraction and a long thermal experience coarsened the β'′ precipitates, a U-type Vickers hardness distribution instead of a W-type Vickers hardness distribution formed in the SSZ. This distribution degraded the service performance of the RPT-FSW joints. An appropriately high pin retraction speed improved the strength of the RPT-FSW joints and the fatigue life of the A356-alloy hollow hubs. •A356 alloy hollow hubs with noise and weight reduction functions were made by Retractable Pin-tool Friction Stir Welding.•Stirred zone in pin retraction zone can be divided to double stirred zone (DSZ) and single stirred zone (SSZ).•Mg/Si phase, nanoscale Si particles, grain size, and dislocation density were investigated by TEM, EBSD, X-ray diffraction.•β'′ phase dominated the variation of joint properties; higher pin retraction speed improved mechanical properties.