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.

The aluminum alloys belonging to the 7000 series are high-strength alloys used in a wide variety of products for weight reduction. They are primarily used in the field of transportation and aerospace. Among these, the A7075 alloy has the highest strength and is expected to be applicable in a wide range of fields, such as aircraft components and sports equipment. However, it has high deformation resistance and is prone to surface defects, which is called tearing. Tearing typically occurs at high temperatures and high ram speeds, and adversely affects productivity. The localized melting of Zn and additive compounds, due to the heat generated during the process, is considered to cause tearing. In this study, the effect of friction, heat, and tearing at the tool—metal interface was mitigated by improving the die surface quality. The reduced friction eliminated recrystallization by preventing the temperature from increasing to recrystallization temperature. In addition, an AlCrN coating was adopted instead of nitriding to improve the die surface quality. The tearing size and heat generated when using the AlCrN coating were found to be limited. Moreover, the grain size observed in the tearing region on the extruded surface was small. The simulations using the shear friction coefficient m observed from friction tests indicate that the use of the AlCrN coating improved the material flow. Thus, the AlCrN coating is considered effective for reducing friction at the interface and preventing the recrystallization of the extruded surface. From the aforementioned results, it can be inferred that a die coating can reduce the tearing sensitivity and increase the productivity of the A7075 alloy.

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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.

This article investigated the tribological performance of the specially formulated chlorine-free lubricant in strip drawing of advanced high strength steel. Four different lubrication conditions (dry, chlorine-free lubricant, chlorine additive lubricant, and mineral base lubricant) at two sliding speeds (10 and 100 mm/min) were carried out to observe the friction coefficients of the die-workpiece interface in the strip drawing test. The main difference among these lubricants was the contents of chlorine and sulfur additives. The die and workpiece materials were SKD11 and JSH780R, respectively. The results showed that the combination of chlorine and sulfur additives provided the best tribological behaviors. In addition, only the small amount of sulfur content could establish a bond with metal surfaces. However, the higher sulfur content could interact with metal surfaces, because it was influenced by the increased temperature (higher sliding speed) and adsorption.