When we go across the metal series from scandium there will be small change in the 1st ionization energy and the 2nd ionization energy, it happens because electrons build up occurs  immediately in d-sub-shells due to which  the 4s electrons are shields in the nucleus and which minimize the effective nuclear charge from one element to other . There is a rapid increase in 3rd and 4rth ionization energy,because of unpaired electrons of 5d subshell, in singly occupied orbitals.and due to this the inter electron repulsion occurs  When the 6th and the further electrons enter in the shell due to which it has to share its orbital which is already occupied, and there is a less requirement of energy to remove  electron.