Thixoforming is a semi-solid metal forming process. It involves the transient behaviour of semisolids that depends on the solid fraction and the microstructure of the material before forming. Finite element simulations of thixoforming require constitutive equations accounted for this transient behaviour in relation with the material microstructure. In this paper, we propose a model based on a micromechanical approach and using homogenization techniques. It is an extension from a previous model initially developed to capture the isothermal and steady-state semisolid behaviour. We showed that the model describes very well the qualitative transient response highlighted via step-change up and step-change down in shear rate. In particular, the instantaneous response after a rapid change in shear rate is found to be associated with a constant internal structure and to follow a Newtonian behaviour. This result is in good agreement with some literature experimental results but contradicts others which reveal a shear thickening behaviour.