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AL vs Steel

 
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the Dude



Joined: 05 Dec 2003
Posts: 3608
Location: Saskatoon

PostPosted: Tue Jan 29, 2013 3:01 pm    Post subject: AL vs Steel Reply with quote

I stole this from Peter Straub. Any other thoughts on it?

6061-T6 has a yield strength of 276 MPa, and an ultimate strength of 310MPa and has a density of 2.7 g/cc

1020 Steel (Cold Rolled) is 350/420 MPa, and a density of 7.87 g/cc

(There are stronger steels, but this is the type you are probably most likely to pull of the shelf at Metal Supermarket to fabricate a bumper or such - there are stronger Aluminums too... 7075-T6 aluminum is 435/670 MPa, compare that to 503/572 MPa for 4130 Chromo!)

Steel is 26% stronger yield strength, 35% stronger ultimate. Yield is what matters, as that's the point that it permanently deforms. So, you want to up-size your aluminum over steel by 26% to match "strength per given wall thickness"

0.120 wall steel = 0.15 wall Aluminum
1/4" steel = 5/16" aluminum
3/16" steel = 1/4" Aluminum

Steel is 191% more dense than Aluminum. When you add on the 26% more material to match steel's strength, the steel is still 130% heavier than the up-sized aluminum (that's 2.3 times heavier for the same strength)

Bottom line, Aluminum is much stronger than steel for the same weight, or same strength for less than 1/2 the weight. That's why there are no steel airplane in the sky!

The PROBLEM, however, is that much of the strength of 6061 aluminum comes from the heat treating. When you weld it, it looses a LOT of strength... it goes from all 276 MPA Yeild all the way down to 55MPa yield and down to 124MPa ultimate strength in the heat affected zones of the weld. You MUST re-heat treat 6061 AFTER fabrication to attain the strength.
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Douglas S.



Joined: 06 Aug 2011
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Location: Regina

PostPosted: Tue Jan 29, 2013 3:48 pm    Post subject: Reply with quote

I haven't looked into it too closely, but there are a number of companies that have been making aluminum skid plates and armour for Jeeps for years now. It's reported to slide over rocks more easily than steel and I haven't heard of many failures.
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the Dude



Joined: 05 Dec 2003
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Location: Saskatoon

PostPosted: Tue Jan 29, 2013 3:56 pm    Post subject: Reply with quote

I have been reading pirate on the subject as well and lots of good (and bad) to say about it.

There are lots of 7075 links out there but they are $$$$$$

Anybody know of a heat treater in Saskatoon? Not so concerned on my front bumper as it will have a steel sub frame, but the rear with the tire carrier could be an issue.
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TROY



Joined: 21 Oct 2006
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PostPosted: Tue Jan 29, 2013 5:14 pm    Post subject: Reply with quote

aluminum is stronger for the weight of it.... but the lose of strength from welding, special tools need to work with it and it does not like to bend or be bent back into place, just breaks.
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CW Fab



Joined: 08 Nov 2006
Posts: 266
Location: Saskatoon

PostPosted: Wed Jan 30, 2013 8:15 pm    Post subject: Reply with quote

TROY wrote:
aluminum is stronger for the weight of it.... but the lose of strength from welding, special tools need to work with it and it does not like to bend or be bent back into place, just breaks.


The same could be said about many different types of steel.

Design is the key with ANY material, you could use Titanium for your bumper and have it fail if it was poorly designed. The problem with people is they are never the ones at fault when things break, it's always the material or (insert excuses here).

Do you have any sketches of what you want to build for a bumper?
As for local heat treating I know Hitachi had their own oven, I'm sure most reputable machine shops will have one or have good connections with one.
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Conrad_Turbo



Joined: 20 Jun 2004
Posts: 309
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PostPosted: Wed Mar 13, 2013 11:03 am    Post subject: Reply with quote

Just saw this thread now.

Each design dictates it's type of material. If you're worried about deflection then you have to factor in other things, more than just tensile and yield strength, you have to figure in modulus of elasticity of the material as well. Controlling deflection is important when dealing with skid plates, the advantage to steel is that it has a modulus of elasticity typically 3x higher than aluminum. So you'd need a thicker aluminum skidplate to reduce the amount of deflection to that of a steel skidplate, which would negate your weight savings by going with aluminum. Also the aluminum skidplate would hang lower due to the increased thickness (not by much...but it would be lower than a steel skid plate to offer the same protection). However there are a lot of pro's to using aluminum as a skidplate too, heck I've even cut out a few 1/4" 6061-T6 skidplates...

Material fatigue is based on the design. Fatigue testing aluminum, even with a very minor load will cause the part to crack within it's life. Whether it be a day or 100 years...it will eventually crack. However a steel part correctly designed will never fatigue or crack, however this may be overbuilt for some situations.

Aluminum does love to be bent, 5052-H32 is an example, if it's heat treated 6061-T6 then not so much... It's just like trying to take a knife blade and fold it into a 90 degree bend...it's not going to happen, because the knife blade is heat treated, as is the 6061-T6 aluminum.

Welding heat treated material, whether it be steel or aluminum, is going to result in that welded joint being weaker than the heat treated base material. Welding is just a focused way of annealing a heat treated material, which isn't necessarily desirable but that's just what happens.
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