Automotive vehicle disc brake rotors are an example of a manufactured article that may comprise a cast iron (or other durable, friction wear material) annular rotor body attached to a lower density metal hub. Of course, many other manufactured articles comprise, or could comprise, a relatively heavy and durable component that is subjected to loading stress and wear, and a lighter component serving another function.
In the case of the vehicle disc brake rotor, a hub is used to attach the rotor body to a vehicle wheel. The hub may be a round cylindrical body (sometimes shaped like a hat) attached to the wheel and aligned with the axis of rotation of the wheel. Attached to the bottom of the hat (for example) and extending radially outwardly is the annular rotor body or disc (brake frictional surface). In operation of the vehicle a hydraulically or electrically actuated brake caliper member, positioned around the outer circumferential edge of the rotor, presses friction pads against opposite sides (cheeks) of the annular rotor disc when it is desired to stop rotation of the wheel. The complimentary hub and rotor body portions of such brake rotors may be cast of a single metal alloy and formed as an integral article. Or the portion subjected to wear may be formed separately of a suitable metal composition and later attached to the complementary portion made of a lighter material. Thus, a complete and integral brake rotor may be formed, for example, of cast iron, or the rotor portion may be made of cast iron and the hub be made of an aluminum or magnesium alloy. An advantage of forming such an article of a relatively heavy and durable portion and a lighter material portion is the overall weight of the article may be reduced.
A brake disc is subject to frictional heating and to induced vibrations when brake pads are pressed against opposite cheeks of the rotating part. Accordingly, the annular rotor body portion of a disc brake may be formed with radial vents so that cooling air may be pumped through the rotating brake body. And sometimes it is desired to incorporate one or more annular inserts in the annular rotor body to produce columbic friction between contacting surfaces of the embedded insert(s) and surrounding rotor body metal to dampen noise otherwise transmitted by the vibrating brake rotor. These additional structural features of the annular rotor body have significantly complicated the manufacture of low mass articles such as vented and sound dampened brake rotors.