cast vs forged aluminum

[bearbvd@domain.elided (Greg Hermann)]

Message from:
"Hardwick, Robert C" <Robert.Hardwick@domain.elided>

>Greg,
>
>You've started some very interesting discussions on high performance
>engine building.  The work I do with aluminum is all at lower
>temperatures.  Still I find the following topic curious:
>
>Greg said:
>
>2618 Aluminium (the strongest of the piston forging alloys) is a fair bit
>softer than the usual piston casting alloys.
>
>OK, I know nothing about 2618, but any casting being stronger than any
>forging sounds very odd!  What about voids, porosity etc and no
>preferential grain direction?

The 2618 is EXTREMELY stronger than any of the casting alloys designed for
pistons--probably by a factor of 4, or maybe more. Talking about tensile
strength here.   All the mechanical properties which you mention are also
VASTLY superior in a forged piston.   The ability of a forged piston to
transfer heat is also vastly superior to that of a cast piston--again due
to denser material and better grain structure.

HOWEVER-- the aluminium casting alloys, while rather brittle, are also
considerably HARDER than any of the forging alloys.  And cast piston alloys
also have a much greater resistance to "galling"--a wiping of the surface
of the metal which can occur under sliding friction conditions (and which
closely resembles welding).

SO--if nothing else, cast aluminium pistons DO have much better wear
resistance than forged aluminium pistons do!!

The hypereutectic casting alloys are the hardest--and most brittle of all
piston alloys--These alloys are so-called because they have higher than the
soluble concentration of silicon in them--when the metal solidifies,
particles of elemental silicon precipitate out throughout the matrix of the
casting--and the hardness and wear resistance of the silicon particles
which are at the surface is what gives the hyper-e alloy pistons their wear
resistance. Unfortunately--the interuption to the crystal structure of the
metal caused by the silicon particles is what makes them brittle.

Thus--oem manufacturer have usually used cast pistons on account of their
better wear properties, while race engine builders tend to use forged
pistons for their strength and heat transfer, whilst ignoring the wear
problems.

To me, the best of both worlds is a forged piston (for strength) with a
low-friction, hard surface coating to improve the wear well beyond the wear
performance of even a cast piston.

These coatings are very much a currently developing technology/science, so
the composition of the coatings tends to be proprietary, and their
application to automotive engines is pretty recent stuff--but I know of at
least one firm which specializes in such coatings which is working BIG TIME
with people like GM to get their coatings into some oem applications on
pistons--so I am inclined to say that there is tremendous potential for
productive use of these coatings.

Regards, Greg

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