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.
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. 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.