This page is far from a comprehensive tutorial; it’s just some tips and some links to more authoritative information. It’s intended for people who want to learn to weld aluminum, but have little no experience in welding aluminum, or even in welding in general.

This was a description of me a couple of months ago. In my attempts to learn to weld aluminum, I gathered all the information I could find from a lot of different sources – the very simplistic and under-informative manual that came with the welder, lots of reading on the web, basic welding books with very short sections on aluminum, and very advanced books that were written for engineers which had more equations and formulas than practical welding advice.

Then using what I had learned, coupled with a lot of trial and error, I eventually figured out how to get two pieces of aluminum to stick together without cracking, warping, shriveling, or breaking. Along the way I made several key discoveries that would have saved me a lot of trial and error time if someone had just told me about them. I thought I’d share the little I do know and maybe it’ll help someone out there learn to weld aluminum faster than they would have otherwise.

What equipment do you need?

A TIG welder

A TIG (GTAW) welder. Most sources say a TIG (Tungsten Inert Gas) welder, also called a GTAW (Gas Tungsten Arc Welder), is the best method of welding aluminum. I’ve heard aluminum can also be welded with a MIG welder or a stick welder or even a with a gas torch.

Since I’ve only used the TIG for aluminum, that’s what I’ll be writing about. TIG welders are fairly expensive and it’s hard to justify buying even the lowest quality units unless you are making money with your welding. The more expensive units ($6000) have a bunch of features that make doing high quality welding on aluminum possible.

We have a bottom-of-the-line ($2500) Hobart welder that is described as good for the hobbyist or farmer. As tempting as it was to blame the machine while I was making charred bits of twisted metal instead of neatly welded joints, I came to realize that adequate welds can be made, even with a cheapo machine.

What do you get when you spend the extra money on a welder? More amperage (meaning the ability to weld thicker metal), water cooling (I don’t know what advantage this provides, but the gas hood glows orange on our air cooled unit when it’s at maximum output, and it’s only 165A), square wave AC (this allow grinding a ceriated tungsten to a point for a more stable arc), frequency adjustment, and adjustment of the ratio of positive to negative current for better cleaning or penetration. Since my machine has none of these features, I can’t offer any advice on how to make use of them.

PPE

Good welding gloves. I have crappy welding gloves and the painful blisters to prove it.
A good welding helmet. I hear the gold tinted auto darkening helmets are the best. I have a $20 helmet with a tiny window that falls off my head when I flip it down.

Argon

Argon gas. Mixes will not work for aluminum with the exception of an Argon / Helium mix. Don’t take the tank from you MIG welder to use on your TIG welder – it won’t work at all. You will just make a bunch of burnt metal and soot.

Aluminum welding rod

Aluminum welding rod. I got the 4043, which seems to be the most recommended. There is a good chart at http://www.tinmantech.com on which rod to use for which alloys as well as a ton of excellent metalworking and aluminum welding information. At this point I don’t have any idea how to tell one alloy from another, and I’m not doing any mission critical welding, so don’t worry about it. The 4043 has been working well for me.

A stainless steel brush

A dedicated stainless steel brush that you only use for aluminum. Write “aluminum” on it so it doesn’t get used for anything else.

A place to weld

A metal bench would be nice. I don’t have one. Stopping your weld to put out a fire is a pain in the ass. This happens to me all the time.

A plant sprayer

A squirt bottle with water. This is not for cooling the work, it’s for putting out small fires that aren’t big enough to use a fire extinguisher on. Cooling aluminum rapidly may cause it to crack in or near the weld.

A fire extiguisher

A fire extinguisher might not be a bad idea if you don’t want to get fired for burning down the shop.

A long sleeve shirt

It’s VERY important: a heavy long sleeve cotton work shirt. TIG welding produces more UV radiation than any other welding process. The first time I used the TIG I was wearing a tee shirt. I used the welder for 10 min if even that. I was painfully burned with blisters and peeling skin on my biceps and neck. I just takes a few minutes to do some serious burning.

Work holding

Clamps or Vise Grips or whatever your going to use to hold your work in place and some blocks or bars of aluminum or copper to use as heat sinks.

That’s enough of the shopping list. On with the useful tips

Tips

Tip #1 — Clean the aluminum

This is the most important tip I have. I read this in several places before I began to practice welding, but it didn’t seem so sink in and I wasted a lot of metal by trying to weld two pieces of dirty aluminum together. ALUMINUM THAT LOOKS BRAND NEW AND CLEAN IS ACTUALLY DIRTY. IT’S NOT LIKE STEEL.

A wandering arc -You can’t get a puddle started without burning through or distorting the metal
Your filler won’t blend into the puddle, instead it rolls into a difficult to re-melt ball.
The aluminum seems to have surface tension, like beads of water on a waxed surface.
When trying to join 2 pieces the edges curl away from each other and form an even bigger gap.

Here’s why

Here’s what’s happening: Aluminum quickly forms an more or less invisible coating of aluminum oxide. Aluminum oxide melts at three times the temperature of aluminum. When you try to weld uncleaned aluminum, the aluminum under the aluminum oxide coating will melt but the aluminum oxide coating will stay solid and act as a membrane, much like a water balloon. When you finally succeed in penetrating the coating, the very runny aluminum inside will flow out all at once, much like a bursting water balloon.

First, I spray the aluminum with acetone. Don’t use brake cleaner unless it’s 100% acetone. If you need convincing, check this out — Brewer Racing Frames. Thanks for the tip, Andrew!
Then, I rinse the aluminum in water, just in case there’s any nasty residue. The aluminum should be completely dry before welding, including all of the nooks and crannies.
Then, I use a stainless-steel brush (make sure the brush is stainless, I’ve read this is important) to scrub the aluminum shiny clean around the area to be welded.

Some articles I’ve read suggest that the aluminum should be scrubbed in one direction only to avoid working contaminants into the aluminum. I don’t always follow this rule and I haven’t noticed any problems stemming from it, but I’m not working on anything too critical and I’m far from an expert. I’ve also read that 3M Scotchbrite pads are a good way to prep aluminum. If you do not weld on the aluminum immediately after cleaning, you should give it a touch up cleaning before you start to weld. I’ve read eight hours exposure after cleaning is the maximum acceptable without re-cleaning.

Tip #2 — Clamp your work to a heatsink

A heat sink made of copper or aluminum whenever possible. Aluminum transmits heat very well. Once the area you trying to weld gets hot enough to melt, the rest of the work is likely to be so hot that it’s shrinking and warping. Using a heat sink under the area being welded will absorb some of the heat and help keep the work from warping.

Tip #3 — Preheat before welding

This makes it a LOT easier to weld aluminum. This is not a subject that is without controversy. The issue is that some aluminum is heat treated, and by heating and cooling heat treated aluminum it will get softer.

I’ve read opinions ranging from “heat treated aluminum should never be preheated” and “preheating is a crutch for inexperienced welders”, to the opposite extreme, “aluminum should always be preheated to prevent cracking”.

Recommended preheating temperatures range from 275 deg. F, to 500 deg. F. I suspect that many of these opinions are correct in their own contexts. The proper procedure likely varies for a welding a space shuttle door in a vacuum chamber and welding a cracked cylinder head.

One thing I know for sure is welding thicker pieces of aluminum with our 165A welder without preheating is impossible. I once tried to weld two pieces of 8 mm thick aluminum together without preheating. The result were a very shallow and weak weld, a circuit breaker that tripped twice and the welder overheated and shut down after every two inches welded.

I don’t have an oven handy, so I use a propane torch aimed at the heat sink I clamp the work to and an infrared thermometer to tell when it’s hot enough. I usually can’t get the work any hotter than 350 deg., so that’s the temperature I use. I’ve considered getting a cheap used electric oven or an electric hot plate but haven’t yet. I don’t use the torch directly on the work. I don’t know if it would cause a problem or not, I feel more comfortable heating the heat sink instead.

Tip #4 — If the tungsten gets contaminated, stop welding and fix it

When the tungsten gets touches the weld pool or the filler, the arc becomes unstable and the weld quality goes way down. This happens to me a lot, unfortunately. The best method for fixing this is to remove the tungsten, lay it on a flat surface with the contaminated part hanging over the edge, hit the contaminated part of the tungsten (it will snap right off), reinstall the tungsten, change the polarity to DCEP (direct current electrode positive), strike an arc on some scrap metal to re-ball the tungsten, switch back to AC high, and you’re ready to weld again. By the way, KEEP YOUR GLOVES ON WHILE YOU DO THIS! Otherwise you gonna have a nasty burn. This only takes about 30 seconds once you’ve done it a few times.

Tip #5 — Fit the parts together as tightly as possible

Leave no gaps, or as close to no gaps as you can manage. When using a MIG welder, I’ve found it fairly easy to fill gaps between the two pieces of steel being welded. However, I’ve found it very hard to do this while welding aluminum with the TIG. The heat from the TIG is very localized.

When there is a gap, and heat is applied to the metal on one side, the metal pools on that side, but metal on the other side stays solid. You can alternate from side to side to get both sides to pool, but when I do this, I usually end up with a wider gap than when I started.

The only way I’ve found to fill a gap is to “slop” a glob of filler across the gap, then work on the filler until it melts into both sides, then build off of the filler. This is hard to do. You can save a lot of time by using a file or die grinder and making the part fit together better before you start welding. The tighter the pieces are pressed together and the fewer the gaps, the easier the welding is.

Tip #6 — A few “rules of thumb”

Use one amp per .001″ of material thickness. Set the amperage a higher than the maximum you expect to use and use the foot pedal to back it down.
Use pure tungsten for aluminum if you have a cheaper (non-square wave) welder like me.
Use a 2% ceriated tungsten ground to a point if you have a more expensive (square wave) welder
Use a 1/16″ pure tungsten for 30 to 80 amps
Use a 3/32″ pure tungsten for 60 to 130 amps
Use a 1/8″ pure tungsten for 100 to 180 amps
Use 15 to 20 CFH Argon flow
Use a filler rod size equal to the tungsten size. Adjust the tungsten to project from the hood a distance roughly equal to the diameter of the tungsten.
The arc length should be roughly equal to the diameter of the tungsten.

The first 3 tips were my major breakthroughs while learning to weld. They were the things I wished somebody had told me before I began my frustrating experimentation. There are, of course, many other things to know about welding aluminum, but there are many more complete articles available on the Internet. Follow the links below for some of my favorites.

Here are some great resources I’ve found:

http://www.tinmantech.com This guy does some amazing stuff and has written a bunch of excellent articles. You can read for hours and hours on this site; there is a lot of content. He has an oxy / acetylene bias, but his site is fascinating.