First the aluminum round bar is machined on a lathe, the corners are removed, the diameter of the round bar will be smaller than the square size, in this case, depending on the aluminum alloy or grade, the aluminum can be cold forged or hot forged on a drawing table for drawing pulling the square shape through a series of steel or tungsten carbide shapes and round dies designed to round those corners it may take multiple draws to make the round shape.
Extrusion in an extruder is possible, but unlikely. Most extruders use round blanks, but square blanks can be upset into rounds in a container and extruded to smaller diameters
In the worst case, remelt it in a crucible and pour it into a round mold, although the mechanical properties (strength) may change.
