An ultrasound sonotrode (101), the first end of which is adapted to be connected to a mechanical vibrations source, equipped with a working tip (105,205,405,805) at the opposite end of the sonotrode (101), equipped with a body (104) with a cooling jacket (103), sealed at the place of contact with the body (104) of the sonotrode (101) with the use of the first seal (106) and the second seal (107), characterized in that according to the invention the first seal (106) is placed at a distance less than or equal to 20 mm from the node of the standing wave excited in the sonotrode in the working conditions, and the second seal (107,207,407,507,607) is equipped with a resilient element (108,208,408,508,608) and is located at a distance less than or equal to 20 mm from the working tip (105,205,405,805). A method for metal alloying, in which the material is melted on the working tip (105, 205, 405, 805) of the sonotrode excited to mechanical vibrations, according to the invention characterized in that a sonotrode according to the invention is used.