I conducted this test in an effort to see actual results of what happens to unprotected Aluminum when subjected to Ultrasonic cleaning. The test sample is a piece of T-6 grade Aluminum I got from work, and applied a 500 finish to, then went over it with Happich Simichrome Polish to achieve as smooth of finish as possible. The above photo shows the finished piece before being subjected to any treatments. It's size is approx. 2 1/2" X 1 1/2" X 1/2" thick.
This was after 9-1/2 minutes in warm water with a small amount of Palmolive Dish Soap. There is not much change except for some very light, hard to see speckling.
This was after 21 minutes exposure to the Ultrasonic cleaner. More profound frosting of the metal has occurred, along with some enhanced speckling. The frosting appears to have taken on the circular pattern of the plastic tray it was resting on with multiple speckles occurring at 9, 12, and 3 O'clock positions around it.
This was after a full 30 minutes of Ultrasonic exposure. Much more profound frosting has occurred, along with deeper speckles. This would no doubt effect the finish of an Aluminium framed pistol if it had any degree of polish on it. While this test wasn't very scientific, I would conclude that it would be risky to expose any Aluminum firearm parts to Ultrasonic cleaning. I wish the photos were better, but you can see the Ultrasonic treatment did in fact effect the finish, which is what I was trying to determine. Bill T.