INFLUENCE OF PROCEES PARAMETERS ON DENSITY OF PARTS PRODUCED BY SELECTIVE LASER SINTERING

Transcription

1 INFLUENCE OF PROCEES PARAMETERS ON DENSITY OF PARTS PRODUCED BY SELECTIVE LASER SINTERING * Dr. Sushant Negi, ** Miss. Pallavi Chauhan, *** Dr. Sunil Thakur * Department of Mechanical Engineering, AP Goyal Shimla University, Shimla, India negiindia@gmail.com ** Department of Chemistry, AP Goyal Shimla University, Shimla, India chauhanpallavi26@gmail.com *** Department of Mechanical Engineering, AP Goyal Shimla University, Shimla, India sunilthakur.nith@gmail.com ABSTRACT Selective laser sintering (SLS) is one of Additive Manufacturing (AM) process, which has become popular due to its ability to use variety of powder material. Current application of SLS is not limited around prototyping as it is moving rapidly towards the fabrication of end-user products. At present, many sectors such as aerospace, automotive, artistic and biomedical are using 3D solid models (prototypes) for visual inspection, concept evaluation and kinematic testing. Therefore, SLS built parts should have better strength (i.e. density) in order to assure functional requirement for model testing or other purposes. Surface quality of AM fabricated parts very much depends on the various sintering process parameters. Hence, this article provides a brief overview of selective laser sintering (SLS), and how the process parameters namely, part bed temperature, scan speed and scan spacing influence the density of produced parts. Keywords: Selective laser sintering, laser power, scan speed, glass filled. INTRODUCTION Selective laser sintering process was developed at the University of Texas at Austin in 1986, and commercialized by DTM Corporation [1], has become popular due to the fact that the SLS process fabricates the parts without using support structure and has the ability to use a variety of powder materials. Materials that can be built in SLS include: polycarbonate (PC), nylon, nylon/glass composite, wax, ceramics, trueform (TM), elastomeric and metal-polymer powders [2, 3]. Nowadays SLS fabricated prototypes are increasingly used as functional parts that require good strength (i.e., density). However, the parts produced by SLS have low strength. It is observed that density of the produced parts very much depends on the sintering process parameters, there are a number of input parameters that can be controlled and varied to improve final part quality. Some of these input parameters (as shown in Fig. 1a) are bed temperature, layer thickness, scan spacing, scan 39

2 speed, scan length, powder characteristics and laser parameters (power density, pulse duration, pulse frequency etc.) [4]. In this present work, an attempt is made to determine the influence of laser power, scan speed, and scan spacing i.e. energy density on density of test parts. Face centred central composite design (CCD) of experiments was used to plan experiments. RSM was used to analyze and predict the effect of these parameters on roughness of test parts. 2. EXPERIMENTATION 2.1 SLS Process and Parameters Selection The SLS process involves fabricating solid parts by fusing powdered materials with the help of a CO 2 laser beam. A very thin layer of powder material is spread with a roller over the part build surface and preheated to a temperature slightly below its melting point. A laser beam follows the cross-section on the powder surface to selectively sinter and bond it in order to produce a layer of the part as presented in Fig. 1b. After completing one layer, successive layers of powder are deposited and sintered until the whole part is completed and then the un-sintered powder is removed from the parts and recycled. The advantages included the ability to use a variety of thermoplastic powders, easy postprocessing, require no no support structure. Disadvantages include abrasive surface of sintered models and costly process [5, 6]. In the present experimental work, process parameters namely, laser power, scan speed, and scan spacing were selected due to their dominant influence on part quality. The range of laser power, scan speed and scan spacing were finalized based on the maximum energy density (mentioned below) and opinions of experts. The energy density that affects the part quality can be calculated according to the given equation: [7, 8, 9]. P ED (1) V S S Where, ED = energy density (J/mm 2 ), P = laser power (W) V = scan speed (mm/s) S s = scan spacing (mm) Literature suggested that the proper sintering will not take place if value of E is below 1 J/cm 2. Whereas, it was noticed that polymer degradation starts, when value of E is above 4.8 J/cm 2. Therefore, range of these parameters has been selected by keeping all these things in mind. Different process parameters and their values which have been finalized for experimentation are presented in Table 1. All other parameters despite selected process parameters were kept fix throughout in the present work. Table 1 Different process parameters and their values used for experiment 40

3 Variable Parameters Values Laser power (W) 28, 32, 36 Scan Speed (mm/s) 2500, 3500, 4500 Scan spacing (mm) 0.25, 0.35, Sample Preparation and Measurements Parts having 13 mm 3 mm 120 mm cross section are selected as the test specimens (see Fig. 2a). Samples were built using SLS machine and material used was GF polyamide powder ((PA 3200GF) with refresh rate of 40:60; fresh and recycled powders. It is evident that properties of used powder vary from fresh powder because it undergoes through different heating cycle [2]. Therefore, it avoids the phenomena like curling and warpage. Hence in order to avoid this only 40% of fresh powder was used for experimentation. Fig. 1 shows the shape and size of the test sample used to investigate density of parts. Fabricated specimens are shown in Fig. 2. Figure 1 CAD model Figure 2 Fabricated test sample 2.3 Design of Experiment The experiments were designed and conducted by employing RSM approach. This study used the stipulated conditions according to the face-cantered CCD to plan the experiments. A total of 20 experiments were executed at three independent input variables which were varied up to three levels. This experimental work considered the following controllable process parameters to investigate their influence on the density of produced parts; laser power (A), scan speed (B), scan spacing (C). The selected process parameters with their working range are presented in Table 1. The experimental design matrix in terms of coded factor is summarized in Table 2. 41

4 Table 2 Experimental design matrix Run Laser Power (watts) Scan Speed (mm/s) Scan Spacing (mm) RESULTS AND DISCUSSION A total of 20 samples were prepared and tested as per designed plan presented in Table 2. Further analysis of variance (ANOVA) was carried out on collected data. Fig. 3 and Fig. 4 clearly show that laser power, scan speed and scan spacing have a significant effect on the density, and their respective effects have been discussed in below section. 3.1 Effect of laser power Effect of laser power on density can be seen through Fig. 3 and Fig. 4. With the increase in laser power from 28 W to 36 W density exhibited increasing trend. As the laser power increases, it transfers most of its energy to material to get it properly sintered and consequently a close packed model is generated, which leads to the improvement in density of sintered part. 3.2 Effect of scan speed Fig. 3 and Fig. 4 reveal the effect of scan speed on density. With the increase in scan speed from 2500 to 4500 mm/s, density exhibited decreasing trend. The main reason behind this phenomena is that when the scan speed increases, the energy absorbed by the sintered material at a 42

5 unit time and a unit area decreases, and thus poor packing of the particles which leads to poor density. Figure 3 Influence of various process parameters on density. Figure 4 3D response surface graph between laser power and scan speed 3.3 Effect of scan spacing Fig. 3 shows the effect of scan spacing on density of built samples. It has been observed that with the increase in scan spacing from 0.25 mm to 0.45 mm there is decrease in density. It can be explained by the fact large scan spacing causes poor packing of the particles. Therefore, the tendency of the layers to curl and to cling with roller increases restricts the next layer from proper sintering [2], which subsequently results in a decrease in density. 43

6 3.4 Optimization Optimization was carried out to find out the optimum values by keeping the density in maximum range. Accordingly, the optimum working conditions to maximize density are presented in Table 3. Table 3 Optimization results for minimizing density S.No Laser Power (Watt) Scan speed (mm/s) Scan spacing (mm) CONCLUSIONS In this study, density of SLS built GF polyamide parts are investigated using RSM tool. From the above analysis, the following conclusions can be drawn: With the increase in laser power there is increase in the density of the parts. On the contrary with the increase in scan speed and scan spacing there is a decrease in density of sintered parts. A strong interaction has been observed between laser power and scan speed. Moreover, optimal results can be obtained using optimized working conditions; laser power 36 Watt, scan speed 2500 mm/s, scan spacing 0.25 mm. Results from this study would help to produce the end user parts with required density. REFERENCES [1]. Wang, R.J., Wang, L., Zhao, L. and Liu, Z., Influence of Process Parameters on Part Shrinkage in SLS International Journal of Advanced Manufacturing Technology, vol. 33, no. 5-6, pp , [2]. Singh, S., Sachdeva, A. and Sharma, V.S., Investigating Surface Roughness of Parts Produced by SLS Process, International Journal of Advance Manufacturing Technology, DOI /s z, [3]. Gibson, I. and Shi, D., Material Properties and Fabrication Parameters in Selective Laser Sintering Process, Rapid Prototyping Journal, Vol. 3, No.4, pp , [4]. Chatterjee, A.N., Kumar, S., Saha, P., Mishra, P, K., and Choudhury, A.R., An Experimental Design Approach to Selective Laser Sintering of Low Carbon Steel, Journal of Materials Processing Technology, vol. 136, pp , [5]. Juster, N.P., Rapid Prototyping Using the Selective Sintering Process, Assembly Automation, vol. 14, no. 2, pp.14-17,

7 [6]. Yang, H.J., Hwang, P.J and Lee, S., A Study on Shrinkage Compensation of the SLS Process by Using Taguchi Method, International Journal of Machine Tools and Manufacture, vol. 42, no. 11, pp , [7]. Jain, P.K., Pandey, P.M., and Rao, P.V.M., Effect of Delay Time on Part Strength in Selective Laser Sintering, International Journal of Advance Manufacturing Technology, Vol. 43, pp , [8]. Raghunath, N. and Pandey, P.M., Improving Accuracy Through Shrinkage Modelling by Using Taguchi Method in Selective Laser Sintering, International Journal of Machine Tools and Manufacture, Vol. 47, pp , [9]. Senthilkumaran, K., Pandey, P.M. and Rao, P.V.M., Influence of Building Strategies on the Accuracy of Parts in Selective Laser Sintering, Materials and Design, Vol. 30, pp ,