Accurate modelling of the thermal history in 3D printing technologies such as Directed Energy Deposition and Laser Powder Bed Fusion (LPBF) is essential for stress and warpage prediction since the main driver of the mechanical problem are the thermal gradients. The thermal model is usually formulated in the fixed frame of reference of the laboratory, as this only requires the solution of a diffusion equation. However, the small support of the heat source relative to its speed and the part's length scale result in a highly transient problem, especially in LPBF. A high fidelity simulation requires a prohibitively small timestep and the use of many CPU hours (Patil2021). Alternatively, posing the thermal problem in the reference frame of the laser eliminates the time dependency of the heat source. On the one hand, for a constant scanning speed and a long track length the system quickly reaches steady state, allowing for the use of big time steps in the numerical simulation (VanElsen2007). On the other hand, this change of coordinate system introduces new challenges such as the application of boundary conditions on moving surfaces and the treatment of the additional convection term. In this work, both reference frames are considered: the thermal problem around the melt pool is posed in the laser reference frame and the laboratory frame is considered in order to impose boundary conditions on the fixed surfaces. The proposed scheme attempts to exploit the advantages of both. REFERENCES Patil2021: Patil, N., Ganeriwala, R., Solberg, J. M., Hodge, N. E., & Ferencz, R. M. (2021). Bench- mark multi-layer simulations for residual stresses and deformation in small additively manufactured metal parts. In Additive Manufacturing (Vol. 45, p. 102015). Elsevier BV. https://doi.org/10.1016/j.addma.2021.102015 VanElsen2007: Van Elsen, M., Baelmans, M., Mercelis, P., & Kruth, J.-P. (2007). Solutions for modelling moving heat sources in a semi-infinite medium and applications to laser material processing. In International Journal of Heat and Mass Transfer (Vol. 50, Issues 23–24, pp. 4872–4882). Elsevier BV. https://doi.org/10.1016/j.ijheatmasstransfer.2007.02.044