pistons: forged vs cast

i don't know if this is the correct group, but...can someone describe
(or better yet point me to a webpage) the difference between the
forging and casting process? also, it is possible to look at a piece
(in this case an aluminum alloy auto piston) and determine if it is
cast or forged? thanks.
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You ought to spend some time reading the links on this web site.
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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.
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Some bicycle parts are advertised as having been made by the revolutionary "melt forging" process.
Which is just like casting, except that it sounds more expensive.
Reply to
Andy Dingley
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.
4MLA1FN wrote:
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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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I cast pistons all the time and use F332. This is a piston alloy developed by Alcoa that is aged at 400F for 7 hours after casting. Alan
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Alan Black

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