Tearing mechanism for high-strength Al-Zn-Mg-Cu alloys in hot extrusion

In hot extrusion, tearing plays an important role on surface quality of profiles. In this study, extrudability of 7075 aluminum alloy without tearing was quantified by extrusion limit diagram, which is a relationship between temperatures and speeds. Tearing appearance and distribution of elements were investigated to assess tearing mechanism. According to the diagram, tearing easily occurrs at high temperature and high speed conditions. Micro-solid bridge was observed at tearing surface to indicate the localization of melting of soluble Al ₂ CuMg and MgZn ₂ compounds because temperature increased by friction coupled with tension stress at a die region. During hot extrusion, recrystallization affects the grain growth, intermetallic compounds were formed at grain boundaries reducing bonding strength between grains promoting tearing sensitivity. Zinc and magnesium concentration is high at the vicinity of grain boundaries because of diffusion into aluminum-matrix whereas insoluble Al ₇ Cu ₂ Fe particles were large and remained at grain boundaries resulting in tearing. Grain refinement can decrease a size of insoluble compounds and promote scattering of soluble compounds. Therefore, to reduce tearing sensitivity, grain refinement has to be controlled during hot extrusion process by controlling chemical composition of billet and friction at bearing region. This causes temperature to increases and thus, induce recrystallization.