Influence of cooling rate on microstructure development of AlSi9MgMn alloy

Aluminum alloys are widely applied in automotive, aircraft, food and building industries. Multicomponent technical AlSi9MgMn alloy is primarily intended for high cooling rate technology. Controlled addition of alloying elements such as iron and manganese as well as magnesium can improve mechanical and technological properties of final casting in dependence from cooling conditions during solidification. The aim of this investigation is characterization of AlSi9MgMn alloy microstructure and mechanical properties at lower cooling rates than those for which this alloy was primarily developed. Thermodynamic calculation and thermal analyses revealed solidification sequence in correlation to microstructure investigation as follows: development of primary dendrite network, precipitation of high temperature Al 15 (Mn,Fe) 3 Si 2 and Al 5 FeSi phases, main eutectic reaction, precipitation of intermetallic Al 8 Mg 3 FeSi 6 phase and Mg 2 Si as a final solidifying phase. Influence of microstructure features investigation and cooling rate reveals significant Al 15 (Mn,Fe) 3 Si 2 morphology change from Chinese script morphology at low, irregular broken Chinese script morphology at medium and globular morphology at high cooling rate. High manganese content in AlSi9MgMn alloy together with high cooling rate enables increase of Fe+Mn total amount in intermetallic Al 15 (Mn,Fe) 3 Si 2 phase and encourage favourable morphology development, all resulting in enhanced mechanical properties in as-cast state.


Introduction
Economic casting production in automotive industry implies application of sophisticated production technologies such as high pressure die casting (HPDC) which assures achieving of good surface finish, high dimensional accuracy and excellent mechanical properties [1,2]. Aluminium alloys have found their wide application due to enhanced microstructural, mechanical and technological properties as a reply to high quality requirements of automotive industry [3,4]. Innovative multicomponent AlSi9MgMn (Silafont-36) is the first one with low iron and intentionally high manganese content developed for structural automotive parts production using HPDC [5].
Addition of particular alloying elements such as magnesium as well as manganese and iron can improve mechanical and technological properties of casting [6][7][8][9][10][11][12][13][14][15][16]. Magnesium is one of essential alloying elements commonly added to Al-Si castings to increase their tensile strength accompanied with the degradation ductility [6][7][8][9]. Iron containing phase evolution and therefore solidification sequence of Al-Si alloys was described in detail elsewhere [6][7][8][9][10]. An emphasis was given on AlxMnyFezSiu phase evolution in two main manners: firstly through high temperature peritectic reaction of Al3Fe (Al6Mn) in liquid due to Al↔Si and later Si↔Mn position exchange in crystal lattice and secondly by independently nucleation of AlxMnyFezSiu phase [8,9]. Iron is usually considered as an inevitable impurity element in aluminum alloys and is mainly precipitated in the form of Al15(Fe,Mn)3Si2 or Al5FeSi crystals [11]. Phase Al5FeSi is naturally hard and brittle precipitate in three-dimensional platelet form, which usually acts as a stress raiser and interferes with liquid flowing in interdendritic channels during solidification [12]. Generally, Al5FeSi is the primary phase and the precipitation approaches the solidification onset of eutectic Al-Si when the cooling rate is increased [13,14]. Nevertheless Al15(Fe,Mn)3Si2 crystals occur during eutectic solidification with αAl and usually appear in polyhedral or Chinese script morphologies. The polyhedral structure forms as a primary phase [15], whereas the Chinese script structure forms at a relatively lower temperature as a coupled eutectic structure together with αAl [16]. Transition elements content and there ratio have significant influence on evolution of the Al-(Mn,Fe)-Si phase [9,17], as well as cooling rate.
Understanding of solidification mechanism of AlSi9MgMn alloy and resulting microstructure represents a challenge due to number of influenced parameters. Microstructure development is strongly dependent from chemical composition and thermodynamic conditions during solidification process [17][18][19][20]. Crystallization kinetics influence on elements interaction intensity, followed with change in intermetallics´ chemical composition and morphology. Innovative multicomponent AlSi9MgMn alloy is the first one with low iron and intentionally high manganese content. Transition elements content and there ratio have significant influence on morphology of the Al-(Mn,Fe)-Si phase. HPDC technology favourably affect the microstructure development through evolution of intermetallic Al15(Mn,Fe)3Si2 phase in globular / polyhedron manner and thus enhancing mechanical properties of castings such as ductility and toughness. Synergy of thermodynamic calculation and thermal analyses reveals solidification sequence with corresponded temperatures in correlation to microstructure investigation as follows: development of primary dendrite network, precipitation of high temperature Al15(Mn,Fe)3Si2 phase, main eutectic reaction, intermetallic iron-magnesium phase precipitation Al8Mg3FeSi6 and secondary eutectic phase αAl + Mg2Si as a final solidifying phase [21][22][23][24][25][26][27][28].
An impact of cooling rate during solidification process is revealed as microconstituents´ morphology, size and distribution correlated to the mechanical properties and theirs anisotropy [27][28][29][30]. Corresponding heat treatment and carefully designed regime comprehend to improved strength, better machinability of the material, increased hardness, improved dimensional stability, eliminated residual stresses caused during casting [29,30].
Since investigated multicomponent AlSi9MgMn alloy intended for HPDC characterized with high cooling rates and therefore thin wall castings, has not been classified in European Norm (EN) [31], its solidification sequence upon different cooling condition has not been determined. The objective of this investigation is complete characterization of solidification behaviour of multicomponent AlSi9MgMn alloy. Therefore, correlation of characteristic temperatures in solidification process at different cooling rates and obtained microstructure will comprehend to estimation of applicability of this material for thick-walled safety critical parts and sets in automotive industry.

Experimental
Multicomponent technical AlSi9MgMn alloy is investigated using multidisciplinary approach in order to determine solidification sequence, characteristic temperature of phase transformation/precipitation and corresponding mechanical properties.
The AlSi9MgMn melt is prepared by melting of AlSi9MgMn (Silafont-36) charge material in electro-resistant furnace KONČAR-Rpa-70LX. Comparison of chemical composition of AlSi9MgMn alloy compared with commonly used AlSi9Mg alloy is shown in Table 1. Comparison of alloys reveals higher and wider range for silicon content and significantly higher manganese content in AlSi9MgMn alloy. Limitation for Fe content is stronger, while required Mn content is 5-8 times higher in AlSi9MgMn alloy when compared to common AlSi9Mg alloy. Usual ratio of Fe and Mn in similar common casting AlSi9Mg alloy corresponds to 1:0.5. In AlSi9MgMn alloy completely different ratio of Fe and Mn can be noticed. Required Mn amount is also several times higher than Fe amount in order to enhance Al-Fe,Mn-Si reaction.
After melting process, degassing was performed in order to remove captured residual hydrogen. Degassing was performed with nitrogen (N2) using MTS-1500-FOSECO equipment with following parameters: flow Q=10 l/min, working pressure p= 0.5 MPA and degassing duration t=45 min. Gas index as a measure for process efficiency was calculated on the base of Archimedes' principle using following formula: where: ρair -density of the sample solidified in the air ρvacuum -density of the sample solidified under the low vacuum (80 kPa) Obtained gas index after treatment was i = 1.13% which indicate high density of an alloy after the degassing process.
Melt treatment consists from inoculation and modification with commercial master alloys in the wire form. Applied inoculant was AlTi5B1 with the target addition of 1wt.% / 1kg melt. As a modifying agent AlSr10 master alloy was added to the target content of 90 ppm of Sr.
The chemical composition of the the prepared AlSi9MgMn melt was investigated using optical emission spectrometer ARL-3460 with indicated uncertainty of measurements obtained by certification with reference material.
Thermodynamic calculations of equilibrium diagram of AlSi7MgCu alloy has been performed by Thermo-Calc software TCW 5.0, with database TTAL7.
Solidification behaviour was investigated in situ using thermal analysis (TA) during solidification process in different moulds material and geometry in order to achieve different cooling conditions and corresponding test bars for mechanical testing. Measurements were repeated 2 or more times depending on the geometry of the casting and the mould material until repetitive results were obtained, which were then presented as a referent obtained values. Data acquisition was continuous during the whole solidification process starting from the pouring temperature. DAQ module and LabView software enables automatic conversion and continuous observation of the cooling curves. In order to obtain different 5 cooling rates different mould material were used. Casting geometries were adopted in order to obtain casting shape and size which allow the production of test samples for mechanical investigation except for standard croning cell (Quick Cup equipped with NiCr-Ni thermocouple). Figure 1 shows used mould type with indication of achieved cooling rate. The highest cooling rate characteristic for HPDC (63 °C/s) was not measured due to limitation of the HPDC machine. Cooling curve of AlSi9MgMn in this case was calculated using MAGMASOFT ver. 4.4 software for HPDC on the base of material properties, machine parameters (melt temperature, temperatures of mould halfs, piston rates in first two casting phase, specific pressure during the third casting phase, time of mould filling, melt rate during first casting pahse, etc.) and casting geometry. Casting geometry is shown in Figure 2. Samples for metallographic investigation were taken from all castings in order to compare developed microstructures (light microscopy) and to investigate chemical composition (SEM-EDS analysis) of characteristic phases related to corresponding cooling rate. Samples were prepared by standard metallographic preparation procedure by grinding and polishing, followed by etching in 0.5% HF. Metallographic analyses cover light microscopy analysis (Olympus GX 51) and microstructural investigations using standard method at scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM-EDS, Tescan Vega TS 5136 MM). Standard SEM-EDS analysis enables mapping analysis of the characteristic region using SE detector.
Mechanical properties investigations were performed on testing machine MTS 810, at room temperature T = 20 °C in accordance to EN 10002-1:1998 [32]. Brinell hardness investigation was performed on WOLPERT DIA TESTOR 3a with sphere of 5 mm and 2500N impression force during 15 s.

Results and discussion
Compared overview of chemical composition of AlSi9MgMn alloy with high Mn content required by manufacturer norm [5] and real investigated sample is given in Table 2. Comparison of chemical compositions values did not brought out any deviation from values requested by manufacturer norm [5]. Iron and manganese values, significantly differ from those in common aluminium alloys, and therefore encourage formation of complex Al-(Mn,Fe)-Si phase instead of harmful Al5FeSi phase.
Numerical modelling of equilibrium phase diagram (TCW 5.0) resulted in prediction of AlSi9MgMn alloy solidification sequence as follows in Figure 3 Figure 4 and Table 3.  Simple thermal analysis was performed at cooling curves in corresponding moulds. An example of cooling curve analysis for permanent ASTM mould (18.7 °C/s) is shown in Figure  5.  Table 4. Microstructural investigation enables correlation of STA data with microconstituents´ appearance. The focus of investigation was pointed to the Fe-bearing phases due to different morphologies and different Mn:Fe content in correlation to cooling rate. Therefore, the appearance of characteristic microconstituents and theirs' morphologies at three different cooling rates according to solidification interval (12.4, 18.7. and 63 °C/s) is shown in Figure 6.  (Table 4) due to both local undercooling and bulk liquid composition which allow its direct nucleation. Unfavorable needle like Al5FeSi, magnesium bearing phase Al8Mg3FeSi6 developed on Al5FeSi needles and secondary eutecic phase Mg2Si in characteristic ramified morphology distributed at the grain boundaries were detected at lower cooling rates (12.4, 18.7 °C/s).
Typical Fe bearing phase in AlSi9MnMg alloy develops as a Al5FeSi and Al15(Mn,Fe)3Si2. Even at low cooling rates high Mn content favours development of independently precipitated Al15(Mn,Fe)3Si2 phase. Increase of cooling rate comprehend to transformation of Al15(Mn,Fe)3Si2 phase morphology from coarse broken Chinese script at low cooling rate, across regular Chinese script morphology at medium one to polyhedron morphology at high cooling rate. Microstructure investigation included analysis of Fe and Mn atomic ratio in different morphologies, as shown in Table 5. Performed microstructural investigations indicate significant differences in morphology and relationship between Mn and Fe as variable constituents of Al15(Mn,Fe)3Si2 phase in dependence from cooling rate, as shown in Figure 7. Mechanical properties values were compared for typical cooling rates, as indicated in Table 6. Increase of cooling rate influence on decrease of Mn:Fe ratio and increase on total Mn+Fe amount in investigated phases'.  Increase of cooling rate acts favourably to increase of yield and tensile strength, elongation and Brinell hardness due to change of microconstituent's morphology.
Although the target medium cooling rate indicates the highest Mn:Fe ratio and the lowest Mn+Fe value resulting in regular Chinese script morphology of Fe-bearing phase, obtained strength and hardness reveals high values in as-cast state. Determination of differences in microstructure development during solidification process and characterization of thermodynamic, microstructural and mechanical properties of multicomponent AlSi9MgMn alloy at low and medium cooling rate conditions comprehends to evaluation of overall applicability of this material for thick-walled safety critical parts and sets in automotive industry solidified at lower cooling rate.