Hello Ju Cheng ,

I am assuming that the destruction of the threads in the aluminum, AL7075, mass by the steel bolt occurs as a result of the ultrasonic tranducer being turned on. My guess is that the vibration from the excited transducer is causing the steel bolt to come loose, and scour the aluminum thread. Have you thought of using Locktite threadlocker on the bolt threads? Another possible solution is to install a steel insert, such as a Heli-Coil or other inserts, in the aluminum mass. In other words, pick a Heli-Coil whose internal thread size matches the thread size of your steel bolt, then drill and tap the aluminum mass to match the external thread size of the Heli-Coil. Install the Heli-Coil, break off the installation tang with a pin punch or cold chisel, then screw your steel bolt into the Heli-Coil.

By the way, is the threaded hole in the aluminum mass a through hole or a blind hole? The Heli-Coil would probably work best with a blind hole.

Here is the link for the various inserts:

https://www.google.com/search?q=helicoils+for+aluminum&rlz=1C1CHZL_enUS753US753&oq=Helicoil&gs_lcrp=EgZjaHJvbWUqCQgDEAAYQxiKBTIMCAAQABhDGLEDGIoFMgkIARAAGEMYigUyCQgCEAAYQxiKBTIJCAMQABhDGIoFMgwIBBAAGEMYsQMYigUyDAgFEAAYQxixAxiKBTIJCAYQABhDGIoFMgcIBxAAGIAEMgcICBAAGIAEMgcICRAAGIAE0gEJOTg3M2owajE1qAIAsAIA&sourceid=chrome&ie=UTF-8

Regards,

Thomas Cuff