welding aluminum

Hi all, I have a need to weld some frameworks in Aluminum 6061 & 6063 extrusions and plate, up to 1/4" and some extrusion to 1/2" plate. There's
almost enough of it to consider getting the equipment to weld it myself. This is hobby stuff not commercial so part of the deal is to be able to do it myself. Here's the problem. Never done it before. Years ago did a reasonable amount of gas welding in steel, mostly tube work race car and bike chassis, and some general buzz box stick stuff. What's the best method to weld this stuff? Mig? I've been looking at a lincoln 135 or 175 with an AL mod. would an average welder get a good result with either of these machines? Would a mig machine of this scale give a decent structural weld? Is there something else I should look at?
Is anything from ( big intake of breath) HF able to do the job? thanks...
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The most cost effective solution would be a 250 - 300 amp DC Stick welder and a Readywelder spoolgun.
http://www.readywelder.com /
They may look like toys, but believe me they work very well.
You will also need a bottle of Argon or argon-helium mix.
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I like the job my buddy does with the tig welder. Another friend of mine has built a lot of race cars - and is building one now with an aluminum chassis (a "clone" of a Lotus 7 - not an abomination like a Locost, but actually to accurate lotus 7 dimensions, modified to make out of aluminum angle, and running a V twin bike engine.) It is ALL being rivetted, as he says with welded 6061T6, unless you can re-heat-treat it (which is NOT a do-it-yourself job) you have no idea WHAT you have after welding. So, he's building it according to "aircraft standards" - all joints rivetted and gussetted.
So, for a "decent" weld, I like TIG. For one you trust your life to? Rivet.
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Does he have anything online about this? I'm curious about how he modified the 7 chassis so it could be made out of angle.
Ed Huntress
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On Sun, 21 Nov 2004 05:54:15 GMT, "Ed Huntress"

Nothing online, and it has not been proven yet - but this guy has repaired, rebuilt, and reproduced more sevens over the years than likely anyone else in Canada. He's got all the jigs.
The "angle seven" is being built by his son, actually, (with Dad's help) for one of the "formula grassroots" type races - something like Formula 2005? where you get to spend up to $2005 US to build a car and then race it. He expects to also drive it on the street.
The seven is built mostly of square steel tubing (mild steel at that) - much of it 1" square.
The "Angle Seven" is made of 6061T6 or 6061T651 1 1/2 inch (I think) angle and will have the driveshaft tube fully triagulated into the front bulkhead.
He estimates the material cost for the chassis, with some carefull scrounging, to be about $200 Canadian plus rivets. Mostly stainless steel Pop rivets
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modified
It sounds interesting. If he puts any photos of it online, let us know.
Ed Huntress
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On Sun, 21 Nov 2004 20:08:30 GMT, "Ed Huntress"

He won't, but I might.
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building
do-it-yourself
The idea of making a space frame with L-channel raises all sorts of engineering questions, but the bottom line will be how well it actually performs -- and what it winds up weighing.
As a matter of interest, the radius of gyration for L-channels sucks in a major way, compared to tubes, which means that it's unlikely to resist buckling nearly as well as a tube-based space frame. That's the major rub, I would think. But there are more issues that this design raises as well.
However, a finished Lotus 7 is actually a hybrid chassis, with shear-panel design combined with space frame. And the torsional weakness in a 7 (Mk. II or Mk. III; I don't recall how the Mk. IV stresses out, but almost all replicas are Mk. IIIs) is in the cockpit bay, anyway, which can generously be called parallel girders with a shear-panel floor. That's true of many open space-frame cars.
So a good, tightly riveted cockpit bay, made of L-channels, could make the car stiffer than the original. I don't know. It would take a complete FEA analysis, or a model analysis, to tell.
In any case, if the car is raced, they'll find out a lot about it from the way it performs on the track. And I'd find it very interesting, as I'm sure many other people who follow chassis design will find it interesting.
Also for the record, a true space frame is the design concept, among all design concepts for car chassis, which benefits LEAST from use a low-density material: aluminum over steel. There have been aluminum-tube space frames (1960s Bobsy sports-racer; Porsche 917; a special made by the Locost folks), but, in general, the slight weight savings have never been worth pursuing. In theory, if not in practice, a steel space frame versus an aluminum one should come out weighing exactly the same, for equal stiffness and strength.
But there are issues other than the pure, simplified engineering theory involved, and aluminum may provide a fairly large advantage with channel. It will help the column stiffness and help prevent buckling in compressed members.
We'll find out when they get it on the track.
Ed Huntress
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On Mon, 22 Nov 2004 02:43:25 GMT, "Ed Huntress"

The car is supposed to weigh in at around 700 lbs. 1.5" angle replacing 1" square tube for most of the chassis, and 3X6 more or less for the bottom "frame rail" that a real lotus does not have (OK, it has a 1X1 tube) and he figures it will be roughly as stiff as the real seven.
He (the father) has built and raced numerous sevens over the last several decades and is intimately familiar with its features and shortcomings..
He has also built several planes,(and has been working on several "texas parasol" type planes - of rivetted aluminum angle construction ) so is also familiar with riveted aluminum construction
As you say - it will be interesting to see it actually perform on the track.

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I suggest you get a copy of "Design of Weldments", Blodgett. Read, especially, his discussions on columns, buckling and torsional rigidity. Particularly note the comparison of open (e.g. angle) vs closed (e.g. tube) sections in torsion.
Through your reading, keep in mind that aluminum is 1/3 the weight of steel but also has 1/3 the elastic modulus.
Ted
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The nice thing about it though, is if you can give up stiffness in one dimension, you gain 3 times the stiffness in the other (think edgewise).
What I *really* love is beryllium. A shame it's expensive and poisonous.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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It doesn't apply with space frames, Tim. A "pure" space frame loads its members only in tension and in compression. Thus, aluminum versus steel is a wash. Aluminum offers no weight advantage, neither for stiffness nor for strength.
In practice, the reality is not far from that, if the space frame is properly triangulated throughout. Most space frame cars are good at the front and rear bays, but they depart from a true space frame in the cockpit bay.
There are several issues here, which Ted has alluded to. First, there is some torsional loading in actual space-frame members. In the cockpit bay, there often is a *lot* of torsional loading. With round or square tubes, once again, aluminum offers no advantage here. But it can offer an advantage with other structure shapes, such as the L-channels (angle aluminum) that were being discussed. There, the PLATE stiffness and strength, which is what you're talking about, comes into play. The stiffness and strength in a plate vary with the cube of the thickness. An L-channel subject to bending or torsion presents a complex resistance, and plate stiffness is one part of it.
Again, that isn't true with tubes. If the limiting factor in a space-frame design is resistance to columnar buckling, aluminum's advantage in plate stiffness is a help, because thicker-wall or larger-diameter tubes have more resistance to buckling. This is something that designers try to avoid today. But the Lotus 7, like other Chapman designs before the early '60s, tend to have long, thin tubes loaded in compression. Aluminum could help, theoretically.
But a better answer is to produce a better design, in which the frame is less likely to fail by buckling. The Caterham folks, who build the Lotus 7 under license, have done just that. They re-designed the frame with the aid of finite-element analysis.
Ed Huntress
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Ah yes, good point. I haven't been following the thread that deeply ;-)
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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Ed Huntress wrote:

How, Ed? Long thin tubes approach the Euler formula in which only elastic modulus and radius of gyration matter. For example
2S_r I_tube 2 .05 radius of gyration for 2"OD, .05" wall 0.6896557112 2S_r I_tube 2 .15 radius of gyration for 2"OD, .15" wall 0.6562202374
Not much change there but the equal weight .05" wall steel tube would have three times the elastic modulus of the .15" aluminum tube and, in fact, a slightly larger radius of gyration.
Ted
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Because the tubes could be of larger diameter and *also* have thicker walls, for the same weight -- thus producing a higher value "r."
In fact, that's how the Bobsy was designed, with aluminum tube space frames, back in the '60s.
Ed Huntress
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Ed Huntress wrote:

I picked 2" as an example but the same comparison applies to any OD until the walls become so thin that other factors become involved.

That doesn't neccessarily make it right.
Ted
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walls,
frames,
Correct me if I'm wrong, Ted (and this is stuff I'm 'way rusty about), but the larger diameter tube made possible by lower density of aluminum will have a higher value for the radius of gyration than will a steel tube of the same weight, and of smaller diameter.
Correct?
Ed Huntress
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Ed Huntress wrote:

Correct but misleading. Of course the radius of gyration increases with increasing diameter but the increase in diameter is not simply "made possible by lower density of aluminum". One could instead choose to make the wall thinner for steel but keep the same diameter. You might want to take a look at "Design of Weldments", Blodgett, Sec. 2.5. I'd be surprised if you didn't have *that* book.
Ted
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but
the
I don't, but I still have a couple of Mechanics and Statics-and-Dynamics books from college, as old as I am. The formulas for moment of inertia and radius of gyration are the same but the Earth rotated the opposite way in those days, which makes it confusing. <g>
About those tubes, Euler's formula and much of the rest are long lost in my memory, but my non-mathematical recollection of the situation is this. Increasing resistance to buckling for a given *weight* of of a given tubular material, by increasing tube diameter and thinning the walls, is self-limiting. You reach a degree of wall thinness at which the diameter (and the curvature) of the tube becomes a lesser factor and the thin walls begin to behave more like a plate loaded in compression, on edge. In other words, plate stiffness in compression-induced bending begins to cross curves with the sectional stiffness of the large-diameter tube.
Aluminum in the form of a tube, having 1/3 the density of steel and also 1/3 the stiffness (roughly), can be made larger in diameter (for the same material weight and length) because the relative wall thickness remains greater, even when the tube is somewhat larger in diameter. In other words, you can take advantage of aluminum's far greater plate stiffness, per pound of material, and, in doing so, you can increase the diameter of the tube somewhat. You can increase diameter and still have considerably thicker walls than you'd have with steel tubes of the same weight. The total effect is an increase in the radius of gyration for the aluminum tubes over the steel tubes, because of the greater diameter, before you start to cross curves with the plate stiffness of the walls.
To get back to the space frames for cars, and what I originally said about there being no theoretical advantage in performance for an aluminum-tube space frame, the things I've just said above don't change that. The performance of a space-frame car chassis is limited by stiffness, not by strength, and the actual resistance to buckling should never come into play at all, in a properly designed, fully triangulated space-frame chassis -- until you crash. <g>
Ed Huntress
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Ed Huntress wrote:

You might want to get it. It's put out by Lincolm (the welder people) and, like most of their books, is both good and cheap. As well as unusually readable theoretical stuff, there's a lot of charts, tables, worked problems and comparisons of designs.

Correct. As with most things, if you go to extremes, you get into trouble. Just where that point occurs requires working through a particular case. My point is that it is not clear that going to aluminum will improve stiffness - you need to do the calculations for a particular case.
BTW, similar considerations apply to torsion.
Ted
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