As per your suggestion, instead of solid I have considered the material in the fluid stage. Now when I follow these steps (From models, select multiphase ... then select your model .. VOF, or a mixture ... then define your two phases. Select the materials you created ... and then go to the phase interaction tab .. there you will find three tabs .. the second one is for mass transfer. Increase the number of mass transfer modes to 1. then you can select melting,-solidification.) instead of melting and solidification, I can see the option of evaporation and condensation, which is obvious because of how can liquid melt further, did we miss something?