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Welded Aluminium Structure

Welded Aluminium Structure

(OP)
I am looking into the possibility of a welded, sheet metal aluminium chassis to be mounted in an aircraft, possibly outside. Probably, the walls will be around 1/8" (3mm) thick. Probably, I would use 5052-H32 or 5086-H32, which is bendable, and has reasonable annealed strength. This would be an optical device so it will be designed for rigidity. It will be very, very strong.

How difficult will it be to get this certified safe for flight? Has anyone any experience with this?

Another option is a casting, which I understand can be another can of worms.

I have not started modelling yet. I am staring at an arrangement in 3D CAD.

--
JHG

RE: Welded Aluminium Structure

welding isn't uncertifiable ... the difficulty is in the design, referencing the welding specs, detailing certificated welders, ... details.

I'd consider 6061 as a "good" weldable Al.

Not for helicopter ?

Design to a low stress level, there's potential that you may have to test (to ultimate) the flyable specimen. another possibility is welding specimens (details) to test.

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

"How difficult will it be to get this certified safe for flight? Has anyone any experience with this?"

Look at the certification regulations related to the type aircraft you are proposing to install it on. If in the US, Part 23 for normal, utility, aerobatic or commuter airplanes, Part 25 for transport airplanes, Part 27 for small helicopters or Part 29 for large helicopters.

RE: Welded Aluminium Structure

(OP)

Quote (rb1957)


welding isn't uncertifiable ... the difficulty is in the design, referencing the welding specs, detailing certificated welders, ... details.

I'd consider 6061 as a "good" weldable Al.

Not for helicopter ?

Why 6061 as opposed to 5052? I want to bend it.

It might go on a helicopter.

--
JHG

RE: Welded Aluminium Structure

Drawoh,
If you use 6061 condition O, you will find that it is reasonably easy to bend, not quite as malleable as 5052 but close enough to cold bend without too much trouble. Then you can heat treat it to T6, If you even want to.
For some reason Aircraft people associate strain hardened alloys like 3003 and 5052 with things like fuel tanks and formed cowlings.
B.E.

You are judged not by what you know, but by what you can do.

RE: Welded Aluminium Structure

As part of obtaining FAA approval for your rotorcraft component you will need to submit analysis documentation showing it meets FAR part 29 requirements. You can use welding or casting processes to manufacture your optical platform. But there are specific requirements in FAR part 29 for using each one, including analysis factors (FAR 29.621) and manufacturing QA procedures (FAR 29.605).

RE: Welded Aluminium Structure

You can bend and weld 6061T6, but use 50xx if you prefer.

My caution about helicopters was that they are nature's fatigue machines, so more caution is required.

Why not a machined structure ? (finding qualified welders, who do good work, is going to be at least as hard as finding a good machine shop)

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

(OP)
rb1957,

Machining out of billet is my fall-back strategy. I suspect though that by the time I find a qualified welder and set up the paper trail, the welding may still be cheaper. As noted above, this is an optical device with a laser. By the time I make it rigid enough to stay aligned under all the shock and vibration the helicopter imposes, it will be very strong.

My managers want this thing to look cool. I will find out how determined they really are when I propose casting it.

--
JHG

RE: Welded Aluminium Structure

There are many welding shops capable of welding 6061 aluminum according to practices acceptable to your DER / FAA authority.
Any bending of 6061 should be done in the O condition, especially if it is 1/8" thick as you say.
Welding of 6061-T6 aluminum tends to cause local annealing around the welded area, plus residual stresses that can deform the part.

Hence...

Complete all bending operations in O condition, complete all welding in O condition, leave the structure in the fixture you used for aligning the parts for welding, and put it all in a heat-treatment bath. I would dispense with the heat-treatment if I could structurally test the finished welded assembly and show that it doesn't fail under ultimate load. Local deformations aren't important at ultimate load, provided nothing breaks, but at working loads, of course, any deformation is unacceptable, and then you would be required to use heat-treated material. Annealed 6061 is very soft.

STF

RE: Welded Aluminium Structure

Sparweb,
It would appear that you and I are on the same page, If Drawoh makes it in condition O and it passes all of Drawohs tests, he is ahead of the game.
If it does distort, it only needs enough heat treat to make it strong enough. Using heat treated material defeats the purpose unless you can accept the large bend rads. Taking T6 quenching, then forming in the Aq condition circumvents this, But this presumes that Drawohs contractor has heat treat facilities.
B.E.

You are judged not by what you know, but by what you can do.

RE: Welded Aluminium Structure


Unless there is some unusual geometry or strict weight requirements, for the typical helicopter loads associated with role equipment, I would be looking at making it out of 6061-T4 and welded as is, especially given that your design criteria is stiffness. As for welding, in New Zealand for this type of kit we wouldn't control the weld procedures and would just call out for each weld assembly to proof loaded (not sure if the FAA DER's can use that approach), it just depends on the expected number of units.

The other point is this what type of use on the helicopter will it get, most role equipment tends to be on and off the helicopter frequently, so addressing fatigue (given that strength of these units they easily grow a visually inspectible crack before being critical length) is often picked up by requiring visual inspections for damage to the kit on installation &/or 100 hourly as finding the damage relating to handling / stowage of the kit off the frame is likely be a bigger problem.

RE: Welded Aluminium Structure

If you are indeed designing for stiffness, aluminum has no weight advantage over steel, which may be easier to fabricate.

Mike Halloran
Pembroke Pines, FL, USA

RE: Welded Aluminium Structure

Steel is suitable indeed, but aluminum should have a greater resistance to corrosion in the "typical" external environment of a rotorcraft.
We don't know what facilities are at Drawoh's disposal for finishing & painting the parts when they are complete, but many kinds of durable finish can be used.
Some people swear by the commercially-available phosphate-bath plus powder coating, though to my knowledge there is no Mil-spec process for this. Such a finish would be ideal for low-stress high-stiffness steel components - much less suitable for heat-treated aluminum parts.

STF

RE: Welded Aluminium Structure

(OP)
MikeHalloran,

I need to keep weight down. If this were a tubular space frame, there would be no advantage for aluminium over 4130 steel. This is an enclosed chassis. The wall of an aluminium chassis will be thicker and stiffer.

--
JHG

RE: Welded Aluminium Structure

I am not a fan of working with O condition sheet and heat treating it, because of the potential of warping.

Are we talking about bending sheet or welding it ... there seem to be two discussions going on !?

I'm alittle confused about the material choice ... if you're comparing 0.04" Al with Steel, then the Al will be lighter but the Steel will be stiffer, if you're comparing 0.12" Al with 0.04" Steel then the two should be much the same (weight and stiffness).

Could we see something of what this looks like ?

IMO welded Al doesn't look "cool" ... "cool" might be a composite, with all the extra difficulties that has.

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

(OP)
rb1957,

Steel has approximately three times the mass and three times the stiffness of aluminium. In a truss frame loaded in pure tension and compression, there is no difference in stiffness between the two structures. If the structure is loaded in bending, the aluminium piece will have a higher second moment of area. The aluminium structure will have a higher stiffness to weight ratio.

The aluminium structure would be a sealed sheet metal box. The walls would be at least .10" (2.5mm) to control warping, and to assure that there is room to grind off edges. There appears to be a requirement for this thing to look cool. I don't have a good gut feeling for warping. I understand that steel is better, but I don't think I can take advantage of its superior strength and stiffness.

--
JHG

RE: Welded Aluminium Structure

we can debate the relative merits of the different materials ... it'll come down to how you make it ... ie you won't have an optimal stress everywhere in the structure, you'll have a structure as complicated (or simply) as fits the manufacture of it.

0.1" thk Al is really thick.

a "box" is rarely an efficient way to react pressure ... so already the design has compromised weight for other reasons.

a thick walled box is rarely an efficient structure.

give us a clue as to what we're looking at ... how big? how many (1 off?) ?

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

oh...

Rb1957's last post poses a very important question to YOU: The design/configuration complexity is an all-important aspect.

In several cases I have seen weld assembly designs that could be far-more-easily reproduced as machined components [3D model required, simplified for machining purposes]. These assemblies could easily be simplified to a single machined part... and/or with some odd details fastened to the machined base component.

In a few rare cases I have seen where weld assemblies... such as for complex pneumatic system components... could only be made using multiple sheet-metal weld details and perhaps some castings.

In some cases, perhaps, a lost-wax casting could also be employed for the base part and/or complex details. The base/detail component[s] is[are] made from one-or-more stereolithography wax/plastic detail[s] [including risers, sprues, vents, etc]; which are coated with multiple layers of ceramic compound to build a thick shell for containing the casting metal; which is then heated to a temperature which both hardens the ceramic and 'burns-out' the core wax/plastic. The ceramic mold is then filled with a typical casting alloy [A356, etc] at a foundry. Finishing the part becomes a simple matter of trimming off casting elements/flash, heat treatment [if necessary]; and then pressure testing the casting for general soundness.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

RE: Welded Aluminium Structure

At the risk of sounding like a broken record (or worse), I'll quote myself "... leave the structure in the fixture you used for aligning the parts for welding, and put it all in a heat-treatment bath..."

There's no other way to guarantee that all the holes line up if you release the assembly from the fixture too soon, if the assembly was welded. Now that I re-read this I'm sure I have a specific configuration in mind - but maybe not at all what the OP has in mind. I'm rethinking my assumptions, but still believe the "camera box" in question doesn't need to contain air pressure. I expect the OP would have mentioned it, given how important this would be if it was. I think it needs to be sealed to to "keep the rain out", but there's no reason I couldn't be wrong.

Hmm, I wonder if there aren't features in the OP's concept that are driving higher and higher thickness of certain parts... seeking higher stiffness... sometimes these pursuits go in the direction of the Tacoma Narrows bridge, or the Fokker D.7 monoplane. I have seen this happen, many times, with 1-off external mods on rotorcraft.

Going back to the first posting: is this going inside or outside? I would want to know where it's mounted, before figuring out HOW it's mounted, and then determine how it must be protected.

STF

RE: Welded Aluminium Structure

It would be helpful to see a sketch of the optical platform with a few dimensions. Other details such as production rate/quantities would also be helpful for providing guidance on what manufacturing approach might be best. If weight and structural stiffness are important, a composite material might be best. There are many low cost methods for prototyping composite parts. If the hollow box platform is not too large, it can be rapid prototyped in a single piece with thin walls (~.06") using laser sintered aluminum.

RE: Welded Aluminium Structure

In his original post Drawoh said this was a chassis. To me, that word, means a base for other things to be mounted upon. You then have the considerations of how much load do those things generate ?
B.E.

You are judged not by what you know, but by what you can do.

RE: Welded Aluminium Structure

"...a welded, sheet metal aluminium chassis to be mounted in an aircraft, possibly outside."

"...this is an optical device with a laser. By the time I make it rigid enough to stay aligned under all the shock and vibration the helicopter imposes, it will be very strong..."

"I need to keep weight down. If this were a tubular space frame, there would be no advantage for aluminium over 4130 steel. This is an enclosed chassis. The wall of an aluminium chassis will be thicker and stiffer. "

"The aluminium structure would be a sealed sheet metal box. The walls would be at least .10" (2.5mm) to control warping, and to assure that there is room to grind off edges."

Based on the information provided so far, all we know is that the structure is a platform for mounting "an optical device with a laser", somewhere inside or outside on a helicopter, where weight and stiffness of the optical platform are primary concerns.

RE: Welded Aluminium Structure

(OP)

Quote (tbuelna)

...

"The aluminium structure would be a sealed sheet metal box. ...

Thanks.

Design has barely started, and I am looking at all my options. I have worked up a cool looking casting with a fibreglass cover. If management is determined for this to look cool, this is the way. I have contacts who rapid prototype wax patterns. Machined from billet is a good way to do one piece. It will cost more if we do a lot of them. I have designed welded sheet metal chassis in the past, but they were not mounted in aircraft. Whatever I do, I will make it optically rigid, with the optical alignment not affected by mounting in the aircraft.

Sheet metal is a near net shape fabrication method with fairly thin walls. There are ways to make it look good. I just want to know if anyone has experience handing something like this to a DAR.

Making a casting look good, making machining from billet look good and making sheet metal look good are three very different tasks. I need to pick one and stick with it.

--
JHG

RE: Welded Aluminium Structure

why wouldn't you 3D print the prototype ? odd to choose casting when weight was previously mentioned as a design goal.

once you define your design you may be able to see how sheet-like it is, and maybe welding or conventional assembly will be options.

welding is a perfectly valid assembly method, as long as you're aware of the pitfalls.

You've mentioned pressure ... what stress level do you expect the box to work at ?

If the structure is going to have a fiberglass fairing cover, then it can look as ugly as a (insert desired reference here) if you want. I'd make the structure efficient as I can afford, and let the fiberglass fairing look pretty.

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

(OP)
rb1957,

I did not mention pressure. Almost certainly, this thing will have a breather desiccator to ensure there is no pressure. All I need to do is keep optics clean.

3D printing a prototype is a good idea if I plan to do a casting. I need to solve whatever problems my fabrication method causes.

The chassis is visible so far, so it need to look good. Mounting a five sided cover over a flat optical platform is a reliable way to re-align optics upon assembly, and probably upon installation as well.smile

--
JHG

RE: Welded Aluminium Structure

For "prettiness" points, take a look at Leica airborne optical scanners. Though designed for internal mounting, the style is functional too. smile

STF

RE: Welded Aluminium Structure

So the optical equipment will be located inside a sealed volume?

Desiccant breather systems are a great way to eliminate moisture within an enclosed volume while equalizing pressure with the outside atmosphere as it changes. Just remember that the desiccant element must be replaced on a regular basis. And the air within the sealed volume at assembly of the system will have some humidity, and should be purged using something like dry nitrogen.

RE: Welded Aluminium Structure

(OP)
tbuelna,

We purge these things with nitrogen. Equalizing pressure is the important thing. I was shocked when I worked out the force of a sealed enclosure even at 10,000ft.

--
JHG

RE: Welded Aluminium Structure

"The aluminium structure would be a sealed sheet metal box." and "this thing will have a breather desiccator to ensure there is no pressure." ... it'll be a tricky control to ensure no pressure in a sealed box ... as the plane climbs you'll need to remove "air" for the box, as it descends you'll have to add it ... tricky, but not impossible (nor ludicrously complex).

If you need to visualise the structure I'd 3D print it, whether casting or not. These days I'd've thought that our 3D models were good enough representation.

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

(OP)
rb1957,

One of our manufacturing guys told me that once I had designed something, it was his job to figure out how to manufacture it, and he could do it any way he damn well pleased. Strictly speaking, he was right, and almost certainly, he knew better.

In over thirty years of mechanical design, I do not recall ever designing something significant where I did not know how it would be fabricated. Usually, this would be machining. Obviously, it must be feasible to fabricate your part by your chosen method. Less obviously, you want to take advantage of your manufacturing technology. Machined parts can be very accurate. Sheet metal allows you to design thin-walled, 3D, near net shapes at low cost. Castings can contain all sorts of weird curves, and can be very complex with minimal cost impact. In all cases, you have to be able to get tooling in and out, including your welder. In many cases, you have an up-front tooling or programming cost.

Any time I have a structure or mount at the point where it can be visualized, I know how it will be fabricated. Right now, I am visualizing a casting, with an FRP cover. I have started off fabrication drawings, I have done some tolerance stacks, and it looks like I am in trouble. I need to figure out accurate interface surfaces, as well as airworthiness. Early in the design process is the time to have these problems. I can adapt the design to solve them, or at worst, I can switch to another design concept.

--
JHG

RE: Welded Aluminium Structure

there is a cart and a horse ...

designing something without an eye to how it's going to manufactured is a short cut to a redesign.

your ME/planner is a little off base ... if you design it with welds, it has to be welded. he can come back to you and bitch and demand a design change. we'll often get design change requests "we want to make this a machining, rather than s/m, 'cause the s/m shop is full and the machine shop needs work" ... sign

we get the same comments here ... talk to one guy in the s/m shop about how to do something, then the next guy says "I can't bend this" ... "well, Benny said he could" ... "you can't design a structure for a specific mech to build for you" ... sigh (go back and reread every Dilbert, again).

another day in paradise, or is paradise one day closer ?

RE: Welded Aluminium Structure

Why not a simple sheet metal riveted enclosure ?
- cheap
- easy to do stress calculations
- easy to modify if necessary
- very simple to add local re-enforcements, stringer, angles etc....

I mean we have been building rivet sealed airframes for more then 80 years !

RE: Welded Aluminium Structure

You don't say anything about alignment accuracy and retention. Requirements for these in optical systems tend to drive to 6061. We tend to CNC stuff, because it's presumably easier to anneal out all the stresses so that alignment doesn't shift over time/temperature. For our systems, flatness and planarity are absolute requirements, so even if you can do a bang-up job of welding/annealing, it's likely that you'll still have to lap mating interfaces to achieve and maintain the desired initial alignments. We've had systems that required lapping because machining stresses couldn't be completely removed.

Note that everything is relative, including stiffness. While Al has great stiffness for what it's supposed to do, it's not really stiff enough for some applications, where you need to go something like steel.

TTFN
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