A numerical solution is presented, capable of handling nucleation, growth and coarsening, and likewise dissolution, of hardening precipitates in Al–Mg–Si alloys during ageing, welding and post weld heat treatment (PWHT). By coupling this to a separate strength model for shearing and bypassing of particles by dislocations, the evolution of the macroscopic yield stress at room temperature can be calculated at each time step. Following testing of the model against reliable hardness and transmission electron microscope (TEM) data obtained from dedicated Gleeble simulation experiments, the connectivity on a micro/macro level throughout a multistage manufacturing route is illustrated by means of a numerical example. These simulations show how a past process step influences the microstructure and strength evolution in the subsequent process steps due to interactions between different groups or classes of particles that form at various temperatures.