J. MRV AR, M. PRIJANOVI^ TONKOVI^: THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ... 801–807 THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ON THE SOLIDIFICATION CHARACTERISTICS OF THE ALUMINIUM ALLOY AlSi7Mg0.3 VPLIV LEGIRNIH DODATKOV IN PROCESNIH PARAMETROV NA ZNA^ILNOSTI STRJEV ANJA ALUMINIJEVE ZLITINE AlSi7Mg0,3 Jakob Mrvar 1 , Marica Prijanovi~ Tonkovi~ 2* 1 University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, Ljubljana, Slovenia 2 Vi{ja strokovna {ola Strojni{tvo, [olski center Novo mesto, Slovenia Prejem rokopisa – received: 2024-06-22; sprejem za objavo – accepted for publication: 2024-10-24 doi:10.17222/mit.2024.1225 We have investigated the aluminium alloy AlSi7Mg0.3, which is used in the production of castings. Melting was carried out in an electric resistance furnace. Casting was done into measuring cells with thermocouples for simple thermal analysis. Six sam- ples that differed in terms of the additives used for grain refinement and inoculation, with two different holding times, were cast. An alloying material AlTi5B1 was used for grain refinement. The inoculation effect on the solidification was studied with the addition of the inoculant AlSr10. The temperature of the melt in the furnace was 750 °C and the melting time was one hour and ten minutes. Firstly, the samples were cast, the melt was cooled, solidification occurred and finally it was cooled down in the solid state. During the entire cooling process, the temperature as a function of time was measured and the cooling curves were obtained. After the casting, samples for metallographic inspection were prepared. The cooling curves show that in samples with a longer holding time, the additions of either the inoculant or the grain refinement agent, had a better effect. This is evident with a higher maximum liquidus temperature T Lmax in sample Ti10 and lower maximum eutectic temperature T E1max in sample Sr10 in comparison to the basis alloy. The same we can conclude from the microstructure analysis of the samples. The samples with inoculant and with a longer holding time, possessed a finer eutectic structure ( Al+ Si), than the samples with the shorter holding time. The same applies for the samples with grain-refinement additions. The estimated grain sizes were smaller in the sample with the longer holding time. Keywords: aluminium alloy, AlSi7Mg0.3, grain refining agent, inoculant, simple thermal analysis V ~lanku avtorji opisujejo preu~evanje aluminijeve zlitine AlSi7Mg0,3, ki se uporablja za izdelavo ulitkov. Taljenje je potekalo v elektrouporovni pe~i. Talina je bila ulita v merilne celice s termo~leni za enostavno termi~no analizo. Odlitih je bilo {est vzorcev, ki so se razlikovali po dodatkih, ki se uporabljajo za udrobnjevanje (zmanj{evanje velikosti) kristalnih zrn in modificiranje. Odliti so bili pri dveh razli~nih ~asih zadr`evanja. Za udrobnjevanje zrn je bila uporabljena predzlitina AlTi5B1. Za preu~evanje u~inka modificiranja na strjevanje je bilo uporabljeno sredstvo AlSr10. Temperatura taline v pe~i je bila 750 °C, ~as taljenja pa eno uro in deset minut. Najprej so bili vzorci uliti, talina se je ohlajala, pri{lo je do strjevanja in nazadnje se je ohladila v trdnem stanju. Med celotnim postopkom ohlajanja so avtorji raziskave merili temperaturo v odvisnosti od ~asa in pridobili ohlajevalne krivulje. Po litju so bili vzorci pripravljeni za metalografsko analizo. Ohlajevalne krivulje ka`ejo, da so v vzorcih z dalj{im ~asom zadr`evanja dodatki inokulanta ali sredstva za udrobnjevanje bili u~inkovitej{i. To je razvidno iz vi{je maksimalne likvidus temperature T Lmax v vzorcu Ti10 in ni`je maksimalne temperature evtektika T E1max v vzorcu Sr10 v primerjavi z osnovno zlitino. Enako lahko zaklju~imo iz mikrostrukturne analize vzorcev. Vzorci z dodatkom inokulanta so v primeru dalj{ega ~asa zadr`evanja imeli bolj fino evtektsko strukturo ( Al+ Si), kot vzorci s kraj{im ~asom zadr`evanja. Enako velja za vzorce z dodatki za udrobnjevanje. Ocenjene velikosti zrn so bile manj{e pri vzorcu z dalj{im ~asom zadr`evanja. Klju~ne besede: aluminijeva zlitina, AlSi7Mg0,3, udrobnilno sredstvo, modifikator, enostavna termi~na analiza 1 INTRODUCTION Aluminium is the most frequently used metal among non-ferrous metals. It is used in the form of technically pure aluminium as well as in alloys with Si, Mg, Cu, Mn, Zn, Fe, etc. The addition of the alloying elements influ- ences the mechanical properties, castability, workability and corrosion resistance. Aluminium alloys can be di- vided into two groups: wrought alloys and casting al - loys. 1,2 Al-Si is a popular aluminium alloy. It is suitable for gravitational casting into moulds that can be made of sand mixtures or steel. It is also used in low-pressure die casting as well as high-pressure die casting. 3 Casting in the sand has lower cooling speeds in comparison with casting into steel dies. Melt quality can be controlled with a cooling curve and a temperature-time diagram (T-t), which is typical output from simple thermal analy- sis 4,5 . By increasing the cooling speed, undercooling is affected, which has an impact on the alloy micro- structure. A higher cooling speed affects the crystalliza- tion and as a result the distribution and size of the eutectic phases is finer and dendritically formed grains are smaller. The microstructure of an alloy is also af- fected by the chemical composition, grain refining, mod- ification of the eutectic and heat treatment. 6,7 Materiali in tehnologije / Materials and technology 58 (2024) 6, 801–807 801 UDK 669.715:669-14 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek Mater. Tehnol. *Corresponding author's e-mail: marica.prijanovic@sc-nm.si (Marica Prijanovi~ Tonkovi~) By adding a grain-refining agent, we introduce heter- ogeneous nucleus into the melt, that enables crystalliza- tion of the primary mixture crystals Al P . This causes the formation of fine crystal grains. Adding the inoculant in- fluences the eutectic solidification especially on the for- mation of Si . It changes from the coarse Si phase shape into finer Si within the eutectic. 8 For grain refining of the primary crystal grains Al , titanium and boron are used, while agents based on sodium, antimony and stron- tium are used for the inoculation. Thus, an alloy with ad- ditives for grain refining and inoculation has a finer microstructure, including the finer primary mixture crys- tals Al P and finer eutectic ( Al + Si ), has improved me- chanical properties, such as strength, hardness, impact toughness. 9–11 Depending on the amount of silicon, Al-Si alloys are distributed into three groups: hypoeutectic alloys with 4–7 w/% Si, eutectic alloys with 10–13 w/% Si and hypereutectic alloys with a ratio of 14–25 w/% Si. Dur- ing the solidification process, primary mixture crystals Al P begin to crystalize from the melt. These crystals are dendrite shaped. During the solidification of primary crystals, the remaining melt is enriched with silicon. This follows a eutectic solidification where the rest of the melt gets solidified into a eutectic ( Al + Si ). 6,12–13 Al-Si can contain other alloying elements, for exam- ple, the hypoeutectic alloy AlSi7Mg0.3. It is mainly used in casting processes. This type of alloy is corrosion resis- tant, has good castability and welding properties, pres- sure strength and workability. Chemical composition and alloy’s properties are stated in the standard EN 1706: 2010 (ENAC-42100). 14 Generally, before casting the alloy AlSiMg0.3, enough grain-refining agent and inoculant are added into the melt. The grain-refinement agent increases the num- ber of heterogenous nuclei in the melt. This influences the number of crystal grains formed during the solidifi- cation process. 13 The Mn content in alloy (up to 0.6 w/%) disables the formation of phase -Al 5 FeSi, which is undesired be- cause of its acicular shape that can cause fatigue notch factor and inferior mechanical and corrosion proper - ties. 11,13 The grain refinement of AlSi7Mg0.3, was studied by Anna Knaislová and coworkers. 15 The test required the use of two types of inoculants: inoculant AlTi5B1 and AlTi3B1. Their results show that the finest dendritic structure is in samples that contained a grain-refinement agent of 0.01 w/%. The difference between AlTi5B1 and AlTi3B1 in terms of grain-refinement effect is minimal. That means that further increasing of the grain-refine- ment agent would not contribute to additional beneficial effect on the microstructure of the casting. Grain refining of Al-Si alloys was studied by Agnes and coworkers. They used Al-10 % Ti and Al-4 % B for refining. Re- search shows that the grain size decreases with increas- ing Ti concentration. Al-4%B can react with traces of Ti and form TiB 2 . 16 The microstructure of alloy AlSi7Mg0.3 consists of a solid solution of primary crystalline Al P , the eutectic ( Al + Si ) and a second eutectic ( +Mg 2 Si). The primary aluminium grains have a dendritic form. First eutectic consists of a solid solution of Al and Si , which is com- pressed into a lamellar shape. Si appears on the plane of the metallographic cut in the acicular shape where the needles are irregular. The form of Si in the eutectic can also be changed by heat treatment, where we also get a globular eutectic besides the phase and precipi - tates. 17–21 The goal of this research was to study the solidifica- tion process of aluminium alloy AlSi7Mg0.3, with addi- tions of a grain-refining agent and an inoculant, during two different holding times, using simple thermal analy- sis. Then the microstructure of samples was studied with an optical microscope. 2 MATERIALS AND METHODS The samples were made from a block of aluminium alloy. Each weighed from 355 g to 370 g. This material was put into cups for melting. The melting was carried out in an electric resistance furnace produced by Bosio. The melting time was one hour and ten minutes. The temperature was set up to 750 °C. After the melting pro- cess, the oxide layer was removed and then the melt was alloyed with a grain-refinement agent and the inoculant. J. MRV AR, M. PRIJANOVI^ TONKOVI^: THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ... 802 Materiali in tehnologije / Materials and technology 58 (2024) 6, 801–807 Figure 1: a) Prepared material – cups, measuring cells, weighed pieces of alloy, b) simple thermal analysis schematic view After the alloying, the melt was stirred with steel rod. After stirring the cups were returned to the furnace for a specific holding time. After a certain holding time the oxide layer was again removed. Next the pouring of melt into the measuring cells followed. There simple thermal analysis was conducted. Figure 1 shows the prepared material for melting and casting. The picture shows cups, pieces of alloy for each charge, as well as the measuring cells for simple thermal analysis with type K (Ni-NiCr) thermocouples, on which are connectors. These are connected to the measuring card, which is connected to a computer. The computer follows the tem- perature in relation to time. Alloy AlTi5B1 was used for grain refinement and al- loy AlSr10 was used as the inoculant. Table 1 represents the additives, holding times of the tested samples and their designation. Table 1: Description of tested samples and their designation Sample des- ignation Description B Primary alloy, AlSi7Mg0.3 (no additions) Ti1 Addition of AlTi5B1 (1 min holding time) Ti10 Addition of AlTi5B1 (10 min holding time) Sr1 Addition of AlSr10 (1 min holding time) Sr10 Addition of AlSr10 (10 min holding time) TiSr Addition of AlTi5B1 and AlSr10 (1 min hold- ing time) The amount of grain refinement and inoculant addi- tives were determined according to recommendations from the literature. 22 The amount of alloying material used for grain refinement was in all cases 3 g (approxi- mately 0.8 w/%), and the inoculant 1 g (approximately 0.3 w/%). Chemical analysis of the primary alloy was carried out with an x-ray fluorescence spectroscopy (XRF analy- sis) with a device manufactured by ThermoSCIENT- IFIC. The model used was Niton XL3. Table 2 shows a chemical composition of the aluminium alloy before the melting process. During the solidification process we studied the cool- ing and solidification process with a simple thermal anal- ysis. For this a type-K thermocouple was used. After the solidification of the castings, some samples were cut to be metallographically analysed. The metallographic analysis was made with an Olympus BX 61 light micro- scope. 3 RESULTS AND DISCUSSION This article studies the effect of grain-refinement agent and inoculant on the alloy AlSi7Mg0.3 solidifica- tion. Six different samples were produced for the test. They differed in the type of additive with two different holding times in which the refinement agent and inoculant were exposed in the melt. Thus, we prepared a primary melt, a melt with the grain-refinement agent, a melt with the inoculant, and a melt with both grain re- finement and inoculant. The tested alloys were cast into the measuring cells for a simple thermal analysis. After the cooling, cast samples were studied using a simple thermal analysis and observation of the microstructure. Samples for microstructural analysis were taken from the casting of the thermal analysis mea- suring cells. Figure 2a and 2b shows a cross-section of the measuring cell, and the position of surface for metallographic analysis (Figure 2c). The cut samples were grinded. After the grinding process, the two-step polishing followed. In the first step the samples were polished on a wool base with a 3-μm diamond suspension. In the second step a neoprene base was used and for the polishing agent a suspension of colloid silicon dioxide with grain size of 0.04 μm was added. The obtained temperatures and times during the so- lidification process of all the samples were presented as cooling curves using OriginPro. Figure 3 shows a cool- ing curve for the base alloy (mark B). J. MRV AR, M. PRIJANOVI^ TONKOVI^: THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 801–807 803 Figure 2: a), b) Presentation of the cut casting of the measuring cell, c) area intended for metallographic analysis Table 2: Chemical composition of the alloy before melting Chemical composition (w/%) Si Fe Cu Mn Mg Cr Zn Ti Al 6.84 0.115 0.011 0.066 0.342 <0.001 <0.001 0.125 rest Figure 3 shows the casting temperature T P (623 °C). During the cooling, the melt’s temperature decreases. The melt starts to solidify. Important points on the cool- ing curve are also undercooling T Lmin (611.1 °C), the for- mation of a nucleus for the growth of crystal mixture Al , that start with the crystallization of the melt at T Lmax (611.7 °C). The crystals Al grow to the eutectic temper- ature T E1min (566 °C) where undercooling occurs and T E1max (568 °C) where the eutectic starts to solidify E 1 ( Al E1 + Si ). At temperature T E2 (549 °C) the solidification of the next eutectic begins. This is eutectic E 2 ( Al E2 + Si +Mg 2 Si). With a solidus temperature T S (535 °C), the solidification is complete. Table 3 shows all the collected temperatures of im- portant points from the cooling curves for the analysed samples. Table 3: Characteristic temperatures from the cooling curves of the tested samples Temper- ature Sample (°C) B Ti1 Ti10 Sr1 Sr10 TiSr T P 623.0 660.0 674.0 691.0 689.0 687.0 T Lmin 611.1 612.2 612.0 609.5 609.5 612.1 T Lmax 611.7 612.7 612.5 611.2 611.0 612.6 T E1min 566.0 567.5 568.4 563.5 562.5 562.0 T E1max 568.0 568.5 569.5 565.5 564.5 565.0 T E2 549.0 549.0 549.5 549.5 549.0 549.0 T S 535.0 538.0 537.0 537.5 534.5 536.3 T L 0.6 0.5 0.5 1.7 1.5 0.5 T E1 2.0 1.0 1.1 2.0 2.0 3.0 Figure 4 shows the comparison of the cooling curves of all the samples. The graph presents the cooling curve of the base alloy sample B and alloys with different addi- tives and holding times in the base melt. In sample Ti1 and Ti10 the precipitation of the primary mixture crys- tals Al P starts at higher temperatures than than the one in the base alloy because of the already present heteroge- neous germs in the melt. If we compare the base alloy and the sample Sr1 and Sr10, we can conclude that the liquidus and eutectic tem- peratures are lower than in sample B. Among all the samples, the sample TiSr has the lowest temperature of the first eutectic T E1 , which is 562 °C and thus the most ennobled state among the analysed alloys. The addition of a grain refinement and inoculant af- fect the curing temperature interval. Thus, by adding the grain refinement AlTi5B1, the interval solidification T L -T S increases compared to the base alloy sample B. The interval also increases when an inoculant is added. The temperature difference between T L -T S increases the most in the TiSr sample, where both additives are added to the melt (Figure 5). The tested alloys were metallographically studied with an optical microscope. Figure 6 shows the micro- structures of the tested samples. In Figure 6B there are larger dendritic shaped Al P grains, eutectic E 1 ( Al E1 + Si ) and E 2 ( Al E2 + Si +Mg 2 Si). The size of Al P grains is smaller in the case of samples with the grain-refinement agent addition Ti1 and Ti10 (Figure 6Ti1 and 6Ti10). Dendrites Al P have decreased in size. In the samples with added inoculant Sr1 and Sr10 the shape of the eutectic phase Si (Figure 6Sr1 and 6Sr10) changes. It is precipitated in the finer form. The Sr10 J. MRV AR, M. PRIJANOVI^ TONKOVI^: THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ... 804 Materiali in tehnologije / Materials and technology 58 (2024) 6, 801–807 Figure 4: Comparison of the cooling curves of all measured samples Figure 3: Cooling curve of the sample mark B Figure 5: Difference between liquidus and solidus temperature J. MRV AR, M. PRIJANOVI^ TONKOVI^: THE EFFECT OF ALLOYING ADDITIVES AND PROCESS PARAMETERS ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 801–807 805 Figure 7: Comparison of samples' microstructures: B, Ti10, Sr10, TiSr Figure 6: Comparison of microstructures of samples: B, Ti1, Ti10, Sr1, Sr10, TiSr sample in comparison to Sr1, has a higher amount of finer eutectic, which was well modified. Sample TiSr (Figure 6TiSr) was alloyed with both the grain-refinement agent and the inoculant. There are small Al grains crystalized in the microstructure. Si in the eutectic has precipitated in a finer globular form in comparison to the base sample B. Besides the eutectic and primary mixture crystals Al P dendrites, also a lighter phase can be detected in the microstructure. This phase is based on iron. It is nor- mally formed in sharp forms. In microstructure of base sample B, eutectic phase is in form of rough lamellas, the grain size of the Al P is around 138 μm (Figure 7B). This eutectic is reformed by addition of the inoculant AlSr10 (Figure 7 Sr10). By adding the alloy AlTi5B1, the size of primary mixture crystals Al P crystals (Figure 7 Ti10) decreases. In case of sample TiSr, a vivid reduc- tion in lamellas of Si inside the eutectic is shown. The addition of inoculant is seen in the TiSr sample, where we have eliminated the Si phase in fibrous form in the eutectic. Also, smaller dendrites Al P is visible in Fig- ure 7 TiSr, the grain size is around 62 μm. As can be seen from Figure 7, the AlSi7Mg0.3 alloy consists of primary mixed crystals of Al P , which solidify first. Crystallization of the Al 8 Mg 3 FeSi 6 intermetallic phase follows. The rest of the melt solidifies eutectically. First, the eutectic E 1 ( Al E1 + Si ) is formed, and then, within the framework of the ternary eutectic, the remain- ing melt in the microlast independent solidification areas crystallizes into E 2 ( Al E2 + Si +M g 2 Si). The addition of a grain refining agent and inoculant are shown in the Figure 7 marked TiSr, where in the microstructure we have a smaller size of crystalline Al P and the eutectic Si is precipitated in a spherical form. Al P crystals are reduced due to the addition of AlTi5B1 refining agent. After the addition of grain refiners AlTi5B1 particles TiAl 3 and TiB 2 are formed. The result- ing precipitates act as grain growth inhibitors (TiAl 3 ) and as heterogeneous nucleation sites (TiB 2 ) for the matrix nucleation. 23 The shape of the precipitated Si in the eutectic is in- fluenced by the addition of the AlSr10 inoculant. This causes a change in the morphology of Si in the eutectic phase. The basic mechanism of the added AlSr10 is the adsorption of strontium onto the preferred Si growth sur- faces. Sr is adsorbed on twinning planes. Therefore, crystal growth in preferred directions is inhibited. As a result, Si grows in the eutectic in a more rounded and fine-grained form. 4 CONCLUSIONS Aluminium alloys are frequently used in mechanical engineering due to their high strength and low density. The desired mechanical properties can be achieved in the cast state with appropriate melt treatment. To produce the best-possible quality of the castings in the cast state, the quality of the melt has to be monitored during the production process. This can be done by a simple ther- mal analysis. Thus, we analysed the solidification pro- cess of the alloy AlSi7Mg0.3, to which was added the grain-refinement agent in the form of the alloying mate- rial AlTi5B1 and the inoculant AlSr10. The results of this study are the following: • The cooling curves of the simple thermal analysis show that by adding AlTi5B1, the liquidus tempera- ture moves up to higher values, while recalescence decreases with liquidus temperature. The indicator for an effective refinement is thus an absolute value T Lmin , which should be as high as possible at low recalescence (T Lmax – T Lmin ). • By adding strontium within the addition of AlSr10 al- loying material caused a change (modification) of the eutectic phase Si in the alloy, which can be observed on the cooling curve where the eutectic temperatures move to lower values. • The cooling curve also shows that samples with a longer holding time that were added either the inoculant or the grain refinement agent, have a more efficient performance. This is evident with a higher maximum temperature T Lmax in sample Ti10 and lower temperature T E1max in sample Sr10 in compari- son to the samples with a shorter holding time. • In the microstructure are primary mixture crystals Al P dendrites as well as two eutectics: eutectic 1 ( Al E1 + Si ) and eutectic 2 ( Al E2 + Si +Mg 2 Si). 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