Aluminum is not easy to weld, but it can be done if you know what you’re doing. At Wiley we know a few things about welding, so if you don’t mind we’re going to show off a little.
Understand the Problem
Welds made in aluminum experience two types of cracking. These are hot cracks and stress cracks. We’ll cover stress cracks first and then move on to hot cracks.
Stress cracks in aluminum are caused by cooling and contraction. (Unlike stress cracking in steel welds hydrogen isn’t a factor as there’s no hydrogen in aluminum.)
Aluminum has both good thermal conductivity and a high expansion coefficient. Apply a gas torch and heat spreads out quickly, making the metal expand as it flows.
In welding the pieces being joined are both melted locally, and usually some filler wire is added. The metal solidifies as the torch moves away, leaving what were separate pieces fused together. Aluminum melts, (and also freezes,) at 1,221°F, so after the weld is fixed the pieces continue cooling and contracting. That puts the metal under tension, which can lead to cracks.
Hot cracks are also a result of solidification, but they have a different mechanism and happen at a higher temperature. Hot cracking is mainly a function of base material and filler material chemistry. It’s also much more common than stress cracking.
The issue is that aluminum used in fabrication is not pure. Instead, it’s alloyed with other elements like copper, silicon, magnesium and magnesium silicide.
Three of these melt at much higher temperatures than aluminum, the exception being magnesium with a melting point of 1,202°F. The result is that, rather than transitioning quickly from liquid to solid, aluminum alloys go through a mushy phase as they cool.
The width of this mushy temperature range, (technically, it’s the “coherence range”,) is determined by the proportions of alloying materials used. The wider it is the more hot cracking the weld will suffer.
Stress cracking can be reduced or even eliminated by:
- Pre-heating the pieces being joined so they expand uniformly.
- Putting less heat into the weld. That can be as simple as going faster, or it might mean adopting more sophisticated techniques like pulsing.
Preventing hot cracking entails looking at material chemistry, and to a lesser extent, weld geometry.
Every aluminum alloy has a solidification cracking curve. For each possible alloying element, this shows how the probability of cracking is driven by the percentage added.
What’s really interesting, for anyone who wants to weld aluminum anyway, is that the relationship is not linear. For silicon, magnesium and magnesium silicide crack sensitivity peaks at around 1% and then declines. When copper is the alloying element sensitivity peaks at about 3%.
Armed with this knowledge, a strategy for welding aluminum starts to emerge. First, look at the composition. If the proportions of alloying elements are away from that for peak sensitivity you should be okay. Just be sure to use a filler wire with a composition that doesn’t shift the overall mix towards the peak.
If however you want to weld pieces where the composition puts them close to peak sensitivity you can use filler wire to your advantage. By picking one with a composition very different to the pieces you’re welding you can shift the alloy percentages in the weld pool away from the peak.
This is where weld geometry can help. If you’re butt welding two square-edge pieces you won’t be adding much filler. In that situation you’ve little scope for diluting the alloy percentages. However, give the edges a generous bevel and you’ll be adding much more filler. Then you can use the filler to change the percentages of alloying elements in the weld pool, and so move away from peak sensitivity.
Not Easy, but Possible
Some people will tell you you can’t weld aluminum. Well as we’ve explained here, that’s not correct. It’s difficult because aluminum welds are prone to cracking, but with a little knowledge of the underlying science it’s possible to find workarounds. Trust us on this, we know what we’re doing.