PEEK composites during selective laser sintering

Accepted Manuscript High temperature rheological behavior and sintering kinetics of CF/PEEK composites during selective laser sintering Mengxue Yan, Xiaoyong Tian, Gang Peng, Dichen Li, Xiaoyu Zhang PII:

S0266-3538(18)30639-0

DOI:

10.1016/j.compscitech.2018.06.023

Reference:

CSTE 7279

To appear in:

Composites Science and Technology

Received Date: 18 March 2018 Revised Date:

17 June 2018

Accepted Date: 21 June 2018

Please cite this article as: Yan M, Tian X, Peng G, Li D, Zhang X, High temperature rheological behavior and sintering kinetics of CF/PEEK composites during selective laser sintering, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.06.023. 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.

ACCEPTED MANUSCRIPT High Temperature Rheological Behavior and Sintering Kinetics of CF/PEEK Composites during Selective Laser Sintering Mengxue Yan1, Xiaoyong Tian1*, Gang Peng1, Dichen Li1, Xiaoyu Zhang2

University, 710054, China

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1, State Key Laboratory of Manufacturing Systems Engineering, Xi’an Jiaotong

* Corresponding author: [email protected]

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2, Beijing Institute of Spacecraft System Engineering, CAST, Beijing, China

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Abstract: As a kind of high performance polymer with excellent mechanical strength, high temperature property and chemical resistance, the polyether-ether-ketone (PEEK) and its composites are the promising candidates that can satisfy the demands for high stiff and lightweight in aerospace industry. So it is very attractive to fabricate PEEK

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and its composites parts with additive manufacture technology, especially the selective laser sintering (SLS), due to its advantage on the fabrication of the parts with complex geometries. However, the strengths of the PEEK and its composites prepared

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by SLS are obviously lower than their injection molded parts and the laser sintering

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kinetics of the PEEK composites is seldom studied. In this paper, to fabricate the carbon fibers (CF) reinforced PEEK composites with high strength by SLS, the sintering kinetics of CF/PEEK composites was thoroughly studied based on the high temperature rheological behavior. A novel effective melting zone was defined by combining the simulated temperature distribution with the viscosity-temperature relationship and used to predict the process planning. Finally, the calculation results were validated by employing the simulation parameters in experiments and the tensile

ACCEPTED MANUSCRIPT strength of CF/PEEK composites reached 109±1 MPa with an elasticity modulus of 7365±468 MPa, which is 85% higher than injection molded pure PEEK. Therefore, methods in this work could be considered as a complement to the numerical analysis

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of SLS process and the reinforced CF/PEEK composites may be used in aerospace industry for the structure optimization and lightweight design with complex geometries.

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Key words: PEEK; carbon fiber reinforced PEEK; Selective Laser Sintering;

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Additive Manufacturing; Temperature distribution; Rheological Behavior 1. Introduction

Selective Laser Sintering (SLS), a powder bed based Additive Manufacturing (AM) process, could fabricate polymer and its composites with complex geometries

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without the need for any tooling or molding process, as it builds up parts layer by layer based on computer-aided design models [1-3]. Especially for the crystalline and semi-crystalline polymers, the SLS products can be directly used as end use parts with

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good mechanical strength [4]. Until now, polyamide and its composites were widely

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utilized by SLS processes, which proved the addition of the reinforced material, such as fiber and nano-fillers, can improve the mechanical or physical properties of laser sintered polymeric parts [3, 5-8]. However, the mechanical strength and thermal property of polyamide and its composites cannot fulfil all challenging requirements of industrial applications any more, especially in the aerospace industry. At the same time, as a typical semi-crystalline polymer with outstanding mechanical performance, high temperature property and favourable biocompatibility, PEEK and its composites

ACCEPTED MANUSCRIPT were much less discussed than nylon in SLS. This may result from the much higher processing temperature (above 300°C) needed for PEEK comparing with polyamide (~160°C), which is a great challenge for the SLS equipment [9]. Meanwhile, the

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mechanical properties of sintered PEEK were obviously lower than their injection molded parts. Although some researches have been done on the preparation of the PEEK composites by adding reinforcement fillers [10-13]. However, the enhancement

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effect was not as obvious as observed in polyamide. As reported in a related literature,

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the tensile strength and tensile modulus of CF/PA12 sintered specimens were increased by 60% and 323% respectively compared with the pure PA12 specimens [5]. While, there are still no literatures reported that the mechanical strength of reinforced PEEK composites prepared by SLS exceeded the injection molded pure PEEK. This

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phenomenon may be caused by the difference between the laser sintering kinetics of polyamide and PEEK with reinforcement filler [14]. Especially, the distinct difference between rheological behaviors of PEEK and polyamide may influence the fusion and

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coalescence of polymer powders during SLS. The viscous sintering of polymer

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powders can be described by the model proposed by Frenkel [15] as shown in Equation.1.

௫మ ோ

ଶ୻

= ଷఎ ‫ݐ‬

(1)

బ

Where x is the length of the sintering neck, R is the original powder radius, Γ is

the surface tension, η0 is the zero shear viscosity of the polymer, and t is sintering time. It can be found that the powder coalescence rates are inversely proportional to

ACCEPTED MANUSCRIPT the zero shear viscosity. Considering the laser scanning speed, the polymer powder was melted and cooled in a very short time during SLS, so it can be concluded that the viscosity is a critical parameter which determines the sintering kinetics. As reported in

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related literature, the viscosity of PEEK melt is much higher than the polyamide (<104 Pa·s) [16-18], especially, the addition of CF would increase the viscosity of composites melt further. Additionally, in the classical reinforcement theory, the

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strength of the composites is also strongly influenced by the interfacial adhesion

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between the fiber and matrix, which shows a strong correlation between the viscosity of polymer melting [19]. However, during all the related research, the heat transfers in the powder bed were mainly analyzed during the numerical models [9, 20-21]. And rheological behaviors of polymer melt and the powder coalescence which are the most

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essential process are ignored.

In the present research, the high temperature rheological behaviors of CF/PEEK were studied. And the zero shear viscosities of the CF/PEEK powder at different

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temperatures were calculated. The previous thermal physical model was coupled with

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the rheological analysis to determine the effective melting zone for optimizing process parameters. The process experiments were conducted to verify the calculation results. And the mechanical properties including tensile and flexible strength of composites were tested to evaluate the optimal CF content and process parameters. 2. Material and methods 2.1 Materials and preparation of the CF/PEEK composites Thermoplastic polyether-ether-ketone with brand name PEEK450PF supplied by

ACCEPTED MANUSCRIPT Victrex plc was employed as the matrix material. And milled carbon fibers with a length of 300-500 µm and a diameter of 7-10 µm (NanJing WeiDa Composite Material co.Ltd, Nanjing, China) was chosen as the reinforcements. The PEEK

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powder was dried in an oven at 120°C for 24 h and the CF powder was heat treated at 300°C for 4 hours. Then these two kinds of powders at the weight ratios of 95/5, 90/10, 85/15 and 80/20 respectively were mixed in a high-speed mixer for two hours

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at the speed of 250 r/min.

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Selective Laser Sintering process of PEEK and CF/PEEK composite powders were carried out on the self-developed SLS equipment. The tensile and flexural specimens were directly built using SLS process according to ISO527-2 test type AB1 and ISO178-2001 respectively. All the samples were built along the X direction, in

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which the powders were spread in the building platform. The processing parameters were as follows: XY double scan mode, scanning speed of 3000 mm/s, scanning interval of 0.12 mm. Powder bed temperature and laser power for preparing the

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specimens were varied according to the powder characteristics.

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2.2 Tests and characterization The tensile tests were performed on the multi-functional statics testing machine

(CMT4304) at a crosshead speed of 1 mm/min. The flexural tests were carried out on the same machine with a test speed of 1 mm/min and a span length of 64 mm. All the tests were performed at the room temperature of 25°C, and each test was executed with five individual specimens. Scanning Electron Microscopy (SEM, S-3000N, HITACHI) was used to

ACCEPTED MANUSCRIPT investigate the microstructure of the composite powder and the fractured surface of composite samples. The particle size distribution of the composites powders was measured by particle sizing instrument (Mastersizer 3000, Malvern).

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Thermogravimetric analysis (TGA) of PEEK was conducted by using the device of STA449C (NETZSCH, Germany) to obtain its thermal decomposition at high temperatures. The PEEK powder was heated from the room temperature to 650°C at a

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constant rate of 10°C/min under the N2 gas environment. The differential scanning

10°C/min from 25 to 400°C.

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calorimetry (DSC, METTLER TOLEDO) tests were conducted at a constant rate of

Rheological measurements of PEEK and CF/PEEK composites powders were performed on a rotational rheometer (AR2000-EX, TAInstruments), equipped with the

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electrically heated plates with a diameter of 25 mm. And the measurement temperatures were set as 350, 360, 370 and 380°C for each sample. The thermal conductivity of composite powders was measured using a hot-disk

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thermal analyzer (TPS-2500S), adopting a transient plane source technique. The

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testing temperature of the pure PEEK and composite powders were set at 320°C and 300°C respectively.

The extinction coefficient α (m-1) of the PEEK characterizes the attenuation of

the laser energy along Z direction during SLS [9, 20] and were measured by the custom-made device in the building chamber and the extinction coefficient α was fitted by the data tested [20]. The powder bed density was tested in the building chamber by following the routine described in previous work [20].

ACCEPTED MANUSCRIPT 2.3 Simulation and calculation model A numerical model with a volumetric heat source was used to simulate the temperature distribution in the powder bed of PEEK and CF/PEEK. The model

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includes an effective volumetric heat source and an integrated testing procedure to determine the material parameters as reference in the literature [20], which accuracy has been validated and used to develop the process efficiency maps and prediction for

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the SLS of PA12 and composites powders [20, 21]. A single-line scanning model was

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considered to reduce computing time and the whole calculation scheme is shown in

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Fig.1.

Fig.1 general scheme of the thermal model and the calculation process [20]

The physical parameters of the materials, including the heat capacities, extinction

coefficient and powder bed densities were obtained by measuring as shown in section 2.2 and the calculated heat capacities and the extinction coefficient of the PEEK are shown in Fig.2. As CF acts as a laser absorber and has little influence on the penetration of CO2 laser in polymer powder bed [20], differences of PEEK and

ACCEPTED MANUSCRIPT CF/PEEK composite powders in the extinction coefficient were ignored to simplify the calculation process. It’s worth noting that the boundary conditions of the preheating temperature of PEEK and composite powders were varied as the different

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thermal properties between them. As observed in the SLS process, the agglomeration between composite powders occurred when the preheating temperature was set the same as the pure PEEK. So the preheating temperature was set as the highest

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temperature to avoid the agglomeration. The details of material parameters and the

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calculation condition are shown in Table.1

-3 -4 -5 -6 -7 200

PEEK/20CF PEEK/15CF PEEK/10CF PEEK/5CF PEEK

250

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Heat capacity (J/gK)

-2

300

350

Transmitted power (W)

0.5

-1

400

0.4

α=11186.22 BEERLAMBERT SLAW (User)

Model

0.3

Equation

Reduced Chi-Sqr

0.425*exp(-a*z) 3.35412

Adj. R-Square

0.2

1.52601

0.98566

0.99164 Value Standard Error 11186.22764 1064.81608

Transmitted powe a Transmitted powe

0.1 0.0

0.0000

0.0002

0.0004

0.0006

Thickness (m)

Temperature(°C)

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Fig.2. the specific heat VS temperature and the extinction coefficient of PEEK Table.1. the material parameters and the calculation condition PEEK/5C

PEEK/10

PEEK/15

PEEK/20

F

CF

CF

CF

0. 415

0.392

0.420

0.447

0.497

0.0926

0.15913

0.15913

0.15913

0.15913

320

305

305

305

305

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Material composites

PEEK

Powder bed density (g/cm3)

thermal conductivity (W/(m·K)) preheating temperature(°C) 3. Results and Discussion

ACCEPTED MANUSCRIPT 3.1 Evaluation of CF/PEEK composite powder for SLS The morphology of the composite powders was investigated to evaluate the mixing condition of the CF and the PEEK powders. As shown in Fig.3a, the PEEK

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powder shows the slightly elongated, round particle with a diameter of 20-40 µm. and the CF are about 7 µm in diameter, and 300-500 µm in length. The desperation of these two-phase is uniform to some extent, only a few PEEK powder and CF

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aggregate together. The result of the particle size distribution test has two peaks as

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shown in Fig.3b. One of the distribution peak is between 10 µm -100 µm, which is consistent with that of the PEEK450PF as reported in the literature [10]. Meanwhile, some composite powders show the size during 100–500 µm and below 10 µm, which is supposed to be the length and diameter distribution of CF. The powder bed picture

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during SLS of the CF/PEEK composite is shown in Fig.3c, which exhibits a flat powder bed and indicates a proper flowability of the composite. The melting and recrystallization of the materials are important properties for SLS and the DSC curves

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in Fig.3d shows the CF/PEEK composite powders exhibit a relatively smooth peak

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compared with the pure PEEK and the onset melting points are basically close with the pure PEEK (about 330°C). While, the recrystallization points of the composites powders are slight lower than the pure PEEK, as the CF prevent the crystallization.

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Fig.3 a) the SEM picture of the composite powder, b) the particle size distribution of the composite powder, c) and the powder bed picture during SLS, d) DSC curves of the PEEK and composite powders

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3.2 The rheological behavior of PEEK and CF/PEEK composite powders The rheological behaviors of the polymer melt are complex, being very

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dependent on temperature, shear rate and material composition. As a kind of typical thermoplastic, the viscosity of its melt presents shear thinning behaver as shown in

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Fig.4a and the viscosities decrease when the shear rate is above a certain critical value. However, during SLS, almost no shear flow is applied. To reflect the real rheological behaviors of polymer during SLS, the viscosities should be measured at a very low shear rate. The plateau value of viscosity at low shear rates (between 0.002-0.02 s-1) could be considered approximately as the zero shear viscosity. Meantime, the viscosities of the polymer also exhibit time dependence due to the post condensation. The zero shear viscosities of PEEK and CF/PEEK composites melt increase slightly

ACCEPTED MANUSCRIPT with the time at each temperature as shown in Fig.4(b-f), which indicates the post condensation happens to PEEK at the temperature above melting point. As the laser scanning speed during SLS is very fast and the time for PEEK maintaining at high

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temperature is very short, the viscosities obtained at the beginning time (about 40 s) should be able to represent the real rheological behaviors of PEEK and its composites during SLS approximately.

10

4

PEEK PEEK/5CF PEEK/10CF PEEK/15CF PEEK/20CF

3

10 -3 10

10

-2

10

-1

10

5

10

4

PEEK/5CF 10

10

3

60

80

100

Time (s)

120

350°C 360°C 370°C 380°C

140

5

10

350°C 360°C 370°C 380°C

4

10

40

60

80

100

Time (s)

120

4

350°C 360°C 370°C 380°C

PEEK

40

140

10

6

60

80

100

120

140

160

10

5

10

4

10

3

d

160

350°C 360°C 370°C 380°C

PEEK/10CF 10

2

40

60

80

100

120

140

160

Time (s) 10

6

10

5

Zero-shear viscosity (Pa.s)

Zero-shear viscosity (Pa.s)

e

PEEK/15CF

10

160

6

10

5

Time (s)

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40

10

0

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c

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Zero-shear viscosity (Pa.s)

Shear rate (1/s)

b

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10

5

Plateau: 0.002-0.02 (1/s)

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a

Zero-shear viscosity (Pa.s)

6

Zero-shear viscosity (Pa.s)

Viscosity (Pa.s)

10

10

f

350°C 360°C 370°C 380°C

PEEK/20CF 4

40

60

80

100

120

140

160

Time (s)

Fig. 4 a) viscosity VS shear rate at the temperature of 350°C b-f) zero-shear viscosity

ACCEPTED MANUSCRIPT VS time at different temperatures The zero shear viscosities of PEEK and CF/PEEK composites powder at different temperatures are shown in Fig.5. It can be found the zero shear viscosities of

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the PEEK and its composites are two orders of magnitude higher than PA12 [16, 17], and all the viscosities of samples decrease with the temperature. In addition, the viscosity of the composites increases significantly with the content of the CF, which

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indicates that to reach the same melt flowability, the temperature of composites with

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CF must be higher than the pure PEEK and the more CF content, the higher temperature is needed to obtain the lower viscosity. For example, to reach the zero shear viscosity of 105 Pa.s, for the composites containing 5wt% and 10wt% CF, the temperature must be higher than 350°C; while, for the composites with 15wt% and

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10

6

high viscosity

10

5

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Initial zero-shear viscosity (Pa.s)

20wt% CF, the temperature must be higher than 360°C.

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10

10

PEEK PEEK/5CF PEEK/10CF PEEK/15CF PEEK/20CF

lower viscosity

4

the viscosity of polyamide melt

3

350

360

370

380

Temperature (°C)

Fig.5 the zero shear viscosity of PEEK and CF/PEEK composites VS temperature 3.3 The sintering kinetics of CF/PEEK during SLS In the previous research and numerical models, the melting zone on the powder bed was defined as the area with temperature above the onset melting point, within

ACCEPTED MANUSCRIPT the assumption that the polymer powder was well fused above onset melting point, which may be appropriate for polyamide due to its relative lower melt viscosity [16-17]. However, as shown in Fig.5, the zero shear viscosities of PEEK and its

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composites are far higher than the polyamide, which would affect the polymer powder fusion and coalescence during SLS. Considering the relationship of the melt viscosities and the sintering kinetics, the melting zone was divided into the high

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coalescence rate zone with low viscosity and low coalescence rate zone with high

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viscosity as shown in Fig.6. In the high coalescence rate zone, the dense microstructure is formed. While in the low coalescence rate zone, the porous microstructure tends to be formed. Owing to the different rheological behaviors between the PEKK and PEEK/CF, the powder coalescence rates and the

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microstructures of the sintered parts are both different at the same temperature. Correspondingly, the process parameters needed for preparing the PEEK and CF/PEEK powder should be varied to obtain the dense microstructure. The effective

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melting zone during SLS also should be redefined according to the zero shear

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viscosity instead of the onset melting point.

Fig.6 general scheme of the process of PEEK and CF/PEEK composites by SLS (a) the XY plane, (b) the Z section

ACCEPTED MANUSCRIPT According to the results of the zero shear viscosities, the boundary of the effective melting zone was extracted from the isoviscous (105 Pa.s) temperature for the composites powder and the results are shown in Fig.b-e, as the viscosity of 105

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Pa.s is generally the ceiling for injection molding [22], in which condition, the composites polymer melting was considered to have enough flowability to form dense structure. While, for the pure PEEK, the boundary of the effective melting zone was

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still extracted from isotherms of the onset melting temperature as shown in Fig.5a,

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because all the viscosities of the pure PEEK are lower than 105 Pa.s. It can be observed that the melting zone from the onset melting point for all the material composites shows little difference (~200 µm). But the effective melting zone from the isoviscous (105 Pa.s) temperature shows much difference. Specifically, the effective

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melting zone in Z direction, named effective melting depth, is up to 225 µm with laser power of 10.9 W for the pure PEEK, While, for the CF/PEEK composites, this value is much less, even the laser power is as much as 18.5 W. It should be noted that under

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this condition, all the maximum temperatures are approaching 500°C, which is very

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close to the decomposition temperature of PEEK (as shown in Fig.7f). So if the higher laser power is conducted, the decomposition of PEEK would happen. For the composites with 5wt% and 10wt% CF, the calculation results are similar with an effective melting depth of 135 µm, which indicates the layer thickness should not be more than this value under this condition. For the composite with 15wt% and 20wt% CF, the effective melting depth is about 110 µm, which suggests a less layer thickness is needed to prepare these two kinds of material. However, the layer thickness of the

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SLS cannot be too small subject to equipment machining accuracy.

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Fig.7 the effective melting zone calculated of PEEK and CF/PEEK (a-e) and the thermogravimetric curve of PEEK (f)

3.4. Experimental validation and process research

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Based on the calculation results, the process experiments were carried out. First,

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the layer thickness was set as 0.15 mm and different laser powers were applied to pure PEEK and the composites powder with 10wt% CF. It is commonly considered the SLS part could be prepared with dense structure with enough laser energy and a typical layer thickness. However, the test results in Fig.8a show that the reinforcing effect of the CF only reflects in the improved tensile modulus when the laser power is higher than 16.7 W. And all the tensile strengths of the composites are lower than the sintered pure PEEK, even when the laser power is high enough. The mechanical

ACCEPTED MANUSCRIPT properties of the PEEK and CF/PEEK prepared with different layer thicknesses are shown in Fig.6b. No big variations between the tensile properties of PEEK prepared with different layer thicknesses are observed, which is consistent with the calculation

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as shown in Fig.7a. That is, the effective melting depth is up to 225 µm when the laser power is 10.9 W. While, there is a huge impact of the layer thickness on the tensile strength of the composites with CF as shown in Fig.8b. The tensile strength of the

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sample with 0.1mm layer thickness is over twice as much as the sample with 0.15mm

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layer thickness. That is because the layer thickness of 0.15mm and 0.2mm is deeper than the effective melting depth (about 135 µm), and the powders with low rate of sintering are existed between layers, which may induce the sintered parts with high

a

5500 5000 4500 4000 3500 3000 2500 6

8

10

12

100 90

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PEEK 10% CF/PEEK

6000

14

16

18

20

PEEK 10% CF/PEEK

80 70 60 50 40 30 20 6

8

10

12

14

18

20

9000

Tensile Modulus 10% CF/PEEK

8000 7000

b

6000 5000 4000 3000 0.10

0.12

0.14

0.16

0.18

0.20

120 PEEK 10% CF/PEEK

100 80 60 40 0.10

0.12

0.14

0.16

0.18

0.20

Layer Thickness (mm)

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Laser power (W)

16

Tensile Modulus (MPa) Tensile Strength (MPa)

6500

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Tensile Strength (MPa) Tensile Modulus (MPa)

porosity and bad interlayer bonding.

Fig.8 the tensile modulus and strength of PEEK and composites with 10wt% CF VS laser power (a, the layer thickness of 0.15mm) and layer thickness (b, the laser power of 18.5 W for CF/PEEK and 10.9 W for pure PEEK)

In addition, the composites with different contents of CF were prepared with the layer thickness of 0.1 mm and laser power of 18.5 W to prove the calculation results and optimize the CF content. As shown in Fig.9, It can be found that the composites

ACCEPTED MANUSCRIPT with 10wt% CF have the best tensile strength and modulus with the optimum tensile strength of 109±1 MPa and an elasticity modulus of 7365±468 MPa, which is apparently higher than sintered pure PEEK parts. When the CF content is more that

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10wt%, the reinforcing effect mainly reflects in the tensile modulus, and the tensile strengths of the composites with 15wt% and 20wt% CF are both lower than pure PEEK. Meanwhile, as shown in Fig.9b, the flexural strength and modulus of the

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composites with different contents of CF show the similar law. When the CF content

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is 5wt%, the maximum flexural strength is 183±4 MPa. Likewise, the flexural strengths of composites with 15wt% and 20wt% CF are both lower than sintered pure PEEK. All the mechanical properties of composites indicate the suitable content of CF is between 5wt% and 10wt%. According to the calculation results as shown in Fig.7d

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and Fig.7e, the effective melting depth of the composites with 15wt% and 20wt% CF is about 110 µm, which is close to 0.1 mm. However, the greater melting depth is needed to re-melting partially the previous layer to obtain a good adhesion between

a

6000 5000 4000

Flexural modulus

3000

200 180 160 140 120 100 80 60

0

5

10

15

20

Flexural strength 0

5

10

15

20

Tensile strength (MPa) Tensile modulus (MPa)

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7000

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Flexural strength (MPa) Flexural modulus (MPa)

the adjacent layers during SLS.

9000 8000

b

7000 6000 5000

Tensile modulus

4000 0

5

120 110 100 90 80 70 60 50

10

15

20

15

20

Tensile strength 0

Carbon fiber content (wt%)

5

10

Carbon fiber content (wt%)

Fig.9 mechanical strengths VS CF content (process parameters: laser power of 18.5 W, layer thickness of 0.1 mm)

ACCEPTED MANUSCRIPT To validate the calculation results and figure out the microstructure formed, the SEM pictures of the fractured sections of the tensile samples were analyzed (as shown in Fig.10). It can be observed the composites with 5wt% and 10wt% show a dense

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structure comparing with pure PEEK (as shown in Fig.10e) and a lot of fiber pullout holes are observed, especially in Fig.8b, which indicates a good adhesion between CF and PEEK. While, the loose structures with high porosity are observed in the

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composites with 15wt% and 20wt% CF. It is worth noting that the most voids appear

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between the adjacent layers, which suggests a worse adhesion between layers induced by the insufficient melting depth. The samples of the pure PEEK and the CF/PEEK composites prepared with optimized process parameters and CF content are shown in

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Fig.10f and a clear shape of the composites specimen can be observed.

ACCEPTED MANUSCRIPT Fig.10 the SEM pictures of the fracture sections of the composites with 5wt%, 10wt%, 15wt% and 20wt% CF (a-d), pure PEEK (e) and the photograph of the samples (f) 4. Conclusions

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In this paper, to fabricate the CF/PEEK composites with high strength by SLS and figure out its sintering kinetics, the high temperature rheological behaviors of CF/PEEK composites were tested and the previous thermal physical model was

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coupled with the zero shear viscosity to determine the effective melting zone. The

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viscosity value of 105 Pa·s was chosen to illustrate the difference between high and low viscosity and thus the effective melting zone denoted the low viscosity zone, which was beneficial to powder fusion. The results showed that the zero shear viscosity increased significantly with the content of CF and the effective melting

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depths of CF/PEEK composites calculated is obviously less than pure PEEK even with the higher laser power. Finally, calculation results were validated by employing the simulation parameters in experiments, the maximum tensile strength of

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composites reached 109±1 MPa with an elasticity modulus of 7365±468 MPa with

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10wt% CF, which is 85% higher than injection molded pure PEEK. Therefore, methods in this work was considered as a complement to the numerical analysis of SLS process and can be used to predict the process planning for the other polymer with high viscosity.

Acknowledgement This work is supported by The National High Technology Research and Development Program of China (863 Program), Grant No. 2015AA042503.

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[2] J. P. Kruth; X.W.T.L. Lasers and materials in selective laser sintering. ASSEMBLY AUTOM 2003, 23, 357-371.

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