2022 Volume 63 Issue 11 Pages 1590-1596
CeNF/Al-based composites were prepared using CeNFs collected by a non-woven aluminum filter, followed by hot extrusion to obtain plates. Gel-like CeNFs were collected by an aluminum non-woven filter and compacted by a warm press to obtain a compressed form with a lighter specific density than pure aluminum. The compressed forms were hot extruded to fabricate bars and plates. Both bars and plates were observed in the macro-and microstructural morphology and XRD measurements. They were not significantly carbonized to graphite only, which was inferred to be present as CeNF under the present experimental conditions. Microstructural observations show that CeNFs are aggregated and present in the pores/cracks between the Al filters in the compressed forms. Al filters and CeNF aggregates were more finely mixed when the material was fabricated into hot extruded plates with a higher extrusion ratio. The maximum tensile strength of the CeNF/Al composite extruded plate was about 1.5 times higher than pure aluminum. In addition, the extruded plates could be cold-rolled by about 30%, and the maximum tensile strength of the extruded sheets was found to be about twice that of pure aluminum.
In many industrial fields, weight reduction of products and components is required. Aluminum and its alloys have been considered one of the means to achieve this. Carbon fibers and ceramic whiskers have been investigated and proposed for fiber reinforcement for aluminum-based composites with various fibers to achieve higher strength and weight reduction.1)
Recently, cellulose nanofibers (CeNF), cellulose derived from plants such as bamboo and wood, have low specific density, a tensile strength of 2 to 5 GPa, and a small thermal expansion coefficient of about 1/50 that of glass. Currently, CeNF is mainly used as a composite material based on resin, such as a reinforcing material for transparent resin in organic EL light-emitting devices.2–4)
The combination of cellulose or CeNFs with metals has been studied to adsorb or retain metal nanoparticles as catalysts.5,6) Still, there are no reports at this research stage on metal-based composites for cellulose-reinforced fibers or cellulose’s inherent low specific gravity.
In our past works, aluminum or its alloy-based composite materials with ceramic particles such as Al2O3 and TiC have been fabricated to obtain higher strength aluminum.7,8) The authors also have fabricated and reported composite materials of aluminum that exhibit, for example, photocatalytic properties of TiO2 and superconducting properties of MgB2 without compromising the properties of the functional composite particles by suppressing the reaction between the composite particles and aluminum matrix.9,10)
CeNFs are carbonized at around 200–400°C.3,11,12) Therefore, it is difficult to fabricate CeNF near the melting point of aluminum because CeNF is completely carbonized by the fabrication method using molten aluminum, which the authors have conventionally used. Therefore, the authors attempted to fabricate a composite material (CeNF/Al composite) using CeNF and aluminum fiber without using molten aluminum and successfully fabricated a bulk form and its extruded bar material.13) In this work, we report on the microstructure and mechanical properties of the CeNF/Al composite, which was successfully modified into an extruded plate with a high extrusion ratio by hot extrusion at a relatively low temperature.
The raw materials used were commercially available non-woven aluminum filter (provided by Unix Corporation, hereafter referred to as the Al filter) and CeNF, a 2% slurry suspension of bamboo pulp (provided by Chuetsu Pulp Industry Co., Ltd.), diluted as needed with deionized water as a stock solution. The mean diameter of fiber in the Al filter is about 80 to 100 µm, and the nominal fiber diameter of CeNF is about 50 nm. Al of 99.99% purity was also used for comparison.
Figure 1 shows the observed images of the used CeNFs and aluminum fiber nonwoven filters. The CeNFs were observed by optical microscopy (OM) and atomic force microscopy (AFM) when the suspension was dropped onto a prepared and dried at room temperature. The lengths of CeNFs are short and submicron, as shown in Fig. 1(a) and (b). The cross-sectional diameters determined by AFM were approximately 20 to 50 nm. Figure 1(c) and (d) show the Al-filter’s appearance and SEM image. The cross-sectional diameter of an individual fiber in the Al-filter has a deviation from 80 to 100 µm from Fig. 1(d), which is not homogeneous. That seems to depend on the manufacturing process of Al-filter.
Morphology of CeNF and non-woven Al-filter used in this work. (a) OM and (b) AFM for CeNF, (c) exterior view, and (d) SEM image for non-woven Al-filter.
Figure 2 shows a schematic illustration of sample preparation. The CeNF suspension was placed in a 100 ml beaker and stirred by a magnetic stirrer for 1 hour to ensure homogeneous dispersion of CeNF. As the next step, an Al-filter was dipped in that beaker containing the CeNF suspension and kept there to collect the CeNF between the fibers of the Al-filter for several minutes. This Al-filter with CeNF is referred to as the CeNF/Al filter. The CeNF/Al filter prepared by the above method was cut into a circular shape with a diameter of approximately 30 mm, and the required number of disks were prepared. After drying in a muffle furnace at 120°C for 30 minutes, the necessary number of CeNF/Al filters were stacked in part C of mold B shown in Fig. 3. 3∼5 CeNF/Al filters were stacked for checking the formability of the CeNF/Al composite material and 7∼10 CeNF/Al filters were stacked for extrusion. The stacked CeNF/Al filters were compressed at room temperature using the steel stem indicated by A from the top of mold B at about 15 MPa for about 10 seconds, heated to about 200°C, and then kept for 60 seconds under the pressure to fabricate a compressed form with a diameter of about 30 mm and a length of about 20 mm. This is referred to as the CeNF/Al composite in this work. Using a horizontal direct extrusion press, 99.99% pure aluminum of approximately 30 mm in diameter and 10 mm in length was placed in front of the CeNF/Al composite in the extrusion container. Using an annular furnace, the extrusion container was externally heated to about 390 ± 10°C. After reaching the set temperature, the CeNF/Al composite and pure Al was extruded at an initial pressure of about 12 MPa and an extrusion speed of about 20 mm/s. The initial temperature of the CeNF/Al composite was about 370°C before extrusion. The extruded bar or plate coming out of the die was cooled by forced air blowing at room temperature. The average cooling rate of extruded materials was about 11°C/s. The bar was with a diameter of about 10 mm, and the plate was about 1.5 mm × 20 mm of a cross-section. Each extrusion ratio was 9.0 for the bar and 23.6 for the plate.
Schematic illustration of method how to collect CeNF by non-woven Al-filter.
Dimensions of compression steel mold for CeNF/Al composite.
Micro-Vickers hardness tester (Mitutoyo HM-101) was used for the evaluation of samples’ hardness with a load of 0.98 N and a holding time of 15 s. Tensile test specimens were prepared by cutting and polishing mechanically from plate specimens with a cross-section of approximately 6.0 mm in width, 0.8 mm in thickness with an accuracy of 1/100 mm, and 17.5 mm of the gauge length. The tensile test was taken place at an initial strain rate of 10−4 s−1 using a Shimadzu DSS-5000 Instron-type tensile test machine. X-ray diffraction measurements (XRD) were performed using a Rigaku Ultima IV with θ/2θ method, 40 kV/40 mA, wave-length of Cu-Kα, scan speed 1°/1 min, and scan angle θ = 10–90°. To confirm the workability of the composite material, a sample of 30% cold-rolled plate material after extrusion was also prepared as comparison material. 99.99% purity of Al was also used after 50% cold rolling and annealing at 400°C for 10 minutes. The density of the CeNF/Al composite and extruded the CeNF/Al composite rod or plate was measured by the Archimedes method (MD-300S, Alfa Mirage), and the volume fraction of CeNF (Vf) in those composite materials was calculated using the law of mixture. The microstructures of those composite materials were observed using an optical microscope (OM, Olympus BX51M), scanning electron microscope (SEM, Hitachi S-3500H), and atomic force microscope (AFM, Hitachi AFM5100N) after mechanical polishing and/or electrical polishing. Scanning low energy electron microscope (SLEEM) images were also observed using a SLEEM detector in the SEM to prevent charge-up.
A visual photograph of a sample of the CeNF/Al composite as described in the experimental procedures and a secondary electron (SE) image observed by SEM are shown in Fig. 4. The 25, 33, and 78% values in the figure indicate the Vf of CeNF. Figure 5 also shows the relationship between the Vf of CeNF and the density of each composite material in this work. The arrows are the results for the CeNF/Al composite, and the other dots are the results for the extruded the CeNF/Al composite described below. The OM images in Fig. 4(a)–(c) show no significant difference except for the surface relief by the stem surface (A in Fig. 3) or the bottom of the mold (B in Fig. 3), which was transferred during the compressing process. The fibrous structure observed as brighter contrast in the SEM images than in the other areas of Fig. 4(d)–(f) which are aggregations of CeNF. In Fig. 4(d), the relatively coarse CeNFs that seem to protrude from the sample surface are shown by white arrows, and the interface between Al filters, indicated by black arrows, is observed as a white boundary. The surface of the Al filter is covered with a fibrous structure, which appears to be aggregates of CeNFs. Considering the manufacturing method, it is considered that CeNFs or their aggregates appear on the sample’s surface coming out from the interface between Al filters.
Morphology of compressed CeNF/Al composites fabricated using 3 Al-filters. Exterior views of CeNF/Al-filter compressed forms of Vf = (a) 25, (b) 33, and (c) 78%. (d)–(f) SE images obtained for the center of (a)–(c), respectively.
Result of density (ρ) of CeNF/Al composites before and after extrusion vs. Vf of CeNF. ○: pure Al, ●: CeNF/Al composites. Arrowed data were obtained for CeNF/Al composites and the others without arrows for extruded composites.
Figure 6 shows an external view of the CeNF/Al composite with a height of about 20 mm produced for extrusion. There are no large visual cracks. Figure 7(a) shows the appearance of the CeNF/Al composite extruded by hot extrusion into a bar with a diameter of 10 mm. Figure 7(b)∼(d) are pictures of the transverse sections of positions of 1∼3 in Fig. 7(a), respectively. The darker regions are distributed and seem to depend on the Al filter, given the complex bellows-like pattern on the surface of Fig. 7(a), especially positions 2∼3. Position 3 near the end of the extruded rod also shows this complex bellows-like pattern. Figures 7(e)∼(g) are enlarged images of the center of sections Figs. 7(b)∼(d) observed by OM. The darker regions are distributed in common in 1∼3 of the extruded rod. These darker regions in 1, which is close to a part of the extrusion tip of Fig. 7(e), are observed as fine dots. The darker areas increase and are distributed homogeneously in region 2 of Fig. 7(f). Region 3, near the end of the extruded rod, also shows this complex bellows-like pattern, but again, the darker regions’ area fraction is smaller and less than those of region 2. The complex bellows-like pattern seen on the bar surface of Fig. 7(a) was also observed in a preliminary experiment in which compressed form made by laminating only Al filters without CeNFs were extruded. So, it is concluded the effect of stacking of Al-filters is not dependent on the existence of CeNFs.
Exterior view of compressed CeNF/Al composite fabricated using 10 Al-filters for extrusion. (a) top and (b) side views. (Vf: 18.3%, ρ: 2.5 g/cm3)
Results of the extruded CeNF/Al composite rod. (a) exterior view of that rod and (b) exterior view of a cross-section of the rod at (b) 1, (c) 2, and (d) 3. Optical micrographs obtained for cross-sections of the rod at the center of (e) 1 in (b), (f) 2 in (c) and (g) 3 in (d).
Figure 8 shows the results of the XRD measurement to confirm the presence of CeNF in those mentioned above extruded the CeNF/Al composite bars. (a) and (b) are ICDD data of graphite and aluminum,14,15) (c) is diffraction obtained from a sheet prepared by drying only CeNF used in this study, and (d) is an extruded CeNF/Al composite bar. The sheet prepared by drying only CeNF in (c) has a strong peak at around 23°. The diffraction peaks of the extruded CeNF/Al composite bar and that of pure aluminum was obtained at almost the same positions, as indicated by the black arrows in the figure. Therefore, it was considered that CeNF was contained in the extruded CeNF/Al composite bars fabricated in this study.
XRD data obtained for each sample. (a) and (b) standard data by ICDD for graphite and aluminum. (c) compressed CeNF without Al-filter and (d) extruded CeNF/Al composite rod.
Figure 9(a) shows the appearance of the extruded CeNF/Al composite plate. Figure 9(b) shows the plate of (a) cut into five sections. As the pure aluminum is extruded in the first, both side of the CeNF/Al composite surface is covered by pure aluminum about 0.1 mm in thickness. In ①, blistering marked by a white arrow was observed on the surface. The Al filters, which are the Al matrix phase of this composite, overlap and create pores/cracks between the Al filters, and those were expanded during the hot extrusion process. However, such blistering was rarely observed in the whole extruded plate.
(a) Exterior view of extruded CeNF/Al composite plate. Its total length was about 1.2 m. (Vf: 15.8%, ρ: 2.5 g/cm3) (b) exterior view of the surface condition of extruded CeNF/Al composite plate for five parts. [1] A-B, [2] B′-C, [3] C′-D, [4] D′-E, [5] E′-F. F′-G is a cut part from the end of the extruded CeNF/Al composite.
Figure 10 shows OM and SEM images obtained for longitudinal cross-sections of extruded CeNF/Al rod and plate parallel to the extruded direction. Many black contrasts are also confirmed parallel to the extrusion direction. The layered structure of CeNFs between the Al filter is observed in the bar in Fig. 10(a), whereas the layered structure is also obscured in the plate in Fig. 10(c). This is also obvious in Fig. 10(b) and (d), which are SEM images of Fig. 10(a) and (c). The CeNF aggregates were observed as brighter or darker contrasts than the Al matrix in the SEM images.
Microstructure of each sample surface. (a) and (b) for extruded CeNF/Al composite rod, and (c) and (d) plate. (a) and (c) OM, and (b) and (d) SE images.
Figure 11 shows XRD results obtained from an extruded CeNF/Al composite plate. In addition to the diffraction peak of pure aluminum, a weak peak was observed around 23°, so an enlarged figure is inserted. It is reported that 2θ = 16.1° is indexed as crystalline α-cellulose (110), 22° as (200), and 34.7° as (004),16) although XRD below 20° was not measured for the CeNF used in this study. 1. There are also no remarkable graphite peaks in the XRD patterns of Fig. 8. There are several reports about the carbonization of CeNFs at around 200–400°C.3,11,12) It has also been reported that crystalline cellulose becomes amorphous when heated to 320°C in water under a pressure of 25 MPa.17) As our research was in the air, it is also unclear whether this phenomenon occurs in the present work. As described in the experimental method, the average cooling rate is estimated about 11°C/s (660°C/min) for the extruded CeNF/Al composites produced in this study. This cooling rate is a fast cooling rate, which in practical fabrication corresponds to the water-cooling rate after the homogenization process18) or the forced fan air cooling rate after extrusion.19) It is speculated that most of the CeNF could be extruded without carbonization even at a temperature of 370°C under the condition where CeNFs or their aggregates were contacted to Al filters with good heat conductivity, completely compressed, and sealed with aluminum.
XRD data obtained for extruded CeNF/Al composite material plate. An inserted graph is enlarged at a lower angle region of XRD than 2θ = 25°.
Figure 12 shows the results of tensile tests on extruded CeNF/Al composite sheets with Vf = 16%. The black solid line in Fig. 12 shows the tensile strength of the extruded plate at about 92 MPa, and the chain line shows the tensile strength of the composite after extrusion and 20% cold rolling at about 130 MPa, which is higher than the 70 MPa of the pure aluminum shown in the thin solid line. The pure aluminum shown here is an extruded plate that was cold-rolled 30% and annealed at 400°C for 10 minutes. It offers relatively high strength and low elongation because the Al matrix as Al-filter is probably not fully recrystallized.
Result of tensile test for each sample. 
: extruded and rolled CeNF/Al-filter composite material plate, 
: extruded CeNF/Al-filter composite material plate, 
: extruded pure Al plate.
Figure 13 shows SEM images of fracture surfaces obtained for extruded CeNF/Al composite and pure aluminum after the tensile test. The fracture surfaces of both samples show an appearance of intragranular fracture: the extruded plate in Fig. 13(a) and the 30% cold-rolled plate after extrusion in Fig. 13(c) both show dimples all over the fracture surface, indicating that the fracture is ductile. In addition, the Al filter was cut off by itself, and the pull-out fracture surface, which is seen in long-fiber composites, was only observed in a small portion of the fracture surface, as shown in the SLEEM image in (e), indicating that the fracture was almost ductile.
Fracture surfaces of samples after tensile tests observed by SEM. (a) extruded CeNF/Al composite plate, (b) extruded pure Al plate, (c) extruded and rolled CeNF/Al composite plate, and (d) a part of “pull-out” morphology of Al-filter in the extruded and rolled sample as (c). (e) SLEEM image obtained for (d) marked by a white square. The pull-outed Al-filter of dark contrast is surrounded by a bright contract of CeNF/Al composite.
The extruded CeNF/Al rod in Fig. 10(a) shows the layered structure, whereas the extruded CeNF/Al rod plate shows finer dots than the rod in Fig. 10(c). A similar feature is also confirmed in the SEM images of Fig. 10(b) and (d), and CeNF aggregates are finer dispersed in the plate in Fig. 10(d) than in the bar in Fig. 10(b). The difference between the two samples is the extrusion ratio by their shapes. The finer distribution of CeNF aggregates is presumed to be caused by a higher extrusion ratio. It means higher plastic deformation during hot extrusion. When the extrusion ratio is high, the Al filters are deformed or partially fractured by higher plastic metal flow during hot extrusion. CeNF aggregates and Al filters have probably been blended into a fine mixture. According to the pre-experiment, there were many pores/cracks between Al filters on the fracture surface when tensile test specimens were taken from CeNF/Al composites without hot extrusion. The tensile strength was also lower than extruded CeNF/Al composite, as shown in Fig. 12. Severe plastic deformation or giant straining process like as Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Accumulative Roll Bonding (ARB) can make nano-ordered crystal grains in the Al matrix. Also, Friction Stir Welding (FSW) is applied to modify mechanical properties by a mixture of ceramic particles.20) It is so-called as Friction Stir Processing (FSP), which introduces into the stirring section to form a hardened layer by partial compositing. The technology of hardened layer formation by partial compositing has been actively studied.21) In the present work, our extrusion and heating during processing is not much higher than ARB, HPT and FSP. Considering those phenomena, each fiber in the Al filters is tightly and mechanically bonded together by hot extrusion. CeNF aggregates are constrained and contained between the Al filters, as shown in Fig. 14(a). As the extrusion ratio is high, the Al filters are deformed or fractured by increased plastic metal flow during hot extrusion shown in Fig. 14(b). Then CeNF aggregates and Al filters are given higher plastic deformation, not just temperature, to blend into a fine mixture before carbonization by a higher cooling rate after extrusion. Thus, the extruded CeNF/Al composite plate shows higher strength and ductile fracture surfaces. The interfacial structure of CeNF/Al and the mixing temperature of CeNF in our composite were not clarified in this study and will be investigated in future work.
Schematic illustrations for (a) compressed CeNF/Al composite or extruded CeNF/Al composite plate at a low extrusion ratio, and (b) extruded CeNF/Al composite plate at a high extrusion ratio.
CeNF/Al composite material with CeNFs collected by a non-woven aluminum filter was prepared, and then hot extrusion was successfully carried out to obtain its plate. The obtained results are summarized as follows.
The authors would like to thank Mr. Masato Nakamura (present: J-TEKT Corp.) for his contribution to preparing the samples for this study, and Mr. Kouichi Onodera, UNIX Co., Ltd. and Ms. Hiromi Hashiba, Chuetsu Pulp & Paper Co., Ltd. for their providing of Al-filters and CeNFs. We also thank the support from the University of Toyama’s Advanced Aluminum International Research Center and the President’s Discretionary Funds of FY2020 and FY2021.
