M. Petrič, J. Medved, S. Kastelic, P. Mrvar, 1 Naravoslovnotehniška fakulteta, Oddelek za materiale in metalurgijo / Faculty for Natural Sciences and Engineering, Department of Materials and Metallurgy, Slovenija / Slovenia Meritve dimenzijskih sprememb med strjevanjem Al-Si zlitin Measurement of dimensional changes of AlSi alloys during solidification Povzetek Namen prispevka je raziskati dimenzijske spremembe ulitka med strjevanjem. Analizirani so bili štirje vzorci. Prvi je bil iz zlitine AlSi12 brez dodatkov, drugi je bil udrobnjen s predzlitino AlTi5B1, tretji modificiran s predzlitino AlSr10 ter četrti udrobnjen in modificiran hkrati. Mikrostrukture vseh vzorcev so bile različne v smislu velikosti kristalnih zrn aAl ter velikosti delcev evtektskega Dilatometrijska analiza je pokazala razlike v krčenju vzorca iz zlitine AlSi12 brez dodatkov in modificiranih vzorcev, kjer je krčenje manjše. Strjevanje AlSi12 zlitine je bilo opisano z uporabo različnih metod, kot so termična analiza, dilatometrijska analiza v povezavi s termodinamskim izračunom faznih ravnotežij izvedeno s Thermo-Calc računalniškim orodjem. Metalografska analiza je bila izvedena z uporabo optične in elektronske mikroskopije (SEM). Analize mikrostrukturnih sestavin so bile izvedene z uporabo energijsko disperzijske spektrometrije (EDS). Ključne besede: strjevanje, Al-Si zlitine, dimenzijske spremembe Abstract Solidification of AlSi12 alloy was described using different techniques such as thermal analysis, dilatometric analysis in accordance with thermodynamic calculation carried out by Thermo-Calc software. Metallographic investigation was made using optic and scanning electron microscopy (SEM). Phase analysis was done by energy dispersive spectroscopy (EDS). The aim of the paper is to investigate dimensional changes of a casting during solidification. Four specimens were analysed. First one was AlSi12 alloy without additions, second is grain refined with AlTi5B1 master alloy, third one modified with AlSr10 master alloy and the last one with both master alloys. Microstructures were different in terms of grain size of orAl phase and size of psi particles. Dilatometric analysis has shown differences in shrinkage of basic alloy and modified alloys where shrinkage is lower. Key words: Solidification, Al-Si alloy, Shrinkage 1 Uvod Strjevanje zlitine AlSi12 je opisano z različnimi tehnikami, kot so termična analiza ter dilatometrijska analiza v skladu s termodinamskim izračunom faznih ravnotežij, izvedenim z računalniškim 1 Introduction Solidification of AlSi12 alloy was described using different techniques such as thermal analysis, dilatometric analysis in accordance with thermodynamic calculation carried out by Thermo-Calc software. Metallographic Livarski vestnik, letnik 63, št. 3/2016 155 orodjem Thermo-Calc. Metalografske preiskave so bile izvedene z optičnim mikroskopom in vrstičnim elektronskim mikroskopom (SEM) opremljenim z energijsko disperzivno rentgensko spektroskopijo (EDS) za določevanje mikrostrukturnih sestavin. Cilj članka je ugotoviti dimenzijske spremembe ulitka med strjevanjem. Analizirani so bili štirje vzorci iz zlitine AlSi12. Uporabljeno je bilo tudi udrobnjevanje in modificiranje taline. Za udrobnjevanje je bila uporabljena predzlitina AlTi5B1, za modificiranje pa predzlitina AlSr10. Prvi vzorec je bil izdelan brez predzlitin, v drugega je bilo dodana predzlitina s titanom, v tretjega predzlitina s stroncijem in v četrtega obe. Mikrostrukture so se razlikovale v smislu velikosti mikrostrukturnih sestavin, predvsem dendritnih zrn aAl ter delcev evtektske faze £Si. Dilatometrijska analiza je pokazala razlike v krčenju vzorcev brez dodatkov in vzorcev z dodatkom stroncija, kjer je krčenje manjše. Strjevanje Al—Si zlitin, opisano s pomočjo termične analize, je dobro dostopno v literaturnih virih [1]. Strjevanje ulitkov je povezano z dimenzijskimi spremembami oziroma krčenjem med strjevanjem, kar lahko privede do livarskih napak, kot je krčilna poroznost. Poroznost je posledica krčenja kovin med strjevanjem. Krčenje med strjevanjem aluminija je kar 7 vol. %, pri zlitinah s silicijem pa se krčenje lahko zmanjša. V literaturi je težko zaslediti poročanja o meritvah dimenzijskih sprememb ulitkov med strjevanjem. Nekateri avtorji poročajo o meritvah dimenzijskih sprememb v ulitku iz merilne celice v povezavi z deformacijami merilne celice in nastanka zračne reže med ulitkom in formo [2]. investigation was made using optic and scanning electron microscopy (SEM). Phase analysis was done by energy dispersive spectroscopy (EDS). The aim of the paper is to investigate dimensional changes of a casting during solidification. Four specimens were analysed. First one was AlSi12 alloy without additions, second is grain refined with AlTi5B1 master alloy, third one modified with AlSr10 master alloy and the last one with both master alloys. Microstructures were different in terms of grain size of aAl phase and size of particles. Dilatometric analysis has shown differences in shrinkage of basic alloy and modified alloys where shrinkage is lower. Solidification of aluminium alloys is well described in literature in terms of thermal analysis [1]. Solidification of castings is connected with dimensional changes or shrinkage during solidification of the castings, which can produce defects such as shrinkage porosity. Shrinkage porosity is a consequence of shrinkage of metal during solidification, which can be up to 7 % (volume fraction) in case of aluminium and is decreased with higher amount of Si in Al-Si alloys. Not many authors have reported about dimensional change measurements of castings during solidification. Some have reported about dilatometry of casting in measuring cell and relationship between casting and mould connected with gap formation [2]. 2 Experimental In order to study such phenomena the permanent measuring cell from calcium silicate brick was produced. The casting from the cell was a 220 mm long bar with the square cross-section of 295 mm2. At the end of the bar two quartz rods were inserted into measuring cell connected to two linear 156 Livarski vestnik, letnik 63, št. 3/2016 155 2 Eksperimentalno delo Z namenom študija pojava krčenja ulitka je bila izdelana merilna celica iz silikatne opeke. Ulitek iz merilne celice je imel kvadratni presek z 295 mm2 ter dolžino 220 mm. Na koncih ulitka sta bili v livno votlino vstavljeni dve kvarčni cevčici, ki sta bili povezani na induktivna merilca pomika, ki sta merila deformacije med strjevanjem in nadaljnjim ohlajanjem. Istočasno je bila v toplotnem središču ulitka izvedena preprosta termična analiza. Štirje vzorci iz zlitine AlSi12 so bili pripravljeni. Uporabljeno je bilo tudi udrobnjevanje in modificiranje taline. Za udrobnjevanje je bila uporabljena predzlitina AlTi5B1, za modificiranje pa predzlitina AlSr10. Prvi vzorec je bil izdelan brez predzlitin in je bil označen z 12, v drugega je bilo dodana predzlitina s titanom in je bil označen z 12T, v tretjega predzlitina s stroncijem in je bil označen z 12S in v četrtega obe, označen z 12TS. 3 Rezultati in diskusija Rezultati preproste termične analize ulitka iz merilne celice so predstavljeni na sliki 1a. Ohlajevalna krivulja (T) vzorca 12 prikazuje potek strjevanja zlitine AlSi12. Ohlajevalna krivulja je v skladu s termodinamskim izračunom faznih ravnotežij, ki je prikazano na sliki 2, in prikazuje delež faz v odvisnosti od temperature. V prvem delu strjevanja se strjujejo dendritno izoblikovani kristali trdne raztopine arAl, temu pa sledi strjevanje faze Al9Fe2Si2. V evtektskem delu ohlajevalne krivulje se strjuje evtektik (arAl + £Si), strjevanje pa se zaključi s strjevanjem faze Al14(Fe,Mn)4(Al,Si)5. Reakcije, ki potekajo med strjevanjem, so prikazane tudi na sliki 1a. Vse omenjene faze so bile najdene v mikrostrukturi in z EDS analizo identificirane. Mikrostrukture vzorcev 12 in variable differential transformers (LVDT), which were measuring the dimensional changes during solidification and cooling. At the same time the thermal analysis was carried out in the thermal centre of the casting. Four samples of AlSi12 alloy were prepared. One was without additions and at others the Ti, Sr and both of additions were added simultaneously in form of master alloys AlTi5B1 and AlSr10. Samples were marked as 12, 12T for Ti addition, 12S for Sr addition, 12TS for Ti and Sr addition. 3 Results and Discussion Results of thermal analysis are presented in Fig. 1a. Cooling curve (T) of sample 12 is showing the solidification of AlSi12 alloy. Cooling curve is in accordance with thermodynamic calculation of solidification presented in Fig. 2. In the first sequence of solidification the aAl precipitates followed by Al9Fe2Si2 phase precipitation. At eutectic part of the curve the solidification of (arAl + £Si) eutectic takes place and solidification is finished by Al14(Fe,Mn)4(Al,Si)5 phase precipitation. The reactions of solidification are shown in Fig. 1a. All phases were detected in microstructure and analysed by EDS-analysis. Microstructure with marked phases is presented in Fig. 3. It can be seen that in modified samples the Al2SrSi2 phase is formed as a consequence of Sr addition [3, 4]. The measurement of dimensional changes of sample 12 is presented in Fig. 1b. It is seen on dilatometric curve (A/) that casting is first expanding due to measuring cell expansion because of warming of a cell. Casting is still liquid at the time. When the solid skin is formed in the casting it starts to move the quartz rods and shrinkage of casting starts and takes place till the room temperature. Derivative dilatometric curve Livarski vestnik, letnik 63, št. 3/2016 155 0 0,000-0,002-0,004E -0,006E §-0,008-< "o-0,010--0,012-0,014- E -2- E 0,5 -i 0,0-0,5-1,0E E -1,5-