Subject: Re: 7075 Aluminum

From: Mike Rehmus <[email-masked]>
Date: 11 May 1996 17:20:13 GMT

The use of 7075 in all cases would seem to be a
no-brainer until you take a look at the strenth
vs temperature curves.  That is why 2024 is to
be preferred for cylinder heads, etc.

Best,

Mike Rehmus

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From: [email-masked] (Edk4)
Date: 12 May 1996 11:31:13 -0400

( I asked about an angle section of 2024 vs 6061 vs 7075 Aluminum)
    
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>> Does yield strength correlate with/to stiffness?

>>For instance, for a given piece of Aluminum angle of the same size, 
>>thickness and profile, which of the 3 alloys, 2024, 6061 or 7075, would 
>>have the greater stiffness, in the sense of rigidity, at room
temperature?

>be preferred for cylinder heads, etc.

 I'm not sure I understand your reply.
  
 Mike, is your answer that there *is* a direct correlation between
 stiffness and yield strength, and that yes, 7075 *is* more rigid than
 2024 or 6061, at normal temperatures?

 ------------------
  Ed Kingsley

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From: Mike Rehmus <[email-masked]>
Date: 14 May 1996 12:17:24 GMT

8I had just made the comment that 7075 suffers significant strength
reduction at eleveated temperatures.

Without taking the time to look up the tables in the metals book, I'd
say "no", to your question.

'Stiffness' is not directly related to, say, yield strength.  (I assume
you are referring to the amount of deflection of a beam held at one end
and from which a load is suspended on the other.)

In general (whoo boy, a generalization from a non-expert) I've found
that within the same metal type (aluminum or steel) the 'stiffness'
is for all practical purposes, the same.  That means that a boring
bar of mild steel will be about as stiff as one of tool steel.

Best regards,

Mike Rehmus

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From: [email-masked] (Stephen Gaudio)
Date: 16 May 1996 08:40:45 GMT

In article <[email-masked]>,
>bar of mild steel will be about as stiff as one of tool steel.

ah, the homemade boring bar. a workmate of mine made a boring bar of
some nice stainless, and he still got hella chatter. i had to laugh.
those cast iron boring bars possess enough (still very small) damping,
enough to fight the onset of regenerative chatter. (the damping comes
>from  the carbon (graphite) grains in the steel...they rub against each
other when deformed, thus dissipating energy instead of elastically
storing it)

if you need damping, just stick a lead weight on it with double-sided
foam tape. or whatever (you'll need a mass AND a damper!)

anyway, to stick to the thread, 7075 has way shitty fracture toughness.
this means that if you have a cracked piece of 7075, and you give it
some elastic energy (by straining it) it will think to itself: "hmm..i
think i'll release some of that energy by propagating this crack
(forming new surfaces + plastic deformation) instead of being in this
state of tension". something in the 2000 series would rather NOT form
the crack (not absolutely, of course, but this is a rough picture). this
is why we make pressure vessels out of 2000 Al...it would rather leak
than burst like a bubble.

also, this means 7075 has terrible fatigue resistance. fatigue is
nothing mysterious: it's just the microscopic propagation of a crack for
every cycle of strain.

as long as i'm here, let's talk about heat treatment: as you recall from
the FAQ, steel resists plastic deformation by holding back dislocation
propagation. grain boundaries do this (cold worked steel has higher
yield) as well as crystal structure (the hardened BCC resists plastic
deformation as opposed to the annealed FCC structure which is friendly
to dislocation propagation.)

(dislocation propagation is like a void in the otherwise perfect crystal
structure which deforms it globally the same way traffic slowly creeps
along by having one car at a time fill the gap)

now aluminum is a totally different animal. the alloying elements in
aluminum are in solution while in the melt, but would rather NOT be at
lower temperatures (like when it's solid!). this is like cold water
holding less sugar. very simple.

now when freshly poured, you can imagine aluminum thinking "wow, i
really want these dissolved elements to come out of solution"....like
crystals forming in syrup, except these are solid crystals in a solid
solution. but, as you know, life in a solid is kind of slow: crystals
don't just pop out of solution overnight (like them sugar crystals!).
so to speed the process, we heat up the aluminum and 'artificially age'
it. this jostles the atoms around enough so the solute atoms can come
together to form crystals. it's quite simple. nothing fancy: just one
solid diffusing through another. (actually, let me say that these aren't
pure crystals per se, but separate *phases*...certain preferred ratios
of mixtures....but you get the idea....)

now here's the fun part: the new phase has a slightly different 
crystal structure than the surrounding aluminum phase. for smaller
sized regions, the surrounding matrix deforms quite readily to
accommodate the ill-fit second phase. medium sized pockets induce a higher
stress, and tend to trap nearby dislocations, preventing their further
propagation and thus overall plastic deformation. finally, really large
phases tend to break away from the surrounding lattice. (this particular
phenomena is called incoherence, not unlike this article).

as you can expect, these incoherent phases are lovely birth sites for
void and cracks, and with every strain, there's a little more crack
growth. this is why aluminum lacks steel's famous 'endurance limit'.

anyway, (i hope i haven't lost you yet because here's where it gets
good), anyway, this heat treatment goes beyond the heating and quenching
of steel. you can't just heat aluminum and say "ok, that's
annealed....let's get coffee". what you've done is transformed the metal into
an incoherent mess (again, not unlike this article). sure, it's going to
have a lower yield, but not by the mechanism you are counting on! ("huh,
can't seem to get the hardness back!" <ahem> )

so there you have it. the story of aluminum.

one more thing: aluminum alloys don't work harden to any practical
degree. people shot peen machined goods and stuff to prevent fatigue
failure. what they're doing is impressing a compressive state of stress
at the surface (remember tempered glass?) which tends to *close* those
teeny-tiny cracks before they have a chance to get really brutal (or at
least, postpone them) 

steve.

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From: Bob Bickers <[email-masked]>
Date: 19 May 1996 23:52:00 GMT

So whats the "good" alloy to use in say motor mounts, 7075 looks good for strenth but is it in 
the long haul? Bob

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From: Mike Rehmus <[email-masked]>
Date: 22 May 1996 03:30:54 GMT

Motor mounts for what application?

7075 suffers from fatigue-related failure (however, I have
no idea how much fatigue is too much for it).

In a model airplane I wouldn't worry too much.  In a full-
size vehicle, I'd be concerned.

Best regards,

Mike Rehmus