https://doi.org/10.1007/s13762-019-02317-3
ORIGINAL PAPER
The mechanical properties of composite materials recycled from waste
metallic chips under different pressures
A. Aslan1 · E. Salur2 · A. Gunes1 · O. S. Sahin1 · H. B. Karadag3 · A. Akdemir4
Received: 26 September 2018 / Revised: 24 February 2019 / Accepted: 5 March 2019 / Published online: 18 March 2019 © Islamic Azad University (IAU) 2019
Abstract
The purpose of this study is to produce composite materials by utilizing the waste metallic chips. In this context, the metal matrix composite materials (MMCs) were produced at different production pressures and the effects of the different pressures on mechanical properties of MMCs were investigated. In the present investigation, spheroidal graphite cast iron (GGG-40) was used as reinforcement material in bronze (CuSn10) matrix system. The MMCs were produced by hot press with 20 wt% GGG-40 reinforcement ratio. The total time required for the production of one specimen was selected as 25 min and tempera-ture was settled at 400 °C. In order to determine mechanical properties and consolidation mechanism of MMCs, Brinell and micro-Vickers hardness, porosity, compression and X-ray diffraction tests were conducted. In addition, microstructure views were examined to determine the consolidation quality of metallic chips. According to experimental results, it was observed that waste metallic chips can be successfully recycled into MMC final parts with approximately 40% porosity and almost 100% strength and 150% hardness with respect to bulk CuSn10 materials. Most of the presented studies in the literature present information about properties of MMCs fabricated by conventional production methods. However, no available data are found about the recycling of bronze-based MMCs which make this study more original. It is also shown in this study that waste metallic chips can be utilized by proposed recycling methodology, which is environmentally friendly in comparison with conventional recycling methods producing harmful gases for earth atmosphere.
Keywords CuSn10 chips · GGG-40 chips · Recycling · Mechanical characterization · Metal matrix composites
Introduction
Heavy metals such as iron (Fe), copper (Cu) and zinc (Zn) accumulate in the soil as industrial wastes (Ghori et al.
2019). Heavy metals gain significance due to their non-degradable nature and pileup in nature creating a harmful environmental issue (Kar et al. 2008; Reza and Singh 2010;
Saleh 2015a, b; Saleh 2016). Due to the rapid consump-tion of natural resources as well as menconsump-tioned problems (Saleh 2018), various industrial companies and researchers have focused on the discovery of new material groups and recycling of existing materials which are found as waste in nature. Few recycling methods are introduced. However, huge amounts of energy consumption are required in con-ventional recycling methods such as melting which affects nature very dangerously (Aslan et al. 2015; Kong and White
2010).
A number of eco-friendly recycling processes have been developed instead of conventional methods which possess low efficiency, high cost and especially risky for environ-ment (Aslan 2014; Aslan et al. 2015; Gronostajski et al.
2000; Kong and White 2010). Literature survey revealed that many researchers concentrated on the recycling of alu-minum (Gronostajski and Matuszak 1999; Guluzade et al.
2013), magnesium (Wu et al. 2009; Ying et al. 2010), cast iron (da Costa et al. 2003) and steel (Karadağ 2012) materi-als and their metallic chips. These metallic chips have been
Editorial responsibility: J Aravind. * E. Salur
esalur@selcuk.edu.tr
1 Department of Mechanical Engineering, Selcuk University, Konya, Turkey
2 Department of Metallurgy and Materials Engineering, Selcuk University, Konya, Turkey
3 Department of Metallurgy and Materials Engineering, Necmettin Erbakan University, Konya, Turkey
4 Department of Aircraft Engineering, Necmettin Erbakan University, Konya, Turkey
5260 International Journal of Environmental Science and Technology (2019) 16:5259–5266
recycled by hot deformation (Szczepanik and Sleboda 1996), extrusion (Fogagnolo et al. 2003; Gronostajski et al. 1997), high-pressure torsion (Abd El Aal et al. 2013) and sintering under pressure techniques (Karadağ 2012; Salur 2017).
It has been stated that the melting and casting process are most commonly used recycling method (Karadağ 2012) which can be applied to almost any metallic chips. These processes may produce contamination problems, which have adverse impacts on the ecological protection, and high-energy consumption is required for these processes (He et al.
2015). In addition, an undesirable oxide layer is formed on the surface of material as a result of the melting and casting process. Oxidized metallic chips surfaces cause very low heat and electrical conductivity (Gronostajski et al. 2000). Aslan (Aslan 2014) reports that melting techniques such as induction furnaces and electric resistance furnaces are wasteful because they cause an oxide layer. Furthermore, toxic gases released during the melting process constitute a serious threat to ecological life and environment (Liu and Diamond 2005; Neto et al. 2009).
In this study, metal matrix composite materials were fab-ricated at three different pressures (480, 640 and 840 MPa) by recycling of CuSn10 and GGG-40 metallic chips with-out melting. In order to produce MMCs, GGG-40 was used as reinforcement material in CuSn10 matrix. The MMCs were produced by hot press with 20 wt% GGG-40 rein-forcement ratio, and production temperature was selected as 400 °C according to preliminary tests. In order to analyze a mechanical behavior of produced MMCs, hardness, poros-ity, compression and XRD tests were carried out. Compres-sion strength values and elastic properties of MMCs were compared with bulk CuSn10. Moreover, consolidation mechanism and structural integrity were evaluated by optic microscope and XRD results. All experimental outcomes were compared with bulk CuSn10. According to experimen-tal results, it was observed that porous and strength meexperimen-tal matrix composite materials were successfully recycled from waste metallic chips by means of the hot pressing. There are two main objectives of this study. These objectives are to contribute to academic studies interested in the recycling topics in the literature and to provide an alternative method for recycling processes used in industrial organizations. It is shown that in this study by the proposed methodology, unlike conventional methods, metallic chips can be con-verted into machine parts without any further treatment.
Materials and methods
In this study, spheroidal graphite cast iron (GGG-40) was used as reinforcement material in bronze (CuSn10) matrix system. Firstly, it was planned to supply and use CuSn10 and GGG-40 material chips as waste. However, since this study is an experimental study, physical and chemical properties of the materials should be known. If the materials are sup-plied as ready, the chemical composition of the materials as well as the manufacturing steps and heat treatments cannot be known. For these reasons, 22-mm-diameter CuSn10 and GGG-40 bars were poured with the desired chemical com-positions. The chemical composition of reinforcement and matrix material is given in Tables 1 and 2, respectively. The bars were transformed into metallic chips by turning lathe with the same machining parameters. Metallic chips with relatively large sizes were milled in a ball mill to obtain smaller and homogeneous metallic chips. These lic chips were sieved with 1–2-mm sieves, and the metal-lic chips between the sieves were used for production. The procedures and stages for obtaining the metallic chips are shown in Fig. 1. In order to provide a homogeneous mixture, the metallic chips with the desired proportion were mixed with conical mixer for 20 min in regard to German’s mixture procedure (German 2005).
The chips mixture was kept in the die for 15 min at con-stant temperature to supply uniform temperature distribu-tion. In the subsequent heating period, the compaction of the metallic chips was achieved by synchronous motion of the dies. So, the total time needed for the manufacturing of one sample was decided as 25 min. More comprehensive infor-mation about production steps and experiments was given in the previous studies (Aslan et al. 2015, 2018). Schematic view of the production stages consisting of mixture process, compaction process and equipment is represented in Fig. 2.
Brinell and micro-Vickers hardness tests were car-ried out to determine consolidation quality of the sys-tem at different locations on the surfaces and bonding between metallic chips, respectively. In order to compre-hend whether sufficient structural integrity was ensured between the metallic chips as well as to determine fracture
Table 1 The chemical
composition of GGG-40 (wt%) Material C Si Mn S Mg P Fe
GGG-40 3.40 2.50 0.13 0.01 0.04 0.08 Balance
Table 2 The chemical composition of CuSn10 (wt%)
Material Cu Sn Zn Pb
behavior, compression tests were applied to MMCs. In addition, elastic properties of produced MMCs were com-pared with bulk CuSn10. Application of the hardness and compression tests is shown in Fig. 3. In the present study, Brinell hardness tester with indenter diameter 5 mm was employed to determine the hardness of the MMCs. A load of 250 kgf was applied for 30 s at room temperature. For micro-Vickers hardness test, a load of 100 g was applied for 20 s. Compression tests were performed on the ‘Instron
8801’ testing machine, and the samples were tested at ambient temperature. The speed of the compression tests was selected as 2 mm/s, and the specimens were prepared according to American Society for Testing and Materials (ASTM/E9-89a) standard. Compression strength, secant modulus, toughness, strain fracture and resilience values of specimens obtained from compression tests were illus-trated in graphs.
Fig. 1 Schematic representation of the transformation from bars to metallic chips
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Results and discussion
Hardness measurements were performed on both vertical and horizontal sections of samples in different locations to provide reliable data. Relative hardness and porosity values are shown in Fig. 4. Proportions of MMC’s val-ues to bulk CuSn10 were represented in vertical axis, and variable production pressures were shown in horizontal axis. Considering Brinell tests results, it was observed that all MMCs exhibited better hardness values than bulk CuSn10 by approximately 60%. Elevating production pres-sure values led to increment in Brinell hardness. Besides, it was monitored that hardness values of each sample were not changed depending on measurement locations. So, mechanical properties of MMCs show homogenous distribution over sample. It was observed that increas-ing production pressure led to relatively easier plastic
deformation of CuSn10 metallic chips. Also, as a result of increasing plastic strain of CuSn10 chips (Aslan et al.
2018; Sahin et al. 2017), strain hardening in the samples was observed as reported by (Jafari et al. 2012; Tang et al.
2004).
Micro-Vickers results showed that metallic chips were consolidated effectively. As it is explained at the Brinell hardness section, micro-Vickers hardness values were better than bulk CuSn10 at the ratio of 40%. Higher hardness val-ues were obtained as expected due to the nature of composite systems as well as harder GGG-40 used as a reinforcement material. Moreover, the reason why the hardness values of MMCs are better than bulk CuSn10 can be explained with plastic deformations of CuSn10 (Witte et al. 2007). Accord-ing to the porosity results, it was determined that the chang-ing production pressure had a minor effect on the porosity values.
During production process, metallic chips contact each other and create friction and shear motion which results in surface hardening. This effect shows itself as micro-hardness increase at points where chips are in contact. This situation is seen in Fig. 5.
In our previous study (Salur et al. 2019), it is shown that the subject MMCs produced under 640 MPa pressure showed higher resistance to drilling. It is also reported that 640 MPa is the pressure to see appropriate plastic deforma-tion phenomena in CuSn10 materials system, while lower pressure cannot result in such effect. The nature of drilling involves both compressive strength and shear strength. How-ever, in compressive test only the resistance to compression is measured. So, it is concluded that applied pressure during production has a little effect on compressive strength on con-dition that the applied different pressures are high enough to form structural integrity.
Fig. 3 a, b Application of compression test and c, d hardness test
Fig. 4 The effect of different production pressures on Brinell–micro-Vickers hardness and porosity results of MMC
The compression stress–strain curve of MMC produced at 820 MPa is shown in Fig. 6a. The relative values of some mechanical properties of MMCs obtained by the compres-sion test results are given in Fig. 6b. It was noticed that compression stress reached about 305 MPa without any permanent damage in the test specimen as shown in Fig. 6a. After exceeding the yield point, the consolidated metal-lic chips were exposed to plastic deformation, and so, the pores in the contact points of metallic chips begun to close until the strain reached to 0.35. The cold deformation which occurred during the closure of the pores led to strain harden-ing mechanism and the strength increased momentarily and then continued to decline again. As a result of the completed closure of the pores, the sample exhibited non-porous mate-rial behavior and the sample reached its maximum stress point. Compressed MMC specimens are shown in Fig. 7, and the shear fracture surfaces for typical ductile materials were observed. The fracture of the samples in this way showed that the pores and metallic chips were homogeneously dis-tributed throughout the material without agglomerate in a particular region. This proves that the selected mixer type and mixing parameters were sufficient to provide a homo-geneous distribution.
The compression strength of the specimen produced at 820 MPa reached to almost the strength of bulk CuSn10. However, it was concluded that the different production pressures did not have an impact on strength significantly. Similarly, it was observed that resilience, toughness, secant
modulus and strain at fracture values are not affected by selected pressures. As shown in Fig. 6b, it was clearly observed that different production pressures had no obvious effect on the mechanical and elastic properties of the MMCs.
X-ray diffraction (XRD) analysis was conducted to evaluate the probable intermetallic compound formation
Fig. 5 Variation of micro-Vickers hardness with respect to different metallic chips zones
Fig. 6 a Stress–strain curve of MMC produced at 820 MPa; b rela-tive values of compression test results of MMC produced at different pressures
Fig. 7 The shear fracture surfaces are characteristics for ductile mate-rials
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as stated by (Saleh 2017). The corresponding X-ray dif-fraction patterns of the specimen produced at 820 MPa are shown in Fig. 8a. The most intense reflection peaks of CuSn10 (JCPDS card no. 44-1477) are observed at 42.8°, 50°, 62° and 73.3° (Juškėnas et al. 2006). In addition, the reflection peaks of graphite (JCPDS card no. 41-1487), lead (Pb) (JCPDS card no. 04-0686) and iron (Fe) are observed as shown in Fig. 8a, and researchers stated that peaks were recorded at the same degrees in X-ray diffraction studies (Dobrzanski and Drak 2004; Li et al. 2017; Saleh and Gupta
2016). When the result of the XRD test and microstructure view were evaluated together, it was observed that consoli-dation mechanism was accomplished by mechanical inter-locking because no intermetallic compounds and no second phase were formed between metallic chips (El-Daly et al.
2013). The structural integrity was achieved through the coverage of GGG-40 chips by CuSn10 chips under tempera-ture and pressure.
The mixture of metallic chips and composite structure was successfully consolidated by using hot press, and the microstructure of selected samples produced with different production pressures is illustrated in Fig. 8b, c. Considering the microstructures of MMCs as shown in Fig. 8b, c, it was observed that no intermetallic phase was formed between the cast iron and bronze components, these two components did not penetrate into each other but the CuSn10 penetrated to voids in the GGG-40 chips and covered the major part of the GGG-40 metallic chips. The pores occurred in the structure
due to the fact that CuSn10 chips could not penetrate to some places because of the zigzag formation of the GGG-40 metallic chips. Although these pores were a desired property for MMC system, it was detected that some large and irregu-lar pores had a negative effect on strength of the specimen.
Conclusion
In this paper, porous and strength metal matrix composite materials were successfully fabricated by recycling method. The mechanical properties of produced MMCs consisting of GGG-40 and CuSn10 waste metallic chips, which were fabricated by hot pressing in three different production pres-sures, were investigated. The following conclusion can be derived from the microstructural characterization and the mechanical test results.
• According to Brinell test results, it was observed that waste metallic chips were successfully transformed into porous and strength MMCs and they exhibited better hardness values than bulk CuSn10. Brinell hardness values of MMCs reached approximately 150% of bulk CuSn10. It can be stated that the varied production pres-sures have little or no effect on the mechanical properties of the MMCs.
• Micro-Vickers results show that metallic chips were con-solidated effectively. Similar to Brinell hardness results,
Fig. 8 a XRD test results of MMC fabricated at 820 MPa; b, c microstructure of the selected MMC at low and high magnifi-cation (50 ×), (200 ×)
owing to effective consolidation and strain hardening mechanism, micro-Vickers hardness values were higher than CuSn10.
• Considering the overall trend, the experiments showed that the porosity was decreased by increasing pressure and the porosity values of MMCs reached about 40% of bulk CuSn10. Increasing production pressure led to bet-ter softening mechanism and structural integrity. Hence, the amount of pore which was located between uncon-solidated metallic chips decreased depending on increas-ing production pressure.
• Compression strength of MMC materials approached to strength of bulk CuSn10. It means that selected produc-tion parameters are appropriate for plastic deformaproduc-tion of CuSn10 chips under pressure. It is concluded that applied pressure during production has a little effect on compres-sive strength on condition that the applied pressure is high enough to form structural integrity.
• In addition, elastic properties, toughness and strain frac-ture slightly affected by variable production pressures and these properties have varied between 40 and 75% of bulk CuSn10.
• According to X-ray diffraction results, no secondary or intermetallic compound phase was detected. The com-bination of X-ray diffraction results and microstructure images showed that the consolidation mechanism was fulfilled by mechanical interlocking.
• Various iron- and copper-based materials are commonly preferred as a bearing and damper element in industrial application. MMC materials which are recycled from metallic chips can be used as a self-lubricating jour-nal bearings, damper and filter with their extraordinary porosity and strength properties.
• As a result of this study, it can be said that hot press-ing can be used in the industry as a method of recyclpress-ing which can be an alternative to conventional methods (melting-based methods).
• •
Acknowledgements This study was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) [Grant Num-ber 113M141] and Scientific Research Projects Coordination Unit (SRPCU) [Project Number 10201039].
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