Due to the simultaneous effects of different physical phenomena that occur on different length and time scales, modelling the fusion and heat affected microstructure of an Additive Manufacturing (AM) process requires more than intelligent meshing schemes to make simulations feasible. The purpose of this research was to develop an efficient high quality and high precision thermal model in wire + arc additive manufacturing process. To quantify the influence of the process parameters and materials on the entire welding process, a 3D transient non-linear finite element model to simulate multi-layer deposition of cast IN-738LC alloy onto SAE-AISI 1524 Carbon Steel Substrates was developed. Temperature-dependent physical properties and the effect of natural and forced convection were included in the model. A moving heat source was applied over the top surface of the specimen during a period of time that depends on the welding speed. The effect of multi-layer deposition on the prediction and validation of melting pool shape and thermal cycles was also investigated. The heat loss produced by convection and radiation in the AM layers surfaces were included into the finite element analysis. As the AM layers itself act as extended surfaces (fins), it was found that the heat extraction is quite significant. The developed thermal model is quite accurate to predict thermal cycles and weld zones profiles. A firm foundation for modelling thermal transport in wire + arc additive manufacturing process it was established.