Source: Copper Development Organization https://www.copper.org/applications/electrical/busbar/bus_table1.html

Source: Aluminum Association https://www.aluminum.org/sites/default/files/aecd13.pdf

Note: Ratings depend upon configuration, air flow, ambient temp, etc. The values depicted are an approximation. Controlled testing is always required to validate.

Other considerations:

Forming the busbar (aluminum has a tendency to crack with very tight radius)

Electroplating the busbar (white rust on aluminum, oxidation is an issue with aluminum)

Configuration of the busbar (vertical or horizontal configuration)