Use of Welding for Aircraft Frames 7

[teknomiks]

I wonder why it is not used instead of machining massive aluminum blocks to frame parts. I understand that bolt to hole and hole to hole connections are safer when compared to welding but machining of an Al plate to a frame part is extremely costly when compared to welding.

Answer to this question may be the change of microstructural properties after welding, crack growth through welding, QC of weld seams in detailed frame parts can be a problem. However, I think that these all can be solved by design solutions like in pressure vessels or other industries. Is there another reason that I missed which makes use of welding non applicable.

Thanks

 

[rb1957]

there could be solutions to the problems you mention, but our industry has different criteria ... we have to be weight aware, we wouldn't weld up a frame from steel plates (geeze, just typing that makes me cringe) and 7075/7050 don't like welding (micro-structure issues you mention).

however, research "friction stir welding" ... a new method that's fashionable (A380) for attaching stringers to the fuse skins without rivets.

 

[CoryPad]

You've really answered the question yourself. Traditional fusion welding techniques like GMAW and GTAW do such terrible things to the high strength Al alloys used for aircraft construction that it is better to use mechanical joining. The newer solid-state welding process friction stir welding has been used for the primary structure of one small aircraft - you can perform an Internet search to learn more about this.

 

[MikeHalloran]

As has been alluded, welding aluminum leaves the HAZ in near-annealed temper.

Also note that CNC mills can run unattended, so the labor cost associated with ~95 pct stock removal is not as great as you might first expect.



Mike Halloran
Pembroke Pines, FL, USA

 

[dicer]

I personally know of some very large titanium parts for an aircraft that are of weldmet construction. Also many jet engine parts are welded as well. Though not Al.
In the old days radial engine mounts were all welded tubing, and any fittings welded to the tubing as well.
We all know it can be done and last look at some of the motorcycle frames made of Al.

 

[RPstress]

Do 328 nacelle engine mounts had titanium tube welded to machined titanium frames. Dornier were very worried about stiffness for dynamic reasons and didn't like the idea of struts with lugs. (Heat and fire requirements make titanium common in more modern nacelles, though it's not usually welded.)

Several older light aircraft fuselages were welded steel tube spaceframe.

It's hard to weld good aluminum. Getting the combination of strength and fracture toughness that makes stuff like 2024-T3 and 7475-T76 so useful is hard enough as it is. You can weld up 2219 quite well, but it's not really good for anything except elevated temperature use. I think the A400M inlet, which is hot air anti-iced/de-iced, is 2219.

To get adequate fracture properties a lot of Al alloys need the -51 (or -52) temper, which has cold word before aging. Re-solution treating and aging would be needed after welding for adequate properties, and it's hard to do cold work on an assembly, never mind the distortion that tends to occur on quench.

NASA has done quite bit of work on weldable aerosapce alloys, including (it tends to be older work):

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660012525_1966012525.pdf

 

[thruthefence]

Here's a link to friction stir welding:

http://www.azom.com/details.asp?ArticleID=1170#_Materials_Suitability

 

[WKTaylor]

teknomiks ...

What you almost described is precision die-forgings. Available in somewhat limited alloys, they have tremendous potentical for material savings and minimal machining... for sizeable production runs of parts.

For onsey-twosy parts... can't beat ecconomy of machining from a forged block or thick plate, but mechanical properties are alway an issue.

FSW has good potential for material savings.. but the metallurgy in the stir/HAZ zone is always an issue.... although can be improved by re-heat treatment and peening

Regards, Wil Taylor

 

[tbuelna]

teknomiks,

Although they're not specifically airframe structures, many highly stressed turbofan engine or gearbox parts are welded using various solid state and fusion processes:

-Shaft and gear blanks are commonly solid state friction welded (direct drive or inertia processes) where there is a large change in diameter.
-Compressor blades are sometimes linear friction welded to the hubs.
-Large hollow titanium fan blades have the skins and honeycomb cores diffusion bonded (solid state process).
-Compressor drums are electron beam or laser welded (fusion process in a vacuum environment).

As the others have noted there are some examples of flight structures using solid state joining:

-Eclipse used friction stir welding to join stringers to skins.
-Boeing uses friction stir welding to build cryo fuel tanks on their Delta IV rocket. They also used conventional TIG welding on the older Delta II alum. cryo tank structures.
-Rockwell used several diffusion bonded titanium pieces on the Space Shuttle.

If you want to get an idea of why few conventionally welded structures are used on aircraft, take a look at sec. 8 & 9 in MIL-HBDK-5 to see what allowables and knockdown factors apply to fusion welded joints.

Hope that helps.
Terry

 

[Solveer]

One point on the above, the FSW process is actually quite a good process to use and similar to welding. I worked on the Eclipse project which was really the first Aerospace application of FSW and the welds (if you can really call them that) were VERY clean. No HAZ to worry about because the material didn't get hot enough. And it's fast too. I think there will be a lot more use for FSW in the future unless everyone starts to use composites. The issue is really one of economics and the problem of not being easy to use on smaller parts (the tool is simply too big).
Not too sure about this, but with FSW you can (to what degree I don't know) bond disimilar metals.

http://www.solveering.com:

Multidisciplinary Engineering Services

 

[plasgears]

Inertia welding should have greater application. We used it on jet engine rotor shafts. It's lighter and more elegant than multiplicity of fasteners.

 

[KENAT]

Welding gets used on some aircraft ancilliary parts, especially those 'disposable' items the military like to hang/drop off theirs.

That said, a lot of them are still riveted though.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[WKTaylor]

teknomiks...

Wichita State University [WSU] has a VERY active R&D Friction Stir Welding [FSW] program in the National Institute for Aviation Research [NIAR]. They receive funding for fundamental aviation research from US government [FAA, DoD] and aero-industry in Wichita KS [aviation capitol of the world]. Engineering MS and PhD candidates are leading the hands-on charge... with an outstanding cadre of PhD-level research instructors guiding the process and interfacing with the outside world. Some incredibly ambitious goals for a university!

One thing they are pursuing is the solid state [FSW] welding of pieces together to form a basic shape that would be be basis for a replacement extrusion or a forged bulkhead with integral reinforcement, etc. Concept is that thin sections have superior grain directionality (orientation) and mechanical properties that could be enhanced (after finish machining) using Laser Shock Peening [LSP]. They are currently attempting to make L, T, U, Z etc sections from thick extruded bar-stock that match the mechanical, SCC and DaDn properties of the extrusion they mimic [replace]. The concept could be expanded to plate with integral stiffener made from bar-stock, for bulkheads, etc....

Go to NIAR website and "search NIAR" for FSW & LSP.

http://www.niar.twsu.edu/

May also find some of the other research fascinating and informative [human factors, composites, aging acft, crash dynamics, mechanical testings, Virtual Reality, etc...].

Regards, Wil Taylor

 

[uGlay]

ORION capsule uses FSW extensively. there's a couple of articles on that also. and yeah, i have also wondered why there are such HUGE pieces being machined instead of assembled. at the end of the day, i guess they have to do whatever it takes to get that building airborne.

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[rb1957]

they probably have a large NC machine lying around, else someone really wanted a large NC machine and used that program to get it.