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Numerical Simulation of Residual Stress during Stamping-forging Forming of 2024 Aluminum Alloy Sheet Metal Da Li, Lei Deng, Xinyun Wang*, Junsong Jin and Juchen Xia State Key Lab of Material Processing and Die&Mould Technology, Huazhong University of Science and Technolog, Wuhan, Hubei, 430074, China *wangxy_hust@163.com Keywords: Residual Stress, Stamping-forging Forming, 2024 Aluminum Alloy, Sheet Metal, Numerical Simulation. Abstract. The unreasonable residual stress field in sheet part has an adverse effect on the dimensional accuracy and performance. A forming method combined stamping and forging was proposed to reduce the residual stress of the sheet part. The residual stress field in 2024 aluminum alloy V-shaped piece after bending and forging was analyzed by the finite element software Abaqus. The results showed that the stamping-forging forming process can significantly reduce the residual stress in round corner of V-shaped piece, and simultaneously decrease springback and improve the dimensional precision of sheet part. Introduction 2024 aluminum alloy is widely used in the manufacture of aerospace vehicle structures because of its excellent mechanical properties and workability [1]. In order to obtain high strength and toughness, the material must be underwent solution and aging treatment, or be plastic formed, after which the residual stress is more or less remained in the alloy. Because the residual stress is in a larger proportion of the yield strength, substantial creep, fatigue failure, stress corrosion cracking and other failure phenomena of part easily occur during use, and seriously affects dimensional accuracy and life of aerospace structures [2]. Therefore, in order to obtain good performance and lightweight aerospace parts, the new forming method of sheet parts with low residual stress is urgently needed to be developed. Several researchers have studied the plastic forming and process parameters of sheet parts and presented some methods to improve the residual stress distribution. R. Greze [3] discussed the effect of temperature during deep drawing on springback from room temperature to 200℃, and found that the increasing temperature tends to decrease the residual stress gradient in the cup wall. M.S. Ragab [4] studied the effect of ironing on the magnitude and distribution of bending residual stress in deep drawn cups, and found that the residual stress can be reduced greatly with small ironing strain. A. Gisario [5] prevented springback by using high power laser to scan the pre-bent aluminium alloy sheet metal, and optimized the residual stress field in the sheet. M. Koc [6] studied two different methods of cold working (compression and stretching) to reduce the residual stress of quenched 7050 aluminium alloy forged block, and found that both methods can reduce the residual stress more than 90%. In recent years, cold stamping-forging forming technology, a new type of sheet metal forming process, is rapidly developed. This technology was first proposed by Japanese researcher Nakano Takashi in the early 1990s [7]. Stamping-forging forming process consists of the characteristics of stamping and forging. It can be used to form the local thickening of complex parts because upsetting, extrusion and other three-dimensional bulk forming processes are integrated into blanking, bending, deep drawing, flanging and other common sheet metal forming processes [8,9]. Based on the deformation characteristics of stamping-forging forming process, a forming method including stamping and forging is proposed to adjust the residual stress field in this paper. The distribution of residual stress in 2024 aluminum alloy V-shaped piece after bending and stamping-forging is analyzed by numerical simulation. And the influence of bending angle and the relative bending radius on residual stress is also discussed. Advanced Materials Research Vols. 602-604 (2013) pp 1903-1909 Online available since 2012/Dec/13 at www.scientific.net ? (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.602-604.1903 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 129.219.96.235, National Cheng Kung University, Tainan, Taiwan-10/01/14,11:48:02) Stress analysis during elastic-plastic bending In the bending process, the bend radius of sheet decreases gradually with the punch be pushed down，while the degree of deformation grows increasingly. The inner and outer edges of sheet firstly reach the yield condition and start to occur plastic deformation. If the punch continues to be pushed down, the fibers yielded in the plate gradually increase from outside to inside. In the round corner of bending deformation zone, the inner and outer layers are a plastic region and the middle layer belongs to the elastic region. That is to say the elastic-plastic deformation occurred in the plate. For the ideal elastic-plastic bended sheet, the bending moment epM is shown in Eq. (1) [10]: 22 ep 1 Mb(2h-) 12 ζσ=. (1) where b is the plate width, h the plate thickness, ζ the thickness of the elastic region, and σ the bending stress. The unloading process is equivalent to imposing a reverse elastic bending moment -epMon the plate, and the residual stress is the superposition of bending stress and elastic unloading stress, which is shown in Fig. 1. (a) (b) (c) Fig.1 Residual stress after elastic-plastic bending in the bending round corner: (a) bending stressσ; (b) elastic unloading stressσ′; (c) residual stressrσ The residual stress can be determined from Eq. (2) [10]： 22 3 (3) 1() 2rs hZ Z h ζζ σσ ??? =?≥?? ?? i 22 3 (3)2 () 2 s rs hZ ZZ h ζσζ σσ ζ ??? =?

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