Rivets or welds on aluminum?

Rivets loosen, welds break. When fastening pieces subject to load or stress which works best?
Not talking gussets, or reinforcements or adding thickness, just attaching one piece to another. The rivet gun is laying right next to the welder, which should I grab?

 

All aluminum aircraft hulls are all riveted. Plain up-set rivets with lock-bolts in high load areas. A riveted assembly is easier to field fix without specialized tools. I think you get a more reliable joint with rivets and more fatigue resistant too. All welded parts require x-ray inspections-there must be a reason for that.

As an engineer with experience in a number of industries (including aerospace) I have designed both riveted and welded structures. Riveting used to be more common, less so now because of availability of portable welding equipment. But rivets only require visual inspection, welding appears to need more rigorous quality control.

I would use rivets, but I would not trust my own welding skill either, perhaps you are better at it than me.

Good luck.

 

Thanks.
Every riveted boat (vintage) I see, leaks through the rivets. "Fish a little, bail a little"
The thread about building a lapstrake boat with rivets made me think if were a good idea to try & build a riveted boat employing a caulk & rivet technique using panels instead of strips.
Butting the cut to shape panels edge to edge & then laying a strip over the joints using rivets & caulk or adhesive.
Yust wundering!

 

The oldest auto ferry in the USA was just retired from Washington state waters, it was built in 1927 and used riveted hulls. It had served Puget sound daily for over seventy years up until just before the Thanksgiving day weekend in November. And though it had be rebuilt and refitted several times, much of the original riveted steel hull was still present. However it so badly degraded it was just determined it was not worth fixing.

When aluminum aircraft structures are riveted they use sealant between all of the surfaces, and in the rivet holes. Yes corrosion starts at the rivet holes, but fatigue cracks can initiate in welds, as well as corrosion. Most people are surprised to learn that rivets are pretty fatigue resistant because the metal around the rivet hole is in compression (caused when the deforming rivet expands in the hold and compressing the metal around the hole). In a production environment (though I do not like them much) they are still a viable and economical manufacturing process.

 

Rivets or welds

Just in case you are interested my aluminum boat has neither, take a look at Duroboat. There are good quality boats that are riveted or welded, but this one is unique and yeilds the best boat where light weight and toughness are necessary. Not sure if this has any benefit to the chioce of rivet or weld for a component or repair of an existing boat but it does illustrate there is another way to do an entire hull for a small boat.

 

Welding is often not a choice since not all alloys of Al are weldable. Even among the weldable alloys, the welds and immeditaely adjacent areas will be weaker than the unwelded parts if the stock was heat treated.

Aircraft are extensively riveted together. Until the latest generation of large aircraft, riveting was used exclusively to join aluminum panels together. This is still true of wings, which always serve as fuel tanks. Leaks are a very minimal problem even though kero is much slipperier than water. In aircraft, adhesive bonding is slowly replacing riveting. For the most part it is not being replaced by welding.

Jimbo

 

I find aluminium a very nice material to work with. It has its difficulties and there are a few tricks.

I have rivetted it, glued it, rivettered/caulked, screwed/caulked, "brazed" and I am finally able to weld it properly.

I think hand welding with less than 3mm material is a real challenge unless the individual is highly practised - my nephew was challenged to cut the tops off two coke cans and join them. He won the bet but it took him more than an hour of tedious spot welding and cooling.

The chosen method depends on the material and the skill level. For brackets and fittings using 3mm and thicker I am finding TIG welding gives a nice result. These parts are not highly stressed so I doubt that fatigue will be a problem in my lifetime. If the joint is properly set up it ends up a nice result.

I do not have the skill level yet to join less than 3mm thick reliably. The objective is to just spot weld initially and then short runs to join it up so it is inevitably slow as you do not want too much heat in one spot causing distortion.

So I feel the best joining method will depend on what you want to construct. It will pay to have the method of joining in mind when you start the design.

Rick W.

 

A cheap rivet gun with the proper die will easily tighten a leaky rivet , at no cost.

We had a Feathercraft "Deluxe Runabout that took quire a pounding with a 40 Merc, rather than the 18 Evinrude ir was built for in 1955.

A quick touch of the gun with a bucking bar behind kept it dry enough to keep on a mooring!

I love repairs that take almost NO time , and NO ca$h, and leave the boat "good as new".

FF

 

You have to distinguish between 'pop'rivits and rivits.

Pop rivits are the ones you use with a gun, I have used those rivits a lot and they work well on the inside of a boat. If overpowered they may stretch and become loose, since they are only a thin allu pipe where a plunger gets pulled into the pipe that stretches the material to form the clamp.

Rivits are solid pieces of ie allu with a head. To 'seal' them in their hole you support the headed side and knock on the other to swell a head that cannot pull through the material. Also, the stem in the hole expands to fill the hole cavity and should produce a watertight fitting. I have used these on some lids on my boat and they exceed my expectations. Not a single one loosened even a bit despite some abuse.

If I have to rivit a boat's hull, this is what I would use. They are more work than the gunned pop rivits, but should be much better. You should also use the correct size rivit for the material thickness and correct spacing between rivits and panel overlap to get optimum strength.

There are welded allu boats, but as mentioned the allu type would be important. One would probably weld the full join length, and overlap with another allu piece that gets welded like a dotted line along the edges to prevent bendages on the weld seam that could cause cracks. Contact an allu boat builder and ask them how what when, just remember to tell us too

 

Some years ago a technique for riveting aluminium skin on planes was developed that did away with the liklihood of solid rivets tearing out of the aluminium.
Instead of drilling the holes to accept the rivets, the holes were "punched" with a tool. This stopped the spreading hairline cracks that come with flexing aluminium.
I havnt seen it used commercially yet, but it was patented. I wonder if it would help with boats?

 

Riveting a hull is great for thin gauge materials, but if you're going to build a bigger boat that is going to live a somewhat violent life it's cheaper to weld. I say that because the process used to rivet the hulls of larger vessels in the pre-1940 era was extremely time consuming and labour intensive. Also, repair was more difficult because neighbouring plates overlapped in a particular way. If you punched a hole in the middle of a plate, you could just patch it, but if you damaged a seam, you would be looking at yanking lots of rivets and replacing the damaged plates or sections of dammaged plate with new pieces. When welded hulls first started to appear, riveted hulls over 25' pretty much disappeared. New build systems evolved to make even more efficient use of welding. Now welding is becoming old fashioned. Why weld when you can glue? No heat distortion, no limit of what materials can be welded to what or how easily (ever tried to weld a pop-can to a piece of 1/2" plate? Or rivet for that matter?)

 

I am very interested in exploring glued aluminum for a home-built skiff.

While I wouldn't use glue in "critical" joints in the first protype, perhaps a skiff could be welded up of heavier material ( 1/8+ ) below the waterline, and for framing, with topsides, seats, decks, airchambers etc. glued, of much lighter material.

However there doesn't seem to be any information on materials and techniques published. A few years ago, I wrote to a major manufacturer of adhesives about this, and they replied "we don't teach people how to build boats."

My limited experiments with West epoxy have had mixed results (I prepped the aluminum by wet sanding, with the material wetted out in resin); 5200 seems to be better, and more flexible.

Any insights, resources, or suggestions would be welcome.

Sal's Dad

 

I have used Loctite products with success.

Surface prep was careful mecahanical clean with fine sanding and acetone wash. Then spray on Loctite 7075 activator on both surfaces followed by a thin bead of Loctite 324 adhesive on one surface. Its sets in a matter of seconds so the compression of the joint had to be done quickly. I would have preferred more time. You need tight dimensional tolerance on the joint because the adhesive spreads rather than clogs. It is quite low viscosity.

With a 20mm overlapping joint on a 0.6mm sheet the aluminium would fail before the joint on the testing I did with fresh joints and up to a few months old.

I used this joint for an underwater application with no mechanical fixing. It did not show any deterioration after 1 year regular dips in in fresh water. However others say the aluminium eventually corrides and the joint fails if used in salt water.

The Loctite adhesive is not cheap but I found it gave a stronger joint than epoxy. THe activator seems to react with the surface.

I have also used a polyurethane glue on aluminium in low load applications and it seems to work well.

If you clean the surface mechanically and then wash with acetone it should be good enough. I would not want to go to sea in a glued boat but should be OK for non critical bits.
Rick W.

 

"I would not want to go to sea in a glued boat but should be OK for non critical bits."


DON'T LEAVE THE GROUND!!! most modern jets have loads of glued surfaces and joints.

FF