Investigation of end cracking and its prevention by a double reduction die in the cold extrusion of automotive parts

End cracking failures occur randomly during cold extrusion of automotive shafts or sometimes even during service. Finite element analysis of this failure shows that cracking is primarily due to a combination of damage during cold deformation and tensile unloading at the exit of the die land. This paper proposes a failure mechanism and a stress-based failure criterion for this cracking. It analyses the original single die design and investigates a double reduction die design, which was patented for the extrusion of brittle materials. It is seen that while the double reduction die reduces end cracking, it increases the probability of chevron cracks at the part centre. Finally, a design of experiments technique is used for determining the optimum die design parameters for the minimum tensile stress state both at the surface and at the billet centre. Production validation experiments confirm the efficacy of the optimal die design in suppressing the initiation of end cracks without the need for special billet shearing or annealing procedures.