Suppose you have samples of various metals, all the same size and shape. And you heat them up to, say, 400 degrees F. Which expand the most and the least? What would be the order of: Copper, gold, mild steel, cast iron, titanium, bronze, aluminum and average stainless steel?
The number you're looking for is the Coefficient of Thermal Expansion. Looked these up and a couple caveats: No listing for stainless, but should be pretty close to regular (carbon) steel. Bronze is mostly copper, but it's expansion will vary some depending on the type of bronze. I found aluminum-bronze. Here are relative expansions (higher number means greater expansion). titanium 1.00 cast iron 1.21 steel 1.63 stainless 1.63 gold 1.65 bronze 1.88 copper 2.04 aluminum 2.73
Given the general descriptions of some alloys in the list, there could be some shuffling of the order....but in general here is my ranking from least expansion to highest: Cast Iron, Ti, Au, Mild Steel, Cu, 316 SS, Bronze and finally Al..... Brock
In general (10-6 in/in/ deg F) Carbon Steel (1020) 6.7 Grey cast iron 6.7 Stainless (304) 10 Aluminum 13 Highest Copper 9.3 Manganese-bronze 11.2 Titanium 5.0 lowest Gold not listed in the book I checked In reality these numbers are just guidelines, the thermal expansion of can vary quite a bit depending on the alloy. Silicone for instance reduces the thermal expansion of aluminum significantly. The thermal expansion rate also changes somewhat depending on the temperature of the metal. Cheers Jim
Ranked in descending order Coefficient of linear expansion per degree C. per the "Pocket Ref" Aluminum .0000231 Bronze .0000175 Stainless Steel (304) .0000173 Copper .0000168 Gold .0000142 Mild Steel .0000120 Cast Iron .0000106 Titanium .0000085
An excellent question, one that engineers who wanted to mate the durability of cast iron cylinder sleeves with a light weight aluminum block struggled with for many years. If I recall GM tried to circumvent the problem by making the block entirely out of aluminum for the Vega (maybe used high silica Al sleeves??). While I am not enlightened on the particulars of how this is achieved in modern engines, logic tells me that comes from a combination of mating the right alloys, controlling cooling within the block and clever engineering solutions. As you can see from the prior responses above, depending on the alloy chosen for say tin bronze vs. aluminum bronze, or what grade of stainless, say austinitic vs. martinsitic the expansion rate can be vastly different for similar materials. So the trick would be to pick the alloys that best match for the conditions. Cast iron has a low expansion rate...a good thing seeing how cylinder temps are actually much higher than the block temps. So then it would be a matter of modifying the aluminum alloy (and/or the iron alloy) so that the expansion rates are more closely matched. Then by controlling the cooling and thus the expansion, one could envision that by keeping the aluminum alloy cooler than the iron, it expands less and things stay relatively well matched. All that being said, I don't think that it is entirely that simple. Because the temps do fluctuate over a wide range (say from start up to normal operating temp), expansion rate control through alloy choice may not be the end all. More factors have to be considered and would venture to guess that there are some engineered solutions that allow for some relative movement between the dissimilar metals. Brock
My old Alfa mechanic used to use a household oven to heat up aluminum heads, in order to remove the bronze valve guides. He got an oven for the shop, as his wife didn't appreciate him using the home oven. The one in the shop also came in handy for keeping pizza warm.
Thermal expansion is a function of temperature. The numbers cited are at specific temperatures or they are the mean value over a range of temperature. In general, useful aluminum has about the same 'alpha' as stainless steels (300 and 400 series). This information is extremely important in the design of expansion joints for heat exchangers in the chemical industry. It is also why fan belts tend not to squeal when the engine warms.
Jaguar (probably others as well) had a lot of trouble with steel valve seats dropping out of aluminum heads in their 12 cylinder engines if overheated. It's interesting to see all the different interpretations of similar information. This helps to explain why there are so many different religions.
Back in the day when I was wrenching on BMW 2002's, Bavarias and CS's the favored technique was similar....we didn't need to heat the head to remove the old guides but used a small air hammer with a tool end that fit inside the guides to buzz them out. Then we used an acetylene torch with a heating head to warm the areas around the holes for valve guides and merely use a valve guide set tool and a plastic hammer to push new guides (which had been stored in the freezer) to the proper depth in the holes. Easy peasy... Brock
Uro- The liners are cooled down with liquid nitrogen to shrink them, and then pressed in. When both come up to room temperature and the liners expand, the aluminum is locked around the cylinder liner. Both expand when the engine heats up from runing, maintaining the lock. Interestingly, COE has been in use since the 1700s in clock pendulums, where two different metals are used to keep the pendulum the same length over a fairly wide range of temperatures. Otherwise the clock gains time when the pendulum shrinks and loses time when it expands. Wooden pendulum rods do not expand or contract much either, and that was often the poor man's solution. Incidentally, pardon me for using your specialty in a post on 599 prices. You might enjoy reading it. Taz Terry Phillips
Interesting, thanks guys. One more question: of all metals (not just the few I listed) does titanium have the lowest COE?
According to the charts I have looked at, Ti has a higher COE than cast iron...7.1 vs. 6.0 10-6 in./in./°F... http://www.lucas-milhaupt.com/htmdocs/brazing_support/everything_about_brazing/materials_comp_chart.html
No, some are lower. Tungsten is the lowest I know of, but there very well may be other metals that have a lower expansion rate. Also, are you limiting your query to pure metals or alloys? You listed some alloys as well as some metals. There are composites (including structural and electrically conductive) that have some properties of metals and a much lower coefficient of thermal expansion. Probably TMI
Well, this thread is getting a little unstable in its orbit SO WHY NOT: How about ceramic and sintered ceramics compared to metals as far as stability with heat?
The numbers I cited are from a materials engineering textbook (I have better resources at work). The book did not say what temperature range they are valid for, but I would suspect they are typical for temperatures from room temperature to about 250 deg F. When you get to temperatures above 400 deg F the expansion coefficient can change. I agree the expansion coefficient of aluminum can vary significantly, I believe the expansion rate I quoted is for an aluminum with very little to no alloys in it. Cheers Jim
No, it does not......... It is important to note that CTE values are also dependant on the temperature range over which they are measured, and the value of CTE is not a constant for any material. Nearly all materials (and absolutely all metals and metal alloys) will expand more (or less) at different temperature ranges. So, identifying the material with "the lowest CTE" is relative to the temperature range and will not be the lowest all the time. That said, when you consider a 'real world' temperature range of say, -25 C to 200 C, the metal with the lowest CTE is an alloy known as "Invar". This is a nickel / iron alloy which was developed specifically for Aerospace for use in applications where extremely low CTE's are required. In fact, over certain temperature ranges, the CTE of Invar is so low that it is essentially "0". The name chosen for this alloy --- Invar --- was taken from the word "invariable" ------ because it is so dimensionally invariable to temperature change.
BTW, I don't mean to be a knit-picker here folks, but the correct expression for this property is "CTE" (for Coefficient of Thermal Expansion) not "COE"......if we're going to talk like Engineers, let's at least use the proper language
And, if we are talking about all materials (not just metals), then there are certain GFC's (Carbon fiber composites) which, when mixed and oriented in specific fashion, have essentially "0" CTE values over a very broad range of temperatures. In Aerospace, these Carbon fiber composites are generally regarded as "the materials with the lowest CTE" ---- as far as structural materials go, that is..... There are other materials with even better (lower) CTE's, but they are so structurally weak, that they have little or no application for anything you would come across in the every day world.
