151

Vol.11 No.3 May 2014Research & Development CHINA FOUNDRY

Effects of laser energy density on forming accuracy and tensile strength of selective laser sintering resin coated sands

*Xu ZhifengMale, born in 1968, Professor. His research and teaching interests mainly focus on the fields of quick casting process, counter gravity casting technology, and preparation of metal matrix composites.E-mail: xuzhf@nchu.edu.cn

Received: 2013-08-22; Accepted: 2014-04-17

*Xu Zhifeng, Liang Pei, Yang Wei, Li Sisi, and Cai ChangchunNational Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology; Nanchang Hangkong University, Nanchang 330063, China

As a representative rapid prototyping method, selective laser sintering (SLS) technology

has received quite extensive attention owing to its application to a vast amount of materials [1]. It has significant prospects in the field of precision sand casting. Using SLS technology, the parts can be made directly according to the original CAD model, which simplifies the process of separating modules and improves the dimensional accuracy of the parts [2]. Moreover, this non-mould rapid precision forming is not limited by the complexity of the parts. In such a case, casting patterns with complex surfaces, variable cross-sections and narrow cavities can be fabricated; which seems impossible or extremely difficult when using the traditional sand casting method [3].

Up to now, SLS technology has been used to

Abstract: Baozhu sand particles with size between 75 μm and 150 μm were coated by resin with the ratio of 1.5 wt.% of sands. Laser sintering experiments were carried out to investigate the effects of laser energy density (E = P/v), with different laser power (P) and scanning velocity (v), on the dimensional accuracy and tensile strength of sintered parts. The experimental results indicate that with the constant scanning velocity, the tensile strength of sintered samples increases with an increase in laser energy density; while the dimensional accuracy apparently decreases when the laser energy density is larger than 0.032 J·mm-2. When the laser energy density is 0.024 J·mm-2, the tensile strength shows no obvious change; but when the laser energy density is larger than 0.024 J·mm-2, the sample strength is featured by the initial increase and subsequent decrease with simultaneous increase of both laser power and scanning velocity. In this study, the optimal energy density range for laser sintering is 0.024-0.032 J·mm-2. Moreover, samples with the best tensile strength and dimensional accuracy can be obtained when P = 30-40 W and v = 1.5-2.0 m·s-1. Using the optimized laser energy density, laser power and scanning speed, a complex coated sand mould with clear contour and excellent forming accuracy has been successfully fabricated.

Key words: selective laser sintering; coated sands; energy density; tensile strength; forming accuracy

CLC numbers: TG221+.1 Document code: A Article ID: 1672-6421(2014)03-151-06

fabricate resin coated sand mould/core using sands with particle size between 50 µm and 150 µm. During the forming process by SLS, the resin between the adjacent particles is melted and partially cured by the local instantaneous high-temperature from a laser. The laser energy density is a key factor influencing the dimensional accuracy and strength of the sintered sand mould/core, the two major features for evaluating the quality of sintered products [4, 5]. SLS technology features high-energy laser and unique scanning process (the laser beam scans in lines and layers and forms a three-dimensional solid by layer superposition). A series of complex physical and chemical reactions occur during the laser sintering of resin coated sand, including the instant transport of heat and mass and the curing of resin film [6]. Ascribed to the short irradiation time of the laser beam and the different absorption extent of the powder material, SLS is prone to generate incomplete curing of resin and no cross-junction between the adjacent layers, leading to non-uniform structure and mechanical properties of the sintered parts [7].

152

Vol.11 No.3 May 2014Research & DevelopmentCHINA FOUNDRY

Fig. 1: Particle morphologies of raw sand and coated sand

Many researchers have investigated the influence of the technical parameters on the sintering effects of different materials, so as to optimize the sintering process. Tang [8] analyzed the effect of individual process parameters (laser power, scanning velocity and powder thickness) on the mechanical properties, surface roughness and dimensional accuracy of coated sand. Using an orthogonal experimental method, Qin [9] obtained the ideal laser power and scanning velocity for laser sintering of coated sand. But these studies mainly focused on the effect of an individual process parameter, e.g. laser power, scanning velocity or powder thickness. However, as mentioned above, during the SLS process complex reactions occur simultaneously in a rather short time, and the formability and strength of laser sintered parts depend on the combined effect of multiple process parameters [10, 11].

Laser energy density E has been used to evaluate the amount of energy acting on coated sands and it can be expressed as [12]:

(1)

where P is laser power, v is scanning velocity, d is scanning interval and f is a correction coefficient for specific experimental conditions. For a constant scanning interval and the same experimental conditions, the laser energy density can

be simplified to E = P/v. The present work aims to investigate the effects of laser

energy density on the strength and forming accuracy of samples fabricated by varied laser power and scanning velocity. Moreover, three-dimensional body scanning analysis was performed to observe the resin curing region and fractured morphology of the sintered samples, which gives a direct interpretation for the relevant variations of formability and accuracy.

1 Experimental procedureSpherical Baozhu sand with the size of 75 to 150 µm and angular coefficient less than 1.1 was used as raw sand for the laser sintering process. The morphology of raw sand can be seen clearly in Fig. 1(a). First, a mixture composed of raw sands, thermoplastic phenolic resin powder (1.5wt.% of sands), methenamine curing agent (14wt.% of resin content) and KH550 coupling agent (1.0wt.% of resin content) was prepared by a blade mixer. Then, a thermal coating process was employed by heating the mixture to 120 °C using a box-type electrical resistance furnace. After cooling, crushing and screening, the sand with uniform coated resin film was obtained, as shown in Fig. 1(b).

(a) Raw sand (b) Coated sand

The laser sintering process was conducted on a rapid prototype machine (AFS-5300) with a forming chamber of 700 mm × 700 mm × 500 mm, a maximum output power of 100 W and a maximum scanning speed of 6,000 mm·s-1. In the experiment, the scanning interval and powder thickness were 0.20 mm and 0.25 mm, respectively. The preheating temperature was 60 °C. To investigate the influence of laser energy density on the sintering effects, the values of laser power and scanning velocity were changed between 10 to 45 W and 0.8 to 2.8 m·s-1, respectively.

The sintering strength at room temperature was measured using a hydraulic universal testing machine for the sample with a standard “8” shape. The microstructure and fracture morphology of the curing zone of the coated sand were observed using a three-dimension ultra-depth digital microscope (KEYENCE VHX-1000, Japan).

2 Results and discussion2.1 Effect of laser energy density on tensile

strength and forming accuracy of sintered part

In the present study, the laser power (P) was set at 12, 18, 24, 30 and 36 W; and the scanning velocity (v) was kept constant as 1.2 m·s-1. According to the formula E = P/v, energy density can be calculated as 0.016, 0.024, 0.032, 0.040 and 0.048 J·mm-2; which represent extra-low, low, moderate, high and extra-high energy density, respectively. Figure 2 shows the changes of measured tensile strength and dimensional accuracy of the samples with different laser energy density. As can be clearly seen, the tensile strength of the sample improves continuously with increasing laser energy density, which indicates the strengthened curing zone between the

153

Vol.11 No.3 May 2014Research & Development CHINA FOUNDRY

Fig. 3: Surface quality of fabricated samples with different laser energy density

Fig. 2: Forming accuracy and tensile strength of samples with different laser energy densities

adjacent layers and enhanced sintering effect for the fabricated specimen. In contrast, the dimensional accuracy is deteriorated seriously with increasing laser energy density when it exceeds 0.032 J·mm-2, which infers an increase in deformation degree.

The dependence of tensile strength and forming accuracy on energy density can be further verified by the corresponding sintered specimen quality, as shown in Fig. 3. When E = 0.016 J·mm-2, the strength of the sample is only 0.21 MPa. Slight

stratification and stage effects were observed, which indicate the weak bond between layers. Moreover, considerable floating sand particles can be found on the sample surface and the resin powder is prone to fall (Fig. 3a). When E reached 0.024 and 0.032 J·mm-2, the strength of the samples increased to 0.27 and 0.34 MPa, respectively. The representative images for E = 0.024 J·mm-2 are presented in Fig. 3(b), where excellent dimensional accuracy and sintering quality can be observed. The surface quality is much improved and no concave or convex defects occurred; the side contour is clear. With a further increase in E to 0.040 J·mm-2, the strength of the sample increases to 0.45 MPa, but the surface of the sample is no longer smooth and it is not easy to clear up the bottom powder (Fig. 3c). Moreover, an obvious deformation occurs, indicating a decrease in sintering accuracy. When E reaches 0.048 J·mm-2, a large amount of irritating smoke emerges during the sintering process, which indicates that part of the resin is carbonized, decomposed and burned severely. Furthermore, the sample bottom is rather irregular and the side profile is fuzzy, accompanying severe warping and deformation. From the above comparisons, it can be concluded that the most suitable range of energy density for laser sintering of coated sand in this experiment is 0.024 to 0.032 J·mm-2 (as indicated in the shaded area in Fig. 2).

(a) E = 0.016 J·mm-2

De-lamination, edge breakage and floating sand(b) E = 0.024 J·mm-2

Clear contour and smooth surface(c) E = 0.040 J·mm-2

Sticky powder, rough and uneven surface

According to the features of “instant heating and fast cooling” of a laser and the different amount of laser output energy, the resin bonded area can be divided into four areas in the coated sand, i.e. unbonded area, softening bonded area, partially hardened area and fully hardened area, as shown in Fig. 4. In the instant heating process, energy is limited on the surface of the coated sand around the laser beam; and then spreads from high to low temperature area by conduction, where the coated sand is heated indirectly to cure the resin. For small value of E with 0.016 J·mm-2, the temperature of the resin during the instant heating process of the laser is lower than the resin softening temperature, leading to insufficient bonded area [Fig. (4a)]. Consequently, it is difficult for the coated sand to become bonded, resulting in serious stratification. Only when the temperature reaches the critical value for resin softening, can

the resin be bonded together. This case occurs when E is between 0.016 and 0.024 J·mm-2 [Fig. (4b)]. However, the bonding strength between the sand and layers is still rather low in this case. As the laser energy E is increased to 0.032 J·mm-2, the sintering temperature increases to the range for resin curing and the sand begins to be bonded together [Fig. (4c)]; so the overall bonding strength is increased. With a further increase in E to 0.040 J·mm-2, the resin is fully cured and the bonding strength reaches the optimal value [Fig. (4d)]. However, charring and decomposition may occur if the heating temperature is too high, and this leads to shrinkage, deformation and reduction of dimensional accuracy and surface quality. So, it is particularly important to select an appropriate laser energy density to ensure the overall quality and performance of coated sand samples.

0 016. 0 024. 0 032. 0 040. 0 048.

0 60

0 55

0 50

0 45

0 40

0 35

0 30

0 25

0 20

0 15

.

.

.

.

.

.

.

.

.

.

1 00

0 75

0 50

0 25

0 00

.

.

.

.

.

%

%

%

%

%

+-

+-

+-

+-

+-

Laser energy density J mm( )-2

Tens

ilest

reng

thM

Pa(

)

Dimensional accuracyTensile strength

Dim

ensi

onal

accu

racy

mm

()

154

Vol.11 No.3 May 2014Research & DevelopmentCHINA FOUNDRY

(a) Not bonded area, E = 0.016 J·mm-2

(c) Partially hardened area, E = 0.032 J·mm-2

(b) Softening bonded area, E = 0.024 J·mm-2

(d) Hardened area, E = 0.048 J·mm-2

Fig. 4: Different resin bonding areas in coated sand samples

Fig. 5: Tensile strength of sample as function of laser power and scanning rate with same laser energy density

2.2 Sintered effect with different laser power and scanning speed

Research results obtained by Zhang Baicheng, et al [13] concluded that equivalent sintering effect was found for specimens fabricated using the same energy density. However, Zheng Haizhong, et al. [14] obtained that the volume density of sintered parts decreases with the simultaneous increase of laser power and scanning speed, even with the same energy density. In this study, different laser power and scanning speed were selected to investigate the resultant sintered effect of coated sand.

Figure 5 illustrates the changes of tensile strength at three different values of laser energy density. When simultaneously increasing the laser power from 15 W to 40 W and scanning speed from 1.0 m·s-1 to 2.67 m·s-1, but keeping the laser energy density constant at 0.024 J·mm-2, the strength of the samples increases slightly at the beginning and then decreases slowly. The resultant tensile strength changes between 0.24 and 0.30 MPa. When E is 0.032 J·mm-2, the strength increases quickly at first with the simultaneous increase of both P and v, and then decreases after reaching its peak value of 0.41 MPa. As E increases to 0.040 J·mm-2, the maximum tensile strength of the sample is about twice (0.5 MPa) as high as that when sintered by energy density of 0.024 J·mm-2 at laser power of

40 W. So Fig. 5 shows that even with the same laser energy density, the strength of samples is not completely equivalent when the P and v values are different.

From the above discussions (referring to Fig. 2 and Fig. 5), it can be inferred that an optimized tensile strength and forming accuracy can be obtained when E is between 0.024 and 0.032 J·mm-2, P = 30 to 40 W and v = 1.5 to 2.0 m·s-1. To give a better

(a) (b)

(c) (d)

E 0 040 J MM= .-2

2 8.2 4.

2 0.1 6.

1 2.0 8.

15 20 25 30 35 40 45

0 55

0 50

0 45

0 40

0 35

0 30

0 25

0 20

0 15

.

.

.

.

.

.

.

.

.

Laser power W( ) Scan speed m s(

)-1

Tens

ilest

rng

thM

Pa(

)e

E 0 032 J MM= .-2

E 0 024 J MM= .-2

155

Vol.11 No.3 May 2014Research & Development CHINA FOUNDRY

illustration of this point, Figures 6 and 7 present two groups of fracture morphologies of sintered samples with the same energy density of 0.032 J·mm-2 but different values of P and v, i.e. P = 15 W, v = 0.75 m·s-1 and P = 40 W, v = 2.0 m·s-1. In the first case, there is mainly point contact between the coated sand. The fracture is prone to occuring at the surface of resin and particles, as indicated by the clear tear trace (as shown in Fig. 6). Moreover, the number of fractures between the small particles is considerable and the section area of resin neck is narrow, which results in a lower overall bonding strength (Fig. 6). In contrast, the neck section of resin in the fracture surface of Fig. 7 is larger and the fractures mainly concentrate on the cross-section of the resin connection bridge, implying the improved overall bond strength and tensile strength. From the above analysis and comparison, it can be concluded that the sintering effects under different conditions are not completely equivalent, even with the same laser energy density. The sintering strength with the low-power and slow scanning speed is smaller than that of high-power and high scanning speed. This can be ascribed to the slow movement of the laser beam and the resultant long exposure time for slow scanning speed, which makes most of the energy transfer from the resin to the coated sand. As a consequence, the overall curing degree of the resin is reduced and the sintering strength is decreased.

3 Practical validation of laser sintering coated sand

A complex coated sand mould (400 mm × 400 mm) was fabricated by laser sintering with P = 40 W and v = 2.0 m·s-1, and other conditions are the same as those in this study. As clearly indicated in Fig. 8, the surface of sample is smooth, with clear contour and side edges.

Fig. 6: Fracture morphologies of coated sand sample at P = 15 W, v = 0.75 m·s-1

Fig. 7: Fracture morphologies of coated sand sample at P = 40 W, v = 2.0 m·s-1

Fig. 8: Complex coated sand mould by selective laser sintering with P =40 W, v = 2.0 m·s-1

4 ConclusionsThe influences of laser energy density on forming accuracy and tensile strength of coated sand by selective laser sintering were investigated. The main conclusions are as follows:

(1) With constant scanning velocity, the tensile strength of the sintered sample is increased with an increase in laser energy density. But, the dimensional accuracy decreases sharply when the laser energy density is larger than 0.032 J·mm-2.

(2) For the sample with sand particle size of 75 to 150 µm and resin content of 1.5wt.%, when the laser energy density is 0.024 J·mm-2, the tensile strength shows no obvious change; but when the laser energy density is larger than 0.024 J·mm-2, the sample strength is featured by the initial increase and subsequent decrease with simultaneous increase of both laser power and scanning velocity. It even fluctuates twice when the laser energy density is increased to 0.040 J·mm-2.

(3) The resin curing regions and fracture morphologies of sintered samples show four different sintered effects, i.e. unbonded area, softening bonded area, partially hardened area and fully hardened area, with the increase of laser energy density, which result in the variations of formability and dimensional accuracy of the sand samples.

(4) In this study, when the energy density is 0.024 to 0.032 J·mm-2, laser power P = 30 to 40 W, and scanning velocity v = 1.5 to 2.0 m·s-1, the excellent tensile strength, dimensional accuracy and best overall quality of the samples can be obtained. When the energy density is 0.024 J·mm-2, with

156

Vol.11 No.3 May 2014Research & DevelopmentCHINA FOUNDRY

P = 40 W and v = 2.0 m·s-1, a satisfactory sintered coated sand mould with complex contour can be obtained.

References[1] Yuan C and Jones S. Investigation of fiber modified ceramic

moulds for investment casting. Journal of European Ceramic Society, 2003, 23: 399-407.

[2] Cheah C M, Lee C W, Feng C, et al. Rapid prototyping and tooling techniques: a review of applications for rapid investment casting. Journal of Advanced Manufacturing Technology, 2005, 25: 308-320.

[3] Casalino G, Filippis L A C, Ludovico A D, et al. An investigation of rapid prototyping of sand casting molds by selective laser sintering. Journal of Laser Applications, 2002, 14: 100-106.

[4] Senthilkumaran K, Pandey P M, and Rao P V M. Influence of building strategies on the accuracy of parts in selective laser sintering. Materials & Design, 2009, 30: 2946-2954.

[5] Yadroitsev I, Bertrand P H, and Smurov I. Parametric analysis of the selective laser melting process. Applied Surface Science, 2007, 253(19): 8064-8069.

[6] Bertrand P H, Bayle F, Combe C, et al. Ceramic components manufacturing by selective laser sintering. Applied Surface Science, 2007, 253(4): 989-992.

[7] Gean V S, Priscila K, Rodrigo A P, et al. Structure and

mechanical properties of cellulose based scaffolds fabricated by selective laser sintering. Polymer Testing, 2009, 28: 648-652.

[8] Tang Y, Fuh J Y H, Loh H T, et al. Direct laser sintering of silica sand. Materials & Design, 2003, 24: 623-629.

[9] Qin Dandan, Dang Jingzhi, Bai Peikang, et al. Experimental study of selective laser sintering on the foundry coated sand. Foundry Technology, 2006, 27: 671-673. (In Chinese)

[10] Casalino G, De Filippis L A C, and Ludovico A. A technical note on the mechanical and physical characterization of selective laser sintered sand for rapid casting. J. Mater. Process Tech., 2005, 166: 1-8.

[11] Kumar S. Selective laser sintering: A qualitative and objective approach. JOM, 2003, 55: 43-47.

[12] Nelson J C. Selective laser sintering: A definition of the process and an empirical sintering model. Ph.D. Dissertation, University of Texas at Austin TX, 1993.

[13] Zhang Baicheng, Liao Hanlin, and Coddet Christian. Effects of processing parameters on properties of selective laser melting Mg-9%Al powder mixture. Materials & Design, 2012, 34: 753-758.

[14] Zheng Haizhong, Zhang Jian, Xu Zhifeng, et al. Effects of laser energy density on densities and microstructures of Nano-A1203/PS composites. Chinese Journal of Lasers, 2006, 33: 1428-1433. (In Chinese)

This work was financially supported by the National Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology, Aeronautical Science Foundation of China (Grant No. 2011ZE56007), the Natural Science Foundation of Jiangxi Province (Grant No. 2010GZC0159), and the High Technology Landing Program of Jiangxi University (Grant No. DB201303014).