Ultrasonic welding device; comprised of a sonotrode attached by a back side to a booster and in turn to a converter suitable for connection to an actuator, handler or robotic arm responsible for its movement, and to which a head is attached; wherein the device is comprised of: an outer sleeve arranged around the sonotrode and which delimits, along with that sonotrode, an annular chamber for the circulation of pressurized air connected through a chamber to a pressurized air inlet port defined in the head; which outer sleeve has a crenellated mouth at a front end, for the outlet of pressurized air at the periphery of a tip of the sonotrode and the peripheral refrigeration of the sonotrode; the outer sleeve has a rear portion assembled in the head with axial displacement, between: - a forward position in which the front end of the outer sleeve stands out frontally with regard to the tip of the sonotrode; forming a pusher that exerts, through the action of some compressible means, a configurable pressure against the parts (P1, P2) to be welded, in a peripheral zone close to the weld, while approaching the sonotrode to the parts to be welded and before the tip of the sonotrode makes contact with the aforementioned parts to be welded, and - a backward position, determined by the advance of the sonotrode during welding while the front end of the outer sleeve keeps the parts (P1, P2) to be welded pressed together, and in which backward position the front end of the outer sleeve is arranged in a coplanar manner, or in a very close plane, to the tip of the sonotrode. an outer sleeve arranged around the sonotrode and which delimits, along with that sonotrode, an annular chamber for the circulation of pressurized air connected through a chamber to a pressurized air inlet port defined in the head; which outer sleeve has a crenellated mouth at a front end, for the outlet of pressurized air at the periphery of a tip of the sonotrode and the peripheral refrigeration of the sonotrode; the outer sleeve has a rear portion assembled in the head with axial displacement, between: - a forward position in which the front end of the outer sleeve stands out frontally with regard to the tip of the sonotrode; forming a pusher that exerts, through the action of some compressible means, a configurable pressure against the parts (P1, P2) to be welded, in a peripheral zone close to the weld, while approaching the sonotrode to the parts to be welded and before the tip of the sonotrode makes contact with the aforementioned parts to be welded, and - a backward position, determined by the advance of the sonotrode during welding while the front end of the outer sleeve keeps the parts (P1, P2) to be welded pressed together, and in which backward position the front end of the outer sleeve is arranged in a coplanar manner, or in a very close plane, to the tip of the sonotrode. - a forward position in which the front end of the outer sleeve stands out frontally with regard to the tip of the sonotrode; forming a pusher that exerts, through the action of some compressible means, a configurable pressure against the parts (P1, P2) to be welded, in a peripheral zone close to the weld, while approaching the sonotrode to the parts to be welded and before the tip of the sonotrode makes contact with the aforementioned parts to be welded, and - a backward position, determined by the advance of the sonotrode during welding while the front end of the outer sleeve keeps the parts (P1, P2) to be welded pressed together, and in which backward position the front end of the outer sleeve is arranged in a coplanar manner, or in a very close plane, to the tip of the sonotrode.