Magnesium (Mg) alloy thick plates welded by friction stir welding (FSW) often exhibit poor formability and uneven microstructures, leading to a significant deterioration in the mechanical properties. In this work, AZ31 Mg alloy plates with a thickness of 20 mm were successfully welded by differential double-shoulder friction stir welding (DDS-FSW). The low-cycle fatigue response behaviour was characterised, and the damage mechanism of the joints were studied. The results indicated that the fatigue life of the DDS-FSW joints in both parameters exceeded that of the FSW joint at 0.3 % strain amplitude, with increases of 200 % and 82 %, respectively. The predominant deformation mechanisms are basal slip and {10 2} twinning. Basal slip is more likely to be activated in the FSW joint, whereas {10 2} twinning is more easily activated in the DDS-FSW joints. Furthermore, the fatigue toughness () and damage transition exponent () could be increased by improving the asymmetric distribution of the stir zone in the joint, the strength of “orientated micro-interface”, and the proportion and distribution uniformity of twins.

中文翻译：

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差动双肩搅拌摩擦焊镁合金厚板接头的低周疲劳行为

通过搅拌摩擦焊（FSW）焊接的镁（Mg）合金厚板通常表现出较差的成型性和不均匀的微观结构，导致机械性能显着恶化。本工作采用差动双肩搅拌摩擦焊（DDS-FSW）成功焊接了厚度为20 mm的AZ31镁合金板材。表征了低周疲劳响应行为，研究了接头的损伤机制。结果表明，DDS-FSW 接头在两个参数下的疲劳寿命都超过了 0.3% 应变幅下的 FSW 接头，分别提高了 200% 和 82%。主要的变形机制是基面滑移和{10 2}孪晶。基底滑移更有可能在 FSW 关节中被激活，而 {10 2} 孪生在 DDS-FSW 关节中更容易被激活。此外，通过改善接头中搅拌区的不对称分布、“定向微界面”的强度以及孪晶的比例和分布均匀性，可以提高疲劳韧性（）和损伤转变指数（）。