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1、Blank holder force (BHF) control in viscous pressure forming (VPF) of sheet metalLeonid B. Shulkina, Ronald A. Posterarob, Mustafa A. Ahmetogluc, Gary L. Kinzeld, Taylan Altanc,*aExxon Production Research Co., PO Box 2189, Houston, TX 77252, USAbExtrudeHone Corp., PO Box 527, Irwin, PA 15642, USAcER
2、C for Net Shape Manufacturing, Ohio State University, Columbus, OH 43210, USAdDepartment of Mechanical Engineering, Ohio State University, Columbus, OH 43210, USAAbstractAn eight-point BHF control system with a flexible blank holder is designed and built as part of an experimental viscous pressure f
3、orming(VPF) machine. This paper describes results of VPF experiments, and addresses several blank holding issues specific to the VPF process.FEM simulations of hydroforming with a multi-point BHF control and an elastic blank holder are conducted to fine-tune the control system as well as to predict
4、the forming loads. The FEM results are compared with experimental VPF results. The viscous medium blow throughphenomenon between the blank holder and the sheet during VPF is described and quantified. # 2000 Elsevier Science S.A. All rights reserved.Keywords: Viscous pressure forming; Blank holder fo
5、rce; Sheet metal1. IntroductionThe quality of sheet metal products is highly dependent upon the rate at which the sheet is drawn into the die. For agiven blank shape and material and final part geometry, it is necessary to optimize the restraining forces applied to the blank 14. For many parts which
6、 are non-symmetric or which have non-uniform material properties, the optimiza- tion of the BHF requires that the pressure distribution on the blank be varied spatially and as a function of time or press ram position. Precise control ofthe BHPmakesit possible to optimize the deep drawing process, wh
7、ich will result in a reduction in the number of parts scrapped. The availabilityof this degree of flexible control will have important appli-cations for use with multi-gage tailor-welded blanks, diffi- cult to form materials, and small production lots.2. Viscous pressure forming (VPF)Viscous pressur
8、e forming (VPF) is a sheet metal forming process developed and patented by ExtrudeHone, Irwin, PA5. VPF offers a potentially simple and versatile approach to soft tooling forming. In its simplest version, it can be thought of as hydroforming where a highly viscousyet flowable semi-solid medium is us
9、ed instead of water. Potential applications of VPF include prototyping and low-to-medium volume production of stretched or drawn sheet metal components, forming of hard-to-form strain sensitive materials, and scratch-free forming of painted or coated sheets 6,7. The VPF project is sponsored by the D
10、efense Advanced Research Projects Agency (DARPA) and participating companies (Pratt however, some wrinkles developed on theflange next to this corner, and a small fracture was observed in the left 908 corner (Fig. 8(c).5.2. Problems observed during preliminary VPF testsPreliminaryVPFtestsshowedsomed
11、iscrepancies betweenthepredictionsofFEMsimulationsandtheforming results and helped usto identify severaldesign shortcomings of the VPF machine. All attempts to form parts with Inconel 718SPF were unsuccessful due to early wrinkling of the blanks which was not observed in the FEM simulations describe
12、d previously. Even though the FEM simulationspredicted that a total BHF ? 4 tons would be sufficient to form the sub-inner panel, severe wrinkling occurred duringFig. 7. BHP distribution recorded by pressure paper: (a) BHF ? 1.25 ton per cylinder, (b) BHF ? 0.5 ton per cylinder, and (c) same as (b)
13、with the top side cylinder disconnected.Fig. 8. VPF of aluminum 6061-O, circles indicate fracture locations for corresponding BHF spatial patterns, numbers show the force of each BHF cylinder in tons.L.B. Shulkin et al./Journal of Materials Processing Technology 98 (2000) 71611the experiments even a
14、t BHF ? 20 tons (capacity of the system).Itwasconcludedthatthe BHF control system could not produce enough BHF for Inconel 718SPF. Therefore, further tests were conducted with materials requiring lower forming pressure and BHF, such as aluminum 6061-O, aluminum 611-T4, and AKDQ steel. The blank geom
15、etry developed by the FEM (Fig. 9,dashed line) was found to be too small since the flange left of the blank holder in the middle of the forming process and the parts experienced severe wrinkling. In Fig. 9, the solid line shows the blank geometry developed experimentally using a trial-and-error proc
16、edure. The blank is somewhat larger than the one developed by the FEM. Using this geometry, aluminum 6061-O blanks were successfully formed with the total BHF ? 8 ton uniformly distributed among all eight cylinders. The same blank geometry was used for further experiments.5.3. VPF tests with constant uniform BHF5.3.1. VPF stages and medium blow through Fig. 10 shows the parts formed from aluminum 6111-T4 blanks with BHF ? 20 ton. The viscous medium pressure was monitored by
