Crystallization and Structure of Alsi10mg0.5Mn0.5 Alloy with Dispersion Strengthening with Al–Fexaly–Sic Phases

metals

Article Crystallization and Structure of AlSi10Mg0.5Mn0.5 Alloy with Dispersion Strengthening with Al–FexAly–SiC Phases

Jarosław Pi ˛atkowski 1,* and Robert Wieszała 2

1 Faulty of Materials Engineering and Metallurgy 1, Silesian University of Technology, 40-019 Katowice, Poland 2 Faculty of Transport 2, Silesian University of Technology, 40-019 Katowice, Poland * Correspondence: [email protected]; Tel.: +48-32-603-44-58

Received: 11 June 2019; Accepted: 25 July 2019; Published: 8 August 2019

Abstract: The paper characterizes a composite with dispersion phases cast via the use of stir casting method on an aluminum matrix. A mixture of aluminum with FexAly and SiC powders was achieved in the process of mechanical alloying and self-propagating high temperature synthesis (ASHS). Chemical composition of agglomerates was chosen in such a way that the strengthening components made up 25% of the mass of the AlSi10Mg0.5Mn0.6 (EN AC-43400) alloy matrix. The characteristic temperatures of crystallization of the tested alloy were measured by thermal analysis ATD (analysis thermal derivative). A change of chemical and phase composition was confirmed in the elements of the intermetallic phase FeAl in the aluminum matrix. A silumin casting structure was achieved, with the matrix including micro-areas of ceramic phases and intermetallic phases, which are characteristic for hybrid strengthening. A refinement of dendrites in solid solution α was found, together with a transition from a binary plate eutectic composition α(Al) + β(Si) into modified eutectic composition.

Keywords: Al-Si cast alloys; ATD thermal analysis; crystallization

1. Introduction Al–Si–Mg–Mn casting alloys are widely applied for casting in many fields of industry, mainly in the automotive and aviation industries, due to their good resistance and plasticity. Sub-eutectic silumins are particularly beneficial as they are characterized by additional beneficial technological properties, which make them the perfect choice for application in thin-walled castings with complex shapes, such as those achieved in gravity or pressure casting processes. Improvement of material parameters, particularly the fatigue properties and tribological properties, can be achieved by modifications that have been widely described in the literature [1–5]. Modifiers for sub-eutectic silumins are simple: Na, Sr, Sb, and complexes in type Al–Ti–C, Al–Ti–B [6–8], or enriched with high-melting carbide forming elements: Cr, Mo, W, Co, V [9,10]. Additional strengthening of solid solution α is possible thanks to the introduction of dispersion particles in FexAly and SiC phases, which are formed as a result of in situ reaction. Ceramic intermetallic phases of FeAl in Al matrices result in hybrid strengthening, which further increases the yield point, creep resistance, and thermal stability of the material, and therefore increases the range of applications, mainly in heavily loaded elements of pistons and cylinder heads of combustion engines. Application of reactions in situ in a system of liquid metal-reacting substance (in the form of a solid body) allows for the preparation of materials with properties close to composite SAP (sintered aluminum powder) when used in casting methods. Achievement of the right morphology and phase composition of the reinforcement is possible with control of the kinetics and factors which are decisive in steering the

Metals 2019, 9, 865; doi:10.3390/met9080865 www.mdpi.com/journal/metals Metals 2019, 9, 865 2 of 8 creation of reinforced dispersion phases. Materials of this type are usually prepared with the use of liquid phase technologies, which are characterized by the fact that the ceramic reinforcing phase is introduced to the liquid metal in presence of not more than 30% of the volume, and the size of particles is bigger than 15 µm [11].

2. Aim and Scope of the Paper The aim of the tests was to prepare an aluminum composite with hybrid strengthening with intermetallic phases from FeAl system and ceramic SiC, and to determine the inﬂuence of composite powders on the modiﬁcation of the casting structure from alloy AlSi10Mg0.5Mn0.5 from series A3XX.X, in accordance with norm ASTM [11]. In order to achieve the stated goal, the scope of the tests included:

- Investigation of the technological and material concepts needed to produce the casted alloy composite modiﬁed with powders FeAl, Al–FexAly, and Al–FexAly–SiC, - Determination of the method by which to produce the powders for modiﬁcation of the aluminum matrix, - Preparation of the technological process for producing composites with varied contents of structural ingredients, - Determination of chemical and phase compositions and the structure of the alloy AlSi10Mg0.5Mn0.5.

The development of the research concept took into account proportion of powder as a modifier in the silumin casting structure. It was also assumed that the composite would be produced in a combination of processes, both ex situ and in situ, to formulate the primary structure. Information from the literature, the results of our own tests, and the new concept of the creation of alloy were all taken into account in the choice of the chemical composition of the composite powders used for modification of the alloy AlSi10Mg0.5Mn0.5 structure. It was assumed that the introduction of intermetallic phase FeAl would have a specific influence on the structure of silumin AlSi10Mg0.5Mn0.5 [12–14]. The method used for preparing composites, which was included in the patent application [15], was adapted for casting samples from sub-eutectic silumins modified with hybrid components of Fe–Al, Al–FexAly, and Al–FexAly–SiC powders.

3. Material and Methodology of Tests The alloy AlSi10Mg0.5Mn0.5 was chosen for tests. It is an alloy widely applied in the automotive and aviation industries, mainly for casting air brakes, wheel cases, gear boxes and compressors, cylinder heads, pistons, and other parts of engines. The dominant technology used in manufacturing these parts is gravity casting to sand molds and casting dies, and also pressure casting for structured cement. The chosen alloy, due to the addition of Mn, is characterized by elevated ductility and resistance to elevated temperatures (200–300 ◦C), low coeﬃcient of thermal expansion, and good resistance to abrasion and corrosion. Methodology of tests was adjusted to the assumed concept of testing the preparation of the silumin composite. Powders were prepared by the method of mechanical alloying and self-propagating high temperature synthesis (ASHS). Powder FeAl was prepared in a mill (Pulverisitte 5) for 0.5 h while mixing the primary powder with the participation of 50% mass. The iron powder used had a granularity of 60 µm, and the aluminum powder, surface oxidized, had a granularity up to 40 µm. FexAly powder (marked as powder-1) was achieved by ASHS and mixed with 50% mass of Al powder. The mixture of powders FexAly–SiC (powder-2) was also mixed with aluminum powder in proportions 50/50 and 70/30. The research material consisted of scrapped castings of AlSi10Mg0.5Mn0.5 alloy, obtained from the pressure casting process from enterprises supplying components for the automotive industry. Metals 2019, 9, 865 3 of 8 Metals 2019, 9, x FOR PEER REVIEW 3 of 7

Structure andand morphology morphology of theof powdersthe powders was determined was determined using light using and scanninglight and microscopy. scanning Compositemicroscopy. powders Composite prepared powders in thisprepared way were in this added way to liquidwere added silumin to in liquid such amountssilumin thatin such the contributionsamounts that the of the contributions composite powdersof the composite were 30 andpowders 50% masswere of30 theand alloy. 50% mass of the alloy. Characteristic parametersparameters ofof siluminsilumin crystallizationcrystallization were were tested tested by by ATD ATD thermal thermal analysis analysis using using a Crystaldigrapha Crystaldigraph PC PC device device (Figure (Figure1). 1).

Figure 1.1. ThermalThermal analysisanalysis by by ATD ATD (analysis (analysis thermal therma derivative)l derivative) method method (standard (standard QC QC 4080): 4080): (a) test(a) stand,test stand, (b) ATD (b) ATD sampler sampler and castings.and castings.

In addition to crystallizationcrystallization tests, microscopicmicroscopic tests were conducted with the use of light and scanning electron electron microscopy. microscopy. Some Some samples samples were were further further melted melted to determine to determine their their structure. structure. The Thestructures structures of the of thetested tested alloys alloys was was observed observed on on metallographic metallographic microsections microsections from from samples cut crosswise and along thethe centralcentral axis.axis. The structures of the surface of samples were observedobserved and registered using an OlympusOlympus GX-71 microscope. Morphology ofof the the powders powders and and local local chemical chemical composition composition of the of alloys the alloys was determined was determined using a scanningusing a electronscanning microscope electron Hitachimicroscope S-3400N Hitachi (Silesian S-3400N University (Silesian of Technology, University Katowice, of Technology, Poland) withKatowice, an EDX Poland) attachment with an by EDX Noran attachment company by and Noran the Voyager company program. and the Voyager program. Preliminary analysisanalysis ofof the the prepared prepared samples samples was wa thes the basis basis with with which which to maketo make corrections corrections of the of waythe way the compositethe composite aluminum aluminum alloy alloy was was produced. produced.

4. Results of Tests and Their Analysis Chemical composition of the tested alloy AlSi10Mg0.5Mn0.5AlSi10Mg0.5Mn0.5 before and after introduction of powders isis presented presented in in Table Table1. The 1. chemicalThe chemical composition composition was tested was on tested a FoundryMaster on a FoundryMaster Compact 01L00113Compact emission01L00113 spectrometer. emission spectrometer. The test material The consisted test material of solid samplesconsisted for metallographicof solid samples defects, for preparedmetallographic in observance defects, of prepared appropriate in methodologicalobservance of principles.appropriate The methodological spectrometer used principles. was not ableThe tospectrometer determine theused contents was not of able non-metallic to determine elements, the co mainlyntents of carbon, non-metallic oxygen, elements, sulfur, and mainly nitrogen. carbon, oxygen, sulfur, and nitrogen. Table 1. Chemical composition of tested alloy AlSi10Mg0.5Mn0.5 before and after introduction of the powderTable 1. (%mass).Chemical composition of tested alloy AlSi10Mg0.5Mn0.5 before and after introduction of the powder (%mass). Tested Alloy Si Cu Fe Mn Mg Ni Al alloyTested EN AC-43400 Alloy 9.82 Si Cu 0.08 Fe 0.47 Mn 0.48 Mg 0.47 Ni 0.13 the Al rest alloyalloy EN+ powder-1 AC-43400 9.82 9.88 0.08 0.09 0.47 0.51 0.48 0.44 0.47 0.43 0.13 0.11 the the rest rest alloyalloy + powder-1powder-2 10.039.88 0.09 0.06 0.51 0.53 0.44 0.49 0.43 0.48 0.11 0.12 the the rest rest alloy + powder-2 10.03 0.06 0.53 0.49 0.48 0.12 the rest

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By maintaining similar parameters of melting and casting, the tested silumin was cast to a standard sampler QC4080, and the temperature curves with a time function (T = f(t)) were registered together with the time derivative of temperature (dT/dt = f’(t)). An example graph of thermal analysis of the alloy AlSi10Mg0.5Mn0.5 without adding the powder is presented in Figure2, and, after re-melting Metalsand 2019 adding, 9, x FOR powder-1,PEER REVIEW is presented in Figure3. Crystallization temperatures for all tested alloys4 of 7 are presented in Table2.

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composites (point E). As in the case of Al dendrites, the iron contained in the powders may have delayedFigure the 2. crystallizationGraph of ATD thermalprocess, analysis causing of a alloy decrease AlSi10Mg0.5Mn0.5: in temperature. (a) without addition of powder Figure 2. Graph of ATD thermal analysis of alloy AlSi10Mg0.5Mn0.5: (a) without addition of withExample characteristic microstructures points, (b )of with alloy addition EN AC-43400 of powder-1, before and (cthe) with implementation addition of powder-2 of powder with are powder with characteristic points, (b) with addition of powder-1, and (c) with addition of powder-2 showncharacteristic in Figure 3a,d points. and after the use of powder-1 in Figure 3b,e, and powder-2 in Figure 3c,f. with characteristic points.

By maintaining similar parameters of melting and casting, the tested silumin was cast to a standard sampler QC4080, and the temperature curves with a time function (T = f(t)) were registered together with the time derivative of temperature (dT/dt = f’(t)). An example graph of thermal analysis of the alloy AlSi10Mg0.5Mn0.5 without adding the powder is presented in Figure 2, and, after re-melting and adding powder-1, is presented in Figure 3. Crystallization temperatures for all tested alloys are presented in Table 2.

Table 2. Characteristic crystallization temperatures for alloy AlSi10Mg0.5Mn0.5 without addition of powder and after addition of powders (°C).

Point A B C D E temperature TE(Mn) T(Al) TEmin. TE TE(Mg) alloy EN AC-43400 648 578 565 568 550 alloy + powder-1 652 559 562 566 540 Figure 3. Microstructure of alloy AlSi10Mg0.5Mn0.5: (a,d) in initial condition (with various Figure 3. Microstructurealloy + powder-2 of alloy AlSi10Mg0.5Mn0.5: 650 560 568 ( a,d) 570 in initial 539 condition (with various magniﬁcations);magnifications); (b(,be,)e with) with addition addition of powder-1; of powder-1; (c,f) with (c,f addition) with addition of powder-2; of powder-2; (a–c—light microscope(a–c—light

Olympusmicroscope GX-71, Olympusd–f—scanning GX-71, d–f electron—scanning microscope electron Hitachimicroscope S-3400N). Hitachi S-3400N). As can be seen from the temperatures shown in Table 2, the crystallization of the manganese-richEutectic eutectic occurrence composite α(Al) +occu β(Si)rred and in Al the dendrites temperature in the rangeAlSi10Mg0.5Mn0.5 of 648–652 °C alloy for all are tested shown in alloys.Figure It should 4. be noted that the introduction of powders into the tested alloy did not change the temperature of this eutectic composite, which crystallized as the original one. Another exothermic effect was observed related to the crystallization of dendrites of the solid solution (Al)—point B. It is noticeable that the introduction of powders into the alloys significantly reduced this temperature by about 18 °C. This may be dictated by the effect of powder suppression of the dendrite aluminum nucleation process. This passivating effect may be based on powder binding of aluminum dendrites in a manner not yet known. However, as can be seen from Table 2, the effect of such suppression of the nucleation process is the local overcooling of the crystallization front, which results in a decrease in temperature on the cooling curves. It should also be noted that these powders did not affect the temperature of the eutectic crystallization (point D). Another temperature reduction caused by the introduction of powders concerns the crystallization of magnesium-rich eutectic

Figure 4. Microstructure of alloy AlSi10Mg0.5Mn0.5: (a,d) in initial condition (with various magnifications); (b,e) with addition of powder-1; (c,f) with addition of powder-2; (a,b,c—light microscope, d,e,f—scanning microscope).

5. Discussion The characteristic values of crystallization temperatures of alloy AlSi10Mg0.5Mn0.5 modified with powders FexAly and FexAly–SiC, where temperature Tstart was similar for all experiments (around 740 °C), show that similar conditions of casting for the tested alloys were preserved. Figures 1 and 2 show that at temperature of about 650 °C, there was a significant exothermic effect visible (point B). From analysis of phase balance system of Al–Mn and Al–Si–Mn, it can be assumed that this was the temperature of nucleation start and eutectic crystallization, which included a primary intermetallic phase rich in Mn (probably Al6Mn). Addition of powders to the alloy did not significantly change the crystallization temperature of this eutectic composite. This temperature was the highest for the initial alloy, where it equaled 578 °C. The addition of powders

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Table 2. Characteristic crystallization temperatures for alloy AlSi10Mg0.5Mn0.5 without addition of

powder and after addition of powders (◦C).

Point A B C D E Metals 2019, 9, x FOR PEER REVIEW 5 of 7 temperature TE(Mn) T(Al) TEmin. TE TE(Mg) composites (pointalloy E). EN As AC-43400 in the case of 648Al dendrites, 578 the iron 565 contained 568 in the 550powders may have delayed the crystallizationalloy + powder-1 process, causing 652 a decrease 559 in temperature. 562 566 540 Example microstructuresalloy + powder-2 of alloy 650EN AC-43400 560 before 568 the implementation 570 539 of powder are shown in Figure 3a,d and after the use of powder-1 in Figure 3b,e, and powder-2 in Figure 3c,f.

As can be seen from the temperatures shown in Table2, the crystallization of the manganese-rich eutectic composite occurred in the temperature range of 648–652 ◦C for all tested alloys. It should be noted that the introduction of powders into the tested alloy did not change the temperature of this eutectic composite, which crystallized as the original one. Another exothermic effect was observed related to the crystallization of dendrites of the solid solution (Al)—point B. It is noticeable that the introduction of powders into the alloys significantly reduced this temperature by about 18 ◦C. This may be dictated by the effect of powder suppression of the dendrite aluminum nucleation process. This passivating effect may be based on powder binding of aluminum dendrites in a manner not yet known. However, as can be seen from Table2, the e ffect of such suppression of the nucleation process is the local overcooling of the crystallization front, which results in a decrease in temperature on the cooling curves. It should also be noted that these powders did not affect the temperature of the eutectic crystallization (point D). Another temperature reduction caused by the introduction of powders concerns the crystallization of magnesium-rich eutectic composites (point E). As in the case of Al dendrites, the iron contained in the powders may have delayed the crystallization process, causing Figure 3. Microstructure of alloy AlSi10Mg0.5Mn0.5: (a,d) in initial condition (with various a decrease in temperature. magnifications); (b,e) with addition of powder-1; (c,f) with addition of powder-2; (a–c—light Example microstructures of alloy EN AC-43400 before the implementation of powder are shown microscope Olympus GX-71, d–f—scanning electron microscope Hitachi S-3400N). in Figure3a,d and after the use of powder-1 in Figure3b,e, and powder-2 in Figure3c,f. Eutectic occurrence αα(Al)(Al) ++ ββ(Si)(Si) and and Al Al dendrites dendrites in in the the AlSi10Mg0.5Mn0.5 AlSi10Mg0.5Mn0.5 alloy alloy are are shown shown in Figurein Figure 4. 4.

Figure 4.4. MicrostructureMicrostructure of of allo alloyy AlSi10Mg0.5Mn0.5: (a (a,d,d) )in in initial initial condition condition (with various magnifications);magniﬁcations); ( (bb,,ee)) with addition of powder-1; ( c,f) with addition of powder-2;powder-2; (a,b,c—light microscope, d,e,f—scanning microscope).

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5. Discussion The characteristic values of crystallization temperatures of alloy AlSi10Mg0.5Mn0.5 modified with powders FexAly and FexAly–SiC, where temperature Tstart was similar for all experiments (around 740 ◦C), show that similar conditions of casting for the tested alloys were preserved. Figures1 and2 show that at temperature of about 650 ◦C, there was a significant exothermic effect visible (point B). From analysis of phase balance system of Al–Mn and Al–Si–Mn, it can be assumed that this was the temperature of nucleation start and eutectic crystallization, which included a primary intermetallic phase rich in Mn (probably Al6Mn). Addition of powders to the alloy did not significantly change the crystallization temperature of this eutectic composite. This temperature was the highest for the initial alloy, where it equaled 578 ◦C. The addition of powders FexAly and FexAly–SiC caused the value of this temperature to decrease, respectively, to 559 ◦C (by application of powder-1) and to 560 ◦C (by application of powder-2). Temperature of binary eutectic crystallization, α(Al) + β(Si), TE, was on similar level and equaled around 570 ◦C, preceded by a few degrees decrease of temperature (TEmin) from 562 to 568 ◦C. After the end of eutectic crystallization α(Al) + β(Si), there was an exothermic heat effect observed on the ATD crystallization curve, which probably came from triple eutectic crystallization, which included intermetallic phase Mg2Si (TE(Mg)). This eutectic crystallization occurred at a temperature of about 550 ◦C, and, after modification with powders, a decrease of its crystallization temperature was observed to a value of about 540 ◦C. The end of the crystallization of the given alloy (Tsol.) was observed at a temperature of about 535 ◦C, and the addition of the applied powders caused a decrease of this value to about 523 ◦C. It should also be pointed out that on the ATD curves, there were no such exothermic effects observed, which was connected with the crystallization of phases rich in iron or too small to be observed. However, on the basis of the Al–Fe and Al–Fe–Si phase equilibrium system, it can be assumed that for sub-eutectic content in silumins, the multi-component, eutectic AlXFeYSiZ will crystallize, and, in a state of thermodynamic disequilibrium, will precipitate in the form of long and sharp-edged plate–needle shapes. Therefore, this should be taken into account so that such precipitations could change their disadvantageous morphology into more rounded forms. The presented, chosen results of the tests of alloy AlSi10Mg0.5Mn0.5 with dispersion strengthening with intermetallic phases from FeAl systems show the structure of a new type of alloy. It should be assumed that due to the characteristic morphology of phases rich in iron and the homogenous structure of the alloy, the aluminum composite should possess good mechanical properties and resistance to friction wear. However, any explanation for the existence of particles that strengthen the solution, and thus result in an increase in mechanical properties, has not yet been investigated. It is therefore necessary to carry out additional studies on the effect of the addition of powders on the strengthening of the solution and the size of its structural components. Results of the tests show that the assumed material concept for preparing a composite was correct, and there is no similar material in the accessible literature data. In the presented chosen scope of technological tests, in order to prepare a composite with the influence of a hybrid component, the mechanism of modification of the silumin structure with sub-eutectic ingredients was not taken into account, and can therefore be the topic of further research. It should be anticipated that the structural effects and the mechanism of modification of sub-eutectic silumins will be connected with morphology, structure, phase composition, and the ingredient percentage contents that will be introduced to the liquid silumins. In continuation of these tests, the plan is to introduce a larger influence of modifying phases to the composite.

6. Conclusion On the basis of conducted tests, the following conclusions were made: Metals 2019, 9, 865 7 of 8

1. Application of the assumed technological procedure of manufacturing the designed dispersion structure of the composite showed a signiﬁcant decrease of crystallization temperature of the dendrites of the solution (Tliq). The decrease of temperature was of about 18 ◦C. 2. Introduction of the aluminum powders FexAly and FexAly–SiC achieved by ASHS process did not cause a change of crystallization temperature of the eutectic composite, which included a phase rich in Mn and the binary eutectic α(Al) + β(Si).

3. As a result of the introduction of FexAly and FexAly–SiC powders, a decrease in the crystallization temperature of the complex eutectic composites was observed, which probably included the intermetallic phase Mg2Si (a decrease of about 10 ◦C). There was also a temperature Tsol. decrease observed (of about 12 ◦C), and therefore there was an extension of the crystallization process after the addition of FeAl powder. 4. The suggested technological procedure of composite preparation on the basis of sub-eutectic silumin AlSi10Mg0.5Mn0.5 showed refinement of the dendrites of solution α(Al), and transition of plate eutectics α(Al) + β(Si) into modified eutectics. This was confirmed by microstructures. 5. Due to insufficiently small exothermic effects from the iron content (around 0.5% mass) and introduced powders FexAly and FexAly–SiC, the ATD thermal tests should be completed with calorimetric analysis DSC.

6. For a more complete vision of the inﬂuence of modiﬁcation with FexAly and FexAly–SiC powders, there should be tests of mechanical properties conducted.

Author Contributions: Conceptualization, J.P. and R.W.; methodology, J.P.; software, J.P.; validation, R.W.; formal analysis, J.P. and R.W.; investigation, J.P.; resources, J.P.; data curation, R.W.; writing—original draft preparation, R.W.; writing—review and editing, R.W.; visualization, J.P. and R.W.; supervision, R.W.; project administration, J.P.; funding acquisition, J.P. Funding: The publication is supported under the rector’s professorial grant. Silesian University of Technology, 11/030/RGP18/0216. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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