Hi wynn-Would it be practical to use "helicoil" thread inserts and threadlock((LOCTITE) to overcome the vibration problem?
Best of Luck-TWWeld

Wynn...1/2" aluminum is usually OK for threading application such as yours.  You should probably consider using fine threads instead of course for your vibration issue, though overall load capacity of the fine threads will likely be less in this alloy.

You could also consider NutSerts (clinch nuts) which work similar to Pop rivets, leaving a nut into which you can screw a fastener.  These work very well in aluminum.

As for materials, definitely use stainless steel fasteners, preferably 316.

Agree with TWWELD about LocTite.  Works well for vibration.

Wynn - I agree with TWWELD(Structural)and Ron(Structural)on the Loc-tite and stainless steel but would take it one step further.  My experience in aluminum engine blocks tells me that you would be better off going to a thread former such as Tap-tite.  Due to their shank design they are very resisitant to vibration/s, many times 3 - 10 times better.

A 3/8-16 stainless taptite will form threads in 6061 aluminum very easily.  It has a major diameter of 0.3810" and should be have an as cast hole of 0.3466" - 0.3425" at the midpoint of the length of engagement.  The ideal would be an as cast hole with a 1 degree draft, or a tapered drilled hole (best guess right now would be 0.359" - 0.356" at the top and 0.343" - 0.340" at the bottom; check the math.  If the hole is drilled be careful of the drill speed as it can work harden the interior surface of the hole.

Under these conditions you might see a thread forming torque of about 90 Lb-in - 110 Lb-in and an assembly torque of about 350 Lb-in - 450 Lb-in.

Remember one test is worth 10,000 opinions.

Bill P.
CSFT, Inc.