Welding aluminum is very different when compared to welding steel, both in technique and in results.
Welding aluminum generally will weaken it, and for a few different reasons. The main reason is that heat will affect the temper of the aluminum, which can result in a loss in yield strength of roughly half in many cases if measures aren’t taken to correct it.
Actually, pretty well all metals will weaken in some way when welded, including steel. To really get the full strength, you’ll need to take some additional steps.
In this post, I’ll share some examples of how welding can affect the strength of aluminum and some best practices to keep your welds as strong as possible.
How Much Welding Aluminum Decreases Its Strength
For the sake of argument, I made the table below based on info from ESAB (the world’s largest producer of welding equipment) that spells out exactly how much strength you can lose in aluminum from welding it.
In this table, I’m listing the minimum yield strengths. This is the minimum allowable point at which the metal will start to fatigue and permanently deform when you pull on it. Also, the type of weld being compared is a groove weld (when you weld it end to end from either side and properly bevel the edges before welding).
Essentially, this is a worst-case scenario, where you’re pulling on a weld joint and you have no mechanical connection.
|Grade||Yield of Base Alloy||Yield As-Welded||Loss in Yield|
|3003-H34||29 ksi||7 ksi||76%|
|5052-H34||24 ksi||12 ksi||50%|
|6061-T6||40 ksi||18 ksi||55%|
As you can see, the loss in yield strength overall is pretty significant.
Importance of Filler Material
This is a critical element to strong aluminum welds, and it’s up to you to control.
For each grade of aluminum, you need to have a filler that will maximize the strength of your weld. It’s actually the filler that will help your welds to stay (relatively) strong.
It’s pretty well always a bad idea to not use filler when you’re TIG welding aluminum. Without the right filler, your welds will be super weak and extremely prone to cracking.
The filler rod needs to be able to handle aluminum’s tendency to crack at the weld as it cools down (hot cracking). The chemistry is determined by the base material,
Aluminum Temper and Why it’s So Important
One of the main factors to consider when it comes to welding aluminum is whether or not it’s heat treatable.
Why is this important?
It all comes down to the Heat-Affected Zone (HAZ). Heat-treatable alloys will be partially annealed (softened) in the HAZ, whereas non-heat treatable alloys will be fully annealed in the HAZ. In other words, non-heat treatable alloys are more susceptible to being lower strength.
For the sake of simplicity, here’s a little table of the common grades of aluminum, so if you know the grade then you’ll know if it’s affected by heat treating.
|Grade Series||Heat Treatable|
|4xxx||No, with some exceptions|
In other words, if you have a piece of 6061 aluminum that you’re welding, it’s heat treatable. If you’re working with 5052 aluminum, it is not heat treatable.
Can Aluminum be Heat Treated to Restore Weld Strength?
The direct answer to this is, technically yes, if it’s a heat treatable grade.
However, the filler that you’re using may not be heat treatable. Also, heat treating aluminum is not as easy as heat treating steel.
For one, the heat treating needs to be done in a controlled furnace. It’s not like steel, where you can develop an eye for the colors and do it with a torch. Aluminum doesn’t glow, and the soak, ramp times, and temperatures are particularly fussy.
Also, aluminum will sag quite a bit during the heat treating and artificial aging, making this generally impractical for weldments. It’s also really easy to overage aluminum if you don’t have experience with this.
So, technically possible, but generally completely impractical.
Most grades of aluminum get a lot of their strength through something called strain hardening.
It’s pretty much exactly what it sounds like. The aluminum is strained, and it gets stiffer/harder.
Generally this is done right at the mill. As the aluminum is rolled as a plate or bar while it’s “cold”, and this aligns the grain structure and works the metal so that it strengthens.
This is a topic worth the read all on its own, but not as it relates to the topic of welding. You generally can’t strain harden weldments simply due to their geometry. In other words, the strain hardening is lost when it’s heated to welding temperatures and you don’t get it back.
Typical Aluminum Welding Issues
Here are some common problems with aluminum welding, along with an explanation of why you get them. Not exhaustive, but some of the common ones.
Cracks down the center of the weld.
This is really common. The issue is normally related to solidification cracking. In other words, as the weld pool solidifies, little cracks start to form. These are often below the surface and are hard to see initially, but are apparent when your weld splits down the seam. Try pushing more filler into the pool, and make sure that you have the right kind of filler for the grade you’re welding.
Cracks around the HAZ.
This could be due to liquation cracking. This is when you get little fractures just outside your bead that can spread all over. You might see this more often when MIG welding thicker plates. If you see this, you probably have the wrong wire alloy.
Cracking that starts at the end of your weld.
This is really common. It’s probably due to something called crater cracking, and it happens because aluminum cools off so quickly that the puddle will sink in (like a crater) and this will cause a ton of stress in the weld. The best way to handle this with a TIG welder is to slowly let off your foot pedal and add an extra two or three drops of filler. If anything, you want your weld to be a little more convex at the end.