Aluminum can be obtained by mining special nodes which spawn in SalvageFields when Salvage nodes split after being hammered.
Note: You will normally get a larger Aluminum node if it spawns from the first time a Salvage node splits. The amount you can expect to gather per Salvage field is dependent on a lot of variables we do not know, so the amount of Aluminum you may get per field can be highly variable.
Aluminum Nodes can be mined with Hammers, Sledge Hammers, or Harvesters. Similar to Iron, the amount of Aluminum in a field is determined randomly as the Salvage Field respawns.