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INFLUENCE OF CHEMICAL COMPOSITION AND IMPURITIES ON MICROSTRUCTURE AND FORMATION OF INTERMETALLIC PHASES IN SELECTED ALUMINUM ALLOYS

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  • Vysoká škola báňská - Technical University of Ostrava

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May20th- 22nd 2020, Brno, Czech Republic, EU
INFLUENCE OF CHEMICAL COMPOSITION AND IMPURITIES ON MICROSTRUCTURE AND
FORMATION OF INTERMETALLIC PHASES IN SELECTED ALUMINUM ALLOYS
José SENA, Jaromír DRÁPALA, Monika LOSERTOVÁ, Kateřina KONEČNÁ,
Gabriela KOSTIUKOVÁ
VSB - Technical University of Ostrava, Ostrava, Czech Republic, EU, jaromir.drapala@vsb.cz
https://doi.org/10.37904/metal.2020.3622
Abstract
Al-Si-Mg alloys account for the biggest percentage of cast pieces in the world and the automotive industry is
one of major consumers of these alloys. The properties of Al alloys are influenced by their chemical
composition, technology of casting, methods of forming and heat treatment. Alloying with Sr, Ti, B etc. can
improve the structure of casts, the presence of Fe, Mn leads to creation of intermetallic phases of
Alx(Fe,Mn)ySiz types. The experimental part complements this contribution with metallography and SEM/EDX
analyses of three different specimens of cast Al alloys with and without heat treatment. Also, metallography
results are given for Al profiles subjected to different working conditions. The mechanical properties of the cast
alloys are evaluated by microhardness. The usability of Al-Si alloys with different elements in the automotive
industry is demonstrated in this work.
Keywords: Aluminum alloys, chemical composition, microstructure, intermetallic phases, microhardness
1. INTRODUCTION
Aluminum is used in a wide range of vehicle parts ranging from heat exchangers to closures; each part requires
unique functional properties for these differing automotive applications and therefore needs various alloying
elements. The key alloying elements differ by alloy family. These additions can be Cu, Mn, Si, Mg, Zn and Sn
[1,2].
For simplification reasons, aluminum alloys have been divided in several categories. These categories are
based on production types and compositions and the primary mechanism of mechanical properties
development, with cast and wrought aluminum alloys based on production types and heat treatable and non-
heat treatable based on the mechanism of properties development [3,4].
The effect of different alloying elements in Al-Si alloys can improve certain properties making it suitable for a
wide variety of uses. For a finer structure and improved electrical conductivity boron is used, while bismuth
increases machinability and chromium improves hardenability. The addition of antimony increases corrosion
resistance in sea water, but the addition of copper decreases this resistance while increasing the strength.
Cobalt together with iron increases the alloy strength properties at high temperatures [4].
The benefits of alloying Al-Si castings with iron are the increase in hot-tear resistance and the decrease in the
tendency of die soldering. This occurs when the molten aluminium welds to the die surface, damaging the die
and resulting in a poor casting surface finish. At least 0.6 wt% of iron is required to avoid die sticking but higher
iron concentration reduces the alloys fatigue properties. Iron also reacts to form several intermetallic phases
like FeAl3, FeMnAl6 and α-AlFeSi. These phases are insoluble and are responsible for improvement in strength
at elevated temperatures but come with the downside of embrittlement of the microstructure.
Iron also decreases the ability to feed, increases the porosity and diminishes the effect of grain refinement.
Therefore, alloys such as A356 have a limit of 0.12 wt% of iron. Manganese is the most preferred element to
May20th- 22nd 2020, Brno, Czech Republic, EU
neutralize the adverse effects of the excess of iron in the melt. Mn is typically added at half the percentage of
iron in the melt. Mn and Fe may be considered isomorphous and alloy chemistry may reflect stoichiometries
favouring the least detrimental insoluble Al-Fe-Mn phases [5,6].
When as-cast Al-Si alloys are subjected to elevated temperatures, they tend to experience growth due to the
precipitation of silicon from the solid solution. Slow solidification of Al-Si alloys results in a coarse
microstructure, in which the eutectic comprises large plates or needles of silicon in the continuous matrix of
aluminum. As the coarse plates of silicon are brittle, the eutectic alloy shows low ductility and tensile strength.
Mechanical properties of Al-Si alloys are largely influenced by the size, form and distribution of second phase
silicon particles, porosity, eutectic morphology and the grain structures. The second phase silicon particles
affect the toughness, strength, ductility and tribological properties. Refinement of primary aluminum, silicon
and eutectic mixture improves the mechanical properties.
Strontium modification effects are longer and the effective time can be extended up to 10 hours by adding 0.05
wt% to 0.2 wt% of beryllium, reducing the oxidation speed of the molten metal and stabilizing the strontium
effect. High Sr master alloys should be added at the lowest practical temperature because they dissolve best
at a low temperature [4,7]. Phosphorus with aluminum produces aluminum phosphide with a high melting point
used as an active under-layer for the crystallization of primary Si. The grain refinement is especially effective
for hypoeutectic Al-Si alloys with a high proportion of solid α solution made available by adding Ti and B to the
molten metal either individually or in a combination [4].
The microstructure of Al-Si alloys depends strongly on both the composition and the casting process. The
rapid cooling causes a fine eutectic structure, small dendrites cells, small arm spacing and small grain size.
Slower cooling rates require eutectic modification to obtain the finely dispersed eutectic silicon. The grain size
and dendrite arm spacing are primarily controlled by cooling and solidification rates. Melt treatment, cooling
conditions heat transfer from the solidifying metal to the mold has a significant influence on the eutectic
morphology, thus playing an important role in obtaining a good casting quality [7].
The main goal of this work is the characterization of the microstructure and properties of three different
aluminum alloys. Results of SEM/EDX analyses, incl. microhardness measurements, have been presented.
2. EXPERIMENTAL MATERIAL AND RESULTS
For the experimental study the following Al alloys have been used:
1) AlSi7Mg alloy, commonly referred to as A356, (sample no. 1) prepared by casting and heat treating
2) AlSi12 eutectic alloy (sample no. 2) prepared by casting
3) AlSi10Mg alloy (sample no. 3) prepared by casting
4) AlMg0.7Si alloy profile with a 30 x 30 mm area (sample no. 4) prepared by casting and forming
5) AlMg0.7Si alloy profile with a 60 x 60 mm area (sample no. 5) prepared by casting and forming
The results of the chemical analyses of all the samples are presented in Table 1.
Table 1 Results of chemical analyzes by OES, average value from three measurements (wt%)
Specimen
Alloy
Si
Fe
Cu
Mn
Mg
Ti
B
Na
Sr
Sample no. 1
AlSi7Mg
7.24
0.152
0.001
0.004
0.287
0.115
0.002
0.001
0.017
Sample no. 2
AlSi12
12.52
0.231
0.028
0,028
0.008
0.121
---
0.003
---
Sample no. 3
AlSi10Mg
13.67
0.940
0.710
0.288
0.58
0.052
---
<0.001
---
Sample no. 4
AlMg0.7Si
0.36
0.190
0.007
0.052
0.350
0.009
---
<0.001
---
Sample no. 5
AlMg0.7Si
0.42
0.200
0.006
0.052
0.450
0.012
---
<0.001
---
May20th- 22nd 2020, Brno, Czech Republic, EU
Figures 1a and 1b present the microstructures of samples no. 1 to no. 5 (the optical microscope).
Figure 1a Microstructures of samples 1 to 3 (OM)
A356 (sample no. 1) is a heat treated and modified silumin that shows the primary aluminum dendrite structure
embedded in Al-Si eutectic in a homogenized and well-organized dendritic structure. The structure contains a
eutectic formed by needles of silicon in the Al solid solution. In sample no. 2 (AlSi12) un-modified Al-Si eutectic
in a form of thin dark needles of un-dissolved Si and primary crystallized Al dendrites are shown. The
Sample no. 1 AlSi7Mg
Sample no 2: AlSi12
Sample no 3:AlSi10Mg
10 μm
30 μm
30 μm
100 μm
100 μm
700 μm
700 μm
500 μm
50 μm
May20th- 22nd 2020, Brno, Czech Republic, EU
microstructure of sample no. 3 (AlSi10Mg) is formed by the eutectic (Al + thin needles of Si) and bigger dark
areas of pure silicon. In all three Al alloys the grey areas (needles, fish-bones or hexagons) are visible, which
represent intermetallic phases containing probably Fe, Mn, Al and Si see white arrows.
Figure 1b Microstructures of samples 4 and 5 profiles 30 x 30 mm and 60 x 60 mm (OM)
In samples no. 4 and no. 5) the matrix is pure aluminum, whereof little particles of another phase are visible.
Table 2 Results of EDX chemical analyzes (at%)
sample no. 1 (AlSi7Mg)
Label
Mg
Al
Si
Ca
Ti
Mn
Fe
Sr
Spectra 1 - 3
67.2
18.6
0.3
14.0
Spectra 4 - 6
1.3
41.5
40.7
1.2
1.0
14.3
Spectra 7, 8
100.0
Spectra 9, 10
99.0
1.0
Figure 2 SEM/BEI microstructure of the sample no. 1
(AlSi7Mg) with positions of the EDX analysis (at%)
Spectra 1, 2 and 3 are formed by sharp particles (Figure 2). The following types of phases have been known
from literature: Al15(Mn,Fe)3Si2, Al12(Mn,Fe)3Si, Al10(Mn,Fe)3Si, Al9(Mn,Fe)2Si2, Al8(Mn,Fe)2Si, Al5(Mn,Fe)Si2.
In our case the most probable intermetallic compound is Al9(Mn,Fe)2Si2 with a theoretical proportion of atoms
Al:(Mn,Fe):Si = 69.2:15.4:15.4, which is in very good
agreement with our results Al:(Mn,Fe):Si 68:14:18. Fe
and Mn are added to avoid the cast to stick to the die.
Sr is added for modification of the sharp structure usually
present in the Al-Si eutectic, which changes to a more
rounded form with less stress concentrator sites. Spectra
4, 5 and 6 are created from globular light particles of IMC
with Sr. There are currently four different phases
associated to the intermetallic Al-Si-Sr composition:
AlSiSr, Al2Si2Sr, Al2Si2Sr3 and Al8Si15Sr4 [8]. In our case
the ratio between atoms Al:Si:Sr 41:41:14, which
corresponds approximately to the known intermetallic
compounds Al2Si2Sr. But the composition of the
intermetallic compound may possibly be influenced by the
presence of Mg, Ti and Ca that were found in the
spectrum. Ti is added for the grain refinement of the alloy
Sample no. 4: AlMg0.7Si
Sample no. 5: AlMg0.7Si
Figure 3 SEM/BEI microstructure of the
sample no. 2 (AlSi12) with positions of EDX
analysis (at%)
May20th- 22nd 2020, Brno, Czech Republic, EU
and Ca have the same structure modification properties as Na with the added benefit of acting as a Fe
neutralizer in recycled Al alloys with a high Fe content.
Spectra 1, 2 and 3 correspond to Al-Si eutectic,
where Si is concentrated. Spectra 4, 5, 6 and 7 are
from bright areas, which belong to Al-Si-Mn-Fe
phase. The exact determination of the formula for
these particles is difficult in this case because of their
small sizes and the influence of the eutectic
surrounding. Spectra 8 and 9 are (α-Al).
Table 4 Results of EDX chemical analyzes (at%)
sample no. 3 (AlSi10Mg)
Label
Al
Si
V
Cr
Mn
Fe
Cu
Spectra 1 - 2
2.8
97.2
Spectra 3 - 4
71.4
11.3
0.3
1.2
5.6
10.1
0.1
Spectra 5 - 6
82.0
10.4
1.2
5.2
1.3
Spectra 7 - 8
98.2
1.8
Figure 4 SEM/BEI microstructure of the sample
no. 3 (AlSi10Mg) with positions of EDX analysis
(at%)
Spectra 1 and 2 bright grey areas and belong to almost pure Si, un-dissolved β phase. Spectra 3 and 4 are
from bright areas, which belong to Al15(Mn,Fe)3Si2 intermetallic compound. These particles contain all the
alloying elements that give this alloy its properties. V and Cr are added to increase strength and Cu to help
with the heat treatment of the alloy. Spectra 5 and 6 are marked in irregular sharp areas. These areas are
composed by Al15(Mn,Fe)3Si2 with 1.3 at% Cu. Spectra 7 and 8 come from the α-solid solution of aluminum.
Table 5 Results of EDX chemical analyses (at%)
sample no. 4 (Al profile)
Label
Mg
Al
Si
Mn
Fe
Spectra 1 - 3
81.1
6.2
0.5
12.1
Spectrum 4
0.3
99.2
0.5
Figure 5 SEM/BEI microstructure of sample
no. 4 (Al profile) with positions of EDX analysis
(at%)
Spectra 1 to 3 belong to the bright irregular areas in the Figure 5, the chemical composition of these spots
answers to intermetallic phase Al11(Fe,Mn)2Si type. Spectrum 4 is the α-solid solution of aluminum with 0.5
at% Si and 0.3 at% Mg.
In Table 6 results of HV microhardness measurements for all samples are presented. It is evident that the
highest microhardness had the alloy AlSi7Mg.
Table 3 Results of EDX chemical analyzes (at%)
sample no. 2 (AlSi12)
Label
Al
Si
Mn
Fe
Spectra 1 3
33.3
66.7
Spectra 4 7
89.5
6.3
0.4
3.8
Spectra 8 9
98.8
1.2
May20th- 22nd 2020, Brno, Czech Republic, EU
Table 6 Microhardness results, average values from five measurements (HV 0.1)
Sample
no.1 (AlSi7Mg)
no. 2 (AlSi12)
no. 3 (AlSi10Mg)
no. 4 AlMg0.7Si (30 x 30)
no. 5 AlMg0.7Si (60 x 60)
HV 0.1
109
66
95
73
80
Figure 6 Grain structure in samples no. 4 (Al profile 30 x 30 on the left) and no. 5 (Al profile 60 x 60 on
the right) color etching
Figure 6 shows different grain sizes and their orientations. The average grain size for sample no. 4 was 69
μm, for sample no. 5 is 60 μm, respectively. Near to the surface of the wall (thickness 3 mm) grains are bigger
than in the central part.
3. CONCLUSIONS
This work deals with three different types of aluminum alloys AlSi7Mg (A356) after casting and heat treatment;
AlSi12 and AlSi10Mg only after casting and alloys AlMg0.7Si (profiles 30 x 30 mm and 60 x 60 mm) after
forming. On the basis of EDX analyses we have found three types of intermetallic compounds: Al9(Mn,Fe)2Si2
in A356, Al15(Mn,Fe)3Si2 in the alloy AlSi10Mg and Al11(Fe,Mn)2Si type in the AlMg0.7Si profiles. In A356
alloyed with strontium we have found Al2Si2Sr intermetallic phases. The highest microhardness has been
observed in AlSi7Mg alloy. The microstructure of all three Al-Si-Mg alloys has been dendritic (α – solid solution
of Al) with the eutectic (Al) + (Si) in a form of thin Si needles. The substructure of Al profiles has shown grains
with different sizes and orientations, the average grain size has been between 50 and 70 μm.
The automotive and aerospace industry have been using these alloys for a long time and thousands of articles
have been written already studying their properties, proposing not only technological changes to the way the
alloys are processed but also changes in processing the castings and in the amount of alloying elements.
ACKNOWLEDGEMENTS
This work was solved in the frame of the project “Preparation and optimalization of properties of
alloy for automotive, electrotechnic and biomedical applications- project No. SP2020/56.
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... % of Fe, the intermetallic phase forms are α-Al15(Fe,Mn)3Si2, Mg2Si and β-AlFeSi. The increasing concentration of Fe in melt B and C caused the formation of β-AlFeSi in higher amounts as α-Al15(Fe,Mn)3Si2 ( Fig. 1) as confirmed in the research works of Ebhota [10], Pezda [14], and Sena [15]. The form and size, morphology, and distribution of β-phases have profound effects on the castability and mechanical properties of the final casts. ...
... % of Fe (Fig. 2). Further analyses by the authors of the research works [14,15] show that these phases are Al5FeSi. The morphology is long and thin platelike (Fig. 3) confirmed by using deep etching (with etcher HCl). ...
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ASM Specialty Handbook: Aluminum and Aluminum Alloys
  • J R Davis
DAVIS, J. R. ASM Specialty Handbook: Aluminum and Aluminum Alloys. Ohio: ASM International, 1993. May20 th -22 nd 2020, Brno, Czech Republic, EU
Aluminum Materials and Technologies from A to Z
  • Š Michna
MICHNA, Š. et al. Aluminum Materials and Technologies from A to Z. Prešov: ADIN, 2007.