Q17Basic Mechanical Engineering

Both are massive-scale industrial metal-forming processes used to permanently deform metal into useful shapes via plastic deformation without melting.

(a) Rolling Process: Rolling is the most widely used deformation process. The raw metal stock (like a large heated ingot or slab) is fed between two massive, heavy cylindrical rollers rotating in opposite directions. The gap between the rollers is physically smaller than the thickness of the incoming metal. As friction grips the metal and pulls it through, compressive forces brutally squeeze the metal, reducing its thickness and elongating it. - Hot Rolling: Done above the recrystallization temperature. Requires less force, refines the coarse grain structure into a finer, stronger one, and is used to make structural beams (I-beams), railway tracks, and thick plates. - Cold Rolling: Done at room temperature. Requires massive force but yields exceptional surface finish, tight tolerances, and strain hardening (used for car body panels and aluminum foil).

(b) Extrusion Process: Extrusion is somewhat analogous to squeezing toothpaste out of a tube. A large, heated cylindrical billet of metal is placed inside a strong container chamber. A powerful hydraulic ram then pushes against the billet, subjecting it to immense compressive pressure. The only escape route for the metal is through a specially shaped hole (die) at the opposite end. As the metal is forced through the die, it permanently takes the cross-sectional shape of the hole, emerging as a long, continuous piece. - Application: Extremely capable of producing highly complex cross-sections that rolling cannot achieve, including hollow tubes. It is extensively used for aluminum alloys to make window frames, structural tubing, and intricate heat sinks.