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On the feasibility of producing polymer–metal composites via novel variant of friction stir processing
G Model JMP-201; No. of Pages 7
ARTICLE IN PRESS Journal of Manufacturing Processes xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical note
On the feasibility of producing polymer–metal composites via novel variant of friction stir processing Ehsan Azarsa ∗ , Amir Mostafapour Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 13 May 2013 Received in revised form 27 June 2013 Accepted 5 August 2013 Available online xxx Keywords: Polymers Composite Friction stir processing
a b s t r a c t In this study a novel variant of friction stir processing was developed for producing of polymer metal surface or bulk composites in order to enhance the mechanical, electrical and thermal properties. For this purpose, a novel tooling system was designed consists of a rotating pin, a stationary shoulder and a heating system located inside the shoulder. In present paper, for preliminary study high-density polyethylene and copper powder was selected as polymeric matrix and metallic additive, respectively. Surface quality, microstructure, ultimate tensile strength and the modulus of elasticity were determined for each prepared sample. From experimental tests, it was found that this approach is an efficient method for producing of polymer–metal composites. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Metal–polymer composites that exhibit the properties of both metals and polymers have been the subject of extensive research last two decades [1]. Physical and mechanical properties of polymeric materials are important in many applications and remarkably influenced by the structures and compositions of the molecular layers. Type of metallic filler, distribution and volume fraction of particles, and nature of techniques used for producing these composites are main factors that determine the mechanical and physical properties. Several techniques have been developed in experimental investigation for producing of polymeric composites. In general, these methods utilize combination of increasing temperature and mixing, such as mechanical milling [2], vacuum arc deposition [3], melt mixing [4] and injection molding [5]. In recent years, much attention has been paid to a new composite fabrication technique named friction stir processing (FSP) [6]. FSP based on friction stir welding (FSW) is a solid-state process for microstructural modifications [7], grain refinement [8], and fabrication of surface layer and bulk composites [9–11]. Recently, FSW and FSP are used on a wide variety of materials, including copper [12], magnesium alloys [13], titanium alloys [14] and steel [15]. However, as conventional friction stir is applied to joining or processing polymeric materials [16–19], it is so difficult to achieve the enhanced mechanical and surface properties [20,21]. It appears
∗ Corresponding author. Tel.: +98 914 107 6849; fax: +98 411 556 5576. E-mail addresses: ehsan [email protected], ehsan [email protected] (E. Azarsa).
that very few studies have been reported on producing polymeric composite using friction stir processing as known by the author [22]. Considering the above facts, the aim of this research was to develop a new variant of friction stir processing in order to fabricate surface layer or bulk composites. Therefore, a novel tooling system was designed to apply on FSP of polymers. The designed tooling system is consists of a shoe (shoulder), a rotating pin and a heating system inside the shoulder. It provides the mixing and joining of polymer molecules together in the presence of heat. The shoulder is stationary relative to pin, whereas in FSW and FSP of metals the shoulder rotates with the pin. In the present article, high-density polyethylene (HDPE) and copper (Cu) powder utilized as matrix and additive metal powder, respectively. Composite materials based on copper particles dispersed in a polymeric material such as HDPE are useful in many fields of engineering [23] since they have enhanced thermal, electrical and mechanical properties [4,24]. Surface quality of prepared samples was investigated through visual inspection. Tensile tests were carried out to evaluate the mechanical properties of fabricated composites. Moreover, in order to make a judgment about the dispersion state of particles in the system, produced samples has been studied by polarized light microscopy.
2. Materials and methods The materials used in this study were commercial grades. HDPE used was Ridge HD 5218 EA supplied by Arak Petrochemical Company. HDPE sheets’ thickness, width and length were 6, 100 and 300 mm, respectively. The copper powder used in this work has
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.08.007
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 1. Photograph of designed friction stir processing tool.
particle size <45 m, with over 99% purity which was made in Germany and packaged by Pourian Chemical Company in Iran. The designed tool in the present work was consisting of three main parts: a cylindrical rotating pin, a stationary shoulder or shoe, and heating system. The photograph of FSP tool is shown in Fig. 1. As it can be seen in this figure, the tool pin is inserted into the shoe shaped shoulder and a heater, which is equipped with a close-loop thermo controller, is located inside the shoe. Through an indicator, this device shows the approximate temperature of the melting area and with a thermal potentiometer, the heat output of the electric heater could be adjusted. Furthermore, a thrust bearing separated the shoe from the pin and its main purpose was to hold the shoulder stationary relative to pin during FSP. The tool pin was made of H13 hot-working steel and shoulder’s material was 7075 aluminum owing to its high thermal conductivity and mechanical strength. The shoulder’s surface was coated with PTFE (Teflon) in order to prevent the stick phenomenon that may occur between hot aluminum and polymeric surface. Tool pin and shoulder dimensions are illustrated in Fig. 2. In order to produce HDPE/copper composite, firstly, a groove with the dimensions of 2 mm × 2.5 mm was machined in the middle of samples (Fig. 3a). Then the copper particles were contrived and compressed into the groove (Fig. 3b). Volume fraction of particles was estimated by dividing the amount of particles in the slot and area over which it was distributed in the matrix [25]. So, in this study the volume fraction of Cu particles was 10%. The next stage is plunging of tool with pin inside the plate for producing composite (Fig. 3c). In contrast to conventional friction stir processing technique, in this approach there is not any requirement to close material upper surface with a FSP-shaped tool, which prevents outpouring of particles, since the tooling system was equipped with a shoe shape shoulder which could satisfy this requirement. Furthermore, this shoulder provides a uniform cooling rate on polymer surface and supply additional heat when it is required. In order to perform composite fabrication process, the HDPE sheets were placed into the designed fixture, which is demonstrated in Fig. 4. Moreover, designed fixture is required to play as a guideline role and prevent any vibration of tool shoulder during process. The process parameters investigated under multiple levels. Shoulder temperatures of 70, 110 and 150 ◦ C were examined. Tool transverse speeds had values of 20, 60 and 120 mm/min. The lists of prepared samples are reported in Table 1. Pin rotational speed was selected to be 1000 rpm because friction stir of polymers is very sensitive to tool rotating speed [19]. Tensile tests were performed by GALDABINI Sun 2500 device with autograph capability at force loading rate of 50 mm/min with maximum head force of
Fig. 2. Drawings of designed FSP tool: (a) pin and (b) shoulder (shoe).
5 kN and samples were extracted from each fabricated composite in accordance with ASTM D638 standard (specimen dimensions were selected according to type 1, which is presented in the table of this standard). It is worth to mention that before conducting tensile test to sample, the back side of plates machine to the depth of composite. Therefore the samples that subjected to the tensile test are completely composited with metal powder. For making judgment about microstructure of fabricated samples in various conditions, a LEICA DMRX polarized microscope was utilized. For microscopic evaluations, samples were cut through a LEICA RM2135 microtome device. 3. Results and discussions 3.1. Feasibility of producing polymer composites The photographs of fabricated samples are shown in Figs. 5 and 6. As it can be seen, when tool transverse speed is 60 mm/min and shoulder temperature is 110 ◦ C the composite can be fabricated without any blister, burr and other surface defects that may be encountered in conventional friction stir processing [19–21]. FSP and FSW of metallic materials are solid-state techniques and temperature of material does not reach to its melting point. However, friction stir processing of polymeric materials is not a solid-state process because polymers consist of molecules of different lengths and the materials do not have single melting point, but melting ranges. Thus, as FSP or FSW applied to polymeric Table 1 List of samples prepared at different processing condition. Sample
S1
S2
S3
Shoulder temperature (◦ C) Transverse speed (mm/min)
110 20
110 60
110 120
S4 70 60
S5
S6
110 60
150 60
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 3. Schematic of novel variant of friction stir processing.
materials, some shorter chains reach their melting point while longer chains do not. Therefore, bits of solid material are suspended in molten material. This results in inhomogeneous distribution of material around pin and even formation of voids or channels inside sheets which significantly reduce the mechanical properties. However, increasing the tool rotational speed, which is generally used in the case of metals is not a proper method to increase the temperature and amount of molten material because polymers has very low thermal conductivity and this will leads to material degradation [26]. So, control of material flow in processing these materials has some difficulty. Therefore, a new tooling system developed in present study supply sufficient heat into stir zone and even heat affected zone, which make this technique a liquid-state process. In other words, supplied heat from shoulder could melt
Fig. 4. Novel variant of friction stir processing of polymers.
whole of the material presented in stir zone and as a consequent render the mixture easy to move and form. From Fig. 5 it is clear that the samples produced with shoulder temperature below softening temperature of HDPE (80 ◦ C) show poor surface quality (sample S4). In low shoulder temperatures the polymer matrix is not melted completely. Lack of the molten material and mixing of copper powder with semi-molten HDPE would lead to outpouring of metal powder, rough surface and inhomogeneous mixing. However, in shoulder temperatures higher than melting range of HDPE (110–130 ◦ C), the material degradation and burning was take placed due to overheating, as it can be seen at the surface of sample S6. Furthermore, in this processing condition large burrs will be formed because melted plastics would be pushed away beneath the shoulder (Fig. 5c). Another problem that encountered during conventional friction stir processing of polymers is to promote uniform cooling rate throughout the processing zone. If outer layers of a plastic sheet cool much quicker than inner, a hard shell is formed. As the inner layers then cool, the materials contract and pull away from the shell. So, large voids will be formed which remarkably reduce the mechanical and surface properties of fabricated composites [21]. A shoe shaped shoulder that designed in novel variant of FSP, provide sufficient time to surface layers of polymer sheets to cool and prevent the quenching of melted polymer crystals. Fig. 6 shows the effects of tool transverse speed on surface quality. An obvious improvement in surface quality can be seen when transverse speed of tool was 60 mm/min. In lower transverse speeds opaque surface observed due to excessive heat input by FSP tool. However, in higher amount of tool travel speed, high cooling rate of polymer crystals lead to relatively poor surface quality. Moreover processing in low shoulder temperatures (S4) or high travel speed (S3) will result in formation of large voids inside stir zone which can significantly affect the mechanical properties of fabricated samples. This can be clearly seen in Fig. 7.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 5. Picture of fabricated samples with various shoulder temperature: (a) S4, (b) S5 and (c) S6.
It is worth noting that another advantage of this novel method is that, in contrast to conventional friction stir processing technique, in present approach there is not any requirement to close material upper surface with a FSP-shaped tool (this tool is similar to FSP tool but there is not any pin on it) which prevents outpouring of particles, because the tooling system was equipped with a shoe shape shoulder which could satisfy this requirement. This mechanism in turn saves the production time and lessens the costs.
3.2. Powder distribution Optical microscopy was used in order to investigate the distribution of the Cu powder in HDPE. The polarized optical microscopy photos of HDPE–Cu composites, fabricated at different process parameters, are shown in Fig. 8. It is known that polymers such as HDPE exhibit dielectric behavior and introduction of metallic filler such as copper could enhance their electrical properties
Fig. 6. Picture of fabricated samples with various transverse speeds: (a) S1, (b) S2 and (c) S3.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 7. Cross section image of fabricated samples: (a) S4, (b) S3 and (c) S2.
significantly. It is also well-established that the higher dispersion state of metallic particles in the system, the higher electrical response of polymer composite and thus higher dielectric constants. Furthermore, the same holds good in respect of mechanical properties. Fig. 8 shows that incorporating the metallic phase inside the polymeric matrix via this variant of FSP has created a composite with well distributed copper powder in it. The good dispersion of copper particles in the system could be attributed to good stirring action of pin, additional heat providing through shoulder and adequate pressure of designed tooling system on processing zone. The effect of processing condition such as transverse speed and shoulder temperature is also shown in Fig. 8. It is clear that in higher transverse speeds such as 120 mm/min (sample S3) the amount of powder agglomerated parts increases. As
the travel speed of tool increases, higher amount of materials process per minute and as a consequent there is not enough time to adequate flow of matrix and eliminate the agglomerated powders. Furthermore, in FSP there is a general trend of decreasing amount of input heat to stir zone as transverse speed increases. Thus it is reasonable to assume that in processing with higher speeds, there would be semi molten materials inside nugget zone which result in inhomogeneous dispersion of copper particles. It also can be seen that decreasing the shoulder temperature exhibits a similar trend, that is, reducing the amount of molten materials inside nugget zone. This leads to formation of large voids and even channels (sample S4). However, in higher shoulder temperature such as 150 ◦ C (sample S6), the fabricated composite does not represent good dispersion state of copper particles which
Fig. 8. Optical polarized microscopy of various samples.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 9. Effect of tool transverse speed on (a) ultimate tensile strength and (b) modulus of elasticity.
Fig. 10. Effect of shoulder temperature on (a) ultimate tensile strength and (b) modulus of elasticity.
could be ascribed to the material degradation due to over heating. 3.3. Mechanical properties of fabricated samples The obtained values of ultimate tensile strength and modulus of elasticity for fabricated HDPE/Cu composites are reported in terms of tool transverse speed and shoulder temperature in Figs. 9 and 10, respectively. It also should be mentioned that the ultimate tensile strength and elastic modulus of parent polymer, which was not processed by FSP, was measured to be 23 and 540 (MPa), respectively. Fig. 11a and b shows the prepared tensile test samples and fractured sample after applying test, respectively. According to results, a 10% increase was observed in the case of ultimate tensile strength of samples fabricated via this variant of FSP approach, whereas the copper/polyethylene composites prepared by other methods [4,24] showed a reduction in ultimate tensile strength value. In addition, modulus of elasticity of fabricated samples shows a 30%
enhancement. These results verify the higher efficiency of novel variant of FSP approach to improve mechanical properties of polymers rather than the other conventional methods. The high values of ultimate tensile strength and modulus of elasticity obtained by this method could be attributed to the good dispersion and higher level of interfacial adhesion between copper particles and polymer matrix, which can improve the mechanical properties [24]. However, we postulate that the effect of this variant of FSP on wetting of filler and bonding of matrix could be other important factors determining the mechanical properties of composites which merits further investigation in future studies. Fig. 9 shows the effects of tool transverse speed on tensile strength and modulus of elasticity. It clearly reveals how, increasing speed to 120 mm/min decrease mechanical properties. Lower travel speeds result in low cooling rate, slower crystals growth and allow polymer to crystallize. Therefore, a polymeric composite with slower crystals growth exhibits enhanced mechanical properties since this could improve the spherulite structure [27]. However,
Fig. 11. Photograph of (a) the prepared tensile test samples and (b) fractured sample after applying test.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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a clear reduction is observed when processing with 20 mm/min transverse speed. Also, it is true that at lower speeds, the time of stirring action is longer which lead to an enhanced state of dispersion; but on the other hand, increasing the travel speed causes the stirring zone to be compressed, which induces higher amounts of thermo-mechanical stress on the polymer leading to the improvement in dispersion state of copper particles [24]. That is why the 60 mm/min transverse speed is the best value for fabricating metal–polymer composites by this variant of FSP. On the role of shoulder temperature in mechanical properties of composites (Fig. 10), it has to be mentioning that processing in temperatures lower than melting point of HDPE would weaken the mechanical properties due to the reasons have discussed in Section 3.1. However, it was expected that there would be a decrease in modulus of elasticity when the shoulder temperature goes up to 150 ◦ C; instead, we observed that there is a general trend of enhancement of elastic modulus as temperature increases. It may be contributed to the wider zone of stir and thus longer chains inside processing area at higher shoulder temperatures. 4. Conclusions In this article, a novel variant of friction stir processing has been developed for producing of metal polymer composites. A tooling system was designed which consists of a rotating pin, a stationary shoe shaped shoulder and a heating system located inside the shoulder. Copper/high density polyethylene composites were successfully fabricated by good distribution of metal particles inside polymeric matrix as well as improved surface quality via novel variant of FSP, which cannot be achieved by conventional methods. It was found that a 10% increase in ultimate tensile strength and 30% enhancement in modulus of elasticity could be achieved for the samples produced at transverse speed of 60 mm/min and shoulder temperatures of 110 ◦ C, which is higher than values observed in conventional methods. This could be ascribed to the good dispersion of metal particles in polyethylene matrix, controlled cooling rate of polymer structure and sufficient heat input into stir zone. Eventually, it can be concluded that this variant of FSP is a very efficient method for fabricating polymer metal surface and bulk composites. Acknowledgement The author gratefully acknowledges Shahram Alyali and Vahid Tavakolkhah for their contributions. References [1] Gungor A. Mechanical properties of iron powder filled high density polyethylene composites. Mater Des 2007;28:1027–30. [2] Zebarjad SM, Noroozi M. Production of polyethylene/carbon nanotube nanocomposite using mechanical milling process and investigation of its microstructure. In: Fibre reinforced composite conference. 2007 [abstract no. 107].
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[3] Zhitomirsky VN, Grimberg I, Joseph MC, Boxman RL, Matthews A, Weiss BZ. Vacuum arc deposition of conductive wear resistant coatings on polymer substrates. Surf Coat Technol 1999;120–121:373–7. [4] Luyt AS, Molefi JA, Krump H. Thermal, mechanical and electrical properties of copper powder filled low-density and linear low-density polyethylene composites. Polym Degrad Stab 2006;91:1629–36. [5] Kanagaraj S, Varanda FR, Zhil’tsova TV, Oliveira MSA, Simoes JAO. Mechanical properties of high density polyethylene/carbon nanotube composites. Compos Sci Technol 2007;67:3071–7. [6] Mishra RS, Ma ZY, Charit I. Friction stir processing: a novel technique for fabrication of surface composite. Mater Sci Eng A 2003;341:307–10. [7] Karthikeyan L, Senthilkumar VS, Padmanabhan KA. On the role of process variables in the friction stir processing of cast aluminum A319 alloy. Mater Des 2010;31:761–71. [8] Ma ZY, Pilchak AL, Juhas MC, Williams JC. Microstructural refinement and property enhancement of cast light alloys via friction stir processing. Scr Mater 2008;58:361–6. [9] Azizieh M, Kokabi AH, Abachi P. Effect of rotational speed and probe profile on microstructure and hardness of AZ31/Al2 O3 nanocomposites fabricated by friction stir processing. Mater Des 2011;32:2034–41. [10] Sharifitabar M, Sarani A, Khorshahian S, Shafiee Afarani M. Fabrication of 5052Al/Al2 O3 nanoceramic particle reinforced composite via friction stir processing route. Mater Des 2011;32:4164–72. [11] Wang W, Shi Q, Liu P, Li H, Li T. A novel way to produce bulk SiCp reinforced aluminum metal matrix composites by friction stir processing. J Mater Process Technol 2009;209:2099–103. [12] Barlas Z, Uzun H. Microstructure and mechanical properties of friction stir butt welded dissimilar pure copper/brass alloy plates. Int J Mater Res 2010;101:801–7. [13] Kohn G, Antonsson S, Munitz A. Friction stir welding magnesium alloys. In: Proceedings of the symposium on automotive alloys, TMS annual meeting. 1999. [14] Ramirez AJ, Juhas MC. Microstructural evolution in Ti–6Al–4V friction stir welds. Mater Sci Forum 2003;426–432:2999–3004. [15] Sato YS, Nelson TW, Sterling CJ, Steel RJ, Pettersson CO. Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel. Mater Sci Eng A 2005;397:376–84. [16] Bilici MK, Yukler AI, Kurtulmus¸ M. The optimization of welding parameters for friction stir spot welding of high density polyethylene sheets. Mater Des 2011;32:4074–9. [17] Bilici MK, Yukler AI. Effects of welding parameters on friction stir spot welding of high density polyethylene sheets. Mater Des 2012;33:545–50. [18] Bilici MK, Yukler AI. Influence of tool geometry and process parameters on macrostructure and static strength in friction stir spot welded polyethylene sheets. J Mater Des 2012;33:145–52. [19] Arıcı A, Sınmaz T. Effects of double passes of the tool on friction stir welding of polyethylene. J Mater Sci 2005;40:3313–6. [20] Scialpi A, Troughton M, Andrews S, De Filippis LAC. Viblade: friction stir welding for plastics. Weld Int 2009;23(11):846–55. [21] Troughton MJ. Handbook of plastics joining. Practical guide. 2nd ed. Norwic, NY: William Andrew, Inc.; 2008. [22] Barmouz M, Seyfi J, Besharati Givi MK, Hejazi I, Davachi SM. A novel approach for producing polymer nanocomposites by in-situ dispersion of clay particles via friction stir processing. Mater Sci Eng A 2011;528(6):3003– 6. [23] Boudenne A, Ibos L, Fois M, Majeste JC, Gehin E. Electrical and thermal behavior of polypropylene filled with copper particles. Composites Part A 2005;36:1545–54. [24] Molefi JA, Luyt AS, Krupa I. Comparison of the influence of copper micro- and nano-particles on the mechanical properties of polyethylene/copper composites. J Mater Sci 2010;45:82–8. [25] Ramadorai U, Newkirk JW, Mishra RS.Proceedings of the 19th international conference on surface modification technologies. 2005. [26] Mostafapour A, Azarsa E. A study on the role of processing parameters in joining polyethylene sheets via heat assisted friction stir welding: investigating microstructure, tensile and flexural properties. Int J Phys Sci 2012;7(4):647– 54. [27] Nielsen LE, Landel RF. Mechanical properties of polymers and composites. 2nd ed. New York: Marcel Dekker, Inc.; 1994.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
ARTICLE IN PRESS Journal of Manufacturing Processes xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical note
On the feasibility of producing polymer–metal composites via novel variant of friction stir processing Ehsan Azarsa ∗ , Amir Mostafapour Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 13 May 2013 Received in revised form 27 June 2013 Accepted 5 August 2013 Available online xxx Keywords: Polymers Composite Friction stir processing
a b s t r a c t In this study a novel variant of friction stir processing was developed for producing of polymer metal surface or bulk composites in order to enhance the mechanical, electrical and thermal properties. For this purpose, a novel tooling system was designed consists of a rotating pin, a stationary shoulder and a heating system located inside the shoulder. In present paper, for preliminary study high-density polyethylene and copper powder was selected as polymeric matrix and metallic additive, respectively. Surface quality, microstructure, ultimate tensile strength and the modulus of elasticity were determined for each prepared sample. From experimental tests, it was found that this approach is an efficient method for producing of polymer–metal composites. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Metal–polymer composites that exhibit the properties of both metals and polymers have been the subject of extensive research last two decades [1]. Physical and mechanical properties of polymeric materials are important in many applications and remarkably influenced by the structures and compositions of the molecular layers. Type of metallic filler, distribution and volume fraction of particles, and nature of techniques used for producing these composites are main factors that determine the mechanical and physical properties. Several techniques have been developed in experimental investigation for producing of polymeric composites. In general, these methods utilize combination of increasing temperature and mixing, such as mechanical milling [2], vacuum arc deposition [3], melt mixing [4] and injection molding [5]. In recent years, much attention has been paid to a new composite fabrication technique named friction stir processing (FSP) [6]. FSP based on friction stir welding (FSW) is a solid-state process for microstructural modifications [7], grain refinement [8], and fabrication of surface layer and bulk composites [9–11]. Recently, FSW and FSP are used on a wide variety of materials, including copper [12], magnesium alloys [13], titanium alloys [14] and steel [15]. However, as conventional friction stir is applied to joining or processing polymeric materials [16–19], it is so difficult to achieve the enhanced mechanical and surface properties [20,21]. It appears
∗ Corresponding author. Tel.: +98 914 107 6849; fax: +98 411 556 5576. E-mail addresses: ehsan [email protected], ehsan [email protected] (E. Azarsa).
that very few studies have been reported on producing polymeric composite using friction stir processing as known by the author [22]. Considering the above facts, the aim of this research was to develop a new variant of friction stir processing in order to fabricate surface layer or bulk composites. Therefore, a novel tooling system was designed to apply on FSP of polymers. The designed tooling system is consists of a shoe (shoulder), a rotating pin and a heating system inside the shoulder. It provides the mixing and joining of polymer molecules together in the presence of heat. The shoulder is stationary relative to pin, whereas in FSW and FSP of metals the shoulder rotates with the pin. In the present article, high-density polyethylene (HDPE) and copper (Cu) powder utilized as matrix and additive metal powder, respectively. Composite materials based on copper particles dispersed in a polymeric material such as HDPE are useful in many fields of engineering [23] since they have enhanced thermal, electrical and mechanical properties [4,24]. Surface quality of prepared samples was investigated through visual inspection. Tensile tests were carried out to evaluate the mechanical properties of fabricated composites. Moreover, in order to make a judgment about the dispersion state of particles in the system, produced samples has been studied by polarized light microscopy.
2. Materials and methods The materials used in this study were commercial grades. HDPE used was Ridge HD 5218 EA supplied by Arak Petrochemical Company. HDPE sheets’ thickness, width and length were 6, 100 and 300 mm, respectively. The copper powder used in this work has
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.08.007
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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ARTICLE IN PRESS E. Azarsa, A. Mostafapour / Journal of Manufacturing Processes xxx (2013) xxx–xxx
Fig. 1. Photograph of designed friction stir processing tool.
particle size <45 m, with over 99% purity which was made in Germany and packaged by Pourian Chemical Company in Iran. The designed tool in the present work was consisting of three main parts: a cylindrical rotating pin, a stationary shoulder or shoe, and heating system. The photograph of FSP tool is shown in Fig. 1. As it can be seen in this figure, the tool pin is inserted into the shoe shaped shoulder and a heater, which is equipped with a close-loop thermo controller, is located inside the shoe. Through an indicator, this device shows the approximate temperature of the melting area and with a thermal potentiometer, the heat output of the electric heater could be adjusted. Furthermore, a thrust bearing separated the shoe from the pin and its main purpose was to hold the shoulder stationary relative to pin during FSP. The tool pin was made of H13 hot-working steel and shoulder’s material was 7075 aluminum owing to its high thermal conductivity and mechanical strength. The shoulder’s surface was coated with PTFE (Teflon) in order to prevent the stick phenomenon that may occur between hot aluminum and polymeric surface. Tool pin and shoulder dimensions are illustrated in Fig. 2. In order to produce HDPE/copper composite, firstly, a groove with the dimensions of 2 mm × 2.5 mm was machined in the middle of samples (Fig. 3a). Then the copper particles were contrived and compressed into the groove (Fig. 3b). Volume fraction of particles was estimated by dividing the amount of particles in the slot and area over which it was distributed in the matrix [25]. So, in this study the volume fraction of Cu particles was 10%. The next stage is plunging of tool with pin inside the plate for producing composite (Fig. 3c). In contrast to conventional friction stir processing technique, in this approach there is not any requirement to close material upper surface with a FSP-shaped tool, which prevents outpouring of particles, since the tooling system was equipped with a shoe shape shoulder which could satisfy this requirement. Furthermore, this shoulder provides a uniform cooling rate on polymer surface and supply additional heat when it is required. In order to perform composite fabrication process, the HDPE sheets were placed into the designed fixture, which is demonstrated in Fig. 4. Moreover, designed fixture is required to play as a guideline role and prevent any vibration of tool shoulder during process. The process parameters investigated under multiple levels. Shoulder temperatures of 70, 110 and 150 ◦ C were examined. Tool transverse speeds had values of 20, 60 and 120 mm/min. The lists of prepared samples are reported in Table 1. Pin rotational speed was selected to be 1000 rpm because friction stir of polymers is very sensitive to tool rotating speed [19]. Tensile tests were performed by GALDABINI Sun 2500 device with autograph capability at force loading rate of 50 mm/min with maximum head force of
Fig. 2. Drawings of designed FSP tool: (a) pin and (b) shoulder (shoe).
5 kN and samples were extracted from each fabricated composite in accordance with ASTM D638 standard (specimen dimensions were selected according to type 1, which is presented in the table of this standard). It is worth to mention that before conducting tensile test to sample, the back side of plates machine to the depth of composite. Therefore the samples that subjected to the tensile test are completely composited with metal powder. For making judgment about microstructure of fabricated samples in various conditions, a LEICA DMRX polarized microscope was utilized. For microscopic evaluations, samples were cut through a LEICA RM2135 microtome device. 3. Results and discussions 3.1. Feasibility of producing polymer composites The photographs of fabricated samples are shown in Figs. 5 and 6. As it can be seen, when tool transverse speed is 60 mm/min and shoulder temperature is 110 ◦ C the composite can be fabricated without any blister, burr and other surface defects that may be encountered in conventional friction stir processing [19–21]. FSP and FSW of metallic materials are solid-state techniques and temperature of material does not reach to its melting point. However, friction stir processing of polymeric materials is not a solid-state process because polymers consist of molecules of different lengths and the materials do not have single melting point, but melting ranges. Thus, as FSP or FSW applied to polymeric Table 1 List of samples prepared at different processing condition. Sample
S1
S2
S3
Shoulder temperature (◦ C) Transverse speed (mm/min)
110 20
110 60
110 120
S4 70 60
S5
S6
110 60
150 60
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Fig. 3. Schematic of novel variant of friction stir processing.
materials, some shorter chains reach their melting point while longer chains do not. Therefore, bits of solid material are suspended in molten material. This results in inhomogeneous distribution of material around pin and even formation of voids or channels inside sheets which significantly reduce the mechanical properties. However, increasing the tool rotational speed, which is generally used in the case of metals is not a proper method to increase the temperature and amount of molten material because polymers has very low thermal conductivity and this will leads to material degradation [26]. So, control of material flow in processing these materials has some difficulty. Therefore, a new tooling system developed in present study supply sufficient heat into stir zone and even heat affected zone, which make this technique a liquid-state process. In other words, supplied heat from shoulder could melt
Fig. 4. Novel variant of friction stir processing of polymers.
whole of the material presented in stir zone and as a consequent render the mixture easy to move and form. From Fig. 5 it is clear that the samples produced with shoulder temperature below softening temperature of HDPE (80 ◦ C) show poor surface quality (sample S4). In low shoulder temperatures the polymer matrix is not melted completely. Lack of the molten material and mixing of copper powder with semi-molten HDPE would lead to outpouring of metal powder, rough surface and inhomogeneous mixing. However, in shoulder temperatures higher than melting range of HDPE (110–130 ◦ C), the material degradation and burning was take placed due to overheating, as it can be seen at the surface of sample S6. Furthermore, in this processing condition large burrs will be formed because melted plastics would be pushed away beneath the shoulder (Fig. 5c). Another problem that encountered during conventional friction stir processing of polymers is to promote uniform cooling rate throughout the processing zone. If outer layers of a plastic sheet cool much quicker than inner, a hard shell is formed. As the inner layers then cool, the materials contract and pull away from the shell. So, large voids will be formed which remarkably reduce the mechanical and surface properties of fabricated composites [21]. A shoe shaped shoulder that designed in novel variant of FSP, provide sufficient time to surface layers of polymer sheets to cool and prevent the quenching of melted polymer crystals. Fig. 6 shows the effects of tool transverse speed on surface quality. An obvious improvement in surface quality can be seen when transverse speed of tool was 60 mm/min. In lower transverse speeds opaque surface observed due to excessive heat input by FSP tool. However, in higher amount of tool travel speed, high cooling rate of polymer crystals lead to relatively poor surface quality. Moreover processing in low shoulder temperatures (S4) or high travel speed (S3) will result in formation of large voids inside stir zone which can significantly affect the mechanical properties of fabricated samples. This can be clearly seen in Fig. 7.
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Fig. 5. Picture of fabricated samples with various shoulder temperature: (a) S4, (b) S5 and (c) S6.
It is worth noting that another advantage of this novel method is that, in contrast to conventional friction stir processing technique, in present approach there is not any requirement to close material upper surface with a FSP-shaped tool (this tool is similar to FSP tool but there is not any pin on it) which prevents outpouring of particles, because the tooling system was equipped with a shoe shape shoulder which could satisfy this requirement. This mechanism in turn saves the production time and lessens the costs.
3.2. Powder distribution Optical microscopy was used in order to investigate the distribution of the Cu powder in HDPE. The polarized optical microscopy photos of HDPE–Cu composites, fabricated at different process parameters, are shown in Fig. 8. It is known that polymers such as HDPE exhibit dielectric behavior and introduction of metallic filler such as copper could enhance their electrical properties
Fig. 6. Picture of fabricated samples with various transverse speeds: (a) S1, (b) S2 and (c) S3.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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Fig. 7. Cross section image of fabricated samples: (a) S4, (b) S3 and (c) S2.
significantly. It is also well-established that the higher dispersion state of metallic particles in the system, the higher electrical response of polymer composite and thus higher dielectric constants. Furthermore, the same holds good in respect of mechanical properties. Fig. 8 shows that incorporating the metallic phase inside the polymeric matrix via this variant of FSP has created a composite with well distributed copper powder in it. The good dispersion of copper particles in the system could be attributed to good stirring action of pin, additional heat providing through shoulder and adequate pressure of designed tooling system on processing zone. The effect of processing condition such as transverse speed and shoulder temperature is also shown in Fig. 8. It is clear that in higher transverse speeds such as 120 mm/min (sample S3) the amount of powder agglomerated parts increases. As
the travel speed of tool increases, higher amount of materials process per minute and as a consequent there is not enough time to adequate flow of matrix and eliminate the agglomerated powders. Furthermore, in FSP there is a general trend of decreasing amount of input heat to stir zone as transverse speed increases. Thus it is reasonable to assume that in processing with higher speeds, there would be semi molten materials inside nugget zone which result in inhomogeneous dispersion of copper particles. It also can be seen that decreasing the shoulder temperature exhibits a similar trend, that is, reducing the amount of molten materials inside nugget zone. This leads to formation of large voids and even channels (sample S4). However, in higher shoulder temperature such as 150 ◦ C (sample S6), the fabricated composite does not represent good dispersion state of copper particles which
Fig. 8. Optical polarized microscopy of various samples.
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Fig. 9. Effect of tool transverse speed on (a) ultimate tensile strength and (b) modulus of elasticity.
Fig. 10. Effect of shoulder temperature on (a) ultimate tensile strength and (b) modulus of elasticity.
could be ascribed to the material degradation due to over heating. 3.3. Mechanical properties of fabricated samples The obtained values of ultimate tensile strength and modulus of elasticity for fabricated HDPE/Cu composites are reported in terms of tool transverse speed and shoulder temperature in Figs. 9 and 10, respectively. It also should be mentioned that the ultimate tensile strength and elastic modulus of parent polymer, which was not processed by FSP, was measured to be 23 and 540 (MPa), respectively. Fig. 11a and b shows the prepared tensile test samples and fractured sample after applying test, respectively. According to results, a 10% increase was observed in the case of ultimate tensile strength of samples fabricated via this variant of FSP approach, whereas the copper/polyethylene composites prepared by other methods [4,24] showed a reduction in ultimate tensile strength value. In addition, modulus of elasticity of fabricated samples shows a 30%
enhancement. These results verify the higher efficiency of novel variant of FSP approach to improve mechanical properties of polymers rather than the other conventional methods. The high values of ultimate tensile strength and modulus of elasticity obtained by this method could be attributed to the good dispersion and higher level of interfacial adhesion between copper particles and polymer matrix, which can improve the mechanical properties [24]. However, we postulate that the effect of this variant of FSP on wetting of filler and bonding of matrix could be other important factors determining the mechanical properties of composites which merits further investigation in future studies. Fig. 9 shows the effects of tool transverse speed on tensile strength and modulus of elasticity. It clearly reveals how, increasing speed to 120 mm/min decrease mechanical properties. Lower travel speeds result in low cooling rate, slower crystals growth and allow polymer to crystallize. Therefore, a polymeric composite with slower crystals growth exhibits enhanced mechanical properties since this could improve the spherulite structure [27]. However,
Fig. 11. Photograph of (a) the prepared tensile test samples and (b) fractured sample after applying test.
Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007
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a clear reduction is observed when processing with 20 mm/min transverse speed. Also, it is true that at lower speeds, the time of stirring action is longer which lead to an enhanced state of dispersion; but on the other hand, increasing the travel speed causes the stirring zone to be compressed, which induces higher amounts of thermo-mechanical stress on the polymer leading to the improvement in dispersion state of copper particles [24]. That is why the 60 mm/min transverse speed is the best value for fabricating metal–polymer composites by this variant of FSP. On the role of shoulder temperature in mechanical properties of composites (Fig. 10), it has to be mentioning that processing in temperatures lower than melting point of HDPE would weaken the mechanical properties due to the reasons have discussed in Section 3.1. However, it was expected that there would be a decrease in modulus of elasticity when the shoulder temperature goes up to 150 ◦ C; instead, we observed that there is a general trend of enhancement of elastic modulus as temperature increases. It may be contributed to the wider zone of stir and thus longer chains inside processing area at higher shoulder temperatures. 4. Conclusions In this article, a novel variant of friction stir processing has been developed for producing of metal polymer composites. A tooling system was designed which consists of a rotating pin, a stationary shoe shaped shoulder and a heating system located inside the shoulder. Copper/high density polyethylene composites were successfully fabricated by good distribution of metal particles inside polymeric matrix as well as improved surface quality via novel variant of FSP, which cannot be achieved by conventional methods. It was found that a 10% increase in ultimate tensile strength and 30% enhancement in modulus of elasticity could be achieved for the samples produced at transverse speed of 60 mm/min and shoulder temperatures of 110 ◦ C, which is higher than values observed in conventional methods. This could be ascribed to the good dispersion of metal particles in polyethylene matrix, controlled cooling rate of polymer structure and sufficient heat input into stir zone. Eventually, it can be concluded that this variant of FSP is a very efficient method for fabricating polymer metal surface and bulk composites. Acknowledgement The author gratefully acknowledges Shahram Alyali and Vahid Tavakolkhah for their contributions. References [1] Gungor A. Mechanical properties of iron powder filled high density polyethylene composites. Mater Des 2007;28:1027–30. [2] Zebarjad SM, Noroozi M. Production of polyethylene/carbon nanotube nanocomposite using mechanical milling process and investigation of its microstructure. In: Fibre reinforced composite conference. 2007 [abstract no. 107].
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Please cite this article in press as: Azarsa E, Mostafapour A. On the feasibility of producing polymer–metal composites via novel variant of friction stir processing. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.08.007