This is a feature of Titanium based materials, namely Titanium Alloys. It is done by:-

1) Forcing the two surfaces to be joined together at an elevated temperature and pressure such that the surfaces are flattened and there is a high atom-to-atom contact area.

2) Atoms then diffuse along the grain boundaries to the voids. This reduces the size of the voids between the materials. This step can occur rapidly due to high rates of grain boundary diffusion.

3) Volume diffusion now occurs, which eliminates the voids completely, leaving a solid material which is essentially of one piece of metal and has no inherent weakness, c.f. Welding.