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Bogdanoff, T., Seifeddine, S., Dahle, A K. (2016)
The effect of SI content on microstructure and mechanical properties of Al-Si alloy.
La Metallurgia Italiana, 108(6)
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THE EFFECT OF SI CONTENT ON
MICROSTRUCTURE AND MECHANICAL
PROPERTIES OF AL-SI ALLOY
Toni Bogdanoff – Jönköping University, School of Engineering, Jönköping, Sweden Salem Seifeddine – Jönköping University, School of Engineering, Jönköping, SwedenArne K. Dahle – Jönköping University, School of Engineering, Jönköping, Sweden
Department of Materials and Manufacturing, Gjuterigatan 5, Box 1026, 551 11 Jönköping, Sweden
Abstract
Al-Si alloys are the most popular casting alloys due to their excellent castability combined with high strength-to-weight ratio. This paper investigates the role of Si content in the range of 6.5 wt. % to 14.4 wt. % on the
microstructure and mechanical properties of Al-Si-Mg casting alloys. All alloys were modified with 90-150 ppm Sr. No grain refiner was added. The samples were produced by directional solidification providing a mi-crostructure that corresponds to mimi-crostructures found in die castings. From the phase diagram and coupled
zone, increasing the Si level up to 14.4 wt. % is expected to start a competition between formation of α-dendrites and a fully eutectic microstructure. However, it is known that Sr-modification shifts the eutectic to higher Si contents. For the lower Si contents, the microstructure of the samples consisted of α-dendrites and
a modified Al-Si eutectic. At 12.4 wt. % Si and above, a cellular eutectic microstructure was observed. No primary Si was observed even at 14.4 wt. % Si. The mechanical properties in terms of yield and tensile strength did not vary remarkably as a function of the Si level unlike the elongation to failure that dropped from 12 % at 6.5 wt. % Si to nearly 6 % at 14.4 wt. % Si; but still the material is exhibiting an elongation to
failure that is far higher than normally expected.
KEYWORDS: Al-Si alloy – Fracture mechanism – Mechanical Properties –
Dendritic eutectic growth – Microstructure
INTRODUCTION:
Al-Si alloys are used in the foundry industry due to their good fluidity, good corrosion resistance and high strength-to-weight ratio. The most important group of Al-Si cast alloys are the ones that have 5 - 12 wt. % Si normally employed in automotive and aerospace industries. Increasing the Si level up to 17 wt. % improves the wear properties and provides higher dimensional stability. Al-Si alloys may result in various
microstructures and properties depending on the chemistry of the alloys, cooling rate and the manufacturing process. While the hypoeutectic Al-Si alloys with up to approx. 12 wt. % Si are characterized by α-dendrites, Al-Si eutectic and other microstructural features such as intermetallics and defects, eutectic Al-Si alloys are a non-faceted/ faceted system with the Si growing irregular in α-Al matrix. Impurities found in these commercial alloys, promote nucleation of α-dendrites and Al-Si eutectic that can occur in different ways; one through a constitutionally undercooled zone and another through nuclei present in the melt. During solidification, a competitive growth between α-dendrites and Al-Si eutectic exists that is dependent on the solidification rate [1-5]. Dendritic growth can affect the concentration of liquid near the dendrite liquid interface and distribution of heterogeneous nuclei in the melt, this influence the nucleation and growth of the Al-Si eutectic.
Solidification of hypereutectic alloys, with Si levels above 12 wt. %, are normally characterized by the formation of primary Si particles, and followed by an Al-Si eutectic. Certainly, when other alloying elements are present, a number of intermetallics could be formed during the solidification [2].
The fracture mechanisms in Al-Si alloys are generally accepted to progress in three stages. The first stage is particle cracking at low strains; it could be cracking of Si or Fe-rich bearing phases etc. During the second
stage, when deformation continues cracked particles generate shear bands that form microcracks. The last stage results in microcracks propagation leading to the final failure [6]. Coarser and longer particles are more prone to cracking while in finer structures the progress of cracking is more gradual due to facing a ductile matrix lowering the crack propagation rate [7]. Besides the matrix and Al-Si eutectic, different kind of Fe-/Cu-/Mg-based intermetallics and defects such as oxide films and porosity may be found in these alloys that can lead to degradation and wide spread in mechanical properties. Besides, even the Si level and particle size may be considered deleterious if high ductility is desired. Standards and literature present that Al-Si alloys above 11 wt. % Si and up to 0.4 wt. % Mg hardly reach elongation to failure above 1-2 % [8-11].
This paper aims henceforward to show that Al-Si alloys with up to 14.4 wt. % Si exhibit an enormous potential that normally is not realized in castings. The study aims thus to widening the knowledge of Al-Si alloys in terms of directional solidification, microstructure and mechanical properties.
EXPERIMENTAL METHODS
Melt and sample preparation Table. 1 – Chemical composition
From base material Al-7%Si-0.4%Mg six alloys with variation of Si concentration were cast, modified with approximately 100-150 ppm Sr; a widely accepted level for appropriate Si particle modification. Chemical composition of each al-loy were obtained using SPECTROMAXx optical emission spectroscopy see Table 1. In order to prepare samples, initial cylindrical rods (length 20 cm, diameter 1 cm) were cast in a permanent copper mold. The cast rods were then re-melted and heated to 710°C for 30 minutes in steel rods coated with graphite under Ar-atmosphere and
subsequently solidified via the gradient solidification technique using a Bridgman furnace. The furnace is mounted to a motorized lifting device. Once the samples are re-melted, the furnace was raised at the pre-scribed speed of 0.3 mm.s-1, while the sample stays in a stationary position. The solidification set-up gener-ated the cast materials with low levels of defects due to directional solidification that push, to some extent, gas and oxides in front of the solidification front. Average SDAS from 10 measurements from the directional solidification direction and 10 from perpendicular direction have been carried out in all conditions.
Tensile test sample preparation
Tensile test samples were produced with a gauge length of 50 mm and a diameter of 6 mm from rods from the Bridgman equipment. Three samples in each condition were tested in a Zwick/Roell Z100 machine equipped with a 100 kN load cell and a clip-on extensometer with 20 mm gauge length. Samples were test until fracture with a constant strain rate of 0.5 mm/min. Microstructural investigations were carried out on tensile test samples using Olympus GX71 inverted microscope equipped with Olympus Stream Motion software.
RESULT AND DISCUSSIONS Microstructure characterization
At 6.5 wt. % Si the microstructure is composed of α-dendrites and modified Al-Si eutectic, see figure 1a. Modification with Sr refines the Si plates and resulted in a fibrous Si morphology. The modification of Al-Si alloys with Sr is explained by two established growth models, impurity induces twinning and twin plane re-entrant edge growth [12-14]. Increasing the Si level up to 11 wt. % the fractions of α-dendrites and Al-Si eutectic are remarkably changed, see Figure 1b. Samples near the eutectic point with a Si level of 12.4 wt. % show more Al-Si eutectic with smaller size of the α-dendrites. Samples with Si content of 13.0 wt. % are characterized by a fully Al-Si eutectic structure growing with a dendritic structure; no α-dendrites is found in the structure see Figure 1c. This because of Al-Si eutectic growth in the coupled zone is more rapid then α-dendrites growth. This is in agreement with other result in the literature [4, 15]. Increasing the Si further changed the Al-Si eutectic from dendritic growth to columnar growth. The boarders around the Al-Si eutectic contains intermetallics and coarser Si due to long range boundary layer build up ahead of solid interface [5]. It is worth mentioning that increasing the Si level from 6.5 to 12.4 wt. % the SDAS decreases from 20 ±2 µm to 8 ±1 µm, even though the cooling/processing conditions was similar.Alloys AIE and AIF exhibit only Al-Si eutectic growth that is assumed to be due to the coupled zone but surprisingly no primary Si was observed when employing directional solidification.
Chemical compositions
Element Si Mg Fe Sr Ti Al AIA 6.55 0.40 0.01 0.0150 0.13 Bal. AIB 10.09 0.40 0.02 0.0104 0.13 Bal. AIC 11.40 0.39 0.02 0.0100 0.13 Bal. AID 12.40 0.39 0.02 0.0140 0.12 Bal. AIE 13.03 0.39 0.02 0.0090 0.12 Bal. AIF 14.43 0.38 0.02 0.0117 0.12 Bal.Mechanical properties
The mechanical properties of Al-Si alloys are dependent on several parameters such as solidification rate, defects and alloying elements. A general observation derived from studying the fracture behaviour of the studied alloys reveals that Si particle cracking is taking place in all samples with increasing numbers of cracks close to the fracture surface; this behaviour is more obvious in some samples of alloy AIC and AID. Cracking of Si particles in these samples are concentrated to the Al-Si eutectic boundaries that offer path for microcracks. Samples with Si level up to 12.4 wt. % show particle cracking, growth of cracks forming voids and plastic deformation of α-dendrites; which is also according to literature [7, 14, 15]. Cracked Si particles and/or Fe-rich phases in the Al-Si eutectic regions, in between the tree-like structure in samples with Si over 13 wt. %, propagate within the Al-Si eutectic that hinders the propagation of the crack. Increasing the Si level from 6.5 wt. % to 10 wt. % resulted in approx. 8 % improved yield strength. Higher Si levels did not show any significant changes in yield strength see Figure 2a, the trends were in agreement with Wang at al [16]. However, a small drop in yield strength is seen for the alloys with Si levels above 13 wt. %. Elongation to failure dropped from 12 % for alloys with 6.5 wt. % Si down to nearly 6 % for alloys with 14.4 wt. % Si. At Si levels of 12.4 wt. %, the elongation to failure showed values approx. 9 % see Figure 2b. This elongation to failure data is far better than the data reported by Dwivedi [11] which showed rapid reduction in ductility increasing Si ranging from 4 to 12 wt. %. The reasons for that behaviour may be the production method; directional solidification usually produce samples that are well fed and contain low amount of defects. The ultimate tensile strength was slightly improved with increased Si level see Figure 2c. The ultimate tensile strength for alloys with 12.4 wt. % Si show larger standard deviations compared to the other alloys due to defects in the fracture surface of the samples. Visual observations from the fracture surfaces revealed obvious gas porosity of sizes approx 100 µm that are certainly participating in strength reduction.
Figure 2- Mechanical properties of the Al-Si series showing result in a) Yield strength, b) Elongation to failure and c) Ultimate tensile strength
(a) (b) (c)
Figure 1- Microstructures for alloy (a) AlA, (b) AlC and (c) AlE
CONCLUSIONS
1. Increasing the Si level up to 12.4 wt. % changed the fractions of α-dendrites and Al-Si eutectic remarkably. Furthermore, increasing the Si level from 6.5 wt. % to 12.4 wt. % and having similar cooling conditions the SDAS decreased from 20 ±2 µm to 8 ±1 µm. The alloys containing levels of Si higher than 12.4 wt. % exhibited only Al-Si eutectic growth; no primary Si particles were observed. 2. The yield strength results show approx. 8 % improvement when increasing the Si level from 6.5 wt.
% to 10 wt. %; further increments did not result in any significant changes.
3. Increasing the Si level from 6.5 wt. % to 14.4 wt. % revealed a positive trend in the ultimate tensile strength despite the variation that is related to the porosity found in the fracture surface.
4. The elongation to failure shows a noteworthy high value; approx. 9 % for alloys with Si level of 12.4 wt. %. Even at Si levels of 14.4 wt. %, it was maintained at an average of approx. 6 %.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support by the Swedish KK-foundation.
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