Induction heater requirements
I had commented, "Talk to an induction heating manufacturer to help you with your project. The combined electrical and thermal design is not trivial. It wouldn't surprise me if you need 50 kW and 50 kHz induction heater".  

Here is further information just to show that the thermal and mechanical considerations are a big part of the design.  

The brass is about, "0.5” thick and a 5” diameter."  Also, the copper is about 0.079 wall thickness.   Both parts need to be heated to about 1200 degrees. So, you will need to apply about 5 times the power in the brass as you will apply to the copper. This will require a special induction coil or coils. 

Since copper and brass both have high thermal conductivity, as you apply heat to the pipes, the heat will be rapidly being removed from the heat zone. The heat conducting down the length of both pipes. Thus, you need to apply heat at a very high rate to over come the heat loss due to conduction away from the heat zone.  

EDIT1 : Using your 48 volts, you will need 1050 amps for 50 kW. 

EDIT2 : There is a minimum power level required which must be greater than the amount of heat flowing away from the work zone. This is a thermal calculation. Additional power then is required to heat the material in the work zone to 1200 degrees. Additional power is then required to offset the induction power supply losses.  

The wattage required has nothing to do with the resistivity or resistance of the workpiece. 

I came up with 50 kW ESTIMATE using my years of experience and observations.