Effect of Die Coating on Tearing in Hot Extrusion of 7075 Aluminum Alloy

Abstract

7000 series aluminum alloy is a high-strength alloy and used in a wide variety of products for the purpose of weight reduction in the field of transportation equipment and aerospace. In particularly, A7075 alloy has the highest strength and is expected to be further applied in a wide range of fields such as aircraft parts and sports equipment. However, it has high deformation resistance and is prone to surface defects called tearing. Tearing affects the productivity because it requires to lower the speed. Tearing is likely to occur under high temperature and high speed conditions. It is thought that the localization of melting of Zn-compounds and additive compounds occur due to heat generated during process leading to tearing. In this study, in order to increase the productivity without tearing, die surface quality was considered to prevent the effect of friction and generated heat at interface between tools and material. In addition, recrystallization was also eliminated by reducing friction because temperature does not increase up to the range of recrystallization temperatures. The AlCrN coating was used to improve die surface quality comparing to Nitriding. It was found that the tearing size and heat generated by using AlCrN coating were small. In addition, grain size was small and observed at the tearing region on the extruded surface. It is thought that the die coating can reduce the effect of friction at interface and prevent the recrystallization of the extruded surface. Furthermore, the localization of melting of Zn-compounds and additive compounds are also decreased. From above results, tearing sensitivity can be decreased by using coating to increase productivity speed of A7075 alloy.