Hi Arthur,
Weld only works if the vertices are coincident within tolerance. If you zoom in closer you see that this is not the case. To Weld this mesh, check first that the normals are pointing in the correct directions, then join it. Unselect the mesh and run:
_AlignMeshVertices _SelectNakedEdges _All _Pause 0.1 _Enter
The _Pause in the macro is that you can select the mesh. This works only post pick, otherwise the _SelectNakedEdges option is not available. After this, _Weld 180 should work as expected.
c.