A void evolution model accounting for stress triaxiality, Lode parameter and effective strain for hot metal forming

Large numbers of voids inevitably exist in the metal ingot due to non-equilibrium solidification. Forging is an important technique to eliminate such internal defects and obtain sound products. However, the research regarding void evolution mechanisms are currently not sufficient to direct the technological design in practical engineering because of the lack of suitable void evolution model, especially in terms of complex stress states. On the basis of a large number of numerical computations, the effect of Lode parameter on the void volume evolution rate is firstly investigated, and then incorporated together with the stress triaxiality T, effective strain E-c and Norton exponent n into a newly proposed void evolution model. At the scale of representative volume element (RVE), the prediction of void evolution calculated from the new model has achieved better accuracy than other analytical models comparing with the simulation results. Radial forging test for rectangular cross-section billet is carried out to verify the accuracy of the proposed void evolution model. Paired screw threads are used to introduce pre-fabricated voids and then embedded into the billet for measuring void volume variation and the distribution of strain around void during forging. The comparison of void evolution between the experimental and analytical solutions also shows that the prediction of void relative volume change by the proposed model is more accurate than those by other models.