How to Best Join Aluminum: Fasteners, Brazing, Soldering or Welding?

Photography Credit: Weldscientist

Anyone who’s ever had a track car has probably done some aluminum fabrication. Whether that was a simple hand-formed bracket or a more complex structure, aluminum’s low cost, ease of availability in big-box stores, high strength-to-weight ratio and mostly benign nature make it the perfect material for even the novice fabricator.

But when it comes to joining two pieces of aluminum, what’s the best method? The answer is that there probably isn’t a single answer, but a range of options that vary based on the material, available time and skill of the fabricator. So the following–while not a complete list–will run through some of the more common options you’ll likely have access to in your home shop, and the pros and cons of each. 

Self-Tapping Screws

Let’s face it, if it weren’t for self-tapping screws, there’d probably be no amateur motorsports hobby. Self-tappers are the zip ties and duct tape of the hardware world, and their low-tech effectiveness has gotten us out of many a jam.

But their simplicity can also be their weakness. The threads of self-tapping screws leverage themselves against the work material with a fairly coarse pitch, so in thinner materials each thread doesn’t get much purchase. For attaching thin materials to thicker materials, they work better. 

We’ll also put regular sheet-metal screws with a separately drilled pilot hole under this heading, but really, who has time to grab two different drills when your screw can just make a hole all by itself?

Pros: Simple, one-operation fastener. Cheap and plentiful. Strong shear resistance. Can be used to attach disparate materials. Can be used as a “blind” fastener (where you have no access to the back of the fastener). Easily removable.

Cons: Strength is highly dependent on material thickness, and it’s not huge in the best of circumstances. For maximum joint strength, thickest material must be away from screw head. Not rotation resistant, and rotation can cause loosening. Creates sharp, pokey thing on opposite side of joint. Removal is possible, but each application weakens the hole a bit. Hole needs to be the right size.

Best for: Emergency repairs, non-mission-critical or low-strength-requirement fabrications, temporary fastening during assembly, when you’ve just given up.

Aluminum Rivets

Rivets are the classy person’s self-tapper, and far more appropriate–and better-looking–for permanent constructions. When used with a backing washer, the rivet places the load of the joint on the face of the material, rather than relying on the bending force of the material to hold the threads as with a sheet-metal screw.

And those backing washers are pretty important. Rivets can be used as “blind” fasteners for attaching workpieces that can only be accessed on one side, but their very nature as ductile, deformable objects makes them inherently susceptible to eventual loosening under load. 

And while rivets are generally fast and easy to use, employing best practices is especially important to get the strongest joint possible. Choosing the proper rivet length for the connection is vital. Rule of thumb: Rivet length should be equal to the total hole depth (including backup washer if employed), plus 1.5 times the rivet diameter. Not that we’d ever recommend using the wrong hardware for a project, but if you’re ever in a jam, slightly too long is better than slightly too short.

Also, when fastening materials of differing hardness, the strongest joint will be achieved when the head (tool end) of the rivet is placed on the softer of the two materials. 

Pros: Excellent strength, finished appearance, can be installed blind, readily available. Expands to fit the hole.

Cons: Requires multiple operations (drill, align, rivet). Highly variable joint strength depending on application (material thickness, washer use, etc.). Allows rotation. Can compress and distort soft materials. More complex removal, but removal generally doesn’t affect the workpiece.

Best for: Lightweight fabrications that don’t need to be disassembled to be serviced. Joints that will experience lot of vibration. (Threaded fasteners tend to loosen under vibrational stresses; rivets are more vibration resistant but don’t deal with shock loads as well as threaded fasteners.)

Bolted Connections

A bolted joint with a threaded fastener and a nut is the ultimate non-welded connection for aluminum–or any other machinable material. Generally, a properly applied bolted connection is as strong or stronger than the material around it when used in sheet or thin plate applications. Properly sized and constructed bolts have excellent tension and shear resistance and are easily removable without lasting effect to the workpiece.

But bolted joints have their downsides. First, the joint must be physically configured so a bolted joint is possible. Typically, this means two flat, parallel surfaces on either side of the joint with good contact between them. For butt joints or anything lacking flat mating surfaces, bolts usually aren’t the answer.

Plus, all threaded fasteners are susceptible to vibrational loosening. This can be combatted with chemical-thread lockers and vibration-resistant washers like Nord-Locks, but it’s still a consideration in high-vibration environments.

Pros: Strong, simple, readily available hardware. Highly calculable strength. Removable. Joint tension can be controlled with high accuracy.

Cons: Requires access to both sides of joint. Lots of weight compared to screws or rivets. Susceptible to vibration, especially if hole hasn’t been reamed to correct size.

Best for: Make bolts and nuts your first choice for any mechanical fastener, then assess whether they’re really what you need or can use in each situation. 

Brazing and Soldering

Brazing and soldering are similar processes. Both join metals via media that melts and adheres to adjacent surfaces before solidifying–all without melting the base metals. The distinction is mostly one of temperature.

Soldering is essentially a low-temperature form of brazing. It typically uses a filler material that melts below 450 degrees, while brazing uses a material that melts above that temperature. In the case of aluminum, most brazing material melts around 800-900 degrees, which is still a few hundred degrees short of aluminum’s melting point.

Right off you can see the advantages of brazing: It’s a non-destructive process that keeps intact the bits that it joins. Think of it as metallic hot glue, and you’re on the right track.

Brazing also allows dissimilar metals to be joined. Need to stick copper to aluminum? Aluminum to steel? Steel to tungsten? All are possible with brazing, and chances are you have a piece of cookware, a bicycle or a firearm that employs one or more such connections.

Experts will swear up and down that a properly brazed joint is just as strong as the base metals it’s joining, but the word “properly” is doing a lot of heavy lifting in that sentence. Proper brazing–especially with aluminum–is an incredibly finicky process that requires surgical cleanliness, zero-tolerance fitting, and saintly patience to achieve those strong-as-the-base-metal joints. Not infrequently you’ll end up with a blob of brazing rod that just falls off the joint, cursing at the late-night TV ad for making it look so easy.

Photography Credit: Cyfac

Pros: Able to join dissimilar metals. Requires no tools more specialized than a torch. Inexpensive materials. Strong joints if you do it right.

Cons: Strong joints only if you do it right–much harder than it looks on TV. Requires meticulous surface prep and precision to form best joints.

Best for: Non-safety-related fabrications with lots of overlap on the joints, tubing connections, panel bonding, thin materials where welding could be too destructive to the workpiece.

MIG Welding

One of the trickier aspects of welding aluminum is its thermal properties. Unlike steel, which covers a broad temperature range as it goes from red hot to soft to liquid to super-runny liquid melting through your shoe, aluminum does all of that within a much more compressed temperature window. Aluminum also absorbs and subsequently sheds heat much faster than steel, meaning the physical properties of the piece you’re working with actually change during the welding process.

Nowhere is this more noticeable than during MIG welding. As with steel, MIG welding aluminum employs a filler wire that’s fed into the joint while also grounding a lot of electricity, creating a very hot arc that produces a molten puddle in the workpiece. This molten puddle is moved along the joint while being fed by the filler wire to form a welded joint that’s strong yet, thanks to the fast rate of cooling, also brittle–so not best for situations that see a lot of vibrations.

Unlike with steel, aluminum MIG welding requires pure argon rather than an argon/CO2 blend, and the softer aluminum wire is usually fed to the welding torch via a spool gun, so the wire doesn’t have to push as far. But aside from that, the basics of steel and aluminum MIG welding are similar.

Except they aren’t. As we mentioned before, aluminum’s thermal profile can make MIG welding frustrating at times. Because it melts so much faster and more aggressively than steel, your weld beads typically need to move much faster than with steel. So fast, in some cases, that you really can’t see into the molten puddle very well. So, you need to line up your weld, strike your arc, and move through it with great precision–mostly on muscle memory, since you can’t really eyeball your way through it like you can with slower-melting steel.

Then there’s the fact that aluminum absorbs heat very rapidly during the process, meaning your weld pass not only has to be quick, it has to accelerate. Otherwise, the end of the weld will be far too hot compared to the beginning.

MIG welding can be frustrating, but once you get the hang of it, it’s fun and rewarding. And best of all, it can be done with low-cost MIG machines. Eastwood’s MIG 175 can be ordered with a spool gun for around $500. Switch wires and gas, and you’re welding steel in a few minutes.

Photograph Courtesy USAF/Elizabeth Baker

Pros: Low cost relative to other welding methods. Very learnable if you know how to MIG steel. Fast. A true molecular connection.

Cons: Welding aluminum of dissimilar thicknesses can be capital-T tricky. Not the prettiest welds in the world, even for skilled operators. Generally unforgiving relative to steel MIG welding. Brittle joint.

Best for: Beginner aluminum welders who can adapt their steel-welding skills to the new material.

TIG Welding

When we think of welded aluminum joints, most of the time we picture TIG welds. That stack of perfectly aligned dimes (or coins of various denominations, depending on your skill level) at the intersection of two pieces of metal is an iconic image, and for good reason: Proper TIG joints are strong and durable, and even crappy TIG joints are pretty darn good. A bumbling novice with a TIG can probably make a stronger joint than a bumbling novice with a MIG, although we don’t recommend entrusting those joints with anyone’s safety.

Like MIG welding, TIG welding melts the area at the joint and then allows it to reform after flowing a melting filler into it. But unlike MIG, TIG gives you far more control over the amount and application point of the heat going into the joint. With TIG, you strike an arc from the tungsten-tipped torch to the workpieces, then control the amperage–and therefore the heat–of that arc in real time during the welding process. So, joining materials of differing thicknesses becomes much easier, as does working with thin materials that would just be obliterated by a MIG welder.

If it sounds like a complex process, that’s because it is. TIG welding requires the operator to maintain the arc with the torch in one hand, apply filler rod with the other, and control the power with a foot pedal. There’s a lot going on, but the upside is that you get to control the pace fairly precisely. 

Photograph Courtesy: USAF/Aaon Jenne

Pros: Despite its complexity, TIG is fairly novice-friendly, mostly because it works in an intuitive way and is very slow-paced. Makes strong, malleable, good-looking joints. Able to join materials of dissimilar thicknesses.

Cons: More expensive than MIG, although prices are coming down and good TIG machines can be had for less than $1000. Requires a lot of consumables. Slow, painstaking process. No matter how good you think you are, there are hundreds of people on Instagram way better than you.

Best for: Projects requiring a high degree of precision. People with patience.

Like what you're reading? We rely on your financial support. For as little as $3, you can support Grassroots Motorsports by becoming a Patron today. 

Join Free Join our community to easily find more Shop Work, aluminum and Welding articles.
Comments
View comments on the GRM forums
Larry
Larry New Reader
12/1/20 9:19 a.m.

What about bonding? That is the preferred method of most manufacturers now. And it is being done by many body shops.

As heat weakens tempered Al and the bonded surface spreads out loads, the joints tend to be stronger. I would love to see more about DIY/grassroots bonding applications. Comparing things like Hysol /Loctite E-60HP and 3M 7333, ease of prep, application, strength, etc.

captdownshift (Forum Supporter)
captdownshift (Forum Supporter) UltimaDork
12/1/20 9:36 a.m.

Fun fact, most hvac duct work is aluminum. Duct tape is clearly the answer. 

jimbbski
jimbbski SuperDork
12/1/20 11:07 a.m.
Larry said:

What about bonding? That is the preferred method of most manufacturers now. And it is being done by many body shops.

As heat weakens tempered Al and the bonded surface spreads out loads, the joints tend to be stronger. I would love to see more about DIY/grassroots bonding applications. Comparing things like Hysol /Loctite E-60HP and 3M 7333, ease of prep, application, strength, etc.

This can work for some joints but not all.  You need surface-to-surface contact to have a strong bond and many times you don't have that. When designing  a part this can be taken into consideration but for one off parts you're making and if weight is a consideration I'd go with a welded joint.

wae
wae UberDork
12/1/20 11:32 a.m.

For a project I just completed, I had a need to bond aluminum roofing flashing to steel angle iron.  There is no space on the inside for a fastener or rivet to stick out so I needed something flush.  In retrospect, I could have drilled and tapped the steel and then used a very short machine screw, but that idea came late.  I used a Loctite construction adhesive on one (PL-375), DAP construction adhesive (Dynagrip MAX) on two, and just regular silicone on one.  They all worked pretty well, but due to the long cure times, clamping is required.  Where I couldn't clamp things properly, I got some pull-back from the springiness of the flashing, but other than that it is pretty solid.  It isn't structural, and I haven't tested it to see exactly how much force it would take to pull the skin off, but it worked pretty well and was quite cheap.

Shaun
Shaun Dork
12/1/20 12:38 p.m.

Really really good double sided tape and rivets has worked for me.  

Larry
Larry New Reader
12/1/20 1:00 p.m.

In reply to wae :

This brings up a good point. For many of us, we will be adding aluminum to a steel structure or composites to an aluminum structure to reduce weight. Welding will not work there, and bonding is stronger than mechanical fasteners. 

While it will not work well for some joints, for joints with the needed surface area, bonding ends up being stronger and lighter than fill rod or solder (spot welds do not add filler, but are not as strong). Panel bonding is very common place now, but structural applications keep growing. 

BA5
BA5 Reader
12/1/20 1:48 p.m.

Bonding with rivets works well, the two methods pick up the slack in the weakness of the other: bonding helps distribute the load over a larger area than rivets generally are able to and can resist loosening over time.  The rivets can help resist peeling and delamination, which is the weakness of a bonded joint.

Djtilly
Djtilly New Reader
12/1/20 4:06 p.m.

You missed solid rivets - The mainstay of old school monocoque racing cars and aircraft.  Biggest con is, of course, that you need access to both sides of the joint.

Leif_In_Calif
Leif_In_Calif New Reader
12/1/20 9:34 p.m.

Also forgot gas welding...the way all those cool cars in the 50's were built. It takes a little practice but the equipment isn't expensive.

frenchyd
frenchyd PowerDork
12/1/20 10:27 p.m.
BA5 said:

Bonding with rivets works well, the two methods pick up the slack in the weakness of the other: bonding helps distribute the load over a larger area than rivets generally are able to and can resist loosening over time.  The rivets can help resist peeling and delamination, which is the weakness of a bonded joint.

3M makes a wonderful bond with the side of rivets. The problem is I use Cleco's as an alignment tool but doing so means some seepage invariably gets on the Cleco's. I haven't found anything that gets the 3M stuff off so it's use em & toss em'.  Expensive. 

I've for non racing applications used mostly pop rivets and a pneumatic riveter to go fast enough to  avoid letting the bond set up before finishing. 
Since some aircraft applications allow "pop-rivets" maybe I shouldn't be so picky.  

Leif_In_Calif
Leif_In_Calif New Reader
12/2/20 10:34 a.m.

Also, this really wasn't fair to brazing. Many British racing cars were brazed in the 50's and 60's because the thin wall tubing used couldn't take the heat of welding. In fact, if you watch the video of the continuation XKSS cars being made today, some frame parts are brazed. Is it idiot proof? Not at all, but the people on this site have skill! 

AJOR
AJOR
12/3/20 10:41 p.m.

In reply to Leif_In_Calif :

Following on from Leif_In_California; Oxy/Fuel Welding of Aluminium goes back over 100 years and was the commonly used technique prior to World War 1. GTAW / TIG Welding came into use early during the Second World War. (1942 I believe). I  look forward to further discussion on this subject with any one on this site who is interested.

triumph7
triumph7 Reader
12/4/20 9:01 p.m.

In reply to frenchyd :

Typically aircraft "pop" rivets are what are called Cherry Max rivets.  They are aluminum (yes, I know there are others) with a stainless pin that stays inside the pulled rivet which allows it to be used as a structural fastener.

pirate
pirate HalfDork
12/5/20 9:50 a.m.
triumph7 said:

In reply to frenchyd :

Typically aircraft "pop" rivets are what are called Cherry Max rivets.  They are aluminum (yes, I know there are others) with a stainless pin that stays inside the pulled rivet which allows it to be used as a structural fastener.

All rivets are not created equal with the type you would find at the big box stores on the lowest rung and the  Cherry Max at the top. There are numerous style heads, and patterns as to how the back of the rivet forms when pulled. The advantage of the more expensive type rivet is the stem is always retained and seals as opposed to cheaper pop rivets where the stem may fall out after being pulled. A real advantage of the stem being retained is the shear strength it adds to the joint. If you go to Aircraft Spruce, Wicks, (there are others) they usually have a section that explains the different types of rivets.

if you use the proper length and diameter rivet the joint can be incredibly strong. An adhesive can make it even stronger. After laying out, drilling holes I deburr both pieces of the metals to be joined put it together with clecos to check fit then clean surfaces with acetone before applying the adhesive. I often times use a bead of Silicone RTV to make the joint waterproof and stronger especially if I think I might ever want to take the two pieces apart in the future. I use a pneumatic rivet puller to insure all rivets are pulled tight and it saves on the arms and hands if you have a lot to pull.  I actually like the way rivets look if done well.

frenchyd
frenchyd PowerDork
12/5/20 11:17 a.m.
triumph7 said:

In reply to frenchyd :

Typically aircraft "pop" rivets are what are called Cherry Max rivets.  They are aluminum (yes, I know there are others) with a stainless pin that stays inside the pulled rivet which allows it to be used as a structural fastener.

Yes I know that but you do bring up a good points. I also have the adjustable counter bore needed to set them flush with the skin. And  have the rated pneumatic riveter required to meet NAS standards. Well it's calibration has long since expired so I can't use it on airplanes legally. 

sevenracer
sevenracer Reader
12/15/20 11:28 a.m.

Ok, so real world application.  I need to re-connect a bracket to an intercooler.  Appears to have been both riveted and glued from the factory (although not totally sure how it was bonded, the material looks like very crystaline aluminum).

 

I am thinking this is a pretty straightforward fix.  Drill out the rivets and replace, plus use something to bond it too.  As long as I don't drill past the intercooler fins and into the bar, I'm golden, right?  Any reason not to use steel rivets vs aluminum?

Our Preferred Partners
YBGHR06I82l9iVc8lZMqC3SaZlKPXZsYJYjTYKBPHPIknfeTAsfOl5dBMyz2MAZB