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

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 Fe_xAl_y 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.

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 [1,2,3,4,5]. Modifiers for sub-eutectic silumins are simple: Na, Sr, Sb, and complexes in type Al–Ti–C, Al–Ti–B [6,7,8], 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 Fe_xAl_y 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 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:

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Investigation of the technological and material concepts needed to produce the casted alloy composite modified with powders FeAl, Al–Fe_xAl_y, and Al–Fe_xAl_y–SiC,

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Determination of the method by which to produce the powders for modification of the aluminum matrix,

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Preparation of the technological process for producing composites with varied contents of structural ingredients,

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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 [12,13,14].

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–Fe_xAl_y, and Al–Fe_xAl_y–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. Fe_xAl_y powder (marked as powder-1) was achieved by ASHS and mixed with 50% mass of Al powder. The mixture of powders Fe_xAl_y–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.

Structure and morphology of the powders was determined using light and scanning microscopy. Composite powders prepared in this way were added to liquid silumin in such amounts that the contributions of the composite powders were 30 and 50% mass of the alloy.

Characteristic parameters of silumin crystallization were tested by ATD thermal analysis using a Crystaldigraph PC device (Figure 1).

In addition to crystallization tests, microscopic tests were conducted with the use of light and scanning electron microscopy. Some samples were further melted to determine their structure. The structures of the tested alloys was observed on metallographic microsections from samples cut crosswise and along the central axis.

The structures of the surface of samples were observed and registered using an Olympus GX-71 microscope.

Morphology of the powders and local chemical composition of the alloys was determined using a scanning electron microscope Hitachi S-3400N (Silesian University of Technology, Katowice, Poland) with an EDX attachment by Noran company and the Voyager program.

Preliminary analysis of the prepared samples was the basis with which to make corrections of the way the composite aluminum alloy was produced.

4. Results of Tests and Their Analysis

Chemical composition of the tested alloy AlSi10Mg0.5Mn0.5 before and after introduction of powders is presented in

Table 1. 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
alloy EN AC-43400	9.82	0.08	0.47	0.48	0.47	0.13	the rest
alloy + powder-1	9.88	0.09	0.51	0.44	0.43	0.11	the rest
alloy + powder-2	10.03	0.06	0.53	0.49	0.48	0.12	the rest

. The chemical composition was tested on a FoundryMaster Compact 01L00113 emission spectrometer. The test material consisted of solid samples for metallographic defects, prepared in observance of appropriate methodological principles. The spectrometer used was not able to determine the contents of non-metallic elements, mainly carbon, oxygen, sulfur, and nitrogen.

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. 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
alloy + powder-2	650	560	568	570	539

.

As can be seen from the temperatures shown in Table 2, 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 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 composites (point E). As in the case of Al dendrites, the iron contained in the powders may have delayed the crystallization process, causing a decrease in temperature.

Example microstructures of alloy EN AC-43400 before the implementation 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.

Eutectic occurrence α(Al) + β(Si) and Al dendrites in the AlSi10Mg0.5Mn0.5 alloy are shown in Figure 4.

5. Discussion

The characteristic values of crystallization temperatures of alloy AlSi10Mg0.5Mn0.5 modified with powders Fe_xAl_y and Fe_xAl_y–SiC, where temperature Tstart was similar for all experiments (around 740 °C), show that similar conditions of casting for the tested alloys were preserved.

Figure 1 and Figure 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 Fe_xAl_y and Fe_xAl_y–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 (T_E(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 (T_sol.) 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 Al_XFe_YSi_Z 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:

• 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.
• Introduction of the aluminum powders Fe_xAl_y and Fe_xAl_y–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).
• As a result of the introduction of Fe_xAl_y and Fe_xAl_y–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 T_sol. decrease observed (of about 12 °C), and therefore there was an extension of the crystallization process after the addition of FeAl powder.
• 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.
• Due to insufficiently small exothermic effects from the iron content (around 0.5% mass) and introduced powders Fe_xAl_y and Fe_xAl_y–SiC, the ATD thermal tests should be completed with calorimetric analysis DSC.
• For a more complete vision of the influence of modification with Fe_xAl_y and Fe_xAl_y–SiC powders, there should be tests of mechanical properties conducted.