Cut 45 degree angle iron
Well, If you were intending to weld them together you have a few choices.


Cut them as close as you can with the chop saw and then grind each joint into the appropriate fit for the 90 degree corner.
Mark with a angle square and cut with a hand hacksaw. Touch up if necessary with minor grinding. 
Use a power hack saw that has an angle clamp to hold the stock accurately and make the cut.


When welding it will be necessary to clamp the parts carefully before applying the arc and rod to start the weld bead. Expect the heating and cooling to make the parts move around some so the clamping has to be solid.

If you were just intending to make nice butted corners and be fastening the angle irons to some other surface such as a work bench edge then you will want to confine the cutting of the ends to the most accurate cut process would be the hand cutting or the power hacksaw.