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Study on the designing rules and processability of porous structure based on selective laser melting (SLM)
Journal of Materials Processing Technology 213 (2013) 1734–1742
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Study on the designing rules and processability of porous structure based on selective laser melting (SLM) Di Wang ∗ , Yongqiang Yang, Ruicheng Liu, Dongming Xiao, Jianfeng Sun School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
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Article history: Received 30 October 2012 Received in revised form 16 April 2013 Accepted 1 May 2013 Available online 13 May 2013 Keywords: Additive manufacturing Porous structure Selective laser melting Unit cell Critical inclined angle Powder adhesion
a b s t r a c t Porous structures are widely used in medical implant, aerospace and other light-weight manufacturing fields. The research on processability and fabricating process are of great importance to laser addictive manufacturing of porous structure, therefore formulating several rules for SLM fabrication of porous structure is necessary. This article had studied the designing rules and the key points to fabricate the porous structure precisely based on selective laser melting (SLM). In order to obtain the fabricating effect of the pre-designed porous structure, besides optimizing fabricating process, there are still a few problems to be solved gradually, including the critical inclined angle, the fabricating resolution, powder adhesion, designing unit cell and porous structure that fit for SLM process. Through the experiments of fabricating overhanging structures with different inclined angles, the critical inclined angle for designing the porous structure was got. Through designing the thin walls and cylinders with different geometrical dimensions, the SLM fabricating resolution is obtained. Then, based on the critical inclined angle and the geometrical resolution, the octahedral unit cell structure and corresponding design rules that fitted for SLM process were proposed. At last, the experiment of fabricating porous structure was conducted and the pore’s sizes were also measured. The results proved that the porous structure can be well fabricated by SLM. This study provides theoretical basis for designing and manufacturing of controllable porous structure based on SLM technique. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The porous structure has excellent property such as low density and large specific surface area, has become a novel functional and structural material. Banhart (2011) and Nakajima (2007) summarized the fabrication properties and applications of porous structures, which have been widely used in filtration, medical implants, and aerospace fields. At present, the research of manufacturing porous material has evoked many scholars’ interests in materials science. Compared with the general metallic material, the structure controlling of porous material is more complicated. Achieving the uniform pore controlling and stable production is the key to the fabrication of porous parts. The porous structure has been applied in many fields, but many key problems are still not completely resolved, for example fabricating process is unstable, experimental repeatability is poor, uniform porous effect are difficult to get. As the important part of additive manufacturing (AM), SLM has the advantages of directly fabricating any complex shape of dense metal parts. SLM is defined in the ASTM-standard and ISO-standard
∗ Corresponding author. Tel.: +86 05913117137; fax: +86 02087114484. E-mail address: [email protected] (D. Wang). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.05.001
(ISO Committee 261 – ‘Additive Manufacturing’) as an additive manufacturing process. Various metal powders can be directly fabricated by SLM process, include stainless steel, titanium and titanium alloys, nickel base super-alloy, cobalt-chromium alloy and so on. Researchers Mumtaz et al. (2008), Murr et al. (2009) and Hauser et al. (2005) studied several kinds of metal powder processed by SLM process, which proved that the relative density of SLM parts could reach nearly 100%. The SLM technique has been widely used in medical and aerospace fields, SLM fabrication of porous structure is a good research and development subject. The fabrication of porous structure that based on laser additive manufacturing has many advantages like widely material choices, shorter process cycle and nearly net-shape fabrication. The principles of fabricating the porous structure by SLM are as follow: a regular porous structure is pre-designed in the CAD model beforehand. In the process of additive manufacturing, the laser beam is controlled to skip pore area and only scan the powder outside the pore area. When the fabrication finished, the residual powders inside the pore are removed to form porous structure. For this method, the pore’s shape, size and distribution are pre-set in CAD modeling stage, then CAD model is input into SLM system and the porous structure will be manufactured afterwards. However, whether the regular porous structure could be obtained is controlled by laser spot diameter, powder particle size, particle
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morphology and the heat affected zone around the micro tracks. Generally speaking, the size of distributing of powder diameter ranges from 10 to 100 m, therefore the CAD pre-set diameter of pore is hardly less than powder diameter. In recent years, using of laser additive manufacturing technology to produce bio-porous structure had been widely reported. Ryan et al. (2008) combined the CAD design, RP technology and investment casting together to manufacture porous structure. First they used the CAD software to pre-build three-dimensional porous structure with certain pores, then formed the framework of model by RP method, and finally used the model as maternal to manufacture porous structure through investment casting technique. The final manufactured porous titanium parts, which had a regular pore structure, had an average pore diameter of 200–400 m and good porous connectivity. This method, which combined the RP technology and investment casting together, could basically realize pre-designed porous structure and ensured the manufacturing accuracy. Yadroitsev et al. (2009) used the SLM device equipped with fiber laser (spot diameter 70 m) to manufacture stainless steel filter with micron-sized pores through SLM process. This porous filter had a regular pore structure with pore size of 150 m and wall thickness of 120 m. This method used the advantages of the SLM process that laser scanned the pre-deposited powder line by line, to make the fine powder melt and solidify rapidly. Through controlling laser scanning spacing to prevent mutual connection between adjacent thin walls, then regular porous structure could be obtained. Mullen et al. (2008) manufactured porous titanium alloy material that could be used as implant, the group also studied the effect of geometric feature of unit cell on the mechanical properties. When the porosity changed from 10% to 95%, the compressive strength increased from 0.5 MPa to 350 MPa, which proved that the porous material with arbitrary porosity and compressive strength could be obtained through SLM process. The research group of Warnke et al. (2009) and Stamp et al. (2009) had also manufactured porous titanium alloy based on SLM process in accordance with the above ideas. Famous SLM equipment manufacturer SLM Solutions GmbH and Frauhofer research institute in Germany had also studied the technique and applications of rapid manufacturing of porous structure. Take the personalized porous titanium alloy used as medical implant as example, each step is vital for clinical application and the recovery of patients from designing to final clinical operation. These steps include CT reverse engineering, CAD designing, laser additive manufacturing, surface treatments and so on. Although SLM can theoretically fabricate any shape of metal parts, the fabricating quality can be different as the design and fabricating parameters change. From the review of the published studies, it could find that the processability and fabricating process of laser addictive manufacturing of porous structure were rarely mentioned. Therefore formulating several rules for SLM fabrication of porous structure is necessary. Su et al. (2012) had discussed the processability and fabricating process of porous fabrication by SLM, but their discussion focused less on the design and process restrictions. This article has studied the SLM fabricating process and several principles to fabricate precise porous structure, which include powder adhesion, overhanging surface fabrication and so on. The obtained results would give theoretical reference for directly and precisely manufacturing porous structure.
2. Experimental methodology 2.1. SLM process and apparatus The apparatus used was self-developed Dimetal-280, which was a pre-commercial SLM workstation with a maximum laser power
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Fig. 1. SLM fabricating and re-coating principle.
200 W continuous wavelength of 1090 nm Ytterbium fiber laser. The building envelop is 280 mm × 280 mm × 240 mm. The scanning system used was a Dual Axis Mirror Positioning System and a Galvanometer optical scanner, which directed the laser beam in the X and Y axis through an F-theta lens. The focusing optics was 163 mm focal length lenses, which produced a focused beam spot size of about 70 m diameter. Since the powder is fully melted during the process, protection of the SLM-processed parts from oxidation is essential, therefore all metal powder processing happened in an Argon or Nitrogen atmosphere with no more than 0.15% O2 . SLM fabricating principle is shown in Fig. 1a, thicker layer is not desirable, as it led to formation of droplets and noncontiguous track, which caused considerable deterioration of part’s quality. For the given laser parameters, the optimum thickness of the powder layer spread by scraper was 20–50 m on SLM machine Dimetal-280. The smoothness and compactness of re-coating effect are important to fabricating high-quality metal parts by SLM. The re-coating result affects the laser irradiation conditions, so re-coater is the key component of SLM equipment. This experiment chose a kind of re-coater of the function to make the re-coated layer flat and compactness, the principle is shown in Fig. 1b, first the powder is distributed uniformly by using the right-angled scraper, then use chamfered scraper to compress the powder compactly.
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of scanning tracks and the effect of intense combination between the tracks. The scanning tracks present the peculiar wave feature of SLM process. There exists trench between two adjacent contiguous tracks, and also exists ridge for single track, which proves that the inter-layer staggered scanning strategy is beneficial to improve the SLM fabricated part’s density. From Fig. 3d, the inter-layer staggered joint effect between two adjacent layers can be clearly observed. The melting and joint effect between two layers is fine. 3.2. Typical porous structure fabrication
Fig. 2. Microscopy of 316L stainless steel powder.
2.2. Experimental material Gas atomized 316L stainless steel powder was used in this experiment. The powder was purchased from Changsha Hualiu Metallurgy Powder Co., Ltd. The powder chemical compositions (mass fraction %) were C (0.03), Cr (17.5), Ni (12.06), Mo (2.06), Si (0.86), Mn (0.3), O (0.09), Fe (balance). The powder was spherical and the powder size distribution (mass fraction %) was <15 m (50%), 15–30 m (40%), >30 m (10%), the mean diameter was 17.11 m, and apparent density was 4.04 g/cm3 . Fig. 2 shows the microscopy of 316L stainless steel powder. 2.3. Experimental procedure Firstly, optimized processing parameters should be obtained. Then porous structure with different cell unit should be designed and fabricated. At the same time, the fabricating quality, main defect and processing rules should be discussed. And the causes of the defects and optimizing method were also studied. At last, the surfaces topography of the specimens were observed under stereomicroscope VHX-600 and scanning electron microscope (SEM) S-3700, the dimensional accuracy of fabricated porous structure was also measured under stereo-microscope VHX-600. 3. Results and discussion 3.1. Dense part fabrication Only when dense part is fabricated, then the strength and the fabricating process of the porous structure could be guaranteed. In order to optimize the fabricating process parameters of 316L stainless steel, the inter-layer stagger and orthogonal scanning strategy was used in this experiment, as shown in Fig. 3a. This scanning strategy was helpful to make the tracks overlap between adjacent tracks and layers. Beal et al. (2006) also proposed to use refilling scanning strategy in SLM fabrication of functionally graded H13/Cu materials where favorable results could be obtained compared with other scanning strategies. Optimized processing parameters of SLM could be got through large amount of time-consuming experiments. When the laser power 150 W, scanning speed 600 mm/s, layer thickness 35 m and scan spacing 80 m, dense 316L stainless steel part could be obtained. The fabricating result of 316L Stainless steel is shown in Fig. 3b–d, in which 3b is the full view, Fig. 3c is the topography of the top surface, and Fig. 3d is the micro observation of the side surface. Fig. 3c clearly shows strips
Three typical porous structures were designed and fabricated as shown in Fig. 4. Fig. 4a and b is designed identically, but the fabricating result are different, Fig. 4a is fabricated in good form, but Fig. 4b–d all have fabricating defects. The porous structure in Fig. 4b had serious powder adhesion defect. The porous structure’s strut in Fig. 4c was destroyed, and in Fig. 4d there was slag inside. It was even jammed when the pore was small enough. The above results indicate that different designs of porous structure could result in different fabricating quality, and the same design may also generate different fabricating quality. In order to obtain porous structure with certain porosity, the designing parameters including pore’s shape, size and distribution should be modified. Several rules should be considered and optimized before fabrication, which would be discussed below. 3.3. Processability for SLM fabrication of porous structure 3.3.1. Overhanging structure Based on the optimized process, overhanging structures with different inclined angles were designed and fabricated. The fabricating effect of the overhanging structures with inclined angle decreased from 45◦ to 25◦ is shown in Fig. 5. It could be seen that when ≥ 40◦ , the overhanging structure was fabricated well, only when ≤ 35◦ , the warping effect happened. Moreover, the smaller of the inclined angle, the more powder would stick to the downward surface of overhanging structure, which made the quality of downward surface worse than the upper surface. Warping defect is due to the thermal stress formed by rapid solidification of melting pool during SLM process. Many researchers have discussed the effect of thermal stresses in the SLS and SLM processes. Kruth et al. (2004) investigated that temperature gradient mechanism could explain the warping phenomenon. When the thermal stress exceeds the strength of the material, then plastic deformation happens. The warping defect of overhanging surface is also due to the lack of supports to secure its firm bonding with the previous layers. Wang et al. (2013) had studied several key factors that could affect the fabricating quality of overhanging surface. Residual stress accumulating is the main cause for forming failure of the overhanging surface. Fig. 6a shows the warping principle during overhanging surface fabrication by SLM, when the warping defect happened, the practical inclined angle between the overhanging part of the present layer and the previous layer is larger than the designed inclined angle . As Fig. 6b shows, when a layer of the overhanging part started to warp, it will influence the practical fabricating layer thickness of the next layer and cause larger warp. When the warp height accumulated to be higher than the pre-set height of the next layer, then it will make part of the fabricating zone have no powder to be recoated, and the whole work piece will become more and more vulnerable. The overhanging surface may suffered from repeated laser scanning, or even the overhanging part would break away from the whole component due to its repeating colliding with the powder scraper. When the warping defect became too serious, the whole forming process had to be stopped. Then it needs to re-design the part or optimize the process to solve the problem. Therefore, when fabricating the overhanging
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Fig. 3. Analysis of the sample’s surface morphology.
surfaces, if the building angles are smaller than the critical inclined angle , the special process is needed. According to the above results, the inclined angle 40◦ and 35◦ are the critical inclined angle for the designing of porous structure with overhanging surface. The fabrication zone could be divided into three zones by 40◦ and 35◦ , including stable fabrication zone, critical fabrication zone and hard fabricated zone as shown in Fig. 7. In order to fabricate porous structure accurately, it requires that the inclined angle of the pore’s strut should be in the stable fabrication zone, therefore, = 40◦ is the critical building angle for designing and manufacturing porous structure in this experiment. If the inclined angle of the unit cell’s strut is less than the critical inclined angle, it would cause poor dimensional accuracy, or even worse, the pores could be jammed by the steel powder. 3.3.2. The geometrical resolution The resolution in SLM production of different geometrical features is an important basis for designing and fabricating porous structure, as the strut’s diameter of the wire frame should not be smaller than the fabricating resolution during the manufacturing of porous structure. However, the strength of porous structure is inversely proportional with the strut’s diameter, so strut’s diameter should not be too small. The typical geometrical features are thin wall and cylinder. This experiment designed and produced thin walls with thickness ranged from 0.15 to 0.5 mm, and the
cylinders with diameter ranged from 0.15 mm to 5 mm. Fig. 8 shows the experiment results of fabricating the typical geometric features by SLM. It can be found that the thin walls with thickness of 0.15–0.5 mm, and cylinder with diameter of 0.15–5 mm could all be fabricated well. Considering the smaller geometrical feature could result in lower mechanical properties, so this experiment did not study on the geometrical features with geometrical size less than 0.15 mm. 3.3.3. Powder adhesion In Fig. 4b and d, there were many powers adhered to the surface of the porous structure. Powder adhesion is an inevitable problem in SLM process, which would leads to bad accuracy of the pore. Moreover, it is hard to deal with the powders adhered to the sides of the pores through post processing ways. Therefore, it is meaningful to control the amount of powder adhered to the pore’s surface by optimizing design and process controlling. While the laser is scanning the metal powder, the laser energy is transferred as heat absorbed by metal powder, which makes the powder melt and bind together. Wang et al. (2011) had studied the laser energy input and its influence on single track, multi-track and multi-layer fabrication during SLM fabrication. When the laser energy input is enough and the metal powder is absolutely melted, then regular and continuous track can be formed; while the laser energy input is insufficient to melt the metal powder, the sintered
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Fig. 4. Typical porous structures fabrication.
status would be resulted. There is a heat affect zone around the melting pool during laser scanning metal powder, which makes the metal powder around the melting pool not fully melted or in sintering status. As Fig. 9 shows, many stainless steel powder particles were adhered on the both sides of the tracks. Serious powder adhesion usually happens when overhanging structure is fabricated, as Fig. 10 shows, when laser irradiate solidsupported zone (point a), where the heat conduction rate is high; however, when laser irradiate powder-supported zone (point b), the heat conduction rate is only 1/100 of the solid-supported zone, and this situation often happens during overhanging surface fabrication by SLM, which was also proved by Kruth et al. (2007). Therefore, when processing parameters are similar, the absorbed energy input will be much bigger when laser irradiate powdersupported zone than laser irradiate solid-supported zone, which leads to the melt pool become too bigger and sinks into the powder as the result of gravity and capillary force. For the above reason, dross will be formed and dimensional accuracy should be very low during SLM fabrication of overhanging surface.
Fig. 5. Experiment of the overhanging structures with different inclined angles.
3.4. Design of unit cell and porous structure The design of unit cell and porous structure not only has a great impact on fabricating quality, but also determines whether the porous structure could be applied to the bio-medical field. The design of unit cell and porous structure include pore shape, size, porosity, pore distribution and the connectivity between each other. Parthasarathy et al. (2011), Patrick and Timothy (2009) and Lin and Wirtz (2007) had introduced different designing ways of porous structures. These porous structures were produced by SLM, which should be of important reference to this experiment. The following factors should be considered when designing, which include SLM process restraint, the critical inclined angle and geometrical resolution. When the strut of unit cell has small inclined angle, it would be difficult to achieve better control of the fabricating quality of the porous structure. In order to avoid the overhanging surface with small inclined angle, it is better to optimize the unit cell to satisfy the requirement of SLM process. Based on the discussion of the critical inclined angle, the authors proposed the proper unit cell as Fig. 11 shows, which could also be called as octahedral unit cell. By adjusting the inclined angle of the pore’s strut, the octahedral structure can theoretically avoid the overhanging surface with small inclined angle, thus can avoid the defects of hanging dross and jamming pore. Through the analysis of the inherent relationship among the radius R of the unit cell, the length of the strut L and the inclined angle of strut, as shown in Fig. 11b, there is a relationship among L, R and without considering the diameter of the strut, as the following equation shows:
R=
1 × L × sin 2 2
(1)
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Fig. 6. The warping principle during SLM fabrication of overhanging surface.
Fig. 7. The fabrication zone is divided by the critical inclined angle.
Fig. 8. The results of SLM fabricating typical geometric features.
Assuming the strut diameter is d, so formula (1) could be modified as the following equation:
R=
Fig. 9. Stainless steel powder particles were adhered to the both sides of the tracks.
1 × L × sin 2 − d 2
(2)
In formula (2), the diameter of strut mainly depends on the laser processing parameters and powder particle size. Due to the thermal effect around the melting pool in the fabricating process, the strut’s diameter is usually larger than the focused spot diameter. When = 45◦ , the radius of the unit cell is the largest, when R = 1/2L − d. Through the formula (2), it could be deduced that when R > 0, then it requires that 1/2 × L × sin2 > d. Combining the experiments in Sections 3.3.1 and 3.3.2 together, the design rules for the
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Fig. 12. The other two design ways of porous structure. Fig. 10. Schematic diagram of overhanging surfaces fabrication by SLM.
unit cell that fit for SLM process were summed up as ⎧structure ◦ ≤ < 90◦ 30 (a) ⎨ (b) related to the device parameters follow: d ≤ 0.15 mm ⎩ 1 × L × sin 2 > d (c) 2 According to the above summarized design rules, the design of unit cell should consider the following three conditions: the critical inclined angle, the fabricating resolution and the mutual constraint among the unit cell’s geometrical parameters. Although this study adopted octahedral porous structure as the unit cell, it is not the only suitable unit cell for SLM fabricating of porous structure. There would be a variety of designs get satisfying unit cell, as long as the designers consider the restraint of the critical inclined angle and the geometrical resolution. Fig. 12 shows other two design ways of porous structure which are also suitable for SLM process. 3.5. Accuracy measurement Based on the design rules of unit cell structure, porous structure was designed with geometrical parameters as follow: the strut diameter 0.3 mm, the length of the strut L = 1 mm, the inclined angle of the strut = 45◦ . The optimized SLM fabricating process parameters were used to manufacture the porous structure and the obtained fabricating effect is shown in Fig. 13. Seen from Fig. 13a, the whole fabrication effect of porous structure is good, however seen from the enlarged view of Fig. 13b and c, some tiny powder particles are stuck to the surfaces of the pore’s
strut, making the surfaces not smooth as the metal parts produced by traditional ways. The reason mainly related to three aspects: the first due to the powder melting and solidifying, there always existed a heat affect zone around the track’s two sides, which easily made powders adhered to the track’s surface. The second reason may due to the small divided scanning area of the cross section in each layer. As the laser scanning time is short in the micro region, the powders sometimes are not completely melted and would stick to the side of the molten pool. The third reason is that when the laser scanned the powder outside the preset pores, the liquid metal would inevitably penetrate into the internal pore and made the powder adhered to the track easily, resulting in low dimensional accuracy and poor surfaces for the final fabricated porous structure. The most difficult task may focus on removing the powders inside the pore when the manufacturing process finished. In order to guarantee the surface quality and mechanical strength of porous structure, the diameter of the pore’s strut should not be designed too small. Table 1 shows the measurement of the pore’s size in Fig. 13. It could be seen that the actual sizes of the pores were a little smaller than the designed value, which may also due to powder adhesion. The above sections discussed the process and the main influential factors of fabricating porous structure by SLM technique. The optimized fabricating process is basis, only when dense stainless steel part is obtained, it can ensure the strength of porous structure. The study on critical inclined angle and the fabricating resolution provided geometrical limitations for the design of porous structure, that is, the inclined angle of the unit cell’s strut cannot be smaller than the critical angle when fabricating metal parts by SLM, otherwise the inside of the porous structure would be jammed by the powder. Meanwhile, the diameter of pore strut must be larger
Fig. 11. The unit cell with octahedral structure.
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Fig. 13. The fabricating effect of porous structure.
than the fabricating resolution. In addition, the design of unit cell structure should also take into account of its mutual constraint of geometrical parameters, as formula (2) explained. The parameters that can affect the fabricating quality of porous structure include spot diameter, powder particle size, energy input and so on. Different researchers would use different laser parameters and experimental materials, which may bring better experimental results. But through the analysis and discuss of experimental results in Section 3.3.3, the conclusion could be drawn that powder adhesion is an inevitable phenomenon in the SLM fabricating process, which affects the surface quality and dimensional accuracy of the porous structure, or even make the pores jammed. So the powder adhesion should be one of the most difficult problems that need to be solved before satisfied porous structure could be obtained.
Table 1 Measurement of the pore size. X direction
Y direction
Measuring point
Width/m
Measuring point
Width/m
1 2 3 4 5 6 Average Design Error
965 999 961 930 946 987 965 1000 35
7 8 9 10 11 12 Average Design Error
976 979 984 976 965 972 975 1000 25
4. Conclusions In order to obtain the desired porous structure accurately by SLM, this article discussed several key issues as below:
(1) Optimizing the process to fabricate dense parts. (2) Obtaining the critical inclined angles under the optimized process parameters. (3) Learning the minimum building size when fabricating thin wall and cylinder geometric features by SLM (fabricating resolution of the SLM equipment). (4) Designing unit cell and porous structure that fit for SLM process based on the value of critical inclined angles and fabricating resolution. (5) Fabricating porous structure and analyzing dimensional accuracy.
Several processing restrictions for SLM fabrication of porous structure were discussed, including overhanging structure, fabricating resolution and powder adhesion. Powder adhesion could be the most troublesome problem for porous structure fabrication by SLM. This experiment focused on the process feasibility for fabricating stainless steel porous structure by SLM. In practical applications, the porous structures would be mainly applied in the medical field and the biocompatibility between titanium alloy and human body is much better. Therefore, the study on SLM fabricating titanium alloy porous structure is of much more significance and should be the authors study direction.
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Acknowledgements The work described in this paper were supported by the National Natural Science Foundation of China (Granted No. 51275179), the Fundamental Research Funds for the Central Universities of China (Granted No. 2012ZB0014), the Natural Science Foundation of Guangdong Province (Granted No. 2012040007516), and the Doctoral Scientific Fund Project of the Ministry of Education of China (Granted No. 20120172120006). References Banhart, J., 2011. Manufacture characterisation and application of cellular metals and metal foams. Progress in Materials Science 46 (6), 559–632. Beal, V.E., Erasenthiran, P., Hopkinson, N., Dickens, P., Ahrens, C.H., 2006. The effect of scanning strategy on laser fusion of functionally graded H13/Cu materials. International Journal of Advanced Manufacturing Technology 30 (9), 844–852. Hauser, C., Sutcliffe, C., Egan, M., Fox, P., 2005. Spiral growth manufacturing (SGM): a continuous additive manufacturing technology for processing metal powder by selective laser melting. In: Proceedings of the 16th Solid Freeform Fabrication Symposium. Solid Freeform Fabrication Symposium, Austin, pp. 1–12. Kruth, J.P., Froyenb, L., Van Vaerenbergha, J., Mercelis, P., Rombouts, M., Lauwers, B., 2004. Selective laser melting of iron-based powder. Journal of Materials Processing Technology 149, 616–622. Kruth, J.P., Mercelis, P., Van Vaerenbergha, J., Craeghs, T., 2007. Feedback control of selective laser melting. In: Proceedings of the 3rd International Conference on Advanced Research in Virtual and Rapid Prototyping, Leiria, Portugal, pp. 521–527. Lin, C.Y., Wirtz, T., 2007. Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process. Journal of Biomedical Materials Research Part A, 272–279. Mullen, L., Stamp, R.C., Brooks, W.K., 2008. Selective laser melting: a regular unit cell approach for the manufacture of porous, titanium, bone in-growth constructs, suitable for orthopedic applications. Journal of Biomedical Materials Research Part B: Applied Biomaterials 89 (2), 325–334.
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Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Study on the designing rules and processability of porous structure based on selective laser melting (SLM) Di Wang ∗ , Yongqiang Yang, Ruicheng Liu, Dongming Xiao, Jianfeng Sun School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
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Article history: Received 30 October 2012 Received in revised form 16 April 2013 Accepted 1 May 2013 Available online 13 May 2013 Keywords: Additive manufacturing Porous structure Selective laser melting Unit cell Critical inclined angle Powder adhesion
a b s t r a c t Porous structures are widely used in medical implant, aerospace and other light-weight manufacturing fields. The research on processability and fabricating process are of great importance to laser addictive manufacturing of porous structure, therefore formulating several rules for SLM fabrication of porous structure is necessary. This article had studied the designing rules and the key points to fabricate the porous structure precisely based on selective laser melting (SLM). In order to obtain the fabricating effect of the pre-designed porous structure, besides optimizing fabricating process, there are still a few problems to be solved gradually, including the critical inclined angle, the fabricating resolution, powder adhesion, designing unit cell and porous structure that fit for SLM process. Through the experiments of fabricating overhanging structures with different inclined angles, the critical inclined angle for designing the porous structure was got. Through designing the thin walls and cylinders with different geometrical dimensions, the SLM fabricating resolution is obtained. Then, based on the critical inclined angle and the geometrical resolution, the octahedral unit cell structure and corresponding design rules that fitted for SLM process were proposed. At last, the experiment of fabricating porous structure was conducted and the pore’s sizes were also measured. The results proved that the porous structure can be well fabricated by SLM. This study provides theoretical basis for designing and manufacturing of controllable porous structure based on SLM technique. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The porous structure has excellent property such as low density and large specific surface area, has become a novel functional and structural material. Banhart (2011) and Nakajima (2007) summarized the fabrication properties and applications of porous structures, which have been widely used in filtration, medical implants, and aerospace fields. At present, the research of manufacturing porous material has evoked many scholars’ interests in materials science. Compared with the general metallic material, the structure controlling of porous material is more complicated. Achieving the uniform pore controlling and stable production is the key to the fabrication of porous parts. The porous structure has been applied in many fields, but many key problems are still not completely resolved, for example fabricating process is unstable, experimental repeatability is poor, uniform porous effect are difficult to get. As the important part of additive manufacturing (AM), SLM has the advantages of directly fabricating any complex shape of dense metal parts. SLM is defined in the ASTM-standard and ISO-standard
∗ Corresponding author. Tel.: +86 05913117137; fax: +86 02087114484. E-mail address: [email protected] (D. Wang). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.05.001
(ISO Committee 261 – ‘Additive Manufacturing’) as an additive manufacturing process. Various metal powders can be directly fabricated by SLM process, include stainless steel, titanium and titanium alloys, nickel base super-alloy, cobalt-chromium alloy and so on. Researchers Mumtaz et al. (2008), Murr et al. (2009) and Hauser et al. (2005) studied several kinds of metal powder processed by SLM process, which proved that the relative density of SLM parts could reach nearly 100%. The SLM technique has been widely used in medical and aerospace fields, SLM fabrication of porous structure is a good research and development subject. The fabrication of porous structure that based on laser additive manufacturing has many advantages like widely material choices, shorter process cycle and nearly net-shape fabrication. The principles of fabricating the porous structure by SLM are as follow: a regular porous structure is pre-designed in the CAD model beforehand. In the process of additive manufacturing, the laser beam is controlled to skip pore area and only scan the powder outside the pore area. When the fabrication finished, the residual powders inside the pore are removed to form porous structure. For this method, the pore’s shape, size and distribution are pre-set in CAD modeling stage, then CAD model is input into SLM system and the porous structure will be manufactured afterwards. However, whether the regular porous structure could be obtained is controlled by laser spot diameter, powder particle size, particle
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morphology and the heat affected zone around the micro tracks. Generally speaking, the size of distributing of powder diameter ranges from 10 to 100 m, therefore the CAD pre-set diameter of pore is hardly less than powder diameter. In recent years, using of laser additive manufacturing technology to produce bio-porous structure had been widely reported. Ryan et al. (2008) combined the CAD design, RP technology and investment casting together to manufacture porous structure. First they used the CAD software to pre-build three-dimensional porous structure with certain pores, then formed the framework of model by RP method, and finally used the model as maternal to manufacture porous structure through investment casting technique. The final manufactured porous titanium parts, which had a regular pore structure, had an average pore diameter of 200–400 m and good porous connectivity. This method, which combined the RP technology and investment casting together, could basically realize pre-designed porous structure and ensured the manufacturing accuracy. Yadroitsev et al. (2009) used the SLM device equipped with fiber laser (spot diameter 70 m) to manufacture stainless steel filter with micron-sized pores through SLM process. This porous filter had a regular pore structure with pore size of 150 m and wall thickness of 120 m. This method used the advantages of the SLM process that laser scanned the pre-deposited powder line by line, to make the fine powder melt and solidify rapidly. Through controlling laser scanning spacing to prevent mutual connection between adjacent thin walls, then regular porous structure could be obtained. Mullen et al. (2008) manufactured porous titanium alloy material that could be used as implant, the group also studied the effect of geometric feature of unit cell on the mechanical properties. When the porosity changed from 10% to 95%, the compressive strength increased from 0.5 MPa to 350 MPa, which proved that the porous material with arbitrary porosity and compressive strength could be obtained through SLM process. The research group of Warnke et al. (2009) and Stamp et al. (2009) had also manufactured porous titanium alloy based on SLM process in accordance with the above ideas. Famous SLM equipment manufacturer SLM Solutions GmbH and Frauhofer research institute in Germany had also studied the technique and applications of rapid manufacturing of porous structure. Take the personalized porous titanium alloy used as medical implant as example, each step is vital for clinical application and the recovery of patients from designing to final clinical operation. These steps include CT reverse engineering, CAD designing, laser additive manufacturing, surface treatments and so on. Although SLM can theoretically fabricate any shape of metal parts, the fabricating quality can be different as the design and fabricating parameters change. From the review of the published studies, it could find that the processability and fabricating process of laser addictive manufacturing of porous structure were rarely mentioned. Therefore formulating several rules for SLM fabrication of porous structure is necessary. Su et al. (2012) had discussed the processability and fabricating process of porous fabrication by SLM, but their discussion focused less on the design and process restrictions. This article has studied the SLM fabricating process and several principles to fabricate precise porous structure, which include powder adhesion, overhanging surface fabrication and so on. The obtained results would give theoretical reference for directly and precisely manufacturing porous structure.
2. Experimental methodology 2.1. SLM process and apparatus The apparatus used was self-developed Dimetal-280, which was a pre-commercial SLM workstation with a maximum laser power
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Fig. 1. SLM fabricating and re-coating principle.
200 W continuous wavelength of 1090 nm Ytterbium fiber laser. The building envelop is 280 mm × 280 mm × 240 mm. The scanning system used was a Dual Axis Mirror Positioning System and a Galvanometer optical scanner, which directed the laser beam in the X and Y axis through an F-theta lens. The focusing optics was 163 mm focal length lenses, which produced a focused beam spot size of about 70 m diameter. Since the powder is fully melted during the process, protection of the SLM-processed parts from oxidation is essential, therefore all metal powder processing happened in an Argon or Nitrogen atmosphere with no more than 0.15% O2 . SLM fabricating principle is shown in Fig. 1a, thicker layer is not desirable, as it led to formation of droplets and noncontiguous track, which caused considerable deterioration of part’s quality. For the given laser parameters, the optimum thickness of the powder layer spread by scraper was 20–50 m on SLM machine Dimetal-280. The smoothness and compactness of re-coating effect are important to fabricating high-quality metal parts by SLM. The re-coating result affects the laser irradiation conditions, so re-coater is the key component of SLM equipment. This experiment chose a kind of re-coater of the function to make the re-coated layer flat and compactness, the principle is shown in Fig. 1b, first the powder is distributed uniformly by using the right-angled scraper, then use chamfered scraper to compress the powder compactly.
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of scanning tracks and the effect of intense combination between the tracks. The scanning tracks present the peculiar wave feature of SLM process. There exists trench between two adjacent contiguous tracks, and also exists ridge for single track, which proves that the inter-layer staggered scanning strategy is beneficial to improve the SLM fabricated part’s density. From Fig. 3d, the inter-layer staggered joint effect between two adjacent layers can be clearly observed. The melting and joint effect between two layers is fine. 3.2. Typical porous structure fabrication
Fig. 2. Microscopy of 316L stainless steel powder.
2.2. Experimental material Gas atomized 316L stainless steel powder was used in this experiment. The powder was purchased from Changsha Hualiu Metallurgy Powder Co., Ltd. The powder chemical compositions (mass fraction %) were C (0.03), Cr (17.5), Ni (12.06), Mo (2.06), Si (0.86), Mn (0.3), O (0.09), Fe (balance). The powder was spherical and the powder size distribution (mass fraction %) was <15 m (50%), 15–30 m (40%), >30 m (10%), the mean diameter was 17.11 m, and apparent density was 4.04 g/cm3 . Fig. 2 shows the microscopy of 316L stainless steel powder. 2.3. Experimental procedure Firstly, optimized processing parameters should be obtained. Then porous structure with different cell unit should be designed and fabricated. At the same time, the fabricating quality, main defect and processing rules should be discussed. And the causes of the defects and optimizing method were also studied. At last, the surfaces topography of the specimens were observed under stereomicroscope VHX-600 and scanning electron microscope (SEM) S-3700, the dimensional accuracy of fabricated porous structure was also measured under stereo-microscope VHX-600. 3. Results and discussion 3.1. Dense part fabrication Only when dense part is fabricated, then the strength and the fabricating process of the porous structure could be guaranteed. In order to optimize the fabricating process parameters of 316L stainless steel, the inter-layer stagger and orthogonal scanning strategy was used in this experiment, as shown in Fig. 3a. This scanning strategy was helpful to make the tracks overlap between adjacent tracks and layers. Beal et al. (2006) also proposed to use refilling scanning strategy in SLM fabrication of functionally graded H13/Cu materials where favorable results could be obtained compared with other scanning strategies. Optimized processing parameters of SLM could be got through large amount of time-consuming experiments. When the laser power 150 W, scanning speed 600 mm/s, layer thickness 35 m and scan spacing 80 m, dense 316L stainless steel part could be obtained. The fabricating result of 316L Stainless steel is shown in Fig. 3b–d, in which 3b is the full view, Fig. 3c is the topography of the top surface, and Fig. 3d is the micro observation of the side surface. Fig. 3c clearly shows strips
Three typical porous structures were designed and fabricated as shown in Fig. 4. Fig. 4a and b is designed identically, but the fabricating result are different, Fig. 4a is fabricated in good form, but Fig. 4b–d all have fabricating defects. The porous structure in Fig. 4b had serious powder adhesion defect. The porous structure’s strut in Fig. 4c was destroyed, and in Fig. 4d there was slag inside. It was even jammed when the pore was small enough. The above results indicate that different designs of porous structure could result in different fabricating quality, and the same design may also generate different fabricating quality. In order to obtain porous structure with certain porosity, the designing parameters including pore’s shape, size and distribution should be modified. Several rules should be considered and optimized before fabrication, which would be discussed below. 3.3. Processability for SLM fabrication of porous structure 3.3.1. Overhanging structure Based on the optimized process, overhanging structures with different inclined angles were designed and fabricated. The fabricating effect of the overhanging structures with inclined angle decreased from 45◦ to 25◦ is shown in Fig. 5. It could be seen that when ≥ 40◦ , the overhanging structure was fabricated well, only when ≤ 35◦ , the warping effect happened. Moreover, the smaller of the inclined angle, the more powder would stick to the downward surface of overhanging structure, which made the quality of downward surface worse than the upper surface. Warping defect is due to the thermal stress formed by rapid solidification of melting pool during SLM process. Many researchers have discussed the effect of thermal stresses in the SLS and SLM processes. Kruth et al. (2004) investigated that temperature gradient mechanism could explain the warping phenomenon. When the thermal stress exceeds the strength of the material, then plastic deformation happens. The warping defect of overhanging surface is also due to the lack of supports to secure its firm bonding with the previous layers. Wang et al. (2013) had studied several key factors that could affect the fabricating quality of overhanging surface. Residual stress accumulating is the main cause for forming failure of the overhanging surface. Fig. 6a shows the warping principle during overhanging surface fabrication by SLM, when the warping defect happened, the practical inclined angle between the overhanging part of the present layer and the previous layer is larger than the designed inclined angle . As Fig. 6b shows, when a layer of the overhanging part started to warp, it will influence the practical fabricating layer thickness of the next layer and cause larger warp. When the warp height accumulated to be higher than the pre-set height of the next layer, then it will make part of the fabricating zone have no powder to be recoated, and the whole work piece will become more and more vulnerable. The overhanging surface may suffered from repeated laser scanning, or even the overhanging part would break away from the whole component due to its repeating colliding with the powder scraper. When the warping defect became too serious, the whole forming process had to be stopped. Then it needs to re-design the part or optimize the process to solve the problem. Therefore, when fabricating the overhanging
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Fig. 3. Analysis of the sample’s surface morphology.
surfaces, if the building angles are smaller than the critical inclined angle , the special process is needed. According to the above results, the inclined angle 40◦ and 35◦ are the critical inclined angle for the designing of porous structure with overhanging surface. The fabrication zone could be divided into three zones by 40◦ and 35◦ , including stable fabrication zone, critical fabrication zone and hard fabricated zone as shown in Fig. 7. In order to fabricate porous structure accurately, it requires that the inclined angle of the pore’s strut should be in the stable fabrication zone, therefore, = 40◦ is the critical building angle for designing and manufacturing porous structure in this experiment. If the inclined angle of the unit cell’s strut is less than the critical inclined angle, it would cause poor dimensional accuracy, or even worse, the pores could be jammed by the steel powder. 3.3.2. The geometrical resolution The resolution in SLM production of different geometrical features is an important basis for designing and fabricating porous structure, as the strut’s diameter of the wire frame should not be smaller than the fabricating resolution during the manufacturing of porous structure. However, the strength of porous structure is inversely proportional with the strut’s diameter, so strut’s diameter should not be too small. The typical geometrical features are thin wall and cylinder. This experiment designed and produced thin walls with thickness ranged from 0.15 to 0.5 mm, and the
cylinders with diameter ranged from 0.15 mm to 5 mm. Fig. 8 shows the experiment results of fabricating the typical geometric features by SLM. It can be found that the thin walls with thickness of 0.15–0.5 mm, and cylinder with diameter of 0.15–5 mm could all be fabricated well. Considering the smaller geometrical feature could result in lower mechanical properties, so this experiment did not study on the geometrical features with geometrical size less than 0.15 mm. 3.3.3. Powder adhesion In Fig. 4b and d, there were many powers adhered to the surface of the porous structure. Powder adhesion is an inevitable problem in SLM process, which would leads to bad accuracy of the pore. Moreover, it is hard to deal with the powders adhered to the sides of the pores through post processing ways. Therefore, it is meaningful to control the amount of powder adhered to the pore’s surface by optimizing design and process controlling. While the laser is scanning the metal powder, the laser energy is transferred as heat absorbed by metal powder, which makes the powder melt and bind together. Wang et al. (2011) had studied the laser energy input and its influence on single track, multi-track and multi-layer fabrication during SLM fabrication. When the laser energy input is enough and the metal powder is absolutely melted, then regular and continuous track can be formed; while the laser energy input is insufficient to melt the metal powder, the sintered
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Fig. 4. Typical porous structures fabrication.
status would be resulted. There is a heat affect zone around the melting pool during laser scanning metal powder, which makes the metal powder around the melting pool not fully melted or in sintering status. As Fig. 9 shows, many stainless steel powder particles were adhered on the both sides of the tracks. Serious powder adhesion usually happens when overhanging structure is fabricated, as Fig. 10 shows, when laser irradiate solidsupported zone (point a), where the heat conduction rate is high; however, when laser irradiate powder-supported zone (point b), the heat conduction rate is only 1/100 of the solid-supported zone, and this situation often happens during overhanging surface fabrication by SLM, which was also proved by Kruth et al. (2007). Therefore, when processing parameters are similar, the absorbed energy input will be much bigger when laser irradiate powdersupported zone than laser irradiate solid-supported zone, which leads to the melt pool become too bigger and sinks into the powder as the result of gravity and capillary force. For the above reason, dross will be formed and dimensional accuracy should be very low during SLM fabrication of overhanging surface.
Fig. 5. Experiment of the overhanging structures with different inclined angles.
3.4. Design of unit cell and porous structure The design of unit cell and porous structure not only has a great impact on fabricating quality, but also determines whether the porous structure could be applied to the bio-medical field. The design of unit cell and porous structure include pore shape, size, porosity, pore distribution and the connectivity between each other. Parthasarathy et al. (2011), Patrick and Timothy (2009) and Lin and Wirtz (2007) had introduced different designing ways of porous structures. These porous structures were produced by SLM, which should be of important reference to this experiment. The following factors should be considered when designing, which include SLM process restraint, the critical inclined angle and geometrical resolution. When the strut of unit cell has small inclined angle, it would be difficult to achieve better control of the fabricating quality of the porous structure. In order to avoid the overhanging surface with small inclined angle, it is better to optimize the unit cell to satisfy the requirement of SLM process. Based on the discussion of the critical inclined angle, the authors proposed the proper unit cell as Fig. 11 shows, which could also be called as octahedral unit cell. By adjusting the inclined angle of the pore’s strut, the octahedral structure can theoretically avoid the overhanging surface with small inclined angle, thus can avoid the defects of hanging dross and jamming pore. Through the analysis of the inherent relationship among the radius R of the unit cell, the length of the strut L and the inclined angle of strut, as shown in Fig. 11b, there is a relationship among L, R and without considering the diameter of the strut, as the following equation shows:
R=
1 × L × sin 2 2
(1)
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Fig. 6. The warping principle during SLM fabrication of overhanging surface.
Fig. 7. The fabrication zone is divided by the critical inclined angle.
Fig. 8. The results of SLM fabricating typical geometric features.
Assuming the strut diameter is d, so formula (1) could be modified as the following equation:
R=
Fig. 9. Stainless steel powder particles were adhered to the both sides of the tracks.
1 × L × sin 2 − d 2
(2)
In formula (2), the diameter of strut mainly depends on the laser processing parameters and powder particle size. Due to the thermal effect around the melting pool in the fabricating process, the strut’s diameter is usually larger than the focused spot diameter. When = 45◦ , the radius of the unit cell is the largest, when R = 1/2L − d. Through the formula (2), it could be deduced that when R > 0, then it requires that 1/2 × L × sin2 > d. Combining the experiments in Sections 3.3.1 and 3.3.2 together, the design rules for the
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Fig. 12. The other two design ways of porous structure. Fig. 10. Schematic diagram of overhanging surfaces fabrication by SLM.
unit cell that fit for SLM process were summed up as ⎧structure ◦ ≤ < 90◦ 30 (a) ⎨ (b) related to the device parameters follow: d ≤ 0.15 mm ⎩ 1 × L × sin 2 > d (c) 2 According to the above summarized design rules, the design of unit cell should consider the following three conditions: the critical inclined angle, the fabricating resolution and the mutual constraint among the unit cell’s geometrical parameters. Although this study adopted octahedral porous structure as the unit cell, it is not the only suitable unit cell for SLM fabricating of porous structure. There would be a variety of designs get satisfying unit cell, as long as the designers consider the restraint of the critical inclined angle and the geometrical resolution. Fig. 12 shows other two design ways of porous structure which are also suitable for SLM process. 3.5. Accuracy measurement Based on the design rules of unit cell structure, porous structure was designed with geometrical parameters as follow: the strut diameter 0.3 mm, the length of the strut L = 1 mm, the inclined angle of the strut = 45◦ . The optimized SLM fabricating process parameters were used to manufacture the porous structure and the obtained fabricating effect is shown in Fig. 13. Seen from Fig. 13a, the whole fabrication effect of porous structure is good, however seen from the enlarged view of Fig. 13b and c, some tiny powder particles are stuck to the surfaces of the pore’s
strut, making the surfaces not smooth as the metal parts produced by traditional ways. The reason mainly related to three aspects: the first due to the powder melting and solidifying, there always existed a heat affect zone around the track’s two sides, which easily made powders adhered to the track’s surface. The second reason may due to the small divided scanning area of the cross section in each layer. As the laser scanning time is short in the micro region, the powders sometimes are not completely melted and would stick to the side of the molten pool. The third reason is that when the laser scanned the powder outside the preset pores, the liquid metal would inevitably penetrate into the internal pore and made the powder adhered to the track easily, resulting in low dimensional accuracy and poor surfaces for the final fabricated porous structure. The most difficult task may focus on removing the powders inside the pore when the manufacturing process finished. In order to guarantee the surface quality and mechanical strength of porous structure, the diameter of the pore’s strut should not be designed too small. Table 1 shows the measurement of the pore’s size in Fig. 13. It could be seen that the actual sizes of the pores were a little smaller than the designed value, which may also due to powder adhesion. The above sections discussed the process and the main influential factors of fabricating porous structure by SLM technique. The optimized fabricating process is basis, only when dense stainless steel part is obtained, it can ensure the strength of porous structure. The study on critical inclined angle and the fabricating resolution provided geometrical limitations for the design of porous structure, that is, the inclined angle of the unit cell’s strut cannot be smaller than the critical angle when fabricating metal parts by SLM, otherwise the inside of the porous structure would be jammed by the powder. Meanwhile, the diameter of pore strut must be larger
Fig. 11. The unit cell with octahedral structure.
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Fig. 13. The fabricating effect of porous structure.
than the fabricating resolution. In addition, the design of unit cell structure should also take into account of its mutual constraint of geometrical parameters, as formula (2) explained. The parameters that can affect the fabricating quality of porous structure include spot diameter, powder particle size, energy input and so on. Different researchers would use different laser parameters and experimental materials, which may bring better experimental results. But through the analysis and discuss of experimental results in Section 3.3.3, the conclusion could be drawn that powder adhesion is an inevitable phenomenon in the SLM fabricating process, which affects the surface quality and dimensional accuracy of the porous structure, or even make the pores jammed. So the powder adhesion should be one of the most difficult problems that need to be solved before satisfied porous structure could be obtained.
Table 1 Measurement of the pore size. X direction
Y direction
Measuring point
Width/m
Measuring point
Width/m
1 2 3 4 5 6 Average Design Error
965 999 961 930 946 987 965 1000 35
7 8 9 10 11 12 Average Design Error
976 979 984 976 965 972 975 1000 25
4. Conclusions In order to obtain the desired porous structure accurately by SLM, this article discussed several key issues as below:
(1) Optimizing the process to fabricate dense parts. (2) Obtaining the critical inclined angles under the optimized process parameters. (3) Learning the minimum building size when fabricating thin wall and cylinder geometric features by SLM (fabricating resolution of the SLM equipment). (4) Designing unit cell and porous structure that fit for SLM process based on the value of critical inclined angles and fabricating resolution. (5) Fabricating porous structure and analyzing dimensional accuracy.
Several processing restrictions for SLM fabrication of porous structure were discussed, including overhanging structure, fabricating resolution and powder adhesion. Powder adhesion could be the most troublesome problem for porous structure fabrication by SLM. This experiment focused on the process feasibility for fabricating stainless steel porous structure by SLM. In practical applications, the porous structures would be mainly applied in the medical field and the biocompatibility between titanium alloy and human body is much better. Therefore, the study on SLM fabricating titanium alloy porous structure is of much more significance and should be the authors study direction.
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Acknowledgements The work described in this paper were supported by the National Natural Science Foundation of China (Granted No. 51275179), the Fundamental Research Funds for the Central Universities of China (Granted No. 2012ZB0014), the Natural Science Foundation of Guangdong Province (Granted No. 2012040007516), and the Doctoral Scientific Fund Project of the Ministry of Education of China (Granted No. 20120172120006). References Banhart, J., 2011. Manufacture characterisation and application of cellular metals and metal foams. Progress in Materials Science 46 (6), 559–632. Beal, V.E., Erasenthiran, P., Hopkinson, N., Dickens, P., Ahrens, C.H., 2006. The effect of scanning strategy on laser fusion of functionally graded H13/Cu materials. International Journal of Advanced Manufacturing Technology 30 (9), 844–852. Hauser, C., Sutcliffe, C., Egan, M., Fox, P., 2005. Spiral growth manufacturing (SGM): a continuous additive manufacturing technology for processing metal powder by selective laser melting. In: Proceedings of the 16th Solid Freeform Fabrication Symposium. Solid Freeform Fabrication Symposium, Austin, pp. 1–12. Kruth, J.P., Froyenb, L., Van Vaerenbergha, J., Mercelis, P., Rombouts, M., Lauwers, B., 2004. Selective laser melting of iron-based powder. Journal of Materials Processing Technology 149, 616–622. Kruth, J.P., Mercelis, P., Van Vaerenbergha, J., Craeghs, T., 2007. Feedback control of selective laser melting. In: Proceedings of the 3rd International Conference on Advanced Research in Virtual and Rapid Prototyping, Leiria, Portugal, pp. 521–527. Lin, C.Y., Wirtz, T., 2007. Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process. Journal of Biomedical Materials Research Part A, 272–279. Mullen, L., Stamp, R.C., Brooks, W.K., 2008. Selective laser melting: a regular unit cell approach for the manufacture of porous, titanium, bone in-growth constructs, suitable for orthopedic applications. Journal of Biomedical Materials Research Part B: Applied Biomaterials 89 (2), 325–334.
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