Apparatus and methods for injecting molten filler material into a hole. The method in accordance with one embodiment comprises: drilling a hole in a composite layer; heating filler material comprising an electrically conductive low-melting alloy to a molten state; inserting a nozzle having an internal channel system into the hole with a gap separating the nozzle and the hole; forcing molten filler material into, through and out of the internal channel system of the nozzle and into the gap; and retracting the nozzle from the hole. The nozzle may be rotary or not rotary.