Simulation of Recrystallisation and Grain Size Evolution in Hot Metal Forming

Transcription

1 Simultion of Recrystllistion nd Grin Size Evolution in Hot Metl Forming Nikoly Bib, Alexnder Borowikow b, Doris Wehge QuntorForm Ltd., 0 Church Rod, Worcester, WR3 8NX, United Kingdom b GMT-Berlin mbh, Gesellschft für Metllurgische Technologie- und Softwreentwicklung mbh Schwnebecker Chussee 5-9, D-1315 Berlin; Abstrct. The pper presents n pproch tht combines the simultion of technologicl processes on two levels. One of them is the finite element model tht predicts mteril flow nd temperture distribution in the workpiece during ny multi-stge deformtion where forming steps cn be seprted by heting/cooling of the billet. The second level is semi-empiricl microstructurl model tht simultes sttic nd dynmic recrystllistion nd grin growth during deformtion or dwelling time. The bckground of the microstructurl model is Sellrs formultion. The model prmeters re derived by prmeteriztion of the experiments done with the mteril specimens for wide rnge of strin, strin-rte, temperture vrition nd puse time. This model hs been integrted in QForm3D softwre nd tested for severl prcticl cses of open nd closed die forging. The cse studies hve shown good greement between the simultion results nd prctice tht hve opened the wy for its implementtion in industry. Keywords: Metl forming, FEM, simultion, microstructure, recrystllistion, nickel lloy. PACS: Bx, z INTRODUCTION These dys the development of metl forming processes is hrdly imginble without FEM simultion. The cpbilities of simultion extend from prediction of mteril flow nd tool stress nlysis to overll optimistion of the technologicl processes including providing required distribution of the mechnicl properties in the finished product. The ltter tsk requires including metllurgicl models into the finite-element metl forming simultion code. The presented pper is dedicted to this tsk tht hs been ccomplished for the needs of erospce industry where the requirements to gurntee product properties re the most criticl. MICROSTRUCTURAL MODEL DESCRIPTION The present work is bsed on Sellrs formultion tht includes semi-empiricl models of dynmic nd sttic recrystllistion nd grin growth [1]. Some prcticl implementtions of this pproch hve been lredy published in our previous works [-6]. The model is bsed on the following ssumptions. Firstly, it is supposed tht dynmic recrystllistion in the deformed mteril strts when the effective strin exceeds certin criticl vlue c tht cn be expressed s follows c 1 0 D Z 3 where D 0 is the initil grin size, Z is the Zener-Hollomon prmeter, re the mteril dependent prmeters. The strin vlue 0,5, t which 50% of the grins re recrystllized is to be determined s follows: c3 T c c4 c D e () 0,5 1 0 (1)

2 where is the strin-rte, T is the temperture, c1...c 4 re the mteril dependent prmeters. The dynmiclly recrystllized shre of the grins X dyn is to be clculted s follows: X dyn 1 e e c e1 0,5 (3) where e 1 nd e re mteril dependent prmeters. A very high grin refining cn be obtined with the dynmic recrystlliztion. The ctul grin size D dyn depends on the ctivtion energy of the mteril, process strin-rte nd temperture. It cn be expressed by the following reltion: D d dyn 1 Z d where d 1...d re the mteril dependent prmeters. Thus ll prmeters mentioned bove ( , c 1...c 4, d 1, d, e 1, e ) re to be determined experimentlly. The sttic recrystllistion cn be specified by the time t 0, 5 when 50% of the grins re stticlly recrystllised. Thus totl shre of stticlly recrystllised grins cn be expressed s: where X t stt 1e h t p to h1 t0,5 Q f f3 RT 0,5 f1 D 0 e f 4 st f5 T e (6) nd t p is the puse time, t 0 is the time till the beginning of the sttic recrystllistion, D 0 is the initil grin size before strting of sttic recrystllistion, Q st is the ctivtion energy for the sttic recrystllistion nd h 1, h, f 1...f 5, g 1...g 4 re the mteril dependent prmeters. The verge dimeter of stticlly recrystllised grins cn be clculted s following: D stt g 1 g D g 0 3 Z g 4 where h 1, h, f 1...f 5, g 1 g 4 re the mteril dependent prmeters of the model nd D 0 is the initil grin size before strting the sttic recrystllistion. After primry recrystllistion the microstructure is not yet in the stte of equilibrium. Further reduction of the grin surfce energy is reched through the grin growth: QKW n RT D) t e (8) where t is the time, D is the grin size vrition, Q KW is the ctivtion energy for the grin growth, nd n re mteril dependent prmeters. (4) (5) (7) PARAMETERS OF THE MODEL FOR INCONEL 718 The model specified bove requires experimentl determintion of the mteril dependent prmeters. It cn be done by series of torsion nd compression tests within the working rnges of the temperture T= C nd the

3 strin-rte s -1 tht corresponds to the most typicl process conditions. Such tests hve been done for Nibsed lloy Inconel 718. Using these experimentlly determined prmeters the mteril model behvior cn be visulised nd verified by the progrm MtILD (Mteril Informtion Link nd Dtbse service) developed by GMT-Berlin. Fig. 1 demonstrtes the possibilities of this tool. Any prmeter of the process (strin, strin-rte, temperture or puse time) cn be continuously vried by mens of the slider control while the other prmeters re kept constnt t certin levels. () FIGURE 1. The shre of dynmiclly () nd stticlly (b) recrystllized grins displyed in MtILD progrm s function of the strin nd the puse time respectively. The results of predicting the grin size fter the deformtion t different vlues of the deformtion nd the temperture hve been compred with experimentl observtions nd they hve shown good correspondence with prcticl observtion s shown, for exmple, in Fig.. (b) FIGURE. Microstructure of Inconel 718 fter deformtion t 1080 C nd strin rte 5 s -1 nd two different vlues of the strin 0.3 () nd 0.5 (b). PRACTICAL IMPLEMENTATION OF THE MODEL The model hs been included into metl forming simultion progrm QForm3D developed by QuntorForm Ltd. It llows simultion of ny metl forming processes performed in ny number of stges with heting/cooling opertions in between. The test technologicl process is the forging of structurl component produced in three opertions. Fig. 3 shows the shpe of the workpiece before the process nd fter ech opertion with the distribution of strin on its surfce. The initil temperture before the process of 1080 C hs been reched by heting it in furnce for 30 min. The initil verge grin size in the billet ws 8 microns. After the first opertion (nosing) it remins unchnged in the hlf of the billet where there ws no deformtion (see Fig 4).

4 b c FIGURE 3. The sequence of the opertions used for production of the structurl component: nosing (), flttening (b), closed die forging (c). Mteril Inconel 718. b FIGURE 4. The grin size fter nosing () nd flttening (b) opertions in trcked points of the meridin crosscut section of the workpiece. Initil grin size of 8 microns is shown on the microgrph nd corresponds to the level represented by letter A.

5 After the flttening opertion nerly the whole volume of the workpiece hs gotten some deformtion nd the grins size hs been reduced. The only exceptions re the ded zones ner the contct with the dies where the deformtion is less thn the criticl vlue c tht is required to strt the recrystllistion. The simultion of the finl stge of the technologicl process (closed die forging) is presented in Fig. 5. The trcked points tht re shown here re plced t depth of 1- mm from the forged prt surfce where the metllogrphic sections hve been done. As we cn see on these pictures the simultion very ccurtely predicts the verge grin size in control points M1 nd M. The grin size in the thinnest re of the forged prt (control point M1) ccording to the microgrph is 13 microns while the simultion shows it is within the rnge of microns (letters H nd I ). In the control point M tht is in the thickest re of the forged prt the grin size is 18 microns s seen on the microgrph while the simultion shows tht is bout 19.5 microns (letter C ). The presented cse s well s severl other test jobs hve proven the model ccurcy nd pplicbility for predicting the grin size in hot forged prts. Besides of Inconel 718 the models for some other steels nd lloys re vilble from the uthors. FIGURE 5. The verge grin size in two control points M1 nd M observed experimentlly on the microgrphs (left) nd predicted by mens of the simultion (right). Arrows show the correspondence of the points on the cut pln nd the legend scle. ACKNOWLEDGMENTS The uthors express their thnks to Mettis Aerospce Plc (UK) who provided the experimentl nlysis nd support for this work. REFERENCES 1... Sellrs, J.A. Whitemn, Recrystlliztion nd Grin Growth in Hot Rolling in Metl Science, P A. Borowikow Modellbetrchtungen zur Ver- und Entfestigung höherfester schweißbrer Feinkornstähle (Model reflections concerning the work hrdening nd the removl of work hrdening of higher-tensile weldble fine grin steels) in TU Bergkdemie Freiberg, Berg- und Hüttenmännischer Tg, 1990

6 3. W. Lehnert, D. C. Nguyen, H. Wehge, Simultion of ustenitic microstructure in rod nd wire rolling of quenched nd tempered steel grdes in Steel Reserch (1995) No W. Lehnert, D. C. Nguyen, H. Wehge, Werkstoffgefüge beim Wlzen von Drht und Stbsthl. (Mteril structure in rod nd wire rolling, in Wire (1993), No W. Lehnert, D. C. Nguyen, H. Wehge, R. Werners, Simultion der Austenitkornfeinung beim Wlzen (Simultion of the ustenite grin refining t the rolling), in Steel nd Iron, 113 (1993) No H. Wehge, U. Skod-Dopp, U. Quitmnn, W. Suer, Stichplnsimultion und optimierung für ds Wrmflchwlzen (Pss schedule simultion nd optimiztion for the hot flt rolling in MEFORM 98. Umformtechnisches Seminr Modellierung von Umformprozessen m Institut für Metllformung der TU Bergkdemie Freiberg, (Seminr for the Metl-Forming Engineering Modelling of metl-forming processes t the institute for Metl Forming of the TU Bergkdemie Freiberg), Februry 1998