Tin Falcon is absolutely right, aluminum can serve perfectly effectively as flywheel material, in many cases, especially in low-torque model, aeronautical, or any application where the drive is thru some fluid or magnetic coupling or any other 'soft' power transfer method.
There is a lot of uninformed opinion, here, or just plain ignorance of metallurgical properties, and the actual torque requirements involved. You are all model engine makers, for Pete's sake! You never actually put your engines under any real significant loads! Of course, one could make an almost entirely aluminum model engine, just use teflon bushings on the shafts. Corrosion can and will happen to some extent between any two dissimilar metals, even worse if there is any source of electron flow, nearby. Even highly polished ferrous metals cannot run on aluminum bearing surfaces without constant forced lubrication, hence the very cheap and successful camshaft running directly on an aluminum head of most modern OHV IC engines. The worst problem with the use of dissimilar metallic bushings is the lower coefficient of linear expansion of nearly all other metals compared to any aluminum block. Thus, the hotter the engine gets the more the bushings loosen up in the their bores, until they can spin, negate any value to having the bushing there. So, if you use ferrous shafts, either, provide constant pressure oiling on aluminum bores, or, again, teflon bushings (other high-lubricity polymers can be used, but teflon is the best choice for steam, up to 500 degrees Fahrenheit). If you insist on using dissimilar metal bushings in aluminum, you must double the interference of the fit, over that which you would use for a ferrous metal bore. Then, you will ruin the aluminum bore, when pressing it in, unless you heat the aluminum block and cool the bushing, before you put them together with silicone or teflon lubricant on the bushing to keep the heat transfer slow enough to permit you time to press it to the bottom of the hole. We have only had success installing such bushings, dry, in unheated aluminum, by using liquid nitrogen to cool the bushing. Then, you can just drop it in the hole, stand back, and listen to it scream as it grows to tighten, permanently, into the bore! Teflon bushings really are the best solution, as any engine, for practical use in any modern auto or loco-motive application, usually needs a sealed crankcase, anyway. Teflon serves both purposes. Yes, we can make practical use of steam engines, today, with the modern metallurgical technologies that they lacked in the "Golden Age of Steam". They are not just toys, or obsolete technology of yesteryear, but the ultimate flex-fuel automotive engine of the future, and they will be made almost entirely out of aluminum, except for shafts, pins and fasteners.
