The paper focuses on thermal analysis of scan welding, a new joining technique redesigned to ensure the composite morphologic, material and mechanical integrity of the weld. This is obtained by real-time control of the temperature field by a proper dynamic heat input distribution on the weld surface. This distribution is implemented in scan welding by a single torch, sweeping the joint surface by a controlled reciprocating motion, and power adjusted by feedback of infrared temperature data in-process. A numerical simulation and an analytical multivariable model of the thermal field in scan welding is established, using real-time identification of its parameters. An adaptive thermal control scheme is thus implemented and validated computationally and experimentally on a robotic Gas-Tungsten Arc Welding setup. The resulting thermal features of scan welding are analyzed in terms of material structure and properties of the weldment.