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AlN composites for automotive applications
Accepted Manuscript Development and characterization of A359/AlN composites for automotive applications Essam A.M. Shalaby, Alexander Yu. Churyumov PII:
S0925-8388(17)32882-7
DOI:
10.1016/j.jallcom.2017.08.154
Reference:
JALCOM 42911
To appear in:
Journal of Alloys and Compounds
Received Date: 5 June 2017 Revised Date:
2 August 2017
Accepted Date: 16 August 2017
Please cite this article as: E.A.M. Shalaby, A.Y. Churyumov, Development and characterization of A359/ AlN composites for automotive applications, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.08.154. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Development and Characterization of A359/AlN Composites for Automotive Applications
National Universit[y of Science and Technology «MISiS», », Leninsky Av. 4, 119049 Moscow, Russian Federation.
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Abstract
A359/AlN composites were successfully manufactured using stir and squeeze techniques. Microstructure, aging
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behavior, hardness, thermal and mechanical properties were inspected. Microstructural investigation showed a uniform distribution of AlN particles in A359 matrix and a significant reduction in porosity. Interfacial analysis indicated considerable bonding between A359 matrix and AlN particles. Hardness, ultimate compressive and yield strength increased as AlN content increased from 5 to 15 wt.%. Furthermore, aging process of A359/AlN composites was accelerated by increasing AlN content. Differential thermal analysis (DTA) revealed that A359/AlN
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composite had lower eutectic temperature than that of A359 alloy. Finite element modeling showed the dependence of the stress induced in A359/AlN composite on the distribution of AlN particles.
1.
Introduction
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Keywords: Microstructure; Aging; Mechanical properties; Thermal analysis; Finite element modeling.
Aluminum matrix composites (AMCs) have received numerous attentions in military, aerospace, automotive
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a
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Essam A. M. Shalaby a*, Alexander Yu. Churyumova
and electronic applications due to their superior mechanical, physical, thermal and electrical properties. AMCs satisfy continuously market needs of lightweight, high durable and performance components. Their immense properties as high strength, thermal and electrical conductivities, corrosion resistance, wear resistance and low thermal expansion nominate it to strongly replace the conventional alloy [1-5]. Regarding to reinforcements, ceramic particles are widely used due to their relatively lower cost as compared to fibers and it can be easily handled
*
Corresponding author: E-mail address: [email protected] (E.A.Shalaby).
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during fabrication processes. Several types of ceramic particles have been incorporated into the aluminum matrix to prepare AMCs. Each type of them was purposely directed to enhance a significant desired property in the produced AMCs; the most common SiC and Al2O3 particles are used to ameliorate properties [6-8]. Different types of TiC,
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TiB2, TiN, Al2O3 and AlN were used to investigate their addition on the mechanical properties, graphite and ZrO2 were introduced for thermal and mechanical betterment [8-10]. Furthermore, influence of B4C [11], fly ash [12], ZrSiO4 [13], TiO2 [14], BN [15], SiC and Si3N4 [1] particles also have been investigated.
Aluminum nitride (AlN) has been considered as an attractive ceramic material due to its excellent mechanical
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and functional properties, such as high strength, high thermal conductivity, high wear resistance and low electrical conductivity [16]. Besides, it has a low density and coefficient of thermal expansion. AlN is mainly used in
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electronics [17, 18], and as reinforcement similar as SiC, Al2O3 [4]. Its superior thermal and mechanical characteristics nominate it to be an important competitive candidate in the industry. It can be used in pistons, brake rotors and brake drums in automotive industries where heat sink and significant wear resistance are required. Also, it can be employed in aerospace structures as semiconductor packaging. Regarding to its potential thermal properties, it has also been applied for refractory applications. Despite the great importance of AlN ceramics, there are only a few works that have studied the effect of their addition on Al-Si alloys. Gajewska et al. [19] have reported that Al
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alloy matrix composite gave higher strength when reinforced with the micro- rather than with submicro-particles. It suggests that the size of the ceramic phase addition can be considered as only one of the factors influencing the composite strength. Fogagnolo et al.[20] have inspected the processing, mechanical strength and hardness of
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aluminum 6061 matrix composite powders reinforced with AlN. Hardness and ultimate tensile strength were extremely increased as compared to that of conventionally mixed composites. Moreover, the production, wetting
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behavior, mechanical properties and microstructure of AMMCs reinforced with AlN ceramic particles have been investigated. Briefly, AMMCs/AlN composites are promising materials for electronic and thermal sink uses [21, 22].
A359 alloy is one of the most important aluminum alloys in automotive industries because of their significant mechanical and casting properties. It can be hardened by the precipitation of Mg2Si phase during aging process that additionally improves the alloy strength [23]. Actually, the most frequently used processing techniques of AMCs are solid or liquid state fabrication techniques [24, 25]. Due to the inherent advantages of the liquid-state processing technique, most of the commercially AMMCs applications are produced by this method. Furthermore, the liquid
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metal process is less expensive, easier to handle than powders and can produce composites in various shapes [26]. Stirring is widespread in AMMCs production [27, 28]. Usually, composites manufactured by stirring technology exhibit a higher porosity percent in comparison with that produced by squeeze casting [28]. Squeeze casting is a
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simple and cost effective process and has superior potential for automated operation at high production rates. [29] This work aims to investigate the influence of AlN addition on microstructure, aging kinetics and thermal and mechanical properties of A359 matrix alloy. Investigation of different properties introduces A359/AlN composites
Experimental methodology
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2.
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as an effective choice for automotive industries such as pistons, brake rotors, and brake drums.
2.1 Materials
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Fig.1 processing technologies used; a) stir casting, b) squeeze casting.
Two steps for material processing were used as shown in Fig.1. In the first step (stir cast process) Fig.1a, AlN
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particles with different weight percent (5, 10 and 15 wt.%) were added as ceramic reinforcements to A359 alloy. The particle size was ~ 40 µm. Stir and squeeze technologies were utilized for composites preparation. First A359 alloy was melted and skimmed. During stirring, the particles were smoothly inserted. Finally, the melt was stirred for 1min and cast into cast graphite mould at pouring temperature of 685 ⁰C. During the next step (squeeze cast process) Fig. 1b, A359/AlN composite was re-melted and poured into a preheated steel mould (200 ⁰C) with an inside diameter of 50 mm and a length of 100 mm. Then 100 MPa pressure was applied within a period of 1 min.
2.2 Microstructure characterization
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The microstructure analysis was accomplished using X-ray diffraction in monochromatic copper Kα-radiation on a Bruker D8 Advance diffract meter and scanning electronic microscope - Tescan Vega 3 LMH microscope.
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2.3 Density and porosity measurements Archimedes’ principle was used to measure the composite density while the rule of mixture was used to calculate the theoretical. Porosity percent was determined using the values of both theoretical and measured
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densities respectively.
2.4 Heat treatment
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Precipitation hardening of A359/AlN composites was accomplished in a Nabertherm furnace in two steps: Solution treatment at 538°C for 8 hr and aging at 155°C. Then, the aging curves were obtained using hardness measurement at a load of 5 kg.
2.5 Thermal analysis
The thermal properties of the squeezed A359 composites were estimated by a Setaram Labsys differential
2.6 Compression test
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scanning calorimeter instrument under the Ar gas protection.
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Gleeble 3800 thermo-mechanical simulator (module Hydrowedge II) was utilized. Stress-strain curves for the
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test specimens (diameter of 10mm and length of 15mm) were accomplished at 10-3 s-1 strain rate.
2.7 Finite element modeling The plastic deformation behavior of the A359/5%AlN composite was investigated using the finite element modeling software Deform 2D. The specimen was divided into 20 000 foursquare elements. After that, the simulation of the deformation process was accomplished at 10-3 s-1strain rate.
3.
Results and discussion
3.1 Microstructure characterization
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Preliminary investigations of microstructure after stirring are demonstrated in Fig.2. A359 matrix alloy microstructure evinces eutectic phase and α-Al (Fig. 2a). The addition of AlN particles into A359 alloy are accompanied with pores formations. Besides, poor wettability between AlN particles and molten A359 matrix alloy
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results in detaching of AlN particles from the matrix after solidification process (Fig. 2b). Moreover, the situation becomes worse when AlN particles are agglomerated where some particles are completely prevented from any
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contact with the matrix (Fig. 2c). That means they are easily detached from the matrix surface leaving large cavities.
Fig.2 Microstructure of stir cast (a) as cast A359 matrix alloy, (b) poor wettability of AlN, (c) agglomeration of AlN
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particles
Fig.3 Microstructure of squeeze cast (a) A359 matrix alloy, (b) A359/5%AlN, (c) A359/10%AlN, (d) A359/15%AlN.
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Fig.3 illustrates the microstructure of finally squeezed A359 matrix alloy and its AlN composites. As shown in Fig 3(b-d), AlN reinforcement particles are well contacted with A359 matrix and uniformly distributed without agglomerations or clusters. Moreover, the microstructure is free from any casing defects like pores or
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cavities. This is attributed to the high pressure that forces the melt to infiltrate into the gap between the particles and interdendritic regions during the squeeze process. The AlN particles, therefore, are wetted, strongly bonded and surrounded by the matrix which results in improved mechanical behavior [30]. Furthermore, AlN particles are observed to be accumulated in the interdendritic regions. This means that α-Al does not nucleate around AlN
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particles and nucleates in the liquid between particles. A possible explanation for this is given in the following: AlN particles have a different thermal diffusivity and a different latent heat of fusion as compared to that of A359 melt.
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Therefore, particles are unable to cool down as the melt. As a result, the temperature of the particles is different than that of the liquid alloy [10, 31].
Table 1 – The average size of dendritic cell in the as cast state and after squeeze casting.
A359
A359/5%AlN
A359/10%AlN
A359/15%AlN
As cast
26±1.2
23±0.9
20±0.8
18±1.1
Squeeze
25±0.9
22±0.7
19±1.1
17±0.9
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The average size of dendritic cells, (µm)
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Quantitative analysis of the microstructure showed that the increase in the amount of particles in the matrix
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alloy leads to a reduction in the average size of dendritic cells. Thus, the average size of dendritic cells for the matrix alloy was 25±0.9 µm, at a content of 5% AlN- 22±0.7 µm at a content of 10% AlN - 19±1.1 µm, at a content of 15%
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AlN- 17±0.9 µm. The reduction in the average size of dendritic cells can be explained by that, the particles act as a barrier to the growth of dendrites during the solidification. It was reported that squeeze casting resulted in the formation of a fine structure due to the higher solidification rate [32]. However, table 1 shows that the average size of dendritic cells is practically unchanged after squeeze casting for all studied alloys, which may indicate that the rate of solidification is slightly changed.
3.2 Phase analyses and interface between A359 matrix and AlN particles
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As illustrated by the results of the X-ray diffraction analysis (Fig. 4a), the structure of the squeezed A359/ 15% AlN composites contain solid solutions of Al, Si and compounds of Mg2Si, MgAl2O4, and AlN. Three dimension (3D) XRD patterns of squeezed A359/AlN composites with different AlN content are illustrated in Fig. 4b. The
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intensity is more pronounced as wt.% AlN increased. It is important to notice that the magnesium is found in two
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substantial phases; Mg2Si and MgAl2O4. The Mg2Si phase has a pivotal role during strengthening.
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Fig. 4 XRD patterns: a) A359/15%AlN composites; b) 3D of A359/AlN composites.
On the other hand, the MgAl2O4 spinel phase enhanced the interfacial bonding and the contact between the matrix and AlN particles [6]. Vicens et al. [33] studied the interfacial surfaces of Al–AlN composites and found MgAl2O4 spinel crystals deposited on it during the liquid infiltration according to Eq. (1).
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2Al + 5Mg + 2SiO2 → MgAl2O4 + 2Mg2Si (1)
The higher melting temperature and the limited interfacial reaction between the melt and ceramic particles can
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improve the wetting and/or bonding. However, excessive reaction due to high-temperature processing may degrade the reinforcement or lead to extensive oxidation [10].
3.3 Density and porosity
The density and porosity percents of A359/AlN composites are declared in Table 2. In the as cast condition, the density of A359/AlN composites decreased with the addition of AlN particles although the later has a higher density (3.26 g/cm3) than the matrix. This can be related to the presence of high amount of entrapped gases in the melt during the stir casting process which resulted in an increment in the porosity percent. On the other hand, a crucial
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decrease in porosity percent after squeeze has been noticed as compared to those of as cast composites. Application of pressure decreases the porosity about 67% for A359/15Wt.% AlN while it is 82% for A359/5Wt.%AlN. The pressure forces the melt moving into the voids and compensates shrinkage cavities.
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(g/cm )
Squeeze As cast Squeeze
Porosity (%)
A359/5%AlN
A359/10%AlN
A359/15%AlN
2.668
2.675
2.677
2.682
2.657
2.572
2.547
2.529
0.041
0.850
1.900
2.788
0.487
4.669
6.662
8.330
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As cast
A359
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Density
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Table 2-The density and porosity percent measurements.
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3.4 Hardness of composites during heat treatment
Fig. 5 Hardness dependence of A359 alloy and its AlN composites on time.
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Figure 5 displays the hardness dependence of squeeze cast A359 alloy and its AlN composites on the aging time at 155°C. Upon aging, there are consecutively precipitated and dissolved coherent (Mg2Si clusters, Guinier– Preston (GP) zones) and semi-coherent (β'' phase, β' phase) precipitates, which leads to the presence of local maxima
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in the aging curve [34]. As seen in Fig. 5, the values of the hardness of the composites are higher than that of the matrix alloy after quenching and in peak Hv during aging. This is related to the presence of AlN particles in their structure, the hardness of which is greater than that of the aluminum solid solution, as well as to the strengthening of the matrix by dislocations that are generated near the particles in the process of solidification. In this case, the
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maximum value of the hardness is attained considerably more rapidly in the composites as compared to the matrix
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alloy.
Table 3- Hardness measurements and optimum time for peak hardness. Hv after solution treatment
Peak Hv during aging
Time to peak (hr)
A359
57
116
12
A359/5% AlN
61
118
10
A359/10% AlN
69
139
5
145
5
A359/15% AlN
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Material
78
Table 3 illustrates that the presence of AlN particles affects the time to peak hardness. The time to peak
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decreases from 12 hr to 5 hr when the AlN content increases to 10 Wt.%. Hence, the kinetics of aging in A359/AlN composites is accelerated. Some other composites exhibited a similar behavior during the aging process [35].
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However, because of the enhanced density of dislocations, more rapid softening occurs during the prolonged aging of composites. The most significant drop in the hardness after 24 hr aging is observed in the composite with 15% AlN (by about 37 HV relative to the maximum value). The drop in the hardness of the matrix alloy was only 10 HV.
3.5 Differential thermal analyses (DTA) Fig.6 presents the DTA record upon cooling and heating of A359 alloy at 10 °C/min. The thermogram shows successive reactions upon solidification: precipitation of Al dendrites (1), of the Al-Si eutectic (2) and of the final eutectic (3). This latter peak was seen to split in two thermal arrests during heating. Three thermal peaks could
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similarly be identified on heating records. The start and maximum temperatures for precipitation were determined
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on all heating and cooling thermograms. After that, they are plotted versus the scanning rate in Fig.7.
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Fig. 6 DTA thermograms recorded upon cooling and heating at 10 °C/min of A359 alloy.
Fig. 7 The dependence of the characteristic temperatures of the A359 alloy on the scanning rate.
The shift of the characteristic temperatures when the scanning rate increases is mainly caused by the thermal resistance in thermocouple measurement of the DTA apparatus [36]. The values obtained by extrapolation to a zero
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scanning rate are thus expected to be the relevant equilibrium temperatures to be discussed below. The peak temperature is about 10 oC above the start temperature upon cooling. The peak temperature upon cooling, which represents the end of eutectic reaction, extrapolates to about the same temperature as the start temperature on
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heating. Fig.8. DTA thermograms recorded upon cooling and heating at 10 °C/min for the A359/AlN composites. The results were similarly determined as mentioned previously for A359 alloy and its composites and are shown in
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table 4.
Fig. 8 DTA thermograms recorded upon cooling and heating at 10°C/min of A359/AlN
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Table 4- Eutectic arrest temperatures for the A359 matrix alloy and its composites Eutectic arrest temperature, °C
A359 matrix
565±1
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Material
A359 / 5 Wt.% AlN
561±1
A359 /10 Wt.% AlN
559±1
A359/ 15 Wt.% AlN
556±1
3.6 Mechanical properties
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Fig. 9 True stress - true strain curves of A ِ 359 composites (a), compressive properties of A359/AlN composites (b).
Compressive properties of A359 alloy and its AlN composites were achieved by stress-strain curves, where each curve represents the average behavior of three test samples as shown in Fig.9a. A359 matrix alloy exhibited a higher value of strain with lower yield and ultimate stress as compared to A359/AlN composites. The yield, ultimate strength as well as the fracture strain of A359/AlN composites as a function of wt.% AlN are demonstrated in
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Fig.9b. A significant improvement in the yield and ultimate compressive strength is associated with the additions of AlN particles into A359 matrix. For example; A359/15% AlN has a 8% and 14.5% increase in yield and ultimate strength, respectively versus A359 alloy. On the other hand, the 5 and 15wt%AlN addition toA359 matrix resulted in about 23 and 55% decrease in ductility, respectively. Similar behaviors were reported in previous works [37-39].
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These compressive test results are related to several factors; metallurgical or processing factors such as hardness, size, shape, wt% of AlN particles, the ability to transfer the stress and to pin the dislocation movement, the strong
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contact between the AlN particles and A359 matrix induced by squeeze process as well as the precipitation hardening [35, 40]. These factors enhance the hardness, yield and ultimate strength of A359/AlN composites, despite it may decrease the ductility.
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Fig.10 Finite element modeling of plastic deformation of the AAMCs with random distribution (a,c) and agglomerated particles: (a,b) strain distribution at 0.5% of overall strain, (c,d) strain distribution at 4 % of overall
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strain (c,d) and stress-strain curves (e).
The particles homogeneous distribution plays a significant role in the deformation processes. It influences on the beginning of plastic deformation. Two finite element models of a compression test of a A359/5 wt. % AlN composite were calculated to investigate the influence of particles distribution on deformation of AAMCs. The
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model is similar to those developed in earlier works by compression of the glassy/crystal composites [41]. The nonplastic particles were randomly distributed in the first model and were consolidated into two agglomerations in the
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second model. As finite element simulations shown, the areas without particles deformed more intensively at the low values of overall strain (lower than 0.7%) (Fig. 10a,b). At higher strains, the absence of the particles is compensated by the work hardening of the aluminum matrix. The strain distribution in both cases became more homogeneous (Fig. 10c,d) and the stress – strain curves were equal (Fig. 10e) at strains higher than 0.7%.
Conclusions
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Based on the results obtained in the current investigation, it is possible to reach the following conclusions: •
Quantitative analysis of the microstructure showed a reduction in the average size of dendritic cells with the
•
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increase of AlN wt.%.
A359 matrix reinforced with different weight percent AlN particles can be properly synthesized by stirring and
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squeeze techniques. Application of pressure during the solidification process significantly reduces the percent of gas porosity incorporated in the melt during stir casting. •
The addition of AlN particles improves the kinetics of aging as the aging time was decreased from 12 hr to 5 hr.
•
Thermal analysis showed that eutectic arrest temperature for A359/AlN composites is decreased as compared to the A359 matrix alloy.
•
The yield and ultimate strength of A359/15 wt.% AlN composites increased to 314 and 473 MPa respectively. While the ductility relatively decreases.
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•
As finite element modeling shown, the distribution of the particles in the structure of the A359/5wt.% AlN composite strongly influences on the stress at the beginning of the plastic deformation.
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Acknowledgements The financial aid of the Ministry of higher education and scientific research of Egypt and the Ministry of Education and Science of the Russian Federation in the Program of NUST«MISiS» (Grant No К1-2014-026)
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are gratefully appreciated.
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35. E.A. Shalaby, A.Y. Churyumov, A.N. Solonin, A. Lotfy, Preparation and characterization of hybrid A359/(SiC+Si3N4) composites synthesized by stir/squeeze casting techniques, Mater. Sci. Eng. A. 674 (2016) 18–24.
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38. E.A. Shalaby, A. Yu. Churyumov, D.H. Besisa, A. Daoud, M.T. Abou El-khair, A comparative study of thermal conductivity and tribological behavior of squeeze cast A359/AlN & A359/SiC composites, J. Mater. Eng. and Perform. 26 (2017) 3079-3089.
39. A.V. Pozdniakov, A. Lotfy, A. Qadir, E. Shalaby, M.G. Khomutov, A.Yu. Churyumov, V.S. Zolotorevskiy, Development of Al-5Cu/B4C composites with low coefficient of thermalexpansion for automotive application, Mater. Sci. Eng.A. 688 (2017) 1-8. 40. R.S. Rana, R. Purohit, V.K. Soni, S. Das, Characterization of Mechanical Properties and Microstructure of Aluminium Alloy-SiC Composites, Materials Today: Proceedings. 2 (2015) 1149-1156.
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41. A.Yu. Churyumov, A.L. Bazlov, A.A. Tsarkov, A.N. Solonin, D.V. Louzguine-Luzgin, Microstructure, mechanical properties, and crystallization behavior of Zr-based bulk metallic glasses prepared under a low
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Highlights Increasing wt.% of AlN in the matrix decreases the size of dendritic cells.
•
Aging process of A359/AlN composites was accelerated by increasing AlN content.
•
A359/AlN composites have higher yield and ultimate strength than A359 alloy.
•
Eutectic temperature of the composites is decreased as compared to A359 matrix.
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S0925-8388(17)32882-7
DOI:
10.1016/j.jallcom.2017.08.154
Reference:
JALCOM 42911
To appear in:
Journal of Alloys and Compounds
Received Date: 5 June 2017 Revised Date:
2 August 2017
Accepted Date: 16 August 2017
Please cite this article as: E.A.M. Shalaby, A.Y. Churyumov, Development and characterization of A359/ AlN composites for automotive applications, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.08.154. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Development and Characterization of A359/AlN Composites for Automotive Applications
National Universit[y of Science and Technology «MISiS», », Leninsky Av. 4, 119049 Moscow, Russian Federation.
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Abstract
A359/AlN composites were successfully manufactured using stir and squeeze techniques. Microstructure, aging
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behavior, hardness, thermal and mechanical properties were inspected. Microstructural investigation showed a uniform distribution of AlN particles in A359 matrix and a significant reduction in porosity. Interfacial analysis indicated considerable bonding between A359 matrix and AlN particles. Hardness, ultimate compressive and yield strength increased as AlN content increased from 5 to 15 wt.%. Furthermore, aging process of A359/AlN composites was accelerated by increasing AlN content. Differential thermal analysis (DTA) revealed that A359/AlN
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composite had lower eutectic temperature than that of A359 alloy. Finite element modeling showed the dependence of the stress induced in A359/AlN composite on the distribution of AlN particles.
1.
Introduction
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Keywords: Microstructure; Aging; Mechanical properties; Thermal analysis; Finite element modeling.
Aluminum matrix composites (AMCs) have received numerous attentions in military, aerospace, automotive
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a
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Essam A. M. Shalaby a*, Alexander Yu. Churyumova
and electronic applications due to their superior mechanical, physical, thermal and electrical properties. AMCs satisfy continuously market needs of lightweight, high durable and performance components. Their immense properties as high strength, thermal and electrical conductivities, corrosion resistance, wear resistance and low thermal expansion nominate it to strongly replace the conventional alloy [1-5]. Regarding to reinforcements, ceramic particles are widely used due to their relatively lower cost as compared to fibers and it can be easily handled
*
Corresponding author: E-mail address: [email protected] (E.A.Shalaby).
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during fabrication processes. Several types of ceramic particles have been incorporated into the aluminum matrix to prepare AMCs. Each type of them was purposely directed to enhance a significant desired property in the produced AMCs; the most common SiC and Al2O3 particles are used to ameliorate properties [6-8]. Different types of TiC,
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TiB2, TiN, Al2O3 and AlN were used to investigate their addition on the mechanical properties, graphite and ZrO2 were introduced for thermal and mechanical betterment [8-10]. Furthermore, influence of B4C [11], fly ash [12], ZrSiO4 [13], TiO2 [14], BN [15], SiC and Si3N4 [1] particles also have been investigated.
Aluminum nitride (AlN) has been considered as an attractive ceramic material due to its excellent mechanical
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and functional properties, such as high strength, high thermal conductivity, high wear resistance and low electrical conductivity [16]. Besides, it has a low density and coefficient of thermal expansion. AlN is mainly used in
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electronics [17, 18], and as reinforcement similar as SiC, Al2O3 [4]. Its superior thermal and mechanical characteristics nominate it to be an important competitive candidate in the industry. It can be used in pistons, brake rotors and brake drums in automotive industries where heat sink and significant wear resistance are required. Also, it can be employed in aerospace structures as semiconductor packaging. Regarding to its potential thermal properties, it has also been applied for refractory applications. Despite the great importance of AlN ceramics, there are only a few works that have studied the effect of their addition on Al-Si alloys. Gajewska et al. [19] have reported that Al
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alloy matrix composite gave higher strength when reinforced with the micro- rather than with submicro-particles. It suggests that the size of the ceramic phase addition can be considered as only one of the factors influencing the composite strength. Fogagnolo et al.[20] have inspected the processing, mechanical strength and hardness of
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aluminum 6061 matrix composite powders reinforced with AlN. Hardness and ultimate tensile strength were extremely increased as compared to that of conventionally mixed composites. Moreover, the production, wetting
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behavior, mechanical properties and microstructure of AMMCs reinforced with AlN ceramic particles have been investigated. Briefly, AMMCs/AlN composites are promising materials for electronic and thermal sink uses [21, 22].
A359 alloy is one of the most important aluminum alloys in automotive industries because of their significant mechanical and casting properties. It can be hardened by the precipitation of Mg2Si phase during aging process that additionally improves the alloy strength [23]. Actually, the most frequently used processing techniques of AMCs are solid or liquid state fabrication techniques [24, 25]. Due to the inherent advantages of the liquid-state processing technique, most of the commercially AMMCs applications are produced by this method. Furthermore, the liquid
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metal process is less expensive, easier to handle than powders and can produce composites in various shapes [26]. Stirring is widespread in AMMCs production [27, 28]. Usually, composites manufactured by stirring technology exhibit a higher porosity percent in comparison with that produced by squeeze casting [28]. Squeeze casting is a
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simple and cost effective process and has superior potential for automated operation at high production rates. [29] This work aims to investigate the influence of AlN addition on microstructure, aging kinetics and thermal and mechanical properties of A359 matrix alloy. Investigation of different properties introduces A359/AlN composites
Experimental methodology
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2.
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as an effective choice for automotive industries such as pistons, brake rotors, and brake drums.
2.1 Materials
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Fig.1 processing technologies used; a) stir casting, b) squeeze casting.
Two steps for material processing were used as shown in Fig.1. In the first step (stir cast process) Fig.1a, AlN
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particles with different weight percent (5, 10 and 15 wt.%) were added as ceramic reinforcements to A359 alloy. The particle size was ~ 40 µm. Stir and squeeze technologies were utilized for composites preparation. First A359 alloy was melted and skimmed. During stirring, the particles were smoothly inserted. Finally, the melt was stirred for 1min and cast into cast graphite mould at pouring temperature of 685 ⁰C. During the next step (squeeze cast process) Fig. 1b, A359/AlN composite was re-melted and poured into a preheated steel mould (200 ⁰C) with an inside diameter of 50 mm and a length of 100 mm. Then 100 MPa pressure was applied within a period of 1 min.
2.2 Microstructure characterization
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The microstructure analysis was accomplished using X-ray diffraction in monochromatic copper Kα-radiation on a Bruker D8 Advance diffract meter and scanning electronic microscope - Tescan Vega 3 LMH microscope.
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2.3 Density and porosity measurements Archimedes’ principle was used to measure the composite density while the rule of mixture was used to calculate the theoretical. Porosity percent was determined using the values of both theoretical and measured
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densities respectively.
2.4 Heat treatment
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Precipitation hardening of A359/AlN composites was accomplished in a Nabertherm furnace in two steps: Solution treatment at 538°C for 8 hr and aging at 155°C. Then, the aging curves were obtained using hardness measurement at a load of 5 kg.
2.5 Thermal analysis
The thermal properties of the squeezed A359 composites were estimated by a Setaram Labsys differential
2.6 Compression test
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scanning calorimeter instrument under the Ar gas protection.
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Gleeble 3800 thermo-mechanical simulator (module Hydrowedge II) was utilized. Stress-strain curves for the
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test specimens (diameter of 10mm and length of 15mm) were accomplished at 10-3 s-1 strain rate.
2.7 Finite element modeling The plastic deformation behavior of the A359/5%AlN composite was investigated using the finite element modeling software Deform 2D. The specimen was divided into 20 000 foursquare elements. After that, the simulation of the deformation process was accomplished at 10-3 s-1strain rate.
3.
Results and discussion
3.1 Microstructure characterization
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Preliminary investigations of microstructure after stirring are demonstrated in Fig.2. A359 matrix alloy microstructure evinces eutectic phase and α-Al (Fig. 2a). The addition of AlN particles into A359 alloy are accompanied with pores formations. Besides, poor wettability between AlN particles and molten A359 matrix alloy
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results in detaching of AlN particles from the matrix after solidification process (Fig. 2b). Moreover, the situation becomes worse when AlN particles are agglomerated where some particles are completely prevented from any
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contact with the matrix (Fig. 2c). That means they are easily detached from the matrix surface leaving large cavities.
Fig.2 Microstructure of stir cast (a) as cast A359 matrix alloy, (b) poor wettability of AlN, (c) agglomeration of AlN
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particles
Fig.3 Microstructure of squeeze cast (a) A359 matrix alloy, (b) A359/5%AlN, (c) A359/10%AlN, (d) A359/15%AlN.
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Fig.3 illustrates the microstructure of finally squeezed A359 matrix alloy and its AlN composites. As shown in Fig 3(b-d), AlN reinforcement particles are well contacted with A359 matrix and uniformly distributed without agglomerations or clusters. Moreover, the microstructure is free from any casing defects like pores or
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cavities. This is attributed to the high pressure that forces the melt to infiltrate into the gap between the particles and interdendritic regions during the squeeze process. The AlN particles, therefore, are wetted, strongly bonded and surrounded by the matrix which results in improved mechanical behavior [30]. Furthermore, AlN particles are observed to be accumulated in the interdendritic regions. This means that α-Al does not nucleate around AlN
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particles and nucleates in the liquid between particles. A possible explanation for this is given in the following: AlN particles have a different thermal diffusivity and a different latent heat of fusion as compared to that of A359 melt.
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Therefore, particles are unable to cool down as the melt. As a result, the temperature of the particles is different than that of the liquid alloy [10, 31].
Table 1 – The average size of dendritic cell in the as cast state and after squeeze casting.
A359
A359/5%AlN
A359/10%AlN
A359/15%AlN
As cast
26±1.2
23±0.9
20±0.8
18±1.1
Squeeze
25±0.9
22±0.7
19±1.1
17±0.9
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The average size of dendritic cells, (µm)
Material
Quantitative analysis of the microstructure showed that the increase in the amount of particles in the matrix
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alloy leads to a reduction in the average size of dendritic cells. Thus, the average size of dendritic cells for the matrix alloy was 25±0.9 µm, at a content of 5% AlN- 22±0.7 µm at a content of 10% AlN - 19±1.1 µm, at a content of 15%
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AlN- 17±0.9 µm. The reduction in the average size of dendritic cells can be explained by that, the particles act as a barrier to the growth of dendrites during the solidification. It was reported that squeeze casting resulted in the formation of a fine structure due to the higher solidification rate [32]. However, table 1 shows that the average size of dendritic cells is practically unchanged after squeeze casting for all studied alloys, which may indicate that the rate of solidification is slightly changed.
3.2 Phase analyses and interface between A359 matrix and AlN particles
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As illustrated by the results of the X-ray diffraction analysis (Fig. 4a), the structure of the squeezed A359/ 15% AlN composites contain solid solutions of Al, Si and compounds of Mg2Si, MgAl2O4, and AlN. Three dimension (3D) XRD patterns of squeezed A359/AlN composites with different AlN content are illustrated in Fig. 4b. The
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intensity is more pronounced as wt.% AlN increased. It is important to notice that the magnesium is found in two
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substantial phases; Mg2Si and MgAl2O4. The Mg2Si phase has a pivotal role during strengthening.
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Fig. 4 XRD patterns: a) A359/15%AlN composites; b) 3D of A359/AlN composites.
On the other hand, the MgAl2O4 spinel phase enhanced the interfacial bonding and the contact between the matrix and AlN particles [6]. Vicens et al. [33] studied the interfacial surfaces of Al–AlN composites and found MgAl2O4 spinel crystals deposited on it during the liquid infiltration according to Eq. (1).
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2Al + 5Mg + 2SiO2 → MgAl2O4 + 2Mg2Si (1)
The higher melting temperature and the limited interfacial reaction between the melt and ceramic particles can
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improve the wetting and/or bonding. However, excessive reaction due to high-temperature processing may degrade the reinforcement or lead to extensive oxidation [10].
3.3 Density and porosity
The density and porosity percents of A359/AlN composites are declared in Table 2. In the as cast condition, the density of A359/AlN composites decreased with the addition of AlN particles although the later has a higher density (3.26 g/cm3) than the matrix. This can be related to the presence of high amount of entrapped gases in the melt during the stir casting process which resulted in an increment in the porosity percent. On the other hand, a crucial
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decrease in porosity percent after squeeze has been noticed as compared to those of as cast composites. Application of pressure decreases the porosity about 67% for A359/15Wt.% AlN while it is 82% for A359/5Wt.%AlN. The pressure forces the melt moving into the voids and compensates shrinkage cavities.
3
(g/cm )
Squeeze As cast Squeeze
Porosity (%)
A359/5%AlN
A359/10%AlN
A359/15%AlN
2.668
2.675
2.677
2.682
2.657
2.572
2.547
2.529
0.041
0.850
1.900
2.788
0.487
4.669
6.662
8.330
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As cast
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Density
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Table 2-The density and porosity percent measurements.
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3.4 Hardness of composites during heat treatment
Fig. 5 Hardness dependence of A359 alloy and its AlN composites on time.
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Figure 5 displays the hardness dependence of squeeze cast A359 alloy and its AlN composites on the aging time at 155°C. Upon aging, there are consecutively precipitated and dissolved coherent (Mg2Si clusters, Guinier– Preston (GP) zones) and semi-coherent (β'' phase, β' phase) precipitates, which leads to the presence of local maxima
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in the aging curve [34]. As seen in Fig. 5, the values of the hardness of the composites are higher than that of the matrix alloy after quenching and in peak Hv during aging. This is related to the presence of AlN particles in their structure, the hardness of which is greater than that of the aluminum solid solution, as well as to the strengthening of the matrix by dislocations that are generated near the particles in the process of solidification. In this case, the
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maximum value of the hardness is attained considerably more rapidly in the composites as compared to the matrix
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alloy.
Table 3- Hardness measurements and optimum time for peak hardness. Hv after solution treatment
Peak Hv during aging
Time to peak (hr)
A359
57
116
12
A359/5% AlN
61
118
10
A359/10% AlN
69
139
5
145
5
A359/15% AlN
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Material
78
Table 3 illustrates that the presence of AlN particles affects the time to peak hardness. The time to peak
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decreases from 12 hr to 5 hr when the AlN content increases to 10 Wt.%. Hence, the kinetics of aging in A359/AlN composites is accelerated. Some other composites exhibited a similar behavior during the aging process [35].
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However, because of the enhanced density of dislocations, more rapid softening occurs during the prolonged aging of composites. The most significant drop in the hardness after 24 hr aging is observed in the composite with 15% AlN (by about 37 HV relative to the maximum value). The drop in the hardness of the matrix alloy was only 10 HV.
3.5 Differential thermal analyses (DTA) Fig.6 presents the DTA record upon cooling and heating of A359 alloy at 10 °C/min. The thermogram shows successive reactions upon solidification: precipitation of Al dendrites (1), of the Al-Si eutectic (2) and of the final eutectic (3). This latter peak was seen to split in two thermal arrests during heating. Three thermal peaks could
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similarly be identified on heating records. The start and maximum temperatures for precipitation were determined
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on all heating and cooling thermograms. After that, they are plotted versus the scanning rate in Fig.7.
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Fig. 6 DTA thermograms recorded upon cooling and heating at 10 °C/min of A359 alloy.
Fig. 7 The dependence of the characteristic temperatures of the A359 alloy on the scanning rate.
The shift of the characteristic temperatures when the scanning rate increases is mainly caused by the thermal resistance in thermocouple measurement of the DTA apparatus [36]. The values obtained by extrapolation to a zero
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scanning rate are thus expected to be the relevant equilibrium temperatures to be discussed below. The peak temperature is about 10 oC above the start temperature upon cooling. The peak temperature upon cooling, which represents the end of eutectic reaction, extrapolates to about the same temperature as the start temperature on
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heating. Fig.8. DTA thermograms recorded upon cooling and heating at 10 °C/min for the A359/AlN composites. The results were similarly determined as mentioned previously for A359 alloy and its composites and are shown in
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table 4.
Fig. 8 DTA thermograms recorded upon cooling and heating at 10°C/min of A359/AlN
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Table 4- Eutectic arrest temperatures for the A359 matrix alloy and its composites Eutectic arrest temperature, °C
A359 matrix
565±1
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Material
A359 / 5 Wt.% AlN
561±1
A359 /10 Wt.% AlN
559±1
A359/ 15 Wt.% AlN
556±1
3.6 Mechanical properties
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Fig. 9 True stress - true strain curves of A ِ 359 composites (a), compressive properties of A359/AlN composites (b).
Compressive properties of A359 alloy and its AlN composites were achieved by stress-strain curves, where each curve represents the average behavior of three test samples as shown in Fig.9a. A359 matrix alloy exhibited a higher value of strain with lower yield and ultimate stress as compared to A359/AlN composites. The yield, ultimate strength as well as the fracture strain of A359/AlN composites as a function of wt.% AlN are demonstrated in
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Fig.9b. A significant improvement in the yield and ultimate compressive strength is associated with the additions of AlN particles into A359 matrix. For example; A359/15% AlN has a 8% and 14.5% increase in yield and ultimate strength, respectively versus A359 alloy. On the other hand, the 5 and 15wt%AlN addition toA359 matrix resulted in about 23 and 55% decrease in ductility, respectively. Similar behaviors were reported in previous works [37-39].
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These compressive test results are related to several factors; metallurgical or processing factors such as hardness, size, shape, wt% of AlN particles, the ability to transfer the stress and to pin the dislocation movement, the strong
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contact between the AlN particles and A359 matrix induced by squeeze process as well as the precipitation hardening [35, 40]. These factors enhance the hardness, yield and ultimate strength of A359/AlN composites, despite it may decrease the ductility.
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Fig.10 Finite element modeling of plastic deformation of the AAMCs with random distribution (a,c) and agglomerated particles: (a,b) strain distribution at 0.5% of overall strain, (c,d) strain distribution at 4 % of overall
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strain (c,d) and stress-strain curves (e).
The particles homogeneous distribution plays a significant role in the deformation processes. It influences on the beginning of plastic deformation. Two finite element models of a compression test of a A359/5 wt. % AlN composite were calculated to investigate the influence of particles distribution on deformation of AAMCs. The
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model is similar to those developed in earlier works by compression of the glassy/crystal composites [41]. The nonplastic particles were randomly distributed in the first model and were consolidated into two agglomerations in the
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second model. As finite element simulations shown, the areas without particles deformed more intensively at the low values of overall strain (lower than 0.7%) (Fig. 10a,b). At higher strains, the absence of the particles is compensated by the work hardening of the aluminum matrix. The strain distribution in both cases became more homogeneous (Fig. 10c,d) and the stress – strain curves were equal (Fig. 10e) at strains higher than 0.7%.
Conclusions
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Based on the results obtained in the current investigation, it is possible to reach the following conclusions: •
Quantitative analysis of the microstructure showed a reduction in the average size of dendritic cells with the
•
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increase of AlN wt.%.
A359 matrix reinforced with different weight percent AlN particles can be properly synthesized by stirring and
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squeeze techniques. Application of pressure during the solidification process significantly reduces the percent of gas porosity incorporated in the melt during stir casting. •
The addition of AlN particles improves the kinetics of aging as the aging time was decreased from 12 hr to 5 hr.
•
Thermal analysis showed that eutectic arrest temperature for A359/AlN composites is decreased as compared to the A359 matrix alloy.
•
The yield and ultimate strength of A359/15 wt.% AlN composites increased to 314 and 473 MPa respectively. While the ductility relatively decreases.
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•
As finite element modeling shown, the distribution of the particles in the structure of the A359/5wt.% AlN composite strongly influences on the stress at the beginning of the plastic deformation.
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Acknowledgements The financial aid of the Ministry of higher education and scientific research of Egypt and the Ministry of Education and Science of the Russian Federation in the Program of NUST«MISiS» (Grant No К1-2014-026)
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are gratefully appreciated.
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41. A.Yu. Churyumov, A.L. Bazlov, A.A. Tsarkov, A.N. Solonin, D.V. Louzguine-Luzgin, Microstructure, mechanical properties, and crystallization behavior of Zr-based bulk metallic glasses prepared under a low
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Highlights Increasing wt.% of AlN in the matrix decreases the size of dendritic cells.
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Aging process of A359/AlN composites was accelerated by increasing AlN content.
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A359/AlN composites have higher yield and ultimate strength than A359 alloy.
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Eutectic temperature of the composites is decreased as compared to A359 matrix.
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