On Fri, 18 May 2012 12:03:42 -0500, Tim Wescott
wrote:

I've always been told that one of the cool things about hypereutectic
aluminum alloy is that its thermal expansion coefficient is significantly
lower than plain ol' aluminum, which makes it a handy material for
pistons. (The other cool thing being that all those silicon particles
can make for a hard, low-wear surface if you machine it correctly).

I was curious yesterday so I went prospecting on Matweb -- it listed
pretty darn near the same expansion rates for 2024 and an alloy with 18%
silicon. It also said the CTE was "derived from similar alloys on
Matweb", which makes me wonder what they mean by "similar". So now I
don't know what to believe.

Anyone know a handy chart of aluminum alloys and their coefficients of
thermal expansion? Google is not my friend in this: when I do a Google
search all I get are enthusiastic articles by gearheads like me. I can
learn all the stuff I already know about the virtues of the stuff, not
any engineering data about those virtues.

I have the data, but it's not a chart. It's in the bound volume of the
_Metals Handbook_ (ASM).

I'll give you any specifics you want, but maybe a short explanation
will be better. You may look askance at this, coming from me rather
than a piston expert, but I asked the same question roughly 30 years
ago when I was preparing a car to race in IT-C and I was Materials
Editor at American Machinist, who would call the engineers all the
time and actually could get them to answer me. g

The forging alloys, including 2024, 6061, and the specialized 2618 (1%
Si) and 4032 (12% Si) used for high-temp aircraft, racing, diesel, and
motorcycle pistons all have thermal coefficients in the range of 11 -
13 microinches/inch/deg. F. So do the common eutectic and
hypereutectic cast-piston alloys: 336 and 390. 390 (16-18% Si) is
around 10, actually, while 336 (11-13% Si) is between 11 and 12.

So, what gives? This is what I was told and what I concluded. What I
was told that there are three things involved: Forged pistons are
meant to run in higher-performance engines, where they run hotter.
They're thicker in various places. And they have a lot of extra metal
around the wrist-pin bosses, because of the shape limitations involved
in forging.

Some of the geometrical issues are vague in my memory, but my response
was that the thermal expansion rate is the same for different
sections. Their response to me was that the thicker sections create
three problems: One percent, say, of a thicker section is greater than
one percent of a thinner section. And with a greater
thickness/diameter ratio, the expanding aluminum produces more force
on the cylinder walls for a given rise in temperature. Third, the
extra force requires more clearance for safety's sake.

Like you, I'd heard for years that forging alloys had much higher
thermal expansion coefficients, and that was the reason. In fact, I've
forgotten what I'm telling you here once or twice, and perpetuated
that myth myself. When I became Materials Editor at _American
Machinist_, and had all the data at my fingertips, I quickly realized
that it isn't true. I discovered what you just discovered, and then I
forgot it. g That was confirmed by the engineers I talked to, at
Ford, and, IIRC, at Federal-Mogul.

So how did the story get started? This is my theory and my conclusion:
The facts of higher temperature operation and the thicker sections
somehow were transmuted into a story about coefficients of expansion.
This is not an uncommon thing among people who are expert at their
mechanical thing (race mechanics) but who are not necessarily
engineers. Then the story got passed from hand-to-hand until it was
taken as gospel.

Anyway, that's what I think. The coefficients are facts, and not just
what I think. And the common misconception is what you and I have both
been told for many years.

--
Ed Huntress