I daven Nusach Sefard but I belong to a a shul that davens Nusach Ashkenaz. When the chazzan reaches kedushah, should I daven along with the tzibbur, or can I say kedushah according to Nusach Sefard quietly to myself. Thank you for your help.

According to Rav Moshe Feinstein zt"l (אגרות משה או"ח ב סי' כג) since Kedusha can only be said with a minyan, it follows that the individual while saying Kedusha must follow the Nusach of the Chazzan. However, according to Rav Avraham Shapira zt"l, the former Chief Rabbi Of Israel, (שו"ת אל-דוד עמ' 44 ) the individual may say silently kedusha according to his own Nusach. In this way he is not deviating from the rest of the Tzibur. [See Also,(משיב דבר סי' יז :ז, יביע אומר חלק ו סי' י' , סעיף ו)] However, should you want to sing along out loud with the Chazzan you should do that in the nusach of the Chazan. May we see the building of the Mikdash in our days and the renewal of the Seder Avoda.