There's a bunch of open source fem solvers e.g. Calculix, Code_Aster, OpenRadioss and probably a few unmaintained forks of (NASA) NASTRAN, but there's no multiphysics package I don't think.
These are at least capable of thermomechanical with fluid-structure coupling. Not all-physics but still multi. True that things with multi species diffusion or electromagnetics are missing, but maybe Elmer can fill the gap.