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Design and fabrication of functionally graded in-situ aluminium composites for automotive pistons
Materials and Design 88 (2015) 1201–1209
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Design and fabrication of functionally graded in-situ aluminium composites for automotive pistons A.G. Arsha, E. Jayakumar, T.P.D. Rajan ⁎, V. Antony, B.C. Pai CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695019, Kerala, India
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Article history: Received 23 January 2015 Received in revised form 15 September 2015 Accepted 16 September 2015 Available online 24 September 2015 Keywords: Aluminium Functionally graded material (FGM) Piston Centrifugal casting
a b s t r a c t Engineering components with location specific properties are being designed and fabricated successfully using functionally graded materials (FGM). The present investigation aims at design, fabrication and evaluation of functionally graded automotive piston using in-situ primary silicon reinforced A390 aluminium composite by centrifugal casting technique with a view of obtaining improved thermo mechanical properties at specific locations. The dies are designed and fabricated so as to obtain the primary silicon rich region towards the head portion of the piston. FGM pistons with A390 and A390-0.5%Mg are produced. They are characterised along the vertical cross section of the piston from piston head towards the skirt by microstructural, chemical, mechanical, thermal and tribological characterisations methods. The results are also compared with that of gravity cast piston. Microstructure and chemical composition analysis of FGM piston shows graded distribution of primary silicon from the head portion of the piston towards skirt and a eutectic composition in the skirt region. That yields an increase in hardness towards the head region. The wear testing revealed that the gradation also resulted in a remarkable enhancement of the wear properties of the piston head. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Functionally graded material (FGM) is an advanced class of engineering materials, which exhibits gradual transitions in the microstructure and composition in a direction. That leads to a desired variation in the functional performance with unique advantages of the smooth transition in thermal stresses across the thickness and minimal stress concentration at the interface between dissimilar materials. Such FGMs are rapidly finding applications in aggressive environments with steep temperature gradients. The major advantages of FGM are their enhanced and location specific functional performance within a component obtained through controlled microstructure [1–4]. FGMs can be processed by physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma spraying, sol–gel technique, and self-propagating high temperature synthesis (SHS). Molten metal infiltration, powder metallurgy, solid free form technology and centrifugal casting methods can also produce FGMs [5–8]. Centrifugal casting is a simple, efficient method for producing graded structures since the liquid metal facilitates the particle/phase mobility aided by centrifugal force during solidification in a spinning mould to form the bulk functionally graded
⁎ Corresponding author at: Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695019, Kerala, India. Tel.: +91 471 2515327. E-mail addresses: [email protected] (A.G. Arsha), [email protected]; [email protected] (T.P.D. Rajan).
http://dx.doi.org/10.1016/j.matdes.2015.09.099 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
component. The difference densities of the matrix and reinforcements also help in gradient structural formation [9]. Aluminium matrix composites have been one of the key research areas in the materials processing field for the past few decades. That offers considerable weight reduction in components of automobile or aerospace compared to the ferrous ones. Instead of the conventional hypoeutectic Al–Si alloys, the hypereutectic Al–Si alloys can be used to produce high performance automotive engine components. The high volume fraction of silicon provides better mechanical and tribological properties [10]. The primary silicon particle dispersed aluminium alloys are considered as in-situ composites since the hard primary silicon particles are generated during solidification. These alloys are suitable for internal combustion engine pistons, engine blocks, cylinder bodies of compressors, pumps and brakes because of their low coefficient of thermal expansion, high hardness and good wear resistance properties. The low expansion group of aluminium–silicon alloys is referred as ‘piston alloy’ which provides the best set of balanced overall properties [11]. In general, the gravity and pressure die casting technique produces components with homogeneous structures. The A390 hypereutectic aluminiumsilicon alloy exhibits excellent wear characteristics, high temperature strength, high modulus of elasticity and low thermal expansion coefficient. It is a promising material for heavy wear conditions [12]. Centrifugal casting is one of the potential methods for obtaining axis symmetry cylindrical parts with the use of centrifugal force [13]. It provides castings with refined microstructures, enhanced mechanical properties, clean cast with very little inclusion and porosity. It is also a flexible method for the production of castings with functionally graded
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Table 1 Optical emission spectroscopy results of A390 alloy ingot, gravity and centrifugally cast FGM piston at specified locations from the piston head towards skirt at (A) 5 mm, (B) 10 mm, (C) 15 mm and (D) 20 mm. Major alloying elements in %
Minor alloying elements in %
Details of alloy A390standards A390 ingot A 390 gravity cast piston A 390 FGM piston A (5 mm) B (10 mm) C (15 mm) D (20 mm)
Al in % Si
Cu
Mg
Fe
Mn
Zn
Cr
Ni
Ti
16–18 18.73 19.14 36.00 28.47 23.66 20.52
4–5 4.09 3.28 3.48 3.67 4.84 4.76
0.4–0.7 0.32 0.62 0.54 0.65 0.73 0.66
1.1 (max) 0.43 0.42 0.44 0.54 0.75 0.52
0.3 (max) 0.06 0.06 0.08 0.06 0.07 0.02
0.2 (max) 0.06 0.04 0.05 0.07 0.05 0.05
0.1 (max) 0.01 0.01 0.02 0.03 0.07 0.08
0.1 (max) 0.01 0.014 0.032 0.055 0.052 0.047
0.2 (max) 0.01 0.001 0.002 0.003 0.003 0.008
74–78 76.21 76.412 59.352 66.447 69.849 73.324
brake rotor disc, etc. [15–17]. Earlier studies have shown that using centrifugal casting the concentration of SiC of 40 vol.% can be segregated for 25–65 μm sized particles [18]. Centrifugal Casting of Aluminium alloy based composite pistons partially reinforced with SiC particles can meet the specific requirements of a piston [19]. The present study aims at design, fabrication and characterisation of in-situ primary silicon reinforced functionally graded piston by the centrifugal casting method. A390 aluminium alloy with 18 wt.% Si was chosen as the base material to fabricate FGM piston. A suitable centrifugal casting mould was designed and fabricated to obtain higher hardness and wear resistance towards the piston head region. Centrifugal cast FGM pistons with A390 Alloy and 0.5wt%Mg added A390 Alloy was fabricated along with the gravity cast pistons. They were characterised and compared with respect to microstructure, hardness, thermal and wear characteristics. 2. Materials and methods
Fig. 1. Schematic diagram of vertical centrifugal casting machine.
microstructures and properties in desired locations of a component. The difference in densities of matrix and reinforcements or different phases leads to the formation of functionally graded materials in composite materials [14]. Hence, this process can be used to make functionally graded automotive components such as engine cylinder liners, piston,
A390 hypereutectic aluminium alloy was selected for the formation of functionally graded piston. Table 1 shows the standard and actual chemical composition of the A390 alloy ingot used in the present study. Apart from the excellent wear resistance and low thermal expansion, the alloy posse's good fluidity because of higher silicon content, which assists in enhanced castability. Pistons were fabricated using A390 and A390-0.5%Mg by gravity and centrifugal casting methods. The aluminium alloy was melted in a resistance heating furnace; alloying additions were carried out followed by degassing and poured into gravity and centrifugal casting moulds. The vertical centrifugal
Fig. 2. (a) Centrifugally cast A390 FGM piston, (b) section of A390 FGM piston showing gradation of primary silicon.
A.G. Arsha et al. / Materials and Design 88 (2015) 1201–1209
Fig. 3. Microstructures of A390 gravity cast piston (a) at piston head and (b) at skirt potion.
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Fig. 5. Microstructures of A390-0.5 Mg gravity cast piston (a) at piston head and (b) at skirt potion.
casting technique was used for making the FGM piston. The centrifugal casting mould was preheated at 250 °C and the molten metal was poured at 760 °C melt temperature into the spinning mould at 800 rpm until the solidification was complete (2–3 min). Fig. 1 shows the schematic representation of vertical centrifugal casing machine used for casting the FGM piston. The die is designed in such a way that the head portion of the piston is oriented towards the inner periphery so that primary silicon particles will get segregated in the head portion. The aluminium alloy and composite pistons were characterised using optical microscopy, SEM, hardness testing, optical emission spectroscopy, differential thermal analysis and tribotester. Optical spectroscopy studies were done by Leica optical microscope. Brinell hardness measurements from piston head to skirt of the as-cast and heat treated specimen have been made using Indentec hardness tester. The standard T6 heat treatment procedure for the alloy is solution treatment at 535 °C for 4 h, quenching in warm water and artificially ageing at 175 °C for 8 h. Optical emission spectroscopy tests were done by using SPECTROMAXX, METEK which confirm the gradation in the chemical composition. Differential thermal analyses were done by using STA 7300, HITACHI. The tribological characteristics were carried out using DUCOM TR- 20 LE wear testing machine. 3. Results and discussion The FGM pistons were fabricated using A390 and A390-0.5% Mg alloy by vertical centrifugal casting techniques and two sets of homogenous gravity cast pistons were made for comparative studies. Fig. 2(a) shows the typical photograph of the as cast A390 piston and Fig. 2 (b) shows the cross sectional view of the piston where the graded distribution of silicon phases are observed in the head region up to 15 mm thickness from the top side. 3.1. Microstructural characteristics
Fig. 4. Microstructures of A390 centrifugally cast FGM piston from piston head towards skirt region (a) at 5 mm (b) at 10 mm (c) at 15 mm (d) at 20 mm (e) at 25 mm (f) at 30 mm.
Optical microscopic studies carried on centrifugal and gravity cast A390 and A 390-0.5% Mg pistons shows remarkable variation of microstructural characteristics. Fig. 3 (a) & (b) show the microstructures in head and skirt portion of gravity cast A390 piston, which contain α-Al dendrites, eutectic Si region and primary silicon particles are equally dispersed in the matrix and there is no variations in distribution of primary Si particles. Mg addition results in the formation of Mg2Si phases which contributes towards the age hardening and thereby enhancing the mechanical properties of the FGM [20]. Fig. 4 shows the microstructure of centrifugally cast piston along the axis of the piston in a distance of 5 mm from piston head towards skirt. Microstructural observation
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Fig. 7. DTA curve of A390 alloy.
Fig. 8. DTA of A390 centrifugally cast FGM piston (A) at piston head portion, (B) at interface and (C) at skirt region. Fig. 6. Microstructures of A 390-0.5 Mg centrifugally cast FGM piston from piston head towards skirt region (a) at 5 mm (b) at 10 mm (c) at 15 mm (d) at 20 mm (e) at 25 mm (f) at 30 mm.
rich region of about 15 mm is observed towards the head portion of the piston. Fig. 5 shows microstructure of A390-0.5% Mg gravity cast piston in (a) head and (b) skirt portion showing uniform primary silicon. However compared to the A390 FGM piston, the 0.5%Mg addition had refined the eutectic silicon phases in the casting. Fig. 6 shows the microstructure of A390-0.5% Mg centrifugal cast FGM piston from head to skirt region, whereas the gradation in the primary silicon is
clearly shows the formation of graded structures of primary silicon rich portion in head of the piston. The thickness of primary silicon particle rich region can be altered by varying the pouring temperature and rotation speed of the centrifugal casting mould. The primary silicon particle
Table 2 Optical emission spectroscopy results of A390-0.5% Mg alloy gravity and centrifugally cast FGM piston at specified locations from the piston head towards skirt at (A) 5 mm, (B) 10 mm, (C) 15 mm and (D) 20 mm. Major alloying elements in %
Minor alloying elements in %
Details of alloy A 390-0.5 Mg gravity cast piston A 390-0.5 Mg FGM piston A (5 mm) B (10 mm) C (15 mm) D (20 mm)
Al in % Si
Cu
Mg
Fe
Mn
Zn
Cr
Ni
Ti
19.14 36.00 29.29 20.89 15.88
3.28 2.52 2.252 2.52 2.36
0.62 1.03 1.00 1.54 1.70
0.42 0.32 0.50 0.56 0.84
0.05 0.05 0.05 0.08 0.08
0.04 0.04 0.02 0.05 0.05
0.01 0.05 0.02 0.04 0.04
0.01 0.27 0.06 0.035 0.013
0.014 0.012 0.062 0.047 0.01
76.443 59.42 66.38 74.021 79.770
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Fig. 9. DTA of A390-0.5 Mg centrifugally cast FGM piston (A) at piston head portion, (B) at interface and (C) at skirt region.
observed. The density of primary Si particle is 2.33 g/cm3 and that of Al is 2.78 g/cm3, hence the low density primary silicon particle always have the tendency to migrate towards the inner periphery in the presence of centrifugal force providing a graded structure. 3.2. Optical emission spectroscopy (OES) analysis Tables 1 and 2 show the optical emission spectroscopy results of both A390 and A390-0.5% Mg pistons respectively. The gravity cast piston shows 19–21% of silicon throughout the casting. OES results show that the chemical composition of A390 alloy and A390-0.5% Mg centrifugally cast piston varies axially from head to skirt portion. In the centrifugally cast piston the concentration of silicon is about 36% near 5 mm from outer periphery. The concentration of Si gradually reduces towards the skirt region. At 10 mm from the piston head 28% of Si is observed and further reduces to 23% at 15 mm and 20.5% at 20 mm from outer periphery. These results reflect the variation of primary silicon as observed in optical microstructures of FGM (Fig. 4). The percentage of copper also varies from head to skirt portion. The head portion shows 3.48 and the skirt region at 20 mm shows 4.76%. This variation is caused mainly due
Fig. 10. Hardness values from piston head towards skirt of the A390 gravity and centrifugally cast FGM pistons in both as-cast and heat treated conditions.
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Fig. 11. Hardness values from piston head towards skirt of the A390-0.5 Mg gravity and centrifugally cast FGM pistons in both as-cast and heat treated conditions.
to the presence of low density silicon towards the head portion and also the copper containing phase which may segregate to the skirt portion which is outer periphery with respect to the centrifugal casting mould. Similarly the percentage of Mg shows marginal variation in the concentration from head to skirt region. The OES analysis of centrifugal cast A390-0.5% Mg FGM piston also shows similar variation in concentration of Si, Cu and Mg as observed for A390 centrifugally cast alloy piston. The enhancement in the concentration of Mg in gravity and centrifugally cast A390-0.5% Mg FGM piston is observed due to the addition of extra 0.5% Mg added into the alloy. 3.3. Differential thermal analysis Thermal characteristics of A390 and A390-0.5% Mg were studied by differential thermal analytical technique and are shown in Figs. 7–9. The DTA was performed at constant heating rate of 10 °C/min. It is possible to track the phase transformation processes occurring during heating or cooling an alloy, by virtue of the large exo- or endo-thermic peaks produced. The DTA Curve of A390 Al alloy shows two major peaks corresponding to the formation of Al and Si from the liquid at 565 °C and
Fig. 12. Wear behaviour of A390 centrifugal cast FGM piston (A) piston head region, (B) interface region and (C) skirt region.
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observed at 505 °C corresponds to the CuAl2 and Cu2Mg8Si6Al5 phase formation as per the reaction (1). Earlier studies on hyper eutectic aluminium silicon alloy and A390 have shown the phase transformation occurring in the similar temperature range [21,22]. The addition of 0.5% Mg to the A390 alloy had lead to the higher intensity peak around 505 °C compared to pure A390 alloy. Liq Al alloy→Al þ Si þ CuAl2 þ Cu2 Mg8 Si6 Al5
ð1Þ
3.4. Hardness behaviour
Fig. 13. Wear behaviour of A390-0.5 Mg centrifugal cast FGM piston (A) piston head region, (B) interface region and (C) skirt region.
the formation of CuAl2 phase at 505 °C (Fig. 7). The centrifugally cast A390 Al alloy and A390 + 0.5%Mg shows three transformation peaks (Figs. 8 and 9). The first transformation peak at 565 °C corresponds to the formation of Al + Si eutectic transformation. The peak at 535 °C corresponds to the Mg2Si phase formation. The transformation peak
The hardness behaviour of the functionally graded piston is evaluated by brinell hardness tester. Brinell hardness values were obtained from both as cast and heat treated (T6 conditions) samples taken from different locations of the sample. The hardness values were taken from the Piston head to skirt portion of A390 and A390-0.5 Mg piston. The Fig. 10 indicates the hardness values of A390 piston in as cast and heat treated condition along the axis of the piston. The minimum hardness value found is 94 BHN, which is similar to conventionally gravity as-cast piston having the hardness in the range 94–99 BHN. The heat treated hardness values of the gravity cast A390 piston is in the range 110–116 throughout the piston. In the case of centrifugally cast A390 piston hardness gradually shows an increase from skirt to head portion with peak value of hardness obtained in the head portion is 135 BHN in heat treated condition. The gradual gradation of hardness value were observed from 120 to 135 BHN from skirt to head portions due to the segregation of primary Si particles in the head region and the maximum
Fig. 14. Stereo images of A390 FGM piston wear samples (a) at 1 kg load, piston head region, (b) at 4 kg load, piston head region, (c) at 1 kg load, skirt region and (d) at 4 kg load, skirt region.
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Fig. 15. Stereo micrographs of 390–0.5 Mg centrifugal casting (a) 1 kg load, head portion, (b) 4 kg load, head portion, (c) 1 kg load, skirt portion and (d) 4 kg load, skirt portion.
standard deviation observed is 4.58. It is evident that the hardness values of centrifugal cast pistons are higher than gravity cast piston. Fig. 11 shows the hardness values of the A 390-0.5% Mg FGM pistons in as cast and heat treated condition It clearly reveals that there is an
increase in hardness values after heat treatment. The addition of 0.5% Mg had resulted in significant enhancement in the hardness values to 188 BHN in the piston head when compared to 135 BHN for the centrifugal cast 390 alloy. The significant improvement in the hardness by the
Fig. 16. SEM photographs of the worm out surface of centrifugaly cast A 390-0.5 Mg FGM pins from piston head and skirt regions at different load conditions (a) at 1 kg (piston head) (b) at 4 kg (piston head) (c) at 1 kg (skirt region) (d) at 4 kg (skirt region).
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Fig. 17. EDS of SEM photographs of the worm out surface of centrifugaly cast A 390-0.5 Mg FGM pins from piston head and skirt regions at different load conditions (a) at 1 kg (piston head) (b) at 4 kg (piston head (c) at 1 kg (skirt region) (d) at 4 kg (skirt region).
addition of Mg is due to the formation more magnesium bearing phases (Mg2Si and Cu2Mg8Si6Al5) in the matrix. The increase in the hardness towards the piston head is due to the presence of higher primary silicon particles. This higher hardness towards the head region contributes for an excellent wear resistance and high temperature capability of the FGM piston. 3.5. Tribological characteristics Figs. 12 and 13 show the wear characteristics of centrifugally cast A 390 and A390-0.5% Mg FGM piston in the head, interface and skirt region. The wear specimen of dimension 6 mm diameter and 30 mm length were taken from three different regions such as head, interface and skirt region of the piston and tested for varying load using pin-ondisc tester. Variation of wear rate is calculated by taking weight loss of the sample taken from various zones of the piston in various load conditions, during pin-on-disc wear testing. It is observed that A390 + 0.5% Mg Al FGM pistons exhibits lower wear rates when compared with pure A390 Al FGM pistons (Figs. 12 and 13). This is due to the presence of more Mg bearing precipitates in the alloy and the formation hard in-situ primary silicon particles. The decrease in wear rate in centrifugal cast FGM piston head is due to the enrichment of primary Si particles and a gradual increase in wear rate is observed from piston head to skirt portion. Fig. 14 (a-d) shows the wear tracks observed on the surface of the A390 alloy FGM piston in head and skirt portions at lower and higher loads. The detailed examination of wear tracks reveals the features associated with abrasive wear which causes scratches on the surface. The abrasive component of the wear mechanism is pointed out by the ploughed grooves inside the wear tracks which are shown in both Figs. 14 and 15 due to the harder (steel disc) surface scratching over the softer (pin) surface [23]. Typical scanning electron micrographs of worn surface of centrifugally cast A390-0.5%Mg added FGM piston
samples are illustrated in Fig. 16. Fine grooves are observed in the head portion whereas the skirt portion shows wider grooves due to higher material removal by ploughing action during wear testing. Fig. 17 shows the Energy dispersion Spectrum (EDS) analysis of the wear piston surfaces of the A 390-0.5%Mg FGM piston wear pin surface. The surface shows higher Fe peak in the head portion due to the more scratching of the disc surface by the hard primary silicon particle in the piston head. In general, the centrifugally cast functionally graded piston provides higher hardness and wear resistance towards the head region. The addition of 0.5% Mg had further enhanced the mechanical and tribological properties.
4. Conclusion In-situ functionally graded Al composite pistons were successfully designed and fabricated by vertical centrifugal casting technique. Two different zones of primary Si rich zone and eutectic silicon rich zones were observed. Higher concentration of primary Si particles gets gradually distributed towards the head region of the piston providing enhanced properties. Magnesium provides substantial strengthening and improvement of precipitation hardening phases of aluminium alloy. Centrifugally cast FGM pistons provide high hardness, thermal resistance and wear properties than that of conventionally gravity cast pistons. Heat treated samples were found to provide superior properties than that of as-cast samples. A higher hardness of 188 BHN was observed in the head portion of the 0.5% Mg A390 FGM piston. Wear test results shows that the outer periphery of the piston is having lower wear rate even at high load (4 kg). Wear resistance gradually decreases from head to skirt portion. These Al FGM pistons containing a large quantity of primary Si particles on the piston head can meet the high temperature requirement and wear resistance of the piston.
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Acknowledgement The authors are grateful to Director and Members of MSTD, CSIRNIIST Trivandrum for all the support and help for carrying out this work. References [1] G. Chirita, D. Soares, F.S. Silva, Advantages of the centrifugal casting technique for the production of structural components with Al–Si alloys, Mater. Des. 28 (2008) 20–27. [2] T.P.D. Rajan, B.C. Pai, Developments in processing of functionally gradient metals and metal–ceramic composites: a review, Acta Metall. Sin. 27 (2014) 825–828. [3] J.W. Gao, C.Y. Wang, Modeling the solidification of functionally graded materials by centrifugal casting, Mater. Sci. Eng. A 292 (2000) 207–215. [4] Z. Yan-boliu, C. ming, W. Kai, Z. Mao-hua, X. Yong, Characteristics of two Al based functionally gradient composites reinforced by primary Si particles and Si/in situ Mg2Si particles in centrifugal casting, Trans. Non-ferrous Met. Soc. of China 20 (2010) 361–370. [5] S.A. Alidokht, A. Abdollah-zadeh, S. Soleymani, T. Saeid, H. Assadi, Evaluation of microstructure and wear behavior of friction stir processed cast aluminum alloy, Mater. Charact. 63 (2012) 90–97. [6] R.S. Rana, Rajesh Purohit, S. Das, Reviews on the influences of alloying elements on the microstructure and mechanical properties of aluminum alloys and luminum alloy composites, Int. J. Sci. Res. Publ. 63 (2012) 90–97. [7] J.W. Gao, C.Y. Wan, Modeling the solidification of functionally graded materials by centrifugal casting, Mater. Sci. Eng. A 292 (2000) 207–215. [8] Z.Y. Ma, A.L. Pilchak, M.C. Juhas, J.C. Williams, Microstructural refinement and property enhancement of cast light alloys via friction stir processing, Scr. Mater. 58 (2008) 361–366. [9] R.M. Mahamood, E.T. Akinlabi, M. Shukla, S. Pityana, Functionally Graded Material: An Overview, Proceedings of the World Congress on Engineering Vol III WCE (2012) 4–6. [10] C.M.L. Huang, L. Xunjia, G.F. Li, Aluminum alloy pistons reinforced with SiC fabricated by centrifugal casting, J. Mater. Process. Technol. 211 (2008) 1540–1546.
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Design and fabrication of functionally graded in-situ aluminium composites for automotive pistons A.G. Arsha, E. Jayakumar, T.P.D. Rajan ⁎, V. Antony, B.C. Pai CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695019, Kerala, India
a r t i c l e
i n f o
Article history: Received 23 January 2015 Received in revised form 15 September 2015 Accepted 16 September 2015 Available online 24 September 2015 Keywords: Aluminium Functionally graded material (FGM) Piston Centrifugal casting
a b s t r a c t Engineering components with location specific properties are being designed and fabricated successfully using functionally graded materials (FGM). The present investigation aims at design, fabrication and evaluation of functionally graded automotive piston using in-situ primary silicon reinforced A390 aluminium composite by centrifugal casting technique with a view of obtaining improved thermo mechanical properties at specific locations. The dies are designed and fabricated so as to obtain the primary silicon rich region towards the head portion of the piston. FGM pistons with A390 and A390-0.5%Mg are produced. They are characterised along the vertical cross section of the piston from piston head towards the skirt by microstructural, chemical, mechanical, thermal and tribological characterisations methods. The results are also compared with that of gravity cast piston. Microstructure and chemical composition analysis of FGM piston shows graded distribution of primary silicon from the head portion of the piston towards skirt and a eutectic composition in the skirt region. That yields an increase in hardness towards the head region. The wear testing revealed that the gradation also resulted in a remarkable enhancement of the wear properties of the piston head. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Functionally graded material (FGM) is an advanced class of engineering materials, which exhibits gradual transitions in the microstructure and composition in a direction. That leads to a desired variation in the functional performance with unique advantages of the smooth transition in thermal stresses across the thickness and minimal stress concentration at the interface between dissimilar materials. Such FGMs are rapidly finding applications in aggressive environments with steep temperature gradients. The major advantages of FGM are their enhanced and location specific functional performance within a component obtained through controlled microstructure [1–4]. FGMs can be processed by physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma spraying, sol–gel technique, and self-propagating high temperature synthesis (SHS). Molten metal infiltration, powder metallurgy, solid free form technology and centrifugal casting methods can also produce FGMs [5–8]. Centrifugal casting is a simple, efficient method for producing graded structures since the liquid metal facilitates the particle/phase mobility aided by centrifugal force during solidification in a spinning mould to form the bulk functionally graded
⁎ Corresponding author at: Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695019, Kerala, India. Tel.: +91 471 2515327. E-mail addresses: [email protected] (A.G. Arsha), [email protected]; [email protected] (T.P.D. Rajan).
http://dx.doi.org/10.1016/j.matdes.2015.09.099 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
component. The difference densities of the matrix and reinforcements also help in gradient structural formation [9]. Aluminium matrix composites have been one of the key research areas in the materials processing field for the past few decades. That offers considerable weight reduction in components of automobile or aerospace compared to the ferrous ones. Instead of the conventional hypoeutectic Al–Si alloys, the hypereutectic Al–Si alloys can be used to produce high performance automotive engine components. The high volume fraction of silicon provides better mechanical and tribological properties [10]. The primary silicon particle dispersed aluminium alloys are considered as in-situ composites since the hard primary silicon particles are generated during solidification. These alloys are suitable for internal combustion engine pistons, engine blocks, cylinder bodies of compressors, pumps and brakes because of their low coefficient of thermal expansion, high hardness and good wear resistance properties. The low expansion group of aluminium–silicon alloys is referred as ‘piston alloy’ which provides the best set of balanced overall properties [11]. In general, the gravity and pressure die casting technique produces components with homogeneous structures. The A390 hypereutectic aluminiumsilicon alloy exhibits excellent wear characteristics, high temperature strength, high modulus of elasticity and low thermal expansion coefficient. It is a promising material for heavy wear conditions [12]. Centrifugal casting is one of the potential methods for obtaining axis symmetry cylindrical parts with the use of centrifugal force [13]. It provides castings with refined microstructures, enhanced mechanical properties, clean cast with very little inclusion and porosity. It is also a flexible method for the production of castings with functionally graded
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Table 1 Optical emission spectroscopy results of A390 alloy ingot, gravity and centrifugally cast FGM piston at specified locations from the piston head towards skirt at (A) 5 mm, (B) 10 mm, (C) 15 mm and (D) 20 mm. Major alloying elements in %
Minor alloying elements in %
Details of alloy A390standards A390 ingot A 390 gravity cast piston A 390 FGM piston A (5 mm) B (10 mm) C (15 mm) D (20 mm)
Al in % Si
Cu
Mg
Fe
Mn
Zn
Cr
Ni
Ti
16–18 18.73 19.14 36.00 28.47 23.66 20.52
4–5 4.09 3.28 3.48 3.67 4.84 4.76
0.4–0.7 0.32 0.62 0.54 0.65 0.73 0.66
1.1 (max) 0.43 0.42 0.44 0.54 0.75 0.52
0.3 (max) 0.06 0.06 0.08 0.06 0.07 0.02
0.2 (max) 0.06 0.04 0.05 0.07 0.05 0.05
0.1 (max) 0.01 0.01 0.02 0.03 0.07 0.08
0.1 (max) 0.01 0.014 0.032 0.055 0.052 0.047
0.2 (max) 0.01 0.001 0.002 0.003 0.003 0.008
74–78 76.21 76.412 59.352 66.447 69.849 73.324
brake rotor disc, etc. [15–17]. Earlier studies have shown that using centrifugal casting the concentration of SiC of 40 vol.% can be segregated for 25–65 μm sized particles [18]. Centrifugal Casting of Aluminium alloy based composite pistons partially reinforced with SiC particles can meet the specific requirements of a piston [19]. The present study aims at design, fabrication and characterisation of in-situ primary silicon reinforced functionally graded piston by the centrifugal casting method. A390 aluminium alloy with 18 wt.% Si was chosen as the base material to fabricate FGM piston. A suitable centrifugal casting mould was designed and fabricated to obtain higher hardness and wear resistance towards the piston head region. Centrifugal cast FGM pistons with A390 Alloy and 0.5wt%Mg added A390 Alloy was fabricated along with the gravity cast pistons. They were characterised and compared with respect to microstructure, hardness, thermal and wear characteristics. 2. Materials and methods
Fig. 1. Schematic diagram of vertical centrifugal casting machine.
microstructures and properties in desired locations of a component. The difference in densities of matrix and reinforcements or different phases leads to the formation of functionally graded materials in composite materials [14]. Hence, this process can be used to make functionally graded automotive components such as engine cylinder liners, piston,
A390 hypereutectic aluminium alloy was selected for the formation of functionally graded piston. Table 1 shows the standard and actual chemical composition of the A390 alloy ingot used in the present study. Apart from the excellent wear resistance and low thermal expansion, the alloy posse's good fluidity because of higher silicon content, which assists in enhanced castability. Pistons were fabricated using A390 and A390-0.5%Mg by gravity and centrifugal casting methods. The aluminium alloy was melted in a resistance heating furnace; alloying additions were carried out followed by degassing and poured into gravity and centrifugal casting moulds. The vertical centrifugal
Fig. 2. (a) Centrifugally cast A390 FGM piston, (b) section of A390 FGM piston showing gradation of primary silicon.
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Fig. 3. Microstructures of A390 gravity cast piston (a) at piston head and (b) at skirt potion.
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Fig. 5. Microstructures of A390-0.5 Mg gravity cast piston (a) at piston head and (b) at skirt potion.
casting technique was used for making the FGM piston. The centrifugal casting mould was preheated at 250 °C and the molten metal was poured at 760 °C melt temperature into the spinning mould at 800 rpm until the solidification was complete (2–3 min). Fig. 1 shows the schematic representation of vertical centrifugal casing machine used for casting the FGM piston. The die is designed in such a way that the head portion of the piston is oriented towards the inner periphery so that primary silicon particles will get segregated in the head portion. The aluminium alloy and composite pistons were characterised using optical microscopy, SEM, hardness testing, optical emission spectroscopy, differential thermal analysis and tribotester. Optical spectroscopy studies were done by Leica optical microscope. Brinell hardness measurements from piston head to skirt of the as-cast and heat treated specimen have been made using Indentec hardness tester. The standard T6 heat treatment procedure for the alloy is solution treatment at 535 °C for 4 h, quenching in warm water and artificially ageing at 175 °C for 8 h. Optical emission spectroscopy tests were done by using SPECTROMAXX, METEK which confirm the gradation in the chemical composition. Differential thermal analyses were done by using STA 7300, HITACHI. The tribological characteristics were carried out using DUCOM TR- 20 LE wear testing machine. 3. Results and discussion The FGM pistons were fabricated using A390 and A390-0.5% Mg alloy by vertical centrifugal casting techniques and two sets of homogenous gravity cast pistons were made for comparative studies. Fig. 2(a) shows the typical photograph of the as cast A390 piston and Fig. 2 (b) shows the cross sectional view of the piston where the graded distribution of silicon phases are observed in the head region up to 15 mm thickness from the top side. 3.1. Microstructural characteristics
Fig. 4. Microstructures of A390 centrifugally cast FGM piston from piston head towards skirt region (a) at 5 mm (b) at 10 mm (c) at 15 mm (d) at 20 mm (e) at 25 mm (f) at 30 mm.
Optical microscopic studies carried on centrifugal and gravity cast A390 and A 390-0.5% Mg pistons shows remarkable variation of microstructural characteristics. Fig. 3 (a) & (b) show the microstructures in head and skirt portion of gravity cast A390 piston, which contain α-Al dendrites, eutectic Si region and primary silicon particles are equally dispersed in the matrix and there is no variations in distribution of primary Si particles. Mg addition results in the formation of Mg2Si phases which contributes towards the age hardening and thereby enhancing the mechanical properties of the FGM [20]. Fig. 4 shows the microstructure of centrifugally cast piston along the axis of the piston in a distance of 5 mm from piston head towards skirt. Microstructural observation
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Fig. 7. DTA curve of A390 alloy.
Fig. 8. DTA of A390 centrifugally cast FGM piston (A) at piston head portion, (B) at interface and (C) at skirt region. Fig. 6. Microstructures of A 390-0.5 Mg centrifugally cast FGM piston from piston head towards skirt region (a) at 5 mm (b) at 10 mm (c) at 15 mm (d) at 20 mm (e) at 25 mm (f) at 30 mm.
rich region of about 15 mm is observed towards the head portion of the piston. Fig. 5 shows microstructure of A390-0.5% Mg gravity cast piston in (a) head and (b) skirt portion showing uniform primary silicon. However compared to the A390 FGM piston, the 0.5%Mg addition had refined the eutectic silicon phases in the casting. Fig. 6 shows the microstructure of A390-0.5% Mg centrifugal cast FGM piston from head to skirt region, whereas the gradation in the primary silicon is
clearly shows the formation of graded structures of primary silicon rich portion in head of the piston. The thickness of primary silicon particle rich region can be altered by varying the pouring temperature and rotation speed of the centrifugal casting mould. The primary silicon particle
Table 2 Optical emission spectroscopy results of A390-0.5% Mg alloy gravity and centrifugally cast FGM piston at specified locations from the piston head towards skirt at (A) 5 mm, (B) 10 mm, (C) 15 mm and (D) 20 mm. Major alloying elements in %
Minor alloying elements in %
Details of alloy A 390-0.5 Mg gravity cast piston A 390-0.5 Mg FGM piston A (5 mm) B (10 mm) C (15 mm) D (20 mm)
Al in % Si
Cu
Mg
Fe
Mn
Zn
Cr
Ni
Ti
19.14 36.00 29.29 20.89 15.88
3.28 2.52 2.252 2.52 2.36
0.62 1.03 1.00 1.54 1.70
0.42 0.32 0.50 0.56 0.84
0.05 0.05 0.05 0.08 0.08
0.04 0.04 0.02 0.05 0.05
0.01 0.05 0.02 0.04 0.04
0.01 0.27 0.06 0.035 0.013
0.014 0.012 0.062 0.047 0.01
76.443 59.42 66.38 74.021 79.770
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Fig. 9. DTA of A390-0.5 Mg centrifugally cast FGM piston (A) at piston head portion, (B) at interface and (C) at skirt region.
observed. The density of primary Si particle is 2.33 g/cm3 and that of Al is 2.78 g/cm3, hence the low density primary silicon particle always have the tendency to migrate towards the inner periphery in the presence of centrifugal force providing a graded structure. 3.2. Optical emission spectroscopy (OES) analysis Tables 1 and 2 show the optical emission spectroscopy results of both A390 and A390-0.5% Mg pistons respectively. The gravity cast piston shows 19–21% of silicon throughout the casting. OES results show that the chemical composition of A390 alloy and A390-0.5% Mg centrifugally cast piston varies axially from head to skirt portion. In the centrifugally cast piston the concentration of silicon is about 36% near 5 mm from outer periphery. The concentration of Si gradually reduces towards the skirt region. At 10 mm from the piston head 28% of Si is observed and further reduces to 23% at 15 mm and 20.5% at 20 mm from outer periphery. These results reflect the variation of primary silicon as observed in optical microstructures of FGM (Fig. 4). The percentage of copper also varies from head to skirt portion. The head portion shows 3.48 and the skirt region at 20 mm shows 4.76%. This variation is caused mainly due
Fig. 10. Hardness values from piston head towards skirt of the A390 gravity and centrifugally cast FGM pistons in both as-cast and heat treated conditions.
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Fig. 11. Hardness values from piston head towards skirt of the A390-0.5 Mg gravity and centrifugally cast FGM pistons in both as-cast and heat treated conditions.
to the presence of low density silicon towards the head portion and also the copper containing phase which may segregate to the skirt portion which is outer periphery with respect to the centrifugal casting mould. Similarly the percentage of Mg shows marginal variation in the concentration from head to skirt region. The OES analysis of centrifugal cast A390-0.5% Mg FGM piston also shows similar variation in concentration of Si, Cu and Mg as observed for A390 centrifugally cast alloy piston. The enhancement in the concentration of Mg in gravity and centrifugally cast A390-0.5% Mg FGM piston is observed due to the addition of extra 0.5% Mg added into the alloy. 3.3. Differential thermal analysis Thermal characteristics of A390 and A390-0.5% Mg were studied by differential thermal analytical technique and are shown in Figs. 7–9. The DTA was performed at constant heating rate of 10 °C/min. It is possible to track the phase transformation processes occurring during heating or cooling an alloy, by virtue of the large exo- or endo-thermic peaks produced. The DTA Curve of A390 Al alloy shows two major peaks corresponding to the formation of Al and Si from the liquid at 565 °C and
Fig. 12. Wear behaviour of A390 centrifugal cast FGM piston (A) piston head region, (B) interface region and (C) skirt region.
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observed at 505 °C corresponds to the CuAl2 and Cu2Mg8Si6Al5 phase formation as per the reaction (1). Earlier studies on hyper eutectic aluminium silicon alloy and A390 have shown the phase transformation occurring in the similar temperature range [21,22]. The addition of 0.5% Mg to the A390 alloy had lead to the higher intensity peak around 505 °C compared to pure A390 alloy. Liq Al alloy→Al þ Si þ CuAl2 þ Cu2 Mg8 Si6 Al5
ð1Þ
3.4. Hardness behaviour
Fig. 13. Wear behaviour of A390-0.5 Mg centrifugal cast FGM piston (A) piston head region, (B) interface region and (C) skirt region.
the formation of CuAl2 phase at 505 °C (Fig. 7). The centrifugally cast A390 Al alloy and A390 + 0.5%Mg shows three transformation peaks (Figs. 8 and 9). The first transformation peak at 565 °C corresponds to the formation of Al + Si eutectic transformation. The peak at 535 °C corresponds to the Mg2Si phase formation. The transformation peak
The hardness behaviour of the functionally graded piston is evaluated by brinell hardness tester. Brinell hardness values were obtained from both as cast and heat treated (T6 conditions) samples taken from different locations of the sample. The hardness values were taken from the Piston head to skirt portion of A390 and A390-0.5 Mg piston. The Fig. 10 indicates the hardness values of A390 piston in as cast and heat treated condition along the axis of the piston. The minimum hardness value found is 94 BHN, which is similar to conventionally gravity as-cast piston having the hardness in the range 94–99 BHN. The heat treated hardness values of the gravity cast A390 piston is in the range 110–116 throughout the piston. In the case of centrifugally cast A390 piston hardness gradually shows an increase from skirt to head portion with peak value of hardness obtained in the head portion is 135 BHN in heat treated condition. The gradual gradation of hardness value were observed from 120 to 135 BHN from skirt to head portions due to the segregation of primary Si particles in the head region and the maximum
Fig. 14. Stereo images of A390 FGM piston wear samples (a) at 1 kg load, piston head region, (b) at 4 kg load, piston head region, (c) at 1 kg load, skirt region and (d) at 4 kg load, skirt region.
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Fig. 15. Stereo micrographs of 390–0.5 Mg centrifugal casting (a) 1 kg load, head portion, (b) 4 kg load, head portion, (c) 1 kg load, skirt portion and (d) 4 kg load, skirt portion.
standard deviation observed is 4.58. It is evident that the hardness values of centrifugal cast pistons are higher than gravity cast piston. Fig. 11 shows the hardness values of the A 390-0.5% Mg FGM pistons in as cast and heat treated condition It clearly reveals that there is an
increase in hardness values after heat treatment. The addition of 0.5% Mg had resulted in significant enhancement in the hardness values to 188 BHN in the piston head when compared to 135 BHN for the centrifugal cast 390 alloy. The significant improvement in the hardness by the
Fig. 16. SEM photographs of the worm out surface of centrifugaly cast A 390-0.5 Mg FGM pins from piston head and skirt regions at different load conditions (a) at 1 kg (piston head) (b) at 4 kg (piston head) (c) at 1 kg (skirt region) (d) at 4 kg (skirt region).
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Fig. 17. EDS of SEM photographs of the worm out surface of centrifugaly cast A 390-0.5 Mg FGM pins from piston head and skirt regions at different load conditions (a) at 1 kg (piston head) (b) at 4 kg (piston head (c) at 1 kg (skirt region) (d) at 4 kg (skirt region).
addition of Mg is due to the formation more magnesium bearing phases (Mg2Si and Cu2Mg8Si6Al5) in the matrix. The increase in the hardness towards the piston head is due to the presence of higher primary silicon particles. This higher hardness towards the head region contributes for an excellent wear resistance and high temperature capability of the FGM piston. 3.5. Tribological characteristics Figs. 12 and 13 show the wear characteristics of centrifugally cast A 390 and A390-0.5% Mg FGM piston in the head, interface and skirt region. The wear specimen of dimension 6 mm diameter and 30 mm length were taken from three different regions such as head, interface and skirt region of the piston and tested for varying load using pin-ondisc tester. Variation of wear rate is calculated by taking weight loss of the sample taken from various zones of the piston in various load conditions, during pin-on-disc wear testing. It is observed that A390 + 0.5% Mg Al FGM pistons exhibits lower wear rates when compared with pure A390 Al FGM pistons (Figs. 12 and 13). This is due to the presence of more Mg bearing precipitates in the alloy and the formation hard in-situ primary silicon particles. The decrease in wear rate in centrifugal cast FGM piston head is due to the enrichment of primary Si particles and a gradual increase in wear rate is observed from piston head to skirt portion. Fig. 14 (a-d) shows the wear tracks observed on the surface of the A390 alloy FGM piston in head and skirt portions at lower and higher loads. The detailed examination of wear tracks reveals the features associated with abrasive wear which causes scratches on the surface. The abrasive component of the wear mechanism is pointed out by the ploughed grooves inside the wear tracks which are shown in both Figs. 14 and 15 due to the harder (steel disc) surface scratching over the softer (pin) surface [23]. Typical scanning electron micrographs of worn surface of centrifugally cast A390-0.5%Mg added FGM piston
samples are illustrated in Fig. 16. Fine grooves are observed in the head portion whereas the skirt portion shows wider grooves due to higher material removal by ploughing action during wear testing. Fig. 17 shows the Energy dispersion Spectrum (EDS) analysis of the wear piston surfaces of the A 390-0.5%Mg FGM piston wear pin surface. The surface shows higher Fe peak in the head portion due to the more scratching of the disc surface by the hard primary silicon particle in the piston head. In general, the centrifugally cast functionally graded piston provides higher hardness and wear resistance towards the head region. The addition of 0.5% Mg had further enhanced the mechanical and tribological properties.
4. Conclusion In-situ functionally graded Al composite pistons were successfully designed and fabricated by vertical centrifugal casting technique. Two different zones of primary Si rich zone and eutectic silicon rich zones were observed. Higher concentration of primary Si particles gets gradually distributed towards the head region of the piston providing enhanced properties. Magnesium provides substantial strengthening and improvement of precipitation hardening phases of aluminium alloy. Centrifugally cast FGM pistons provide high hardness, thermal resistance and wear properties than that of conventionally gravity cast pistons. Heat treated samples were found to provide superior properties than that of as-cast samples. A higher hardness of 188 BHN was observed in the head portion of the 0.5% Mg A390 FGM piston. Wear test results shows that the outer periphery of the piston is having lower wear rate even at high load (4 kg). Wear resistance gradually decreases from head to skirt portion. These Al FGM pistons containing a large quantity of primary Si particles on the piston head can meet the high temperature requirement and wear resistance of the piston.
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