Ceramic Parts Fabricated by Ceramic Laser Fusion

Transcription

1 Materials Transactions, Vol. 45, No. 8 (2004) pp to 2751 #2004 The Japan Institute of Metals Ceramic Parts Fabricated by Ceramic Laser Fusion Hwa-Hsing Tang 1 and Hsiao-Chuan Yen 2; * 1 Department of Mechanical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R.O. China 2 College of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R.O. China Rapid prototyping technology offers an ideal method for manufacturing ceramic workpieces. Not only can it produce parts without tooling, it poses no limitation in shape and complexity of the parts to be fabricated. This article aims to study the manufacturing technologies of a new rapid prototyping process named Ceramic Laser Fusion. A self-developed rapid prototyping system is employed to fabricate complex ceramic parts using slurry composed of silica powder as basic material, inorganic fire-resistant binder as additive, and water as solvent. Threedimensional parts can be made by repeatedly executing the single-layer generating cycle which includes slurry feeding, layer paving, layer drying, remnants cleaning, platform descending and laser scanning. The laser fusion experiment shows that hollow ceramic pump impeller, complicated ceramic fan and turbine blades as well as ceramic molds for metal casting can be made at a production rate of 32 cm 3 /h. Ceramic Laser Fusion is capable of paving very thin layers; green portion provides solid-state support to prevent deformation of workpieces, and can be removed by water or sodium hydroxide solvent. Further advances in stacking thinner layers will lead to the production of parts with fine details and higher precision. (Received March 15, 2004; Accepted June 18, 2004) Keywords: ceramic laser fusion, rapid prototyping, green parts, ceramic parts 1. Introduction Conventional ceramic manufacturing processes involve the technology of powder metallurgy relying on making the green portion with a mold, and then sintering it to be a ceramic part. However, when producing components of complex shapes, such as fan blade and turbine blade, such processes are not only costly but also time-consuming. This poses problems for design modification and product improvement of ceramic parts. Rapid prototyping (RP) technology, a kind of layer manufacturing process, offers an ideal method for manufacturing ceramic workpieces because it can produce parts without tooling and it poses no limitation in shape and complexity of the parts to be fabricated. Polymers and metals can be processed to form complex parts by welding or NC machining for low-volume productions without tooling. In particular, NC machining integrated with well-developed CAD/CAM for producing polymer parts and metal parts normally can achieve compatible, or even better, results compared with RP technology. However, when considering the technical difficulty of cutting and welding ceramic parts, RP technology is more competitive than NC cutting and welding. In the last decade, many researchers have investigated the application of RP technologies to forming ceramic green parts. Most of the developed processes can be called conventional indirect ceramic RP processes. Brady 1 3) proposed a process that employed the Stereo Lithography (SL) method. Slurry comprising a mixture of ceramic powder and photo-curable resin is used as the raw material. The ceramic resin is first exposed under direct ultraviolet light to solidify the liquid-state photo-curable resin. The solidified resin then bonds the ceramic powder to form ceramic green parts. Another technology involving Selective Laser Sintering (SLS) was developed by Subramanian et al. 4) Alumina powder is coated with resin, which is subsequently melted by *Corresponding author, hcyen@ntut.edu.tw laser. The resin acts as a bonding agent of the ceramic powder for forming a ceramic green part. The above two processes and other RP processes, such as Fused Deposition Manufacturing and Laminated Object Manufacturing, are related to bonding ceramic powder with an organic binder, 5 9) which must be burnt out in a furnace during post sintering. As a result, the emitted hazardous gases will pollute the environment and pose a health threat to humans. Furthermore, because the workpiece is subjected to additional shrinkage during post sintering, distortion may inevitably appear if the workpiece is complicated. 10) Direct sintering of ceramic powders such as aluminum oxide, aluminum silicate or zirconium silicate (ZrSiO 4 ) has been conducted by Wirtz. 11) The particle is heated by a carbon dioxide (CO 2 ) laser to a temperature above its melting point. Without using organic binder as one of the ingredients, no hazardous gases are released during the processing. Another direct melting process named Ceramic Laser Fusion (CLF) has been developed by Tang. 12,13) The raw material is slurry made up of ceramic powder, inorganic binder and water, which is dried by infrared radiation, and then fused by a laser. This article aims to study the manufacturing technology of CLF, and then to employ it to fabricate complex ceramic parts. 2. Experiment A series of single-layer and multi-layer experiments were conducted by a self-developed RP machine to determine the proper laser scanning parameters for making ceramic parts. Structural ceramic parts and ceramic shell molds for metal casting were made, and the manufacturing time required when using the CLF process was measured to evaluate the production rate. 2.1 Experimental setup Figure 1 shows the complete CLF RP system, which can

2 Ceramic Parts Fabricated by Ceramic Laser Fusion 2745 (a) Fig. 3 Fabrication procedures of the CLF system. distance, this laser scanning system can acquire smaller spot size, approximately 0.3 mm, to scan fine details. (b) Fig. 1 The complete CLF system. fulfill the requirements for fabricating three-dimensional ceramic components. A commercial laser engraving machine is modified by integrating the existing scanning system with the self-developed paving system and control system. The paving system serves to make thin ceramic green layers, while the laser scanning system selectively scans the specified pattern of each layer, which will be melted to become a single ceramic layer. The control system is in charge of sequence controlling and process monitoring, so that a three-dimensional ceramic component can be produced by repeated stacking ceramic layers. 14) As shown in Fig. 2, the paving system comprises a slurry feeding device, a paving device, a cleaning device, a drying device and an elevating device. The laser scanning system, which consists of a 50 W CO 2 laser and a plotter, has the advantages of fixed laser spot size and wide scanning area. Having a 50 mm focal Fig. 2 Schematic of the paving system. 2.2 Experimental procedures Fabrication procedures Figure 3 displays flow diagram of the CLF process. The fabrication steps include (1) slurry blending, (2) layer paving, (3) drying, (4) platform descending, (5) laser scanning, (6) completion of paving and scanning, (7) green portion removal, and (8) completion of workpiece. Silica powder is the basic raw ceramic material used in this process; it is mixed with an inorganic binder and water, which acts as a solvent to promote uniform mixing. The above-mentioned materials are placed in a blender at specified weight ratios (64% silica powder with 5 mm mean diameter, 3% clay, 3% silica sol, and 30% water) and mixed thoroughly to yield homogeneous slurry, which has to be kept in a damp state for producing the green layer. With the well-mixed slurry, three-dimensional parts can be fabricated by repeated executing the single-layer generating cycle, including slurry feeding, layer paving, layer drying, remnants cleaning, platform descending and laser scanning. Detailed description of the automatic layer generating cycle is as follows.. Slurry feeding and paving: The drying device and the paving device move to the slurry feeding zone one after another; in the meantime, the feeding device is switched on to supply a certain quantity of slurry into the paving device. After feeding, a gate at the bottom of the paving device opens. Then, the paving device moves backwards to pave a thin layer on the working platform, and stops at the cleaning zone located at one end of the paving route.. Cleaning and drying: After paving, cleaning and drying will proceed simultaneously. Upon completion of the drying process, the drying device returns to its original position near the paving device.. Platform descending: The platform descends for a distance of one layer thickness.. Laser scanning: The selected portion of the green layer is scanned by laser with proper parameters. Once scanning is completed, the laser head moves to the origin of the plotter, and the entire mechanism returns to the initial state for the next cycle. After executing the single-layer generating cycle repeatedly, the fabricated three-dimensional part is surrounded by the green portion. The green portion is first roughly removed

3 2746 H.-H. Tang and H.-C. Yen by pressure water, while the fine remains are later dissolved completely in Sodium Hydroxide (NaOH) solvent. Dissolution of the remaining green portion can be enhanced by applying a vibration induced by an ultrasonic machine Laser fusion experiment The key point of RP is to connect the green layers with each other; in other words, the laser scanning parameters should be able to bond the newly paved green layer and the previously scanned layer together. A series of laser fusion experiments for the single-layer generating was conducted to examine the effect of laser scanning parameters on the depth within which the property of the layer will be transformed, also known as the property transformation depth. The laser scanning parameters were set as follows: laser power was 50 W, scanning pitch was 0.2 mm, and scanning speed was from 20 up to 100 mm/s. The slurry was made up of materials at the specified weight ratios as mentioned in section A 1 mm thickness layer was paved and dried, and a 30 mm 30 mm square zone was then scanned by the laser. As shown in Fig. 4, the surface of the scanned area sank to a certain depth because of shrinkage; a definite property transformation depth, which equaled to fusion depth plus sintering depth, was formed simultaneously. The sunk depth, which is the difference between the paving layer surface and the sunk surface, was measured by a coordinating measuring machine (CMM). The property transformation depth is denoted by how thick the remaining layer is after the green portion is completely dissolved in water or NaOH solvent. Such thickness varies with different solvents used for the removal of the green portion. NaOH solvent was employed in this study because of its better dissolution capability. Hence, in this experiment, the remaining thickness of the laser-treated layer after a 90- min ultrasonic vibration in NaOH solvent is denoted the property transformation depth, which was observed by an optical stereo microscope and measured by a micro measurement instrument. After obtaining the relation between laser scanning speed and property transformation depth, a proper scanning speed was selected to fabricate multiple layers and to ensure that individual layers could be connected by laser melting. To investigate the deformation of workpiece during the CLF process, a T-shaped specimen was made. The deformation of its overhanging structure was measured by a CMM Fabricating structural ceramic parts To verify the feature of solid-state support, hollow parts were fabricated, and other complex fan blades and turbine blades were also manufactured to reveal the superiority of CLF in fabrication of ceramic parts Fabricating ceramic shell mold Because ceramic products made by CLF can withstand high temperature, this technique is suitable to make the ceramic shell mold for casting metal parts. According to the required accuracy, the shell of a threedimensional part designed by a CAD software PRO/ ENGINEER could be sliced by executing the slicing function of PRO/ENGINEER. The cross-sectional information thus obtained, including scanning route of the laser, was transformed to a scanning path file used on the CLF RP machine to manufacture the ceramic shell mold. The fabrication of the ceramic shell mold was completed only after the green portion in the cavity and that surrounding the manufactured part were removed by a post treatment. First, the finished block, which comprised the workpiece and the green portion, was taken from the working platform; then most of the green portion was removed by pressure water. To clean the remaining green portion from the ceramic shell mold, the finished part was placed in NaOH solvent and vibrated by an ultrasonic machine for 90 min Measuring layer fabrication time Production rate can reveal the efficiency of the CLF process. The fabrication time can be divided into three periods: pre-treatment time, layering time and post-treatment time. Normally, layering takes more time than the other periods, thus this study focuses only on the CLF layering time. The time needed for layering depends on the number of layers, paving time, and laser scanning time. The paving time is fixed and is the total time required for slurry feeding, layer paving, drying, and platform descending. The number of layers and laser scanning time vary depending on the dimensions of the part to be fabricated. The greater the dimension of the part, the more the number of layers, and the bigger the scanning zone, the longer the scanning time will be. In this study, a part with fixed dimension (30 mm 30 mm 30 mm) and fixed manufacturing parameters (scanning pitch, scanning speed, and slicing thickness) was employed to measure the fixed and the variable time consumed for generating each layer and to determine the production rate of fabricating such part. 3. Results and Discussion Fig. 4 Schematic of the scanned surface. 3.1 Effect of laser scanning parameter on property transformation depth Figures 5 and 6 reveal the sunk depth, property transformation depth and shrinkage at different scanning speeds, where shrinkage was calculated by the following equation: Sðvol%Þ ¼ðD s =D pi Þ100 ð1þ In which S: shrinkage (vol%) D s : sunk depth (mm) D pi : actual thickness of the ith paved layer (mm) In general, the slower the scanning speeds, the greater the property transformation depth will be. For a specific property

4 Ceramic Parts Fabricated by Ceramic Laser Fusion 2747 Depth, Dpt, Ds/mm Scanning Speed, Vs/ mm/s Property Transformation Depth, Dpt/ mm Sunk Depth, Ds/ mm Fig. 5 The relation between scanning speed, property transformation depth and sunk depth. Shrinkage, S/ (%) Fig Scanning speed, Vs/ mm/s The relation between scanning speed and shrinkage. transformation depth, its corresponding scanning speed can be obtained from Fig. 5. During the multi-layer generating, the property transformation depth resulted by laser scanning should be larger than the actual thickness of the paved layer to ensure that two successive layers are connected. This will be discussed in detail in the following sections. 3.2 Calculation of proper property transformation depth for multi-layer generating The plotted line of actual thickness of the paved layers in Fig. 7 was based on 20% shrinkage. A 0.03 mm sunk depth caused by shrinkage during the first layer paving and a 0.15 mm distance caused by the descending of the working platform resulted in a total actual thickness of 0.18 mm for the second paved layer. The shrinkage of the green portion caused by laser melting in each layer can be compensated by the increase in the actual thickness of the green portion in successive layering. After the sixth paving, the actual thickness of the paved layer approached the ultimate value of mm. This phenomenon could be presented by the following equation. 15) D Pi ¼½D P =ð1 SÞŠ ½1 S expðiþš In which D P : platform descending distance (mm) D Pi : actual thickness of the ith paved layer (mm) S: shrinkage (vol%) ð2þ Aactual thickness of paved layer, Dpi/ mm The number of paved layer, i Fig. 7 Actual thickness of the initial paved layers. Fig. 8 Schematic of the overlapping. i: the number of the paved layer Figure 8 illustrates the concept of overlapping for the multi-layer generating. The overlapping between two successive layers can be derived by following equations. Equation (3) shows that shrinkage and the actual thickness of the paved layer have no any effect on overlapping. Vðvol%Þ ¼½ðD pt þ D s D pi Þ=D pt Š100 ¼f½D pt þ D s ðd p þ D s ÞŠ=D pt g100 ¼½ðD pt D p Þ=D pt Š100 ¼½1 ðd p =D pt ÞŠ 100 ð3þ In which D pt : property transformation depth (mm) D p : descending distance of platform (or slicing layer thickness) (mm) D Pi : actual thickness of the ith paved layer (mm) D s : sunk depth (mm) V: overlapping (vol%) Equation (3) can be transformed into D pt ¼ D p =ð1 VÞ ð4þ Given 20% overlapping, a platform descending distance of platform is 0.15 mm, according to eq. (4), the calculated property transformation depth is mm, which can be achieved by a scanning speed of 80 mm/s, as shown in Fig. 5. This parameter combination has been selected for making all parts in the following experiments.

5 2748 H.-H. Tang and H.-C. Yen Fig. 9 T-shaped workpiece. 3.3 Deformation of a T-shaped specimen Figure 9 shows a T-shaped workpiece made by CLF, in which the overhanging structure revealed no deformation after removal of the green portion. The hardened green portion of the CLF process offers solid-state support for overhanging structures of the part to prevent deformation in both upward and downward directions. In contrast, although inherent powder support in the SLS can prevent overhanging structures from downward deformation, it cannot prohibit upward deformation. An experiment conducted by Meiners 15) showed that the upper overhanging portion of a T-shaped part made by an SLS machine deformed upwards after laser melting of metal powder in the SLS process. Similarly, the ceramic resin slurry underneath the overhanging structure undergoing SL process cannot prevent either upward or downward deformation. Thus, constructing solid-state supports is necessary for SL to prevent deformation. Fig. 11 Fan blade made of ceramic. 3.4 Parts fabrication Figure 10 shows two kinds of ceramic pump impellers made by the CLF process. The bigger one is of 30 mm diameter and 8 mm height, while the other is of 15 mm diameter and 4 mm height. During fabrication, overhanging structures could be easily formed because the green portion provided solid-state support. The completed parts were placed in an ultrasonic machine containing NaOH solvent. Ceramic grains in the unscanned hollow portion then dropped gradually to the bottom of the container without leaving any marks at the interface between the part and the support. Manufacturing of ceramic fan blade and turbine blade Fig. 12 Turbine blade made of ceramic. shown in Fig. 11 and Fig. 12, respectively verifies that the CLF process is capable of fabricating parts with complex shapes. The thickness of the ceramic fan blade in Fig. 11 is 1 mm, while that of the turbine blade in Fig. 12 is approximately 0.8 mm. Fig. 10 Pump impellers made of ceramic. 3.5 Ceramic shell mold for metal casting A ceramic shell mold with 2 mm shell thickness was made for casting pure aluminum (melting point 658 C) parts. For convenient removal of the green portion, the mold was separated into the upper and the lower mold. The post-treated ceramic shell mold made by the CLF process is shown in Fig. 13. Before casting, the upper mold and the lower mold had to be bound by ceramic slurry. The completed mold was joined to a hand-made sprue, and then preheated to 600 C. Melted aluminum (730 C) was poured into the mold. After cooling and solidifying the cast aluminum, the ceramic shell mold was broken, followed by the sprue cutting and some proper finishing in accordance with the requirement of its application. Figure 14 shows a completed pure aluminum casting part.

6 Ceramic Parts Fabricated by Ceramic Laser Fusion 2749 Table 1 Time taken for single layer paved by the Ceramic Laser Fusion process. Feeding & Platform Fixed Drying paving descending time Sub-Total Total 35 s 60 s 5 s 100 s Variable Laser scanning 56 s 156 time 56 s s Fig. 13 Ceramic shell mold made by the CLF process. single green layer of 0.15 mm thick. The paving time included 35 s for paving, 60 s for drying, and 5 s for platform descending, while laser scanning for an area of 30 mm 30 mm took another 56 s. Hence, the total time taken for making a single layer was 156 s. Because each layer was 0.15 mm thick, 200 layers were needed for fabricating a cubic part of 30 mm 30 mm 30 mm. The total generating time of this cube was s (520 minutes or 8 hours and 40 minutes), equivalent to a production rate of 0.89 mm 3 /s (32 cm 3 /h). Fig. 14 Pure aluminum casting part. 3.6 Time taken for CLF layering Table 1 illustrates the details of the single-layer generating cycle time. The CLF paving system took 100 s to pave a 3.7 Comparison of CLF with other RP processes Table 2 displays a comparison of the CLF with three RP processes for production of ceramic parts. The main feature of the CLF process is its ability to pave very thin layers because it uses slurry as raw material and it offers inherent solid-state support. This feature distinguishes it from the other three processes. SL uses slurry type material and builds extra solid-state support structures; indirect SLS and direct SLS use powder type material and have inherent powder support. Upward deformation of the scanned portion must be less than the thickness of the paved layer; otherwise, the scraper will destroy the upwardly deformed portions of the workpiece during the paving. As mentioned above, solid-state support can effectively prevent deformation, and is thus helpful for paving a layer with minimum thickness. Similar to a flat plate moving on fluid, the shear force is inversely with the fluid thickness, 16) the smaller the layer thickness, the Table 2 Comparison of CLF with other indirect and direct rapid prototyping processes for production of ceramic parts. Process Stereo Indirect selective Direct selective Ceramic laser lithography 3;10Þ laser sintering 4;10Þ laser sintering 11;18Þ fusion 12;13Þ Solidifying after Polymerization of melting of Solidifying after Solidifying after Principle photo resin polymer-coated melting of ceramic melting of + Post sintering ceramic powder powder ceramic powder + Post sintering Material Slurry Powder Powder Slurry Support Extra solid Inherent powder Inherent powder Inherent solid support structure support support support Support 3D design + construction Physical building No No Drying By immersion in Support removal By force By air blowing By air blowing water or sodium hydroxide solvent Grain size <5 mm >20 mm >20 mm 1500:3 mm Post sintering Yes Yes No No

7 2750 H.-H. Tang and H.-C. Yen bigger the paving force will be. In the meantime, the scanned portion on the top layer of the workpiece will be subjected to a shear force equal to paving force. Solid-state support with enough strength can resist this force. As shown in Table 2, both SL and CLF have solid-state support structures, and are thus capable of paving very thin layer; in contrast, commercial SLS system offers no solid-state support. Even if the SLS system constructs solid-state support structures to prevent upward deformation and to resist a greater paving force during the thin paving process, it can hardly pave a layer thinner than 20 mm, because its layer thickness is limited by the grain size. Conventional SLS process uses powder type material. Huge amount of small grains increases agglomeration, making it impossible to pave a uniform surface automatically. 17) According to Levy et al., 18) commercialized SLS metal powder systems use powder with a grain size up 20 mm. EOS GmbH 19) and Wirtz 11) reported a minimum layer thickness paved by SLS of 20 mm and 50 mm, respectively. The ceramic grain size of slurry used in the CLF process can be of mm or mm scale, even nm scale. Decreasing the grain size to minimize layer thickness is possible; thus reducing the step effect of the fabricated parts can be achieved. The commercial machines employing similar techniques such as tape casting can have a paving layer as thin as 5 mm; 20) accordingly, CLF should be capable of paving a layer of 5 mm thick. Consequently, the SL employs solid-state supports, 10) which can be easily removed by a small force, under the overhanging portions of the part. However, it increases the CAD complexity, and broken marks will be left on the surface of the part upon removal of the supports. Furthermore, it is often difficult to remove supports on complex parts. In contrast, powder supports on the SLS parts do not need special constructions and can be easily removed. Similar to the SLS, the CLF s hardened green portion offers a complete support for the workpiece, needs no special construction, and can be easily dissolved in water or NaOH solvent. Considering the aspects of construction and removal of supports, CLF is superior to SL, as evidenced by the parts fabricated (Fig. 10 Fig. 13). The only shortcoming of CLF is that the extra drying procedure for support hardening takes a lot of time. As seen in the experimental result illustrated in section 3.6, the drying time was 60 s, which amounted to 40% of the processing time. How to reduce drying time is therefore important for commercial application of the CLF process. Compared with other conventional indirect ceramic RP processes, the CLF has the potential to fabricate more precise ceramic parts. The CLF fabricates ceramic parts directly; the green layers are heated to a temperature above its melting point by laser, and then connected together; as shown in Fig. 7, the shrinkage can be compensated by successive paving. There is no additional shrinkage and deformation problem because this process requires no post-sintering treatment. As a result, using this process to make a ceramic part, especially a complex one, can achieve greater precision than conventional indirect RP processes. Furthermore, because CLF offers solid-state support to prevent curling phenomenon caused by laser scanning, and is capable of paving ultra thin layers; it can fabricate parts with finer details and smoother surface than many existing RP processes. 3.8 Perspectives of CLF Capable of precision manufacturing, the CLF is useful for industrial application such as metal precision casting. The feasibility of CLF for manufacturing ceramic shell molds was shown in section 3.5. Parts shown in this paper were made from 0.15 mm layers; nevertheless, if they could be made from mm or 0.01 mm layers, we could obtain very fine contours. Definitely, the interior surface roughness of the ceramic shell mold for casting determines the roughness of the part; and it is very difficult for the interior portion to be post-treated. Our future work aims to develop methods and equipment that can automatically pave thinner layers and manufacture parts with finer interior surface roughness. 4. Conclusion The features of the CLF process are as follows: (1) the raw material used is slurry, which can be paved to form very thin layers, (2) dried and hardened green layers provide solid-state support, which can effectively prevent deformation of parts, and (3) the green portion can be dissolved by water or NaOH solvent. The CLF process can be employed to manufacture ceramic parts of very complex shapes such as turbine blade and ceramic shell mold for obtaining metal parts rapidly by normal casting process. Fabricated samples have verified that the solid-state supports can hold up the hollow or overhanging structure. Results have shown that the production rate achieved is around 32 cm 3 /h. To achieve commercialization of the CLF process, future efforts should be directed toward paving thinner layers, making parts with smoother surface and accelerating the drying speed to increase the production rate. Acknowledgements The authors would like to acknowledge Wen-Hsiang Lin for building the workpieces. This work was supported by the National Science Council of ROC under project no. NSC E REFERENCES 1) G. A. Brady, T. M. Chu and J. W. Halloran: Proc. The 7th Solid Freeform Fabrication Symposium, ed. by D. L. Bourell, (The U. of Texas at Austin, Texas, 1996) pp ) G. A. Brady and J. W. 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