Artem Komarov noted that hot cracking is an incredibly common form of weld defect found in aluminum welds. Almost all defects you may encounter when welding aluminum are in the form of cracks in the weld. Why? Because pure aluminum cannot be welded.
Pure aluminum is too soft for commercial use, so it is alloyed with other elements such as copper, silicon, magnesium, and magnesium silicide to make it workable. These different elements have different melting points — all except magnesium have a higher melting point than aluminum. (Aluminum melts at about 1221 degrees Fahrenheit, which is quite low).
Hot cracking occurs after metal, heated to a liquid form, solidifies. For those interested, you can find a useful table online breaking down welding sensitivity ranges. In principle, every aluminum alloy has a solidification cracking curve. For each possible alloy element added to aluminum, the chart shows what the likelihood of cracking is based on the percentage of that alloy added. This is called a peak.
The crack sensitivity of silicon, magnesium, and magnesium silicide peaks at approximately 1%. When copper is the alloying element, sensitivity peaks at about 3%.
Armed with this knowledge, you can begin to develop an aluminum welding strategy that will help you avoid cracking as much as possible.
First look at the composition of your base metal. If the percentage of alloying elements is below peak sensitivity, everything should be fine. Just make sure you use filler wire with a composition that doesn’t push the overall consistency toward the peak.
However, if you are welding parts with a compound that gives them maximum sensitivity, you can use filler wire to your advantage. Wire with a composition very different from the parts being welded can cause the percentage of alloy in the weld pool to deviate from the maximum.
Joint design can also be a factor in causing or minimizing hot cracking. For example, butt welding two pieces with square edges won’t give you much room to add filler. In this situation, you have little room to dilute the alloy percentage. However, giving the edges a large bevel will allow you to add more filler. This filler changes the percentage of alloying elements in the weld pool, reducing maximum sensitivity. This will help you avoid hot cracking, added Artem Komarov.