Hi,
I assume the pipe wall is boundary surface and not cell zone. The materials property will be that of the material you defined in the cell zone of which the pipe wall belongs to. The material you define in the wall boundary condition is only used if you are providing a virtual thickness to the wall.
Heat transfer during solidification and melting processes involves the transfer of heat via conduction and convection within the fluid and solid regions of the model. You can refer to the following section in user manual to get more insights. 16.4. Energy Equation (ansys.com)
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