3.4.2.1 Sheet Metal Stamping

Sheet metal stamping utilizes sheet products, mostly alloys of steel, stainless steel, aluminum and copper, with steel alloys being predominant. Stampings are suitable at most levels of production, but are most economical where annual production is high. Press productivity and die costs depend on the number of dies and presses required to produce the stamping, which are in turn driven by the complexity of the stamping. The list price of sheet stock is competitive with forging stock, but stamping is not usually as material efficient. Energy consumption is low since the stock is not heated. The range of available alloys is wide, but higher strength materials are often not as formable as lower strength materials. Shape flexibility may therefore be restricted, and process cost increased.

Six factors, usually in combination, give forging advantages over sheet metal stampings.

1. The engineered scrap rate for some types of stampings that are alternatives to forging may be as high as 50% and is occasionally higher. It includes perimeter material in the clamp and binder areas of the die, and openings in the stamping. Engineered scrap is recycled but little of the original purchase price is regained, particularly with sheet steel.
2. Most stampings are made in stages, each requiring a separate die. The die and processing costs are driven by number of dies required.
3. Many applications require that several stampings be separately formed and joined. Manufacturing and tooling costs for the stampings are proportionately increased, and the cost of fixturing and joining is incurred. As parts count increases, forgings become more cost competitive. In some cases forgings have been chosen over one-piece stampings to achieve weight or to gain secondary advantages from shapes that can be forged but cannot be achieved by stamping.
4. Stampings are usually made from stock of uniform thickness, and wall thicknesses can be varied a limited amount throughout the stamping. The capacity to vary thickness depends on the process and cannot always be utilized to optimize the product. The stock thickness of a stamping is therefore usually driven by the mechanical requirements of one critical feature. Alternatives, such as added reinforcements or tailored blanks, require that separate parts be processed and joined. Forging allows more opportunity to tailor feature thicknesses to functional requirements, reducing component weight. This gives forgings an important advantage in applications where minimum weight is critical, such as aerospace applications and automobile suspension members.

5. Assemblies of stampings require features, such as flanges, to facilitate welding or adhesive bonding. These additional features usually increase somewhat the amount of purchased stock and the weight of the end product.

6. Stamping processes work harden the metal to some degree, increasing strength and hardness and decreasing ductility in some areas of the stamping. However, the increases are driven by the process, and usually can not be optimized to the application as is often done in forging. In some cases, work hardening requires intermediate annealing.