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I was motivated to ask by this question: Why are airplanes riveted and not screwed?

Why not welded construction? Is welding these alloys too difficult? I come from a refining background & in a lot of hazardous duties welded construction has become the norm because integrity is easier to guarantee than screws, bolts or rivets.

Just wondering what's different in aviation? Isn't repair, safety, weight & aerodynamics all better with a welded construction?

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    $\begingroup$ Short answer; It is exceptionally difficult to heat-treat a welded aluminum structure back to original strength. In addition to all the other reasons listed below. $\endgroup$ – blacksmith37 Nov 15 '18 at 19:22
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Short answer: High-strength aluminium alloys are tricky to weld correctly. Aluminium is such a fine material for aircraft structures that the need to rivet it is gladly accepted.

Two things are important:

  1. While steel has a temperature range in which it gets more and more runny, aluminium alloys change from solid to liquid within a few degrees. Also, heat conductivity in iron-based alloys is lower than in aluminium, so heating steel locally will keep the surrounding material cooler and more solid compared to aluminium. While welding thin sheets of steel is trivial, it needs lots of experience in aluminium. For very thin sheets, special equipment like a water-cooled copper backing on which the aluminium sheets rest, so their back is cooled, are needed. Also, the melting temperature of steel and titanium is high enough for it to glow long before it melts, while aluminium will melt without giving you any optical hint of its temperature.

  2. High-strength aluminium is produced by progressively aging and precipitation hardening the material. The usual alloys use copper atoms dispersed through the aluminium matrix which locally distort the atomic lattice and strengthen it. If they are heated and rapidly cooled by welding, the copper distribution would be changed and the material would be weakened around the welding area. To harden the finished structure again is rather impractical in most cases, so riveting is the better alternative.

A third speciality is the oxide layer on aluminium, which has a higher melting temperature than the base material. You need an AC TIG welder to disrupt the aluminium oxide layer, so your choice of welding techniques is rather limited.

Also, riveted structures are easier to inspect and to repair. Most repairs need to remove aircraft structure for access, and a riveted structure is easier to disassemble and to put together again after the repair using slightly thicker rivets.

My experience with aluminium welding stopped at 4mm thick sheets; while thicker ones were easy to weld, I never managed to weld thinner ones. You sit in front of your structure and heat the spot where you want to start welding. Watching it through the darkened head screen you wait until the spot under the arc becomes glossy, which signals that the surface has started to melt. Now you need to add your welding wire like crazy to keep the spot from heating more, and get the spot moving. If you fail to do so, a second later you will have a hole under your arc, because the aluminium has molten completely and has fallen away. Doing this with 2mm sheets was a pure exercise in futility for me - the moment the surface became glossy it fell away already.


Edit:

Thanks to @voretaq7 for sharing the link about friction stir welding in the comments! This is made possible by precise positioning of the parts and a computer-controlled welding head and will see wider application in the future. Eclipse Aerospace claims it helps them to avoid 60% of rivets in their jet aircraft.

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    $\begingroup$ Years ago I had an engineering materials professor, specializing in metals and allloys, who, in place of the term "welding" for the chapter, used the phrase "The Ruination of Otherwise Carefully Prepared Metals". $\endgroup$ – J... Jul 18 '15 at 11:22
  • $\begingroup$ Thanks Peter! In aircraft construction what are the typical sheet thicknesses one encounters? I suppose fusalage vs wings vs control surfaces all might have very different thicknesses and depending on Type too but I was just wondering about some general values. If welded wings / fuselage etc. was to happen are we looking at 2 mm or 4 mm or much thicker sections? $\endgroup$ – curious_cat Jul 18 '15 at 12:14
  • $\begingroup$ Also, this sounds like an application where robotics might help? Has it in reality? i.e. Getting the exact temperature and then rapidly and precisely placing the weld bead where it needs to be. How much of welding in aircraft construction has been automated? And has that helped any in making welded Aluminium more practical? $\endgroup$ – curious_cat Jul 18 '15 at 12:15
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    $\begingroup$ @curious_cat: Computer-controlled welding is certainly a big step forward - mind you, compared to the TIG welders 30 years ago the new ones with automatic voltage adjustment for starting the arc and current control are a revelation. And that is still manual welding. But for the computer to do the job correctly, everything has to be carefully prepared. Regarding thicknesses: Most aluminium is less than 2mm thick. It is more efficient to use a paper-thin sheet and rivet reinforcements to it than to use a thicker sheet. $\endgroup$ – Peter Kämpf Jul 18 '15 at 12:37
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    $\begingroup$ It's worth nothing that some aircraft are constructed by welding - at least in part. The increasing use of composites seems to make this less likely though. $\endgroup$ – voretaq7 Jul 19 '15 at 1:27
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Welding has 2 significant drawbacks (1) it generally weakens the parent material the heat in affected zone and (2) use of a single component can lose crack stopping at the change from one section to the next. I heard a story that someone (Lockheed?) had tried the use of diffusion bonding but then suffered major crack development during structural testing.

Having had access to the Comet fuselage used for water tank pressure testing (mounted on the wall of a lab at RAF College Cranwell) the growth of cracks is clearly apparent. In the past, engineers used to drill holes at the end of a crack (when they found them) because the increased tip radius slows the rate of propagation. My father and godfather were both Imperial Airways engineers in the 1930's, 40's and 50's. (OK, the name changed to BOAC i 1940.)

Also, it is almost impossible to test the integrity of a weld without breaking it; like the match factory that tests 100% and only despatches the ones that worked.

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    $\begingroup$ Re. Weld Integrity: What about X-Ray Testing, ultrasonic testing, Radioactive Testing etc.? Are they not trustworthy enough for weld integrity tests? $\endgroup$ – curious_cat Aug 5 '15 at 9:11
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    $\begingroup$ I saw a recent documentary on the Comet, where they went back and re-examined the aerial cutout section from G-ALYP that was recovered, where the crack that destroyed the aircraft was believed to have started. What they found is that in addition to the almost square openings, a wartime riveting technique had been used, that created micro cracks in the skin around the rivet holes. That wasn't a problem for the unpressurized bombers made for the RAF, but was a problem for a pressurized airliner. $\endgroup$ – tj1000 Apr 7 '18 at 5:29
  • $\begingroup$ @tj1000: The punch-riveted fuselage wasn't just a major contributor to the disintegrating Comets, it was pretty much the only significant contributor. If you take a look at the Comet 1's windows, you'll see that they were already quite rounded - certainly not significantly more angular than those of many other jetliners (look at the Caravelle's triangular windows, or the rounded squares or rectangles used nowadays). (1/2) $\endgroup$ – Sean Dec 31 '18 at 18:57
  • $\begingroup$ (2/2) Besides, the Comet's overhead ADF windows (the original suspect in the breakup of both BOAC 781 and SAA 201) were of identical shape and construction (apart from being filled with opaque fiberglass instead of transparent panes) to the cabin windows, and remained rounded squares, even on the later Comets with circular cabin windows... and, yet, no later Comet came apart in midair as a result of fatigue cracking emanating from the ADF windows. It wasn't the switch to circular windows that saved the Comet 4 - it was the elimination of punch riveting. $\endgroup$ – Sean Dec 31 '18 at 19:02
  • $\begingroup$ Actually, it was a combination of both the poor riveting technique starting the cracks, and the squared holes accelerating the stress. In the original tank test, the Comet fuselage split open around a window opening, while Yoke Peter appears to have come apart around the ADF cutout. They would have failed due to the riveting cracks alone, the somewhat squared openings accelerated the process by increasing the stress. Conversely, had the cracks not been present, the squared openings wouldn't have been as much of a problem. $\endgroup$ – tj1000 Jan 14 at 17:19
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Just wondering what's different in aviation? Isn't repair, safety, weight & aerodynamics all better with a welded construction?

I just thought I'd extend with another thing that is used extensively (which you may or not know about since it is less obvious): Adhesives.

While not combining the material in itself like welding, it goes along the same lines but unlike welding generally does not change material properties. Industrial strength adhesives, such as epoxy, can be incredibly strong when applied properly.

This PDF from Henkel has a huge range of products as does this overview from 3M Aerospace.

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    $\begingroup$ Among structural engineers there is a saying: Those who don't know rivet, those who know already something use adhesive, and those who know a lot rivet again. Modern rivets are set wet, meaning that at least some sealant is applied. Ideally, this sealant sticks well and transmits shear. Now you have a bonded joint secured with rivets. That is really the best way currently. Bonded joints must be secured against peeling by adding rivets. $\endgroup$ – Peter Kämpf Jul 19 '15 at 10:01
  • $\begingroup$ @PeterKämpf Maybe that's one advantage of welded over adhesives. Welds don't peel. Are there peel resistant adhesives that need no riveting? $\endgroup$ – curious_cat Jul 19 '15 at 12:33
  • $\begingroup$ @curious_cat: No, all there is are peel-resistant design techniques. But peel forces are impossible to avoid, especially when damage starts somewhere and begins to spread. Rivets help to stop the progression of damage. You can lower peel stresses somewhat by spreading them out by using a thick layer of elastic adhesive, but the strength of this kind of adhesive is rather low. The other way is to stiffen up the parts at the ends of the bonded joint. $\endgroup$ – Peter Kämpf Jul 19 '15 at 13:07
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First of all, most aircraft have many welded components, but they are mostly the frame/structural components. The skin is usually rivetted for two reasons (1) it is much easier to remove or replace a rivetted panel, and (2) the skin-to-structure mating location is in many cases not accessible. In other words, the skin needs to be joined to the frame. That means the welder would have to be able to access that joint. Even if you had access to the joint, it would just be a seam weld, then you would have to weld again on the outer surface to seal the butt joints between panels. Then you would have to grind those seals. Very time consuming and expensive and probably not stronger than rivets anyway.

Also, consider that airplanes flex constantly while they fly. This will play havoc with welded joints, potentially leading to cracking in a thin joint (which skin joints will be). Rivets are much more friendly to moving parts because they have a little give to them.

Normally, you want a lot of surface area for the weld, and with a thin section it is hard to get this. For this reason it would actually be a much better idea to solder or braze the skin on than weld it.

All those things being said, it is very possible to design a plane using welded construction similar to a car using what are called "spot welds". This may yet happen, but the aircraft industry has not got there yet.

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  • $\begingroup$ Spot welding is not too popular/useful as singular method for lightweight construction, as it introduces load peaks. Even today in the automotive industry, spot welding is mainly used together with adhesive bonding to quickly fix parts in place until the adhesive cures to provide structural strength. Line welding is quite well introduced already (see above, Laser beam welding and increasingly friction stir welding). $\endgroup$ – yankeekilo Dec 17 '16 at 9:01
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I came to this thread in search of comparisons between rivets and welds. My curiosity was piqued in reading "The night the Fitz went down" / by Hugh E. Bishop in cooperation with Dudley Paquette. Paquette (captain of the Wilfred Sykes when the Edmund Fitzgerald went down) was critical of welds in ships, which have become almost standard since a comparison of of welds vs. rivets done when the Navy was trying to decide how to build submarines. The Navy concluded welds were much stronger than rivets, so they started welding subs and m\pretty much all the big boats after that. Paquette didn't trust them, because a big boat "works" (flexes and twists) a lot, particularly in storms, and welds are so rigid that where a rivited area would flex with the rest of the ship, a weld would pop right open.

I'm not entirely sure how applicable it is to planes, but it goes to show that "boats and ships do it" may not be as strong an argument as people may think.

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    $\begingroup$ I work in the shipbuilding industry, I was onboard the USCGC WLBB-30 Mackinaw when it struck a seawall (I actually have a piece of the dented steel they cut out and gave to me) on it's maiden voyage. The welds held very well. Welds themselves are actually stronger than the surrounding steel. $\endgroup$ – Ron Beyer Dec 17 '16 at 4:26
  • $\begingroup$ Yes, that's what I said that the Navy concluded. I AS $\endgroup$ – Wendy Dec 17 '16 at 17:16
  • $\begingroup$ Yes, that's what I said that the Navy concluded. I ALSO said that the welds were more rigid, i.e. more prone to brittle fracture. Unless there's a major snafu in construction, any join will perform well on a maiden voyage. But consider this: there were four big boats on the lake on Nov. 10, 1975. The Wilfred Sykes had just been converted to a self-unloader, the Arthur M. Anderson had just been lengthened, and the Roger Blough was only three years old. Guess who didn't have new steel in her (and a loose keel, to boot)? $\endgroup$ – Wendy Dec 17 '16 at 17:44
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In refinery industry ; welding is not easy, and that is for steel which is much easier to weld than aluminum and titanium. ASME Boiler and Pressure Vessel code has a whole section ( Section 5 ) that defines the requirements and conditions for welding refinery steels. One of the biggest problems for any metal is the heat affected zone ( HAZ). The metal in the HAZ has seen the whole range of temperatures from melting to nothing. For aluminum, the weld deposit and some of the HAZ is in the lowest strength condition . I will be brief. To bring the aluminum back to strength , the entire welded structure would need to be solution annealed ( heated to about 900 F ) and rapidly cooled ( like in water). Then aged at about 300 F. This is the common T 6 condition . Consider the problems of supporting the whole structure while hot and the aluminum has the strength of cheese. And then quenching in water. Titanium has its own set of unique welding problems.

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As others have noted, it's because of the properties of aluminum. I'd like to add that there are good alternatives to using aluminum but the only one that's feasible on both performance and price is magnesium.

Northrop's rocket powered XP-79 used a welded magnesium monocoque structure for example.

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    $\begingroup$ Depends how you define "performance." It reacts with water and is highly flammable, both qualities that are fairly sub-optimal in aircraft structures. $\endgroup$ – fooot Feb 19 '16 at 17:57
  • $\begingroup$ @fooot A magnesium alloy called Elektron was very popular in German and Russian aircraft. Corrosion was the biggest problem, and flammability was no issue. $\endgroup$ – Peter Kämpf Feb 19 '16 at 18:46
  • $\begingroup$ @fooot "sub-optimal" heh... :) $\endgroup$ – FreeMan Feb 19 '16 at 20:56
  • $\begingroup$ @fooot Mg is used for helicopter gears and components. Whlie corrosion can be problematic, burning is not. In terms of ductility and formability, Mg is no match for Al. Especially forming thin sheets is problematic. $\endgroup$ – yankeekilo Dec 17 '16 at 8:56

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