1. Brittle metals: such as cast iron and some alloy steels, which have poor ductility and are difficult to form effective connections through vibration.
2. High carbon steel or hardened metal: The hardness is too high, and ultrasonic energy is difficult to deform it plastically.
3. Metals with extremely high melting points: such as tungsten and molybdenum, which require extremely high temperatures to melt, and ultrasonic welding may not provide enough energy.
4. Metals with severe surface oxidation: The oxide layer will hinder energy transfer and cause welding failure.

5. Combinations of dissimilar metals: Metals with large differences in physical properties (such as copper and aluminum) may be difficult to weld due to mismatched vibration frequencies.