UDK 669.715:621.74.043	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 48(6)917(2014)
EFFECT OF AlTi5B1 AND AlSr10 ADDITIONS ON THE FLUIDITY OF THE AlSi9Cu3 ALLOY
VPLIV DODATKOV AlTi5B1 IN AlSr10 NA LIVNOST ZLITINE
AlSi9Cu3
Matej Steinacher1, Franc Zupanic1, Mitja Petric2, Primož Mrvar2
1University of Maribor, Faculty of Mechanical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia 2University of Ljubljana, Faculty of Natural Sciences and Engineering, Aškerčeva cesta 12, 1000 Ljubljana, Slovenia
matej.steinacher@gmail.com
Prejem rokopisa - received: 2014-01-17; sprejem za objavo - accepted for publication: 2014-01-31
This work studies the effect of the AlTi5B1 and AlSr10 additions on the fluidity and the solidification time of the AlSi9Cu3 casting alloy. The fluidity was investigated by determining the flow length in a spiral-shaped mould. The solidification time was measured with a thermocouple positioned at the ingate bottom. An individual pouring into the preheated (200 °C) metallic mould was done at different pouring temperatures ((640, 670, 700, and 710) °C). In all the cases, the fluidity was improved with the increasing pouring temperatures. An addition of the AlTi5B1grain refiner to the basic alloy reduced both the grain size and the fluidity, whilst the solidification time was similar to that of the basic alloy. On the other hand, an addition of the AlSr10 modifier refined the /SSi eutectic phase, increased the fluidity and prolonged the solidification time in comparison to the basic alloy. The fluidity was proportional to the solidification time. Thus, by carrying out a simple thermal analysis and determining the solidification time, it is possible to predict the fluidity.
Keywords: AlSi9Cu3 alloy, grain refinement, modification, fluidity, solidification time
V deluje predstavljen vpliv dodatkov AlTi5B1 in AlSr10 na livnost in strjevalni čas livne aluminijeve zlitine AlSi9Cu3. Livnost smo preiskovali z merjenjem dolžine toka taline v kovinski kokili s spiralno livno votlino, medtem ko smo strjevalni čas merili s termoelementom, ki je bil vstavljen na dnu lijaka. Talino smo pri različnih livnih temperaturah ((640, 670, 700 in 710) °C) ulivali v predgreto kokilo (200 °C). Livnost je v vseh primerih naraščala z naraščanjem livne temperature. Dodatek udrobnilnega sredstva AlTi5B1 k osnovni zlitini je zmanjšal tako velikost kristalnih zrn kot livnost, medtem ko je bil strjevalni čas podoben kot v osnovni zlitini. Dodatek modifikatorja AlSr10 je zmanjšal velikost evtektične faze /SSi, povečal livnost ter podaljšal strje-valni čas v primerjavi z osnovno zlitino. Livnost je bila sorazmerna strjevalnemu času, tako da lahko z enostavno termično analizo in določanjem strjevalnega časa napovemo livnost preiskovane zlitine.
Ključne besede: aluminijeva zlitina, udrobnjevanje, modificiranje, livnost, strjevalni čas
The effect of grain refining on the fluidity of
1 INTRODUCTION	Al-alloys depends on several factors: the type and
amount of the grain refiner, the composition of the alloy,
Aluminium casting alloys are extensively used in the	the holding time and the temperature within the furnace6.
production of thin-walled castings. This is limited by the	Tiryakioglu et al.7 found that an addition of the mass
fluidity of the molten metal. Fluidity is defined as the	fraction w = 0.04 % of Ti, in the form of an AlTi5B1
ability of a molten metal to flow until it ceases due to	master alloy, to the A356 Al-Si alloy did not affect the
solidification1. The fluidity is influenced by many factors	fluidity measured in the sand spiral test. On the other
that can be divided into metallurgical and mould/casting	hand, the fluidity was reduced with the additions of Ti
variables. The metallurgical factors are the composition,	below w = 0.12 %, whilst increased with the Ti additions
superheat and latent heat, the surface tension of the melt,	above w = 0.12 %8. Kwon et al.9 observed an increase in
oxide-film inclusions and the mode of solidification. The	the fluidity when w = 0.03 % Ti was added to the A356
mould/casting factors are the heat-transfer coefficient at	alloy. Lang5 found that the fluidity was significantly
the interface, the mould temperature and the mould	increased with the additions of B, within the range of w
conductivity23.	= 0.04-0.07 %, to the Al-Si alloy.
The most widely used test for measuring fluidity is	The increases in the fluidities of the modified Al-Si
the spiral test where fluidity is quantified by measuring	alloys were expected only when the Si amount was close
the length of the molten-metal flow inside a spiral-	to the eutectic composition. However, Seshadri and
shaped mould. The fluidities of pure metals and alloys	Ramachandran10 found that the fluidity of the modified
differ due to different solidification mechanisms4. Pure	AlSi12 alloy was reduced by 7 % within the sand mould
metals and eutectic alloys have the highest fluidities.	and by 3 % within the iron mould. Venkateswaran et al.11
However, the fluidities of the hypereutectic Al-Si and	studied the effects of the trace elements on the fluidity of
Al-Mg alloys are better than those of the hypoeutectic	the eutectic Al-Si alloy. The fluidity was decreased with
and even eutectic compositions5.	the additions of Na, Na + Sr, Ti, Na + Ti, Na + Sr + Ti,
Figure 1: a) Metallic spiral mould and b) its fluidity sample
Slika 1: a) Kovinska kokila z livno votlino spiralne oblike in b) uliti vzorec
whilst it was increased with the additions of S, Sb, Sb + Ti, S + Ti. Kotte12 found that with an addition of Sr to the Al-Si alloy, the fluidity was less reduced than with an addition of Na.
The purpose of the present study was to investigate the influence of grain refinement and modification on the fluidity and solidification time of the AlSi9Cu3 casting alloy. Thus, the main focus was on measuring the flow lengths and the cooling curves, and determining their mutual influence, which had not been yet investigated for the AlSi9Cu3 alloy. The solidification behaviour of the investigated alloy was also carried out using a simple thermal analysis.
2	EXPERIMENTAL WORK
The commercial AlSi9Cu3 alloy was melted in a Bosio EUP-K 2G/12GG electrical resistance furnace, grain refined with an addition w = G.G6 % of Ti (the AlTi5B1 master alloy), or modified with an addition w = G.G8 % of Sr (the AlSrlG master alloy), heated to a pouring temperature and poured into a metal spiral mould. The fluidities of the basic, grain-refined, and modified alloys were tested at different pouring temperatures ((64G, 67G, 7GG, and 71G) °C) by measuring the flow length inside the spiral mould, which was preheated to a temperature of 2GG °C. The solidification time was measured using a NiCr-Ni thermocouple positioned at the ingate bottom. Figure 1 shows the spiral mould and the shape of a fluidity specimen. The solidification behaviour of the investigated alloy was examined with a simple thermal analysis, when a sample was poured into a Croning measuring cell at a pouring temperature of (68G ± 5) °C. Light-optical microscopy work was done using an Olympus BX61 with the software Analysis Materials Research Lab 5.G.
3	RESULTS AND DISCUSSION 3.1 Solidification behaviour
Figure 2 shows a cooling curve with the characteristic temperatures of the basic AlSi9Cu3 alloy poured into the Croning measuring cell. The characteristic temperatures of the grain-refined and modified alloys are
presented in Table 1. It can be seen that the minimum liquidus temperatures, Tl min, of the basic and modified alloys were approximately the same, while Tl min of the grain-refined alloy was higher by about 5 °C. This is a characteristic of the strong grain refining that does not require significant undercooling before nucleation13. The recalescence during the primary crystallisation, ATl, of the grain-refined alloy was also the lowest (G.5 °C). The eutectic reaction L ^ (cai + /^Si) of the modified alloy took place at the temperature lower by about 6 °C than those of the basic and grain-refined alloys due to the addition of Sr14, which caused the characteristic temperatures to drop. The solidification was completed with two eutectic reactions: L ^ (cai + Mg2Si) at temperature Te1 and L ^ (cai + Al2Cu-0) at temperature Te2. The solidus temperature, Ts, was between 483.5 °C and 486.1 °C.
Figure 2: Cooling curve of the basic AlSi9Cu3 alloy poured into the Croning measuring cell. The marked characteristic temperatures: Tl min - minimum and TL max - maximum liquidus temperature, ATL -recalescence of the primary crystallisation, TE min - minimum and Te max - maximum eutectic temperature, ATE - recalescence of the eutectic crystallisation L ^ (aAl + /SSi), TE1 and TE2 - eutectic temperatures, and TS - solidus temperature.
Slika 2: Ohlajevalna krivulja osnovne zlitine AlSi9Cu3, ulite v merilno celico Croning. Označene karakteristične temperature so: Tl min - minimalna in TL max - maksimalna likvidusna temperatura, ATl - rekalescenca primarnega strjevanja, TE min - minimalna in Te max - maksimalna evtektična temperatura, ATE - rekalescenca evtektičnega strjevanja L ^ (aAl + /SSi), TE1 in TE2 - evtektični temperaturi in TS - solidusna temperatura.
Table 1: Characteristic temperatures of the basic AlSi9Cu3, grain-refined, and modified alloys Tabela 1: Karakteristične temperature osnovne AlSi9Cu3, udrobnjene in modificirane zlitine
Temperature /°C	Tl min	TL max	ATl	TE min	TE max	ATe	Te1	Te2	Ts
Basic	564.8	567.3	2.5	563.2	563.3	G.1	5G2.9	494.1	483.5
Grain-refined	571.6	572.1	G.5	564.5	564.6	G.1	5G6.8	498.8	486.1
Modified	566.3	568.6	2.3	557.2	557.7	G.5	5G3.6	497.4	486.5
Figure 3: Light-optical micrograph of the modified AlSi9Cu3 alloy poured into the Croning measuring cell
Slika 3: Mikrostruktura modificirane zlitine AlSi9Cu3, ulite v Cro-ning merilno celico
The microstructural constituents of the modified alloy were determined using a metallographic analysis, and were the same as in15. Figure 3 shows the micro-structure of the modified alloy. In addition to a a and /^Si, the Ali5(Fe, Mn)3Si2 and AlsFeSi phases were also observed. The Ali5(Fe, Mn)3Si2 phase precipitated prior to the primary crystallisation of aAll5 and the stoichio-metry of this phase usually changes during the cooling16. The AlsFeSi phase was formed during the eutectic reaction15, simultaneously with /^Si. The Mg2Si and Al2Cu-0 eutectic phases were also observed.
Effects of the grain refinement and modification on the grain size and the size of the /^Si eutectic phase were metallographically examined. The average grain size was 911 pm in the basic alloy, only 5G7 pm in the grain-refined alloy and more than 1GGG pm in the modified alloy (Figure 4). The average length of the /^Si eutectic phase was 49.5 pm in the basic alloy, 6G.3 pm in the grain-refined alloy and only 17.4 pm in the modified
Figure 4: Light-optical micrographs using polarised light: a) basic alloy and b) grain-refined alloy	Figure 5: Light-optical micrographs of the: a) basic and b) modified
Slika 4: Mikrostruktura zlitine v polarizirani svetlobi: a) osnovna zli- alloys
tina in b) udrobnjena zlitina Materiali in tehnologije / Materials and technology 48 (2G14) 6, 917-921
Slika 5: Mikrostruktura: a) osnovne in b) modificirane zlitine
Figure 6: Effect of the pouring temperature on the fluidity of the basic AlSi9Cu3, grain-refined and modified alloys
Slika 6: Vpliv livne temperature na livnost osnovne AlSi9Cu3, udrobnjene in modificirane zlitine
alloy (Figure 5). These results showed that both the grain refinement and modification had been effective.
3.2 Fluidity and solidification time
Figure 6 shows the effect of the pouring temperature on the fluidity of the pure AlSi9Cu3, grain-refined and modified alloys. The fluidities of the basic and grain-refined alloy were the same at the pouring temperature of
64G °C, whilst the fluidity of the modified alloy increased by 4 cm. The cooling curves show that the solidification times of the basic and grain-refined alloys were also similar, whilst the solidification of the modified alloy took about IG s more (Figure 7a). Furthermore, the fluidity of the modified alloy in comparison to the basic alloy was also increased by 1 cm or 2 cm at the pouring temperatures of 67G °C or 7GG °C, and by 4 cm at 71G °C. The cooling curves also show these differences (Figures 7b to 7d), indicating that the solidification times of the modified alloy were about 5 s longer at the pouring temperatures of 67G °C or 7GG °C and about IG s longer at 71G °C. The fluidity of the modified alloy was significantly reduced at the pouring temperatures of (67G, 7GG and 71G) °C in comparison to the basic alloy, although the solidification times were similar (Figures 7b to 7d). In all the cases, the fluidity was improved with the increasing pouring temperature because the nucleation and growth of the fine grains at the tip of the flowing metal in the test channel were impeded; hence, the fluidity length increased17. However, the fluidity of the modified alloy was the greatest, whilst the fluidity of the grain-refined alloy was the lowest at all pouring temperatures. This is related to the mechanisms of the grain refinement and modification. For instance, a large number of grains, formed due to the presence of a grain refiner, caused that the critical solid fraction required to stop the molten flow was reached earlier than in the
Figure 7: Cooling curves of the basic AlSi9Cu3, grain-refined and modified alloys at different pouring temperatures (Jp): a) 64G °C, b) 67G °C, c) 7GG °C and d) 71G °C
Slika 7: Ohlajevalne krivulje osnovne AlSi9Cu3, udrobnjene in modificirane zlitine pri različnih livnih temperaturah (rP): a) 64G °C, b) 67G °C, c) 7GG °C in d) 71G °C
basic alloy. However, the mechanism of the grain refinement did not influence the solidification time. The modification of /^Si, where the Sr atoms act as impurities and slow down the growth rate of the /^Si lamellas, extended the time of the eutectic reaction because the metal flow was stopped later than in the non-modified alloy.
4 CONCLUSIONS
This work revealed that the grain refinement and modification of the AlSi9Cu3 alloy influenced both the solidification morphology and fluidity. The average grain size of the grain-refined alloy was about 45 % smaller than in the basic alloy, whilst the /^Si eutectic phase in the modified alloy was about 35 % smaller. In all the cases, the fluidity was improved with the increasing pouring temperature. The fluidity of the modified alloy was the greatest, and the solidification time was the longest at all the pouring temperatures, whilst the fluidity of the grain-refined alloy was the smallest and the solidification time was similar to that of the basic alloy.
The results of this work clearly showed that the fluidity was proportional to the solidification time. It can thus be concluded that, for the investigated AlSi9Cu3, the fluidity can be predicted from the solidification time obtained with a simple thermal analysis. It is anticipated that the same could also be true for other aluminium casting alloys, which will be explored in our future work.
Acknowledgement
This work was partly financed by the Slovenian Research Agency (ARRS), projects 1GGG-G9-31G152 and L2-2269. The authors also wish to thank Mr. Tomaž
Martinčič, University of Ljubljana, Faculty of Natural Sciences and Engineering, for manufacturing the samples.
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