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TiB2 in situ aluminum cast composites
archives of civil and mechanical engineering 14 (2014) 72–79
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Effect of TiB2 content and temperature on sliding wear behavior of AA7075/TiB2 in situ aluminum cast composites H.B. Michael Rajana,n, S. Ramabalanb, I. Dinaharanc, S.J. Vijayd a
Department of Mechanical Engineering, St. Joseph's College of Engineering and Technology, Thanjavur 613403, Tamil Nadu, India b EGS Pillay Engineering College, Nagapattinam 611002, Tamil Nadu, India c Department of Mechanical Engineering, V V College of Engineering, Tisaiyanvilai 627657, Tamil Nadu, India d Centre for Research in Metallurgy (CRM), School of Mechanical Sciences, Karunya University, Coimbatore 641114, Tamil Nadu, India
ar t ic l e in f o
abs tra ct
Article history:
Aluminum alloy AA7075 reinforced TiB2 particulate composites were prepared by the
Received 12 December 2012
in situ reaction of K2TiF6 and KBF4 to molten aluminum. The prepared aluminum matrix
Accepted 19 May 2013
composites (AMCs) were characterized using X-ray diffraction and scanning electron
Available online 28 May 2013
microscopy (SEM). The sliding wear behavior of the AMCs was evaluated using a pin-on-
Keywords:
disc wear apparatus. The effect of TiB2 particulate content (0, 3, 6 and 9 wt%) and
Aluminum alloy
temperature (30, 60, 90, 120, 150, 180, 210 and 240 1C) on wear rate and worn surface of the AMCs were studied. The results indicated that TiB2 particles were effective to enhance the
TiB2 Wear rate Worn surface
wear resistance of the AMCs at all test temperatures studied in this work. The wear rate of the AMCs increased when the applied temperature was increased. The in situ formed TiB2 particles pushed the transition wear temperature by another 30 1C. The wear mode was observed to be abrasive at room temperature and metal flow at high temperature. & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Aluminum alloys are widely used in automobile industries as components of internal combustion engines such as cylinder blocks, cylinder heads and pistons due to light weight, high strength to weight ratio, high thermal conductivity and good corrosion resistance which meets the current demand for fuel efficiency. The applications of aluminum alloys are limited because of poor wear resistance at room and elevated
temperatures [1–3]. It was reported by several investigators that the incorporation of hard ceramic particles including SiC [4], Al2O3 [5], TiC [6], B4C [7], TiB2 [8] and ZrB2 [9] into the aluminum alloys enhanced the wear resistance. Aluminum alloys reinforced with ceramic particles are universally termed as aluminum matrix composites (AMCs). The mechanical and tribological properties of AMCs are influenced by several factors namely the type, size, shape, mass or volume content and spatial dispersion of ceramic
n
Corresponding author. Tel.: +91 9245664761; fax: +91 4362 282471. E-mail addresses: [email protected] (H.B. Michael Rajan), [email protected] (S. Ramabalan), [email protected] (I. Dinaharan), [email protected] (S.J. Vijay). 1644-9665/$ - see front matter & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.05.005
archives of civil and mechanical engineering 14 (2014) 72–79
particles in the aluminum matrix [10]. A clear interface without the presence of porosity and reaction products is required between aluminum matrix and ceramic particle to obtain higher properties. The method of preparation of AMCs has a crucial role to attain a clear interface and homogeneous distribution of ceramic particles [11,12]. Liquid method is often preferred to prepare AMCs owing to its simplicity, economy, easy of adaption and mass production [13]. Synthesizing the ceramic particles within the molten aluminum alloy is called as in situ method of preparing AMCs [14]. It was observed by many investigators that the in situ prepared AMCs were characterized with a clear interface and homogeneous distribution of ceramic particles [15–19]. Literatures reporting on high temperature wear behavior of AMCs are limited compared to the room temperature wear behavior. Some studies on high temperature wear behavior of AMCs were reported in the literatures [20–29]. Hui et al. [20] reported that the incorporation Al2O3 particles into the Al–12Si aluminum alloy enhanced the wear resistance over the entire range of test temperatures during sliding wear. Natarajan et al. [21] showed that the in situ formed TiB2 particles were effective to improve the wear resistance of aluminum alloy AA6063 at elevated temperatures. Kumar et al. [22] observed that the transition temperature to attain severe wear of Al–7Si/TiB2 AMC reached higher and higher when the content of in situ TiB2 particles was increased. Kumar et al. [23] recorded that the addition of in situ TiB2 particles strongly influenced the wear mode from mild to severe wear of Al–4Cu/TiB2 AMC at high temperatures. Jerome et al. [24] noted that the wear rate of Al/TiC AMC increased linearly with the increase in the applied normal load for all temperature ranges employed under his experimental conditions. Rajaram et al. [25] found that the incorporation of graphite particles reduced the wear rate of Al–Si/ graphite AMC at high temperatures due to the formation of glazing layer and oxide layer. Rio et al. [26] determined the critical velocity and temperature for transition between mild and severe wear of AA8090/SiC AMC. Kumar et al. [27] noticed that the reinforcement of fly ash particles pushed up the transition temperature of AA6061/fly ash AMC due to the formation of protective transfer layers of comminuted reinforcing particulates and transferred steel debris from sliding counterface. Zhu et al. [28] studied the high temperature sliding wear behavior of Al/(Al2O3+Al3Zr) AMCs in detail. Zhu et al. [29] concluded that the abrasive wear, oxidation wear and adhesive wear are the main modes in the complex wear mechanisms of Al/(Al2O3+Al3Zr) AMCs at elevated temperatures. In this work, an attempt is made to study the effect of TiB2 particulate content and temperature on wear rate and worn surface of AA7075/TiB2 AMCs prepared by the in situ reaction of K2TiF6 and KBF4 inorganic salts to molten aluminum. Aluminum alloy AA7075 (Al–Zn–Mg–Cu) is one of the strongest aluminum alloys in industrial use today due to high strength to weight ratio and natural aging characteristics. The alloy derives its strength from precipitation of Mg2Zn and Al2CuMg phases [30]. Those precipitates become unstable and begin to dissolve in the matrix when the operating temperature rises above certain limit. Hence, it is essential to evaluate the sliding wear behavior of AA7075/TiB2 AMC at high temperature.
2.
73
Material and methods
Aluminum alloy AA7075 reinforced TiB2 particulate composite was fabricated by the in situ reaction of inorganic salts K2TiF6 and KBF4 with molten aluminum. Castings were obtained with various amounts (0, 3, 6 and 9 wt%) of TiB2. A detailed fabrication procedure is available in existing literatures [31]. Specimens were prepared from the castings to carry out microstructural characterization. The specimens were polished using standard metallographic technique and etched with Keller's reagent. The etched specimens were observed using a scanning electron microscope (JEOL-JSM6390). X-ray diffraction patterns (XRD) were recorded using Panalytical x-ray diffractometer. Rectangular shape specimens of size 6 mm 6 mm 30 mm were obtained from the castings having various amounts of TiB2. The wear rate was measured using a pin-on-disc wear apparatus (DUCOM TR20-LE) as shown in Fig. 1 according to ASTM G99-04 standard. The polished surface of the pin was slid on a hardened chromium steel disc (AISI 52100). The tests were carried out at various temperatures (30, 60, 90, 120, 150, 180, 210 and 240 1C) at a sliding velocity of 1.2 m/s, sliding distance of 1000 m and normal load of 20 N. These experimental conditions were selected based on several trial tests such that the height loss of the specimen will not exceed the limit of the wear tester. A computer-aided data acquisition system was used to monitor the loss of height. The volumetric loss was computed by multiplying the cross section of the test pin with its loss of height. Atleast two tests were carried out for each set of parameter to get a representative data. Worn surface of selected specimens was observed using scanning electron microscope.
3.
Results and discussion
3.1.
X-ray diffraction analysis of AA7075/TiB2 AMCs
The XRD patterns of the prepared AMCs are shown in Fig. 2. The diffraction peaks of in situ formed TiB2 particles are clearly seen and the intensity of the peaks increases as TiB2 content is increased. TiB2 particles are formed as a result of in situ reactions as given in the following equations. The peaks of aluminum in the AMCs as shown in Fig. 2 are slightly shifted to higher 2θ compared to that of aluminum due to the formation of TiB2 phase in the aluminum matrix. It is also observed in Fig. 2 that
Fig. 1 – Pin-on-disc experimental setup.
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archives of civil and mechanical engineering 14 (2014) 72–79
between the TiB2 particles and the matrix is clean and TiB2 particles are well bonded to aluminum matrix. The presence of clear interface can be attributed to the thermodynamic stability of the TiB2 particles and the formation of particles within the melt itself. When a ceramic particle behaves thermodynamically unstable in the aluminum melt, it produces undesirable compounds at the particle to matrix interface [32,33]. A pure interface is needed to increase the load bearing capability of the AMCs. The increase in local melt temperature due to the in situ reaction causes good bonding between the TiB2 particle and the matrix. Fine size of TiB2 particles is seen in Fig. 3b and d. The average size of TiB2 particle was measured to be less than 2 mm. The high nucleation rate and sluggish growth kinetics of TiB2 particles result in fine size of particles. Fig. 2 – XRD patterns of AA7075/TiB2 in situ composites.
there is no formation of any other compounds except TiB2. This indicates that the in situ formed TiB2 particles are thermodynamically stable and the interface between aluminum alloy and TiB2 particles tends to be free. K2 TiF6 þ 13=3Al-Al3 Ti þ 4=3AlF3 þ 2KF
ð1Þ
2KBF4 þ 3Al-AlB2 þ 2AlF3 þ 2KF
ð2Þ
Al3 Ti þ AlB2 -TiB2 þ 4Al
ð3Þ
3.2.
Microstructure of AA7075/TiB2 AMCs
The SEM micrographs of the fabricated AMCs are presented in Fig. 3. Common casting defects such as porosity, shrinkages or slag inclusion are not seen in the micrographs which display the quality of castings. The in situ formed TiB2 particles are distributed nearly homogeneously (Fig. 3a and c) in the aluminum matrix. Such kind of particulate distribution is an essential requirement to achieve enhanced mechanical and tribological properties of the AMCs. The solidification process governs the distribution of TiB2 particles in the aluminum matrix. The density difference between the aluminum matrix and ceramic particle significantly contributes to the distribution of ceramic particles during solidification. When the density of ceramic particle is higher compared to matrix material, it starts to sink in the molten matrix. Suspension of ceramic particles for a long time in the molten aluminum is desired to obtain homogenous distribution. It is reported that if the density difference between matrix and ceramic particle is more than 2 g/cm3, the ceramic particle can suspend for a long time in the melt [31]. In the present work, the density difference between aluminum matrix and TiB2 particle is nearly 2 g/cm3. Therefore, the sinking rate of in situ formed TiB2 particles is negligible and TiB2 particles suspend in the melt for a long time. On the other hand, the wetting action between molten aluminum and TiB2 particles offers resistance to the free movement of TiB2 particles. The above said factors lead to a better distribution of TiB2 particles. The SEM micrographs of the fabricated AMCs at higher magnification are presented in Fig. 3b and d. The in situ formed TiB2 particles exhibit various shapes such as spherical, hexagonal and cubic. It is evident from Fig. 3b and d that the interface
3.3.
Sliding wear behavior of AA7075/TiB2 AMCs
The effect of in situ formed TiB2 particulate content and temperature on wear rate and worn surface of the fabricated AMCs are elaborated in the following sections.
3.3.1.
Effect of TiB2 particulate content
The effect of in situ formed TiB2 particulate content on wear rate of the AMCs is presented in Fig. 4. The value of standard deviation of wear rate results was around 6.84 10−5 mm3/m. The figure shows that when temperature is kept constant the wear rate decreases as a function of the amount of TiB2 in the AMCs. TiB2 particles are effective to enhance the wear resistance of the AMCs. The improvement in the wear resistance of AA7075/TiB2 AMCs by in situ formed TiB2 particles can be explained as follows. The interactions between dislocations and TiB2 particles resist the propagation of cracks during sliding wear. Strain fields are created around TiB2 particles due to the thermal mismatch between the aluminum alloy and TiB2 particle during solidification. Those strain fields offer resistance to the propagation of the cracks and subsequent material removal. The in situ formed TiB2 particles are defect free which preserve their integrity during sliding. The homogeneous distribution of TiB2 particles provides Orowan strengthening [34]. The clear interface and good bonding delay the detachment of particles from the aluminum matrix. Therefore, the wear resistance of the AMCs is enhanced by TiB2 particles. The grain refining action of TiB2 particles can further be considered to play a role in lowering the wear rate. The effect of TiB2 content on the morphology of worn surface of AA7075/TiB2 AMC at room temperature (30 1C) is presented in Fig. 5. The worn surfaces are covered with parallel grooves. The wear mode appears to be abrasive. The depth of the grooves and plastic deformation at the edges of the grooves reduces when TiB2 particulate content is increased. The worn surface of aluminum matrix (Fig. 5a) shows several cutting marks and delamination. The presence of TiB2 particles (Fig. 5b–d) resists the cutting action of counterface asperities and reduces the wear rate. The effect of TiB2 content on the worn surface of AA7075/TiB2 AMC at 240 1C is presented in Fig. 6. All the worn surfaces reveal a large amount of metal flow during sliding which indicates severe wear mode. The metal transfer during sliding between specimen and counterface is higher which corresponds to higher wear rate. The parallel groove features reduces considerably
archives of civil and mechanical engineering 14 (2014) 72–79
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Fig. 3 – SEM micrographs of AA7075/TiB2 in situ composites containing TiB2: (a) 6 wt%, (b) 6 wt%, (c) 9 wt% and (d) 9 wt%.
compared to worn surfaces at room temperature. The wear mode changes from abrasive wear to metal flow. It is further evident from the figure that the depth of subsurface deformation reduces when TiB2 particulate content is increased. TiB2 particles offer resistance to metal flow at high temperature.
3.3.2.
Fig. 4 – Wear rate of AA7075/TiB2 in situ composites as a function of TiB2 content.
Effect of temperature
The effect of temperature on wear rate of the AMCs is presented in Fig. 7. The figure shows that for a constant amount of TiB2 in the AMCs, the wear rate increases as a function of temperature. It is evident from this figure that temperature has significantly influenced the wear rate of AA7075/TiB2 AMCs. This can be attributed to the following causes. The sliding wear is initiated when the relatively moving counter surfaces make asperity-to-asperity contact. The counter face chromium steel disc (hardness¼63 HRC) is relatively harder than the composite specimen. The initial stage of wear consists of fragmentation of the asperities and removal of material due to cutting of hard asperities into the
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Fig. 5 – SEM micrographs of worn surface of AA7075/TiB2 in situ composites at room temperature: (a) 0 wt%, (b) 0 wt%, (c) 3 wt %, (d) 3 wt%, (e) 6 wt%, (g) 6 wt%, (g) 9 wt% and (h) 9 wt%.
softer pin surface. When temperature increases, it leads to softening of aluminum matrix. This results in enhanced penetration of the hard asperities of the counter surface into the pin which causes more metal removal. The effect is well pronounced when the temperature is further increased leading to higher wear rate. The thermal mismatch between the aluminum matrix and TiB2 particle can be considered to play a role in sliding wear. The thermal mismatch creates an interface stress. If the interface stress exceeds the interfacial bond
strength between aluminum matrix and TiB2 particle, cracks will form and TiB2 particles will be pulled off. The rise in temperature boosts interface stress. Hence, more TiB2 particles are pulled off resulting in increased wear rate. It is further evident from Fig. 7 that there is a transition in the wear rate of AA7075/TiB2 AMCs from mild to severe wear. Some investigators observed such transition in their works [20,22,23,26]. The transition wear temperature of AA7075 and AA7075/3 wt% TiB2 AMC is 180 1C. The unreinforced aluminum alloy and AA7075/
archives of civil and mechanical engineering 14 (2014) 72–79
77
Fig. 6 – SEM micrographs of worn surface of AA7075/TiB2 in situ composites at temperature of 240 1C: (a) 0 wt%, (b) 3 wt%, (c) 6 wt% and (d) 9 wt%.
The effect of temperature on the worn surface of AA7075/ 6 wt% TiB2 AMC is presented in Fig. 8. The worn surfaces are covered with compacted debris. The worn surface at room temperature (Fig. 8a) reveals plowing, delamination and parallel grooves running through the worn surface. The worn surfaces (Fig. 8a–d) appear similar till the temperature of the specimen reaches 180 1C. As discussed above, TiB2 particles provide dimensional stability to the aluminum matrix which preserves the AMC during sliding wear. But the worn surface at 240 1C (Fig. 8e) shows large amount of metal flow which indicates severe wear. The removed metal is deposited on the counterface.
4. Fig. 7 – Wear rate of AA7075/TiB2 in situ composites as a function of temperature.
3 wt% TiB2 AMC behaved in a similar manner. The transition wear temperature of AA7075/6 wt% TiB2 AMC and AA7075/9 wt % TiB2 AMC is 210 1C. The increase in TiB2 particulate content pushed the transition wear temperature to 30 1C further under the experimental conditions. This can be attributed to the better dimensional stability of the aluminum matrix due to the presence of hard TiB2 particles. During mild wear, the pin is oxidized and delaminated to form a mechanical mixed layer and slides without any bulk metal transfer. Whereas in severe wear, the bulk metal is transferred from the pin to the steel counter face during sliding [23].
Conclusion
In the present work, AA7075/TiB2 AMCs were prepared by the in situ reaction of K2TiF6 and KBF4 inorganic salts to molten aluminum and the effect of TiB2 particulate content and temperature on wear rate and worn surface of the prepared AMCs were investigated. The results can be summarized as follows:
The XRD patterns of the prepared AMCs showed the
formation of in situ TiB2 particles without the presence of any other deleterious intermetallic compounds. The SEM micrographs revealed a homogeneous distribution of TiB2 particles in the aluminum matrix. The interface between the TiB2 particles and the matrix was clean without the presence of pores and inclusions. TiB2 particles were well bonded to the aluminum matrix
The high content of TiB2 particles in AMCs lead to high wear resistance for a constant temperature.
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Fig. 8 – SEM micrographs of worn surface of AA7075/6 wt%TiB2 in situ composite at various temperatures: (a) 30 1C, (b) 60 1C, (c) 120 1C, (d) 180 1C and (e) 240 1C.
TiB2 particles reduced cutting and plastic deformation of
the AMCs at room temperature (30 1C). The resistance to wear of AMCs was lower at high temperature for a constant amount of TiB2 particles. The increase in TiB2 particulate content pushed the transition wear temperature from 180 1C to 210 1C under the experimental conditions. TiB2 particles offered resistance to metal flow and reduced subsurface deformation of the AMCs at high temperature (240 1C). The applied temperature significantly influenced the wear mode. The wear mode was observed to be abrasive at room temperature and metal flow at high temperature.
Acknowledgment The authors are grateful to the Centre for Research in Metallurgy, School of Mechanical Sciences, Karunya University, Coimbatore, India for providing the facilities. One of the
authors, Dr. I. Dinaharan acknowledges the Department of Science and Technology, Govt. of India for providing INSPIRE fellowship. The authors are also thankful to Mr. T. Mukilan, Mr. T. A. S. Nirmal, Mr. U. Sethusudhan, Mr. A. Raja (Central Research Facilities, Karunya University) and Mr. I. Devamanoharan for their assistance offered to execute the above work.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Effect of TiB2 content and temperature on sliding wear behavior of AA7075/TiB2 in situ aluminum cast composites H.B. Michael Rajana,n, S. Ramabalanb, I. Dinaharanc, S.J. Vijayd a
Department of Mechanical Engineering, St. Joseph's College of Engineering and Technology, Thanjavur 613403, Tamil Nadu, India b EGS Pillay Engineering College, Nagapattinam 611002, Tamil Nadu, India c Department of Mechanical Engineering, V V College of Engineering, Tisaiyanvilai 627657, Tamil Nadu, India d Centre for Research in Metallurgy (CRM), School of Mechanical Sciences, Karunya University, Coimbatore 641114, Tamil Nadu, India
ar t ic l e in f o
abs tra ct
Article history:
Aluminum alloy AA7075 reinforced TiB2 particulate composites were prepared by the
Received 12 December 2012
in situ reaction of K2TiF6 and KBF4 to molten aluminum. The prepared aluminum matrix
Accepted 19 May 2013
composites (AMCs) were characterized using X-ray diffraction and scanning electron
Available online 28 May 2013
microscopy (SEM). The sliding wear behavior of the AMCs was evaluated using a pin-on-
Keywords:
disc wear apparatus. The effect of TiB2 particulate content (0, 3, 6 and 9 wt%) and
Aluminum alloy
temperature (30, 60, 90, 120, 150, 180, 210 and 240 1C) on wear rate and worn surface of the AMCs were studied. The results indicated that TiB2 particles were effective to enhance the
TiB2 Wear rate Worn surface
wear resistance of the AMCs at all test temperatures studied in this work. The wear rate of the AMCs increased when the applied temperature was increased. The in situ formed TiB2 particles pushed the transition wear temperature by another 30 1C. The wear mode was observed to be abrasive at room temperature and metal flow at high temperature. & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Aluminum alloys are widely used in automobile industries as components of internal combustion engines such as cylinder blocks, cylinder heads and pistons due to light weight, high strength to weight ratio, high thermal conductivity and good corrosion resistance which meets the current demand for fuel efficiency. The applications of aluminum alloys are limited because of poor wear resistance at room and elevated
temperatures [1–3]. It was reported by several investigators that the incorporation of hard ceramic particles including SiC [4], Al2O3 [5], TiC [6], B4C [7], TiB2 [8] and ZrB2 [9] into the aluminum alloys enhanced the wear resistance. Aluminum alloys reinforced with ceramic particles are universally termed as aluminum matrix composites (AMCs). The mechanical and tribological properties of AMCs are influenced by several factors namely the type, size, shape, mass or volume content and spatial dispersion of ceramic
n
Corresponding author. Tel.: +91 9245664761; fax: +91 4362 282471. E-mail addresses: [email protected] (H.B. Michael Rajan), [email protected] (S. Ramabalan), [email protected] (I. Dinaharan), [email protected] (S.J. Vijay). 1644-9665/$ - see front matter & 2013 Politechnika Wroc"awska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.05.005
archives of civil and mechanical engineering 14 (2014) 72–79
particles in the aluminum matrix [10]. A clear interface without the presence of porosity and reaction products is required between aluminum matrix and ceramic particle to obtain higher properties. The method of preparation of AMCs has a crucial role to attain a clear interface and homogeneous distribution of ceramic particles [11,12]. Liquid method is often preferred to prepare AMCs owing to its simplicity, economy, easy of adaption and mass production [13]. Synthesizing the ceramic particles within the molten aluminum alloy is called as in situ method of preparing AMCs [14]. It was observed by many investigators that the in situ prepared AMCs were characterized with a clear interface and homogeneous distribution of ceramic particles [15–19]. Literatures reporting on high temperature wear behavior of AMCs are limited compared to the room temperature wear behavior. Some studies on high temperature wear behavior of AMCs were reported in the literatures [20–29]. Hui et al. [20] reported that the incorporation Al2O3 particles into the Al–12Si aluminum alloy enhanced the wear resistance over the entire range of test temperatures during sliding wear. Natarajan et al. [21] showed that the in situ formed TiB2 particles were effective to improve the wear resistance of aluminum alloy AA6063 at elevated temperatures. Kumar et al. [22] observed that the transition temperature to attain severe wear of Al–7Si/TiB2 AMC reached higher and higher when the content of in situ TiB2 particles was increased. Kumar et al. [23] recorded that the addition of in situ TiB2 particles strongly influenced the wear mode from mild to severe wear of Al–4Cu/TiB2 AMC at high temperatures. Jerome et al. [24] noted that the wear rate of Al/TiC AMC increased linearly with the increase in the applied normal load for all temperature ranges employed under his experimental conditions. Rajaram et al. [25] found that the incorporation of graphite particles reduced the wear rate of Al–Si/ graphite AMC at high temperatures due to the formation of glazing layer and oxide layer. Rio et al. [26] determined the critical velocity and temperature for transition between mild and severe wear of AA8090/SiC AMC. Kumar et al. [27] noticed that the reinforcement of fly ash particles pushed up the transition temperature of AA6061/fly ash AMC due to the formation of protective transfer layers of comminuted reinforcing particulates and transferred steel debris from sliding counterface. Zhu et al. [28] studied the high temperature sliding wear behavior of Al/(Al2O3+Al3Zr) AMCs in detail. Zhu et al. [29] concluded that the abrasive wear, oxidation wear and adhesive wear are the main modes in the complex wear mechanisms of Al/(Al2O3+Al3Zr) AMCs at elevated temperatures. In this work, an attempt is made to study the effect of TiB2 particulate content and temperature on wear rate and worn surface of AA7075/TiB2 AMCs prepared by the in situ reaction of K2TiF6 and KBF4 inorganic salts to molten aluminum. Aluminum alloy AA7075 (Al–Zn–Mg–Cu) is one of the strongest aluminum alloys in industrial use today due to high strength to weight ratio and natural aging characteristics. The alloy derives its strength from precipitation of Mg2Zn and Al2CuMg phases [30]. Those precipitates become unstable and begin to dissolve in the matrix when the operating temperature rises above certain limit. Hence, it is essential to evaluate the sliding wear behavior of AA7075/TiB2 AMC at high temperature.
2.
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Material and methods
Aluminum alloy AA7075 reinforced TiB2 particulate composite was fabricated by the in situ reaction of inorganic salts K2TiF6 and KBF4 with molten aluminum. Castings were obtained with various amounts (0, 3, 6 and 9 wt%) of TiB2. A detailed fabrication procedure is available in existing literatures [31]. Specimens were prepared from the castings to carry out microstructural characterization. The specimens were polished using standard metallographic technique and etched with Keller's reagent. The etched specimens were observed using a scanning electron microscope (JEOL-JSM6390). X-ray diffraction patterns (XRD) were recorded using Panalytical x-ray diffractometer. Rectangular shape specimens of size 6 mm 6 mm 30 mm were obtained from the castings having various amounts of TiB2. The wear rate was measured using a pin-on-disc wear apparatus (DUCOM TR20-LE) as shown in Fig. 1 according to ASTM G99-04 standard. The polished surface of the pin was slid on a hardened chromium steel disc (AISI 52100). The tests were carried out at various temperatures (30, 60, 90, 120, 150, 180, 210 and 240 1C) at a sliding velocity of 1.2 m/s, sliding distance of 1000 m and normal load of 20 N. These experimental conditions were selected based on several trial tests such that the height loss of the specimen will not exceed the limit of the wear tester. A computer-aided data acquisition system was used to monitor the loss of height. The volumetric loss was computed by multiplying the cross section of the test pin with its loss of height. Atleast two tests were carried out for each set of parameter to get a representative data. Worn surface of selected specimens was observed using scanning electron microscope.
3.
Results and discussion
3.1.
X-ray diffraction analysis of AA7075/TiB2 AMCs
The XRD patterns of the prepared AMCs are shown in Fig. 2. The diffraction peaks of in situ formed TiB2 particles are clearly seen and the intensity of the peaks increases as TiB2 content is increased. TiB2 particles are formed as a result of in situ reactions as given in the following equations. The peaks of aluminum in the AMCs as shown in Fig. 2 are slightly shifted to higher 2θ compared to that of aluminum due to the formation of TiB2 phase in the aluminum matrix. It is also observed in Fig. 2 that
Fig. 1 – Pin-on-disc experimental setup.
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between the TiB2 particles and the matrix is clean and TiB2 particles are well bonded to aluminum matrix. The presence of clear interface can be attributed to the thermodynamic stability of the TiB2 particles and the formation of particles within the melt itself. When a ceramic particle behaves thermodynamically unstable in the aluminum melt, it produces undesirable compounds at the particle to matrix interface [32,33]. A pure interface is needed to increase the load bearing capability of the AMCs. The increase in local melt temperature due to the in situ reaction causes good bonding between the TiB2 particle and the matrix. Fine size of TiB2 particles is seen in Fig. 3b and d. The average size of TiB2 particle was measured to be less than 2 mm. The high nucleation rate and sluggish growth kinetics of TiB2 particles result in fine size of particles. Fig. 2 – XRD patterns of AA7075/TiB2 in situ composites.
there is no formation of any other compounds except TiB2. This indicates that the in situ formed TiB2 particles are thermodynamically stable and the interface between aluminum alloy and TiB2 particles tends to be free. K2 TiF6 þ 13=3Al-Al3 Ti þ 4=3AlF3 þ 2KF
ð1Þ
2KBF4 þ 3Al-AlB2 þ 2AlF3 þ 2KF
ð2Þ
Al3 Ti þ AlB2 -TiB2 þ 4Al
ð3Þ
3.2.
Microstructure of AA7075/TiB2 AMCs
The SEM micrographs of the fabricated AMCs are presented in Fig. 3. Common casting defects such as porosity, shrinkages or slag inclusion are not seen in the micrographs which display the quality of castings. The in situ formed TiB2 particles are distributed nearly homogeneously (Fig. 3a and c) in the aluminum matrix. Such kind of particulate distribution is an essential requirement to achieve enhanced mechanical and tribological properties of the AMCs. The solidification process governs the distribution of TiB2 particles in the aluminum matrix. The density difference between the aluminum matrix and ceramic particle significantly contributes to the distribution of ceramic particles during solidification. When the density of ceramic particle is higher compared to matrix material, it starts to sink in the molten matrix. Suspension of ceramic particles for a long time in the molten aluminum is desired to obtain homogenous distribution. It is reported that if the density difference between matrix and ceramic particle is more than 2 g/cm3, the ceramic particle can suspend for a long time in the melt [31]. In the present work, the density difference between aluminum matrix and TiB2 particle is nearly 2 g/cm3. Therefore, the sinking rate of in situ formed TiB2 particles is negligible and TiB2 particles suspend in the melt for a long time. On the other hand, the wetting action between molten aluminum and TiB2 particles offers resistance to the free movement of TiB2 particles. The above said factors lead to a better distribution of TiB2 particles. The SEM micrographs of the fabricated AMCs at higher magnification are presented in Fig. 3b and d. The in situ formed TiB2 particles exhibit various shapes such as spherical, hexagonal and cubic. It is evident from Fig. 3b and d that the interface
3.3.
Sliding wear behavior of AA7075/TiB2 AMCs
The effect of in situ formed TiB2 particulate content and temperature on wear rate and worn surface of the fabricated AMCs are elaborated in the following sections.
3.3.1.
Effect of TiB2 particulate content
The effect of in situ formed TiB2 particulate content on wear rate of the AMCs is presented in Fig. 4. The value of standard deviation of wear rate results was around 6.84 10−5 mm3/m. The figure shows that when temperature is kept constant the wear rate decreases as a function of the amount of TiB2 in the AMCs. TiB2 particles are effective to enhance the wear resistance of the AMCs. The improvement in the wear resistance of AA7075/TiB2 AMCs by in situ formed TiB2 particles can be explained as follows. The interactions between dislocations and TiB2 particles resist the propagation of cracks during sliding wear. Strain fields are created around TiB2 particles due to the thermal mismatch between the aluminum alloy and TiB2 particle during solidification. Those strain fields offer resistance to the propagation of the cracks and subsequent material removal. The in situ formed TiB2 particles are defect free which preserve their integrity during sliding. The homogeneous distribution of TiB2 particles provides Orowan strengthening [34]. The clear interface and good bonding delay the detachment of particles from the aluminum matrix. Therefore, the wear resistance of the AMCs is enhanced by TiB2 particles. The grain refining action of TiB2 particles can further be considered to play a role in lowering the wear rate. The effect of TiB2 content on the morphology of worn surface of AA7075/TiB2 AMC at room temperature (30 1C) is presented in Fig. 5. The worn surfaces are covered with parallel grooves. The wear mode appears to be abrasive. The depth of the grooves and plastic deformation at the edges of the grooves reduces when TiB2 particulate content is increased. The worn surface of aluminum matrix (Fig. 5a) shows several cutting marks and delamination. The presence of TiB2 particles (Fig. 5b–d) resists the cutting action of counterface asperities and reduces the wear rate. The effect of TiB2 content on the worn surface of AA7075/TiB2 AMC at 240 1C is presented in Fig. 6. All the worn surfaces reveal a large amount of metal flow during sliding which indicates severe wear mode. The metal transfer during sliding between specimen and counterface is higher which corresponds to higher wear rate. The parallel groove features reduces considerably
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Fig. 3 – SEM micrographs of AA7075/TiB2 in situ composites containing TiB2: (a) 6 wt%, (b) 6 wt%, (c) 9 wt% and (d) 9 wt%.
compared to worn surfaces at room temperature. The wear mode changes from abrasive wear to metal flow. It is further evident from the figure that the depth of subsurface deformation reduces when TiB2 particulate content is increased. TiB2 particles offer resistance to metal flow at high temperature.
3.3.2.
Fig. 4 – Wear rate of AA7075/TiB2 in situ composites as a function of TiB2 content.
Effect of temperature
The effect of temperature on wear rate of the AMCs is presented in Fig. 7. The figure shows that for a constant amount of TiB2 in the AMCs, the wear rate increases as a function of temperature. It is evident from this figure that temperature has significantly influenced the wear rate of AA7075/TiB2 AMCs. This can be attributed to the following causes. The sliding wear is initiated when the relatively moving counter surfaces make asperity-to-asperity contact. The counter face chromium steel disc (hardness¼63 HRC) is relatively harder than the composite specimen. The initial stage of wear consists of fragmentation of the asperities and removal of material due to cutting of hard asperities into the
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Fig. 5 – SEM micrographs of worn surface of AA7075/TiB2 in situ composites at room temperature: (a) 0 wt%, (b) 0 wt%, (c) 3 wt %, (d) 3 wt%, (e) 6 wt%, (g) 6 wt%, (g) 9 wt% and (h) 9 wt%.
softer pin surface. When temperature increases, it leads to softening of aluminum matrix. This results in enhanced penetration of the hard asperities of the counter surface into the pin which causes more metal removal. The effect is well pronounced when the temperature is further increased leading to higher wear rate. The thermal mismatch between the aluminum matrix and TiB2 particle can be considered to play a role in sliding wear. The thermal mismatch creates an interface stress. If the interface stress exceeds the interfacial bond
strength between aluminum matrix and TiB2 particle, cracks will form and TiB2 particles will be pulled off. The rise in temperature boosts interface stress. Hence, more TiB2 particles are pulled off resulting in increased wear rate. It is further evident from Fig. 7 that there is a transition in the wear rate of AA7075/TiB2 AMCs from mild to severe wear. Some investigators observed such transition in their works [20,22,23,26]. The transition wear temperature of AA7075 and AA7075/3 wt% TiB2 AMC is 180 1C. The unreinforced aluminum alloy and AA7075/
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Fig. 6 – SEM micrographs of worn surface of AA7075/TiB2 in situ composites at temperature of 240 1C: (a) 0 wt%, (b) 3 wt%, (c) 6 wt% and (d) 9 wt%.
The effect of temperature on the worn surface of AA7075/ 6 wt% TiB2 AMC is presented in Fig. 8. The worn surfaces are covered with compacted debris. The worn surface at room temperature (Fig. 8a) reveals plowing, delamination and parallel grooves running through the worn surface. The worn surfaces (Fig. 8a–d) appear similar till the temperature of the specimen reaches 180 1C. As discussed above, TiB2 particles provide dimensional stability to the aluminum matrix which preserves the AMC during sliding wear. But the worn surface at 240 1C (Fig. 8e) shows large amount of metal flow which indicates severe wear. The removed metal is deposited on the counterface.
4. Fig. 7 – Wear rate of AA7075/TiB2 in situ composites as a function of temperature.
3 wt% TiB2 AMC behaved in a similar manner. The transition wear temperature of AA7075/6 wt% TiB2 AMC and AA7075/9 wt % TiB2 AMC is 210 1C. The increase in TiB2 particulate content pushed the transition wear temperature to 30 1C further under the experimental conditions. This can be attributed to the better dimensional stability of the aluminum matrix due to the presence of hard TiB2 particles. During mild wear, the pin is oxidized and delaminated to form a mechanical mixed layer and slides without any bulk metal transfer. Whereas in severe wear, the bulk metal is transferred from the pin to the steel counter face during sliding [23].
Conclusion
In the present work, AA7075/TiB2 AMCs were prepared by the in situ reaction of K2TiF6 and KBF4 inorganic salts to molten aluminum and the effect of TiB2 particulate content and temperature on wear rate and worn surface of the prepared AMCs were investigated. The results can be summarized as follows:
The XRD patterns of the prepared AMCs showed the
formation of in situ TiB2 particles without the presence of any other deleterious intermetallic compounds. The SEM micrographs revealed a homogeneous distribution of TiB2 particles in the aluminum matrix. The interface between the TiB2 particles and the matrix was clean without the presence of pores and inclusions. TiB2 particles were well bonded to the aluminum matrix
The high content of TiB2 particles in AMCs lead to high wear resistance for a constant temperature.
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Fig. 8 – SEM micrographs of worn surface of AA7075/6 wt%TiB2 in situ composite at various temperatures: (a) 30 1C, (b) 60 1C, (c) 120 1C, (d) 180 1C and (e) 240 1C.
TiB2 particles reduced cutting and plastic deformation of
the AMCs at room temperature (30 1C). The resistance to wear of AMCs was lower at high temperature for a constant amount of TiB2 particles. The increase in TiB2 particulate content pushed the transition wear temperature from 180 1C to 210 1C under the experimental conditions. TiB2 particles offered resistance to metal flow and reduced subsurface deformation of the AMCs at high temperature (240 1C). The applied temperature significantly influenced the wear mode. The wear mode was observed to be abrasive at room temperature and metal flow at high temperature.
Acknowledgment The authors are grateful to the Centre for Research in Metallurgy, School of Mechanical Sciences, Karunya University, Coimbatore, India for providing the facilities. One of the
authors, Dr. I. Dinaharan acknowledges the Department of Science and Technology, Govt. of India for providing INSPIRE fellowship. The authors are also thankful to Mr. T. Mukilan, Mr. T. A. S. Nirmal, Mr. U. Sethusudhan, Mr. A. Raja (Central Research Facilities, Karunya University) and Mr. I. Devamanoharan for their assistance offered to execute the above work.
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