Mechanical Properties, Surface Structure, and Morphology of Carbon Fibers Pre-heated for Liquid Aluminum Infiltration
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To efficiently produce carbon fiber-reinforced aluminum on a large scale, we developed a special high-pressure die casting process. Pre-heating of the fibers is crucial for successful infiltration. In this paper, the influence of heating carried out in industrial conditions on the mechanical properties of the fibers was investigated. Therefore, polyacrylonitrile-based high-tensile carbon fiber textiles were heated by infrared emitters in an argon-rich atmosphere to temperatures between 450 and 1400 °C. Single fiber tensile tests revealed a decrease in tensile strength and strain at fracture. Young’s modulus was not affected. Scanning electron microscopy identified cavities on the fiber surface as the reason for the decrease in mechanical properties. They were caused by the attack of atmospheric oxygen. The atomic structure of the fibers did not change at any temperature, as x-ray diffraction confirmed. Based on these data, the pre-heating for the casting process can be optimized.
Keywordsadvanced characterization aluminum casting and solidification composites heat treatment metallic matrix
Carbon fiber textiles serve as reinforcement especially in polymer matrices and they are advancing at an accelerated rate. What is something of a rarity today will be affordable and widely used in the near future due to intensive efforts to realize cost-efficient automated production routes. Despite its extraordinary bending and tensile strength (Ref 1), carbon fiber-reinforced polymers exhibit a lack of resistance against temperature and compressive stress. This deficit may be overcome by replacing the polymer matrix with a light metal-like aluminum or magnesium (Ref 2-4). Local reinforcement of highly stressed regions in, e.g., engine parts, could be enabled.
In order to meet the demand for serial production of composites in the automotive and machine tool industry, a special high-pressure die casting process (HPDC) was developed (Ref 5-7). It features a short cycle time and also a short contact time between the molten aluminum alloy and the fibers. Due to the rapid solidification of the melt in the die, the main challenge is to properly infiltrate the fibrous preform. In the HPDC process, which we developed, the carbon fibers are heated by infrared lamps prior to casting to prevent premature freezing of the melt. Carbon fibers exhibit very poor wettability by liquid aluminum alloys (Ref 8, 9) but a proper pre-heating process associated by high pressure can lead to almost complete infiltration of the fiber preform with aluminum. Without pre-heating, on the other hand over 40% of the fabric remain infiltrated (Ref 7). Other method for enhancement wettability is deposition of different coatings like, e.g., Ni-P (Ref 10-12). Non-coated carbon fibers start to oxidize when heated above 600 °C in air (Ref 13). This leads to a decay of the fiber’s extraordinary mechanical properties, which also lowers the potential reinforcing effect.
Several authors, except changes in morphology (Ref 14, 15) describe a change in the properties of carbon fibers after different thermal treatments, i.e., removal of sizing in high vacuum, high pure air, or argon atmosphere (Ref 16-18) and also after the graphitization processes in the presence of metallic coatings (Ref 19, 20). But such laboratory conditions are often much different compared to industrial practice or at least conditions are fitted to actually utilized processing technologies, e.g., gas pressure infiltration. Therefore, the aim of the present work is to identify the effect of the carbon fibers textile pre-heating procedure carried out in industrial conditions using the innovative high-pressure die casting method which was not so far used for fabrication of MMC-reinforced by carbon fibers textiles. Pre-heating was realized by means of infrared lamps in a specific mix of argon and oxygen containing atmosphere. This allows for the determination of a temperature range in which both infiltration and mechanical properties are at a high level.
Heating parameters of IR-lamps and resulting temperatures for the heating of carbon fibers
Power of IR-lamps, kW/m2
Pre-heating time, s
To determine the tensile properties, 20 to 30 single fibers from each fabric were tested according to ASTM standard no. C1557-03 on a MTS Tytron 250 testing machine. The data were interpreted according to Weibull statistics, which is common for ceramic materials. The probability of fracture P F in a brittle material is described by the fracture strength σF, the characteristic strength σ0, and the Weibull modulus m: P F = 1 − exp[−(σF/σ0) m ]. To plot the data in a Weibull plot, the fracture probability was estimated by P F = (i − 0.5)/N, where N is the total number of samples which are arranged in ascending order with the rank i. The characteristic strength and the modulus were determined by the maximum-likelihood estimation. A detailed description of this method is given in the standard DIN EN 843-5.
The surface of the differently heated carbon fibers was studied using a high-resolution Hitachi S5500 SEM. Samples were stitched to the holder using carbon tape and investigated using 10 kV accelerating voltage in SE mode. In addition, XRD patterns of an unheated sample and samples heated to 900 and 1200 °C were obtained using a Rigaku Miniflex II x-ray diffractometer equipped with a Cu cathode (λ (Cu-Kα) = 1.54184 Å). The scan scope (2θ) ranged from 20° to 80° at a scan rate of 4°·min−1.
For all investigations, samples from the center of the fabric, where the maximum temperature was measured, were used.
Measurements on the original untreated fibers reveal a characteristic strength of 4100 MPa, which is very similar to the value of 3950 MPa specified in the producer’s data sheet. Slight differences in characteristic strength at 450 °C (4152 MPa) and 600 °C (3953 MPa) can be attributed to statistical effects. This means there is no significant loss of tensile strength up to a temperature of 600 °C. A further increase of temperature evokes a decline in characteristic strength. At 900 °C, it decreased to 2707 MPa which equals 66% of the original strength. For 1400 °C, the highest temperature was investigated, the characteristic strength drops to 1082 MPa, retaining only 26% of the untreated fiber’s strength.
For the Weibull modulus m, which correlates to the scattering of the data, a value of around m = 5 was found for fibers without pre-heating and with pre-heating to 600 °C. At a temperature of 450 °C, the modulus is increased to 10. For higher temperatures, it is below 5. This increased scattering is due to varying defect sizes.
The strain at fracture displays a decrease at a comparable rate as the tensile strength. The original value of 1.7% is identical to the producer’s specification and reduces significantly for pre-heating temperature over 600 °C. For the temperature range between 900 and 1400 °C, the strain at fracture averages between 59 and 29% of the value for the untreated fiber.
Scanning Electron Microscopy
The observation of the carbon fiber’s surface after different pre-heating temperatures indicates significant changes in their morphology. Degradation increases with temperature. Figure 7(b) shows a fiber after pre-heating to 450 °C. The morphology of the surface appears unchanged. Also at 600 °C, no transformation in the surface morphology can be detected (Fig. 7c). A further increase of the temperature up to 900 °C causes degradation in the form of small cavities or grooves (Fig. 7d). At even higher temperatures, the grooves become more numerous and larger. At a temperature of 1200 °C, the cavities reach a size of up to 2.5 µm in diameter (Fig. 7e). In some cavities, small particles, potentially products of thermal degradation, were spotted. Reaching a pre-heating temperature of 1400 °C, large cavities of irregular shape consume the original surface (Fig. 7f).
Tensile testing clearly shows that the mechanical properties of high-tensile carbon fibers change with increasing temperature of the pre-heating used in the described manufacturing process for MMCs. The decrease in tensile strength and strain at fracture is a result of the fibers’ surface degradation, which was detected by SEM observations. It occurs due to the attack of oxygen. Even though the atmosphere is enriched with argon, it is not possible to completely avoid oxygen. This is in agreement with the results from (Ref 25). At temperatures above 600 °C and in the presence of oxygen, gaseous CO2 is produced (Ref 8), leaving holes in the surface of the fibers. These cavities act like notches and evoke stress concentrations which cause the tested fibers to fail much below their original strength. The particles found inside many cavities may be the residue of the oxidation process.
The oxidation process could be reduced, even for higher temperatures of up to 1100 °C, by heating the fibers in an argon atmosphere of higher quality or in vacuum (Ref 16, 25-27). However, this is not practicable in HPDC.
A change in the atomic structure of the fibers as the reason for the decay of the mechanical properties at temperature over 600 °C can be excluded. No change in the atomic structure could be detected by the XRD measurements. Other authors (Ref 19) found that the graphitization process can start at 900 °C. But the heating time of the fibers was much longer than in the present experiment.
These results indicate that the pre-heating temperature of the fabrics before infiltration in the high-pressure die casting process can be carried out in and over 600 °C but not exceed 900 °C. The utilization of a higher temperature leads to the degradation of the fibers before they are infiltrated. Therefore, the great potential of carbon fibers to act as reinforcement in an MMC cannot be exploited to its full extent. However, in the MMC casting process, the fibers are not exposed to an oxygen containing atmosphere for such a long time. Just 5 s-10 s after the emitters are turned off, the melt infiltrates the fibers. Hence, the thermal damage of fibers in an actual MMC might be less.
The mechanical properties of high-tenacity carbon fibers are not affected by pre-heating to temperatures below 600 °C in an oxygen containing atmosphere. With higher temperatures, tensile strength and strain at fracture decrease more and more. At 1400 °C, the fiber possesses only 24% of its original strength and 29% of its original strain at fracture. Through scanning electron microscopy, these changes were attributed to surface degradation due to oxygen attack. However, Young’s modulus and the atomic structure of the fibers remain unchanged.
This knowledge is crucial to optimize the pre-heating process for the manufacturing of carbon fiber-reinforced aluminum by a novel high-pressure die casting because the pre-heating temperature should be kept between 600 and 900 °C to reach a degree of infiltration of over 95 vol.%, while the excellent properties of CF are preserved. Thus, further studies should be performed to explore behavior of fibers during pre-heating process in the mentioned temperature range in order to find precise and optimized temperature for fibers processing.
The work is part of the Polish-German bilateral Project “3D-textile reinforced Al-matrix composites (3DCF/Al-MMC) for complex stressed components in automobile applications and mechanical engineering” (PAK 258). We would like to thank the Polish Ministry of Science and Higher Education and the German Research Foundation for their financial support.
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