I'm certainly not an expert, but I would think that ultrasonic welding of plastic to metal would not be possible. When you weld metal to metal the ultrasonic energy is used to break up the oxide layer on the metal so that the two metals can form a metallurgical bond. If plastic was used, the force required to break up the oxide layer on the metal would destroy the plastic. The only ultrasonics I have seen much of was putting metal inserts into plastic holes. Once the ultrasonic energy was turned on, the plastic flowed out of the way of the metal like water. Only the knurl on the metal parts held it in to the plastic, not any sort of weld. Also, plastic was not mentioned, that I saw, in the AWS handbook ultrasonic welding section.