You ought to spend some time reading the links on this web site.
http://octopus.freeyellow.com/metlinks.html It is full of a lot of
introductory stuff for people in your position.
Casting is pouring a liquid into a mold and letting it cool into a solid.
Old fashioned ice cube trays that you filled with water and froze were
therefore a form of a casting. The ice cube tray was a mold, and the
molten liquid was the water. The final casting was the ice cube.
You can substitute molten aluminum for the water and a metal mold or a
sand mold for the ice cube tray... and you don't need to stick the stuff
in a freezer to promote solidification.
You can cast plastics and a few ceramics as well.
Forging is mechanical banging or squashing. Hammers can be used to forge
something or big mechanical presses (or other machines)can also do it.
If the object is heated substantially with respect to the melting point,
it can be called "hot forging". And, obviously the forgings without
heating can usually be called cold forging. There is also warm forging,
which is where the relative temperature (compared to the melting point
of the material being forged) isn't so high.
Some cast objects have large grain structures and under some conditions,
they can be seen without too much special work or equipment. Some
castings show mold marks of different kinds, such as a sand mold may
show a granualar surface on surfaces which aren't machined after casting.
I think you'll find that nowadays many such structurally critical
components of engines like pistons and connecting rods are actually
sintered -- neither forged nor cast.
But to add something that should be fairly obvious to JBUCH's (Jim's)
post . . . forging is almost certainly going to yield a much stronger
part than casting because by definition it WORKS the part. In working
the part, however, stresses are induced which can cause deformation when
the part is heated again . . . as in an internal combustion engine. And
whereas he mentions that casting can form crystals that can even be seen
and which can differ in size and type from one portion of a casting to
another (i.e., process control, including cooling, is critical to
creationg of homogenous crystalization) sintering can easily produce a
fairly homogenous crystalization because the size of the powder granules
can be easily controlled -- and varied if necessary.
Oh, forgot to mention that in the case of pistons the last step, after
sintering and postprocess machining, is induction hardening to create a
hard outer surface. Induction surface hardening, as opposed to heat
treating, does not create internal stresses.
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