Microstructures and mechanical properties of AA6061–SiC composites prepared through spark plasma sintering and hot rolling

Microstructures and mechanical properties of AA6061–SiC composites prepared through spark plasma sintering and hot rolling

Materials Science & Engineering A 650 (2016) 139–144 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

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Materials Science & Engineering A 650 (2016) 139–144

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Microstructures and mechanical properties of AA6061–SiC composites prepared through spark plasma sintering and hot rolling X.P. Li a, C.Y. Liu b,n, M.Z. Ma a, R.P. Liu a,n a

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Key Laboratory of New Processing Technology for Nonferrous Metal & Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, China

b

art ic l e i nf o

a b s t r a c t

Article history: Received 26 August 2015 Received in revised form 3 October 2015 Accepted 5 October 2015 Available online 13 October 2015

In this study, AA6061–SiC composites were synthesized through spark plasma sintering and hot rolling. The microstructures of these composites exhibited excellent SiC particle distribution in the AA6061 matrices, high dislocation density, and ultrafine Al matrix grain. The AA6061–SiC composites strengthened with increasing amounts of SiC particles. The yield strength, ultimate tensile strength, and elasticity modulus of the composites were 373 MPa, 414 MPa, and 95 GPa, respectively, when the volume fraction of SiC was 20%. The composites exhibited dislocation, grain boundary, and secondary phase strengthening. The ductility of the composites decreased with increasing amounts of SiC particles, and the fracture mode was ductile. & 2015 Elsevier B.V. All rights reserved.

Keywords: AA6061–SiC composite Spark plasma sintering Rolling Microstructure Mechanical properties

1. Introduction Aluminum-based metal matrix composites (AMMCs) have attracted considerable attention because of their light weight, high strength, high stiffness, and wear resistance [1–3]. Powder metallurgy (PM), which involves mixing reinforcing phases and Al powder followed by sintering, produces AMMCs with a uniform distribution of reinforcements and superior properties because the fabrication process is not affected by the wettability of the reinforcements and Al [4,5]. Spark plasma sintering (SPS), which is characterized by rapid heating and cooling during sintering, has received considerable attention over the past few years because of its notable advantages over conventional PM techniques. In specific, SPS can maintain the nano- and submicron-structures of powder-based materials after consolidation, create a strong adhesion between the reinforcement and the matrix, and fabricate fully dense materials [6–8]. To date, SPS has been successfully used to fabricate several AMMCs, including Al–SiC [8–12], Al–carbon nanotube [12,13], Al–Al2O3 [14], Al–diamond [15], Al–ZrB2 [16], Al–metallic glass [17], Al–B4C [18], Al–SiO2 [19], and Al-intermetallic composites [20–23]. Plastic forming through forging, extrusion, or rolling is usually performed to design AMMCs with improved mechanical properties, such as strength and ductility [24]. However, the microstructures n

Corresponding authors. Fax: þ 86 335 8074545. E-mail addresses: [email protected] (C.Y. Liu), [email protected] (R.P. Liu).

http://dx.doi.org/10.1016/j.msea.2015.10.015 0921-5093/& 2015 Elsevier B.V. All rights reserved.

and mechanical properties of AMMCs prepared through SPS–plastic forming have yet to be systematically investigated. Accordingly, we conducted a systematic investigation of the microstructures and mechanical properties of AMMCs prepared through SPS and hot rolling. AA6061–SiC composites with different contents of SiC were prepared through milling and SPS. The composites were used as raw materials, and their microstructures and mechanical properties after hot rolling were investigated. The strengthening mechanism and fracture mode of the AA6061–SiC composites were also analyzed.

2. Experimental AA6061 alloy (Al–0.8 Mg, 0.4 Si, 0.1 Mn, 0.1 Cu, 0.2 Zn, 0.06 Cr, 0.2 Ti, 0.7 Fe) particles 5–60 μm in size and SiC particles 100 nm– 2 μm in size were used as raw materials. Fig. 1 shows SEM images of the AA6061 and SiC particles used in this study. Matrix powders containing various SiC volume fractions (0%, 5%, 10%, 15%, and 20%) were mixed with a planetary mill in a WC vial. Milling was performed at 300 rpm for 2 h by using WC balls. The weight ratio of balls to powder was fixed to 5:1, and the vial was filled with argon during milling. SPS was performed in an SPS apparatus (SPS-3.20MK-IV). The compaction pressure was 50 MPa, and the heating rate was 50 °C/ min. The sintering temperature and time were set at 560 °C and 3 min, respectively. After SPS, AA6061–SiC composites 30 mm in

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Fig. 1. SEM image of as-received (a) AA6061 and (b) SiC particles.

Fig. 2. Schematic of the production of AA6061–SiC composites.

Fig. 3. SEM image of AA6061–10 vol%SiC powders after 2 h of high-energy ball milling: (a) low magnification and (b) high magnification.

The microstructures of the samples were observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM specimens were prepared via ion milling. Tensile test samples were machined with the tensile axis parallel to the rolling direction. Tensile tests were performed at a strain rate of 5  10  4 s  1 by using an Instron-5982-type test machine. The tests were repeated thrice to confirm the reproducibility and reliability of the results. Fracture surfaces after the tensile tests were also observed via SEM to determine the failure mode.

3. Results and discussion Fig. 4. As-SPSed sample and as-rolled AA6061–SiC composites with various SiC volume fractions.

diameter and 7 mm in height were obtained. Fig. 4 shows an image of an as-SPSed sample. The as-SPSed samples were heated in a furnace at 500 °C for 1 h and then rolled from 7 mm to 1 mm in 20 passes (approximately 85% thickness reduction in total). In every rolling pass, the samples were reheated in a furnace at 500 °C for 1 min. The diameter of the rolling mill was 300 mm, and the rolling speed was 0.2 m/s. Fig. 2 shows a schematic of the production process.

Fig. 3 shows SEM images of the AA6061–10 vol%SiC powder mixture after 2 h of high-energy ball milling. The low-magnification image shows that the AA6061 powder was slightly finer than the unmilled AA6061 powder (Fig. 3(a)). Mixtures with uniform distributions of nano-sized SiC powder on the surface of submicron-sized AA6061 powder were obtained as shown in Fig. 3(b). Fig. 4 shows an image of the as-SPSed samples before and after hot rolling. At a low SiC content, the as-SPSed samples showed a satisfactory forming ability under hot rolling conditions. No considerable edge cracking was observed on the as-rolled AA6061– 5 vol%SiC composites. The number of edge cracks increased as the amounts of SiC particles increased, and some edge cracks appeared on the as-rolled AA6061–20 vol%SiC composite.

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Fig. 5. SEM obtained in BSE imaging mode of (a) as-SPSed AA6061–10 vol%SiC, (b) as-SPSed AA6061–20 vol%SiC, (c) as-rolled AA6061–10 vol%SiC, and (d) as-rolled AA6061– 20 vol%SiC composites.

Fig. 5(a) and (b) shows back-scattered SEM (BSE) images of the as-SPSed AA6061–10 vol%SiC and AA6061–20 vol%SiC composites, respectively. Gray and black regions corresponded to Al and SiC, respectively. The as-SPSed composites appeared as heterogeneous materials consisting of SiC agglomerates separating Al grains. The secondary phase layer along the Al grain boundaries reportedly exhibits weak bonding with a metallic matrix, which may cause poor mechanical properties [25]. Several microvoids also appeared in the Al matrix. Numerous SiC particles were drawn out during sample polishing; hence, some cavities were retained on the sample surface. This finding also indicates the weak bonding between the SiC particles and the Al matrix in the as-SPSed samples. Fig. 5(c) and (d) shows BSE images of the as-rolled AA6061– 10 vol% SiC and AA6061–20 vol% SiC composites, respectively. SiC layers fractured and separated in the AA6061–SiC composites after hot rolling, and SiC fragments became uniformly embedded in the Al matrix. Almost no particle-free zones were found in these samples. Al particles were elongated with respect to the rolling direction, and continuous AA6061 matrices were formed. Fig. 6(a) shows that the microstructure of as-rolled AA6061 was characterized by elongated grains and high dislocation density. Fig. 6(b) shows that most grain boundaries of the AA6061–20 vol% SiC composite were indistinct because of the presence of numerous highly entangled dislocations. Fig. 6(c) shows the selected area diffraction (SAD) pattern of ‘A’ area in Fig. 6(b). The SAD revealed the ‘A’ particle is Al grain. The size of this Al grain is approximately 500 nm which is significantly less than that of as-rolled AA6061. Furthermore, ultrafine (less than 300 nm in mean size) Al grains were also obtained in the AA6061–20 vol% SiC composite as shown in Fig. 6(b). The SAD patterns of ‘B’ particle in [100], [310], and [210] zone axes are shown in Fig. 6(d), (e) and (f), respectively. The SAD results revealed that the ‘B’ particle is SiC. Additional dislocations were observed near the Al matrix–SiC interfaces as shown in Fig.6(b).

The stress–strain curves of the as-rolled AA6061–SiC composites with various SiC volume fractions are shown in Fig. 7. A schematic of the tensile test specimen is shown in the insert of Fig. 7. Fig. 8 shows the mechanical properties, including yield strength (YS), ultimate tensile strength (UTS), elongation (EL), and elasticity modulus (EM), of the as-rolled AA6061–SiC composites. Compared with the as-rolled AA6061, the as-rolled AA6061–SiC composites showed higher strength and lower strain. The YS, UTS, and EM of the as-rolled AA6061–SiC composites increased with increasing amounts of SiC particles, whereas the ductility of the composites showed an opposite trend. The strength of the particulate-reinforced MMCs was calculated as follows [26]:

⎡ Vf (S + 4) ⎤ σ MMC = σm ⎢ ⎥ + (1 − Vf ) ⎣ ⎦ 4

(1)

where sMMC and sm are the strengths of the MMC and the matrix, respectively, and Vf and S are the volume fraction and aspect ratio of the refined particulates, respectively. The S of the as-rolled AA6061–SiC composites was approximately 3 (Fig. 5(c) and (d)). Based on Formula (1), the 0.2% PS of the as-rolled AA6061–SiC composites with 5, 10, 15 and 20 vol% SiC are found to be 230, 239, 247 and 255 MPa, respectively. These values were considerably lower than those of the experimentally obtained composite. The calculation and experimental results indicate that SiC particles also affect the strength of the AA6061 matrices. Three key effects of SiC particles in the Al matrix were observed: (1) the large difference in thermal expansion between the Al matrix (23.86  10  6/K) and the SiC particles (3.8  10  6/K) results in thermal stress during hot working (500 °C rolling in this study) [27]; (2) during rolling, hard SiC particles impede the motion of the soft Al matrix, thereby increasing the dislocation density in the Al matrix near the matrix–reinforcement interfaces (Fig. 6(b)); and (3) during hot deformation, recrystallization grains easily coarsen. SiC particles can impede Al grain boundary

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Fig. 6. TEM of (a) as-rolled AA6061 and (b) as-rolled AA6061–20 vol%SiC composites. SAD patterns observed at (c) ‘A’, and (d), (e) and (f) ‘B’ particle in (b). GBs, grain boundaries; UFG, ultrafine grain.

movement and cause the generation of ultrafine grains after hot rolling. The effects of thermal stress, dislocation density, and number of ultrafine Al grains enhanced with increasing amount of SiC particles, and all of these effects strengthened the as-rolled AA6061–SiC composites. Thus, the UTS of the as-rolled AA6061– 20 vol%SiC composite was as high as 414 MPa. The as-rolled AA6061–SiC composites exhibited dislocation, grain boundary, and secondary phase strengthening.

SEM was performed to clarify the rupture mechanisms in the as-rolled AA6061 and as-rolled AA6061–SiC composites (Fig. 9). All of the as-rolled samples exhibited ductile fracture, with dimples and shear zones. The rupture model consists of a fracture that occurs through the formation and coalescence of microvoids in the entire sample [28]. The dimple in the as-rolled AA6061–20 vol%SiC composite (Fig. 9(c)) was shallower than those in the as-rolled AA6061 (Fig. 9(a)) and as-rolled AA6061–10 vol%SiC composites

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(Fig. 9(b)) possibly because of the smaller elongation of the asrolled AA6061–20 vol%SiC composite than the other samples.

4. Conclusions The microstructure, mechanical properties, strengthening mechanisms, and rupture mechanisms of AA6061–SiC composites prepared by SPS and hot rolling were studied. The following conclusions were drawn:

Fig. 7. Stress–strain curves of as-rolled AA6061–SiC composites with various SiC volume fractions.

(1) The as-SPSed AA6061–SiC composites appeared as heterogeneous materials consisting of SiC agglomerates separating Al grains. (2) The as-SPSed AA6061–SiC composites with low SiC contents showed a satisfactory forming ability under hot rolling at 500 °C, whereas the as-rolled AA6061–20 vol%SiC composite showed edge cracks. (3) SiC layers fractured and separated in the AA6061–SiC composites during rolling, and SiC particles were uniformly distributed in the continuous AA6061 matrix in the as-rolled AA6061–SiC composites. The as-rolled AA6061–SiC composites also exhibited ultrafine Al grains and a high dislocation density. (4) The tensile strength and elasticity modulus of the as-rolled AA6061–SiC composites increased with increasing amounts of SiC particles. The as-rolled AA6061–SiC composites demonstrated dislocation, grain boundary, and secondary phase strengthening. (5) The ductility property of the as-rolled AA6061–SiC composites decreased with increasing amounts of SiC particles, and the fracture mode was ductile.

Acknowledgements

Fig. 8. Mechanical properties of as-rolled AA6061–SiC composites with various SiC volume fractions.

This work was funded by the National Basic Research Program of China (No. 2013CB733000) and Guangxi Natural Science Foundation (No. 2015GXNSFBA139238).

Fig. 9. Fracture surfaces after the tensile test of (a) as-rolled AA6061, (b) as-rolled AA6061–10 vol%SiC, and (c) as-rolled AA6061–20 vol%SiC composites.

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