Manufacturing Aluminium Alloy Bicycle Parts

From a 1992 Bridgestone Cycles catalogue
There are trade secrets involving minute differences in heat, time,
and alloy, but the basic processes are the same, regardless of who's
doing the work.

In cold forging, the alloy is warmed to a temperature just below the
point at which the crystalline structure is changed, then bashed into
shape by means of forging dies (like molds). Brakes can be formed in
one or two bashings of up to 330 tons each, but more complex and
massive parts - crank arms - require up to six whomps of up to 660
tons to reach final form. Cold-forging alloys are high strength to
begin with (cold-forged cranks are often made from 7075-T6; 74,000 psi
[tensile strength]), and the forging process adds grain structure
along the curves of the piece, much like the grain in a crooked tree
branch. Cold-forged parts are typically thinner, lighter, stronger,
more accurately made, and more expensive than cast parts.

In hot-forging, a slightly lower strength alloy (for cranks, around
65,000 psi [tensile strength]) is heated, softened, then stomped into
shape with one fell blow. Hot-forged cranks cost less to make than
cold-forged cranks, mainly because the tooling last longer and fewer
dies are needed.

In Gravity-casting, still another alloy (typically ACIB-T4; 42,000 psi
tensile strength) is melted, then poured into a mold and allowed to
cool naturally. During the cooling, air bubbles gravitate upwards and
out-hence the term. The alloy isn't as strong as that used for hot- or
cold-forging, but it is about 30 percent stronger than the alloy used
in melt-forging. Compared with melt-forged parts, gravity-cast parts
tend to be more expensive, stronger, lighter and less brittle (not
that brittleness or strength are problems with well-made
melt-forgings). Gravity-castings, like hot- and cold-forgings, can be
anodized, and consequentlythe finished pieces cab be difficult to
distinguish from hot- or cold-forgings. Some cranks, such as the
excellent Specialized ST-4, are gravity cast and then, for added
strength, whomped once in a forging die.

Melt-Forging is a high pressure casting, in which molten AC46-T6
aluminum (tensile strength appox. 32,714 psi) is forced into a mold
under roughly 11,378.4 lbs of pressure. This eliminates bubbles much
faster and more economically than in gravity casting. The "forged"
piece is then cooled quickly with water. To compensate for the lower
strength of AC4C-T6 (only 50 percent of typical crank cold-forging
alloys and 75 percent of crank gravity-casting alloys), the parts tend
to be chunkier. Since AC46-T6 cannot be anodized, melt-forged parts
never display the fine finishes possible with hot-forgings,
cold-forgings, or gravity castings. Still, melt-forging has made
relatively lightweight, attractive, reliable components affordable to
people who would otherwise ride steel.