In this type of underfill defect, we would observe that the part produced by the normal forging techniques is unable to reach to the sharp corners or side edges of the die cavity either due to lack of fluidity in material of work piece i.e. flow characteristics or
due to pre-mature solidification of the heated billet as they loose their plastic behaviour in their journey to such narrow and cooled portions present in the die. Preventing the geometrical accuracy of the final work piece to be created. Thus forming undulated surfaces as shown in the simulation provided.
So we need to keep the check on the material physical properties and also the initial temperature of the billet before being forged sufficiently in case when we need to deform the part to good extent.