If i introduce a huge stainless steel piece in the work coil, it has the same effect, the current Decrease
Later Edit:
You are right, the aluminum changes the resonant frequency, i have increased the frequency to 120Khz and came down and the current increased and the aluminum heated, moderately fast.
The induction heater is small now (working coil, capacitor, transformer...) but the controling circuit is a microcontroler, a PWM circuit some opamps, and analog comparators and the tranzistors are 50amps 600V IGBT.
First i need to understand how it works at low power, and after that i will increase the power up to 4Kw