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Semisolid-rolling and annealing process of woven carbon fibers reinforced Al-matrix composites
Accepted Manuscript Title: Semisolid-rolling and Annealing Process of Woven Carbon Fibers Reinforced Al-matrix Composites Author: Junjia Zhang Shichao Liu Yiping Lu Li Jiang Yubo Zhang Tingju Li PII: DOI: Reference:
S1005-0302(17)30010-5 http://dx.doi.org/doi:10.1016/j.jmst.2017.01.002 JMST 889
To appear in: Received date: Revised date: Accepted date:
4-7-2016 16-8-2016 22-8-2016
Please cite this article as: Junjia Zhang, Shichao Liu, Yiping Lu, Li Jiang, Yubo Zhang, Tingju Li, Semisolid-rolling and Annealing Process of Woven Carbon Fibers Reinforced Al-matrix Composites, http://dx.doi.org/10.1016/j.jmst.2017.01.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Semisolid-rolling and Annealing Process of Woven Carbon Fibers Reinforced Al-matrix Composites Junjia Zhang, Shichao Liu, Yiping Lu, Li Jiang, Yubo Zhang*, Tingju Li*
Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
[Received 4 July 2016; Received in revised form 16 August 2016; Accepted 22 August 2016] *Corresponding authors. Tel.:/Fax: +86 411 84708940. E-mail addresses: [email protected] (Yubo Zhang); [email protected] (Tingju Li).
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Semisolid-rolling method was successfully developed to prepare the Ni-coated woven carbon fibers reinforced Al-matrix composite. Due to the appropriate matrix flowability and rolling pressure, the Al-matrix could infiltrate into the woven fibers sufficiently and attach to the reinforcements closely forming a smooth interface. The rolling speed of 4 rad/min offered a subtle equilibrium between the heat transfer and the material deformation. The covering matrix should be controlled at semisolid state to provide a better infiltration behavior and a protective effect on the carbon fibers. With the addition of fibers, an improvement for more than 25% was obtained in the bending strength of the materials. Furthermore, the woven carbon fibers could strengthen the composite in multiple directions, rather than only along the fiber longitudinal directions. The annealing process promoted the Ni coating to react with and to diffuse into the matrix, resulted in an obvious increase of the bending strength. Key words: Metal-matrix composites; Woven carbon fiber; Semisolid-rolling process; Annealing; Mechanical property 1. Introduction Carbon fibers reinforced Al matrix composites (Cf/Al) have received considerable attention because of their high specific strength, outstanding ductility and superior thermal conductivity[1]. They are expected to have specific application possibilities in the field of automobile and aerospace industries[2,3]. According to the different morphologies of the carbon fibers, the Cf/Al can be mainly classified into three categories: short carbon fibers reinforced Al-matrix composites, continuous carbon fiber bundles reinforced Al-matrix composites and woven carbon fibers reinforced Al-matrix composites[4‒6]. The short fibers offer the composites a better isotropic property, since the short reinforcements are able to keep uniform distribution in all directions. The continuous fiber bundle improves the unidirectional strength of the composite, and therefore, it is more suitable for using in the pipe, the rod and also the wire materials. The woven structure could ensure a more stable reinforcement distribution in the matrix, and effectively reduces the fiber clustering defects. 2
The major problem in fabrications of these high performance composites is to obtain a well-bonded interface between the matrix and reinforcements[7‒9]. Due to the poor wettability, infiltration fails and porosity defects often occur under the typical casting conditions. Although the temperature rise could improve the wettability of fibers obviously, it promotes the formation of Al4C3 phase at the same time. This undesirable interface reaction product is detrimental to the composites by greatly reducing their mechanical and physical properties[10]. In order to get a well-bonded interface, deposition of coatings on reinforcements and application of pressure during fabrication are the most common practices. Metals, such as Ni and Cu elements, are widely used as coating materials, because the liquid Al can easily wet these solid metallic coatings at typical casting temperature[3,11‒13]. The refractory ceramic materials are also used to coat the fibers for retarding the harmful interfacial reactions, due to their inertness towards both the matrix and fibers[14‒16]. Electroless plating and electro plating are adopted for the metallic coating, while the ceramic coatings are usually obtained through the vapor deposition and the sol-gel process. Recently, an increasing number of experts in the Cf/Al field have focused on the advancement of fabrication processes[17]. The appropriate pressure is introduced into the process by many different methods. Gas pressure infiltration method uses gas pressure to facilitate the molten matrix infiltrating into the fiber bundles, which is hold in vacuum conditions[18]. Daoud reported that the application of the gas pressure could force the matrix attaching closely to the carbon fibers, and effectively improved the interface bonding degree of the composites[19,20]. Squeeze casting method uses the mechanical pressure, rather than the gas pressure to improve the infiltrating ability, and the mechanical pressure is usually higher than the gas pressure. Hajjari et al.[21,22] discovered that the appropriate mechanical pressure was approximately 30 MPa to prepare carbon fibers reinforced Al-matrix composites. Due to a relatively low preparation temperature, which can effectively reduce the interfacial reactions, spark plasma sintering method is also widely used. In addition to the above fabrication methods, centrifugal infiltration, ultrasonic infiltration, powder metallurgy process 3
and other methods have also been successfully developed[23‒27]. However, it is still difficult to prepare bulk composites and maintain continuous production. In this study, mechanical pressure was introduced by a semisolid-rolling method, which would provide a beneficial exploration for further continuous production technology. The woven carbon fibers were selected as the reinforcements. The work focused on suitable preparation technologies as well as the two-dimensional strengthening effect. The effect of annealing on the interface characteristic and the bending strength of the composite were also systematically investigated. 2. Experimental Procedures 2.1 Materials and Pre-treatment Commercial pure aluminum (99.7 wt.%) was used as metal matrix, and the experimental plain weave of polyacrylonitrile-based carbon fiber was produced by the Institute of Coal Chemistry, Chinese Academy of Sciences. There were approximately 3000 fibers per bundle, and the strength was more than 3.5 GPa. The woven fibers were heated at 773 K for 45 min to get rid of the epoxy resin glue from the surface [28]. Then, they were coated with a smooth and uniform Ni/P coating layer by electroless method, more details were reported in Ref. [6]. The obtained coatings had a thickness of ~0.5 μm and a phosphorus content of ~10 wt%, and it would offer a better wetting behavior during the manufacturing process. 2.2 Composite Fabrication The composite samples were prepared through a rolling process, it was conducted with the co-rolling of a solid Al back-sheet, a woven carbon fiber, and a Al top-film: the coated woven carbon fibers (120 mm × 60 mm, 670 K) was placed in the middle of the solid Al back-sheet (150 mm × 80 mm × 2 mm, 620 K). The molten Al was then poured from the tundish to cover the woven fibers. The rolling process was started with the covering aluminum at three different states: (1) liquid state (~960 K), (2) mushy state (~930 K), and (3) solid state (~900 K). The roller spacing is 4 mm, and the diameter of the roll wheel is 250 mm. The rolling speed of 1‒6 rad/min was respectively used as to investigate the appropriate technological parameter. For comparison, the matrix without reinforcements was also prepared under the same 4
conditions. 2.3 Microstructures and Mechanical Properties To investigate the infiltration results, the composite samples were sectioned, and the microstructures were observed using a Zeiss Supra 55 scanning electron microscope (SEM). The mechanical property of the samples were analyzed by a three-point bending test, which were conducted using a span length of 20 mm and a crosshead speed of 1.2 mm min-1.
The experimental bending samples (50 mm ×10
mm ×3.5 mm) were cut from the composites along different directions to evaluate the two-dimensional strengthening effect of the woven carbon fibers. After the measurements, SEM was used to reveal the microstructural features of the failed samples. 2.4 Heat Treatment In order to improve the mechanical property, the composites, as well as the matrix prepared under the same conditions were heated at 750 K for different times. The microstructure of the interfaces was observed using the SEM. The bending specimens were then cut from the annealed materials. The composite specimens owned the carbon fibers oriented parallel (and/or perpendicular) to their longitudinal direction. For each condition, three specimens were respectively tested. 3. Results and Discussion 3.1 Composite Fabrication and Microstructures Fig. 1 shows the macro-morphologies of the composite samples obtained with different rolling speeds. When the speed was slow (1 rad/min), many severe crack defects would form during the rolling process, as shown in Fig. 1(a). This generation of the crack was caused by long contact time between Al-matrix and steel roller. Because of the high heat conductivity coefficient and the certain rolling pressure, a large deformation of the Al-matrix occurred at low temperature. With the rolling speed increased to 2 and 3 rad/min, the contact time became shorter; as a result, the crack defects were significantly reduced (Fig. 1(b) and (c)). The subtle equilibrium between the heat transfer and the material deformation was obtained when the rolling speed reached 4 rad/min. The macro-morphology of the composite sample is shown in 5
Fig. 1(d), a flat and smooth surface can be observed without any obvious crack defects. As the rolling speed further increased to 5 rad/min, the sample had a slight warpage (Fig. 1(e)), which could be attributed to the differential deformation between the upper and lower parts of the composite. Since the upper part of the sample, which was made up of the original top-film, had a higher temperature than the lower part, which was made up of the original back-sheet, heat transfers would occur in the internal system of the composite during rolling, as well as between the composite and the rollers. When the speed of rolling was high, the rolling time was too short to get an insufficient heat transfer. Under such conditions, a larger deformation would take place at the upper part, and the sheet composite was down-warped. As shown in Fig. 1(f), this warpage tendency was enhanced with the rolling speed increasing to 6 rad/min. The morphologies for the composites obtained with different states of covering Al are shown in Fig. 2. When liquid Al (~960 K) was used as the raw covering materials, the woven carbon fibers were well infiltrated by the Al matrix with a good fiber distribution, and no apparent porosity or significant casting defects were observed in most areas (Fig. 2(a)). However, in the internal woven fibers part, the un-infiltrated defects remained after the forming process. This can be attributed to that the high temperature (~960 K) of the covering Al delayed the solidification process, and the good flow ability would be kept for too long time. Therefore, under the rolling pressure the liquid Al would flow away from the rollers, rather than infiltrate into the woven carbon fibers, which significantly weakened the infiltrating ability and leaded to the final un-infiltrated defects. The rolling process was then carried out with the covering Al in a mushy state (~930 K), and the microstructure of the obtained composite is shown in Fig. 2(b). Since mushy state Al had a poor flow ability, it could not escape from the rollers during the forming process. The rolling pressure promoted the Al-matrix to fill up all the interspaces; as a result, a sufficient infiltration effect was achieved without any remaining un-infiltrated region. The fiber-Ni, Ni-Al and also fiber-Al interfaces are all 6
smooth and uniform without any discontinuities, and no noticeable void can be observed even in a higher magnification micrograph in Fig. 2(b), which means a good bonding between the carbon fibers and the matrix. In addition, there are no fiber clusters or fiber missing defects appearing in the microstructure, and the overall presence of the intact woven carbon fibers is obvious. Since this appropriate Al state together with the sufficient rolling pressure will not change the distribution of the fibers at the forming process, the two different kinds of fiber distribution directions are easily distinguished. Fig. 2(c) shows the microstructure of the composite sample obtained with the just solidified Al (~900 K) as raw covering material. As the solid Al could not flow away, they were pressed into the woven fibers efficiently, and no un-infiltrated region remained. However, the fiber fractures can be easily found, meaning that the reinforcements have suffered serious damages. When the solid matrix underwent a large deformation, the woven carbon fibers in the matrix were difficult to retain in their original location. The warp and weft fibers were disorganized, and cannot be distinguished. The fiber missing defects also appeared at the interface of the reinforcements and the matrix. Furthermore, although the solid Al matrix had a high temperature (~900 K), it was still too hard for the carbon fiber which owned a poor shear strength. Many fibers were cut off into short fibers. Therefore, the temperature of 900 K is relatively low for preparing the composite. According to the above observations and analysis, the rolling speed of 4 rad/min and the covering Al temperature of ~930 K are identified as the optimal parameters in the present study. To further investigate the fiber morphology in the composite, the matrix was dissolved by the concentrated sodium hydroxide solution. After the matrix complete dissolution, the woven fibers were fully exposed, as shown in Fig. 2(d). The reinforcements maintained their original appearance, and there were no obvious fiber fractures or dislocation occurrences, which indicated that the mushy state could keep an appropriate flow ability to protect the carbon fibers from large shear stress. Therefore, this semisolid rolling method is adequate to obtain the composite with a better microstructure. 7
3.2 Bending Properties In order to investigate the two-dimensional strengthening effect of the woven carbon fibers on the composite, two samples (50 mm × 10 mm × 3.5 mm) were respectively cut from the materials along different directions, and the schematic diagram of the sampling method is shown in Fig. 3(a). The carbon fibers in sample A are oriented parallel (and/or perpendicular) to the edge of the sample (Fig. 3(b)), while the carbon fibers in sample B have an orientation angle of 45° (Fig. 3(c)). The three-point bending tests were carried out, and the bending stress-deflection curves of the composite and the matrix are shown in Fig. 4(a). The results showed that the composites with different distribution fibers both had an obvious increase in the bending strength, though the carbon fibers only had a high specific strength along their longitudinal direction. It means that the woven form reinforcements are able to strengthen the composite to resist the stress from multiple directions, rather than only along the two fiber longitudinal directions. Since the Al matrix had a good plastic deformation ability, there was no fracture failure occurring during the bending test. Even the deflection reached up to 9 mm, all the curves could keep a integrate shape without any sudden breakages. The bending stress of the Al-matrix reached the maximum of 97 MPa at the deflection of 6.8 mm, then, kept steady with little fluctuation. Owing to the addition of woven carbon fibers, the bending strength of the composite materials evidently increased to 126 MPa (sample A) and 122 MPa (sample B), having an improvement of 29.9% (sample A) and 25.8% (sample B), respectively. This improvement can be attributed to the high specific strength of carbon fibers and also the excellent interface bonding resulting from the appropriate fabrication process. Because of the different distribution of the fibers in composites, a difference in stress trend appears between sample A and sample B. The bearing stress of sample A has a rapid increase before the deflection reaches ~4.5 mm, and then it maintains around 126 MPa until a slow decrease at the deflection of ~7.5 mm. The bearing stress of sample B consistently remains a continuous rising tendency all along the deflection process without any decrease occurring. 8
The microstructures of the samples after three-point bending test were investigated by SEM, as shown in Fig. 4(b) and (c). For sample A, two kinds of carbon fibers, the longitudinal ones and the latitudinal ones, can still be easily distinguished. The longitudinal one, which locates perpendicular to the bending direction in Fig. 4(b), had an obvious prevention effect on the deformation, and caused an immediate resistance to the bending load once the deformation started. As a result, the curve of sample A in Fig. 4(a) shows a rapid increase at the beginning of the deflection. The decrease appearing at large deflection (~7.5 mm) may be attributed to that the addition of the latitudinal fibers had nothing to do with the strengthening in this bending direction, but introduced many interfaces, which were considered sensitive to the stress. Lots of cracks can be observed in the magnified picture in Fig. 4(b), they all locate along the bending direction, and perpendicular to the longitudinal fibers. With the increase of bending degree, the initial tiny crack defects existed in the matrix or on the interface would become larger and propagate along the load direction. When being encountered with the longitudinal carbon fibers, the propagation was blocked, and the bearing stress had a significant improvement. Then, as the deformation was large enough, the cracks deflected and propagated along the interface, and went across the fibers at last[13]. Fig. 4(c) shows the microstructure of the failed sample B. Since both kinds of the original carbon fibers in sample B had an orientation angle of 45° to the sample edge, no longitudinal direction fiber existed to firsthand prevent the deformation. The bending stress of sample B was lower than that of sample A when the deflection was small. With the increase of the deflection, the stress was able to transfer and distribute to both two kinds of fibers; as a result, the curve of sample B in Fig. 4(a) exhibits a continuous and stable rising tendency. Since all carbon fibers were useful to prevent the deformation, sample B was able to offer a better stability at the large deflection situation. As can be seen in the magnified picture in Fig. 4(b), no obvious crack defect was observed, and the carbon fibers were still attaching to the matrix closely after the test. According to the above analyses and comparison, the woven carbon fibers could 9
offer almost the same strengthening effects on samples A and B, which implies that the woven form of reinforcements are much suited to be used in plate and jar-shaped materials for two-dimensional strengthening. 3.3 Annealing Process The strengths of the composite and the matrix are related to the annealing treatment[29], and Fig. 5(a) shows the relationship between the annealing time and the bending strength under 750 K. The bending strength of the Al-matrix had a slight change before 90 min. According to the component analysis, there existed a small amount of Fe (0.15 wt%) and Si (0.06 wt%) in the matrix as impurity elements, the precipitation and phase change might take place during the annealing. Then, an obvious annealing softening occurred, which could be caused by the reduction of the dislocations, and also the recovery and recrystallization process. According to Hall‒ Petch relationship[30], when the annealing time was long enough (beyond 90 min in this work), the matrix bending strength would have a significant decrease. The bending strength of the composite is also shown in Fig. 5(a): it appears in the trend of initial increase and subsequent decrease, and the maximum is achieved at 120 min. The difference of the strength value between the composite and the matrix, which implies the strengthening effect resulting from the fiber addition is shown in Fig. 5(b). It has an increase until reaching the maximum value at 150 min. Due to the low temperature and the rapid cooling, a rapid solidification would take place during the manufacturing process. As a result, there was no enough time for the Ni coating to react with or diffuse into the matrix, and the coatings all remained in their original form, as shown in the magnification micrograph in Fig. 2(b). The annealing treatment played a positive role in enhancing the interface combination by the diffusion of the Ni and the Al-Ni intermetallic reaction [31], and the stress could be transferred from matrix to fibers more effectively. However, with long time of annealing, the Ni layer would completely diffuse into the matrix, and the carbon fibers contacted the matrix directly without protection. As reported before, when carbon fibers were long time exposed to the Al matrix at high temperature[32], the brittle phase Al4C3, which is harmful to the composite, would be produced, and it could severely damage the 10
carbon fibers and weaken the interfacial bonding. The strengthening effect had an decrease when the annealing time was too long, as shown in Fig. 5(b). The micromorphology of the composites after annealing for different time is shown in Fig. 6. For the composite without annealing, the Ni coatings can be easily observed (Fig. 6(a)). They closely cover on the fiber reinforcements and tightly combine to the Al matrix, which means that the Ni coating could offer a better wetting behavior, and the appropriate rolling pressure would not peel the Ni coatings off from the fibers. The thickness of the Ni layer is about 0.5 μm, the same as that of the original coating. It is strongly suggested that no obvious reaction or diffusion occurred during the preparation process, which is in good agreement with our above speculation. The microstructure of the composite after 30 min annealing is shown in Fig. 6(b). The interface between the Ni layer and the Al matrix turned to be unsmooth and of irregular shaped, implying that a diffusion took place at the annealing process. Because of this diffusion, the thickness of the original coating had a significant increase. As shown in the magnification micrograph in Fig. 6(b), the thickness of the coating at some part reached ~2.0 μm, and the coating dispersed from the fibers was also found in the microstructure. With the increase of the annealing time to 90 min, the diffusion of the Ni layer became more sufficient, as observed in Fig. 6(c). Lots of Ni atoms did not deposit around the individual fibers any more in a ring-shaped form. They were separated from the fiber surface and segregated together near the woven carbon fibers. The segregation region was easily found with a light color. The quantitative analysis demonstrates that this region mainly contains Al and Ni elements, and their atomic ratio is approximately 4:1, which may imply that the Ni atoms mainly exist in the form of intermetallic compounds [33,34]. Fig. 6(d) shows the micromorphology of the composites after annealing for 150 min. No serious Ni segregation can be observed in the microstructure, which means that the diffusion process tends to close. The vast majority of Ni atoms had dissolved into the matrix and became incorporated as a strengthening element, and this dissolution could bring a positive role to the matrix by increasing its hardness and strength[15]. The carbon fibers contacted Al matrix directly and the interface was still 11
smooth without any defects. Because of the sufficient diffusion and the limited harmful interface reaction, the addition of the woven fibers could offer the largest improvement (37 MPa) in the bending strength. Therefore, appropriate diffusion has been proven to be positive to improve the strengthening effect of the carbon fibers. 4. Conclusions (1) A semisolid-rolling method was successfully developed to prepare the woven carbon fibers reinforced Al-matrix composite. Due to the appropriate flowability and reasonable pressure, the Al-matrix could infiltrate into the woven without any obvious defects. (2) The optimal rolling speed was proven to be 4 rad/min, which resulted in a subtle equilibrium between the heat transfer and the material deformation to obtain a flat and unbroken composite macrostructure. The semisolid state Al used as the covering materials was able to improve the infiltration behavior and protect the carbon fibers. (3) The addition of the woven carbon fibers provided an obvious improvement in the bending strength of the material for more than 25%. Furthermore, the strength of composites were strengthened in multiple directions, rather than only along the fiber longitudinal direction. (4) The annealing could strengthen the composite by promoting the Ni coating to react with and to diffuse into the matrix. On the other hand, it also weakened the composite by promoting the grain growth and harmful interface reaction. The best effect was achieved at the annealing time of 120 min, at which point the bending strength reached the maximum value of 140 MPa.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51271042 and 51501027), the Fundamental Research Funds for the Central Universities, the Key Laboratory of Basic Research Projects of Liaoning Province Department of Education (No. LZ2014007), the Natural Science Foundation of Liaoning Province (No. 2014028013), and China Postdoctoral Science Foundation (No. 2015M570246). 12
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Figure captions Fig. 1 Macro-morphologies of the composites obtained with different rolling speeds: (a) 1 rad/min; (b) 2 rad/min; (c) 3 rad/min; (d) 4 rad/min; (e) 5 rad/min; (f) 6 rad/min. Fig. 2 the morphologies of the composites obtained with different covering Al:(a) liquid covering Al, (b) mushy covering Al, (c) solid covering Al and (d) stripping the woven carbon fibers from the Al-composite using concentrated sodium hydroxide solution. Fig. 3 schematic diagrams of (a) sampling method, (b) fiber distribution in sample A and (c) fiber distribution in sample B. Fig. 4 results of the three-point bending tests: (a) stress-deflection curves; (b) micro-structure of the sample A; (c) micro-structure of the sample B. Fig. 5 Effect of annealing time on the bending strength: (a) bending strength of composite and matrix; (b) improvement resulted from the fibers addition. Fig. 6 Micromorphologies of the annealed composites after different time: (a) 0 min; (b) 30 min; (c) 90 min; (d) 150 min.
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S1005-0302(17)30010-5 http://dx.doi.org/doi:10.1016/j.jmst.2017.01.002 JMST 889
To appear in: Received date: Revised date: Accepted date:
4-7-2016 16-8-2016 22-8-2016
Please cite this article as: Junjia Zhang, Shichao Liu, Yiping Lu, Li Jiang, Yubo Zhang, Tingju Li, Semisolid-rolling and Annealing Process of Woven Carbon Fibers Reinforced Al-matrix Composites, http://dx.doi.org/10.1016/j.jmst.2017.01.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Semisolid-rolling and Annealing Process of Woven Carbon Fibers Reinforced Al-matrix Composites Junjia Zhang, Shichao Liu, Yiping Lu, Li Jiang, Yubo Zhang*, Tingju Li*
Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
[Received 4 July 2016; Received in revised form 16 August 2016; Accepted 22 August 2016] *Corresponding authors. Tel.:/Fax: +86 411 84708940. E-mail addresses: [email protected] (Yubo Zhang); [email protected] (Tingju Li).
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Semisolid-rolling method was successfully developed to prepare the Ni-coated woven carbon fibers reinforced Al-matrix composite. Due to the appropriate matrix flowability and rolling pressure, the Al-matrix could infiltrate into the woven fibers sufficiently and attach to the reinforcements closely forming a smooth interface. The rolling speed of 4 rad/min offered a subtle equilibrium between the heat transfer and the material deformation. The covering matrix should be controlled at semisolid state to provide a better infiltration behavior and a protective effect on the carbon fibers. With the addition of fibers, an improvement for more than 25% was obtained in the bending strength of the materials. Furthermore, the woven carbon fibers could strengthen the composite in multiple directions, rather than only along the fiber longitudinal directions. The annealing process promoted the Ni coating to react with and to diffuse into the matrix, resulted in an obvious increase of the bending strength. Key words: Metal-matrix composites; Woven carbon fiber; Semisolid-rolling process; Annealing; Mechanical property 1. Introduction Carbon fibers reinforced Al matrix composites (Cf/Al) have received considerable attention because of their high specific strength, outstanding ductility and superior thermal conductivity[1]. They are expected to have specific application possibilities in the field of automobile and aerospace industries[2,3]. According to the different morphologies of the carbon fibers, the Cf/Al can be mainly classified into three categories: short carbon fibers reinforced Al-matrix composites, continuous carbon fiber bundles reinforced Al-matrix composites and woven carbon fibers reinforced Al-matrix composites[4‒6]. The short fibers offer the composites a better isotropic property, since the short reinforcements are able to keep uniform distribution in all directions. The continuous fiber bundle improves the unidirectional strength of the composite, and therefore, it is more suitable for using in the pipe, the rod and also the wire materials. The woven structure could ensure a more stable reinforcement distribution in the matrix, and effectively reduces the fiber clustering defects. 2
The major problem in fabrications of these high performance composites is to obtain a well-bonded interface between the matrix and reinforcements[7‒9]. Due to the poor wettability, infiltration fails and porosity defects often occur under the typical casting conditions. Although the temperature rise could improve the wettability of fibers obviously, it promotes the formation of Al4C3 phase at the same time. This undesirable interface reaction product is detrimental to the composites by greatly reducing their mechanical and physical properties[10]. In order to get a well-bonded interface, deposition of coatings on reinforcements and application of pressure during fabrication are the most common practices. Metals, such as Ni and Cu elements, are widely used as coating materials, because the liquid Al can easily wet these solid metallic coatings at typical casting temperature[3,11‒13]. The refractory ceramic materials are also used to coat the fibers for retarding the harmful interfacial reactions, due to their inertness towards both the matrix and fibers[14‒16]. Electroless plating and electro plating are adopted for the metallic coating, while the ceramic coatings are usually obtained through the vapor deposition and the sol-gel process. Recently, an increasing number of experts in the Cf/Al field have focused on the advancement of fabrication processes[17]. The appropriate pressure is introduced into the process by many different methods. Gas pressure infiltration method uses gas pressure to facilitate the molten matrix infiltrating into the fiber bundles, which is hold in vacuum conditions[18]. Daoud reported that the application of the gas pressure could force the matrix attaching closely to the carbon fibers, and effectively improved the interface bonding degree of the composites[19,20]. Squeeze casting method uses the mechanical pressure, rather than the gas pressure to improve the infiltrating ability, and the mechanical pressure is usually higher than the gas pressure. Hajjari et al.[21,22] discovered that the appropriate mechanical pressure was approximately 30 MPa to prepare carbon fibers reinforced Al-matrix composites. Due to a relatively low preparation temperature, which can effectively reduce the interfacial reactions, spark plasma sintering method is also widely used. In addition to the above fabrication methods, centrifugal infiltration, ultrasonic infiltration, powder metallurgy process 3
and other methods have also been successfully developed[23‒27]. However, it is still difficult to prepare bulk composites and maintain continuous production. In this study, mechanical pressure was introduced by a semisolid-rolling method, which would provide a beneficial exploration for further continuous production technology. The woven carbon fibers were selected as the reinforcements. The work focused on suitable preparation technologies as well as the two-dimensional strengthening effect. The effect of annealing on the interface characteristic and the bending strength of the composite were also systematically investigated. 2. Experimental Procedures 2.1 Materials and Pre-treatment Commercial pure aluminum (99.7 wt.%) was used as metal matrix, and the experimental plain weave of polyacrylonitrile-based carbon fiber was produced by the Institute of Coal Chemistry, Chinese Academy of Sciences. There were approximately 3000 fibers per bundle, and the strength was more than 3.5 GPa. The woven fibers were heated at 773 K for 45 min to get rid of the epoxy resin glue from the surface [28]. Then, they were coated with a smooth and uniform Ni/P coating layer by electroless method, more details were reported in Ref. [6]. The obtained coatings had a thickness of ~0.5 μm and a phosphorus content of ~10 wt%, and it would offer a better wetting behavior during the manufacturing process. 2.2 Composite Fabrication The composite samples were prepared through a rolling process, it was conducted with the co-rolling of a solid Al back-sheet, a woven carbon fiber, and a Al top-film: the coated woven carbon fibers (120 mm × 60 mm, 670 K) was placed in the middle of the solid Al back-sheet (150 mm × 80 mm × 2 mm, 620 K). The molten Al was then poured from the tundish to cover the woven fibers. The rolling process was started with the covering aluminum at three different states: (1) liquid state (~960 K), (2) mushy state (~930 K), and (3) solid state (~900 K). The roller spacing is 4 mm, and the diameter of the roll wheel is 250 mm. The rolling speed of 1‒6 rad/min was respectively used as to investigate the appropriate technological parameter. For comparison, the matrix without reinforcements was also prepared under the same 4
conditions. 2.3 Microstructures and Mechanical Properties To investigate the infiltration results, the composite samples were sectioned, and the microstructures were observed using a Zeiss Supra 55 scanning electron microscope (SEM). The mechanical property of the samples were analyzed by a three-point bending test, which were conducted using a span length of 20 mm and a crosshead speed of 1.2 mm min-1.
The experimental bending samples (50 mm ×10
mm ×3.5 mm) were cut from the composites along different directions to evaluate the two-dimensional strengthening effect of the woven carbon fibers. After the measurements, SEM was used to reveal the microstructural features of the failed samples. 2.4 Heat Treatment In order to improve the mechanical property, the composites, as well as the matrix prepared under the same conditions were heated at 750 K for different times. The microstructure of the interfaces was observed using the SEM. The bending specimens were then cut from the annealed materials. The composite specimens owned the carbon fibers oriented parallel (and/or perpendicular) to their longitudinal direction. For each condition, three specimens were respectively tested. 3. Results and Discussion 3.1 Composite Fabrication and Microstructures Fig. 1 shows the macro-morphologies of the composite samples obtained with different rolling speeds. When the speed was slow (1 rad/min), many severe crack defects would form during the rolling process, as shown in Fig. 1(a). This generation of the crack was caused by long contact time between Al-matrix and steel roller. Because of the high heat conductivity coefficient and the certain rolling pressure, a large deformation of the Al-matrix occurred at low temperature. With the rolling speed increased to 2 and 3 rad/min, the contact time became shorter; as a result, the crack defects were significantly reduced (Fig. 1(b) and (c)). The subtle equilibrium between the heat transfer and the material deformation was obtained when the rolling speed reached 4 rad/min. The macro-morphology of the composite sample is shown in 5
Fig. 1(d), a flat and smooth surface can be observed without any obvious crack defects. As the rolling speed further increased to 5 rad/min, the sample had a slight warpage (Fig. 1(e)), which could be attributed to the differential deformation between the upper and lower parts of the composite. Since the upper part of the sample, which was made up of the original top-film, had a higher temperature than the lower part, which was made up of the original back-sheet, heat transfers would occur in the internal system of the composite during rolling, as well as between the composite and the rollers. When the speed of rolling was high, the rolling time was too short to get an insufficient heat transfer. Under such conditions, a larger deformation would take place at the upper part, and the sheet composite was down-warped. As shown in Fig. 1(f), this warpage tendency was enhanced with the rolling speed increasing to 6 rad/min. The morphologies for the composites obtained with different states of covering Al are shown in Fig. 2. When liquid Al (~960 K) was used as the raw covering materials, the woven carbon fibers were well infiltrated by the Al matrix with a good fiber distribution, and no apparent porosity or significant casting defects were observed in most areas (Fig. 2(a)). However, in the internal woven fibers part, the un-infiltrated defects remained after the forming process. This can be attributed to that the high temperature (~960 K) of the covering Al delayed the solidification process, and the good flow ability would be kept for too long time. Therefore, under the rolling pressure the liquid Al would flow away from the rollers, rather than infiltrate into the woven carbon fibers, which significantly weakened the infiltrating ability and leaded to the final un-infiltrated defects. The rolling process was then carried out with the covering Al in a mushy state (~930 K), and the microstructure of the obtained composite is shown in Fig. 2(b). Since mushy state Al had a poor flow ability, it could not escape from the rollers during the forming process. The rolling pressure promoted the Al-matrix to fill up all the interspaces; as a result, a sufficient infiltration effect was achieved without any remaining un-infiltrated region. The fiber-Ni, Ni-Al and also fiber-Al interfaces are all 6
smooth and uniform without any discontinuities, and no noticeable void can be observed even in a higher magnification micrograph in Fig. 2(b), which means a good bonding between the carbon fibers and the matrix. In addition, there are no fiber clusters or fiber missing defects appearing in the microstructure, and the overall presence of the intact woven carbon fibers is obvious. Since this appropriate Al state together with the sufficient rolling pressure will not change the distribution of the fibers at the forming process, the two different kinds of fiber distribution directions are easily distinguished. Fig. 2(c) shows the microstructure of the composite sample obtained with the just solidified Al (~900 K) as raw covering material. As the solid Al could not flow away, they were pressed into the woven fibers efficiently, and no un-infiltrated region remained. However, the fiber fractures can be easily found, meaning that the reinforcements have suffered serious damages. When the solid matrix underwent a large deformation, the woven carbon fibers in the matrix were difficult to retain in their original location. The warp and weft fibers were disorganized, and cannot be distinguished. The fiber missing defects also appeared at the interface of the reinforcements and the matrix. Furthermore, although the solid Al matrix had a high temperature (~900 K), it was still too hard for the carbon fiber which owned a poor shear strength. Many fibers were cut off into short fibers. Therefore, the temperature of 900 K is relatively low for preparing the composite. According to the above observations and analysis, the rolling speed of 4 rad/min and the covering Al temperature of ~930 K are identified as the optimal parameters in the present study. To further investigate the fiber morphology in the composite, the matrix was dissolved by the concentrated sodium hydroxide solution. After the matrix complete dissolution, the woven fibers were fully exposed, as shown in Fig. 2(d). The reinforcements maintained their original appearance, and there were no obvious fiber fractures or dislocation occurrences, which indicated that the mushy state could keep an appropriate flow ability to protect the carbon fibers from large shear stress. Therefore, this semisolid rolling method is adequate to obtain the composite with a better microstructure. 7
3.2 Bending Properties In order to investigate the two-dimensional strengthening effect of the woven carbon fibers on the composite, two samples (50 mm × 10 mm × 3.5 mm) were respectively cut from the materials along different directions, and the schematic diagram of the sampling method is shown in Fig. 3(a). The carbon fibers in sample A are oriented parallel (and/or perpendicular) to the edge of the sample (Fig. 3(b)), while the carbon fibers in sample B have an orientation angle of 45° (Fig. 3(c)). The three-point bending tests were carried out, and the bending stress-deflection curves of the composite and the matrix are shown in Fig. 4(a). The results showed that the composites with different distribution fibers both had an obvious increase in the bending strength, though the carbon fibers only had a high specific strength along their longitudinal direction. It means that the woven form reinforcements are able to strengthen the composite to resist the stress from multiple directions, rather than only along the two fiber longitudinal directions. Since the Al matrix had a good plastic deformation ability, there was no fracture failure occurring during the bending test. Even the deflection reached up to 9 mm, all the curves could keep a integrate shape without any sudden breakages. The bending stress of the Al-matrix reached the maximum of 97 MPa at the deflection of 6.8 mm, then, kept steady with little fluctuation. Owing to the addition of woven carbon fibers, the bending strength of the composite materials evidently increased to 126 MPa (sample A) and 122 MPa (sample B), having an improvement of 29.9% (sample A) and 25.8% (sample B), respectively. This improvement can be attributed to the high specific strength of carbon fibers and also the excellent interface bonding resulting from the appropriate fabrication process. Because of the different distribution of the fibers in composites, a difference in stress trend appears between sample A and sample B. The bearing stress of sample A has a rapid increase before the deflection reaches ~4.5 mm, and then it maintains around 126 MPa until a slow decrease at the deflection of ~7.5 mm. The bearing stress of sample B consistently remains a continuous rising tendency all along the deflection process without any decrease occurring. 8
The microstructures of the samples after three-point bending test were investigated by SEM, as shown in Fig. 4(b) and (c). For sample A, two kinds of carbon fibers, the longitudinal ones and the latitudinal ones, can still be easily distinguished. The longitudinal one, which locates perpendicular to the bending direction in Fig. 4(b), had an obvious prevention effect on the deformation, and caused an immediate resistance to the bending load once the deformation started. As a result, the curve of sample A in Fig. 4(a) shows a rapid increase at the beginning of the deflection. The decrease appearing at large deflection (~7.5 mm) may be attributed to that the addition of the latitudinal fibers had nothing to do with the strengthening in this bending direction, but introduced many interfaces, which were considered sensitive to the stress. Lots of cracks can be observed in the magnified picture in Fig. 4(b), they all locate along the bending direction, and perpendicular to the longitudinal fibers. With the increase of bending degree, the initial tiny crack defects existed in the matrix or on the interface would become larger and propagate along the load direction. When being encountered with the longitudinal carbon fibers, the propagation was blocked, and the bearing stress had a significant improvement. Then, as the deformation was large enough, the cracks deflected and propagated along the interface, and went across the fibers at last[13]. Fig. 4(c) shows the microstructure of the failed sample B. Since both kinds of the original carbon fibers in sample B had an orientation angle of 45° to the sample edge, no longitudinal direction fiber existed to firsthand prevent the deformation. The bending stress of sample B was lower than that of sample A when the deflection was small. With the increase of the deflection, the stress was able to transfer and distribute to both two kinds of fibers; as a result, the curve of sample B in Fig. 4(a) exhibits a continuous and stable rising tendency. Since all carbon fibers were useful to prevent the deformation, sample B was able to offer a better stability at the large deflection situation. As can be seen in the magnified picture in Fig. 4(b), no obvious crack defect was observed, and the carbon fibers were still attaching to the matrix closely after the test. According to the above analyses and comparison, the woven carbon fibers could 9
offer almost the same strengthening effects on samples A and B, which implies that the woven form of reinforcements are much suited to be used in plate and jar-shaped materials for two-dimensional strengthening. 3.3 Annealing Process The strengths of the composite and the matrix are related to the annealing treatment[29], and Fig. 5(a) shows the relationship between the annealing time and the bending strength under 750 K. The bending strength of the Al-matrix had a slight change before 90 min. According to the component analysis, there existed a small amount of Fe (0.15 wt%) and Si (0.06 wt%) in the matrix as impurity elements, the precipitation and phase change might take place during the annealing. Then, an obvious annealing softening occurred, which could be caused by the reduction of the dislocations, and also the recovery and recrystallization process. According to Hall‒ Petch relationship[30], when the annealing time was long enough (beyond 90 min in this work), the matrix bending strength would have a significant decrease. The bending strength of the composite is also shown in Fig. 5(a): it appears in the trend of initial increase and subsequent decrease, and the maximum is achieved at 120 min. The difference of the strength value between the composite and the matrix, which implies the strengthening effect resulting from the fiber addition is shown in Fig. 5(b). It has an increase until reaching the maximum value at 150 min. Due to the low temperature and the rapid cooling, a rapid solidification would take place during the manufacturing process. As a result, there was no enough time for the Ni coating to react with or diffuse into the matrix, and the coatings all remained in their original form, as shown in the magnification micrograph in Fig. 2(b). The annealing treatment played a positive role in enhancing the interface combination by the diffusion of the Ni and the Al-Ni intermetallic reaction [31], and the stress could be transferred from matrix to fibers more effectively. However, with long time of annealing, the Ni layer would completely diffuse into the matrix, and the carbon fibers contacted the matrix directly without protection. As reported before, when carbon fibers were long time exposed to the Al matrix at high temperature[32], the brittle phase Al4C3, which is harmful to the composite, would be produced, and it could severely damage the 10
carbon fibers and weaken the interfacial bonding. The strengthening effect had an decrease when the annealing time was too long, as shown in Fig. 5(b). The micromorphology of the composites after annealing for different time is shown in Fig. 6. For the composite without annealing, the Ni coatings can be easily observed (Fig. 6(a)). They closely cover on the fiber reinforcements and tightly combine to the Al matrix, which means that the Ni coating could offer a better wetting behavior, and the appropriate rolling pressure would not peel the Ni coatings off from the fibers. The thickness of the Ni layer is about 0.5 μm, the same as that of the original coating. It is strongly suggested that no obvious reaction or diffusion occurred during the preparation process, which is in good agreement with our above speculation. The microstructure of the composite after 30 min annealing is shown in Fig. 6(b). The interface between the Ni layer and the Al matrix turned to be unsmooth and of irregular shaped, implying that a diffusion took place at the annealing process. Because of this diffusion, the thickness of the original coating had a significant increase. As shown in the magnification micrograph in Fig. 6(b), the thickness of the coating at some part reached ~2.0 μm, and the coating dispersed from the fibers was also found in the microstructure. With the increase of the annealing time to 90 min, the diffusion of the Ni layer became more sufficient, as observed in Fig. 6(c). Lots of Ni atoms did not deposit around the individual fibers any more in a ring-shaped form. They were separated from the fiber surface and segregated together near the woven carbon fibers. The segregation region was easily found with a light color. The quantitative analysis demonstrates that this region mainly contains Al and Ni elements, and their atomic ratio is approximately 4:1, which may imply that the Ni atoms mainly exist in the form of intermetallic compounds [33,34]. Fig. 6(d) shows the micromorphology of the composites after annealing for 150 min. No serious Ni segregation can be observed in the microstructure, which means that the diffusion process tends to close. The vast majority of Ni atoms had dissolved into the matrix and became incorporated as a strengthening element, and this dissolution could bring a positive role to the matrix by increasing its hardness and strength[15]. The carbon fibers contacted Al matrix directly and the interface was still 11
smooth without any defects. Because of the sufficient diffusion and the limited harmful interface reaction, the addition of the woven fibers could offer the largest improvement (37 MPa) in the bending strength. Therefore, appropriate diffusion has been proven to be positive to improve the strengthening effect of the carbon fibers. 4. Conclusions (1) A semisolid-rolling method was successfully developed to prepare the woven carbon fibers reinforced Al-matrix composite. Due to the appropriate flowability and reasonable pressure, the Al-matrix could infiltrate into the woven without any obvious defects. (2) The optimal rolling speed was proven to be 4 rad/min, which resulted in a subtle equilibrium between the heat transfer and the material deformation to obtain a flat and unbroken composite macrostructure. The semisolid state Al used as the covering materials was able to improve the infiltration behavior and protect the carbon fibers. (3) The addition of the woven carbon fibers provided an obvious improvement in the bending strength of the material for more than 25%. Furthermore, the strength of composites were strengthened in multiple directions, rather than only along the fiber longitudinal direction. (4) The annealing could strengthen the composite by promoting the Ni coating to react with and to diffuse into the matrix. On the other hand, it also weakened the composite by promoting the grain growth and harmful interface reaction. The best effect was achieved at the annealing time of 120 min, at which point the bending strength reached the maximum value of 140 MPa.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51271042 and 51501027), the Fundamental Research Funds for the Central Universities, the Key Laboratory of Basic Research Projects of Liaoning Province Department of Education (No. LZ2014007), the Natural Science Foundation of Liaoning Province (No. 2014028013), and China Postdoctoral Science Foundation (No. 2015M570246). 12
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Figure captions Fig. 1 Macro-morphologies of the composites obtained with different rolling speeds: (a) 1 rad/min; (b) 2 rad/min; (c) 3 rad/min; (d) 4 rad/min; (e) 5 rad/min; (f) 6 rad/min. Fig. 2 the morphologies of the composites obtained with different covering Al:(a) liquid covering Al, (b) mushy covering Al, (c) solid covering Al and (d) stripping the woven carbon fibers from the Al-composite using concentrated sodium hydroxide solution. Fig. 3 schematic diagrams of (a) sampling method, (b) fiber distribution in sample A and (c) fiber distribution in sample B. Fig. 4 results of the three-point bending tests: (a) stress-deflection curves; (b) micro-structure of the sample A; (c) micro-structure of the sample B. Fig. 5 Effect of annealing time on the bending strength: (a) bending strength of composite and matrix; (b) improvement resulted from the fibers addition. Fig. 6 Micromorphologies of the annealed composites after different time: (a) 0 min; (b) 30 min; (c) 90 min; (d) 150 min.
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