Best Al Alloy casting?

I am designing a new component that will be sand or investment cast. The alloy grade needs to be readily machinable but most importantly it needs to be tough. Ideally the component would be machined from bar stock and while that is feasable for a few prototypes it's not approriate for production. What cast grade and heat treatment would be closest to wrought bar stock?
All feedback appreciated.
Flyn

Take a close look at ZA-27. Close to the strength of CI, but can be sand, plaster, investment and die casted depending on your volume requirements and machining costs. Bar stock is available for prototyping but you will lose the "skin" effect you get from castings. Machines well.
see properties http://www.mrt-castings.co.uk/design_za27.html http://www.mrt-castings.co.uk/design_za12.html http://www.eazall.com/materialproperties.aspx
suppliers http://metals.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.eazall.com%2F
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Thanks George. Very interesting as I hadn't previously considered Zinc alloys. They may well have other applications for me but in this particular case I was looking at Aluminium alloys because mass is critical. I didn't realise that zinc alloys are approximately twice as heavy as Al! Didn't realise it was so strong either. We actually have a number of cast steel and stainless parts that I may reconsider in zinc alloys in future.
Any similar recommendations in Aluminium?

It would help a lot if you indicated what bar stock that you are thinking about. There is a big difference between 1100 aluminum and 7136-T76511. In general, wought aooys have much better stength and toughness than cast alloys. State your needs.
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Sure they can.
The Policy writer is not a structural Engineer. Should something happen Im sure they will come out with a structural engineer to check the installation. Its then they will determine if he is covered.

Hi Flyn,
There are other casting alloy can be considered in your case if mass is important, such as titanium, you can contact me at snipped-for-privacy@gmail.com, we can talk about it,

Hi Flyn,
There are other casting alloy can be considered in your case if mass is important, such as titanium, you can contact me at snipped-for-privacy@gmail.com, we can talk about it,