This article addresses visualization of the thermally generated distributions of material structure and properties in the manufactured products, and provides a methodology for offline design of the process conditions in virtual source thermal techniques. These are implemented by timesharing a single heat source, scanning the external surface of the processed part to generate a flexible heat input distribution, and thus a specified temperature field yielding the desired thermal quality of the product. A numerical simulation of generic thermal processing is developed, integrating a solid conduction description to a flow model in molten regions. This computational model is validated by a comparative thermal study of classical and a novel scan welding process, in which the torch reciprocates along the joint to provide uniform thermal features. Virtual source processing is currently simulated for other thermal manufacturing processes and their real-time control with infrared thermal feedback.