money spent, lessons learned and small fires set while building Bearhawk fuel tanks
HiYa!
So... I'm building this airplane
It's a Bearhawk, designed by Bob Barrows
The airplane has 4 fuel tanks, two in each wing
They are built of aluminum, and, well... they can't leak... because it's fuel and it's in a wing that is above you and leaks up there would be bad
Everyone tells me I need to weld aluminum with a MIG welding rig
That's DC electricity covered with an inert gas to keep the molten aluminum from oxidizing
I'm certain that it works really well because that's what everyone uses
Way back in the corner of my mind I think I remember something about welding aluminum with an oxy/acetylene torch
Searching the Internet I run across a book called "How to weld damn near anything" by Jesse James
I've seen Monster Garage and I figure if Jesse James can do what he does, then I can do it too
There isn't too much about welding aluminum with an oxy/acetylene torch
Jesse says it can be done
And frankly, I really dislike welding with electricity
When you do electrical welding you gotta cover up... everything
You see... welding with electricity throws off ultraviolet light that will give you a nasty sun burn
So, you gotta cover up, everything
And you have to use really thick gloves
So... it's hot in the Summer in my garage
Wearing all that stuff, long pants, long sleeve shirts and face guard helmet makes it ever so much hotter
With oxy/acetylene all you gotta cover is your eyes... that and stop from time to time to make sure nothing in the garage is on fire
So I did more research
Welding is different than soldering
Soldering (like silver solder) involves melting a different metal than you are putting together
Best I can tell there are two things in soldering
1) The solder has to melt below the melting point of the metal you are soldering
2) The solder must stick to the metal you are soldering
The problem with soldering is that you are using a metal that is different from the metal that you are putting together to hold stuff together
Rarely (read this as never) will you find a metal that meets criteria 1 & 2 and has the same strength characteristics as the metal you are hooking together
And aluminum is really picky
If the aluminum is tempered then heating it will kill the temper
This is why you don't do this process on any tempered aluminum
For the bearhawk (and possibly many other airplanes) you use a special "weldable" aluminum
I'm using 5052H-32 (not sure what the H-32 means here but it is billed as "weldable aluminum for fuel tanks"
Anyway, I imagine that there is some "aluminum solder" that is available but I didn't use that
There are a variety of problems with welding aluminum.
Listed below are 4 or 5 problems:
OK, so cleanliness is a good idea in general but for aluminum there are several different meanings to "clean"
Meaning #1: Remove all oxidized aluminum from the surface
Yep, sitting around the shop the surface of the aluminum will oxidize (I'm sure that there is a way to keep this from happening, like that clear plastic film that the aluminum comes covered in from your supplier, but once you start working on a fuel tank that stuff has to go and then, well, you get oxidation
There is an water-based relatively mild acid that you can use on aluminum that will work really well
The stuff I buy is called (NAME OF THE STUFF HERE) which is (KIND OF ACID HERE) that you can apply with a brush, wait a few minutes, and wash with clean water
But, there is a problem
You run the risk of solving one problem and creating another
Meaning #2: Keep other stuff from contaminating the scene
Using this acid wash to remove oxidation you will need a way to apply the acid, right?
If you use a brush and there is anything to "snag" on(for instance, a cut edge), it will snag on it
You probably won't be able to see the result (a bit of brush material stuck in a tiny, sharp, hangnail-like snag along the edge) with your naked eye
BUT you will be able to see it when you start welding
It will evidence itself as you heat the part by becoming a tiny, little, glowing, pesky, hole-making, difficult to repair or fill in, ember of carbonized brush
I figured this out about 6 months into welding aluminum
So, next time you use something else, like a paper towel (REALLY bad idea, leaves tiny bits of junk EVERYWHERE)
Or a cloth, equally bad
I suppose you could "spray" it on with one of those pump-sprayer-mister things like household cleaners come in but, think about it, you are turning loose a fine, breathable cloud of corrosive mist... maybe not the best idea?
The acid wash works really well if you can avoid the "contamination introduced while removing contamination" problem
I reserve that to polished aluminum parts
Meaning #3: Don't introduce contamination while shaping parts.
Here's the problem: you are fitting two edges together and you want them to be a nice match.
How do you trim them so that they fit nicely? Well, if you sand them using, say, an orbital sander you will crud-up the surface with stuff off the abrasive paper like the grit used by the manufacturer, the abrasive used to hold the grit onto the backing material or the backing material itself. Sometimes actually mashing the grit right into the rather soft aluminum. Filing the edges with a hand file produces an un-oxydized aluminum surface ready for welding but it leaves thousands of tiny sharp edges just waiting for something to get close enough to snag on and produce one of those pesky embers.
About 9 months in I discovered that using an air-powered angle die grinder with a metal filing bit worked like a dream. Didn't leave much of an edge like happens with an abrasive wheel on an angle grinder. Seems that an abrasive wheel on an angle grinder does nearly as much melting and shoving of aluminum as it does removing aluminum. The edge kinda rolls over and when you get through, a .050 inch edge has a 1/4 inch "hangnail" of aluminum that was either pushed over or melted and shoved over. And the abrasive wheel can tend to deposit grit and/or adhesive that holds the grit together once again causing those pesky little embers.
You need something to keep the atmospheric oxygen from oxidizing the molten aluminum. Official MIG welders use a continuous bath of some inert gas to keep the oxygen away. I suppose you could do the welding in a vacuum chamber but that's above my pay grade and is best reserved for working with more valuable stuff like titanium.
Instead of working in a vacuum or flooding the weld with shielding inert gas you can use a compound that is a solid at normal temperature, a molten liquid at just below the melting temperature of aluminum and easily removed after you are finished. Oh... and it can't chemically combine with the aluminum while it is melted. This miracle compound is called "flux".
Aside from that, its simple... right? RIGHT!
At first I tried some special aluminum welding rods that had the flux that was inside the center of the rod. They were expensive, they were too large (about 3/16 of an inch) for the kind of work I was planning to do and I didn't have enough control.
I found a compound that my local welding shop would order for me. Harris Al-Braze 1070, aluminum powdered brazing flux. It comes with a massive list of health and safety warnings about all the unknown long-term effects of breathing the vapors.
A side note: These days EVERYTHING has warning labels. Coffee cups have warnings that coffee is hot, every bridge on any road has a sign warning about how bridges freeze before before roadways... even in Florida. My point is this: it's like the boy who cried "wolf" too many times. At some point the warnings become so lengthy and voluminous that you just start ignoring them all together because taking the time to read them would take all of your available time so your choice is to ignore them or do nothing with your life but read all those warning labels.
So... I bought the Harris Al-Braze 1070 and broke down and bought a gas mask for volatiles from home depot. I also have some Soviet surplus nuclear/biological gas masks but they completely cover your face and make it hard to use the special welding goggles you need to use when using the flux. I use the Soviet stuff when I am doing the brakes on my roadster.
Anyway, the Harris Al-Braze 1070 is relatively inexpensive (about $10 for 8 ounces) and when mixed with denatured alcohol into a thin paste it goes a long way. I apply the flux to the parts to be welded and the welding rods just before I do the welding. Notice I mentioned welding rods... this is something that I'll cover next... what I use as a welding rod.
what I use for a welding rod
Rather than spending a lot of time looking for welding rods of different sizes and aluminum composition I decided to see what I could do with the aluminum welding wire that you buy at home depot that is intended for use in a MIG welder. I bought a roll and started playing around. A single strand was too thin to handle so I started using a drill to twist a number of strands together. I tried 2 and 4. As it turns out, for the kind of welding I am doing two wires twisted together with a drill and then cut to ~24 inch sections works really well.
I twist the wire by first anchoring one end (I use a vise that is in my garage), feeding out about 20 feet, wrap it once around a 10 pound sledge hammer that I set out in my driveway and then feeding out enough wire to get me back to the vice. I secure both ends in the vise. I made a hook that I can put in my drill out of a 16 d nail with the head cut off. At the end that is out in my driveway I remove that end and attach it to the hook. Pulling the wire taught I run the drill and twist the wires together. The welding wire breaks easily so it takes a while to get a feel for it. The tighter the twisting the better but it works pretty well even if it isn't. After a while you'll get the hang of it. It's also important to keep the right tension on the wire because of you don't the twisted wire will not be straight making it harder to use during welding.
If you remember the "you've got to keep it clean" discussion above then you will be able to guess that you also have to keep these twisted rods free of lint and other things that fall from the air in a shop. I wash my newly created welding rods after I cut them to length and then I store them in a section of 4 inch sewer line with end caps to keep the dust out.
Flux and flustration
So... now you know about the flux and you know about the welding rods I use. The obvious next questions involves how to apply the flux. You want flux on the parts to be welded and on the welding rod. I've tried all manner of things including brushes and dipping to get the flux on the part and on the rods. What finally worked out works really well for me. Here's what I do: I wear those disposable mechanic's gloves (the ones that are immune to petrochemicals) when I weld (much nicer than using those giant, bulky gloves that you need for using an electric welder). I apply the flux to the part by simply sticking my finger in the flux paste and wiping it on the pieces I want to weld.
I've got a bucket of soapy water available when I weld that I use to rinse the flux off my hands before I pick up the torch to weld.
The basic problem is that the flux is REALLY corrosive. And it attracts water out of the air so it makes a corrosion double kill by keeping its corrosive self moist in order to promote corrosion on everything. Which reminds me... let's say you have done some welding and you messed up a bit of it so you want to grind it down. Wash the flux off FIRST! If you don't then the process of grinding sends this corrosive cloud of vapor all over the shop settling on everything and, you guessed it, corroding all your cool metal-based stuff.
After you finish welding, let the metal cool. When it does, the flux will form a hard crust that is difficult to remove right after you finish. The trick is to have patience. If you can leave it out and go do something else until the next day it'll just rinse off by pouring water on it. If you are in a hurry, alcohol and a stainless steel brush works ok but it does take some elbow grease.
Iron is a polite metal. As you heat up iron it starts out black, then turns red and then as it gets hotter and hotter it gets brighter and brighter... orange then yellow then really light yellow. Now that's nice. It gives you clear and strong signals about how hot it is getting.
Aluminum, on the other hand, does none of that polite stuff. Start heating it up and it just stands there and stares at you. Kinda like you aren't even there. It stands there, nice and aluminum colored all the way up to the point where it melts (BAM!) and drops out onto the floor. The afore-mentioned flux is your only saving grace... well the flux gives and it takes away. It gives: the flux melts just before the aluminum becomes molten so it acts like a sign, it also takes away: when you heat it up it glows BRIGHT YELLOW hiding the aluminum in a wall of yellow and making it impossible to see and therefore impossible to weld. You can't see the welding rod, you can't see the aluminum parts... you can't see ANYTHING!
What you need to solve this problem is to get the right kind of goggles/glasses/safetyGlasses. I bought a pair that are designed specifically to filter out the yellow glow of the flux. I hesitated to buy them because the folks that sold them wanted $70+ for a single pair. They look like they ought to cost, maybe $15 on the outside. I drug around for a couple of weeks and then decided I had to make the dive. I did, and I will NEVER go back! They work like a charm. I still think they are priced like BMW factory parts but, well, they do work.
So, you may ask yourself, why do I like to use this nasty, dangerous, view-blocking flux? Well, it's inexpensive, it's available locally and I can make it work reasonably well for me.
Image 4 found on the "Bearhawk fuel tank construction illustrations" page (below) are illustrations of how this can cause problems.
Also covered on the "Bearhawk fuel tank construction illustrations" page (below).
Bearhawk fuel tank construction illustrations
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jimrowanisdr.jimrowan@gmail.com