Effect of Cooling Rate on Microstructure and Mechanical Properties in Al-Si Alloys

Transcription

1 Proeedings of the 12th Interntionl Conferene on Aluminium Alloys, Septemer 5-9, 2010, Yokohm, Jpn 2010 The Jpn Institute of Light Metls pp Effet of Cooling Rte on Mirostruture nd Mehnil Properties in Al-Si Alloys Suk Bong Kng 1, Jing Zhng 1,2, Shouren Wng 1, Jehyung Cho 1, V. U. Stetsenko 3 1 Kore Institute of Mterils Siene, 66 Sngnm-dong, Chngwon, , Kore 2 Key Lortory of Liquid Struture nd Heredity of Mterils, Ministry of Edution, Shndong University, 73 Jingshi Rod, Jinn, , Chin 3 Institute of Tehnology of Metls, Ntionl Ademy of Siene of Belrus, 11 Bilynitskg-Biruli, Mogilev, , Belrus Aluminum-silion se lloys re widely used in erospe struturl pplition nd the utomoile industry due to their low therml expnsion oeffiient, high wer resistne nd good stility. They n e divided into hypoeuteti, euteti nd hypereuteti lloys ording to the mount of silion ontent. The morphology nd size of primry silion nd euteti silion ffet mehnil properties in Al-Si lloys. In this study, the miro-struturl vrition of Al-Si lloys hs een rried out with vrition of ooling rtes in ingot sting nd horizontl ontinuous sting proesses. The influene of ooling rte on mirostruture of ingot sting Al-Si lloy ws investigted. The effet of fored ooling devie nd ddition of rw metl with superdispersed mirostruture ws evluted on the view point of the size ontrolling of primry silion nd euteti silion in Al-Si lloys during horizontl ontinuous sting proess. The influene of het tretment on tensile properties ws lso rried out. Keywords: Al-Si lloys, ooling rte, mirostruture, mehnil properties, horizontl ontinuous sting 1. Introdution Al Si lloys re used extensively in erospe struturl pplition nd utomoile industry euse of exellent rsion resistne, low oeffiient of therml expnsion, nd high strength-to-weight rtio. Euteti or ner-euteti Al Si lloys provide the est overll lne of properties whih re referred to s piston lloys [1]. Mehnil properties nd sting qulity of Al Si lloys strongly depend on the size nd the morphology of silion rystls, seondry dendrite rm sping (SDAS) nd lso the numer of defets[2-4]. The morphology of Si phse during solidifition my e sensitive to ooling/growth rte nd to ertin speifi impurity dditions[5-7]. Modifition of the Al Si lloys n e hieved in two different wys: y ddition of ertin elements (hemil modifition) or with rpid ooling rte (quenh modifition). In this study, Al-Si lloys were prepred in two sting proesses: ingot sting with different thikness step lok nd horizontl ontinuous sting. The mirostruture nd mehnil properties of Al-Si lloys in different ooling rte were lso investigted. 2. Experimentl The lloys for ingot sting re AP2 lloy nd A390 lloy. Tle 1 is the hemil omposition of those two lloys. The se lloy ws melted t temperture of 800 in grphite ruile. After degssed y Ar gs, the melt ws poured into steel mould. The mould is designed for four different thikness step lok, whih is 40mm, 20mm, 10mm nd 4mm, respetively. The sting temperture is 695 for AP2 lloy nd 780 for A390 lloy, whih orresponds to the Al-Si phse digrm. Different thiknesses were designed to otin different ooling rtes. After the lultion of omputer, the ooling rtes were estimted to e 11.6 /s (4mm thikness), 9.2 /s (10mm thikness), 5.7 /s (20mm thikness) nd 3.3 /s (40mm thikness), respetively.

2 676 Euteti nd hypereuteti inry Al-Si lloys were used in horizontl ontinuous sting(hcc). The mterils of Al-18%Si lloy nd Al-12%Si lloy were proessed y HCC equipment. The horizontl ontinuous sting ws performed t the temperture rnge of Tle 1 Chemil omposition of AP2 nd A390 lloy (wt. %) Alloy Si Cu Mg Fe Al A Bl. AP Bl. 3. Results nd disussion 3.1 Ingot sting of Al-Si lloy Fig. 1 is the mirostruture of AP2 lloy t different ooling rtes. White dendriti one is α-al nd gry interonneted need-like one is euteti silion. As shown in these photos, oth α-al nd euteti silion eome to e finer with inresing ooling rte. Seondry dendrite rm sping (SDAS) of α-al hnges from 40μm to 10 μm. This is oinidene with the following formul d 13840v 2 L 7562v (1) = L where d is SDAS of α-al nd v L lol solidifition rte[8]. The morphology of euteti silion does not hnge, whih is still need-like. But it eomes to e muh finer s short rod-like form with rpid solidifition. d Fig. 1 Mirostruture of AP2 lloy y ingot sting () 40mm ()20mm () 10mm (d) 4mm Mirostruture of A390 lloy is shown in Fig. 2. A series of morphologies of primry silion were oserved in A390 lloy: plte-shped rystl of hexgonl form, othedrl equi-xed rystls, str-like rystls ontining two to five rditing twin plnes, nd more or less spheril shpes, et. The men size of primry silion dereses from 40 μm to 20 μm s the ooling rte inreses. Interior mirostruture of euteti ell lso eomes to e muh finer in rpid ooling rte. The

3 677 morphology of euteti silion hnges from orse need-like shpe to fine rod-like form nd the SDAS of α-al eomes to e muh smller. The tendeny is s sme s the one in AP2 lloy. d Fig. 2 Mirostruture of A390 lloy y ingot sting () 40mm ()20mm () 10mm (d) 4mm Higher ooling rte provides igger underooling, whih mkes the ritil rdius of nuleus smller. This mens it is esier to form more nulei. On the other hnd, there is no enough time for grins to grow into orse forms. Both of these two spets re ttriuted to the resultnt finer mirostruture t higher ooling rte. 3.2 Horizontl ontinuous sting of Al-Si lloy Al-12%Si lloy The sting of luminum lloys for extrusion illets nd rolling sls hs priniplly een rried out y the semi-ontinuous sting proess (diret hill sting proess). Horizontl ontinuous sting hs mny dvntges suh s lower investment ost, higher flexiility, longer sting times nd so on. However, sting defets suh s mirosegregtion nd porosity re usully present in the s-st mirostruture whih n led to the deteriortion of mehnil properties.the enter of the st rs ws hosen to ompre euse it is the most frilty prt in horizontl ontinuous sting lloy. The following pitures re hosen s the sme position. There re two ooling system in horizontl ontinuous sting: single nd doule ooling. Alloys proessed y single ooling solidified in the ir fter drwn from the rystllizer. So the ooling rte is not rpid enough to mke fine mterils. Diret wter spry method is dopted in doule ooling proess to otin higer ooling rte. The wter jet n e spyed from mould to hill the lloys drwn from the rystllizer whih is known s indiret hill. The temperture of ooling wter is out 20 nd the wter onsumption is 5-15m 3 /h. Fig. 3 is the SEM of the lloys proessed y single nd doule ooling horizontl ontinuous sting proess. () nd () re the views of α-al in low mgnifition while () nd (d) re the pitures of euteti silion in lrger mgnifition. Both α-al nd euteti silion eome to e finer. Men SDAS of α-al hnges from 100 μm to 30 μm. The euteti silion ws modified from orse needle-like one to fine firous form. Doule ooling

4 678 method mkes ooling rte muh more rpid, whih results in igger underooling. This indues finer mirostruture of lloys fter seondry ooling. Tensile properties of two different ooling onditions in Al-12% Si n e seen in Tle 2. Both the yield strength nd ultimte tensile strength inrese oviously fter diret wter ooling, espeilly for the UTS, whih inreses y 90 MP. By introduing ooling wter, the het n onvet rpidly in the ross-setion of the illet. So the higher ooling rte of melt results in igger underooling. This indues finer struture nd enhned mehnil properties. 25μm d 25μm Alloy Fig. 3 Mirostruture of Al-12%Si lloys t different ooling onditions (), () single ooling (), (d) doule ooling Tle 2 Tensile properties of Al-12Si lloy in different ooling onditions Stte Yield strength (MP) UTS (MP) Elongtion (%) Single ooling F Doule ooling F Fig.4 SDAS of the enter of Al-12%Si lloys with different mount of rw metl () 0% () 30% () 50% Conventionl modifiers re ville to refine the primry or the euteti silion, ut it is inevitle to inrese the mount of impurity. Rpid ooling is nother method to modify Al-Si lloy esides the elements suh s Sr. This rw metl with superdispersed mirostruture ws proessed y

5 679 rpid ooling rte, whih the lloy hs SDAS s 13μm. Fig.4 is the SDAS of lloys with different mount of rw metl. As the mount of rw metl inreses, the SDAS eomes to e muh smller nd the differene etween the enter nd the surfe eomes to e smller, too. This mens the mirostruture eomes to e more uniform due to its role s modifier. Both α-al nd euteti silion eome to e finer fter dding rw metl s shown in Fig.5. The lloy with 30% rw metl hs no ig differene with the one with 50% rw metl. 200μm Fig.5 Mirostruture of Al-12% Si lloys with different mount of rw metl () 0% () 30% () 50% The rw metl dded for horizontl ontinuous sting proess ts s modifier to refine the euteti luster. The modifier disperses uniformly in the lloy to form potentil nuleus for solidifition. It is esy to nulete in smll underooling. So when the surfe strts to solidify, mny nulei ome out to grow up efore the solidifition front. The solidifition front moves quikly from the surfe to the enter for medium dimeter of illets (70mm). So the mirostruture eomes to e uniform nd fine in ross-setion of the lloy. The ddition of rw metl up to 50 perent does not help in further struturl refinement. This is due to the ft tht s the mount of inoulnt prtiles prtiipting in nuletion (t given underooling) inreses, so does the ltent het relese leding to relesene t smller underooling, therey lowering the effiieny of the grin refiner Al-18%Si The usul method to refine primry silion is to dd the element P. But it is inevitle to introdue extr impurity. Rpid ooling is one of wys to modify Al-Si lloy to otin fine mirostruture. In this study, the size of primry silion in rw metl proessed y rpid ooling is out 20 μm. This lloy ws put into the ontinuous sting lloy melt to t s modifier.as to heredity, silion in rpid ooling ingot n keep smll size during growth fter remelting. These smll silion phse n e the nuleus in horizontl ontinuous sting Al-Si lloys. More nulei men more grins nd finer struture. This n e proved in Fig. 6, the mirostruture of lloys with dding rw metl. The verge size of primry silion dereses from 40 μm to out 30 μm fter dding 15 perent rw metl. And the euteti silion lso eomes to e finer. Inresing the ontent of rw metl to 30 perent, there is no ig hnge in the mirostruture for ontinuous sting.. Fig. 6 Mirostruture of Al-18%Si lloys with different mount of rw metl

6 680 ()0% () 15% () 30% The sme sitution ws found for the results of mehnil properties, s shown in Fig. 7. The lloy with 15 perent rw metl hs the est properties of strength. The melt ontining 30 perent rw metl does not otin further struturl refinement. This is due to the ft tht s the mount of inoulnt prtiles prtiipting in nuletion (t given underooling) inrese, so does the ltent het relese leding to relesene t smller underooling, therey lowering the effiieny of the grin refiner Fig. 7 Mehnil properties of Al-18%Si lloys with different mount of rw metl 4. Conlusions From these oservtions, the following onlusions re drwn. 1. For ingot sting, the mirostruture of AP2 lloy nd A390 lloy eomes to e finer s the ooling rte inreses. 2. Mirostruture differs t different ooling onditions in horizontl ontinuous sting proess. After doule ooling, Al-12% Si lloys shows finer mirostruture nd etter mehnil properties. 3. Rw metl with superdispersed mirostruture proessed y rpid ooling n modify oth Al-12%Si lloy nd Al-18%Si lloy. The optimum mount of rw metl is 30 perent for Al-12%Si nd 15 perent for Al-18%Si lloy. Referenes [1] X. G. Song, X. F. Bin, J. X. Zhng nd J. Zhng: J. Alloy. Compd. 479 (2009) [2] A.M.A. Mohmed, A.M. Smuel, F.H. Smuel nd H.W. Doty: Mter. Des. 30 (2009) [3] H. C. Lio, Y. Sun nd G. X. Sun: Mter. Si. Eng. A 335 (2002) [4] M. Zeren: J. Mter. Proess Tehnol. 169 (2005) [5] S. Z. Lu nd A. Hellwell: J. Cryst. Growth. 73(1985) [6] H. Singh, A. M. Gokhle, A. Tewri, S. Zhng nd Y. Mo: Sript Mter. 61 (2009) [7] O. EI Seie, A. M. Smuel, F.H. Smuel nd H.W.Doty: Mter. Si. Eng. A 486 (2008) [8] G. F. Mi, X. Y. Liu, Z.J. Zhu nd H. W. Wng: Trnstions of Nonferrous Metls Soiety of Chin. 17 (2007)