A friction stir welding (FSW) metalworking model is proposed which partitions flow through distinct deformation zone geometries around the pin probe and beneath the shoulder. These geometries form under the effects of the temperature field, constitutive properties, strain rate and pin tool geometry in these zones. Inadequacies in this flow are related to specific FSW defect types. An initial volume of material equal to the projected area of the pin tool probe is assumed to enter the FSW processing zone. An excess material function is presented in terms of controllable process parameters to describe flowing material added to this initial volume due to temperature and pressure conditions around the pin tool. A forcing function defines the state of applied stresses which partitions the flow through these distinct zones to fill the cavity behind the pin tool. These are equated using the equations of motion for a multi-body dynamic system. Only the concept of a flow-partitioned deformation zone model is presented in this paper. Experimental validation of the model is in progress and the results will be made available in the future.