Reheating 150 mm Billets of A356 Alloy for Thixo-Processing

Transcription

1 Internatinal Scientific Cllquium Mdelling fr Electrmagnetic Prcessing Hannver, March 24-26, 23 Reheating 15 mm illets f 356 lly fr Thix-Prcessing F. Dughier, M. Frzan, S. Lupi bstract The paper deals with the pssibility f btaining high prductin rates f a thixfrming inductin reheating system fr 356 ally billets by the use f multi-stage heating transients which allw t fulfil the temperature cnstraints characterising the prcess. These cnstraints becme particularly stringent with the recent increase f billet dimensins up t 15 mm diameters fr vlume prductin. In the paper tw-stage temperature transients cnstituted by tw heating perids f equal length, each fllwed by equal saking intervals f cnvenient duratin are analysed. Intrductin In the last ten years thix-frming has been accepted as a reliable prcess fr manufacturing net shape cmpnents, especially fr the autmtive industry. The realisatin f the thix-frming prcess requires the reheating f the billets in their semi-slid state with a very accurate temperature cntrl. In the case f the hypeutectic 356 ally, in rder t btain gd mechanical prperties, the eutectic must be cmpletely remelted. This means t adpt fr the inductin reheating prcess a pwer-time heating pattern which can assure the cmplete eutectic melting with a maximum temperature difference f ± 3 ºC in the billet. In rder t fulfil such stringent cnstraints, the heating f a 356, 15 mm diameter billet with cnstant pwer density in a single-stage inductr requires abut 2 minutes. s knwn, reductin f heating times with the same final temperature difference in the billet crss-sectin can be btained by the use f multi-stage heating prcesses. [1] In this paper tw-stage temperature transients cnstituted by tw heating perids f equal length each fllwed by equal cnvenient saking intervals are analysed. In this way each billet can be heated frm rm temperature t the target cnditins, with a cnsiderable reductin f heating times, in independent heating mdules. Each mdule can be cnstituted either by a single cil individually cntrlled in which the billet remains fr the entire heating cycle, r by tw inductrs with different cntrlled pwer inputs. This slutin can assure a cntinuus utput and a mre ratinal use f the frequency cnverters. 1. Reheating requirements Many papers have dealt with the inductin reheating f 356 ally billets fr thixfrming. The basic principle f this prcess is the reheating befre defrmatin at temperatures between the slidus and liquidus pints fr btaining a fine unifrm glbular micrstructure by minimising the temperature difference between the center and the surface parts f the wrkpiece. 177

2 1,,8 Slid Fractin,6,4, t ( C) 6 62 Fig. 1 Relatinship between temperature and slid fractin fr 356 [7] Taking int accunt that generally a slid fractin f abut 5% is recmmended fr thixfrming and that the eutectic remelting temperature is 575 ºC, frm the relatinship between temperature and slid fractin shwn in fig.1 a temperature distributin in the billet f 583 ± 3 ºC has been assumed as target at the end f the heating cycle. 1 ll temperature cycles analysed in the fllwing will satisfy the abve cnstraints and have been calculated making reference t the characteristics f the material shwn in figure 2. density(g/cc) uburn University Metals Handbk Pechiney resistivity(hm cm) 3.5E-5 3.E-5 2.5E-5 2.E-5 1.5E-5 1.E-5 5.E-6.E+ uburn University Metals Handbk Pechiney Specific heat(j/ C g) uburn University Metals Handbk Thermal cnduc.(w/ C cm) uburn University.5 Metals Handbk Catalg Pechiney Elektrwärme Fig. 2 Electrical and thermal characteristics f 356 [5,8] Nn negligible discrepancies can be bserved n the thermal cnductivity values given by different surces [5,8]; in the fllwing the values given in [8] will be used fr calculatins and the influence n the heating cycles f the abve differences will be examined. 1 ccrding t the literature the temperatures crrespnding t the slid fractin f 5% and 55% are 584 C and 576 C respectively. 178

3 2. Single-step reheating prcess In rder t fulfil the abve stringent reheating specificatins, in experimental setup and industrial plants the heating system is generally based n a mdular multi-inductr cnfiguratin cmprising mutually independent heating mdules. Each mdule is prvided by a single inductr, supplied by a cntrlled pwer input variable in a wide range, in which each billet underges the cmplete reheating cycle. It is knwn frm the inductin heating thery, the temperature difference between the center and surface parts f the wrkpiece - fr fixed billet dimensins and material characteristics depend n cil gemetry, inductr supply frequency and specific induced pwer. In particular, when heating with cnstant induced pwer a lng cylindrical wrkpiece, assuming cnstant electric and thermal characteristics and neglecting surface lsses, the temperature differential ( ϑmax ϑmin ) in the crss-sectin is given by: D P F(m) (1) ϑ ϑ = max min 4 λ with: D billet diameter; P induced pwer density; λ - thermal cnductivity; F(m) functin f the parameter m = D 2 δ ranging frm.5 t 1 as m increases; δ - current penetratin depth. Eq. (1) shws that the temperature differential required fr the thix-frming can be btained nly with very lw pwer density values and, crrespndingly, with lng heating times t,s t T, C temperature t=165 s t=145 s 12 P,w/cm P t=115 s 8 f,hz r, cm Fig. 3 Heating times with cnstant specific pwer density fr a 15 mm diameter billet Fig. 4 Influence f thermal cnductivity n temperature distributins (f=5 Hz; P =6.4 W/cm²; data frm[5]; frm [8]) In practice, eq. (1) cannt give reliable results fr the thix prcess due t the strng variatin f the material electrical and thermal parameters with temperature and the cnsiderable per cent weight f the surface thermal lsses n the ttal induced pwer. Mrever, the inductr and wrkpiece end-effects must be taken int accunt. Tday, hwever, pwerful numerical calculatin 1D and 2D prgrams allw the designer t take int accunt all these factrs. Fig. 3 gives the calculated values 2 f heating times and cnstant specific pwer density required t fulfil the abve mentined requirements 2 Values calculated with the 1D prgram ELT fr a billet 15 mm diameter, 3 mm length. 179

4 as a functin f the inductr supply frequency, when inductr and billet have the same length, while fig. 4 shws the influence n the final temperature distributins at 5 Hz fr the different thermal cnductivity values given in fig.2. pssibility f reducing the heating times can be btained in the single step prcess by a cntinuus clse cntrl f the pwer induced in the billet, using high pwer values at the beginning f the heating and reducing the specific induced pwer twards the end f the cycle t few per cent f the value used at the beginning [5]. This prcedure requires, hwever, a frequency cnverter with cntrlled pwer utput variable in a very wide range and, cnsequently, leads t higher installatin csts fr the same utput. 3. Multi-step reheating prcess Many recent researches have been perfrmed n multi-step prcesses, in which the verall effects f the reheating parameters such as ttal reheating time, pwer density values, duratin and temperatures f the heating, saking r pwer-ff stages have been examined. ll experiments refer t billets f 76 mm diameter with different lengths, with ttal times ranging frm 12 t 16 minutes. Hwever, they fcused mainly their attentin n the verall metallurgical effects f the reheating, but d nt give sufficient data (e.g. induced pwer density, supply frequency) fr crrelating the chice f the reheating parameters (duratin and temperature f hlding intervals) t the final results. [6,7,8,9,1] In the fllwing we will analyse fr a 15 mm diameter billet a particular tw-step prcess in which the reheating transient is cnstituted by tw heating intervals f equal length, fllwed by tw pwer-ff stages in turn f the same length, as shwn in fig. 5. T, C Temperature T, C t, s r, cm Fig. 5 Tw-step reheating prcess: temperature distributins vs. time and radial crdinate [t 1 =175 s, P =34 W/cm²; t 1 =25 s (P ff); t 2 =175 s, P =3.5 W/cm²; t 2 =25 s; final: ϑ ϑ C ] This special reheating prcess allws t reduce cnsiderably the ttal time and t perfrm the reheating in individual heating units cnstituted by tw inductrs, each supplied by an independent frequency cnverter (r pwer supply) whse pwer varies in a relatively narrw range. Mrever, a cnvenient chice f the temperature at the end f each heating stage can assure a sufficient hlding time ver 575 C t btain the required fine glbular micrstructure, as stated in [9]. The results given by the 1D mdel, shwn in fig.5, have been checked and refined by a 2D calculatin 3, which is particularly imprtant fr the exact evaluatin f the heating max min 2 3 Calculatins made with the package FLUX2D 18

5 pwer necessary in the secnd heating stage, taking int accunt the inductr length and the wrkpiece end effect. The results btained are shwn in figure 6, which gives the temperature pattern in the billet crss-sectin and the inductr and wrkpiece active pwer during the reheating cycle; they refer t an inductr 45 mm length, 26 mm internal diameter, excited at 5 Hz. C C C ϑ max 585 min b) ϑ max = 573 C; ϑmin = 563 C; c) ϑ max = 58 C; ϑmin = 574 C; a) = C; ϑ = 546 C; kw 12 Ttal pwer 1 D 8 6 illet pwer 4 2 C C ϑ max 587 min e) ϑ max = 586 C; ϑmin = 581 C; d) = C; ϑ = 58 C; 1 2 Time, s3 4 f) Fig. 6 Temperature distributins and active pwer at different heating stages 4 [a: t=175 s; b: t=2 s; c: t=28 s; d: t=375 s; e: t=4 s] They substantially cnfirm the pssibility f perfrming the reheating f a 15 mm diameter billet in relatively shrt times, cmplying with the very stringent specificatins required in the thix-frming prcess, by adpting the particular tw-step prcess prpsed in the paper. Mrever, the particular characteristics f the reheating cycle, cnstituted by tw heating perids and tw saking stages f equal length, give the pssibility f subdividing each heating mdule in tw inductrs which can be individually supplied with nearly cnstant pwer values (fig.6-f). 4 Symbls: - billet axis; -C central crss-sectin; - billet edge; -C billet surface; D pint f ϑ max 181

6 Similar results can be btained at different frequencies (e.g. 3 Hz r 7 Hz) by a cnvenient chice f heating and saking times and suitable pwer density values. Cnclusins In this paper the analysis f the inductin reheating prcess f 15 mm diameter billets fr thix-frming has been develped. particular tw-step reheating cycle has been adpted which has the fllwing characteristics: - cnsiderable reductin f the ttal time f the reheating cycle - heating mdules cnstituted by tw inductrs supplied by nearly cnstant pwer - cntinuus utput with n dead intervals - ptimal hlding time in the final step t btain the required glbular micrstructure. References 1. Geisel H.: Induktives schnellerwärmen mit abgestufter leistung, wt-z. ind. Fertig, (197), vl.6, n.1, Cremer R., Schiele J.:"Verbesserte anlagentechnik fuer die blzen-erwärmung in das slidus/liquidusintervall", Elektrwärme internatinal, (1996), vl.54, Juergens R.: "Cntrlled heating fr the thix-prcessing f aluminium", UIE XIII Cngress n Electricity pplicatins (irmingham, UK), (1996), June, MII 17-MII Kaprans P., Gibsn R., Kirkwd D., Sellars C.: "Inductin heating and partial melting f high melting pint thixfrmable allys", IV Int. Cnf. n Semi-Slid Prcessing f llys and Cmpsites (Sheffield, UK), (1996), June, Juergens R.: Przessgeführte induktive erwärmung vn thix-legierungen aus aluminium, Elektrwärme Internatinal, (1998), n.1/march, Jae Chan C., Hyung Jin P., yung Min K.: "The influence f inductin heating n the micrstructure f 356 fr semi-slid frging", Jurnal f Materials Prcessing Technlgy, (1999), vl.87, Jung H.: "The inductin heating prcess f semi-slid aluminium allys fr thixfrming and their micrstructure evaluatin", Jurnal f Materials Prcessing Technlgy, (2), vl.15, akhtiyarv S., Overflet R., Tedrescu S.: "Electrical and thermal cnductivity f 319 and 356 aluminum allys", Jurnal f Materials Science, (21), vl.36, Jung H., Kang C.: "Inductin heating prcess f an l-si aluminum ally fr semi-slid casting and its resulting micrstructure", Jurnal f Materials Prcessing Technlgy, (22), vl.12, Zqui E., Shenata M., Paes M., Ka V., Es-Sadiqi E.: "Mrphlgical evlutin f SSM 356 during partial remelting", Materials Science and Engineering, (22), vl. -325, uthrs Dughier F., Frzan M., Lupi S. Labratry fr Electrheat - Dept. f Electrical Engineering University f Padua (Italy) 182