come far combaciare due mesh stl?

[pigalle86]

hello to all, I press that I am a real sw neophyte and that I need to process mesh to carry out studies at the fem. By digitizing a 3d object, I segmented two different materials by exporting stl surfaces that I need to recompose into a single solid whose surfaces fit perfectly to avoid errors in fem analysis. which function can I use in sw to achieve this result?
Thank you very much