some of the Chinese manufacturers use special powder for repairing leakage of pipe and fittings after casting. they heating the pipe wall and then add this powder on the surface of leakage area.
Most of these alloy powders will have an eutectic composition such that the melting point is low. Eutectic composition of Ni-Si-B is very common. Nickel goes well with ferrous alloys including cast iron. Silicon and boron are added to suppress the melting point, and render high fluidity (so that it flows through the flaws).