A computational design-of-experiments study of hemming processes for automotive aluminium alloys

Hemming is a three-step sheet-folding process utilized in the production of automotive closures. It has a critical impact on the performance and perceived quality of assembled vehicles. Using a two-dimensional finite element model, this paper presents a design-of-experiments (DOE) study of the relationships between important hemming process parameters and hem quality for aluminium alloy AA 6111-T4PD flat surface-straight edge hemming. The quality measures include roll-in/roll-out of the hem edge as well as the maximum true strain on the exposed bent surface. The finite element (FE) model combines explicit and implicit procedures in simulating the three forming subprocesses (flanging, pre-hemming, and final hemming) along with the corresponding springback (unloading). The results show that the pre-hemming die angle and the flanging die radius have the greatest influence on hem edge roll-in/roll-out, while pre-strain and the flanging die radius impact the maximum surface strain significantly. The computational DOE results also provide the basis for process parameter selection to avoid hem surface cracking and particular insights for achieving acceptable formability.