1. Introduction
Ceramics have high strength and modulus, excellent abrasion and chemical resistance, and particularly outstanding thermal resistance1,2). They are mainly applied to parts used in extreme environments such as severe friction or high stress load conditions at high temperatures. They can be applied to structures and parts in aerospace, automobile and energy fields that require high working temperatures such as gas turbines, engines, batteries, and heat exchangers3,4). Applying ceramics to these applications can improve efficiency by increasing the operating temperature and reducing losses in the system5).
Sintering, a traditional method of manufacturing ceramics, is performed in the following order: 1) powder preparation (granulation), 2) compression molding, 3) machining of green body, 4) sintering, 5) post annealing and finishing, etc. (Fig. 1). Powder preparation refers to a process of granulating to facilitate densification by adding additives such as a binder and a lubricant. After that, molding is performed by compressing the powder to produce a green body. Since shaping becomes difficult after sintering due to increased strength, the green body is machined primarily to form the desired shape beforehand. Such machining work requires consideration of the shrinkage that occurs in the subsequent sintering process, and more detailed features are made in the finishing process after sintering. Sintering, which is the main step, is a process of producing a densified sample while decomposing organic material, facilitating grain growth and removing pores through spontaneous reactions using variables such as temperature, time, pressure, and atmosphere. Currently, the process of manufacturing a commercial ceramic involves many steps.
In order to improve the quality of ceramics and simplify the complex process, additive manufacturing using a laser has recently been studied. The additive manufacturing process uses a laser heat source and powder, and the manufacturing takes place layer by layer as the head moves to form a designed 3D structure6). Since the commercial sintering process is a process performed in a chamber, there are restrictions in place and size of parts, whereas additive manufacturing can be performed freely in a space where robots can move and be utilized for maintenance and repair of parts. Additive manufacturing can be divided into two groups according to the configuration of the process: indirect additive manufacturing (I-AM) that integrates the powder preparation, compression molding and green body machining steps; and direct additive manufacturing (D-AM) that integrates the processing and sintering steps as well (Fig. 1)7). The former, I-AM, includes processes such as fused deposition modeling (FDM), stereolithography (SLA), direct inkjet printing (DIP), layer-wise slurry deposition (LSD), and laminated object manufacturing (LOM). In the I-AM method, after manufacturing a 3D-molded body with a binder, densification is performed through a commercial sintering process. On the contrary, D-AM is classified as directed energy deposition (DED) by ASTM, and laser engineered net shaping (LENS) developed by Optomec is a well-known example. Depending on the company, it is also used in various names such as direct metal deposition (DMD), 3D laser cladding, and laser based metal deposition (LBMD). Nonetheless, D-AM forms a melt pool using a high- power laser heat source without a binder and coaxially sprays the powder to produce a densified deposition at once. With no separate sintering process required, parts can be manufactured quickly through fewer steps using D-AM.
However, since ceramic materials are sensitive to thermal shock and have low fracture toughness, the level of technical difficulty for applying to additive manufacturing is higher than that of metal or polymer materials. With increasing demand of DED in manufacturing of parts and the rapid development of related technology, studies are being conducted to apply the DED method to oxide ceramics overseas. In South Korea, there have been reports on additive manufacturing of metal or metal matrix composites using the DED method8-10), but there have been no domestic studies on AM only using ceramic powders reported so far, Therefore, this study aims to summarize the issues that need to be addressed for domestic researchers in ceramic manufacturing by the DED method.
2. Results & Discussion
2.1 Ceramics Manufactured by DED: Effects of Process Parameters
The DED process system delivers the laser beam and powder simultaneously as shown in Fig. 2. The laser beam creates a melt pool on the surface of the base material, and the powder is fed into the melt pool. The process parameters include powder feeding speed, laser power, scan speed and hatch spacing, and such parameters affect the shape, thermal profile and microstructure of the deposited layer. Al2O3 is a representative oxide ceramic. Since it has a melting point of 2072 ℃, the temperature of the melt pool is higher than that of general metal materials. In addition, the thermal conductivity of Al2O3 is 6 W/m·K at 1000 ℃, whereas that of metal materials tends to fall in the range of 40 to 500 W/m·K at the same temperature11). Moreover, Al2O3 has a higher laser absorption rate than general metals with a laser absorption rate of 0.7712). Due to their low thermal conductivity and high laser absorption, oxide ceramics can be melted and deposited with a relatively lower laser power than metals. However, ceramics typically have significantly lower tensile stress than compressive stress. When shrinkage occurs in the peripheral portion solidifying faster a tensile stress is applied to the central portion solidifying more slowly, resulting in cracking. The studies conducted on the effects of the process parameters for additive manufacturing of shapes and the effects of the process parameters for reducing the cracks caused by stress are as follows.
Fig. 3 shows two samples produced by additive manufacturing using an oxide ceramic material, MgAl2O4, under the same conditions (scan speed, total heat input, hatch spacing, and powder feed rate) except for laser power. A wall-shaped sample was fabricated by continuously moving the nozzle left and right alternately as shown in the deposition pathway in Fig. 2, using a 6- axis robot. The longer the interlayer idle time is, the more energy is consumed for reheating, thereby increasing the total energy applied to the deposited layer. This in turn increases residual stress and deformation, resulting in cracking in the ceramic material. Therefore, performing the process continuously using residual heat without idle time helps to reduce cracking.
Fig. 3
Sample appearance Images fabricated with a constant power of 700 W (a)-(b); and with a modulated power pattern (c)-(d)
The samples in Fig. 3 (a) and (b) are fabricated with a constant laser power of 700 W; the samples in Fig. 3 (c) and (d) are fabricated with a modulated power pattern while maintaining the same total heat input. The power pattern is shown in Fig. 4, and the modulated power exerted a higher power to the lower layer in consideration of the thermal conductivity of the ceramic material. At the lower layer, heat is relatively easily released to the base material, but once a deposited layer is formed, the passage through which heat can be transferred is limited. Due to the low thermal conductivity of the ceramic, residual heat remains in the previous layer. Therefore, under constant speed and power conditions, the width of the wall widened toward the upper layer as shown in Fig. 3 (b). This means that an increase in temperature of the melt pool reduced the viscosity and expanded the size of the melt pool. Since the feeding of the powder was constant, the height of one layer decreased as the width of the melt pool expanded. Based on the design, a wall with a height of 20 mm was supposed to be formed by regularly stacking 40 layers with a hatch interval of 0.5 mm. However, as it proceeded up to the upper layer, the width of the melt pool increased due to accumulation of residual heat, unable to ensure a sufficient height. As the distance from the tip of the nozzle to the surface of the deposited layer increases, melting and deposition of the powder became more difficult (Fig. 3(a) to (b)). When using a modulated pattern in which the power is reduced in the upper layer while maintaining the total heat input, it was possible to carry out deposition according to the designed shape as shown in Fig. 3(d). Therefore, it is necessary to control the process parameters in consideration of the thermal conductivity of the ceramic material and the shape of deposition.
The study by Niu manufactured a sample with almost no cracks with a layer thickness of 0.8 mm or more by depositing the Al2O3 material using the DED method13). Table 1 shows the DED process conditions and the number of cracks observed on the side of each sample. As the layer thickness increases, the number of required layers and the total energy input decrease. Cracking can be suppressed primarily by reducing thermal stress. Suppressing the diffusion and aggregation of impurities can reduce cracking by improving the solidification rate of the melt pool. In other literature, cracking decreased with increasing scan speed, and wall structures without cracks were fabricated at scan speeds of 700 mm/min or higher. As the scan speed increased, the thermal stress decreased, and the fracture strength increased14).
Table 1
Various process parameter conditions and the number of surface cracks in each sample13)
The study by Yu analyzed the effect of scan patterns on the deformation and quality of parts15). The raster pattern and offset from inside to outside patterns in Fig. 5 (a) and (b) showed larger thermal gradient and thermal deformation than the offset from outside to inside and fractal patterns in Fig 5 (c) and (d). Research on deposition patterns to reduce thermal gradient and thermal deformation is also required.
Fig. 5
Single layer patterns for multilayer additive manufacturing. (a) Raster, (b) offset from inside to outside, (c) offset from outside to inside, and (d) fractal15)
The process parameters of DED also affect the microstructure16). The microstructure formed by solidification of the melt pool is quite different from the microstructure formed by the general sintering process17,18). As the melt pool solidifies, heat escapes in the direction opposite to the scan direction within the layer, and most of the heat is lost by heat conduction toward the base metal19). According to the solidification theory, the grains grow in the opposite direction to the heat flow. During the solidification process, small grains disappear and large grains grow resulting in the final microstructure with the distribution of columnar grains stretched in the height direction of deposition. The size of the columnar grains is tens of μm in the width direction and several hundred μm in the height direction, which is larger than that in the structure made of a metal material manufactured by the DED method20,21). According to the primary dendrite spacing model proposed by W. Kurz, the grain size is proportional to G-0.5 (where, G is the temperature gradient)22). Since ceramics tend to accumulate significant amount of heat due to its lower thermal conductivity than metals, the temperature gradient appears relatively smaller than that of metals at the solid-liquid interface, and larger grains are formed.
Thus, various process parameters in DED affect not only the macroscopic shape, but also the defects such as cracking and porosity, the microstructures, and the mechanical properties. Therefore, it is important to control and optimize the process parameters, and real-time monitoring technology is required for this23). Although the makers of additive manufacturing equipment provide the modules for real-time monitoring, but in many cases, the modules only store the generated data in real-time and do not immediately reflect them to the process through feedback. In order to improve the quality of manufactured parts, the technology to control of the process parameters such as heat input in real-time by analyzing the size of the melt pool is required. Also, in order to improve the precision of the parts, the working distance from the tip of the nozzle to the surface of the melt pool must be controlled in real-time. In addition, research on the technology for modifying the scan patterns to suppress cracks and deformations through monitoring of temperature gradients and thermal profiles is required. Since the process parameters of DED are diverse and related to one another, further studies should be conducted to understand the effects of the process parameters.
2.2 Ceramics Manufactured by DED: Trend of Material Technology
Ever since Balla12) reported the possibility of deposition with the Al2O3 ceramic material, various oxide ceramics such as Al2O3-ZrO2 24), Al2O3-YAG-ZrO2 25), zirconate-titanate26), calcium phosphate bioceramics27) and the MgAl2O4 transparent ceramics28,29) have been manufactured by the DED method. Since considerable thermal stress is exerted due to the nature of DED involving local heating of the material, research has been conducted to improve the toughness of the ceramic material to overcome the high brittleness. In sintering, additives such as ZrO2 are used to improve the toughness of Al2O3. When ZrO2 solidifies in its liquid phase, it changes from t-ZrO2 to m-ZrO2 through stress induced phase transformation during cooling, which increases the toughness of the material and suppresses cracking30,31). More- over, the residual stress due to the difference in the coefficient of thermal expansion between the Al2O3 and ZrO2 materials plays a positive role in suppressing crack propagation. The study by Wu manufactured Al2O3- ZrO2 samples by DED, and addition of ZrO2 appeared to reduce cracking as shown Fig. 632).
Fig. 6
xAl2O3-yZrO2 oxide ceramics fabricated by DED (unit of x, y is mol%). (a) 100Al2O3, (B) 95Al2O3-5ZrO2, and (c) 90Al2O3-10ZrO2 32)
Fig 7 is a binary phase diagram of Al2O3-ZrO2 where the location of each microstructure shown in Fig. 8 is marked in a red dotted line. Fig. 8 shows the photographs of different microstructures with varying ZrO2 contents, and white arrows indicate build direction32). In all samples, the grains are vertically stretched. According to Thakur, the size of grains is considerably reduced compared to pure Al2O3, and the grain growth direction changes depending on the ZrO2 content33). In pure Al2O3, grains of Al2O3 continuously grow during the solidification process. With ZrO2 added (Fig. 8 (b) to (e)), an eutectic reaction takes place as primary Al2O3 grains are form first to appear in dark gray and ZrO2 and Al2O3 are formed at the same time below the eutectic temperature to appear in light gray. ZrO2 is formed at the grain boundaries of primary Al2O3, and in the composition of 63Al2O3-37ZrO2 at the eutectic point, the eutectic reaction takes place immediately in the liquid phase, and Al2O3 and ZrO2 form a fine lamellar structure as shown in Fig. 8 (f) with no formation of primary Al2O3 grains. The fracture toughness value was the highest in the composition at the eutectic point with 5.1 MPa·m1/2, which was 17% higher than that of pure Al2O3 32).
Fig. 8
Microstructure of Al2O3-ZrO2 ceramics. (a) 100Al2O3, (b) 95Al2O3-5ZrO2, (c) 90Al2O3-10ZrO2, (d) 80Al2O3-20ZrO2, (e) 70Al2O3-30ZrO2, and (f) 63Al2O3-37ZrO2 32)
The study by Pappas used MgO powders as an additive to Al2O3 for analyzing the effects of the parameters on porosity29). Pore size distribution analysis revealed that the use of MgO nanopowder having an average particle size of 50 nm or less significantly reduced the number of large pores of 30 μm or more in size compared to the use of MgO having a size of several tens of μm. In addition, the use of high laser power of 485 W instead of 275 W reduced the overall porosity. However, changing the laser mode (continuous wave, pulse, etc.) while maintaining the average power of the laser did not have a significant impact on the porosity. It is thermodynamically stable for Al2O3 and MgO to react and form a MgAl2O4 phase. Since this material stays transparent despite its polycrystallinity, it has potential as a new method of manufacturing transparent ceramics28).
2.3 Ceramics Manufactured by DED: Ultrasonic Application Technology
Since rapid solidification in the DED process creates a large thermal gradient and results in cracking and degradation of mechanical properties as mentioned earlier, additives such as ZrO2 and Y2O3 were used to refine the grains of Al2O3 and improve the toughness. However, it is difficult to ensure local homogeneity as different materials are mixed in. Therefore, the effect of ultrasonic vibration using ultrasonic devices has been proposed by researchers5,34-36). An ultrasonic device is attached to the bottom of the base material to apply ultrasonic vibration during the melting and solidification process of the DED process. Ultrasound creates acoustic streaming in liquid substances absorbing acoustic frequencies and vacuum cavitation. This effectively homogenizes the material dispersion, mitigates thermal gradients and thermal stresses, reduces cracks, and refines grains37,38).
The study by Hu manufactured Al2O3-ZrO2 ceramics with and without ultrasonic vibration2). As shown in Fig. 10, the grain size was refined by ultrasonic vibration and the eutectic structure existing at the grain boundary was homogenized. This change in structure demonstrated enhanced mechanical properties (hardness, wear resistance, and compressive strength) as suggested by the results of the compression test and the fracture surface analysis in Fig. 11. The sample manufactured by DED with ultrasonic vibration had higher compressive strength and ductility than the sample manufactured without ultrasonic vibration. In the fracture surface analysis, only intragranular fractures were observed on the fracture surface of the sample manufactured without ultrasonic vibration, whereas intergranular fractures were observed as well on the fracture surface of the sample manufactured with ultrasonic vibration.
Fig. 10
Microstructure of Al2O3-ZrO2 manufactured by DED without ultrasonic vibration (a), and with ultrasonic vibrations (b)2)
Fig. 11
(a) Compression test result showing the effect of ultrasonic vibration (marked in UV). Fracture surface of compression test samples without ultrasonic vibration (b), and with ultrasonic vibrations (c)2)
The study by Yan analyzed the effect of ultrasonic vibration on cracking and porosity35,36). Since the energy of ultrasonic vibration helps reduce the thermal gradient, cracks decreased with increasing power. On the other hand, the porosity decreases until the ultrasonic power is 100 W, but the porosity slightly increases when the power exceeds 100 W35). This is because the adequate power deepens the melt pool and promotes the streaming of the melt pool by the effect of the acoustic streaming while the strong power causes the evaporation of the material and impedes the flow of the melt pool. The cracks were reported to be refined by ultrasonic vibration with an average eutectic spacing of 70 nm, and the sample demonstrated excellent properties with a hardness of 16.22 GPa and a fracture toughness of 7.67 MPa·m1/2 36).
3. Conclusions
While sintering, the commercial manufacturing method of oxide ceramics, involves complex processing steps, the DED additive manufacturing using laser can be performed in simple steps with no limitation in terms of space, and the technology can be applied for easy repair, overhaul, and maintenance of parts. However, ceramics have a higher melting point, lower thermal conductivity, and higher brittleness than metals. Therefore, during the DED process involving local heating of materials, ceramics are vulnerable to temperature gradients and thermal stresses and are prone to cracks. Therefore, various studies have been conducted to reduce defects, such as pores and cracks, and increase toughness. The summarized results of the studies are as follows:
1) Various process parameters (laser power, scan speed, powder feeding speed, scan pattern and hatch spacing) affect not only the cracking and porosity, but also the microstructure and mechanical properties. Therefore, the process parameters must be controlled to lower the thermal stress on the manufactured parts. In the future, real-time monitoring technology based on real-time analysis is required to improve the quality of parts.
2) Oxide additives such as ZrO2 and Y2O3 were mixed with Al2O3 to reduce cracking and porosity. The additive forms a lamellar eutectic structure at the boundary of primary Al2O3 grains through eutectic reaction. The additives refine the grains to improve mechanical properties, and the eutectic structure suppresses the propagation of cracking.
3) Due to the nature of the DED process involving local application of energy and fast solidification, it is difficult to achieve homogenization. Therefore, a technique using ultrasonic vibration has been studied. The acoustic streaming and cavitation affect the behavior of the melt pool; reduce porosity and cracking; and improve toughness and strength at the same time.
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