Note that the weld illustrated in Fig. 3 is a partial-penetration weld, and would only cover a small portion of the joint area at the left side of the joint overlap, leaving the rest of the joint unbonded. Also note that the shape of the external portion of the weld-bead is such that it can help to spread stresses that might otherwise be concentrated at the sharp corner if the weld-bead were not there. However, the value of that weld is limited to just the area of the joint-overlap where the physical weld is located. The rest of the overlap has no support, unless a second weld were added at the other end of the joint-overlap. Additionally, due to the high weld-heat used to melt the base-metal in the weld area, distortion of the metal in the rest of the joint overlap is quite common.
By comparison, the braze joint shown in Fig. 2 covers the entire surface in the overlap area between the two components, and, because of the even heating of the part in the vacuum brazing furnace used to join the assembly, no distortion of the lower sheet portion of the joint occurred.
But — due to the small size of the brazing fillet (often also referred to as the braze-meniscus because of its small size), there is no large fillet to help spread the stresses that will concentrate at that sharp corner. This is where the designers need to become educated re braze-design! Because there is no large fillet to help spread the stress, the base metal itself needs to be shaped so as to spread the stress, rather than depending on a large fillet to do so. Thus, a proper design for the joint would look like the one illustrated in Fig. 4, where the sharp corner is removed.