Why does aluminum expand more than copper

For common materials like many metals and compounds, the thermal expansion coefficient is inversely proportional to the melting point. Copper’s melting point is less than iron’s, so its thermal expansivity is greater.

It expands more because some metals expand more than others due to differences in the forces between the atoms / molecules. In metals such as iron the forces between the atoms are stronger so it is more difficult for the atoms to move around . In brass the forces are a little weaker so the atoms are free to move about more. Same for aluminum and copper. Hope this helps.