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3DP process for fine mesh structure printing
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Powder Technology 187 (2008) 11 – 18 www.elsevier.com/locate/powtec
3DP process for fine mesh structure printing Kathy Lu ⁎, William T. Reynolds 1 Virginia Polytechnic Institute and State University, Materials Science and Engineering Department, 211B Holden Hall-M/C 0237, Blacksburg, VA 24061, USA Received 12 September 2007; received in revised form 18 December 2007; accepted 20 December 2007 Available online 4 January 2008
Abstract Three dimensional printing (3DP) is a unique technique for creating complex shapes. However, printing feature sizes at less than 500 μm with high integrity and intricate structures have not been possible. In this study, TiNiHf shape memory alloy (SMA) powder was printed into 3D mesh structures of 300 μm wire width. Effects of printing layer thickness and binder saturation level on the integrity and dimensional accuracy of the 3D mesh structures were evaluated. 35 μm printing layer thickness and 170% binder saturation level offer the highest mesh structure integrity. Also, 35 μm printing layer thickness results in the smallest dimensional deviation from the designed 200 μm mesh width with the smallest standard deviation. Overall, 35 μm printing layer thickness and 170% binder saturation level are the most preferred printing condition for the designed 3D mesh structure. © 2008 Elsevier B.V. All rights reserved. Keywords: 3D printing; Mesh structure; Printing layer thickness; Binder saturation level; Integrity; Dimensional accuracy
1. Introduction Since the mid-1990s, various digital forming techniques have emerged at the realization of digital control, more advanced materials, and the demand for very specific performances and structure designs [1–6]. Three dimensional printing (3DP) is a unique technique that prints complex 3D structures that cannot be produced by other means, especially for rapid prototype purpose [7–9]. During 3DP, the solid model, formatted into a [.stl] file (standard triangle language), is converted by a slicing routine into a compilation of twodimensional slices representing the 3D part. The slice file is further formulated into instructions that control the movement of the 3D printing components. The powder is spread by a counter-rotating roller onto a build platform inside a build box. By means of ink-jet printing technology, a printhead, containing an array of binder fluid jets, rasters across the layer of the powder and deposits binder droplets in those locations defined by the current 2D slice of the solid model. Subsequently, the build platform advances downward by one layer thickness and a new layer of powder is spread, which is then printed by the ⁎ Corresponding author. Tel.: +1 540 231 3225; fax: +1 540 231 8919. E-mail address: [email protected] (K. Lu). 1 Tel.: +1 540 231 3225; fax: +1 540 231 8919. 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.12.017
printhead. This procedure is repeated layer after layer until the 3D part is completed. After the designed 3D geometries are printed, the particles are held together by the binder used. The printed part can be removed from the surrounding unbound powders. However, the printed structures are not strong enough to be used directly and need to be sintered to densify the matrix. 3DP has demonstrated the capability of fabricating parts of a variety of materials, including ceramics, metals, and polymers with an array of unique geometries [9,10]. However, substantial work is still needed to explore the minimum feature sizes that can be printed and the ability of forming intricate structures. This is mainly because the minimal feature sizes and the integrity of the structures are affected by numerous factors such as binder droplet size and printing layer thickness. Also, suitable techniques need to be developed to evaluate the quality of the fine features. Shape memory alloy (SMA) is a group of novel materials that demonstrate the ability to return to some previously defined shape when subjected to appropriate thermal or stress-induced procedures [11]. The phase transformation strains have the potential to relieve the thermal and mechanical stresses during the repeated thermal cycles and restore the distorted matrix shape to its original by shape memory effect. Currently, TiNiHf is considered to be the most attractive candidate as elevated temperature SMA due to its excellent workability, thermal
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glass/ceramic matrix. The AutoCAD design of the 3D mesh structure is shown in Fig. 2. The diameters of the three rings shown are 5.0, 10, and 15 mm, respectively. The numbers of the radial wires in the horizontal layers are 16, 8, and 4 from the top to the bottom. Square wires of 200 μm width are used in each horizontal layer. Round wires of 200 μm diameter are used inbetween the layers. The reason for using the round wires inbetween the horizontal layers is to ensure that the horizontal and the vertical wires is connected with a known interfacial area (the round wire cross-section area). Otherwise, the curvature from the horizontal wire can create more complicated connecting areas. Fig. 1. Particle size distribution of gas-atomized TiNiHf powder.
2.3. Mesh structure evaluation cycling stability, and the ability to absorb large amount of strain energy [12–16]. One of the promising applications for the TiNiHf SMA is a metal/glass composite that integrates a defined TiNiHf SMA mesh structure into a glass/ceramic matrix for high temperature thermal cycling applications such as solid oxide fuel/electrolyzer cells. The phase transformation strains from the TiNiHf can be used to offset the thermal stresses in the glass matrix imposed by other solid oxide cell components during cooling. The shape memory effect during heating to the operating temperature has the potential to close cracks that may have formed in the glass matrix during a previous cooling cycle. To realize these predicted functions, the first step is to create desired 3D mesh structures from the TiNiHf alloy. Since TiNiHf alloy powder can be obtained by gas atomization, 3DP presents itself as a promising technique for creating the mesh structures needed. This study is focused on 3D printing of TiNiHf particles into mesh structures for such purpose. The printing variables are examined based on the originating sources: 3D printer related, powder-related, and binder-related. Printing layer thickness and binder saturation level (binder to pore volume ratio for a given printing volume) are identified as the primary variables affecting the 3DP capability and systematically evaluated in order to achieve the highest 3D mesh structure integrity and minimum dimensional variation. 2. Experimental Design 2.1. TiNiHf powder In this study, gas-atomized Ti35Ni50Hf15 (mol%, abbreviated as TiNiHf) powder (Crucible Research Co., Pittsburgh, PA) of smaller than -635 mesh size was used for 3D mesh structure printing. The TiNiHf particle size distribution was characterized by laser light scattering (Horiba, LA-950, Irvine, CA). The measured particle size distribution is shown in Fig. 1. It can be observed that 100% particle sizes are below 20 μm and the mean particle size is about 5.50 μm. The particle size distribution is close to normal distribution.
The printed 3D mesh structures are loosely bound by the binder used and do not have high integrity. A small load cell has to be used in the integrity evaluation. Since the printed samples are brittle and experience compressive stress during the mesh structure-glass composite application, a Texture Analyzer test console equipped with a 5 kN load cell (Stable Micro Systems, Surrey, UK) was used for compression failure test. The console was set to record compressive force and the crosshead was lowered monotonically at a speed of 0.1 mm/min. The shape of the force vs. displacement curve was recorded and the peak force was determined from the curve as the 3D mesh structure breaking force. Breaking strength is calculated as the ratio of breaking force to the average cross section area of the three layers in the mesh structure horizontal direction. To evaluate the dimensional accuracy of individual wires, an optical microscope (ME300TZ-3P, Amscope, Chino, CA, USA) was used to measure the wire width from the top-down view. Each reported wire width was measured at ten different radial wire locations. 3. Results and Discussion 3.1. 3D printing variable evaluation As mentioned, 3DP process is affected by numerous variables. In order to obtain 3D printed structures with high integrity, small feature sizes, and accurate dimensions, each factor needs to be considered. By the correlation with the major aspects of 3DP, the variables can be classified into three categories: 3D printer-related variables, powder-related variables, and binder-related variables. These three groups of printing variables are listed in Tables 1, 2, and 3, respectively.
2.2. Mesh structure design A gradient 3D mesh structure was designed based on the application requirement of TiNiHf SMA mesh structure in
Fig. 2. Designed 3D mesh structure to be printed.
K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18
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Table 1 3D printer-related printing variables Variables
Definition
Effect
Range
Cap Cleaning Frequency Wipe Cleaning Frequency Full Prime Frequency Spreader Reverse Speed Spreader Traverse Speed
Number of printing cycles between cleaning of printhead cap Number of printing cycles between cleaning of the sponge wipe for printhead Number of printing cycles between fully refilling printhead with binder Travel speed of printbed after dispensing of binder
Cleanliness of printhead cap
1-5 cycles/cleaning 2 cycles
Cleanliness of printhead sponge wipe
1-5 cycles/cleaning 2 cycles
Consistency of binder drop volume
1-5 cycles/cleaning 1 cycle
Travel speed of printbed after drying
Dwell time of binder before drying; 0.5-5.0 mm/sec affect the dimensional tolerance of the meshes Smoothness of powder layer for next printing 0.5-5.0 mm/sec cycle; related to powder packing density and particle size
Tables 1–3 also list the range of the 3D printing variables and how they affect printed structure quality. Even though all the above variables affect the printed structure integrity and resolution, some of the printing variables can be pre-determined. First of all, 3DP process requires careful selection of particle size and size distribution. Suitable particle size range is dependent on the feature sizes to be printed, the printed mesh structure integrity desired, and the availability of specific powders. If the particle size is too large, it will not create strong enough mesh structures and the fine features desired. For example, if a 50 μm thick feature is desired, then each printing layer would only be one particle size thick for a 50 μm size powder and is not practically possible. However, other difficulties can arise if the particle size is too small. Small particles, due to their poor flowability, cannot be spread into thin and even layers. The spreader roller can press the particles into the previous layers in the printbed. Also, the particles should have a reasonable range of size distribution. The largest size should meet the integrity and feature size requirement and the smallest particle size should not affect powder flow. The normal size distribution used in this study is preferred for higher particle packing density in comparison to monosize distribution. Binder is another important variable to consider. It should have low viscosity and high stability during the 3DP process. If the binder viscosity is too high, it can easily clog the printhead nozzles, which have about 74 μm size. Also, the binder should be stable during the 3DP process with no significant chemical reaction. Otherwise, it can affect the wetting capability on the particle surfaces and subsequently the integrity of the mesh structures. In this study, an acrylic-based aqueous binder with
Value Adopted
2.0 mm/sec 2.0 mm/sec
17 wt% concentration was used for printing the SMA powder and is pre-determined based on the binder availability. Cap cleaning frequency, wipe cleaning frequency, full prime frequency, spreader reverse speed, and spreader traverse speed were determined by trial and error before the designed 3D mesh structures were printed and the values are given in Table 1: 2 cycles/cleaning for the cap, 2 cycles/cleaning for the wipe, 1 cycle for each full prime. The spreader reverse speed and the spreader traverse speed are 2.0 mm/s. Although the theoretical packing density should be ~64% for the smaller than –635 mesh powder based on loose random packing principle, actual packing density of the SMA powder in the printbed is far below that. The powder packing density is measured to be 35%. This is because the SMA powder in the printbed is not as densely packed as the bulk powder, likely due to the roller spreading effect and the loose packing of particles in the spread layer when the printing layer is thin. Also, feed powder to printing layer thickness ratio needs to be properly pre-set so that an even layer is spread for each printing cycle. For the SMA powder, this value is again identified by trial and error to be 1.5 (Table 2). For the variables listed in Table 3, drying time is determined to be sufficient at 60 s and drying power to be sufficient at 90% based on printing of simple wires with the SMA powder used. Binder drop volume is determined to be at 211.5 pico-liter by the 3D printer based on the powder packing density, printing layer thickness, and binder saturation level. Assprayed binder droplet size is directly determined by the printhead nozzle size and reflected by the binder drop volume. This value is the same for all the conditions studied, approximately 74 μm.
Table 2 Powder-related printing variables Variables
Definition
Effect
Particle Size and Size – Feature size, powder packing density Distribution Powder Packing Density Ratio of powder volume to total Binder saturation volume Printing Layer Thickness Thickness of each layer to be Dimensional tolerance of printed mesh structures printed Feed Powder to Layer Thickness Ratio
Ratio of feed powder to layer thickness
Range
Value Adopted
–
5.5 μm, normal distribution 30%
0-100%
N2 times of 20 μm particle size 35 μm 50 μm Should be high enough to spread new layers but not so high to cause 1.0-2.0 1.5 shifting of layers
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Table 3 Binder-related printing variables Variables
Definition
Effect
Range
Value Adopted
Binder Volume percent of binder printed vs. Higher saturation creates stronger bonds between particles; but too high a 40-200% Saturation Level pore volume among particles saturation creates lateral spreading and poor dimensional tolerance Drying Time and Power Binder drop volume
Drying time and power between Integrity and shape retention of 3D mesh structures successive layers Average binder drop volume per jet Integrity and dimensional resolution of mesh structures spray
The major variables that need to be further studied in the 3D printing of the TiNiHf powder are printing layer thickness and binder saturation level. These two parameters also play important roles in determining the integrity, dimensional accuracy, and minimal feature sizes that can be printed for the TiNiHf powder. In order to print the 3D mesh structures with good integrity and accurate shape, thin printing layer thickness should be used. However, it has to be greater than the SMA particle size. Since the average particle size is 5.5 μm and the maximum particle size is 17.0 μm, the printing layer thickness is chosen at 20, 35, and 50 μm in this study for further evaluation. Binder saturation level is closely related to powder packing density, binder-particle interaction, and thus binder distribution in the 3D printed meshes. Under the same printing layer thickness, higher saturation level results in higher volume of sprayed binder. However, there is no specific knowledge related to the TiNiHf powder. Since the binder saturation varying range is 40-200%, three levels were selected in this work: 55%, 110%, and 170%. It should be pointed out that the 3D printer allows greater than 100% binder saturation level (excessive binder amount than what is needed to fill the pores among the particles). After the 3D printing, the mesh structures are pre-cured for one hour at 100% heater power of the 3D printer before they are removed from the printbed. A subsequent cure of one hour at 170 °C is done in an oven to completely remove the water in the binder solution and fully bind the particles. 3.2. Mesh structure integrity evaluation
55% 110% 170% Time: 0-90 s 60 s, Power: 0-100% 90% Binder specific 211.5 pico-liter
better powder spreading. However, our aim is to print mesh structures with 200 μm wire width. Higher printing layer thickness will result in fewer printing layers. To achieve high 3D mesh integrity, the number of printing layers should be as high as possible. With this consideration, printing layer thickness of 50 μm is selected to evaluate three binder saturation levels: 55%, 110%, and 170%. Six samples were printed for each condition. However, 55% binder saturation level produced 3D mesh structure that is too fragile to be removed from the printbed. All the samples broke into pieces during transfer from the printbed to the sample container, as shown in Fig. 3. This means that 55% binder saturation level is too low to form the designed 3D mesh structure. When the binder saturation level was increased to 110%, the 3D mesh structures printed broke partially. The framework of the 3D structure was intact but some vertical connecting wires broke. The image of the printed 3D mesh structures at 110% binder saturation level is shown in Fig. 4(a). This means 110% binder saturation level produces improved mesh structures than the 55% binder saturation level. However, the vertical, roundshaped wires connecting the horizontal layers are still weak. When the binder saturation level was increased to 170%, all the samples were strong enough to be removed from the printbed as shown in Fig. 4(b). In order to quantitatively compare the integrity of the 3D mesh structures, three printing layer thicknesses, 20, 35, and 50 μm, are evaluated at 110% and 170% binder saturation levels. All the other printing parameters are kept the same as discussed in 3.1. From Fig. 5, it can be observed that at all the
For a given powder with fixed particle size and size distribution, higher printing layer thickness generally ensures
Fig. 3. 3D printed TiNiHf mesh structure with 50 μm printing layer thickness and 55% binder saturation level.
Fig. 4. 3D printed TiNiHf mesh structure with (a) 110% binder saturation, and (b) 170% binder saturation. The samples are printed with 50 μm printing layer thickness.
K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18
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170% binder saturation level are the desired condition for printing the highest integrity 3D mesh structures. 3.3. 3D printing dimensional accuracy
Fig. 5. Breaking strength of printed TiNiHf 3D mesh structures.
printing layer thicknesses, the 3D mesh structures printed using 170% binder saturation level have higher breaking strength than those of the mesh structures printed using 110% binder saturation level. The error bar represents the standard deviation of the breaking strength. Mesh structures with 170% binder saturation level have smaller breaking strength variation than those of the mesh structures printed using 110% binder saturation level. This means higher binder saturation level is beneficial for the studied 3D wire meshes and offers more binding function among the particles. Also, 170% binder saturation level offers more consistent 3D mesh structure integrity. The effect of the printing layer thickness on the integrity of the 3D mesh structures can also be analyzed from Fig. 5. For both 110% and 170% binder saturation levels, the 3D mesh structures printed with 35 μm printing layer thickness are stronger than those with 20 μm and 50 μm printing layer thicknesses. Also, the 3D mesh structure printed with 35 μm printing layer thickness and 170% binder saturation level shows the smallest breaking strength variation. Based on our observation, this can be explained from the lateral and vertical binder spreading differences. In general, the binder spreading distance in the horizontal direction is much longer than that in the vertical direction. This is because the binder drop has certain size when it reaches the powder bed and the spreading is in all horizontal directions. For the vertical spreading, the binder only progresses from the top to the bottom of the printing layer. The detailed lateral and vertical binder spreading rates will be detailed in future studies using 3D microscope. For the 20 μm printing layer thickness, the wire width is 10 times of the vertical spreading distance. The vertical direction will be saturated with the binder before the lateral binder spreading is complete. The poor lateral spreading causes low integrity 3D mesh structures. For the 35 μm printing layer thickness, the binder is able to complete the binder spreading in both the lateral and vertical directions within similar time span. This contributes to the highest breaking strength observed. For the 50 μm printing layer thickness, the wire width is only 4 times of the printing layer thickness. Binder spreading in the lateral direction will be complete before that in the vertical direction. As a result, the binder will likely have incomplete penetration in the vertical direction. Based on the breaking strength evaluation, it can be concluded that 35 μm printing layer thickness and
In addition to the 3D mesh structure integrity, the printed wire thickness and its deviation from the designed 200 μm width are other important factors to study. Fig. 6 shows the average wire width and the corresponding standard deviation under different printing conditions. All the printed wire width is larger than the designed 200 μm value. Actually, the dimensional deviation is ~50% or higher. While the dimensional deviation should be substantially less when the printed feature size increases, it is still important to be able to predict the printed wire width. Future effort will be devoted to narrow the dimensional gap between the designed and the printed structures. Under the same binder saturation level, 35 μm printing layer thickness yields the smallest dimensional deviation from the designed 200 μm width in comparison to the other two printing layer thicknesses. The wires printed with 20 μm printing layer thickness show the largest deviation from the designed 200 μm wire width. Also, the wire width printed with 35 μm printing layer thickness has the smallest standard deviation overall. This difference can be understood as follows. For the 20 μm printing layer thickness, the designed wire width is 10 times of the printing layer thickness. The binder penetrates quickly to the bottom of the layer, but the previous layer printed prevents the binder from further spreading. In the lateral direction, the binder spreads without such limitation. This results in larger wire width than desired. For the 50 μm printing layer thickness, the binder is able to spread in the vertical direction before being completely consumed. This results in lower wire width deviation. 35 μm printing layer thickness allows the binder to finish spreading in both directions at approximately the same time and offers the closest dimension to the designed wire width. From Fig. 6, it can be concluded that 35 μm printing layer thickness is preferred for the realization of good wire dimensional tolerance. Combined with Fig. 5, it confirms that 35 μm printing layer thickness is the desired 3D printing condition for the TiNiHf powder. As the binder saturation level increases at the same printing layer thickness, the wire width generally becomes thicker even
Fig. 6. Printed wire width measured by optical microscopy.
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K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18
though the difference is small. This is because the binder saturation level affects binder spreading distance. Higher binder saturation level leads to longer binder spreading distance. In general, 35 μm printing layer thickness shows minimal wire width at different binder saturation levels. Optical micrographs of the printed wire meshes under different conditions are shown in Figs. 7–9. It can be seen that under the same binder saturation level, the wire meshes printed with 20 μm printing layer thickness show more dimensional variation in comparison to those printed with 35 and 50 μm printing layer thicknesses. There are some large protruded areas from the 20 μm printing layer thickness wire. Even though the
Fig. 8. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 35 μm.
micrographs only show local areas, the general trend is the same for all the wires examined. 3.4. Optimal 3D mesh structure
Fig. 7. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 20 μm.
The binder saturation level and the printing layer thickness can be understood schematically as shown in Fig. 10. During 3D printing, binder is jetted from the printhead with a certain drop size. However, the total binder amount jetted per layer is proportional to the total volume of the powder to be printed. After it reaches the printbed, the binder spreads into the interstitial sites of the particles in vertical and lateral directions.
K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18
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Fig. 11. 3D mesh structure printed with 35 μm printing layer thickness and 170% binder saturation level.
Fig. 9. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 50 μm.
When the powder layer is too thin, such as the case for the 20 μm printing layer thickness, there is less than desired powder thickness to accommodate the binder at the studied saturation levels since the powder layer beneath the current printing layer is saturated with the binder already from the prior printing cycle. This causes excessive lateral flow of the binder and larger and uneven wire width. When the printing layer thickness is too large, such as 50 μm, uneven lateral spreading becomes less. However, there is more binder jetted into the printbed because of the larger powder volume per layer. The binder might have less than sufficient time to diffuse vertically to the previously printed layer while the binder drop is large enough for lateral spreading. As a result, larger wire width still results, with much less wire width variation. At the optimal printing layer thickness, such as 35 μm, binder vertical spreading and lateral spreading proceed and finish at approximately the same time. This very desirably leads to smallest wire width and the smallest wire width variation. Since the binder is distributed all within the wire mesh structure, it also leads to stronger 3D wire mesh structures. The actual binder spreading process requires in-situ observation, which will be discussed in future work. Based on the above observations and understanding, the 3D mesh structures are printed as shown in Fig. 11 under the optimal conditions: 35 μm printing layer thickness, 170% binder saturation. It reproduces the designed 3D mesh structures in Fig. 2 with high accuracy and integrity. Clearly, 3D printing is capable of producing near net-shape intricate mesh structures when the printing condition is optimized. There are no other techniques available to produce mesh structures in such complexity and resolution.
Fig. 10. Different printing layer thickness and binder spreading cases.
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4. Conclusions In this work, 3DP variables are systematically analyzed in order to print TiNiHf 3D mesh structures. Two major 3D printing variables, printing layer thickness and binder saturation level, are evaluated. At the same printing layer thickness, breaking strength increases with the binder saturation level up to 170%. At the same binder saturation level, 35 μm printing layer thickness yields 3D mesh structures with the highest integrity and the lowest dimensional deviation. 35 μm printing layer thickness and 170% binder saturation level are the optimal 3D printing parameters for the TiNiHf 3D mesh structures. This study demonstrates the unique capabilities of the 3DP technique in printing intricate and 300 μm wire width structures. Acknowledgment This material is based upon work supported by the Department of Energy under Award Number DE-FC07-06ID14739. References [1] J. Stampfl, H. Fouad, S. Seidler, R. Liska, F. Schwager, A. Woesz, P. Fratzl, Int. J. Mater. Prod Technol. 21 (2004) 285.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
W. Bauer, R. Knitter, J. Mater. Sci. 37 (2002) 3127. J.A. Lewis, G.M. Gratson, Materials Today 7 (2004) 32. Z.S. Rak, CFI Ceram. Forum Int. 77 (2000) E25. S. Schroeder, Adv. Mater. Process. 159 (2001) 32. B.A. Tuttle, J.E. Smay, J. Cesarano, J.A. Voigt, T.W. Scofield, W.R. Olson, J.A. Lewis, J. Am. Ceram. Soc. 84 (2001) 872. E. Sachs, M. Cima, P. Williams, D. Brancazio, J. Cornie, J. Eng. Ind. 114 (1992) 481. X.W. Yin, N. Travitzky, P. Greil, Int. J. Appl. Ceram. Technol. 4 (2007) 184. H. Seitz, W. Rieder, S. Irsen, B. Leukers, C. Tille, J. Biomed. Mater. Res. 74B (2005) 782. S.A. Uhland, R.K. Holman, M.J. Cima, E. Sachs, Y. Enokido, Mater. Res. Soc. Symp. Proc. 542 (1999) 153. F. Fremond, S. Miyazaki, Shape Memory Alloys, Springer-Verlag Wien, New York, NY, 1996, pp. 69–142. X.L. Meng, Y.F. Zheng, W. Cai, L.C. Zhao, J. Alloys Compd. 372 (2004) 180. X.L. Meng, Y.F. Zheng, Z. Wang, L.C. Zhao, Scr. Mater. 42 (2000) 341. X.L. Meng, W. Cai, Y.F. Zheng, Y.X. Tong, L.C. Zhao, L.M. Zhou, Mater. Lett. 55 (2002) 111. X.L. Meng, Y.F. Zheng, Z. Wang, L.C. Zhao, Mater. Lett. 45 (2000) 128. G. Airoldi, S. Pireda, M. Pozzi, A.V. Shelyakov, Mater. Sci. Forum 327-328 (2000) 135.
Powder Technology 187 (2008) 11 – 18 www.elsevier.com/locate/powtec
3DP process for fine mesh structure printing Kathy Lu ⁎, William T. Reynolds 1 Virginia Polytechnic Institute and State University, Materials Science and Engineering Department, 211B Holden Hall-M/C 0237, Blacksburg, VA 24061, USA Received 12 September 2007; received in revised form 18 December 2007; accepted 20 December 2007 Available online 4 January 2008
Abstract Three dimensional printing (3DP) is a unique technique for creating complex shapes. However, printing feature sizes at less than 500 μm with high integrity and intricate structures have not been possible. In this study, TiNiHf shape memory alloy (SMA) powder was printed into 3D mesh structures of 300 μm wire width. Effects of printing layer thickness and binder saturation level on the integrity and dimensional accuracy of the 3D mesh structures were evaluated. 35 μm printing layer thickness and 170% binder saturation level offer the highest mesh structure integrity. Also, 35 μm printing layer thickness results in the smallest dimensional deviation from the designed 200 μm mesh width with the smallest standard deviation. Overall, 35 μm printing layer thickness and 170% binder saturation level are the most preferred printing condition for the designed 3D mesh structure. © 2008 Elsevier B.V. All rights reserved. Keywords: 3D printing; Mesh structure; Printing layer thickness; Binder saturation level; Integrity; Dimensional accuracy
1. Introduction Since the mid-1990s, various digital forming techniques have emerged at the realization of digital control, more advanced materials, and the demand for very specific performances and structure designs [1–6]. Three dimensional printing (3DP) is a unique technique that prints complex 3D structures that cannot be produced by other means, especially for rapid prototype purpose [7–9]. During 3DP, the solid model, formatted into a [.stl] file (standard triangle language), is converted by a slicing routine into a compilation of twodimensional slices representing the 3D part. The slice file is further formulated into instructions that control the movement of the 3D printing components. The powder is spread by a counter-rotating roller onto a build platform inside a build box. By means of ink-jet printing technology, a printhead, containing an array of binder fluid jets, rasters across the layer of the powder and deposits binder droplets in those locations defined by the current 2D slice of the solid model. Subsequently, the build platform advances downward by one layer thickness and a new layer of powder is spread, which is then printed by the ⁎ Corresponding author. Tel.: +1 540 231 3225; fax: +1 540 231 8919. E-mail address: [email protected] (K. Lu). 1 Tel.: +1 540 231 3225; fax: +1 540 231 8919. 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.12.017
printhead. This procedure is repeated layer after layer until the 3D part is completed. After the designed 3D geometries are printed, the particles are held together by the binder used. The printed part can be removed from the surrounding unbound powders. However, the printed structures are not strong enough to be used directly and need to be sintered to densify the matrix. 3DP has demonstrated the capability of fabricating parts of a variety of materials, including ceramics, metals, and polymers with an array of unique geometries [9,10]. However, substantial work is still needed to explore the minimum feature sizes that can be printed and the ability of forming intricate structures. This is mainly because the minimal feature sizes and the integrity of the structures are affected by numerous factors such as binder droplet size and printing layer thickness. Also, suitable techniques need to be developed to evaluate the quality of the fine features. Shape memory alloy (SMA) is a group of novel materials that demonstrate the ability to return to some previously defined shape when subjected to appropriate thermal or stress-induced procedures [11]. The phase transformation strains have the potential to relieve the thermal and mechanical stresses during the repeated thermal cycles and restore the distorted matrix shape to its original by shape memory effect. Currently, TiNiHf is considered to be the most attractive candidate as elevated temperature SMA due to its excellent workability, thermal
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K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18
glass/ceramic matrix. The AutoCAD design of the 3D mesh structure is shown in Fig. 2. The diameters of the three rings shown are 5.0, 10, and 15 mm, respectively. The numbers of the radial wires in the horizontal layers are 16, 8, and 4 from the top to the bottom. Square wires of 200 μm width are used in each horizontal layer. Round wires of 200 μm diameter are used inbetween the layers. The reason for using the round wires inbetween the horizontal layers is to ensure that the horizontal and the vertical wires is connected with a known interfacial area (the round wire cross-section area). Otherwise, the curvature from the horizontal wire can create more complicated connecting areas. Fig. 1. Particle size distribution of gas-atomized TiNiHf powder.
2.3. Mesh structure evaluation cycling stability, and the ability to absorb large amount of strain energy [12–16]. One of the promising applications for the TiNiHf SMA is a metal/glass composite that integrates a defined TiNiHf SMA mesh structure into a glass/ceramic matrix for high temperature thermal cycling applications such as solid oxide fuel/electrolyzer cells. The phase transformation strains from the TiNiHf can be used to offset the thermal stresses in the glass matrix imposed by other solid oxide cell components during cooling. The shape memory effect during heating to the operating temperature has the potential to close cracks that may have formed in the glass matrix during a previous cooling cycle. To realize these predicted functions, the first step is to create desired 3D mesh structures from the TiNiHf alloy. Since TiNiHf alloy powder can be obtained by gas atomization, 3DP presents itself as a promising technique for creating the mesh structures needed. This study is focused on 3D printing of TiNiHf particles into mesh structures for such purpose. The printing variables are examined based on the originating sources: 3D printer related, powder-related, and binder-related. Printing layer thickness and binder saturation level (binder to pore volume ratio for a given printing volume) are identified as the primary variables affecting the 3DP capability and systematically evaluated in order to achieve the highest 3D mesh structure integrity and minimum dimensional variation. 2. Experimental Design 2.1. TiNiHf powder In this study, gas-atomized Ti35Ni50Hf15 (mol%, abbreviated as TiNiHf) powder (Crucible Research Co., Pittsburgh, PA) of smaller than -635 mesh size was used for 3D mesh structure printing. The TiNiHf particle size distribution was characterized by laser light scattering (Horiba, LA-950, Irvine, CA). The measured particle size distribution is shown in Fig. 1. It can be observed that 100% particle sizes are below 20 μm and the mean particle size is about 5.50 μm. The particle size distribution is close to normal distribution.
The printed 3D mesh structures are loosely bound by the binder used and do not have high integrity. A small load cell has to be used in the integrity evaluation. Since the printed samples are brittle and experience compressive stress during the mesh structure-glass composite application, a Texture Analyzer test console equipped with a 5 kN load cell (Stable Micro Systems, Surrey, UK) was used for compression failure test. The console was set to record compressive force and the crosshead was lowered monotonically at a speed of 0.1 mm/min. The shape of the force vs. displacement curve was recorded and the peak force was determined from the curve as the 3D mesh structure breaking force. Breaking strength is calculated as the ratio of breaking force to the average cross section area of the three layers in the mesh structure horizontal direction. To evaluate the dimensional accuracy of individual wires, an optical microscope (ME300TZ-3P, Amscope, Chino, CA, USA) was used to measure the wire width from the top-down view. Each reported wire width was measured at ten different radial wire locations. 3. Results and Discussion 3.1. 3D printing variable evaluation As mentioned, 3DP process is affected by numerous variables. In order to obtain 3D printed structures with high integrity, small feature sizes, and accurate dimensions, each factor needs to be considered. By the correlation with the major aspects of 3DP, the variables can be classified into three categories: 3D printer-related variables, powder-related variables, and binder-related variables. These three groups of printing variables are listed in Tables 1, 2, and 3, respectively.
2.2. Mesh structure design A gradient 3D mesh structure was designed based on the application requirement of TiNiHf SMA mesh structure in
Fig. 2. Designed 3D mesh structure to be printed.
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Table 1 3D printer-related printing variables Variables
Definition
Effect
Range
Cap Cleaning Frequency Wipe Cleaning Frequency Full Prime Frequency Spreader Reverse Speed Spreader Traverse Speed
Number of printing cycles between cleaning of printhead cap Number of printing cycles between cleaning of the sponge wipe for printhead Number of printing cycles between fully refilling printhead with binder Travel speed of printbed after dispensing of binder
Cleanliness of printhead cap
1-5 cycles/cleaning 2 cycles
Cleanliness of printhead sponge wipe
1-5 cycles/cleaning 2 cycles
Consistency of binder drop volume
1-5 cycles/cleaning 1 cycle
Travel speed of printbed after drying
Dwell time of binder before drying; 0.5-5.0 mm/sec affect the dimensional tolerance of the meshes Smoothness of powder layer for next printing 0.5-5.0 mm/sec cycle; related to powder packing density and particle size
Tables 1–3 also list the range of the 3D printing variables and how they affect printed structure quality. Even though all the above variables affect the printed structure integrity and resolution, some of the printing variables can be pre-determined. First of all, 3DP process requires careful selection of particle size and size distribution. Suitable particle size range is dependent on the feature sizes to be printed, the printed mesh structure integrity desired, and the availability of specific powders. If the particle size is too large, it will not create strong enough mesh structures and the fine features desired. For example, if a 50 μm thick feature is desired, then each printing layer would only be one particle size thick for a 50 μm size powder and is not practically possible. However, other difficulties can arise if the particle size is too small. Small particles, due to their poor flowability, cannot be spread into thin and even layers. The spreader roller can press the particles into the previous layers in the printbed. Also, the particles should have a reasonable range of size distribution. The largest size should meet the integrity and feature size requirement and the smallest particle size should not affect powder flow. The normal size distribution used in this study is preferred for higher particle packing density in comparison to monosize distribution. Binder is another important variable to consider. It should have low viscosity and high stability during the 3DP process. If the binder viscosity is too high, it can easily clog the printhead nozzles, which have about 74 μm size. Also, the binder should be stable during the 3DP process with no significant chemical reaction. Otherwise, it can affect the wetting capability on the particle surfaces and subsequently the integrity of the mesh structures. In this study, an acrylic-based aqueous binder with
Value Adopted
2.0 mm/sec 2.0 mm/sec
17 wt% concentration was used for printing the SMA powder and is pre-determined based on the binder availability. Cap cleaning frequency, wipe cleaning frequency, full prime frequency, spreader reverse speed, and spreader traverse speed were determined by trial and error before the designed 3D mesh structures were printed and the values are given in Table 1: 2 cycles/cleaning for the cap, 2 cycles/cleaning for the wipe, 1 cycle for each full prime. The spreader reverse speed and the spreader traverse speed are 2.0 mm/s. Although the theoretical packing density should be ~64% for the smaller than –635 mesh powder based on loose random packing principle, actual packing density of the SMA powder in the printbed is far below that. The powder packing density is measured to be 35%. This is because the SMA powder in the printbed is not as densely packed as the bulk powder, likely due to the roller spreading effect and the loose packing of particles in the spread layer when the printing layer is thin. Also, feed powder to printing layer thickness ratio needs to be properly pre-set so that an even layer is spread for each printing cycle. For the SMA powder, this value is again identified by trial and error to be 1.5 (Table 2). For the variables listed in Table 3, drying time is determined to be sufficient at 60 s and drying power to be sufficient at 90% based on printing of simple wires with the SMA powder used. Binder drop volume is determined to be at 211.5 pico-liter by the 3D printer based on the powder packing density, printing layer thickness, and binder saturation level. Assprayed binder droplet size is directly determined by the printhead nozzle size and reflected by the binder drop volume. This value is the same for all the conditions studied, approximately 74 μm.
Table 2 Powder-related printing variables Variables
Definition
Effect
Particle Size and Size – Feature size, powder packing density Distribution Powder Packing Density Ratio of powder volume to total Binder saturation volume Printing Layer Thickness Thickness of each layer to be Dimensional tolerance of printed mesh structures printed Feed Powder to Layer Thickness Ratio
Ratio of feed powder to layer thickness
Range
Value Adopted
–
5.5 μm, normal distribution 30%
0-100%
N2 times of 20 μm particle size 35 μm 50 μm Should be high enough to spread new layers but not so high to cause 1.0-2.0 1.5 shifting of layers
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Table 3 Binder-related printing variables Variables
Definition
Effect
Range
Value Adopted
Binder Volume percent of binder printed vs. Higher saturation creates stronger bonds between particles; but too high a 40-200% Saturation Level pore volume among particles saturation creates lateral spreading and poor dimensional tolerance Drying Time and Power Binder drop volume
Drying time and power between Integrity and shape retention of 3D mesh structures successive layers Average binder drop volume per jet Integrity and dimensional resolution of mesh structures spray
The major variables that need to be further studied in the 3D printing of the TiNiHf powder are printing layer thickness and binder saturation level. These two parameters also play important roles in determining the integrity, dimensional accuracy, and minimal feature sizes that can be printed for the TiNiHf powder. In order to print the 3D mesh structures with good integrity and accurate shape, thin printing layer thickness should be used. However, it has to be greater than the SMA particle size. Since the average particle size is 5.5 μm and the maximum particle size is 17.0 μm, the printing layer thickness is chosen at 20, 35, and 50 μm in this study for further evaluation. Binder saturation level is closely related to powder packing density, binder-particle interaction, and thus binder distribution in the 3D printed meshes. Under the same printing layer thickness, higher saturation level results in higher volume of sprayed binder. However, there is no specific knowledge related to the TiNiHf powder. Since the binder saturation varying range is 40-200%, three levels were selected in this work: 55%, 110%, and 170%. It should be pointed out that the 3D printer allows greater than 100% binder saturation level (excessive binder amount than what is needed to fill the pores among the particles). After the 3D printing, the mesh structures are pre-cured for one hour at 100% heater power of the 3D printer before they are removed from the printbed. A subsequent cure of one hour at 170 °C is done in an oven to completely remove the water in the binder solution and fully bind the particles. 3.2. Mesh structure integrity evaluation
55% 110% 170% Time: 0-90 s 60 s, Power: 0-100% 90% Binder specific 211.5 pico-liter
better powder spreading. However, our aim is to print mesh structures with 200 μm wire width. Higher printing layer thickness will result in fewer printing layers. To achieve high 3D mesh integrity, the number of printing layers should be as high as possible. With this consideration, printing layer thickness of 50 μm is selected to evaluate three binder saturation levels: 55%, 110%, and 170%. Six samples were printed for each condition. However, 55% binder saturation level produced 3D mesh structure that is too fragile to be removed from the printbed. All the samples broke into pieces during transfer from the printbed to the sample container, as shown in Fig. 3. This means that 55% binder saturation level is too low to form the designed 3D mesh structure. When the binder saturation level was increased to 110%, the 3D mesh structures printed broke partially. The framework of the 3D structure was intact but some vertical connecting wires broke. The image of the printed 3D mesh structures at 110% binder saturation level is shown in Fig. 4(a). This means 110% binder saturation level produces improved mesh structures than the 55% binder saturation level. However, the vertical, roundshaped wires connecting the horizontal layers are still weak. When the binder saturation level was increased to 170%, all the samples were strong enough to be removed from the printbed as shown in Fig. 4(b). In order to quantitatively compare the integrity of the 3D mesh structures, three printing layer thicknesses, 20, 35, and 50 μm, are evaluated at 110% and 170% binder saturation levels. All the other printing parameters are kept the same as discussed in 3.1. From Fig. 5, it can be observed that at all the
For a given powder with fixed particle size and size distribution, higher printing layer thickness generally ensures
Fig. 3. 3D printed TiNiHf mesh structure with 50 μm printing layer thickness and 55% binder saturation level.
Fig. 4. 3D printed TiNiHf mesh structure with (a) 110% binder saturation, and (b) 170% binder saturation. The samples are printed with 50 μm printing layer thickness.
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170% binder saturation level are the desired condition for printing the highest integrity 3D mesh structures. 3.3. 3D printing dimensional accuracy
Fig. 5. Breaking strength of printed TiNiHf 3D mesh structures.
printing layer thicknesses, the 3D mesh structures printed using 170% binder saturation level have higher breaking strength than those of the mesh structures printed using 110% binder saturation level. The error bar represents the standard deviation of the breaking strength. Mesh structures with 170% binder saturation level have smaller breaking strength variation than those of the mesh structures printed using 110% binder saturation level. This means higher binder saturation level is beneficial for the studied 3D wire meshes and offers more binding function among the particles. Also, 170% binder saturation level offers more consistent 3D mesh structure integrity. The effect of the printing layer thickness on the integrity of the 3D mesh structures can also be analyzed from Fig. 5. For both 110% and 170% binder saturation levels, the 3D mesh structures printed with 35 μm printing layer thickness are stronger than those with 20 μm and 50 μm printing layer thicknesses. Also, the 3D mesh structure printed with 35 μm printing layer thickness and 170% binder saturation level shows the smallest breaking strength variation. Based on our observation, this can be explained from the lateral and vertical binder spreading differences. In general, the binder spreading distance in the horizontal direction is much longer than that in the vertical direction. This is because the binder drop has certain size when it reaches the powder bed and the spreading is in all horizontal directions. For the vertical spreading, the binder only progresses from the top to the bottom of the printing layer. The detailed lateral and vertical binder spreading rates will be detailed in future studies using 3D microscope. For the 20 μm printing layer thickness, the wire width is 10 times of the vertical spreading distance. The vertical direction will be saturated with the binder before the lateral binder spreading is complete. The poor lateral spreading causes low integrity 3D mesh structures. For the 35 μm printing layer thickness, the binder is able to complete the binder spreading in both the lateral and vertical directions within similar time span. This contributes to the highest breaking strength observed. For the 50 μm printing layer thickness, the wire width is only 4 times of the printing layer thickness. Binder spreading in the lateral direction will be complete before that in the vertical direction. As a result, the binder will likely have incomplete penetration in the vertical direction. Based on the breaking strength evaluation, it can be concluded that 35 μm printing layer thickness and
In addition to the 3D mesh structure integrity, the printed wire thickness and its deviation from the designed 200 μm width are other important factors to study. Fig. 6 shows the average wire width and the corresponding standard deviation under different printing conditions. All the printed wire width is larger than the designed 200 μm value. Actually, the dimensional deviation is ~50% or higher. While the dimensional deviation should be substantially less when the printed feature size increases, it is still important to be able to predict the printed wire width. Future effort will be devoted to narrow the dimensional gap between the designed and the printed structures. Under the same binder saturation level, 35 μm printing layer thickness yields the smallest dimensional deviation from the designed 200 μm width in comparison to the other two printing layer thicknesses. The wires printed with 20 μm printing layer thickness show the largest deviation from the designed 200 μm wire width. Also, the wire width printed with 35 μm printing layer thickness has the smallest standard deviation overall. This difference can be understood as follows. For the 20 μm printing layer thickness, the designed wire width is 10 times of the printing layer thickness. The binder penetrates quickly to the bottom of the layer, but the previous layer printed prevents the binder from further spreading. In the lateral direction, the binder spreads without such limitation. This results in larger wire width than desired. For the 50 μm printing layer thickness, the binder is able to spread in the vertical direction before being completely consumed. This results in lower wire width deviation. 35 μm printing layer thickness allows the binder to finish spreading in both directions at approximately the same time and offers the closest dimension to the designed wire width. From Fig. 6, it can be concluded that 35 μm printing layer thickness is preferred for the realization of good wire dimensional tolerance. Combined with Fig. 5, it confirms that 35 μm printing layer thickness is the desired 3D printing condition for the TiNiHf powder. As the binder saturation level increases at the same printing layer thickness, the wire width generally becomes thicker even
Fig. 6. Printed wire width measured by optical microscopy.
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though the difference is small. This is because the binder saturation level affects binder spreading distance. Higher binder saturation level leads to longer binder spreading distance. In general, 35 μm printing layer thickness shows minimal wire width at different binder saturation levels. Optical micrographs of the printed wire meshes under different conditions are shown in Figs. 7–9. It can be seen that under the same binder saturation level, the wire meshes printed with 20 μm printing layer thickness show more dimensional variation in comparison to those printed with 35 and 50 μm printing layer thicknesses. There are some large protruded areas from the 20 μm printing layer thickness wire. Even though the
Fig. 8. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 35 μm.
micrographs only show local areas, the general trend is the same for all the wires examined. 3.4. Optimal 3D mesh structure
Fig. 7. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 20 μm.
The binder saturation level and the printing layer thickness can be understood schematically as shown in Fig. 10. During 3D printing, binder is jetted from the printhead with a certain drop size. However, the total binder amount jetted per layer is proportional to the total volume of the powder to be printed. After it reaches the printbed, the binder spreads into the interstitial sites of the particles in vertical and lateral directions.
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Fig. 11. 3D mesh structure printed with 35 μm printing layer thickness and 170% binder saturation level.
Fig. 9. Optical micrographs of 3D printed TiNiHf square wire with binder saturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thickness is 50 μm.
When the powder layer is too thin, such as the case for the 20 μm printing layer thickness, there is less than desired powder thickness to accommodate the binder at the studied saturation levels since the powder layer beneath the current printing layer is saturated with the binder already from the prior printing cycle. This causes excessive lateral flow of the binder and larger and uneven wire width. When the printing layer thickness is too large, such as 50 μm, uneven lateral spreading becomes less. However, there is more binder jetted into the printbed because of the larger powder volume per layer. The binder might have less than sufficient time to diffuse vertically to the previously printed layer while the binder drop is large enough for lateral spreading. As a result, larger wire width still results, with much less wire width variation. At the optimal printing layer thickness, such as 35 μm, binder vertical spreading and lateral spreading proceed and finish at approximately the same time. This very desirably leads to smallest wire width and the smallest wire width variation. Since the binder is distributed all within the wire mesh structure, it also leads to stronger 3D wire mesh structures. The actual binder spreading process requires in-situ observation, which will be discussed in future work. Based on the above observations and understanding, the 3D mesh structures are printed as shown in Fig. 11 under the optimal conditions: 35 μm printing layer thickness, 170% binder saturation. It reproduces the designed 3D mesh structures in Fig. 2 with high accuracy and integrity. Clearly, 3D printing is capable of producing near net-shape intricate mesh structures when the printing condition is optimized. There are no other techniques available to produce mesh structures in such complexity and resolution.
Fig. 10. Different printing layer thickness and binder spreading cases.
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4. Conclusions In this work, 3DP variables are systematically analyzed in order to print TiNiHf 3D mesh structures. Two major 3D printing variables, printing layer thickness and binder saturation level, are evaluated. At the same printing layer thickness, breaking strength increases with the binder saturation level up to 170%. At the same binder saturation level, 35 μm printing layer thickness yields 3D mesh structures with the highest integrity and the lowest dimensional deviation. 35 μm printing layer thickness and 170% binder saturation level are the optimal 3D printing parameters for the TiNiHf 3D mesh structures. This study demonstrates the unique capabilities of the 3DP technique in printing intricate and 300 μm wire width structures. Acknowledgment This material is based upon work supported by the Department of Energy under Award Number DE-FC07-06ID14739. References [1] J. Stampfl, H. Fouad, S. Seidler, R. Liska, F. Schwager, A. Woesz, P. Fratzl, Int. J. Mater. Prod Technol. 21 (2004) 285.
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