24.-26.5.2005, Hradec nad Mravicí Oxidatin f Liquid Al-7Si Allys Cntaining Strntium and Magnesium S. M. Miresmaeili Lecturer in Dept f Metallurgical and Materials Engr, Faculty f Engineering, Rajaei University, Tehran, 16788, Iran, E-mail: s.m.miresmaili@srttu.edu Abstract Oxidatin behaviur f small amunts f Al-7Si allys melts cntaining strntium and magnesium has been investigated using therm gravimetrical analysis (TGA) experiments. The results f TGA experiments shwed that Sr-additins increases the xidatin rate f the Al-7Si allys melt significantly. Nticeable cntent f SrO cntaining mixed xides was fund, using scanning electrn micrscpe (SEM), in the xidised surfaces after even shrt xidatin perids. The rapid rate f Sr-lss during the xidatin perids als shwed that Sr was highly reactive in the small melts fr xidatin. The xidatin f large Al-7Si allys melts was als studied by analyzing the melts surface layer at different perids. The surface xide layers cnsisted f Al 2 O 3 and Al 2 O 3.MgO but was fund t cntain n detectable SrO. Als, in cntrast with the small melts, the relatively slw rate f Sr-lss during the xidatin perid shwed that strntium has a gd stability in a large melt. These reveal that in such a case the tendency f Sr fr xidatin is nt nticeable. Keywrds: Al-Si allys, Oxidatin, Sr-mdificatin, Magnesium 1. INTRODUCTION The mdificatin treatment f Al-Si allys is assciated with an increase tendency t prsity frmatin [1-5]. T explain this tendency, many authrs have suggested that Srmdificatin increases the xidatin rate and/r the melt inclusin cntent f Al-Si allys [6-9]. Cntrary, Miresmaeili et al [3-4], have suggested that the strntium xide can nt be respnsible fr the increase in prsity cntent. They have fund that in bth mdified and unmdified allys, xide films f Al 2 O 3 and Al 2 O 3.MgO play a significant rle in pre frmatin and the Sr-cntaining xides d nt take part in pre frmatin. Despite widespread researches, the nly investigatins t quantify the effect f Srmdificatin n the melt xidatin rate f Al-Si allys are thse carried ut by Gruzleski et al [10-11]. Emadi and Gruzleski [10] fund that Sr-mdificatin increases the rate f xidatin f binary Al-7Si ally at 740 C by three times. Hwever in anther wrk, Gruzleski and Yuen [11] have reprted that Sr-mdificatin decreases the rate f xidatin f Mgcntaining Al-Si allys. They have fund that in the absence f Sr, the A356 and A357 samples gained substantial amunts f weight thrugh xidatin. Samples cntaining Sr had significantly lwer weight gains. The literature shws that there is n agreement abut the effect f Sr-mdificatin n the xidatin rate f Al-Si allys in particular in the presence f Mg which itself has a cnsiderable effect n the xidatin f Al-Si allys. The purpse f the present study is t btain new results by develping a mre precise technique t quantify the interactin f Sr and Mg n the xidatin f Al-7Si allys. In the present study, the xidatin f small and large amunt f liquid Sr-mdified Al-7Si allys is studied. 1
24.-26.5.2005, Hradec nad Mravicí 2. EXPERIMENTAL PROCEDURE 2.1. Oxidatin f Small Al-7Si Allys Melts Melting and allying was carried ut in a 3kg capacity silicn carbide crucible using an electrical resistance furnace. Binary Al-7Si, ternary allys Al-7Si-0.35Mg (A356), Al-7Si- 0.03Sr and Al-7Si-0.1Sr, and quaternary allys Al-7Si-0.35Mg-0.03Sr (0.03Sr-mdified A356) and Al-7Si-0.35Mg-0.1Sr (0.1Sr-mdified A356) were used, prepared by adding pure silicn, magnesium and Al-10%Sr master ally t 99.7% aluminium. In facilitating detectin f the heavy Sr-rich cmpunds in Al-Si allys micrstructures (either intermetallics r xides), using BSE image that emphasises the differences in atmic weight f elements, the allys cntained very lw cncentratin f ther heavy elements Fe, Cu, Mn and Cr. The cast samples f the allys were machined t a truncated cne shape weighing abut 47g. Therm gravimetrical analysis (TGA) was used t study the xidatin behaviur f the liquid Alallys. In measuring the weight change due t xidatin, an AP250D series OHAUS- ANALYTICAL PLUS recrding balance was used. It was capable f measuring weights up t 62g with a reslutin f 10µg. 2.2. Oxidatin f large Al-7Si Allys Melts Melting and allying f 2.5kg weight f the allys were carried ut in the 3kg capacity silicn carbide crucibles using the electrical resistance furnace. This amunt f melt is regarded as the large melt, in cmparing with the 47g melts samples. Since the TGA methd is nt capable t study precisely the xidatin rate f the large quantity f melts, in such a case the effect f Sr-additin n the xidatin f the large melts was carried ut by analyzing the melt surface xide layer. After Sr-additin the melts were held fr 20 minutes at C fr cmplete disslutin f strntium. In analyzing the the xidized surfaces, samples f the melt surface layers were taken just after the 20-minute perid f disslutin f strntium. The melts were then held fr anther 300 min perid at C and the sampling frm the melts surface layers were made every 60 minutes using preheated metallic meshes. In analyzing Sr-cncentratin after the Sr-additin sequence and during the melts hlding perids, samples frm the melt were als taken every 60 min. 2.3. Metallgraphic Observatins A Philips XL30 scanning electrn micrscpe (SEM) was used fr micrstructure bservatin and X-Ray micranalysis was perfrmed using a LINK ISIS-Oxfrd Energy dispersive spectrscpy (EDS) analyzer linked t the SEM. In evaluating the amunt f different xides, the surface xide films after shrt and lng perids f xidatin, were studied by analyzing different areas f the xide film using the spt and selected area methds. In rder t reduce the errrs caused by the matrix beneath the xide film, ZAF crrectin was made, with reference t the standard pure element. The cmpsitin f the allys after different hlding perids were als analyzed using the ptical emissin spectrmetric analysis methd. 3. RESULTS AND DISCUSSIONS 3.1. The Small Melts The weight gains f Al-7Si allys cntaining different amunts f the elements Sr and Mg versus time at C is pltted in Fig.1. As can be seen, Sr and Mg-additins increase nticeably the xidatin rate f the liquid Al-7Si ally althugh the effect f even 0.03%Sr n the increased melt xidatin rate is higher than that f 0.35%Mg. Figure 2 shws the variatin f cncentratin f Sr disslved in the 47g A356 ally samples versus time at 2
24.-26.5.2005, Hradec nad Mravicí C. Mre than 80% f strntium faded during the 1000min hlding perid at C, revealing that in a small Al-7Si ally melt which has a high rati f xidatin surface area t melt vlume (surface mdulus), Sr is very reactive, prbably fr xidatin. The SEM-EDS micranalysis pattern btained frm the upper surface film f an Al-7Si-0.35Mg-0.03Sr ally melt, which held fr 240min at C is shwn in Fig.3. As seen, the xidised surface cnsists f cmplex f SrO.MgO.Al 2 O 3 xides, shwing the high reactivity f Sr and Mg fr xidatin in a small melt. Weight gain (µ g/cm 2 ) 1800 0.35%Mg, 0.1%Sr 1600 1400 0.1%Sr 1200 0.35%Mg, 0.03%Sr 1000 0.03%Sr 800 600 0.35%Mg 400 n Sr and Mg 200 0 0 300 600 900 1200 Time (min) Fig.1: The effect f Sr and/r Mg-additin n the xidatin rate f Al-7Si ally at C 0.03 0.025 0.02 Sr% 0.015 0.01 0.005 0 0 200 400 600 800 1000 1200 Time(min) Fig.2: Variatins f cncentratin f Sr in the small Al-7Si-0.35Mg-0.03Sr ally melt versus time at C Fig.3: SEM-EDS micranalysis pattern btain frm the xide film f Al-7Si-0.35Mg-0.03Sr ally after 240 min xidatin at C 3
24.-26.5.2005, Hradec nad Mravicí The quantitative SEM-EDS analyses were als used t estimate the cmpsitin f the xidised surfaces. Cnsequently, by ding mass balance calculatins, the cmpsitin f the xidised surfaces in liquid 0.03%Sr-mdified A356 ally at different perids f xidatin at C has been determined as in Fig.4. Again it shws that in the small melts, the elements Sr and Mg were bth chemically reactive fr xidatin. Wt % 100 80 60 40 20 0 Al2O3 SiO2 MgO SrO 0 40 80 120 160 200 240 Time (min) Fig.4: Variatins f cmpsitin f the xidised surface f the Al-7Si-0.35Mg-0.03Sr ally with time at C In rder t explain the high tendency f Sr fr xidatin, resulted in its significant influence n increased xidatin rate f the small melts, the stability f the melt xidatin prducts can be cnsidered. The standard Gibbs free energy f frmatin fr single xides can be used t evaluate their stability. Equatins (1) and (2) shw the energy fr the Al-Sr-O system [12]. 2 1 1 Al (liquid) + O2 (g) = Al2 O 3, G = - 452.5 kjml -1 (1) 3 2 3 C 1 Sr (slid) + O2 = SrO, G = - 492 kjml -1 (2) 2 C It can be cncluded that, because f having higher negative free energy at C, SrO is mre stable than Al 2 O 3 thus the frmatin f SrO in Al-Si melts is likely. In additin, there may be ther allying elements in Al-Si melts. In cmparing the thermdynamic affinity f different allying element fr xidatin, Table.1 shws the thermchemical prperties f sme prbable xides in Al-melts at C. The quantity f K C in Table.1 is the equilibrium cnstant fr the reactins f frmatin f the different single xides at C. As seen, elements such as Ca, Be, Mg and Sr have mre stable xide than alumina, thus the frmatin f their xide in Al-Si allys melt is likely. It is hwever imprtant t nte that, there is a high affinity fr Al 2 O 3 and ther xides t frm mixed xides [12]: Al 2 O 3 + SrO = Al 2 O 3.SrO, Al 2 O 3 + MgO = Al 2 O 3.MgO, G = -151 kjml -1 C (3) G = -38 kjml -1 C (4) Accrding t equatins 3 and 4 the relatively high negative magnitude f standard free energies f frmatin f the mixed xides indicates that the xidatin f the elements Sr and 4
24.-26.5.2005, Hradec nad Mravicí Mg, in the Al-Si allys wuld be thrugh the frmatin f these mixed xides. Equatin (5) and (6) shws the standard free energy f frmatin f mixed xide in the Al-Sr-O and Al- Mg-O systems: 1 1 1 1 Sr(l) + Al(l) + O2 = SrAl2 O 4, G = -482.7 kjml -1 (5) 4 2 2 4 C 1 1 1 1 Mg(l) + Al(l) + O2 = MgAl2 O 4, G = -473 kjml -1 (6) 4 2 2 4 C As seen because f having higher negative free energies at C, SrAl 2 O 4 and MgAl 2 O 4 are mre stable than Al 2 0 3 thus the frmatin f these mixed xides in the melt xide film is prbable. The mixed xide f Al 2 O 3.SrO is als mre stable than the Al 2 0 3.MgO spinel, thus by increasing the xidatin perid, the system appraches equilibrium, as Al 2 O 3 and Mg.Al 2 O 3 react gradually with Sr and Mg t frm SrO.Al 2 O 3 and prbably Al 2 O 3.SrO.MgO. Figures (4) als shws that the xidatin f small Al-7Si-0.35Mg-0.03Sr ally melt started thrugh the frmatin Al 2 O 3 film and cnsequently fllwed by further frmatin f Al 2 O 3 and reactin f the alumina film t MgO.Al 2 O 3, SrO.Al 2 O 3 and Al 2 O 3.SrO.MgO. Table 1: Thermchemical prperties f prbable xides in Al-melts [12-13, 15] Element Oxide Density f xide (g/cm 3 ) PBR factr G (kj) K C C Al Al 2 O 3 2.8-4 1.28-452.5 4.45 10 23 Mg MgO 3.7 0.78-497 10 26 Si SiO 2 2.45 2-364 10 19 Sr SrO 4.7 0.65-492 5.16 10 25 Ca CaO 3.3 0.65-529 1.4 10 27 Na Na 2 O 2.27 0.57-341 6.6 10 17 Be BeO 1.28 1.71-511 4.8 10 26 It is imprtant t nte that the presence f these mixed xides is nt the nly respnsible fr enhancement f xidatin rate. The Pilling-Bedwrth rati (PBR) [13] in Table.1 has been calculated t shw the xide/ metal vlume rati and it can mstly be referred t shw the self-prtectivity f xide films. Accrding t Table.1, elements such as Ca, Mg and Sr, which have mre stable xide than alumina, because f the nn-prtective nature f their xide films, may increase the xidatin rate f Al-melts. Cnversely, because f frming a prtective xide film, Be decreases the xidatin rate f Al-allys melts [14]. In explaining figures 1, it can be cncluded that SrO is nt as prtective as an alumina film and it increases significantly the melt xidatin rate by nt nly frmatin f SrO cntaining mixed xides but als by facilitating cnditin fr further xidatin f Al, Mg and ther allying elements. In ther wrds, when SrO is frmed in the melt surface layer, the surface layer may be mre prus whereby Sr causes the nticeable increased in the xidatin rate. 5
24.-26.5.2005, Hradec nad Mravicí 3.2. The Large Melts Variatins f cncentratin f Sr disslved in 2.5kg liquid Al-7Si-0.35Mg-0.03Sr ally versus hlding time at C is shwn in Fig.5. As seen in cmparing with the small melts in which mre than 30% f Sr lst after 300min xidatin (fig. 2), the Sr-lss in the large Al- 7Si-0.35Mg-0.03Sr ally melt was nt nticeable (abut 8% after 300min), shwing that in such a case, the rate f xidatin f Sr was relatively slw. This shws that strntium has gd stability in large Al-7Si allys melt. The systematic analysing f the melt surface layers at different stages f the hlding perids at C using SEM-EDS and linescan micranalysis shwed that the surface layers f the 0.03 and 0.1%Sr-mdified Al-7Si and A356 allys cnsisted f Al 2 O 3, MgO (nly in A356 ally) and minr amunt f SiO 2 but were fund t cntain n detectable SrO. Hwever, Sr-rich intermetallic cmpunds were fund t be present in the surface layer. Sr% 0.03 0.025 0.02 0.015 0.01 0.005 0 0 60 120 180 240 300 Time (min) Fig.5: Variatins f cncentratin f Sr in the large Al-7Si-0.3 5Mg-0.03Sr ally melt versus time at C The BSE image f a plish sectin f the surface film f the 0.1%Sr-mdified A356 ally after 300min xidatin at C and an assciated linescan micranalysis pattern are shwn in Fig.6. The distributin f the elements shws that the prus regin is the surface xide film and the cmpsitin f the surface film crrespnds t prbably Al 2 O 3.MgO. There is als a sharp rise in the cncentratin f the elements Sr and Si in the regin f the bright particles and a marked fall in the cncentratin f Al and O, revealing that the bright particles were Sr-rich intermetallics but nt SrO cntaining xides. Figure 6-C als shws the SEM-EDS micranalysis pattern btained frm a selected area f the surface xide film (windw in fig. 6-a) and nt cntains f the bright particles. As seen, the amunt f Sr in the surface layer was nt sufficient t be detected by SEM-EDS analysis, indicating that SrO is absence in the xidised surface. Hwever, the existence f Si peak reveals the pssibility f SiO 2 frmatin in the xidised surfaces. These shw that in the large Al-7Si allys melt that has lw surface mdulus fr xidatin, in cntrast with small melt, Sr is nt a reactive element fr xidatin. 6
24.-26.5.2005, Hradec nad Mravicí Fig. 6-a: BSE image f a plished sectin Fig. 6-b: linescan micranalysis pattern Fig.6-c: SEM-EDS micranalysis pattern btained frm a selected area (windw) Fig.6: SEM images f xide film f 2.5kg Al-7Si-0.35Mg-0.1Sr ally melt after 300min xidatin at C Accrding t the thermdynamic studies, althugh the frmatin f SrO cntaining mixed xides in the xidised surface is lgical, this may be stated when the xidatin reactins reach t an equilibrium cnditin. It may be hwever, a quick r slw prcess depends n the kinetic f the reactins. Thus in explaining the different xidatin behaviur f Sr in the small and in the large melts the kinetically aspects f xidatin f Sr needs t be analyzed. While xygen can nt diffuse t the Al-allys [3], the xidatin f disslved elements in the Al-allys melts will ccur by diffusin f the elements t the surface layers. The prcess f diffusin f Sr t the surface layer can gradually ccur if there is a shrt diffusin distances; it may be achieved when there is sufficient rati f xidatin surface area t melt vlume (surface mdulus). In a small melt, this rati is significantly higher than that f a large melt. This rati is determined t be 26 mm -1 fr the 47g melt, and 0.38 mm -1 fr the 2.5kg melt. It shws that the area f xidatin surfaces in the large melt is smaller than that f the small melts by abut 68 times, resulting in the nticeably slwer rate f xidatin f Sr in the large melt than that in the small melt (cmpare figs. 2 and 5). This indicates that the equilibrium 7
24.-26.5.2005, Hradec nad Mravicí cnditin fr xidatin reactins in the large melts which lead t frmatin f SrO cntaining xides may be achieved in significantly lnger perids. 4. CONCLUSION 1-In a small melt which has high rati f melt surface area t melt vlume, Sr is a very reactive element in the Al-7Si ally and raises the melt xidatin rate significantly. 2-In a large melt which has lw surface mdulus fr xidatin, Sr has a gd stability and is nt reactive fr xidatin. 5. ACKNOLEDGEMENT Financial and in-kind supprt received frm Rajaei University is gratefully acknwledged. 6. REFERENCES 1-J.E. Gruzleski and B.M. Clsset; The treatment f liquid Al-Si allys, AFS, Des Olaines, IL, 1990 2-S.M. Miresmaeili, W.D. Griffiths, J. Campbell, S.G. Shabestari and S.M.A. Butrabi; ''Precipitatin f Sr-rich Intermetallic Particles and their Effect n Pre Frmatin in A356 Ally'' T be in published in the Jurnal f Metallurgical and Materials Transactin. A. 3-S.M. Miresmaeili, S.G. Shabestari and S.M.A. Butrabi; "Effect f Melt Filtratin n Prsity Frmatin in Sr-mdified A356 Aluminium Ally" Int. J. Cast Metals Res., 2003, Vl. 16, number 6, pages 541-548. 4-S.M. Miresmaeili, S.G. Shabestari and S.M.A. Butrabi; "The Effect f Sr-mdificatin Treatment n Prsity Frmatin f Reduced Pressure 319 Al-ally Castings" Canadian Metallurgical Quarterly, 2003, Vl. 42, number 2, pages 245-252. 5-S.G. Shabestari, S.M. Miresmaeili and S.M.A. Butrabi; "Effects f Sr-mdificatin and Melt Cleanliness n Melt Hydrgen Absrptin f 319 Aluminium Ally" Jurnal f Materials Science, 2003, Vl. 38, pages 1901-1907. 6-J.E. Gruzleski; "The art and science f mdificatin: 25 years f prgress", AFS Trans, 1992, Vl. 100, pages 673-683. 7-L. Liu, A.M. Samuel, F.H. Samuel, H.W. Dty and S. Valtierra; "Influence f xides n prsity frmatin in Sr-treated Al-Si casting allys", Jurnal f Material Science, 2003, Vl. 38, pages 1255-1267. 8-J. Ansn, J.E. Gruzleski and M. Stucky; "Effect f Sr-cncentratin n micrprsity in A356 Al-Ally", AFS Trans, 2001, Vl. 108, pages 243-258. 9-M. Garat, G. Laslaz, S. Jacb; "The state-f-art f the use f antimny, sdium and strntium mdified Al-Si casting allys", AFS Trans, 1992, Vl. 100, pages 821 832. 10-D. Emadi and J.E. Gruzleski; "Melt xidatin behaviur and inclusin cntent in unmdified and Sr- mdified A365 Ally: Their rle in pre nucleatin", AFS Trans, 1996, Vl. 104, pages 763-768. 11-P.Yuen, R.Drew, J.E.Gruzleski and K.Dennis; "Effects f strntium n the xidatin behaviur f mlten aluminum allys cntaining silicn and magnesium", 6th Internatinal AFS Cnference, Mlten Aluminum Prcessing, Orland, FL, USA, 2001 12-O. Kubaschewski and C.B. Alcck, Metallurgical Thermchemistry, Internatinal Series n Materials Science and Technlgy, Vl. 24, Pergamn Press, Lndn, 1979 13-N.B. Pilling, R.E.Bedwrth; "The xidatin f metals at High Temperatures", J. Inst. f Metals, 1923, Vl. 29, pages 529-591. 14- Metals Handbk, American sciety fr metals, Vl 15, Casting 15- JESSNER, G.Hawley, The Cndensed Chemical Dictinary, 10 th ed, Van Nstrand Reinhld C Inc, New Yrk, N.Y.10020, 1981 8