Basic Principles of Welding

If your were to take two ice cubes from the refrigerator, the outer surfaces under the heat of the day will begin to melt to ice water. Place the two wet cubes one on top of the other back in the refrigerator and within a short time the two cubes are welded together to form one block of ice. The addition of heat has melted portion of the two parts to be joined and they both cool down to one structure, the melted section becoming an intimate part of the bond.

This is the basic principle of welding - the base materials being joined are melted at the abutting faces and become all or part of the weld.

Compare this with the glueing of two cubes of wood. The wood forms no part of the bond and adhesion is achieved by the strength and intimate contact of the glue with the two abutting surfaces. This is more akin to the processes of soldering and brazing, where a metallic cement melting at a lower temperature than the parts being joined flows and solidifies in intimate contact with the components to form a bond.