First of all my fusion is a 2018 and still under warranty. The pads are fine. The problem is many groves on the rotors. The front ones are like new. I have been around cars all my life from building complete race cars to the machine shop and welding business. There is such a thing as bad castings which I believe these are. Thanks for your input.
Hi Ed. Well, "first of all"

...I did not say there is no such thing as a "bad casting" or bad rotor. I clearly stated "...defective/'faulty' brake rotors are rare". Of course there is such a thing as "bad castings", but they are also rare. Both are true statements. And all you originally stated was "the rear rotors are bad". You did not tell us what you felt was wrong with the rotors (cracked, warped, heavily grooved?) until now. If you want good ideas and not blind Internet guesses, all of that is important information to know.
And we can not know if your car is still within warranty unless we ask. For example: While I accidentally typed "2017" in my first reply, a 2018 model purchased in mid-2017 to mid-November 2017, would be out of warranty at this point.
And also, keep in mind we can not know how much car knowledge you have, how long you have been around cars or that you have built complete race cars.
You asked for ideas as to what you can do. I'm attempting to tell you. You can ask the Service Manager why the warranty claim is being denied. Ask them why the rear rotors are so heavily grooved at ~19,000 miles when the pads are "fine" and "The front ones are like new". Are they claiming owner negligence? Lack of maintenance? Environmental issues? Are they claiming it was a problem with the calipers? Are they claiming you raced the car?
I am not making any judgements, only asking...
What exactly are they claiming excludes it from warranty coverage?
Something caused "many grooves on the rotor". What do they claim it was? That is where you need to start your case.
If they are claiming it is not a defective rotor, but something else that is not warrantied, you need to find out what they claim it was. Then you need to convince them that the rotor was defective in material or workmanship or that whatever the other claimed root-cause is covered under warranty. So you need to put your knowledge from many years of being around cars and building complete race cars to convince them.
In addition, if you can not get any satisfaction from the Service Department/Service Manager at your Dealership, you can call Ford Customer Relations and plead your case to them.
That is what you can do.
If other members here have more advice to offer, I am sure they will jump in to help.
Let us know how you make out and good luck.