Why can't copper and aluminum wires be joined together

Cu-al potential is different, cu-Al contact part will accelerate the oxidation of the aluminum wire due to the galvanic cell reaction, long time cu-Al contact will be bad.

Compared with copper, aluminum is more active. The connection of two kinds of cables will accelerate aluminum oxidation (that is, rust) and affect its use. When copper and aluminum conductor are directly connected, the contact surface of the two metals is easy to form electrolyte under the action of moisture in the air, carbon dioxide and other impurities, thus forming a galvanic cell with aluminum as the negative pole and copper as the positive pole, so that aluminum produces electrochemical corrosion, resulting in the copper and aluminum connection of the contact resistance increased.