The strong metallic bonding in aluminum is due to a large number of delocalized electrons that are shared among the metal atoms, creating a "sea" of electrons that strongly attracts the positively charged metal ions. 

In metallic bonding, the outermost electrons of metal atoms are not bound to any specific atom but are free to move throughout the entire structure. These electrons form a "sea" or "cloud" of negative charge. The positively charged metal ions (cations) are attracted to this "sea" of delocalized electrons, creating a strong electrostatic attraction that holds the metal atoms together.