KR118: Warning: materials in neighbor cell threads (3 and 2) of interior zone 19 are of different types (aluminum and water-liquid). This problem MUST be fixed before solving!
In FLUENT, a Wall/Wall-shadow pair must separate solid/fluid regions. Between fluid/fluid regions, we can have an INTERIOR zone type.
It is also possible to convert a solid zone to a fluid and vice versa.
The trick is that you can always turn a solid into a fluid, because the wall/wall-shadows will already exist. After you turn the solid into a fluid, you can then change the Wall/wall-shadow into an INTERIOR (Note: the wall-shadow will disappear when you do this).
On the contrary, if you have a fluid region that you want to turn into a solid, you must FIRST turn the INTERIOR zone separating the regions into a wall/wall-shadow pair. This is done by changing INTERIOR into WALL. The shadow will be created automatically. Then you can change the Fluid zone into a SOLID zone.
The warning message indicates which zones (fluid/solid) you are interested in converting, and the INTERIOR zone which must be converted into a wall