Yunhong Ling,Hn Wu,Zhohu Lin,Qingping Liu,Zhihui Zhng

a The Key Laboratory of Bionic Engineering,Ministry of Education,Jilin University,Changchun 130025,China

b School of Mechanical and Aerospace Engineering,Jilin University,Changchun 130025,China

Keywords:3D printing Shear-induced alignment Carbon fiber Aluminum matrix composites Powder metallurgy

ABSTRACT Carbon fiber reinforced aluminum composites with ordered architectures of shear-induced aligned carbon fibers were fabricated by 3D printing.The microstructures of the printed and sintered samples and mechanical properties of the composites were investigated.Carbon fibers and aluminum powder were bonded together with resin.The spatial arrangement of the carbon fibers was fixed in the aluminum matrix by shear-induced alignment in the 3D printing process.As a result,the elongation of the composites with a parallel arrangement of aligned fibers and the impact toughness of the composites with an orthogonal arrangement were 0.82% and 0.41 J/cm2,respectively,about 0.4 and 0.8 times higher than that of the random arrangement.

1.Introduction

Due to their low weight,high ductility and excellent strength,aluminum(Al)and aluminum alloy matrix composites have attracted increasing interest in the automotive,aerospace and national defense fields[1].Numerous studies on aluminum reinforced with different materials have been conducted,such as carbon fiber(CF)[2–8],carbon-nanotubes[9–14],SiC[15],B4C[16],graphite fiber[17],graphene[18,19]and so forth.The low cost and superior mechanical properties of carbon fibers are far better than those of other reinforcements[20].Thus,CFs/Al composites have been studied intensively.

The fabrication processes of CFs/Al composites include such methods as squeeze casting[21–23],gas pressure infiltration[24],centrifugal infiltration[25],liquid process[26]and spark plasma sintering(SPS)[1].Hasan Ali et al.[21]reinforced aluminum alloy 6061 with alternate carbon fiber/aluminum layers by laminate squeeze casting and the Vickers hardness was increased by~50%.A similar structure was found in Qiurong Yang et al.'s work[22]that used Cu-coated woven carbon fibers as reinforcement in the production of CFs/Al composites by SPS.Their results showed that the CFs had no significant influence on the bearing compression of the composites,but the ductility was enhanced by two times,and the tensile strength was also significantly improved.Due to the special advantages of gas pressure infiltration[24],the ultimate strength of the compactness of the CFs/Al composites was increased by three times.

To further improve the mechanical properties of CFs/Al composites,many researchers have focused on the interface of the aluminum and the carbon fibers.Se-Il Oh et al.[26]enhanced the wettability of aluminum on carbon nano-fibers by forming Al–Cu compounds,reducing the interfacial energy between the Al and Cu,and increasing the hardness and ultimate tensile strength by 131% and 102.3%,respectively.Ni-coated carbon fibers have also been widely used to impede the interfacial reaction of aluminum and carbon fibers[4,22].Rams et al.[4]used Ni-coated carbon fibers to reinforce Al 6061 alloys and reported that the formation of Al4C3was suppressed due to the formation of eutectic Ni–Al and Al3Ni on the fiber surface.The interface of the aluminum and carbon fibers was tightly bonded,and the hardness and elastic modulus of the CFs/Al composites had obvious improvement.

The above studies suggest that focus should be placed on improving the wettability and hindering the interfacial reaction of Al and CFs by coating Cu or Ni on the surface of the carbon fibers for CFs reinforced Al based composites.Designing different structural layers between the Al and CFs to evaluate the mechanical behaviors is also important as the current structures are largely limited to enhancing the mechanical properties of the composites.To achieve a more complex structure,we created a new method for preparing CFs/Al composites,which combines 3D printing with the powder metallurgy process.

3D printing has attracted enormous attention due to its high accuracy,low cost and short manufacturing time.Various materials can be printed using different techniques[27],such as metal[28–30],ceramic[31,32],cement[33,34],polymer[35,36],and biological materials[37,38],which allows it to be applied in varied fields.

In the present study,we printed four samples with different arrangements of aligned CFs in Al layers using a modified SLA based 3D printer,arranging the fibers in random,parallel,round,and orthogonal ways,as shown in Fig.1.After debonding and sintering,we investigated the microstructures,interface reaction,and mechanical properties of the CFs/Al composites.

2.Experimental section

2.1.Materials preparation

In this study,photosensitive resin with 18–22 cPas(25°C)viscosity,28–32 Dynes/cm surface tension,and<0.5 μm filtration accuracy,obtained from Shenzhen Yangcai Digital Technology Co.Ltd,China,was used as the adhesive.Spherical aluminum powder provided by Beijing Xing Rong Yuan Technology Co.Ltd,China,with a 48 μm particle size and 99.7%purity was used as the matrix material.Carbon fibers(1.76 g/cm3)with an average diameter of 7 μm and an average length of 2 mm supplied by Shanghai Kajite Chemical Technology Co.Ltd.China were used as reinforcement.

To reduce gas cavities and improve the mechanical properties of the composite materials,the quantity of the photosensitive resin should be minimized under the premise that the sample is completely cured.After a number of experiments,the quantity fraction was:Al-72.3 wt%;photosensitive resin-27.2 wt%;carbon fiber-0.5 wt%.0.5 wt%was the highest carbon fiber content that could be added to the aluminum matrix.If the carbon fiber content was higher than 0.5 wt%,it would aggregate in the mixed slurry due to the relatively large length-diameter ratio.

According to the above ratio,0.12g carbon fibers,6.00g photosensitive resin and 16.00g aluminum powder used as each layer were weighted and stirred continuously with a glass rod in a beaker until they became a uniformly dispersed slurry.

2.2.3D printing and sintering process

Fig.2(a)shows the printing process of the orthogonal and parallel arrangements.The samples were printed according to the following procedures:first,the resin container was lowered 1 mm,then the mixed slurry was poured into it;the resultant height was slightly higher than the plane of the building platform.Second,the scraper groove with the scraper inserted was moved horizontally to the right at a speed of 5 mm/s until the edge of the recovery groove,then the scraper was detached from the scraper groove,cleaned of the surface-adhering slurry,and reinserted into the scraper groove when the scraper groove returned to the original position.Third,the layer was cured for 10 min by DLP projector.Following the above steps,printing and curing layer by layer,we achieved parallel arrangement of the carbon fibers.In the orthogonal arrangement,the adjacent fiber layers were perpendicular to each other.The only difference between the printing processes for the parallel and orthogonal fiber arrangements was that the resin container was rotated 90°counterclockwise when lowered 1 mm.After curing,the orthogonal arrangement was printed.

Fig.2(b)shows the printing process of the round arrangement.Briefly,after the resin container was lowered and the slurry was poured,the scraper groove without the scraper in it was moved to the center of the resin container and the scraper was inserted.Then the resin container was rotated 180°counterclockwise at 0.05 rad/s and cured for 10min.And the printing of round arrangement was finished.

The random arrangement was directly cured without induction treatment for comparison.

All the samples consisted of 15 layers of slurry with a total thickness of 15 mm.After printing,they were dried in a vacuum oven at 50°C for 4h.

The dried printing-samples were put into a graphite die to debond and sinter in vacuum.Fig.3 shows the integral sintering process,which is described as follows:from 20°C to 330°C in 135 min with a heating rate of 2°C/min,and a heat preservation of 60 min.During the sintering,most of the photosensitive resin was slowly pyrolysed to generate carbon dioxide volatilization with increment of temperature.With the aim of discharging the rest of the photosensitive resin and preheating thecomposite,the sample was heated up to 450°C for 80 min with a heating rate of 1.5°C/min and the heat was maintained for 60 min.Finally,the composite was sintered at 1100°C for 55 min,with a heating rate of 10°C/min,and a heat preservation of 40 min to enhance the wettability between the aluminum and the carbon fiber due to the interfacial reaction[4],and cooled to room temperature with the furnace.In order to achieve the aligned arrangement of the fibers in the aluminum,hot pressing was not applied during sintering.The printed and sintered samples were fabricated,as shown in Fig.4.

Fig.1.Schematic diagram of printing device and fiber arrangement.

Fig.2.Schematic diagram of 3D printing processes:(a)the yellow dotted line represents the parallel arrangement and the black solid line represents the orthogonal arrangement;(b)the round arrangement.

Fig.3.Schematic diagram of integral sintering process.

2.3.Measurements

Rheological measurements were taken using a Rheometer MCR 702(TwinDrive-Anton Paar)with a tapered plate.A scanning electron microscope(SEM,ZEISS EVO 18,Carl Zeiss AG,Cambridge,England)was used to reveal the orientation features of the fibers in cross-sections after printing and sintering.A MAXima_X XRD-7000 X-Ray diffraction system using Cu Kα radiation and a scan speed of 4°/min was used in the analysis phase.A Charpy impact tester was used to measure the impact strength of 55×10×10 mm V-shaped incision specimens.An electronic universal testing machine was used to test the tensile strength.The tensile samples were cut like a dog-bone with a gauge length of 36 mm,width of 10 mm,and thickness of about 2 mm,using a wire cutting machine.The long axis of the tensile and impact samples was parallel to the carbon fiber for the parallel arrangement composites,and the samples of the rest of the arrangements were cut in the same position.All the tests were conducted at room temperature and each arrangement was measured at least three times.Fig.5 shows the dimension of the tensile and impact tests.

3.Results and discussion

3.1.Microstructure

Due to the flow behavior,mixed slurry has a great influence on the print process.For this reason,rheological measurements were taken,as shown in Fig.6.

Fig.4.Carbon fiber reinforced aluminum matrix composite:(a)printed sample;and(b)sintered sample.

Fig.5.The dimension diagram of specimens:(a)tensile and(b)impact test.

Fig.6.Viscosity of the slurry of aluminum powder,carbon fibers and photosensitive resin.

Fig.6 shows that the viscosity of the mixed slurry gradually decreases as the shear rate increases,indicating that the mixed slurry has the property of shear thinning,which may be conducive to the fiber alignment arrangement in the print process[39].

Fig.7 shows the SEM microstructures of the cross-section of the varied arrangements of the print samples.In Fig.7(a)and(b),the carbon fiber-aluminum powder and the aluminum powder-aluminum powder are in contact with each other,and there is some resin bond around the contact to fix them together.Fig.7(a)also shows that the carbon fiber has some small grooves on the surface,which can improve the integral strength of the printed samples by enhancing the surface adhesion with the photosensitive resin through mechanical interaction.The microstructures of all arrangements–(c)random,(d)parallel,(e)round,and(f)orthogonal–are shown in Fig.7(c)~(f).It is easy to see that the printing process has a major impact on the arrangements of the carbon fibers.

Fig.7(c)shows that the carbon fibers directly cured without any shear induction are very disorganized,however,as shown in Fig.7(d)all the carbon fibers are almost identical in orientation.This is because the slurry on the resin container moves together due to the viscosity when the slurry on the building platform is driven to the right by the scraper,and the carbon fibers in its interior are subjected to the shearing force in the same direction so that the orientation of all fibers is nearly parallel.Fig.7(e)shows the almost uniform orientation of fibers as well,differing from Fig.7(d)in that fibers observed from top to bottom exhibit a certain curvature,as shown by the yellow arrow,mainly because the direction of the shear force changes circumferentially inside the slurry.Thus,the round arrangement of carbon fibers can be constructed in the printed sample.

Although the orientation of the carbon fibers is totally different in the parallel and round arrangements,it is the same in the adjacent layers.Fig.7(f)shows the interface orientation of the fibers of adjacent layers is almost 90°,due to the orthogonal shear force.This suggests that printing the next layer of carbon fibers in a different arrangement does not affect the layout of the previous layer printed,which ensures the stability of the arrangement of the carbon fibers in aluminum so that more arrangements can be designed to improve the mechanical properties of the composites.

Fig.7.The section SEM images of printed samples:(a)the bonding between carbon fiber and aluminum powder and(b)the bonding between aluminum powder and aluminum powder;(c)the random,(d)parallel,(e)round and(f)orthogonal arrangement.

Fig.8.The section SEM images of sintered samples:the yellow arrows showing the bond between carbon fiber and aluminum matrix.

The post-sintering SEM images of longitudinal sections of samples in Fig.8 and Fig.9 show the bond between the carbon fiber and the aluminum powder,and the microstructure of the(a)random,(b)parallel,(c)round,and(d)orthogonal arrangement after sintering(as indicated by the yellow arrows).In Fig.9,the arrangement of fibers is nearly the same as in Fig.7(c–f).Combining these with Figs.1 and 7,it can beconcluded that the fabrication method of the CF/Al composite can achieve a well-defined ordered arrangement of carbon fibers in aluminum.Compared to Refs.[1,7],it results in a richer structure of carbon fibers in aluminum.

Fig.9.The section SEM images of sintered samples:the microstructure of(a)random;and(b)parallel;and(c)round;and(d)orthogonal arrangement.

Fig.10.X-ray diffraction of the mixture of carbon fiber and aluminum powder,and the random,parallel,round and orthogonal sintering arrangement.

Fig.10 shows XRD scans for sintered samples with different arrangements and mixtures of carbon fibers and aluminum powder.In comparison to the spectrum of the mixtures,the aluminum carbide peaks increase,which is attributed to the carbon fibers reacting with the aluminum to form aluminum carbide at high temperatures.Similar results were reported by Stephen F[18].In addition,the diffraction peaks for the samples with different arrangements are similar,indicating that the arrangement of the carbon fibers does not greatly influence the change of phase during the sintering process.

3.2.Tensile mechanical properties

Fig.11 shows the stress-strain curve of the(a)random,(b)parallel,(c)round,and(d)orthogonal arrangement,with tensile strength and elongation values of 12.85Mpa,0.59%;15.20Mpa,0.82%;17.3Mpa,0.75%;and 15.6Mpa,0.74%,respectively.The well-defined ordered arrangement has a significant value in enhancing the tensile property;however,in terms of improvement,there are obvious differences among them.

Qiurong,Yang et al.[1]and Hajjari et al.[21]studied the fracture behaviors of the carbon fiber/aluminum composite.From their reports,it can be concluded that good interface bonding and the propagation and deflection of cracks play a predominant role in enhancing the tensile property of the composite.From this perspective,compared with Fig.12(b)~(d),Fig.12(a)shows relatively few round holes resulting from the pull-out of carbon fibers.It is because there are too few fibers parallel to the load direction in the random arrangement.The random orientation of the fibers causes a strong local interface reaction between the carbon fibers and the aluminum powder,resulting in the formation of a brittle compound(Al4C3).As shown by the XRD analysis,the local interface bond is so strong that the brittle fracture can be seen on the flat surface in the magnified image.

Fig.12(b)shows that the number of round holes resulting from the pull-out of fibers and the orientation is nearly identical.The parallel arrangement shows relatively good tensile properties.When the sample is loaded,cracks are generated from defects due to the concentration of stress,then propagate along the vertical direction of the loading axial line.When encountering the carbon fibers,the cracks are deflected and propagate along the interface between the fibers and the aluminum matrix resulting in the pull-out of fibers,as shown in the magnified image.In the process of propagation and deflection,the stress becomes less,which greatly improves the ductility of the composite.

Fig.11.The stress-strain curve of the different carbon fiber arrangements.

Fig.12(c)shows the round arrangement has more round holes than the random arrangement shown in Fig.12(a),the highest tensile strength,and less elongation than the parallel arrangement shown in Fig.12(b).This is because the orientation of the fibers is different from the direction of the tensile stress,which makes it more difficult for the fibers to be pulled out.Meanwhile,the local stress is so concentrated that the brittle fracture,as shown in the magnified image,is similar to that of Fig.12(a).However,its elongation is higher than in the random arrangement,which is ascribed to the round arrangement having more pull-out fibers than the random arrangement.

In terms of the orthogonal arrangement,its tensile stress is almost the same as that of the parallel arrangement,as shown in Fig.11,however,the elongation is lower.

Fig.12(d)shows some holes in the distribution of the tensile stress and the arrangement of carbon fibers vertical to the direction of the tensile stress(as indicated by yellow arrows).According to the analysis of the parallel arrangement,the failure mechanism of the orthogonal arrangement can be explained as follows.The fibers parallel to the direction of tensile stress can significantly improve the ductility through the propagation and deflection of cracks.However,for those fibers vertical to the direction of the tensile stress,the cracks propagate almost directly along the interface of the carbon fibers and the aluminum without any deflection,resulting in the exposure of fibers(as indicated by the yellow arrows),which slightly improves the ductility of the composite.Overall,half the fibers are in the parallel direction,and play a major role in reinforcement,but the reinforcing effect of the other half in the vertical direction is far less.Thus,the orthogonal arrangement exhibits a lower elongation than the parallel arrangement.With respect to the approximately the same tensile strength,it may be due to the entire interface bonding strength being nearly the same as the parallel arrangement.

3.3.Impact test

Fig.13 shows the impact toughness values of the different arrangements:the random,parallel,round and orthogonal arrangement are 0.23J/cm2,0.32 J/cm2,0.28J/cm2,and 0.41 J/cm2,respectively,showing that the well-defined ordered arrangement of fibers has a great effect on improving the impact toughness of composites.

Fig.14 shows the fracture appearance of impact specimens in order to analyze the mechanism of fiber arrangements to resistance impact.

The magnified images of Fig.14(a)–(d)show that all fracture fibers exhibit a similar smooth fracture surface.It can be concluded that the carbon fiber may resist the impact by absorbing the impact energy,which leads to the sudden concentration of stress resulting in the flat fracture.

By observing the low magnification of the varied arrangements,it can be found that,as Fig.14(a)shows,the fiber destruction of the random arrangement is less than that of the other arrangements,which indicatesthat the random arrangement absorbs minimal impact energy,resulting in it having the weakest impact toughness.However,Fig.14(b)shows that for the parallel arrangement,the carbon fibers are divided into segments after impacting.This is mainly because the orientation of the fibers is perpendicular to the direction of impact,causing the impact energy to be absorbed in the radial direction of the fibers,as Fig.15(a)shows.Therefore,the parallel arrangement mainly resists the impact through the carbon fibers absorbing the impact energy in the radial direction.

Fig.12.Fracture appearance of(a)random;(b)parallel;(c)round;and(d)orthogonal arrangement.

Fig.13.Impact toughness values of(a)random;and(b)parallel;and(c)round;and(d)orthogonal arrangements.

For the round arrangement,as Fig.14(c)shows,the fiber destruction is relatively small compared to the parallel arrangement,which means that when there is a certain gradient between the orientation of the fibers and the direction of the impact,the fiber arrangement absorbs less energy,resulting in lower impact toughness than in the parallel arrangement.

As Fig.14(d)shows,in terms of the orthogonal arrangement,with the highest impact toughness,fibers parallel to the direction of impact are most severely destroyed(as indicated by the A arrow),while fibers in the orthogonal direction suffer little destruction(as indicated by the B arrow).

As Fig.15(c)shows,when the orthogonal arrangement is impacted,the axial direction of the fibers absorbs the great mass of the impact energy,to resist the impact.Combined with Fig.14(b),it can be seen that the axial direction of the fiber is more impact resistant than the radial direction.Therefore,its impact toughness is the highest.

Combined with the tensile test analysis,by optimizing the process parameters,the orthogonal aligned carbon fiber reinforced aluminum matrix composite can be improved to provide better tensile properties and impact toughness to meet varying engineering requirements.

4.Conclusion

In the present study,four differently arranged carbon fiber reinforced aluminum matrix composites were successfully fabricated by combining 3D printing with shear-induce alignment and the powder metallurgy process.The microstructure of the carbon fiber arrangements,and the mechanical properties and failure mechanisms of the prepared composites were investigated.The results of microstructure observation and mechanical property testing can be concluded as follows:

(1)The feasibility of the proposed process is demonstrated by successfully producing composites with highly ordered structures of aligned fibers.In mechanical properties,the composites with ordered structures are superior to that of the random arrangement.

(2)Results of the mechanical tests show that the parallel arrangement has the highest elongation,which is about 0.4 times higher than that of the random arrangement,and the orthogonal arrangement has the highest impact toughness,which is about 0.8 times higher than that of the random arrangement.This shows that the fiber arrangement can anisotropically improve the mechanical properties of the fiber reinforced composites.

Fig.14.Impact appearance of(a)random;and(b)parallel;and(c)round;(d)orthogonal arrangements.

Fig.15.Schematic illustration showing the resistance of different fiber arrangements under impact condition:(a)parallel;and(b)round;and(c)orthogonal arrangement,and the yellow and green arrows represented fiber resistance impact of the top and bottom layers,respectively.

Conflict of interest

None.

Acknowledgments

This work was supported by the Projects of National Key Research and Development Program of China (2018YFA0703 300,2018YFB1105100,2018YFC2001300),the National Natural Science Foundation of China(5167050531,51822504,91848204),Key Scientific and Technological Project of Jilin Province(20180201051GX),Program for JLU Science and Technology Innovative Research Team(2017TD-04).