Hi Christoph,
In Workbench Engineering Data, it is possible to define temperature dependent Thermal conductivty.
In your case, the Thermal conductivity is changing after metling and solidifies. So after solidfication when body reaches to room temperature it should still use thermal conductivity of solid material. For this you can define two material models. Inbetween the loadsteps(transition from powder to melt), you can use MPCHG command to change the material of solid elements.
If you are using Workbench Mechanical, you can insert command object under analysis setting.
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