After casting and solidification, Al-cast products undergo heat treatments to achieve the required strengthening. The heat treatments consist of solution heat treatment followed by quenching and artificial ageing. Although fast cooling from the solution heat treatment is needed to get relevant alloying elements (Mg, Si and Cu) in supersaturated solid solution, the gradients of temperature in real products produce internal stresses that can degrade the mechanical properties of the cast piece. A part of these internal stresses can relax during the following ageing treatment when forming the strengthening phases. This work presents the analysis, quantification and modelling of the mechanical response coupled to the microstructure evolution during the relaxation and strengthening processes. Cast specimens of AlSi7Cu0.5Mg are solution heat treated, quenched and aged at 180, 200 and 230°C for different times. The solid solution and aged samples are submitted to a constant strain and the mechanical stresses are measured over 5 hours at different temperatures. Additionally, the samples after solution heat treatment and ageing are analyzed for the different precipitation states using differential scanning calorimetry, hardness and transmission electron microscopy. The alloy exposed at the ageing temperatures experiences: 1) stress relaxation due annihilation of dislocations that were produced during artificial straining, and 2) precipitation strengthening by the formation of Q‘ and β’’ phases. The developed model to simulate stress relaxation and precipitation consists of two parts. Firstly, it is considered that the dislocation movement is the predominant cause of plastic deformation and its relaxation. A physically based mean-field model is developed where specific dislocation reactions are combined with constitutive equations for the stress and take into account the interaction with precipitates. Secondly, all the phases formed during all the heat treatments are simulated using the software MatCalc®. The materials conditions after the casting process are generated with a Scheil-Simulation, while in the solid state all calculations related to primary and secondary precipitates are done using the Classical Nucleation Theory. Finally, the model predicted that the relaxation of the stresses is almost negligible at 180°C of ageing temperature due to the combination of low velocity of dislocations and the pinning effect of the secondary phases.