A device (1, 1') for welding metal parts by means of ultrasound, in particular for welding strands, a.) comprising a sonotrode (3, 3') with a sonotrode head (3.1, 3.1'), which can be excited by a vibration generator (9, 9a, 9b) to perform torsional vibrations in relation to a torsion axis (B, B'), wherein b.) at least one welding surface (3.4, 3.4') is arranged circumferentially on the sonotrode head (3.1, 3.1'), as seen in relation to the torsion axis, and c.) an anvil (18, 18') with a compacting surface (18.1, 18.1') is present, it being possible for this to be arranged opposite the welding surface of the sonotrode (3, 3'), in a fixed state in relation to the same, wherein d.) in the case of an opposite arrangement, the welding surface (3.4, 3.4') and the compacting surface (18.1, 18.1') delimit a compacting space (8, 8'); provided for accommodating parts which are to be welded, in a direction perpendicular to the torsion axis (B, B'), characterized in that e.) the sonotrode (3, 3') is designed, and coupled to the vibration generator (9, 9a, 9b), such that the sonotrode (3, 3') as a whole can be excited to perform torsional vibration with a negligibly small longitudinal-vibration fraction by (i) the vibration generator (9, 9a, 9b) being designed for generating longitudinal vibrations and arranged perpendicularly to the torsion axis (B, B') of the sonotrode (3, 3'); (ii) the vibration generator (9, 9'), for exciting the torsional vibrations, interacting with the sonotrode (3, 3') via a torsional vibrator (4, 4'), which is coupled to the sonotrode (3, 3') in respect of torsional vibration; and (iii) the vibration generator comprising an activator (9.1, 9a.1, 9b.1) which is in contact with the torsional vibrator (4, 4'), at a radial distance from the torsion axis (B, B')_. a.) comprising a sonotrode (3, 3') with a sonotrode head (3.1, 3.1'), which can be excited by a vibration generator (9, 9a, 9b) to perform torsional vibrations in relation to a torsion axis (B, B'), wherein b.) at least one welding surface (3.4, 3.4') is arranged circumferentially on the sonotrode head (3.1, 3.1'), as seen in relation to the torsion axis, and c.) an anvil (18, 18') with a compacting surface (18.1, 18.1') is present, it being possible for this to be arranged opposite the welding surface of the sonotrode (3, 3'), in a fixed state in relation to the same, wherein d.) in the case of an opposite arrangement, the welding surface (3.4, 3.4') and the compacting surface (18.1, 18.1') delimit a compacting space (8, 8'); provided for accommodating parts which are to be welded, in a direction perpendicular to the torsion axis (B, B'), e.) the sonotrode (3, 3') is designed, and coupled to the vibration generator (9, 9a, 9b), such that the sonotrode (3, 3') as a whole can be excited to perform torsional vibration with a negligibly small longitudinal-vibration fraction by (i) the vibration generator (9, 9a, 9b) being designed for generating longitudinal vibrations and arranged perpendicularly to the torsion axis (B, B') of the sonotrode (3, 3'); (ii) the vibration generator (9, 9'), for exciting the torsional vibrations, interacting with the sonotrode (3, 3') via a torsional vibrator (4, 4'), which is coupled to the sonotrode (3, 3') in respect of torsional vibration; and (iii) the vibration generator comprising an activator (9.1, 9a.1, 9b.1) which is in contact with the torsional vibrator (4, 4'), at a radial distance from the torsion axis (B, B')_. (i) the vibration generator (9, 9a, 9b) being designed for generating longitudinal vibrations and arranged perpendicularly to the torsion axis (B, B') of the sonotrode (3, 3'); (ii) the vibration generator (9, 9'), for exciting the torsional vibrations, interacting with the sonotrode (3, 3') via a torsional vibrator (4, 4'), which is coupled to the sonotrode (3, 3') in respect of torsional vibration; and (iii) the vibration generator comprising an activator (9.1, 9a.1, 9b.1) which is in contact with the torsional vibrator (4, 4'), at a radial distance from the torsion axis (B, B')_.