are you using a reducing flame to melt your metal?? Start with that. When open pouring and melting alloys if you have to much oxygen in your flame you will get oxides and porosity.
a reducing flame is just a bit gas rich. This is assuming that you are using acetylene when melting and not electricity.
Also don’t skimp on the flux. You need enough borax in your crucible to be able to see it flowing a bit across the molten metal. This is assuming your using a ceramic crucible.
Also try making your alloy before using it in a casting. Alloy the metals and turn them into a round or bar ingot then role it out into thin sheet. This way you are not over heating the metal in the casting crucible while trying to mix it. It will also reduce coring (when metals do not alloy evenly producing an inconsistent malleability over the casting)
when the metal starts to flow you need to cast as quickly as you can, spending to much time waiting for metals to alloy together will cause issues.
Are you using a vacuum assist or centrifuge.