F. Zupanič1, M. Steinacher2, T. Bončina1 1 University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia / Univerza v Mariboru, Fakulteta za strojništvo, Maribor, Slovenija 2 Impol 2000, d. d., Slovenska Bistrica, Slovenija / Slovenia Lita mikrostruktura nove aluminijeve zlitine AA 6086 As-cast microstructure of a novel Al-Mg-Si alloy Povzetek V raziskavi smo preiskali mikrostrukturo nove zlitine AA 6086, ki je modificirana aluminijeve zlitine 6082 z bakrom, silicijem in cirkonijem. Drogove zlitine s premerom 282 mm smo polkontinuirno lili s postopkom HTAS (Hot Top - Air Slip, litje s kokilo v obliki poroznega grafitnega obroča). Vzorce z glave in noge izbranega droga smo metalografsko pripravili in preiskali z različnimi metodami, kot so svetlobna mikroskopija, vrstična elektronska mikroskopija z mikrokemično analizo, diferenčna termična analiza ter rentgenska fazna analiza. Mikrostruktura je bila sestavljena iz enakoosnih kristalnih zrn aluminijeve trdne raztopine aAl ter različnih mikrostrukturnih sestavin v meddendritnih prostorih in po kristalnih mejah. Z metalografsko analizo smo identificirali sedem faz: a-AlMnSi, Mg2Si, Al3Zr, Si2Zr, b-Si, Al2Cu in Q-AlCuMgSi. Od teh je največ faze a-AlMnSi, ki vsebuje še nekaj Fe in Cr. Faza Mg2Si ima obliko pismenk. Cirkonij je v glavnem vezan v Al3Zr ter delno tudi v Si2Zr. Faze Al2Cu, b-Si in Q-AlCuMgSi so bile navzoče v otočkih, ki so nastali z večfazno evtektično reakcijo preostale taline ob zaključku strjevanja. Procese pri strjevanju smo modelirali z uporabo Scheilovega modela, ki je dal dobro ujemanje z eksperimentalnimi podatki. Na osnovi analize rezultatov eksperimentov ter modeliranja smo podali verjetni potek strjevanja obravnavane zlitine. Ključne besede: aluminijeva zlitina, cirkonij, lito stanje, mikrostruktura, strjevanje Abstract We have investigated the microstructure of a modified aluminium alloy 6082 with additions of copper and zirconium. The billets of this alloy, with the diameter of 282 mm, were semi-continuously cast using HTAS (Hot Top - Air Slip). Samples from the bottom and head of a billet were prepared by metallographic methods, and investigated using different methods, such as light microscopy, scanning electron microscopy with microchemical analysis, differential scanning calorimetry and X-ray diffraction. The microstructure consisted of equiaxed dendritic grains of aluminium solid solution a-Al and different phases in the interdendritic regions and crystal grains. The metallographic investigation revealed seven additional phases: a-AlMnSi, Mg2Si, Al3Zr, Si2Zr, p-Si, Al2Cu in Q-AlCuMgSi. Among them, the highest volume fraction belong to a-AlMnSi that also contained some Fe in Cr. The phase Mg2Si was in the shape of Chinese script. Zirconium was mainly present in Al3Zr and partly in Si2Zr. The phases Al2Cu, p-Si in Q-AlCuMgSi were present with the islands that formed via a multicomponent eutectic reaction of the remaining liquid during the last stage of solidification. The processes during solidification were modelled by using the Scheil model, which gave rather good matching with the experimental data. By analysing the results of the experiments and of the Scheil model, we proposed a probable course of solidification of the investigated alloy. Keywords: aluminium alloy, zirconium, as-cast condition, microstructure, solidification 126 Livarski vestnik, letnik 66, št. 2/2019 1 Uvod Izdelava zelenih vozil, za katera je značilna manjša poraba goriva in zmanjšanje izpustov toplogrednih plinov, je nuja za avtomobilsko industrijo. Eden izmed ukrepov za dosego tega cilja je zmanjšanje mase vozil. To se lahko doseže z zamenjavo težjih jeklenih delov z deli, izdelanih iz lažjih, visokotrdnostnih in hkrati žilavih aluminijevih zlitin. Visokotrdnostne zlitine serije 7xxx (Al-Zn) imajo majhno preoblikovalnost in žilavost. Zato je cilj razviti nove visokotrdnostne zlitine iz serije 6xxx (Al-Mg-Si), ki imajo v osnovnih izpeljankah manjšo trdnost, vendar odlično žilavost in korozijsko obstojnost [1, 2]. Aluminijeve gnetne zlitine, ki temeljijo na sistemu Al-Mg-Si (serija 6xxx), se polkontinuirno lijejo in nato preoblikujejo, končne lastnosti pa dobijo z ustrezno toplotno obdelavo. Pri litju drogov se velikokrat uporablja metoda litja s kokilo v obliki poroznega grafitnega obroča, skozi katerega se med litjem vpihava mešanica olja in zraka, ki ustvari zračno režo med ulitkom in kokilo, HTAS (kratica izvira iz angleškega poimenovanja Hot Top - Air Slip), ki je shematično prikazan na sliki 1 [3]. Čeprav dobijo zlitine na osnovi Al-Mg-Si končne lastnosti šele po preoblikovanju in toplotni obdelavi, ima litje zelo velik vpliv na končne lastnosti. Prevelikih pomanjkljivosti pri litju ni mogoče popolnoma odpraviti z nadaljnjimi postopki. Pomembna je začetna velikost in oblika kristalnih zrn, porazdelitev elementov in mikrostrukturnih sestavin, vrste faz, ki nastanejo ob koncu strjevanja, ter napake, ki se pojavijo na površini in v notranjosti drogov. Pri razvoju nove zlitine je bila optimirana celotna sestava, predvsem pa je pomemben sočasen dodatek cirkonija in bakra. Dodatek cirkonija že lahko vpliva na velikost kristalnih zrn v litem stanju, čeprav 1 Introduction The manufacturing of green vehicles, which are characterised by lower fuel consumptions and greenhouse gas emissions, is a necessity for the automotive industry. One of the measures is the replacement of heavy steel parts with parts made of lighter, high-strength and, at the same time, extremely tough aluminium alloys. High-strength alloys from the 7xxx series (Al- Zr) have a small forming ability and toughness. Therefore, it is a goal to develop high-strength 6xxx alloys (Al-Mg-Si), which have basically lower strength, but excellent toughness and corrosion resistance [1, 2]. Aluminium wrought alloys, based on the system Al-Mg-Si (6xxx), are normally semi- continuously cast, and then formed, and the final properties are obtained by heat treatment. The method is often used for casting of billets [3]. It is shown schematically in Fig. 1. Despite of the fact, that the final properties of Al-Mg-Si alloys are obtained after forming and heat treatment, the casting process has a very important influence on the final properties. Further processing steps cannot remove all deficiencies by casting. The initial grain size, grain shape, distribution of the elements and microstructural constituents, types of phases that formed during final stages of solidification, and defects that form at the surface and inside the bars are very important. By development of a novel alloy, complete chemical composition of the alloy was optimised. The most important are zirconium and copper additions. Zirconium can decrease the size of crystal grains in the liquid state. However, it is that the presence of zirconium in the alloy can reduce the efficiency of Al-Ti-B grain refiners. It is 127 Livarski vestnik, letnik 66, št. 2/2019 Sl. 1. Shematičen prikaz postopka litja po postopku HTAS Fig. 1. Schematic presentation of HTAS-process (Hot Top - Air Slip) je tudi znano, da dodatek cirkonija zmanjša učinek običajnega udrobnila, kot je npr. zlitina Al-Ti-B. Predvsem je pomembno, da pri višjih temperaturah tvori disperzoide Al3Zr, ki močno zmanjšajo nagnjenje k rasti kristalnih zrn pri toplotni obdelavi in vročem preoblikovanju [4, 5]. Tako prispeva cirkonij k višji trdnosti zlitin zaradi disperzijskega utrjanja osnove ter zaradi utrjanja s kristalnimi mejami, ki je mnogo močnejše, če so kristalna zrna zelo majhna. Glavni utrjevalni učinek v zlitinah Al-Mg-Si predstavljajo izločki b' in b". Ti izločki so metastabilne variante ravnote~ne faze Mg2Si. Zato se tudi ti izločki pretvorijo v fazo Mg2Si, še posebej, če se zlitina zadržuje dalj časa pri povišanih temperaturah (med temperaturo staranja in temperaturo solvus). Ob dodatku bakra tem zlitinam dobimo v bistvu kombinacijo med zlitinami 6xxx in 2xxx. V zlitinah prevladuje faza Q-AlCuMgSi in ne faza Q-Al2Cu, ki je večinska v zlitinah 2xxx. Pri staranju v trdni raztopini nastajajo important that zirconium forms Al3Zr dispersoids at higher temperatures [4, 5]. These dispersoids considerably decrease the tendency to crystal growth during heat treatment and hot plastic deformation. In this way, zirconium contributes to strengthening of the alloys by dispersion hardening and hardening by crystal grains. Namely, the latter hardening is more important when the crystal grains are fine. The strengthening effect in Al-Mg-Si alloys is achieved through p' in P" precipitates. This precipitates are metastable variants of the equilibrium Mg2Si phase. These precipitates are turned to Mg2Si when the alloy is held longer times at higher temperatures (between the aging and solvus temperatures). By the addition of copper to Al-Mn-Si we obtain a combination of 6xxx and 2xxx alloy. In these alloys dominates Q-AlCuMgSi and not ©-Al2Cu, which is predominant in 2xxx alloys. During aging Q' precipitates are formed in the a-Al, which strengthen the matrix in addition to P'' precipitates [6]. By modification of Al-Mg-Si alloys by the additions of zirconium and copper, some strengthening mechanisms are enhanced, leading to tensile strengths close to that of 7xxx alloy, but with much higher toughness. The main aim of this contribution is to determine in detail the cast microstructure of the novel high-strength Al-Mn-Si alloy that contains zirconium and copper, and to explain the course of solidification on the basis of the experimental results and results of the Scheil solidification model. 2 Experimental work The alloy was prepared in an induction furnace (Junkers) at temperature of 760 °C. The fraction of zirconium was 0.16 wt.%, which was added with the master alloy 128 Livarski vestnik, letnik 66, št. 2/2019 izločki Q', ki poleg faze b" ae dodatno utrjujejo trdno raztopino [6]. Torej se zaradi dodajanja cirkonija in bakra zlitinam Al-Mg-Si okrepijo mehanizmi utrjanja, kar lahko vodi do nateznih trdnostih, ki so skoraj enakovredne nateznim trdnostim zlitin 7xxx, vendar ob mnogo večji žilavosti. Glavni cilj tega prispevka je natančno opredeliti lito mikrostrukturo nove visokotrdnostne zlitine Al-Mg-Si (AA 6086), ki vsebuje cirkonij in baker, ter podati potek strjevanja na temelju eksperimentalnih rezultatov in rezultatov modeliranja strjevanja s Scheilovim modelom. 2 Eksperimentalno delo Priprava taline je potekala v indukcijskih pečeh (Junkers) na temperaturi 760 °C. Delež cirkonija, ki se je dodajal s predzlitino AlZr10, je znašal 0,16 mas. %. Tako pripravljena talina je bila prelita v zadrževalno peč, v kateri je bila 90 min na temperaturi med 730 in 740 °C. Drogove te zlitine s premerom 282 mm smo polkontinuirno lili po postopku HTAS, dodatek AlTi5B1 je bil 3 kg/t taline. Sestava zlitine je v tabeli 1. Diferenčna termična analiza (DTA) je bila izvedena na napravi Mettler Toledo 851, pri kateri smo v argonu vzorec segrevali in ohlajali s hitrostjo 10 °C/ min. Predzlitino AlZr10 ter vzorce v ulitem stanju smo metalografsko preiskali s svetlobnim mikroskopom Nikon EPIPHOT 300 in programsko opremo analySIS ter vrstičnima elektronskima mikroskopoma AlZr10. The melt was poured into the holding furnace for 90 minutes at temperatures between 730 in 740 °C. The billets with the diameter of 282 mm were semi-continuously cast with a HTAS method. The addition of grain refiner AlTi5B1 was 3 kg/t of melt. The chemical composition of the alloy is given in Table 1. The differential thermal analysis was carried using Mettler Toledo 851. The sample was heated and cooled with the rate of 10 °C/min. The master alloy AlZr10 and samples in the as- cast condition were metalographically examined using a light microscope Nikon EPIPHOT 300 and software analySIS, and two scanning electron microscopes FEI Helios Nanolab 650 in Sirion 400 NC. In SEMs, we also carried out microchemical analysis using EDS. The x-ray diffraction was done at synchrotron Elettra (Trst, Italy), by using X-rays with a wavelength of 0.099996 nm. A software Thermocalc and database we have simulated solidification according to the Scheil model. 3 Results and discussion 3.1 Metallographic analysis The microstructure of the master alloy AlZr10 consisted of aluminium solid solution a-Al and particles of the tetragonal Al3Zr phase, which was in form of platelets (Fig. 2). Their length typically exceeded 200 mm, while their thickness was ten times smaller. X-ray diffraction revealed also the Al13Fe4 Tabela 1. Kemijska sestava modificirane aluminijeve zlitine 6082 Table 1. The chemical composition of the modified 6082 alloy Element Si Fe Cu Mn Mg Cr Zn Ti Zr mas. % wt.% 1.31 0.24 0.25 0.71 0.86 0.16 0.15 0.03 0.16 129 Livarski vestnik, letnik 66, št. 2/2019 FEI Helios Nanolab 650 in Sirion 400 NC. Prav tako smo izvedli mikrokemično analizo EDS v vrstičnih elektronskih mikroskopih. Rentgenska fazna analiza je bila narejena na sinhrotronu Elettra (Trst, Italija), pri čemer smo uporabili rentgensko sevanje z valovno dolžino 0,0999996 nm. S programom Thermocalc in uporabo baze podatkov TCAL5 smo simulirali potek strjevanja po Scheilovem modelu. 3 Rezultati in diskusija 3.1 Metalografska analiza Mikrostruktura predzlitine AlZr10 je bila sestavljena iz osnove a-Al in delcev tetragonalne faze A^Zr, ki je imela obliko ploščic (slika 2). Dolžina ploščic je večinoma presegala 200 mm, debelina pa je bila okoli desetkrat manjaa. Rentgenska fazna analiza je pokazala, da je v predzlitini tudi faza Al13Fe4 (tabela 2), saj je v predzlitini 0,25-0,330 % Fe. Na sliki 3 je svetlobnomikroskopski posnetek mikrostrukture zlitine AA 6086, iz phase (Table 2). Namely, the master alloy contained 0.25-0.30 % Fe. Al13Fe^ mm -, ^^^Bnljk ¿S^** a-Al Al3Zr x jp^ A J 200 um Sl. 2. Mikrostruktura predzlitine AlZr10 (SM). Fig. 2. The microstructure of the master alloy AlZr10 (light micrograph). Figure 3 shows a light micrograph of the modified alloy 6082 in the as-cast condition. The microstructure consisted of equiaxed dendritic grains of a-Al. The grain size was 235 mm, determined using the linear Tabela 2. Mrežni parametri faz v nm v zlitini 6086 (pdf - powder diffraction file) Table 2. The lattice parameters of phases in a modified alloy 6082, given in nm (pdf - powder diffraction file) Glava droga / Head of the bar Noga droga / Butt of the bar Predzlitina AlZr10 Master alloy AlZr10 Al, pdf 04-0787 a = 0,40464 ± 0,00024 a = 0,40480 ± 0,00012 a = 0,40482 ± 0,00006 ZrSi2, pdf 32-1499 a = 0,37460 ± 0,00369 b = 1,47837 ± 0,00688 c = 0,36344 ± 0,00390 a = 0,37069 ± 0,00115 b = 1,47500 ± 0,00246 c = 0,36559 ± 0,00079 Mg2Si, pdf 35-0773 a = 0,63447 ± 0,00071 a = 0,63489 ± 0,00059 A^Zr, pdf 65-5706 a = 0,40040 ± 0,00156 c = 1,72983 ± 0,00044 a = 0,40125 ± 0,00147 c = 1,73132 ± 0,00057 a = 0,40110 ± 0,00081 c = 1,72727 ± 0,00024 alfa-AlMnSi, pdf 87-0528 A,4 01MnSin74 a = 1,26231 ± 0,00277 a = 1,26364 ± 0,00091 AlnFe4, pdf 29-0042 a = 0,77502 ± 0,00216 b = 2,41501 ± 0,01078 c = 0,39955 ± 0,00190 130 Livarski vestnik, letnik 66, št. 2/2019 katere je razvidno, da je bila mikrostruktura sestavljena iz enakoosnih dendritnih kristalnih zrn aluminijeve trdne raztopine aAl. Velikost kristalnih zrn, opredeljena z linearno metodo, je bila 235 mm. Mikrostruktura glave in noge droga je bila zelo podobna. Pravzaprav nismo mogli najti pomembnih razlik, zato vsi rezultati in razprava veljajo za celotni drog. Sl. 3. Lita mikrostruktura zlitine AA 6086, izdelane po postopku HTAS (SM) Fig. 3. The as-cast microstructure of the alloy Al-Si-Mg, with additions of Zr and Cu, cast by HTAS-method (light micrograph) Ostale faze so navzoče v meddendritnih prostorih in so jasneje opazne na mikroposnetku z odbitimi elektroni (slika 4). Z različnimi metodami metalografske analize smo identificirali sedem faz: a-AlMnSi, Mg2Si, A^Zr, Si2Zr, b-Si, Al2Cu in Q-AlCuMgSi. Od teh je največ faze a-AlMnSi, ki vsebuje še nekaj Fe in Cr. Ta faza je svetlosive barve. Faza Mg2Si je črna in ima obliko pismenk. Cirkonij je v glavnem vezan v Al3Zr ter delno tudi v Si2Zr. Obe fazi sta na mikroposnetkih z odbitimi elektroni najbolj svetli. Dokaz o obstoju obeh faz, ki vsebujeta cirkonij, je na sliki 5, kjer je prikazana porazdelitev elementov intercept method. The microstructures of the head and bottom of the billets were almost identical, because it was not possible to find important differences. Therefore, all results and discussion are valid for the whole billet. Other phases that were present in the interdendritic areas are clearly visible in the backscattered electron image (Fig. 4). By using different metallographic techniques, seven phases were identified: a-AlMnSi, Mg2Si, A^Zr, Si2Zr, p-Si, A^Cu in Q-AlCuMgSi. Among these phases, a-AlMnSi prevailed, which contained also Fe and Cr. This phase has a bright- gray colour in Fig. 4. Mg2Si is black, and has a shape of Chinese script. Zirconium is mainly in Al3Zr, but also in Si2Zr. Both phases appeared the brightest in backscattered electron micrographs. A proof for the existence of two phases containing Zr comes from the distribution of alloying elements in Fig. 5. It is clear that Si2Zr crystalizes on the preexisting Al3Zr-particles during solidification. A^Cu, p-Si and Q-AlCuMgSi were present in islands formed during the multi-phase 'f * islands A i otočkijj M J WlM ;r 4 HV WD curr dst mod± tilt 11/17/2014 mag S 10.00 kV 3.7 mm 0.80 nA CBS All 0° 11:02:21AM 300x Sl. 4: Lita mikrostruktura zlitine AA 6086, izdelane po postopku HTAS (SEM, slika z odbitimi elektroni) Fig. 4. The as-cast microstructure of the alloy Al-Si-Mg, with additions of Zr and Cu, cast by HTAS-method (light micrograph) 131 Livarski vestnik, letnik 66, št. 2/2019 v meddendritnem prostoru. Kaže, da faza Si2Zr kristalizira na Al3Zr pri strjevanju. Faze Al2Cu, b-Si in Q-AlCuMgSi so bile navzoče v otočkih, ki so nastali z večfazno evtektično reakcijo (slika 6). Z rentgensko fazno analizo (slika 7) smo identificirali in opredelili mrežne parametre za a-Al, tetragonalni Al3Zr, a-AlMnSi, Mg2Si in Si2Zr. Lege vrhov faze AlMgSiCu se skoraj v celoti ujemajo z a-AlMnSi. Ker faza a-AlMnSi prevladuje, bi na osnovi rentgenske fazne analize težko sklepali, da je v zlitini Q-AlMgSiCu. Pojavi se nekaj manjših vrhov, ki morda pripadajo Al2Cu. Po drugi strani pa rezultati kažejo, eutectic reaction, taking place during the terminal stages of solidification (Figure 6). With X-ray diffraction (Figure 7), we have identified a-Al, tetragonal Al3Zr, a-AlMnSi, Mg2Si in Si2Zr, and determined their lattice parameters (Table 2). The peak positions of AlMgSiCu and a-AlMnSi almost completely coincided. Since the phase a-AlMnSi prevailed in the microstructure, X-ray diffraction does not give conclusive proof for the presence of AlMgSiCu. There were also few peaks belonging to Al2Cu. On the other hand, few peaks confirmed the presence of p-Si. Nevertheless, the fractions of AlMnSiCu, Al2Cu and p-Si was very small, a-Al cx-AlMnSi - N 7 Sl. 5. Elektronski mikroposnetek z odbitimi elektroni in porazdelitev elementov v tem območju Fig. 5. The backscattered electron micrograph and distribution of the elements in the same area 132 Livarski vestnik, letnik 66, št. 2/2019 Sl. 6. Elektronski mikroposnetek z odbitimi elektroni območja, ki se je strdilo nazadnje. Fig. 6. The backscattered electron micrograph of the area, where the last stages of the solidification took place Slika 7. Rentgenska fazna analiza zlitine 6086 v litem stanju, ki kaže, da se vrhovi več faz prekrivajo. Fig. 7. X-ray diffraction of the modified alloy 6082, showing that the peaks of several phases coincided 500 550 600 650 700 750 Temperatura/°C Sl. 8. Segrevalna DTA-krivulja. Začetno stanje je bilo lito stanje, hitrost segrevanja je bila 10 °C min-1 v argonu Fig. 8. The heating DTA-curve. The initial condition was as-cast condition, the heating rate was 10 °C min-1 in argon 133 Livarski vestnik, letnik 66, št. 2/2019 da je navzoč tudi manjši delež silicija. Majhne deleži Q-AlMgSiCu, Al2Cu in b-Si smo potrdili z EDS pri 5 kV. Te faze se lahko brez težav raztopijo pri nadaljnjem homogenizacijskem žarjenju. 3.2 Razvoj mikrostrukture med strjevanjem Strjevanje pri litju drogov gotovo ne poteka ravnotežno. Veliko bolj je verjetno strjevanje, ki je blizu Scheilovemu modelu [7]. Pri tem modelu se privzame, da ni difuzije v trdnem, medtem ko ima preostala talina homogeno sestavo. Ravnotežje se doseže le na fazni meji trdno-tekoče. Model lahko napove kristalne izceje v trdnih raztopinah in faze, ki nastanejo pri strjevanju. Izračun za dano zlitino je pokazal, da poteka strjevanje preko devetnajstih reakcij, pri katerih se pojavi petnajst trdnih faz. V dejanskih razmerah ni nujno, da bi se pojavile vse faze, ker morda ni pogojev za nastanek vseh faz, poleg tega pa lahko nekatere faze izginejo pri reakcijah peritektičnega in prehodnega tipa. Pri metalografski analizi tudi ne moremo ugotoviti vseh nastalih faz, če je njihov fazni delež zelo majhen. Vse faze, ki smo jih identificirali pri metalografski preiskavi, je napovedal tudi Scheilov model. Faze so se praviloma pojavile v okviru večfaznih reakcij. Pri segrevanju potekajo reakcije v obratnem zaporedju kot pri ohlajanju. Namig o številu reakcij lahko dobimo pri segrevanju raziskovane zlitine iz začetnega litega stanja. V ta namen smo izvedli DTA-analizo s hitrostjo segrevanja 10 °C min-1, nastala segrevalna krivulja pa je prikazana na sliki 8. Na krivulji je zaporedje številnih vrhov, ki se med seboj deloma prekrivajo; označeni so s številkami od 1 do 6. Največji je vrh 6, pri katerem se pri segrevanju tali a-Al, pri ohlajanju pa a-Al nastane. thus these phases can easily dissolved during subsequent homogenization. 3.2 Microstructure evolution during solidification The solidification of the billets definitely does not take place under equilibrium conditions. It is much more probable that the solidification problem is closer to the Scheil model [7]. The Scheil solidification model supposes that there is no diffusion in the solid state, and that the remaining liquid possesses the homogeneous chemical composition. The equilibrium is attained only at the solid-liquid interface. The model can predict the microsegregation in the solid solutions and phases that can form during solidification. The calculation for the present alloy has shown that the solidification process should take place over 19 reactions, and that 15 different solid phases can form. It is not necessary that during solidification all predicted phases appear, and it is even less probable that all appear in the as- cast microstructure. Namely, some phases do not have convenient conditions for their nucleation, while some phases can disappear during peritectic and transition reactions. In addition, if the volume fraction of a phase is very small, it is difficult to identify it using metallography. It is important to stress that all experimentally identified phases were predicted by the Scheil solidification model. These phases usually appeared by the multiphase reactions. During segregation, the reactions take place in the opposite order than upon cooling. An indication about the number of reactions can be obtained by heating of the investigated alloy from the initial as-cast condition. For that purpose, a DTA-analysis was carried out at the heating rate of 10 °C 134 Livarski vestnik, letnik 66, št. 2/2019 Kot omenjeno, strjevanje poteka v nasprotni smeri. Od faz, ki smo jih našli v mikrostrukturi, se prvi pojavi tetragonalni Al3Zr pri temperaturi nekoliko nad 700 °C. Torej nastane Al3Zr kot primarna trdna faza. Na DTA-krivulji ni ustreznega vrha, povsem mogoče zato, ker je fazni delež Al3Zr zelo majhen. Pri približno 640 °C nastane a-Al, s peritektično reakcijo, pri kateri talina reagira z Al3Zr (vrh 6). V ravnotežnih razmerah bi se moral Al3Zr popolnoma porabiti. Na nekaterih delcih Al3Zr se morda nukleira a-Al, medtem ko ostale delce Al3Zr rastoči dendriti a-Al potiskajo v preostalo talino; to je v bistvu v meddendritni prostor, v katerem najdemo Al3Zr po strjevanju. Faza a-AlMnSi začne nastajati z evtektično reakcijo pri približno 630 °C. Pri tem se vrh 5 prekriva z mnogo večjim vrhom 6, tako da vrh 5 ni samostojen. Pri vrhu 4 se pojavi faza Si2Zr, ki delno prekrije ploščice Al3Zr (glej sliko 5). Vrh 3 je povezan z nastankom faze Mg2Si. Ta raste v okviru heterogenega zloga skupaj z a-Al. Čeprav se hkrati z Mg2Si izloča še več faz, raste lokalno ločeno in ima videz binarnega evtektika, v katerem ima faza Mg2Si obliko pismenk. Pri vrhu 2 nastane v zadnjih območnih preostale taline faza Q-AlCuMgSi, povsem na koncu pa še fazi b-Si in Al2Cu (vrh 1). Obe fazi nastaneta pri večfaznih evtektičnih reakcijah, ki so značilne za zaključke strjevanj v večkomponentnih sistemih. S kombinacijo metalografske analize litega stanja, termične analize in Scheilovega modela strjevanja lahko racionalno razložimo potek strjevanja sodobne večkomponentne zlitine. min-1, and the DTA heating curve is shown in Fig. 8. The curve shows the sequence of several peaks that partly overlap. They are indicated by numbers from 1 to 6. The highest is the peak 6. This peak is caused by melting of a-Al, however, during cooling a-Al is formed. As mentioned before, the solidification takes place in the opposite direction. Among the phases that were identified in the microstructure, the first phase that appears in the tetragonal Al3Zr that should form at temperature slightly above 700 °C. Therefore, Al3Zr forms as a primary phase. DTA-curve does not indicate any corresponding peak. It is likely that the volume fraction of Al3Zr is too small. At around 640 °C forms a-Al, via a peritectic reaction, at which the liquid phase reacts with Al3Zr (peak 6). Under equilibrium conditions, all V Al3Zr should be consumed. The aluminium grains a-Al may form on some Al3Zr-particles, while other remaining Al3Zr are pushed by the growing dendrites into the remaining liquid; that is in the interdendritic regions, where Al3Zr is found after solidification. Phase a-AlMnSi starts to form around 630 °C by a eutectic reaction. The peak 5 overlaps by the much greater peak 6; therefore, peak 5 is not standalone. At peak 4, appears Si2Zr that partly covers Al3Zr platelets (see Figure 5). Peak 3 is related to the formation of Mg2Si. This phase grows in the form of heterogeneous microstructural constituent together with a-Al. In spite of the fact that several phases crystallize from the liquid together with Mg2Si, Mg2Si grows together with a-Al in separate regions as a binary eutectic, and has the shape of Chinese script. Peak 2 indicates the formation of Q-AlCuMgSi. At the terminal stages of solidification form also p-Si in Al2Cu (peak 1). Both phases formed by multiphase eutectic reactions that are 135 Livarski vestnik, letnik 66, št. 2/2019 4 Zaključki Z raziskavo nove aluminijeve zlitine AA 6086, smo prišli do naslednjih zaključkov: • mikrostruktura je bila sestavljena iz enakoosnih kristalnih zrn aluminijeve trdne raztopine aAl ter različnih mikrostrukturnih sestavin v meddendritnih prostorih in po kristalnih mejah; • v meddendritnem prostoru smo identificirali sedem faz: a-AlMnSi, Mg2Si, Al3Zr, Si2Zr, b-Si, Al2Cu in Q-A lCuMgSi; • največ je bilo faze a-AlMnSi, ki je vsebovala še nekaj Fe in Cr. Faza Mg2Si ima obliko pismenk. Cirkonij je v vezan v Al3Zr in v Si2Zr; • faze Al2Cu, b-Si in Q-AlCuMgSi so nastale z večfazno evtektično reakcijo preostale taline ob zaključku strjevanja; • proces strjevanja poteka preko šestih stopenj. Faze, ki smo jih identificirali, smo napovedali s Scheilovim modelom strjevanja. Vseh faz, ki jih je napovedal Scheilov model strjevanja, pa nismo odkrili v mikrostrukturi. Zahvala Delo je bilo izvedeno v okviru Slovenske pametne specializacije, program Materiali in tehnologije za nove aplikacije, MARTINA, OP20.00369, in infrastrukturnega programa UM I0-0029, ki ga financira ARRS. Zahvaljujemo se Sincrotrone Elettra, Trst, Italija in Luisi Barba za izvedbo rentgenske fazne analize. typical for the final stages of solidification in multi-component systems. The combination of the metallographic analysis of the as-cast condition, thermal analysis and Scheil solidification model allowed us to rationally explain the solidification of a modern multicomponent aluminium alloy. 4 Conclusions The investigation of the alloy Al-Mg-Si, modified by zirconium and copper, lead us to the following conclusions: • The as-cast microstructure consisted of equiaxed dendritic grains of aluminium solid solution a-Al and different microstructural constituents in the interdentritic regions and crystal grains. • We have identified seven phases in the interdendritic regions: a-AlMnSi, Mg2Si, Al3Zr, Si2Zr, p-Si, Al2Cu in Q-AlCuMgSi. • The phase a-AlMnSi prevailed; containing also Fe in Cr. Mg2Si had a form of Chinese script. Zirconium was present in Al3Zr and Si2Zr. • Al2Cu, p-Si in Q-AlCuM gSi formed via multi-phase eutectic reaction during terminal stages of solidification. • The solidification process took place over six stages. The Scheil solidification model predicted the phases that were identified in the microstructure. On the other hand, not all phases were found in the microstructure that were predicted by the Scheil model. Acknowledgments The work was carried out in the framework of the Slovenian smart specialization, programme Materials and Technologies for New Applications, MARTINA, No. OP20.00369 and Infrastructure programme 136 Livarski vestnik, letnik 66, št. 2/2019 UM I0-0029 financed by the National Research Agency ARRS. Reference / References 1. H.J. McQueen, O.C. Celliers, Application of hot workability studies to extrusion processing .3. Physical and mechanical metallurgy of Al-Mg-Si and Al-Zn-Mg alloys, Can. Metall. Q., 36 (1997) 73-86. 2. Y. Himuro, K. Koyama, Y. Bekki, Precipitation behavior of zirconium compounds in Zr- bearing Al-Mg-Si alloy, in: W.J. Poole, M.A. Wells, D.J. Lloyd (Eds.) Aluminium Alloys 2006, Pts 1 and 2: Research through Innovation and Technology, Trans Tech Publications Ltd, Zurich-Uetikon, 2006, pp. 501-506. 3. R. Vertnik, M. Založnik, B. Šarler, Solution of transient direct-chill aluminium billet casting problem with simultaneous material and interphase moving boundaries by a meshless method, Engineering Analysis with Boundary Elements, 30 (2006) 847-855. 4. L. Li, Y. Zhang, C. Esling, H. Jiang, Z. Zhao, Y. Zuo, J. Cui, Crystallographic features of the primary Al3Zr phase in as-cast Al-1.36wt% Zr alloy, Journal of Crystal Growth, 316 (2011) 172-176. 5. L. Litynska, D. Abou-Ras, G. Kostorz, J. Dutkiewicz, TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy, Journal of Microscopy-Oxford, 223 (2006) 182-184. 6. D.J. Chakrabarti, D.E. Laughlin, Phase relations and precipitation in Al-Mg-Si alloys with Cu additions, Prog. Mater. Sci., 49 (2004) 389-410. 7. M. Pellizzari, Thermodynamic modeling for the alloy design of high speed steels and high chromium cast irons, Mater. Tehnol., 44 (2010) 121-127. AKTUALNO / CURRENT Koledar livarskih prireditev 2019 in 2020 Datum dogodka Ime dogodka Lokacija 18.-20.09. 2019 WFO-Technical Forum in 59. 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