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Aluminum Workshop: A hotshot’s guide to hot short
- By Frank Armao
- December 8, 2014
- Article
- Aluminum Welding
Q: A few months ago, you wrote that 6061 is “hot short.” I’m not familiar with this term. Can you explain it?
A: Hot short is a standard metallurgical term, but I shouldn’t have assumed that everyone would be familiar with it.
If a material is hot short, it means that the material is prone to cracking when it is at a temperature close to its melting point. In welding, this cracking takes place as the weld solidifies and cools.
What causes hot shortness? Well, all metals form compounds with other metals and other elements. Some of these compounds have melting points higher than the metal’s, but many of these compounds have melting points lower than the metal’s. These compounds tend to segregate to the grain boundaries of the metal. As the weld is solidifying, these compounds in the weld and the heat-affected zone (HAZ) are still liquid for a time after the metal is solid. These liquid grain boundaries have no strength, so if you put a stress on them, such as the residual stresses in a weld, they can pop open and form a crack.
Many alloy systems are prone to hot shortness. However, in most alloy systems, the low-melting-point compounds are formed between the metal and residual, or “tramp,” elements, such as sulfur, boron, and lead. In steels, hot shortness is caused by high sulfur levels. Therefore, the tendency for hot shortness can be, and has been, minimized by improved melting practices, which result in very low levels of these tramp elements.
Aluminum is different. It is very reactive and forms low-melting-point compounds with almost all other metals. So in aluminum alloys, hot shortness isn’t caused by tramp elements, it is caused by the alloying elements, such as copper, magnesium, and silicon, that we add intentionally. The tendency for hot shortness changes dramatically with the amount of these alloying elements added. Specifically, aluminum with about 1 percent silicon and 1 percent magnesium added is prone to hot shortness. Unfortunately, this composition describes almost all the 6XXX alloys, and they are all hot short to some degree.
Do you want to prove this to yourself? Just gas tungsten arc weld (GTAW) 6061 without adding filler metal. It will crack every time. How do you get away from this cracking? Use filler metals like 4043 and 5356, which have very different compositions, to modify the chemistry of the weld to one that isn’t hot short.
About the Author
Frank Armao
Aluminum Consulting Inc.
440-479-0239
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