Gr composites through friction stir processing

Materials Letters 265 (2020) 127437

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Development of high strength and high electrical conductivity Cu/Gr composites through friction stir processing R. Bheekya Naik a, K. Venkateswara Reddy a, G. Madhusudhan Reddy b, R. Arockia Kumar a,⇑ a b

Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal 506004 India Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India

a r t i c l e

i n f o

Article history: Received 30 October 2019 Received in revised form 11 January 2020 Accepted 28 January 2020 Available online 29 January 2020 Keywords: Cu-Gr composite Electrical conductivity Hardness Surface composites

a b s t r a c t The Copper/Graphene (Cu/Gr) composite was prepared with single-pass friction stir processing. The composites were prepared by fixing the tool rotational speed and by varying the traverse speed. The stir-zone of processed specimens were characterized for its microstructure, hardness and electrical conductivity. Microstructural investigation showed that graphene has uniformly distributed in the Cu-matrix. The hardness increased by up to 40% after FSP. A minimal decrease in electrical conductivity was observed after friction processing. The study showed FSP is a promising method to improve the mechanical properties of Cu without much deterioration in electrical conductivity. Ó 2020 Elsevier B.V. All rights reserved.

1. Introduction Copper (Cu) has extensive applications in electrical and sliding contact parts such as electric brushes, electromagnetic gun rails pantograph sliders, and resistance spot-welding electrodes due to high electrical conductivity. However, Cu has poor resistance to abrasion and low hardness which leads to more wear and quick replacement of the components. To overcome these problems, several researchers have worked on reinforcing second-phase particles such as Zr2O3, Al2O3, TiB2 etc., and graphite nano-particles, carbon fibers etc. (carbon reinforcements), in to the Cu-matrix. In recent times graphene (Gr) has become a potential reinforcement owing to its high modulus, good thermal conductivity, and high electrical conductivity [1]. However, it is a challenging task to uniformly distribute Gr to utilize the promising properties of Gr [2]. The particle agglomeration and poor wettability with metals are problems encountered while developing composites through liquid metallurgy route [1,3]; whereas achieving proper mixing was reported as a challenge in powder metallurgy route [4]. Chen et al., [3] worked on plasma sintering to fabricate Gr-Cu composite and observed defects in the final composite. Friction stir processing (FSP), a modification of friction stir welding process, is a promising and efficient technique to modify the surface microstructures. Mishra et al. [5] produced surface composites on aluminum substrate and observed that process parameters such as, rotating speed, ⇑ Corresponding author. E-mail address: [email protected] (R. Arockia Kumar). https://doi.org/10.1016/j.matlet.2020.127437 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.

traverse speed, and tool profile have a remarkable outcome on fabrication of surface composite layer. Researchers across the globe produces composites such as Cu-SiC, Aluminum–Al2O3, AZ31– MWCNT, Cu-CNT and copper–graphite etc., using FSP [5–7]. In the present work, Cu/Gr composites were fabricated with FSP with an aim to improve mechanical properties without deteriorating the electrical conductivity of Cu. 2. Methods and materials The commercial pure Cu (99.9%, M/s Andhra Metals, Hyderabad) plate of dimensions 100  40  4 mm3 was chosen as a work-piece. The size of Gr reinforcement was 10–20 lm (ARCI, Hyderabad Fig. 1a). The structure of carbon was confirmed as graphene by Raman spectra (Fig. 1c). The holes of size /2mm and 3 mm depth were drilled on the copper plate and they were filled with graphene (Fig. 1a) to prepare Cu/Gr surface composite. The size and number of holes are enough to accommodate 8 vol% of graphene and can be calculated using the formula area of the holes/(pin length of tool  pin diameter). Initially, a pin-less tool was used to close the hole opening to prevent spilling of reinforcements [6]. The FSP tool (Fig. 1b) of shoulder /18 mm with straight cylindrical tool pin of /3 mm and pin length 6 mm was chosen for experiments. The tool rotational speed (900 rpm) kept constant and traverse speed varied from 50 to 200 mm/min. Specimens were extracted from the central region of FSP zone for all characterizations. The polished specimens were etched with a chemical solution containing 5 ml FeCl3, 2 ml HNO3 and 93 ml

2

R.B. Naik et al. / Materials Letters 265 (2020) 127437

intense stirring of the tool locally raised the temperature, thus enabled the plastic flow of the material. Moreover, the generated heat was also good enough to recrystallize the stir zone and it was evident from the microstructures. An increase in travel speed lowers the heat input to a region and shortens the time of exposure of that region to high temperatures [7]. This, in turn, reduces the time available for grain growth resulting in the reduction of grain size. Also, the presence of Gr acts as a preferential site for nucleating new grains further by pinning effect hinders grain growth, thereby assists grain refinement. Further, there was no segregation of particles and no formation of the intermetallic compound at the interfaces. It was reported in the literature that the improper selection of FSP parameters resulted in particle segregation [6]. Also, any reaction results in a decrease of bond strength between the reinforcement and matrix. However, the present study shows that the selected FSP process parameters were sufficient enough to uniformly distribute the Gr in the Cu matrix, further resulted in an interface with no reaction. 3.2. Micro-hardness of Cu/Gr composite

Fig. 1. a) SEM image of graphene particles, b) FSP tool, c) Raman spectrum of the Graphene.

H2O. The microstructure was observed under scanning electron microscope (SEM) (TESCAN VEGA3LMU) at different areas of the stir zone and the grain-size was measured through Hayn’s linear intercept method. The micro-hardness was measured using Vicker’s hardness tester at a load of 100 g for dwell of 15 s at various locations of the stir zone. The electrical conductivity measurements were performed (10 mm  10 mm  4 mm) using SIGMATEST 2.069, FOERSTER (resolution +/ 0.1% of measured value) device. The conductivity was measured in the unit of % IACS (International Annealed Copper Standard).

Fig. 3(a) shows the hardness of various samples (base metal (BM) and Cu/Gr composite) developed in this study. The hardness of base metal was measured to be 73HV. The hardness values in SZ (i.e., composite formed zone) was much higher than that of BM. The maximum hardness (125HV) was observed in the sample obtained with 200 mm/min traverse speed. The increase in hardness could be due to microstructure refinement and the very presence of Gr particles according to the rule of mixtures [8]. The well-known Hall-Petch relation interlinks grain diameter with the mechanical properties of metallic materials. The reduction in grain size increased the fraction of grain boundaries, which increases the resistance to the dislocation movement, thereby contributes to the strengthening of the matrix. The thermal expansion coefficients (CTE) between Gr and Cu (in-plane CTE is 6  10–6 K 1 for Gr at 300 K and 24  10–6 K 1 for Cu) induces the lattice distortion with a high dislocation density at the interface [9]. The thermal contraction differences between the Cu matrix and the Gr particles result in a quench hardening effect, and the homogeneous distribution of Gr particles in the Cu matrix invokes Orowan strengthening [7]. The reduction in grain size with the increase in traverse speed was the reason for the increase in hardness. Finally, fine and uniformly distributed Gr particles and the absence of defects (e.g., porosity) ensured the increase in hardness of the composite.

3. Results and discussion 3.3. Electrical conductivity of Cu/Gr composite 3.1. Microstructure of Cu/Gr composite Fig. 2(b–e) presents the SEM micrographs of FSP’ed Cu/Gr composite processed with varying tool traverse speed from 50 to 200 mm/min. The visual inspection of fabricated composites (Fig. 2b–e) shows that FSP has successfully (i.e. without defects like wormholes, or tunnels) produced Cu/Gr composite. The traverse motion of rotating tool plasticized Cu that enabled the movement of material from the advancing side to the retreating side. The rotating action of the tool disperses the compacted Gr particles into the plasticized Cu. In all the processed conditions, homogeneous distribution and good interfacial bonding between Gr particles and Cu matrix was observed. The distribution and presence of the Gr in the stir zone was confirmed through SEM-EDS analysis and obtaining Raman spectra (Fig. 2f and g). The measured base metal grain size was 40 lm (Fig. 2a). The grain size in the stir zone of composites much lesser than the initial grain size. It is observed that increasing the traverse speed led to a reduction in the grain size from 2.1 lm to 1.5 lm (Table 1). The

The conductivity value of Cu in this study was 98.5%IACS, while the conductivity of surface composites as high as 97.4% IACS (Fig. 3b). The change in electrical conductivity with the traverse speed was insignificant. It has been reported that the electrical conductivity directly proportional to the grain size [10]. The finegrained material has a large fraction of grain boundary, which acts as a scattering center for electron movement, thereby reduces conductivity. It is evident from Fig. 2(b–e) that fine grains are obtained after FSP; however, there is a minimal change in electrical conductivity. Takata et al. [10] noticed a significant improvement in the strength of Cu by accumulative roll bonding without impairing the electrical conductivity. This observation is in line with the present study. The electrical conductivity of Cu-Gr composite fabricated through various routes [1–4] are compared (Fig. 3c). It is evident from the figure that the electrical conductivity of surface composites prepared via FSP higher than the reported values. An increase in Gr content decreased the conductivity when it was fabricated

R.B. Naik et al. / Materials Letters 265 (2020) 127437

3

Fig. 2. SEM microstructures of Cu (a) and Cu-Gr composites obtained with tool traverse speeds (TS) of (b) 50, c) 100, d) 150, e) 200 mm/min, (f) X-ray energy spectrum (EDS) of Cu-Gr composite obtained with the TS, and (g) Raman spectrum Cu-Gr composite obtained with the TS 50 mm/min.

Table 1 Average grain size of base metal and surface composites. Condition

Avg. grain size(mm)

Decrease in grain size (%)

Base metal FSP, 50 mm/min FSP, 100 mm/min FSP, 150 mm/min FSP, 200 mm/min

40 2.1 1.8 1.7 1.5

– 94.75 95.5 95.75 96.25

through other routes. Though the composites developed in the present study containing higher amounts (8 vol%) of graphene, their electrical conductivity retained above 97%IACS. The present study demonstrates that FSP can be successfully used to improve the strength of the copper without deteriorating the electrical conductivity. 4. Conclusions The following are conclusions of the present study. 1. The use of tool rotational speed 900 rpm and traverse speed 50, 100, 150, and 200 mm/min was observed to be optimum to produce defect-free Cu-Gr surface composite through friction stir processing.

2. The grain size was reduced from an initial 40 mm to 1.5 mm. The increase in traverse speed led to a decrease in grain size. 3. There was about a 40% increase in the hardness after FSP. The maximum hardness (125HV) was measured for the surface composite developed with a traverse speed of 200 mm/min. 4. The decrease in electrical conductivity after FSP and with the traverse speed was observed to be minimal. The maximum difference between the electrical conductivity of Cu (98.5%IACS) and Cu-Gr composite (97.4%IACS) was only about 1.1% IACS.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors greatly acknowledge the support rendered by DRDO-DMRL in conducting FSP and ARCI, Hyderabad for providing graphene and for the support in electrical conductivity measurements.

4

R.B. Naik et al. / Materials Letters 265 (2020) 127437

Fig. 3. (a) Variation of hardness across the stir-zone samples processed with travel speeds of 50, 100, 150, and 200 mm/min (b) Electrical conductivity of surface composites prepared with different traverse speeds and (c) results of present study compared with the reported literature.

References [1] Lidong Wang, Ziyue Yang, Ye Cui, Bing Wei, Weidong Fei, Graphene-copper composite with micro-layered grains and ultrahigh strength, Sci. Rep. 7 (1) (2017) 41896, https://doi.org/10.1038/srep41896. [2] R. Jiang, X. Zhou, Q. Fang, Liu Z Copper-graphene bulk composites with homogeneous graphene dispersion and enhanced mechanical properties, Mater. Sci. Eng., A 654 (2016) 124–130, https://doi.org/10.1016/j. msea.2015.12.039. [3] F. Chen, J. Ying, Y. Wang, S. Du, Z. Liu, Effects of graphene content on the microstructure and properties of copper matrix composites, Carbon 96 (2016) 836–842, https://doi.org/10.1016/j.carbon.2015.10.023. [4] Qiao Xiao, Xiaoou Yi, Bo Jiang, Danqing Yi, In-situ synthesis of graphene on surface of copper powder by rotary CVD and its application in fabrication of reinforced Cu-matrix composites, Adv. Mater. Sci. 2 (2) (2017), https://doi.org/ 10.15761/AMS10.15761/AMS.1000123. [5] R.S. Mishra, Z.Y. Ma, I. Charit, Friction stir processing: a novel technique for fabrication of surface composite, Mater. Sci. Eng., A 341 (2003) 307–310, https://doi.org/10.1016/S0921-5093(02)00199-5.

[6] H.R. Akramifard, M. Shamanian, M. Sabbaghian, M. Esmailzadeh, Microstructure and mechanical properties of Cu/SiC metal matrix composite fabricated via friction stir processing, Mater. Des. 54 (2014) 838–844. [7] H. Sarmadi, F.A.H. Kokabi, S.M. Seyed Reihani, Friction and wear performance of copper–graphite surface composites fabricated by friction stir processing (FSP), Wear 304 (2013) 1–12, https://doi.org/10.1016/j.wear.2013.04.023. [8] H.S. Kim, On the rule of mixtures for the hardness of particle reinforced composites, Mater. Sci. Eng., A 289 (2000) 30–33, https://doi.org/10.1016/ S0921-5093(00)00909-6. [9] J.N. Wei, Z.B. Li, F.S. Han, Thermal mismatch dislocations in macroscopic graphite particle-reinforced metal matrix composites studied by internal friction, Phys. Status Solidi A 191 (1) (2002) 125–136, https://doi.org/10.1002/ 1521-396X(200205)191. [10] N. Takata, S.H. Lee, N. Tsuji, Ultrafine grained copper alloy sheets having both high strength and high electric conductivity, Mater. Lett. 63 (2009) 1757– 1760, https://doi.org/10.1016/j.matlet.2009.05.021.