Effects of Processing Parameters on the Forming ... - Springer Link

Appl Compos Mater (2013) 20:927–945 DOI 10.1007/s10443-012-9310-7

Effects of Processing Parameters on the Forming Quality of C-Shaped Thermosetting Composite Laminates in Hot Diaphragm Forming Process X. X. Bian & Y. Z. Gu & J. Sun & M. Li & W. P. Liu & Z. G. Zhang

Received: 30 July 2012 / Accepted: 5 December 2012 / Published online: 24 January 2013 # Springer Science+Business Media Dordrecht 2012

Abstract In this study, the effects of processing temperature and vacuum applying rate on the forming quality of C-shaped carbon fiber reinforced epoxy resin matrix composite laminates during hot diaphragm forming process were investigated. C-shaped prepreg preforms were produced using a home-made hot diaphragm forming equipment. The thickness variations of the preforms and the manufacturing defects after diaphragm forming process, including fiber wrinkling and voids, were evaluated to understand the forming mechanism. Furthermore, both interlaminar slipping friction and compaction behavior of the prepreg stacks were experimentally analyzed for showing the importance of the processing parameters. In addition, autoclave processing was used to cure the C-shaped preforms to investigate the changes of the defects before and after cure process. The results show that the Cshaped prepreg preforms with good forming quality can be achieved through increasing processing temperature and reducing vacuum applying rate, which obviously promote prepreg interlaminar slipping process. The process temperature and forming rate in hot diaphragm forming process strongly influence prepreg interply frictional force, and the maximum interlaminar frictional force can be taken as a key parameter for processing parameter optimization. Autoclave process is effective in eliminating voids in the preforms and can alleviate fiber wrinkles to a certain extent. Keywords Thermosetting composite . Hot diaphragm forming process . C-shaped . Prepreg . Defect

X. X. Bian : Y. Z. Gu (*) : J. Sun : M. Li : Z. G. Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China e-mail: [email protected] Y. Z. Gu e-mail: [email protected] W. P. Liu National Engineering and Research Center for Commercial Aircraft Manufacturing, Commercial Aircraft Corporation of China, Ltd, Shanghai 200436, People’s Republic of China

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1 Introduction Using prepreg materials, traditional hand lay-up process is often adopted to manufacture composite components of aircrafts. With increasing use of composites in airplanes, this labor-intensive and time-consuming process is being challenged. A promising manufacturing process with significant time and cost saving is automatic tape laying (ATL) technique [1]. It provides more efficient lay-up for geometrically huge and complex composite parts, but has restrictions on high-curvature components. One alternative way is to firstly produce flat prepreg stack with the application of ATL, and then make the sheet deform to the curved geometry of tool, which can be achieved by means of hot diaphragm forming process. Finally, the preforming curved part is cured in autoclave or oven under applied pressure. This technique can fabricate curved composite parts with good quality and excellent stability, and has been successfully implemented in B777 and V-22 composite I-stringers as well as A400 wings [2–6]. Initially applied to thermoplastic matrix composites, hot diaphragm forming process has been investigated by many researchers. For thermoplastic prepreg stacks, the processing temperature is empirically determined to be 30 °C higher than the matrix melting temperature, which indicates that the hot diaphragm forming is one-step process for forming and solidifying. Some studies showed that forming a stack of thermoplastic prepreg can be influenced by many factors, such as processing parameters [4, 5, 7–11], material systems and configurations [12], diaphragm types [5] as well as the part geometries [11–13]. Nowadays hot diaphragm forming process is prevailing in aircraft structure manufacture using thermosetting prepreg [14–16]. Different from thermoplastic composite processing, thermosetting matrix does not have obvious flowability during hot diaphragm forming process, but is heated to decrease the matrix viscosity sufficiently for enabling the prepreg to be formed [2]. The process temperature of thermosetting prepreg stack for hot diaphragm forming is much lower than resin flowing and curing temperatures. Therefore, for thermosetting composites, the hot diaphragm forming process is just a pre-fabricating process which must be followed by the curing process in oven or autoclave to obtain composite product. The two steps both have strong influences on the manufacturing quality of composites. However, most works reported have been focused on the final part during autoclave process, such as thickness measurement, defect evaluation and mechanical test. The studies on the effects of processing parameters on thermosetting prepreg preform quality in hot diaphragm forming process before autoclave processing are rare, and the relationship between the diaphragm forming and the autoclave process also needs to be explored. During hot diaphragm forming process, the deformation to the tool geometry is achieved by heat and vacuum pressure, and in-plane deformation behavior between adjacent plies and compaction behavior through-thickness direction occur to form curved structure. The inplane deformation behavior where plies slide relative to each other can be tested by investigating the interlaminar friction through a one-dimensional ply-pull-out experiment. The interlaminar friction is very important to understand the inter-ply deformation and laminate wrinkling, which is the main defect during forming curved structure. Studies have shown that processing conditions [2, 9, 12, 15–18] and prepreg material types [16, 19] significantly influence interlaminar friction. The laminate compaction behavior can influence resin flow and dimensional stability, which is also strongly affected by processing conditions [20]. Some basic works published by Scherer et al. [18, 21] and Hou et al. [22– 24] have successfully transferred one-dimensional prepreg behavior to the stamp forming of two- or three-dimensional composites by measuring or analyzing several forces occurred in the transfer of a flat laminate into a hemispherical cavity. These works are meaningful for the


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investigation of hot diaphragm forming of C-shaped parts. Thus, in order to obtain comprehensive understanding for diaphragm forming process of curved prepreg composite and process optimizing method, the relationship between processing parameters and prepreg forming behaviors should be studied, and the details about the relevant defects need to be analyzed. This work aims to investigate the effects of processing temperature and vacuum applying rate on the forming quality of C-shaped thermosetting composite laminates during hot diaphragm forming process. C-shaped carbon fiber/epoxy prepreg stacks were produced using a home-made hot diaphragm forming equipment with different process temperatures and vacuum conditions. In this paper, process temperature range was 40–80 °C based on the property of the studied epoxy system and vacuum conditions were set according to the experimental experience in Ref. [5] in which a pressure application rate range of 1 bar/min to 5 bar/s for material APC-2/AS4 was adopted. The thickness distribution and the defects of the preforms after diaphragm forming process, including fiber wrinkling and voids, were investigated. For understanding the forming process of the defects, both interlaminar friction and compaction behavior of the prepreg stacks were experimentally analyzed at different processing conditions. Moreover, autoclave technology was used to cure the preforms to investigate the changes of the defects on the preforms before and after autoclave processing.

2 Experiment 2.1 Materials The studied material is unidirectional carbon fiber/epoxy resin X850 prepreg (Cytec Co.) with 35 % resin weight content. The nominal cured thickness per ply of the prepreg is 0.19 mm. In hot diaphragm forming process, the diaphragm material is SL850 (Airtech Co.) with breaking elongation of 450 %, tensile strength of 82 N/mm2 and maximum operational temperature of 204 °C. For better understanding of the investigated material system, epoxy resin was separated from the carbon fiber prepreg through acetone extraction method and tested to get the trend of the resin viscosity over temperature. 2.2 Hot Diaphragm Forming Process The forming process was implemented in a diaphragm forming facility, as shown in Fig. 1a, involving two systems, i.e. heating system and vacuum system. To achieve desired forming temperature, the heating power of the lamps, the distance between the lamps as well as the distance between the lamps and the stack surface were adjustable. The C-shaped preforms investigated in this study were produced on a mould with the geometry shown in Fig. 1b. The tool was machined to an angle of 90° with corner radius of 8 mm. A silicone rubber heating film was located inside the mould to preheat the mould, as shown in Fig. 1a. The vacuum system consists of one vacuum pump, two vacuum gauges, one gas flow meter and one control valve. Using the control valve and the gas flow meter, different vacuum pressure applying rates were obtained. Before the diaphragm forming process, flat prepreg stacks were prepared in the size of 300×180 mm at ambient temperature and the stacking sequence of the laminate was [45/ −45]8 cross-ply. A vacuum pressure of approximately 0.1 MPa was applied for 15 min every four plies during lay-up the stack to consolidate the hand–laid stacked prepreg layers and

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Fig. 1 (a) The schematic presentation of diaphragm forming facility; (b) Geometry of the tool

remove entrapped air. The flat prepreg stack between two pieces of diaphragms was placed on the mould, forming a sealed vacuum chamber, as described in Fig. 1a. Five thermocouples were attached to the sample and the mould, to give a direct measurement of the upper surface temperature of the sample and the mould. During diaphragm forming process, the mould was preheated accompanied by the flat stack under vacuum sealed state with the double diaphragms, and then the stack was heated by the infrared radiation lamps. After the stack was heated to the set temperature, vacuum in the vacuum box (Fig. 1a) was applied and the flat preform progressively deformed into C-shaped prepreg laminate based on the geometry of the tool (Fig. 1b). More detailed description of the hot diaphragm forming process using this device can be found in our previous work [25].

Fig. 2 (a) Geometry of C-shaped preform; (b) The positions for micrographic measurements


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Fig. 3 Ply Pull-Out Test: (a) Schematic of the device; (b) Geometry of the specimen

Under vacuum pressure in the vacuum box, flat stack conforms to the profile of mould to get curved structure, and the time spending on the deformation process needs to be properly controlled. In our studies, the deformation process finished when the vacuum pressure reached 0.06 MPa for all cases. To quantify the deforming rate of the flat stack, vacuum pressure applying rate ν in the vacuum box was defined, which was estimated by dividing 0.06 MPa by the deforming time τ. n½kPa= min ¼ 60=t

ð1Þ

2.3 Characterization of the Forming Quality of Prepreg Preforms To study the influences of process temperature and forming rate on the processing quality of C-shaped prepreg stacks, three preforms were produced under 45 °C, 60 °C and 80 °C with a vacuum applying rate of 400 kPa/min, while another three preforms were produced under 50 °C with vacuum applying rates of 400 kPa/min, 50 kPa/min, 25 kPa/min, respectively. Fig. 4 Slippage on the ends of the preform

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Fig. 5 The apparatus for compaction behavior test

The C-shaped preforms made in this study have a flange length of 40 mm and a web length of 100 mm, as shown in Fig. 2a. The forming quality of the prepreg laminate was characterized by visual inspection for the surface morphology, measurement of the thickness distribution and micrograph observation of the cross-sections inside the preform. Different positions measured to get thickness information are shown in Fig. 2a. To quantify the thickness difference between the corner regions and the flat regions of the C-shaped prepreg laminate, the thickness distribution coefficient δ was defined as follows: d ¼ Tc = Tf

ð2Þ

where Tc is the corner thickness, Tf is the mean value of web and flange thickness. In this paper, the investigated process temperature for hot diaphragm forming process is 45–80 °C, which is much lower than resin curing temperature. Therefore, the formed preform is uncured and the defects in it are not fixed. To preserve the states of the defects in preform and avoid the changes of the defects resulted from cutting and polishing operations during sample preparation, slow-curing method was employed for the preform. Firstly, the preform was slowly cured under 120 °C for 12 h in oven without any exerted pressure. This curing condition can make the curing degree reach 70 % and ensure maintaining the original states of the defects during sample preparation. Then the cured preform was cut to get samples from different positions, as shown in Fig. 2b. The cross sections of Fig. 6 Schematic diagram of the bagging procedure for autoclave processing


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Table 1 Thicknesses of preforms with different processing temperatures Processing temperature/°C

Thickness of flat prepreg stack/mm

Thickness of preform/mm

Thickness distribution coefficient δ

flange

web

corner

45

1.63±0.02

1.62±0.02

1.60±0.01

1.55±0.02

0.96

60

1.58±0.01

1.57±0.00

1.57±0.01

1.52±0.02

0.97

80

1.60±0.01

1.59±0.02

1.58±0.02

1.51±0.02

0.95

samples were wet ground with successively finer silicon carbide paper, from 600 to 2,000 grit, and wet polished with chromium oxide. Finally, using an optical microscope (Olympus BX51M) the polished cross-sections were observed and void volume fractions were measured through the digital microscopy and image analysis to study the defects formed during the diaphragm forming process. 2.4 Ply Pull-Out Test During hot diaphragm forming process, the prepreg material deforms into the desired shape in a predictable way by forcing all layers to slip against each other. In this study, the slipping friction behavior between non-cured prepreg layers at different conditions was measured using the testing device, as shown in Fig. 3a. It was designed to be mounted in a standard tensile testing machine (SANS Ltd. Co., 5KN), and its configuration was similar with the apparatuses used by Ersoy et al. [15] and Martin et al. [19]. The specimens (Fig. 3b) used for measuring interply friction were prepared by folding two 370×35 mm prepreg strips which enclosed two 190×35 mm plies. The short prepreg strips were pulled out from the long ones. The testing area was 100 mm×35 mm. Since both sides of the short ones underwent frictional effect, the load measured was theoretically equal to twice value of inter-ply frictional force. More details can be found in Ref. [25]. The temperature and pulling rate during the tests were set according to hot diaphragm forming process parameters. The pressure of 0.1 MPa was applied to the test area with the

Fig. 7 Measured compaction behavior of prepreg stack at different temperatures

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Fig. 8 Surface morphology of preforms produced at (a) 45 °C; (b) 60 °C; (c) 80 °C

application of four calibrated spring-screw sets. The samples were tested under 45 °C, 60 °C and 80 °C with a cross-head speed of 1 mm/min, while others were examined under 50 °C with cross-head speeds of 1 mm/min, 0.2 mm/min and 0.1 mm/min, respectively. A minimum of six specimens were tested for each condition. As depicted in Fig. 4, there is a slippage between plies on the ends of the preform (just four plies is shown in Fig. 4), which results from the difference in the radius of curvature of the plies during the forming. The interlaminar slippage was quantified in the way similar to that in Ref. [9]. Thus, the crosshead speed γ used for the ply pull-out tests was calculated as follows: g ½mm= min ¼ σ=7t

ð3Þ

where σ is the relative slippage between the upper ply and the bottom ply, τ is the deforming time.


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Fig. 9 Optical micrographs of preforms at corner region with different processing temperatures: (a) 45 °C; (b) 60 °C; (c) 80 °C

In this way, the cross-head speed was determined based on the actual prepreg slipping velocity between adjacent plies with different vacuum pressure applying rate during hot diaphragm forming process [25]. The cross-head speeds of 1 mm/min, 0.2 mm/min and 0.1 mm/min in this paper correspond to the vacuum pressure applying rates of 400 kPa/min, 50 kPa/min, 25 kPa/min, respectively. 2.5 Compaction Behavior Test For thermosetting matrix composites, the consolidation process has a close relationship with the compaction behavior of prepreg stack. In this study, the compaction behavior of prepreg

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Fig. 10 Interlaminar friction behavior at different temperatures

stack was tested under different processing conditions to investigate the changes of thickness during hot diaphragm forming process. The testing apparatus was designed to install on a standard compression testing machine (Fig. 5), which was similar to that used in Ref. [20]. The sample of prepreg stack with [+45/ −45]8 and the size of 150×150 mm was placed into the testing mould. Then the sample was compressed using the mechanical testing machine, and the change of sample thickness was recorded. Testing temperatures were 45 °C, 60 °C and 80 °C, and cross-head speed was 1 mm/min. 2.6 Autoclave Processing Autoclave technique with steel male mould was employed for curing the C-shaped prepreg stack after the diaphragm forming process. The bagging procedure for Cshaped preform is displayed in Fig. 6. In autoclave processing, temperature was increased from room temperature to 180 °C and then the temperature was held for 120 min. After that, the temperature was decreased to ambient temperature. External pressure 0.6 MPa was applied at the beginning of increasing temperature stage, and was held until the temperature was decreased to 65 °C. Note that vacuum pressure 0.1 MPa was relieved when the external pressure rose to 0.14 MPa. These curing cycles are recommended by the manufacturer of X850 prepreg. Similar to the way conducted for the hot diaphragm forming, the cured C-shaped laminates were studied by means of visual inspection for surface quality, thickness measurement and optical micrograph for defects.

Table 2 Maximum interlaminar frictional force at different temperatures tested at 1 mm/min Temperature/°C

45

60

80

Respective maximum frictional force/N

188.8

51.5

46.8

Average maximum frictional force/N

187.8±1.4

50.1±3.4

50.4±6.3


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Fig. 11 X850 resin viscosity over temperature

3 Results and Discussion 3.1 Effect of Processing Temperature on Diaphragm Forming Process In hot diaphragm forming process, processing temperature determines resin flowability as well as the deformability of prepreg, thus influences the forming quality of preform. In this paper, to study the influence of forming temperature on hot diaphragm formed C-shaped thermosetting composite parts, C-shaped prepreg laminates were respectively produced under 45 °C,60 °C and 80 °C with a vacuum applying rate of 400 kPa/min. Table 1 shows the thicknesses of preforms produced at different forming temperatures. From the results, it can be found that the temperature variation has little effect on the thickness change. The reason is that under 0.1 MPa pressure, the compaction behaviors of the prepreg stacks at different temperatures are almost same, as shown in Fig. 7. Furthermore, the corner is generally thinner than the flat section (Table 1). The phenomenon can be attributed to the additional tensile stress superimposed by the diaphragm on the corner region, where the diaphragm stretches more significantly than that on the flat section. Surface morphology of preform close to the mold side is shown in Fig. 8 and the surface quality looks different at different processing temperature. It can be observed Fig. 12 Void distributions in preforms produced under different processing temperatures

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Fig. 13 Vacuum pressure vs. time with different forming rates

from these photos that the preform produced at 45 °C has some wrinkles, while the preforms produced at 60 °C and 80 °C both have good surface quality without wrinkle. Similar results also can be found from the micrographs of the crosssections inside the preforms, as shown in Fig. 9. Fiber wrinkles appear on the inner layer on the tool side, only for the sample processed at 45 °C, as indicated by the elliptical circle in Fig. 9a. This fiber buckling could affect the performance of cured composite component [11]. In order to further understand the formation of fiber wrinkle, the friction behaviors between prepreg plies under the corresponding temperatures were measured. Figure 10 shows the load–displacement behaviors of the representative specimens under 45 °C, 60 °C and 80 °C. For all the tested temperatures, the load increases rapidly up to a level and thereafter the load–displacement curve levels off. To simplify the results, the maximum value of interlaminar frictional force was extracted from the curves and the average maximum frictional forces of all the samples for each condition were listed, as shown in Table 2. As can be seen, the maximum interlaminar frictional force drops approximately 137 N from 45 °C to 60 °C, while the decrease is less than 5 N from 60 °C to 80 °C. Additionally, as can be seen from Fig. 11, the resin viscosity shows a drop trend with increasing temperature from 40 °C to 60 °C and basically remains the same from 60 °C to 80 °C, which agrees with the result of interlaminar friction over the temperature range. Thus, higher processing temperature results in lower interlaminar frictional force, which corresponds to higher deformability, i.e. sliding easier

Table 3 Thicknesses of preforms with different vacuum applying rates Vacuum applying rate/(kPa/min)

Thickness of flat prepreg stack/mm

Thickness of preform/mm

Thickness distribution coefficient δ

flange

web

corner

400

1.61±0.01

1.63±0.01

1.61±0.02

1.54±0.01

0.96

50

1.63±0.02

1.58±0.01

1.59±0.01

1.51±0.02

0.95

25

1.58±0.00

1.57±0.01

1.57±0.01

1.46±0.01

0.93


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Fig. 14 Surface morphology of preforms produced at (a) 400 kPa/min; (b) 50 kPa/min; (c) 25 kPa/min

between plies. Appropriate interlaminar friction can make each ply of prepreg deform consistently and avoid the formation of wrinkles. It explains the influence of processing temperature on the defect of preform, and the maximum interlaminar frictional force is an important value to determine the processing temperatures in hot diaphragm forming process. Figure 12 presents the void contents of flat sections and corner sections of the samples produced at different processing temperatures in hot diaphragm forming. The voids mainly came from entrapped air during hand lay-up process. The void content decreases with increasing temperature, indicating that higher temperature makes it easier to remove the entrapped air between plies during diaphragm forming process. In addition, the void content in the corner is lower than that in the flat sections, which may be caused by the additional compaction behavior at the corner and is consistent with the corner thinning effect (Table 1).

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Fig. 15 Optical micrographs of preforms produced at (a) 400 kPa/ min; (b) 50 kPa/min; (c) 25 kPa/ min

3.2 Effect of Forming Rate on Diaphragm Forming Process Different vacuum applying rates can change the shaping rate of curved structure, and so change the interply slipping behavior of prepreg. In this paper, to study the influence of forming rate on the forming quality of hot diaphragm formed C-shaped preform, the Cshaped prepreg laminates were produced under 50 °C with vacuum applying rates of 400 kPa/min, 50 kPa/min and 25 kPa/min, respectively. The curves of vacuum pressure in the vacuum box vs. time with different forming rates are shown in Fig. 13. As can be seen, all curves present approximately linearly trend in the vacuum range of 0–0.06 MPa, indicating the validity of the definition of vacuum pressure applying rate using Eq. (1).


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Fig. 16 Interlaminar friction behavior at different pulling rates

Table 3 shows a decrease in thickness of prepreg preform with decreasing forming rate. Since there is negligible resin flow at 50 °C, this rate-dependant behavior may be attributed to creep response of [+45/−45] laminates [26]. In addition, the corners are also found to be thinner than the flat sections, and the thinning degree increases with decreasing vacuum applying rate. Similar to the case with forming thermoplastic prepreg stack, it indicates that with slow forming rate, the thickness uniformity decreases [5]. Figure 14 presents the surface states of preforms produced at 50 °C under different forming rates. The preform produced under 400 kPa/min shows some wrinkles at the corner. Visual inspection of the preform produced under 25 kPa/min shows good surface quality and no signs of wrinkles. The results are confirmed by their optical micrographs, as shown in Fig. 15. As indicated by the elliptical circles, the preforms produced at 400 kPa/min and 50 kPa/min have fiber wrinkles on the corners close to the mould side, while that produced at 25 kPa/min presents no wrinkles. This result agrees with the observations for forming thermoplastic composites in Refs. [10, 27] that faster forming rates increase the severity of buckling. For further investigating the influence of vacuum applying rate on hot diaphragm forming process, the interply friction behavior was tested at 50 °C with different pulling rates, simulating different forming rates. The results of the representative specimens at 400 kPa/min, 50 kPa/min and 25 kPa/min are shown in Fig. 16, and the corresponding maximum interlaminar frictional forces and the average maximum frictional forces of all the samples for each condition are given in Table 4. It is indicated that the interply friction behavior is pulling rate dependent, i.e. higher pulling rate yields higher interply frictional force value, which agrees with the experimental results in Ref.[15]. The maximum interlaminar frictional force at 1 mm/min, in accordance with 400 kPa/min forming rate, is twice as that at 0.1 mm/min (25 kPa/min). At the same processing temperature, vacuum applying rate corresponds to the rate of flat laminate deforming to a

Table 4 Maximum interlaminar frictional force at different forming rates tested at 50 °C Vacuum applying rate/(kPa/min)

400

50

25

Respective maximum frictional force/N

110.2

61.7

48.3

Average maximum frictional force/N

106.8±8.3

61.7±3.6

50.7±3.4

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Fig. 17 Void distribution in preforms produced under different vacuum applying rates

desired C-shaped structure, so high interply frictional force means that interlayer sliding rate is hard to keep up with forming rate and that fiber wrinkle is difficult to avoid. The void contents at different positions in the preforms produced under different vacuum applying rates are shown in Fig. 17. The vacuum application rates seem to have little impact on the void content. The voids in the corner are also fewer than those in the flat sections. It is interesting to note that high temperature with high forming rate and low forming rate with low temperature seem to give similar interply friction behavior. For example, the frictional behavior at 60 °C with 400 kPa/min is similar to that at 50 °C with 25 kPa/min, and their maximum interlaminar frictional forces are almost identical. Moreover, both of the produced preforms with small interlaminar frictional force have good surface quality without wrinkles. Therefore, the interply friction behavior has strong effect on the forming process and defects, and the maximum interlaminar frictional force can be used as a key parameter for optimizing processing conditions. 3.3 Change of Defects After Autoclave Processing As mentioned above, prepreg preform produced using hot diaphragm forming process needs to be further cured under higher temperature and pressure, such as autoclave process. During autoclave processing, resin flows and redistributes in composite, and fiber bed is compacted obviously. These behaviors would significantly influence the final quality of composite part [28]. In order to study the changes of C-shaped preforms after autoclave process, such as

Fig. 18 Surface morphology of cured C-shaped laminate after autoclave processing using the preforms produced with different diaphragm forming process conditions: (a) 50 °C with 400 kPa/min, (b) 60 °C with 400 kPa/min


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Fig. 19 Optical micrographs of cured C-shaped laminate after autoclave processing using the preforms produced with different diaphragm forming process conditions: (a) 50 °C with 400 kPa/min, (b) 60 °C with 400 kPa/min

fiber wrinkles and voids shown in aforementioned experimental results, the preforms produced with different diaphragm forming process were cured in an autoclave using steel male mold (Fig. 6). The surface morphology of final parts after autoclave processing is shown in Fig. 18. It is demonstrated that fiber wrinkles on the corner of prepreg preform decrease under autoclave processing. For example, Fig. 18a corresponds to the preform produced at 50 °C with 400 kPa/min, and it has better surface quality than the uncured prepreg preform (Fig. 14a). It should be noted that the cured C-shaped laminate in Fig. 18a still has some wrinkles, indicating that the wrinkle defect formed during diaphragm forming process might still exist to a certain degree after high pressure and temperature curing process. Thus, this kind of defect should be avoided during diaphragm forming process. In addition, there are not any obvious voids inside the cured laminates for all studied preforms, as shown in Fig. 19. It is proposed that void defects in preforms can be significantly eliminated through autoclave technology, and voids are not very important defects for considering during hot diaphragm forming process. The results indicate that autoclave process is effective in eliminating voids in the preforms and can alleviate fiber wrinkles at the corner to a certain extent. For obtaining curved composite with high quality at corners, it is essential to optimize process parameters in hot diaphragm forming and produce prepreg preforms without wrinkles.

4 Conclusions In this study, C-shaped prepreg preforms produced in a home-made diaphragm forming laboratory facility were used to study the influences of process temperature and vacuum applying rate on the preform processing quality, including thickness and defects. The influences of process parameters were analyzed by means of prepreg interlaminar friction behavior test and compaction behavior test. Furthermore, the impact of autoclave processing on the curing quality was studied to give a complete understanding of the hot diaphragm forming process. The results show that the temperature variation has little effect on the dimensional change of the preform, while the thickness decreases in a degree with slow forming rate. And the thickness of preforms at the corner is thinner than those at the web and the flange for all studied cases. Temperature and vacuum applying rate have great impacts on the surface

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quality of performs. Low temperature and high forming rate easily cause poor sliding of prepreg layers and large frictional force, which are the main reasons for fiber wrinkling at the corner of the preform. It is found that high temperature with high forming rate and low forming rate with low temperature have similar interply friction behavior and maximum interply frictional force. The maximum interply frictional force is suggested as a key parameter for optimizing processing conditions. Autoclave processing is effective in eliminating voids in the preforms and can alleviate fiber wrinkles at the corner to a certain extent. For obtaining curved composite with high quality at corner, it is essential to optimize process parameters in hot diaphragm forming and produce prepreg preform without wrinkles. Acknowledgments This work was supported by funding from the Fund of National Engineering and Research Center for Commercial Aircraft Manufacturing (Project No. SAMC11-JS-07-220), the National 973 Program of China [Project No. 2010CB631100].

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