Extruded aluminum and cast aluminum use different alloys. Melting down old castings will give you better results than melting extrusions. If you don't provide enough riser (which seems to be the case with that ingot you made), then the resulting billet will be porous. Flux is necessary; degassing helps a lot. Bad results in turning are probably due to lack of heat treatment; raw aluminum tends to be extremely gummy, and doesn't machine well at all. But if all you want is round stock for turning, try to find some drops of factory-made 6061 t6 round rod; it's going to work better than anything you cast and try to heat-treat yourself, and it's not really that expensive, considering.