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Brainstorming ideas aluminum auto chassis construction 4

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formula94lt1

Automotive
May 6, 2007
17
US
Hi all this is my first post. I was thinking about how aluminum is conventionally used to make automotive chassis for certain kit cars by welding aluminum panels together. I was only interested in this way of construction since it seems more feasible to create such a thing in my garage than hydroforming. As far as I know 6061 t6 is the most commonly used which is a medium strength alloy of aluminum that is weldable and corrosion resistant but of course suffers from a pretty high(40+%) loss in strength around the welds. This takes it from being a medium strength alloy to something quite a bit less. So I set out to find some other way of getting the strength to weight benefits of aluminum alloy without any sort of welding. This also opened up the options of aluminum that could be used to non-weldable alloys such as 2024 and 7075 which are stronger and significantly stronger respectively. The later is actually another reason why wanted to persue another form of construction other than welding. Using these stronger alloys naturally allows for less material to be used for a lighter weight design and/or for a stronger design. I looked shortly into riveting which sounded like more of a pain than welding just to get to the point of being able to drive them in. Then I thought of bonding with adhesive like with composites. So I looked into epoxys and such and finally found something specifically for bonding aluminum by loctite called "H8000." I read a review where they were replacing welds with this stuff so I had to give it serious consideration. So after some simple math I found it not unreasonable to overlap the panels a certain amount as a function of thier thickness and tensile strength and bond them. Then I realised that I would need some way to overlap the panels at different angles depending on how that panel fit in relation to the ones around it. After tinkering with a few ways to do this I considered one way best. That's to take two panels and join them at their edge at an angle then take two more slightly thinner panels (bending them to the same angle) that overlap the two forementioned in the front and the back just enough to provide equal shear strength to the larger panels when bonded. Then I ran into the problem of the alloy I wanted to use(7075 t6), and most other aluminum alloys of appreciable strength, not having a high enough percentage of elongation to make a sufficiently short radius bend to contour to the surfaces of the larger panels. Now Ive thought of using composite panels to sandwhich the aluminum ones together but the rate of thermal expansion is far too different between the two not to cause serious problems and thats if the adhesive would bond to the composite as well as the aluminum if at all. SO FINALLY for my questions... Can 7075 t6 be heated without changing its strength to make a very small radius bend? Any guide to a temp to radius bend for various thicknesses? Obviously the point is to not have any flex in the joining of the aluminum panels and any radius at all in the front and back sandwhiching panels hinders that as apposed to welding, but Im not sure exactly how much and that if there was some radius to the joining 7075 t6 panels would they be less rigid than welding a weak alloy? Any alternative solutions are welcome of course.... sounds like this one has a few flaws.
 
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I also thought about having a 2024 or 7075 solid bar join the two panels with slits machined into them lengthwise at the appropriate angles, but cutting all the bar stock correctly to match at junctions of multiple panels would be hard. Not to mention heavier and much more expensive...
 
formula94lt1,

Aluminium 6061 and 7075 can be bent in the annealed state, then heat treated.

I tried to move a tent once that had poles made of 7001-T6 (7005-T6?). The first indication I had that I was doing anything wrong was when the material snapped. I played with the broken pieces afterwards, and I found it impossible to bend or twist. Any force from my pliers and the stuff shattered into pieces. I suspect 7075-T6 would be similar. Perhaps someone out there has tried this?

7075-T6 is used extensively on aircraft, which do not experience a lot of fender benders at stoplights and in parking lots.

Why not weld an aluminium that has good annealed strength, like 5086? 5086 is much more corrosion resistant than 7075, and you are probably not losing much in the way of impact resistance. The body shop guys will be able to hammer out dents.

JHG
 
metengr top link well done have a star.

5086, 0r 5083 are great choices, weldable aluminium used extensively in the marine industry.

But I would ask an important question, are you really concerned about strength? If so why? Or is really stiffness that you are interested in?

 
I am after less deflection per a given load. However, I thought stiffness and strength went somewhat hand in hand. Generally it seems the stronger the materials the more rigid the structure. Am I wrong in that assumption? Usually, I thought, this is why most preffer chromoly space frames as opposed to mild steel when it can be afforded. Thanks for the link by the way...
 
Stiffness isn't necessarily an intrinsic material characteristic. It is also a function of the geometry. Just because something is stiffer doesn't mean that it's stronger.

So if it's stiffness you're after, you have a lot more to consider than just the strength of the material.

V
 
formula94lt1,

All aluminium grades in all states of heat treatment and work hardening have an elastic modulus of around 10Mpsi, or 70GPa. All carbon steel grades are around 29Mpsi, or 200GPa. Heat treatment and work hardening increase the strength, not the stiffness of the material.

I have seen electronics boxes fabricated from 2024-T4 aluminium by aircraft people. The quality of the punching and drilling was awful, although they were careful to place relieving radii in the corners. It was obviously done in a hangar somewhere. They bent with very large bend radii to avoid cracking the material. This appears to be a rare skill. Sheet metal shops do not want to use this material. They like 5052-H32 a lot. Consider the effect of workmanship. 7075-T6 with cracks in it is not very strong.

7075-T6 usually is machined.

JHG
 
Sorry, I laid a bit of a trap for you..
I think there are some basic things about materials that will really help you. Stiffness is measure of how much deflection you get for a given load. So for example many glasses are much stiffer than steel or aluminium, but would you make your chassis from glass? Strength is a measure of the maximum load the material can take before it fails, in glasses this is less than steel or aluminium.

An important thing to understand is that the grade of any metal, will not change its stiffness (maybe by a tiny amount) so the stiffness of all of your grades of aluminium you mention will be exactly the same. The strength does vary from grade to grade, so some will take more load than others before they fail.

Another really interesting point is that the stiffness of steel is about 3 x the stiffness of aluminium, and the density of steel is about 3x the density of aluminium. So their specific stiffnesses are boradly the same.
Engineers use some tricks with cross sectional area to up the specific stiffness of any member, hence the prevalence of oversized tubes on bicycles.

So for vehicles you could compare the use of aluimium for car chassis and bodies woth the HSLA steel auto bodies that are also in use.

I hope this makes sense. I think really you should get access to some undergraduate level materials science/engineering texts of which there are many.



 
Blimey drawoh you type fast! Whilst I was writing my missive you popped yours in there, so I apologise for repeating everything you just said!

I would add one thing, the term "elastic modulus" that drawoh mentioned, this is the term used to measure stiffness. Also known as youngs modulus, or just E.

 
For a lightweight construction look at the old tube frame cars of the mid 60s, some of them are quite light, using steel tube.
After that we get into riveted aluminum sheet construction. Check out a mid 70s Chaparal or Lotus to see what you can do with structural rivets and aluminum sheet.
Trying to build a bonded metal structure at home is a very tricky process; surface preparation and the bonding process are extremely critical to get a good structure.
One advantage of a riveted construction is that if you don't like how someting turned out you can drill them out and try something different. With adhesives you are pretty much stuck with what you have.
 
"With adhesives you are pretty much stuck with what you have. "


Nice!
 
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