Most people are familiar with a phenomenon called stress corrosion cracking (SCC). This happens when the wrong filler metal or base material is used in combination with inappropriate environmental conditions.
As we all know, any aluminum alloy with a magnesium content of more than 3% must undergo SCC treatment. This includes most 5XXX alloys, such as 5083 and 5086, which are plates and plate alloys. However, you must remember that almost all common Al 5XXX filler alloys (such as 5356, 5183, and 5556) also contain more than 3% Mg. No matter which aluminum alloy is used for welding, any of them may be susceptible to SCC under certain conditions.
However, the sensitivity to SCC mainly depends on four factors:
1. The alloy must contain at least 3% Mg.
2. There must be stress (the residual stress around the weld is almost always high enough to meet this requirement).
3. Electrolytes must be present, such as salt water.
4. At a certain moment, the weld must have been exposed to a service temperature higher than 150 degrees Fahrenheit for a considerable period of time (hundreds of hours).
Remember, all four conditions do not have to happen at the same time. Once the weld is exposed to high service temperatures, a sensitive microstructure will be formed. Even if the use temperature decreases, the presence of factors 2 and 3 is sufficient to cause SCC.
What is SCC like? It is usually a zigzag crack in the center of the weld. Depending on the stress level, it can be short or long. In fact, the length of the SCC I saw was 24 inches.
Here are three tips to avoid SCC:
If the operating temperature is higher than 150 degrees Fahrenheit, do not use the aforementioned filler metals with higher Mg content. This is true even if you are using base metals that are not subject to SCC restrictions, such as 6XXX alloys (such as 6061). It can still be cracked.
If 5XXX fillers must be used at higher operating temperatures, please use special filler metals such as 5554 with a magnesium content of less than 3% and not subject to SCC. However, their strength is not as high as 5356 and other high-magnesium fillers.
Generally, if you want to weld 6XXX alloys such as 6061 to achieve higher temperatures, 4043 is usually the best choice.
To sum up, no matter whether it is soldered or not, do not use high-magnesium-based materials (such as 5083) at operating temperatures higher than 150 degrees Fahrenheit.
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