Fusion welding symbol
There is no standard symbol for fusion welding. Fusion welding includes approximately 25 different welding processes in all.
Your definition of fusion welding is not entirely correct. The AWS defines fusion as "the melting together of filler metal and base metal, or of base metal only to produce a weld".
If you are looking for a welding symbol for your application, you need the correct weld symbol which will be based on joint geometry. If you want to specify an autogenous weld (no filler material) you can state that in you drawing notes and reference it. Or, you could place your requirement in the Supplementary Data area of your weld symbol, see AWS publication A2.4 Standard Symbols for Welding, Brazing and Nondestructive Examination.