As in the title, I need some indication about the optimal tightening torque to choose when screwing in a SS screw (A2-70, say) into an aluminium threaded hole. In particular, I am concerned with metrical dimensions (M8, M10 and M12).
The most likely point of failure is tearing out the female thread so the strength of the aluminium will be the limiting factor. For more details see this page
The actual torque which equates to a certain pull-out force will depend on both the tensile strength of the aluminium (which will depend on the alloy and temper condition) and the coefficient of friction between the two, which itself will depend on the quality of the threads.
It is also worth noting that torque, on it's own, isn't a great way to prevent self unscrewing and if this is an issue some sort of self locking fastener or locking adhesive are usually required.
Also it is often better when using threaded fasteners into softer materials or into a cast or machined part to use studs secured with locking adhesive and apply the torque with a nut as this significantly reduces the risk of damaging the thread.
A thread locking compound will also help to mitigate galvanic corrosion between dissimilar metals.
You need to determine the minimum thread engagement for full thread strength requirements of the weaker material in the assembly.
Equations and calculators for thread pullout pet industriy standards, See: http://www.engineersedge.com/fastener_thread_menu.shtml
You should also evaluate the environment and corrosion risks of SS and Aluminum in your application.