It is a hot topic among coin collectors right now. The US Mint sued the owner of the 1974-D aluminum cent, Randy Lawrence, to return the coin because:
The Mint claimed that authority was never granted for production of the experimental test pieces and that the coin in fact belonged to the US Government.
While this may seem like a governmental overreach and possibly an abuse of power, it is actually good news. For example, I have not been granted permission to make Jack Daniels. So, I am heading to Lynchburg, TN to claim my whiskey.
Here is the Coin Week article (quoted above) for more information.
This has me concerned for another reason. This photo is of two coins I found while going through some old wheat pennies.
The coin on the left is a 1964-D cent struck on an incorrect metal, possibly a silver dime planchet. Though, it doesn't appear to be to a dime planchet as the full design is present. (A dime planchet would be too small to accommodate the full Lincoln cent design.) What if this is also aluminum?
I suppose I will never know, as the US Mint only sought the return of the 1974-D aluminum cent after it was certified. I would rather keep my penny and wonder at its composition. (It could also be a regular copper cent that was coated with zinc. In my high school chemistry class we did an experiment where we took pennies coated in zinc sulfate and blasted them with propane torches, it turned them a golden color.)
Here is a video of the process of turning a penny "silver" and then "gold".