A method for direct chill casting of a metal or metal alloy, comprising: supplying a fluid melt comprising a molten metal or molten metal alloy to a direct chill mold having an inlet and an outlet; cooling the fluid melt in the mold to obtain a billet having a molten core forming an inwardly tapering sump and a solid outer shell that thickens at increasing distance from the mold outlet; applying vibrational energy to the fluid melt in the molten core sump of a billet exiting the mold with a device positioned within the mold; injecting a flow of a purge gas into the fluid melt in the molten core sump of the billet by a purge gas feed unit extending into the mold; applying vibrational energy to the solid outer shell of the billet beyond the outlet of the mold in a region of the tapering sump; removing the billet from the mold outlet; and further cooling the billet beyond the mold outlet to obtain a solid billet, wherein the device applying vibrational energy and being positioned in the mold and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to a ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold, and wherein applying vibrational energy to the solid outer shell of the billet in the region of the tapering sump includes applying the vibrational energy from a plurality of vibrational energy sources located in a plurality of positions around the circumference of the billet. supplying a fluid melt comprising a molten metal or molten metal alloy to a direct chill mold having an inlet and an outlet; cooling the fluid melt in the mold to obtain a billet having a molten core forming an inwardly tapering sump and a solid outer shell that thickens at increasing distance from the mold outlet; applying vibrational energy to the fluid melt in the molten core sump of a billet exiting the mold with a device positioned within the mold; injecting a flow of a purge gas into the fluid melt in the molten core sump of the billet by a purge gas feed unit extending into the mold; applying vibrational energy to the solid outer shell of the billet beyond the outlet of the mold in a region of the tapering sump; removing the billet from the mold outlet; and further cooling the billet beyond the mold outlet to obtain a solid billet, wherein the device applying vibrational energy and being positioned in the mold and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to a ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold, and wherein applying vibrational energy to the solid outer shell of the billet in the region of the tapering sump includes applying the vibrational energy from a plurality of vibrational energy sources located in a plurality of positions around the circumference of the billet.