Chevron cracking during full forward impact extrusion of aluminum alloy EN AW 7075 in dependence of heat treatment condition and tribological system

In addition to the growing demand for energy efficiency, the current and future legal emission limits in the transport sector lead to an increasing demand for weight-reduced components. By substituting conventional materials with weight-reduced components, such as the aluminum alloy EN AW 7075 (AlZn5.5MgCu), emissions during vehicle service life can be reduced. Chevron cracks, also called internal cracks, can be caused by a variety of reasons during cold extrusion processes and pose a great, non-visible challenge for product quality [1]. However, the cause effect relations between the heat treatment condition and the tribological system in regard to damage development have not yet been sufficiently analyzed. In order to examine these relations, the first forming step of a ball pivot was investigated using full forward impact extrusion. Experiments with different tribological systems were investigated by varying lubricant and die treatment with regard to friction and wear reduction. In addition, the effect of the workpiece formability on crack formation was examined by comparing the T6 and soft annealed state of EN AW 7075 as workpiece materials. During this study, the microstructure of both conditions was investigated to highlight differences. A validated FE process simulation using the simulation software Forge NxT 3.2 accompanied the process in order to evaluate the stress state during the full forward impact extrusion process and its relation to the occurrence of chevron crack formation. In the experiments conducted during this study, chevron cracking occurred during full forward impact extrusion of EN AW 7075 in state T6, while in soft annealed condition none appeared. Unlike the heat treatment condition, the tribological system had no impact on the occurrence of chevron cracking but on the severity of the cracks.