Aluminium alloys in the 5xxx series of Al-Mg alloys are readily weldable and have good corrosion resistance in marine environments but, the higher magnesium content means that cold work and ageing can make it sensitive to stress corrosion cracking (SCC).
Alloy 5083 doesn’t require coating for use in marine conditions, provided it is not sensitised to corrosion.
A study was undertaken to assess the performance of both MIG and electron beam welds in alloy 5454, which contains 2.6% magnesium, and in alloy 5083, which contains 4.6% magnesium. The MIG welds used consumables with different magnesium contents while the electron beam welds were autogenous. Some of the material samples were heat treated before all were tested for SCC as u-bend specimens in 3% sodium chloride, to mimic submersion in seawater.
Localised corrosion was found in the MIG welded aluminium alloy grades related to segregation of magnesium to the weld toe. Stress corrosion was found in the sensitised 5454 welds that had filler wire containing over 3.7% magnesium, but not in the heat affected zone (HAZ), sensitised plate or weld metals with less than 2.8% magnesium. Stress corrosion was also not found in the unsensitised specimens. The 5454 alloy also showed weld metal pitting in all of the MIG welds where the magnesium content was more than the plate material‘s 2.6%.
The 5083 alloy used a matching consumable (with 4.8% magnesium). SCC was found to occur in the sensitised MIG weld metal and HAZ, but not in the unsensitised weld metal or in the electron beam weld metal or HAZ.
As a result of these tests, it was determined that sensitised 5083 should not be used in conditions that could lead to stress corrosion, but 5454 could be used if welded autogenously or with a matching filler metal.
Friction stir welding (FSW), invented at TWI in 1991, has seen significant developments in the ensuing years and is now commercially used to weld aluminium alloys in industries including shipbuilding. FSW has revolutionised how ships can now be built from prefabricated modules without corrosion to the weldments in alloys used by the shipping industry (such as 5xxx and 6xxx series).
When these FSW welds in 5xxx and 6xxx series alloys were compared to TIG welds, they were found to perform as well or better. SCC was found in anodically polarised slow strain rate tests, but this was under severe conditions and no such susceptibility was found in u-bend specimens.