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Nowadays thin-walled tube rotary draw bending in small bending ratio is a production process widely used in advanced industries such as aerospace and automotive. Cross section ovality, wall thickness changing during tube bending are the main inevitable defects in this process. The purpose of this research is to obtain the smallest bending ratio and maximum pressure applicable in hydro-rotary draw bending of thin-walled aluminum alloy 8112 tube using failure criterion. For this purpose, the equivalent plastic strain at the critical extrados region used for necking prediction. Concluded results showed that this failure criterion by a maximum difference of 12.5% from experimental tests, is a useful method for predicting the necking onset in the bending process. Moreover, the effects of bending ratio and internal pressure on the defects such as cross section ovality and thickness changing are investigated with simulation in the ABAQUS software and experimental methods. The maximum ovality is not located at the mid-cross section of bent tube unexpectedly and regardless of the internal pressure and bending ratio, occurs at the cross-section with an angle of approximately θ=33°. The minimum achievable amounts of ovality at R/D1.6, R/D1.8 and R/D2 were 11.42%, 7.72% and 4.35% respectively. Furthermore, bending ratio and internal pressure had noticeable effects on the cross section of the bent tubes, so that as the bending ratio or pressure increased, cross-section ovality and the thickening of the tube wall at the intrados decreased, but contrary to bending ratio, as the internal pressure increased, extrados thinning increased.

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Nowadays, thin-walled tube bending (D/t≥20, D-tube diameter and t-tube thickness) in the critical bend ratio (R/D≤2, R bend radius) is a widely used manufacturing process in the aerospace industry, automotive, and other industries. During tube bending, considerable cross-sectional distortion and thickness variation occurs. The thickness increases at the intrados and reduces at the extrados. Also in some cases, when the bend die radius is small, wrinkling occurs at the intrados. In the industry, the mandrel is used to eliminate wrinkling and reduce cross-sectional distortion, which the choice of the mandrel depends on, tube material, bending angle, radius tube and bending radius. However, in the case of a close bend die radius, using the mandrel avoided. Because the mandrel, in addition to the cost of the process, the thinning of the wall increases at the extrados and this is undesirable in the manufacturing operation. So, in the present study regarding to development of tube hydroforming, internal fluid pressure is used instead of the mandrel. Therefore, the purpose of the feasibility study, observation and analysis of the formation of tube bending process, the tube rotary draw bending process with two of the mandrel and the internal fluid pressure is simulated by software ABAQUS.

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One of the forming pipes methods is the rotary draw bending process. Today, bending of thin-walled pipes with low radius of curvature is widely used in the automotive, military and aerospace industries, which is used to bend high-strength pipes. In this paper, at first the necessary models were created to simulate the bending process of the rotary pipe, and then the necessary mechanical and physical properties for stainless steel 304 and elastomers were determined. Then, experimental and numerical study of the forming force and changes in pipe wall thickness were performed. The process simulation was analytically performed using polyurethane elastomeric mandrels and nitrile rubber based on ABAQUS finite element software on 304 steel. The results show a good agreement between simulation and experimental results. Finally, the effects of process parameters including mandrel type, pipe diameter and bending radius were analyzed on the maximum forming force by factorial analysis. The results showed that the maximum forming force for both types of mandrel materials is obtained for pipes with small diameter and high curvature radius. Also, the bending forces increase 5 times by 30%increasing the bending radius, for pipes with smaller diameters. In addition, in equal diameter and radius of bending, the bending forces in the case of using polyurethane mandrel are 25% more than nitrile mandrel.