Paper describes influence of grain refining and of modification on feeding behavior of Al – alloys. Research was made with the AlSi9Cu3 alloy. Three samples of basic, grain refined, and modified alloy were cast by the newly developed method for determining susceptibility to shrinkage and into the Quick cup measuring cells for simple thermal analyses which were simultaneously done with all these samples. Castings were then analyzed with X-raying, density measurements, visually observed and metallographically examined. Also grain size was determined with light microscopy and polarized light.Ovaj rad opisuje utjecaj dodatka cjepiva i modifikatora na ponašanje napajanja Al-legura. Istraživanja su
provedena na AlSi9Cu3 leguri. Tri su uzorka, iz osnovne legure, cijepljene i modificirane, lijevana u „Quick Cup“ mjerne ćelije primjenom novo razvijene metode za određivanje osjetljivosti na stezanje pri skrućivanju. Jednostavna toplinska analiza provedena je simultano u sva tri uzorka. Odljevci su potom ispitani određivanjem gustoće, vizualnom kontrolom i metalografskom analizom. Također, veličina zrna je određena optičkom mikroskopijom u polariziranom svjetlu

J. Medved

M. Petrič

P. Mrvar

Hrčak - Portal of scientific journals of Croatia

M. PETRI^, J. MEDVED, P. MRVAREFFECT OF GRAIN REFINEMENT AND MODIFICATIONOF EUTECTIC PHASE ON SHRINKAGE OF AlSi9Cu3 ALLOYReceived – Prispjelo: 2009-11-23Accepted – Prihva}eno: 2010-01-15Preliminary Note – Prethodno priop}enjeINTRODUCTIONAll metals and alloys with exception of Bi areshrinking during cooling, mostly during the solidifica-tion process. In order to avoid shrinking porosity, cast-ing process, and gating and feeding system must beproperly designed. Solidification front must move to-wards the feeding system where solidification is thencompleted. No separate liquid pockets where solidifica-tion ended without feeding should exist. In order to meetthese demands the feeders and feeder necks must havesuitable modules to keep metal liquid during transport tothe solidification front 1.There exist two types of solidification, exogenousand endogenous.Exogenous solidification is typical for pure andeutectic alloys that solidify at constant temperatures.Solidification starts there on interface with the mouldwhere solid layer of metal is formed, and solidificationfront is then moving towards the center of casting or to-wards the area of the highest temperature. Exogenoussolidification type is further divided into three sub-groups, i.e. solidification with smooth shell, dendriticsolidification, and spongy-layer solidification.Endogenous solidification is typical for alloys withhypo and hypereutectic compositions that are solidify-ing in a temperature range. Solidification starts through-out the whole volume of the casting, and solid shell oninterface with the mould is formed more slowly. Thereare two subtypes of endogenous solidification. One ismushy, and the other one is shell-forming type 2. So-lidification types are mixed in all the real cases. Solidifi-cation starts with one prevailing type which is changedinto another one after some time. This mechanism de-pends on cooling rate since this is changed during the so-lidification process 2, 3.Type of solidification has also influence on feedingability of alloy. Alloys with smooth-wall type of solidi-fication have the best feeding abilities which result thatfeeder neck remains liquid during the whole solidifica-tion process and thus transport of liquid metal to the so-lidification front is enabled. Feeding ability of alloys islower with the endogenous dendritic type and it be-comes even worse with the mushy-type solidificationwhere flow of liquid metal is disturbed by floating crys-tals that are already present in liquid metal. Feedingabilities of alloys dependent on solidification type arepresented in Figure 1.Nucleation potential has big influence on shrinkageporosity, as well. When alloy is properly grain-refined ithas higher nucleation potential than the basic alloy andMETALURGIJA 50 (2011) 2, 127-131 127Paper describes influence of grain refining and of modification on feeding behavior of Al – alloys. Research wasmade with the AlSi9Cu3 alloy. Three samples of basic, grain refined, and modified alloy were cast by the newlydeveloped method for determining susceptibility to shrinkage and into the Quick cup measuring cells for sim-ple thermal analyses which were simultaneously done with all these samples. Castings were then analyzed withX-raying, density measurements, visually observed and metallographically examined. Also grain size was deter-mined with light microscopy and polarized light.Key words: AlSi9Cu3 alloy, shrinkage, solidification morphology, sinkUtjecaj usitnjavanja zrna i modifikacije eutekti~ke faze na stezanje pri skru}ivanju AlSi9Cu3 legu-re. Ovaj rad opisuje utjecaj dodatka cjepiva i modifikatora na pona{anje napajanja Al-legura. Istra`ivanja suprovedena na AlSi9Cu3 leguri. Tri su uzorka, iz osnovne legure, cijepljene i modificirane, lijevana u „Quick Cup“mjerne }elije primjenom novo razvijene metode za odre|ivanje osjetljivosti na stezanje pri skru}ivanju. Jedno-stavna toplinska analiza provedena je simultano u sva tri uzorka. Odljevci su potom ispitani odre|ivanjem gu-sto}e, vizualnom kontrolom i metalografskom analizom. Tako|er, veli~ina zrna je odre|ena opti~kommikroskopijom u polariziranom svjetlu.Klju~ne rije~i: AlSi9Cu3 legura, stezanje pri skru}ivanju, morfologija skru}ivanja, uvlakaISSN 0543-5846METABK 50(2) 127-131 (2011)UDC – UDK 669.2/8:669.715:539.377:536.425=111M. Petri~, J. Medved, P. Mrvar, Faculty of Natural Sciences and Engi-neering, University of Ljubljana, Sloveniathe course of crystallization is taking place with highernumber of nuclei. This results in number of primary crys-tals and better distribution of shrinking porosity 2.Grain refining has beneficial effect on feeding ability ofan alloy since smaller crystals in melt are more movableand can more easily reach the solidification front 4.Interdendritic feeding is more problematic in later se-quences of solidification process due to smaller formeddendrites but it has advantage due to finer shrinkage po-rosity distribution and lower porosity fraction 5. On theother hand, grain refining can worsen feeding ability,causing endogenous type of solidification 2. Conse-quence of endogenous type is also sink of casting walls assolid casting shell is not firm enough to stay the pressure,resulting from shrinking during the solidification process.Feeding ability and porosity are influenced by modi-fication, as well. Gas porosity is usually consequence ofdissolved hydrogen in the melt that increases with addi-tions of modifying agents 5, 6. In some cases, this iseven advantageous since gas porosity can compensateshrinkage during the solidification. Modification canalso lead to higher amount of shrinking porosity as Sradditions cause irregular solidification front resulting inentrapped liquid areas that are not fed and what is caus-ing micro shrinkage porosity. Such defects do not ap-pear if sodium is used as modifying agent 5.EXPERIMENTALTest casting for determining the feeding ability wasdeveloped on basis of castings that R. Hummer 7 usedfor studying feeding ability of cast iron. Test casting wascomposed of four cubes with dimensions 30×30×30mm. Each cube was connected to feeder by feeder neck.Feeder necks had different dimensions that resulted indifferent modules. Module of cubes was 5 cm, and mod-ules of feeder necks are presented in Table 1. 3D CADmodel is presented in Figure 2a.Analyses of feeding ability as functions of grain re-finement and modification have been performed withthe AlSi9Cu3 alloy. Grain refining agent was theAlTi5B1 master alloy with additions of 0,05 wt. % ofTi. AlSr10 master alloy with addition of 0,01 wt. % Srwas used as modifying agent. The first samplerepresented basic alloy, the second one was grain-re-fined alloy, and the last one was made of modified alloy.Samples of 1000 g melt were prepared and cast intomould for test casting and into the Quick Cup measuringcell for simple thermal analysis.Test castings were then analyzed with light micro-scope, X-rayed with YXLON MU 2000 X-ray device,while density measurements were made with the Mohrscales. Amount of shrinking porosity together with thefraction of sink as due to shrinkage during the solidifica-tion process were measured. Castings from the QuickCup measuring cell were analyzed with Olympus BX 61light microscope and grain sizes of Al grains and sizesof Si eutectic phase were determined using the programanalysis 5,0.RESULTS AND DISCUSSIONMacro analysis of test castings made of the basic andthe grain-refined alloy showed that there was great sinkon upper sides of cubes and lateral sides of feeders. Sinkin the castings made of the modified alloy occurred insmaller degree only on cubes. Figure 2b is presenting128 METALURGIJA 50 (2011) 2, 127-131M. PETRI^ et al.: EFFECT OF GRAIN REFINEMENT AND MODIFICATION OF EUTECTIC PHASE ON SHRINKAGE...Figure 1 Metal flow from feeder to casting, dependingon type of solidification: smooth shell (a), den-dritic (b) and mushy (c) 2Table 1 Sizes and modules of feeder necksNo. of cubeModule of feederneck / cmDimensions of feederneck / mm1 1,25 5 × 5 × 102 1,88 7.5 × 7.5 × 103 2,5 10 × 10 × 104 3,75 15 × 15 × 10(a)(b)(c)Figure 2 CAD geometry of test casting with various fee-der necks (a) and test casting of basic alloywith sink on cubes and feeders (b)test casting of the basic alloy where sink can be seen.Sink was the lowest on the No. 4 cube that had the high-est module of feeder neck and metal could flow fromfeeder to cube for longer time. Reason for sink is usuallyendogenous mushy solidification where solid shell atwall of the casting is not firm enough to stay pressure ofmelt in the casting.Size, shape, and position of shrinkage porosity weredetermined by X-ray analysis. Sink was found too.Shrinkage porosity had form of larger holes in the sam-ple of basic alloy, while in the grain-refined alloy it wasfinely dispersed in the area of final solidification – cen-ter of cubes. Lower amount of shrinkage porosity wasobserved in the casting of modified alloy, and there wasalso smaller sink of cube walls. Figure 3 is showingX-ray images of No. 1 cubes of all the three samples.Density measurement showed similar results asX-ray analyses. Table 2 presents results of density mea-surements for cubes and feeders with correspondingshrinkage porosity and also sink as result of alloyshrinkage. Basic alloy sample showed the lowest den-sity in the No. 3 cube and the highest one in the No. 2cube. Same results were obtained with light-microscopeanalysis of cross sectioned cubes and feeders, shown inFigure 4. Similar results were obtained with thegrain-refined sample where the lowest density had theNo. 3 cube and the highest one the No. 1 cube. It wasseen in Figure 4 that the No. 3 cube had the highest frac-tion of shrinkage porosity which corresponded with thedensity measurements, while the lowest fraction ofshrinkage porosity had the No. 4 cube but there ap-peared huge sink that represented loss of density, thusthe highest density had the No.1 cube. The highest den-sity in the sample of modified alloy had the No. 1 cube,and it could be seen that there was no sink. The lowestdensity had the No. 4 cube. It could be seen that therewas sink only on the Nos. 2, 3 and 4 cubes. Densities inthe modified sample were in average for 0,02 kg/dm3lower than in the other two samples, and reason for thisis higher gassing of the alloy that caused gas porositywhich compensated shrinkage porosity.Results obtained with the cross-sectioned castings arecomparable with the results obtained with the densitymeasurements and X-ray analyses. It can be seen that incubes made of the basic alloy, the shrinkage porosity wasthe highest in the No. 3 cube and the lowest in the Nos. 1and 2 cubes. It was also found that there was almost noshrinkage porosity in feeders in all the cases but sink wasobserved with the basic alloy and the grain-refined alloy.This suggested that feeding system for test casting wasnot correctly designed. Shrinkage porosity was better dis-tributed and was finer due to grain refinement of the grainrefined alloy. Amount of porosity was the highest in theNo. 3 cube and the lowest in the No. 4 cube. Also sinkwas observed there. Densities were slightly higher in thebasic alloy, followed by the grain-refined alloy. Reasonfor this was sink as density loss in density measurements.If sink would not be taken in account in density measure-ments, density would be the same and this confirmed thatthere was greater sink in the grain-refined alloy as resultof changed solidification type to the mushy one. Amountof shrinkage porosity in the modified alloy was lowerthan in the other two alloys but it was found that there waspresent gas porosity throughout the whole volume ofcubes and feeders. This was consequence of Sr additionthat caused higher gassing of Al melt 8. Additional gas-sing was caused due to 10 minutes delayed pouring.Formed gas porosity has compensated shrinkage porosityduring the solidification so there was no sink on feedersand on the No. 1 cube and fraction of shrinkage porositywas lower, as well. This result corresponded to references5 and 6 too.Cooling curve with characteristic temperatures andcorresponding reactions of the basic alloy that waspoured into the Quick Cup measuring cell is presented inFigure 5 while all the cooling curves are presented inFigure 6. Table 3 presents characteristic temperatures,where TP (°C) is the pouring temperature. It can be seenthat minimum liquidus temperatures TL min (°C) of thebasic and of the modified alloy are approximately thesame, they are 562,3 °C for the basic alloy and 565,0 °Cfor the modified one. Liquidus temperature is about10 °C higher with the grain-refined alloy, which is resultMETALURGIJA 50 (2011) 2, 127-131 129M. PETRI^ et al.: EFFECT OF GRAIN REFINEMENT AND MODIFICATION OF EUTECTIC PHASE ON SHRINKAGE...Figure 3 X-ray photos of No. 1 cubes: basic alloy (a),grain-refined alloy (b), and modified alloy (c)Table 2 Densities of cubes and feeders for all thesamples / kg/dm3No. of cube 1 2 3 4Basic alloyCube density 2,646 2,655 2,622 2,650Feeder density 2,656 2,653 2,670 2,655Grain refined alloyCube density 2,646 2,633 2,618 2,641Feeder density 2,649 2,644 2,661 2,638Modified alloyCube density 2,652 2,609 2,579 2,556Feeder density 2,629 2,644 2,661 2,652of grain refinement where under-cooling is not neededfor formation of nuclei 3. Also recalescence in primarycrystallization, TL (°C), was lower with the grain-re-fined alloy (0,8 °C), followed by the basic and the modi-fied alloy. It was 2,6 and 1,2 °C, respectively. TL max (°C)is the maximal liquidus temperature. Eutectic reaction istaking place at the minimum eutectic temperature, TE min,being 562,6 °C only with the modified alloy since it isfor 6 °C lower due to Sr modification 5 that is causingtemperature drop. There was found also recalescencewith the eutectic reaction, being TE = 0,5 °C. Solidifi-cation is completed with reactions representing solidifi-cation of remnant liquid into ( + Mg2Si) eutectic attemperature TE2 (°C) and ( + Al2Cu – ) eutectic attemperature TE3 (°C). Solidus temperature TS (°C) isthus between 488,9 and 473,6 °C.Microstructural constituents in the basic alloy weredetermined with metallographic analyses and takenfrom references 8 and 9. Figure 7 is presenting mi-crograph of the basic alloy, cast into the Quick Cup mea-suring cell, with microstructural constituents. Next tophases Al and Si also (Alx(Fe,Mn)ySiz) iron containingphase was found. It precipitated at the beginning of so-lidification. During cooling process, the stoichiometryof this phase changed 9. There were observed alsoMg2Si and Al2Cu- eutectic phases in the micro-structure.Grain sizes and sizes of eutectic Si phase were mea-sured metallographically in polarized light and in brightfield. Effects of grain refinement and modification weredetermined. Grain sizes were determined according tothe ASTM E 112-96 standard. Grain size in the basic al-loy from the Quick Cup measuring cell was 841 m, inthe grain-refined alloy only 469 m, and in the modifiedone 1000 m. Sizes of eutectic Si phase were measuredmicroscopically in bright field. The average particle sizein the basic alloy was 44,41 m, in the grain-refined one56,16 m, and in the modified one only 11,63 m. Theseresults show that grain refining with Ti and modificationwith Sr were successful. Figure 9 is presentingbright-field micrographs of the basic and the modifiedalloy from the Quick Cup cell.CONCLUSIONSIt was found that grain refining and modification ofthe AlSi9Cu3 alloy affect the solidification morphology130 METALURGIJA 50 (2011) 2, 127-131M. PETRI^ et al.: EFFECT OF GRAIN REFINEMENT AND MODIFICATION OF EUTECTIC PHASE ON SHRINKAGE...Figure 4 Cross sections of cubes and feeders: basic alloy(a), grain-refined alloy (b), and modified alloy (c)Figure 5 Cooling curve of basic alloy from Quick Cupmeasuring cell with characteristic temperaturesand reactions during solidificationFigure 6 Cooling curves of all the three samples fromQuick Cup measuring cellTable 3 Characteristic temperatures from cooling curves for each alloyTemperatures / °C TpTL minTL maxTLTE minTE maxTETE 2TE 3TSBasic alloy 678,4 562,3 564,9 2,6 562,6 562,6 0 505,1 490,9 473,6Grain-refined alloy 638,7 572,1 572,9 0,8 565,1 565,1 0 510,7 502,5 488,3Modified alloy 635,8 565,0 566,2 1,2 556,5 557,0 0,5 505,6 497,3 488,9Figure 7 Bright-field micrograph of the basic alloy, castinto the Quick Cup measuring cell, with defi-ned microstructural constituentsand feeding ability too. Shrinkage porosity in the testcasting made of the basic alloy was coarse in the centerof cubes where melt solidified the last. Shrinkage poros-ity was much finer and better dispersed in the grain-re-fined alloy thus having favorable influence on mechani-cal properties. Sink appeared on top walls of cubes andside walls of feeders of these two alloys. Reason for thiswas endogenous mushy-type of solidification while un-der-pressure in the casting and feeder during the solidifi-cation process represented additional unfavorable influ-ence. Density of the basic alloy was slightly higher thanthat of the grain-refined one, and reason for this was thatsink that was taken as density loss in the density mea-surements, otherwise densities would be the same. Fur-ther, it was also found that sink was greater in thegrain-refined alloy than in the basic one as consequencethat type of solidification changed into mushy one. Sinkin the modified alloy was present only on the Nos. 2, 3and 4 cubes but none was observed on the No. 1 cubeand on feeders. There was also lower shrinkage porositybut higher fraction of gas porosity that was consequenceof the Sr modification. Gas porosity has compensatedshrinking and sinks during the solidification, so therewas lower shrinkage porosity but densities were alsolower than in the other two alloys because of high gasporosity. Since shrinkage porosity occurred mainly incubes and not in feeders, it proved that solidificationstarted in feeders and was completed in cubes, and thatwas wrong. It was obvious that the feeding system oftest castings was incorrectly designed. In further analy-ses of feeding ability, the feeding system for test cast-ings should be changed.REFERENCES1 P. Mrvar, M. Trbi`an, J. Medved, Metalurgija 40 (2001) 4,81-84.2 W. Glat, W. Konzelman, H. Orbon, ed.. Aluminium Gu-sslegirungen. Duesseldorf: Vereinigung Deutscher Schmel-zhuetten, 1988.3 J.G. Sylvia, Cast Metals Technology. Des Plaines: Ameri-can Foundrymen’s Society, Inc, 1990.4 R. Cook, Grain refinement of aluminium – silicon foundryalloys. London: London & Scandinavian metallurgical Co.Limited, 1998, 9-12.5 R. Cook, Modification of aluminium – silicon foundry al-loys. London: London & Scandinavian metallurgical Co.Limited, 1998, 12-14.6 J.R. Davis, ed.. ASM Specialty Handbook, Aluminiumand Aluminium Alloys. s.l.: ASM International, 1993,220-224.7 R. Humer, Livarski vestnik 37 (1990) 2, 33-52.8 L. Bäckerud, G. Chai, J. Tamminen, Solidification characte-ristics of aluminium alloys. vol. 2: Foundry alloys. Stoc-kholm: Skanaluminium, 1986, 151-194.9 B. Markoli, S. Spai}, F. Zupani~, Aluminium 80 (2004) 1/2,84-88.Note: The responsible translator for English language is prof. dr. A.Paulin, Slovenia.METALURGIJA 50 (2011) 2, 127-131 131M. PETRI^ et al.: EFFECT OF GRAIN REFINEMENT AND MODIFICATION OF EUTECTIC PHASE ON SHRINKAGE...Figure 8 Micrographs in polarized light: basic alloy (a),grain refined alloy (b)Figure 9 Bright-field micrographs: basic alloy (a), modi-fied alloy (b)

Utjecaj usitnjavanja zrna i modifikacije eutektičke faze na stezanje pri skrućivanju AlSi9Cu3 legure

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia