During friction stir lap joint welding, hooks are systematically formed at the interface of dissimilar Al2024/Ti-6Al-4V plates. Various hooks dimensions and shapes are observed according to the welding parameters (welding speed, rotational speed and double-pass). Based on digital image correlation (DIC) during tensile shear tests, it is found that hooks features govern the fracture behaviours and, hence, the mechanical properties of the overall welds. In particular, three fracture behaviours are obtained: at the interface, in the Al2024 and across the Ti-6Al-4V plate, which was never reported in dissimilar Al/Ti studies. The aims of this work are (i) to study the influence of FSW parameters on the mechanisms of hooks formation and (ii) to understand the role of both AS and RS hooks on the mechanical behaviour of the welds, by the means of two new approaches. First, hooks are investigated by X-ray tomography which provided some innovative insight into the material flow (3D viewing). Based on these qualitative and quantitative results, hooks are used as tracers and two new scenarios of AS and RS generation are proposed. Then, the contribution of both hooks on the stress distribution is clearly revealed by some calculations of the stresses reached around AS and RS hooks. Contrary to RS hook, AS hook is sharp and, hence, the stress reached at the top of AS hook is characterized by a stress concentration. The results showed that the fracture behaviours are greatly influenced by hooks size and the formation of microstructural defects inside the Ti-6Al-4V, which depend on the welding parameters. In conclusions, this work revealed the relationship between the achievable parameters, the resulting hooks dimensions and the fracture behaviours. In addition, based on the understanding of the material flow, another FSW parameters are proposed to optimize the mechanical properties.

在摩擦搅拌搭接焊接过程中，在不同的Al2024 / Ti-6Al-4V板的界面处系统地形成了钩子。根据焊接参数（焊接速度，转速和双道次），可以观察到各种弯钩的尺寸和形状。基于拉伸剪切试验期间的数字图像相关性（DIC），发现弯钩特征控制着断裂行为，从而决定了整个焊缝的机械性能。特别是，获得了三种断裂行为：在界面处，在Al2024中以及在整个Ti-6Al-4V板上，这在不同的Al / Ti研究中从未报道过。这项工作的目的是（i）研究FSW参数对弯钩形成机理的影响，以及（ii）了解AS和RS弯钩在焊缝机械性能中的作用，通过两种新方法。首先，通过X射线断层摄影术对钩子进行了研究，这为材料流（3D观察）提供了一些新颖的见解。基于这些定性和定量结果，将挂钩用作示踪剂，并提出了两种新的AS和RS生成方案。然后，通过对AS和RS钩周围达到的应力进行一些计算，可以清楚地揭示出两个钩对应力分布的贡献。与RS钩相反，AS钩很锋利，因此，在AS钩顶部达到的应力的特征在于应力集中。结果表明，Ti-6Al-4V的钩形尺寸和内部微结构缺陷的形成对断裂行为的影响很大，这取决于焊接参数。总而言之，这项工作揭示了可达到的参数，所产生的弯钩尺寸和断裂行为之间的关系。此外，基于对材料流的理解，提出了另一个FSW参数来优化机械性能。