Definition - What does Hydraulic Expansion mean?
Hydraulic expansion is a process that ensure a tight fit between the outside diameter of a tube (or pipe) and the inside diameter of another hole or cylinder. Hydraulic expansion creates a tight fit by using a liquid to deform the inner material until it is in contact with the outer material. This may prevent future corrosion caused by liquids that would otherwise leak.
Corrosionpedia explains Hydraulic Expansion
Hydraulic expansion creates a very tight fit between two round objects. To perform hydraulic expansion, a tube with an outside diameter relatively close in size to, but smaller than, the inside diameter of another round object must be selected. Then, the tube is inserted into the larger round object and filled with a liquid, typically water. The two tubes are sealed off and the water is pressurized. The pressure of the water on the inside walls of the inside of the tube is made great enough to plastically deform the inner tube. The tube continues to deform until it comes into contact with the outside round object, thus creating a tight fit.
One main advantage of hydraulic expansion is time. Tubes can be rapidly pressurized and deformed to the desired size. It is also a relatively clean process because the liquid is usually water, not hydraulic oil.