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Al2O3-based melt growth ceramics (MGCs) possess fascinating application potential in the aerospace industry for their superior high-temperature performance, inherent thermochemical stability, and outstanding oxidation resistance at temperatures close to their melting point under oxidizing environments. The main preparation methods of Al2O3-based MGCs, including the Bridgman method and the laser floating zone method, are difficult to meet comprehensive requirements of high density, high toughness, and complex shape. Therefore, it is imperative to develop effective near-net shaping techniques for high-efficiency and high-quality preparation of Al2O3-based MGCs. Directed energy deposition (DED) is a kind of additive manufacturing method, which can truly realize the long-term pursuit of "molding-sintering integration" in the field of ceramic preparation. The melting-solidification process of DED technology gives Al2O3-based MGCs more compact, unique microstructure and better comprehensive mechanical properties. Furthermore, the bottom-up and layer-by-layer deposition methods rids DED of mold restrictions, implying the feasibility of rapidly preparing ceramics with arbitrary shapes. Dr. Yun-Fei Huang, Prof. Dong-Jiang Wu, Dr. Da-Ke Zhao, Prof. Fang-Yong Niu, Guang-Yi Ma from Dalian University of Technology, China, wrote a paper "Investigation of melt-growth alumina/aluminum titanate composite ceramics prepared by directed energy deposition" on IJEM. In this article, the authors have introduced the formation mechanism of solidification defects, and the relationship between microstructure characteristics, mechanical properties and energy input during DED of Al2O3/Al6Ti2O13 composite ceramics. The directed energy deposition-laser based (DED-LB) system used in the experiment is depicted in figure 1. It mainly consists of a Nd:YAG continuous laser, a three-cylinder powder feeder, a CNC machine, and a circulating cooling water system.
Figure 1. Diagram of DED-LB system.
2. Solidification Defects
Pores are one of the major solidification defects in most DED-LB MGCs. Pores are divided into gas cavities and shrinkage cavities, both of which are potential crack sources. The existence of pores easily induces the nucleation and propagation of macro cracks and seriously weaken the mechanical properties of MGCs. In this article, Prof. Dong-Jiang Wu gave a detailed introduction to the formation mechanism of gas cavity and shrinkage cavity.
Since the specific free energy is higher between solid/gas than between solid/liquid and liquid/gas, the gas in the molten pool is squeezed to the liquid phase, as shown in figure 2. Once the concentration reaches a certain level, the gas nucleates and forms bubbles near the solid/liquid interface. The bubbles grow continuously through diffusion and coalescence and then escape from the molten pool under the action of buoyancy. Large bubbles with high movement velocity are more likely to escape from the molten pool in time. In contrast, some small bubbles, whose movement velocity is lower than the solidification rate of molten pool or even fail to move due to the viscosity resistance, are entrapped in the melt and form gas cavities.
Figure 2. Simplified schematic diagram of the gas cavity formation process: from the appearance of gas bubbles to imprisonment after solidification.
Shrinkage cavities are widely believed to be caused by the untimely filling of the liquid phase during DED-LB rapid solidification. A simplified schematic diagram of the formation process is illustrated in figure 3. In the solidification process of molten pool, the primary phase precipitates first, and the surrounding residual liquid phase adheres to the solid phase for solidification and contraction. In the absence of timely supplement from the liquid phase, the voids generated by solidification and contraction gradually expand into shrinkage cavities as the solidification progresses.
Figure 3. Simplified schematic diagram of the shrinkage cavity formation process: gradually expanding contraction voids.
3. Microstructure Characteristics
The energy input has a marked influence on the solidification behavior of the molten pool, so the microstructure of Al2O3/Al6Ti2O13 composite ceramics are sensitive to changes in energy input. The molten pool with an energy input lower than 0.36 W*min2 g-1mm-1 had poor fluidity and short duration, resulting in the incomplete development of a primary α-Al2O3 phase. Therefore, the cellular α-Al2O3 phases in figures 4(a1)-4(b2) are isotropic growth. There was a serious heat accumulation phenomenon due to the remelting of the previous deposition layer during the DED process. The heat accumulation intensified with increasing energy input, leading to a decrease in the temperature gradient between adjacent deposited layers. Under the condition of a certain solidification rate, the decrease in temperature gradient intensified the composition supercooling degree at the front of solid/liquid interface. The intensified composition supercooling induced cellular bulges that extended further into the liquid phase, which promoted the directional growth of the primary α-Al2O3 phase along the deposition direction. Therefore, the aspect ratio of the primary α-Al2O3 phase increased and exhibited an anisotropic growth characteristic after the energy input exceeded 0.45 W*min2 g-1mm-1 (figures 4(c1)-4(d2)).
With the energy input increases, more and more α-Al2O3 phases developed secondary dendrites and transformed from cellular to cellular dendrite morphology. The transformation of the α-Al2O3 phase morphology with energy input was also highly correlated with composition supercooling. The high transverse solute diffusion rate caused by large composition supercooling stimulated the generation of secondary dendrite arms, resulting in the transformation of the cellular α-Al2O3 phase into a cellular dendrite morphology with a hexagonal substructure. As a result, the α-Al2O3 phase experienced an evolution of cellular (figures 4(a1)-4(b2))-cellular coexisting with cellular dendrite (figures 4(c1)-4(d2))-cellular dendrite (figures 4(e1)-4(e2)) when the energy input increased from 0.27 W*min2 g-1mm-1 to 0.63 W*min2 g-1mm-1. Moreover, the secondary dendrite arms of the α-Al2O3 phase continued developing, which was attributed to the prolonged retention time and improved fluidity of the molten pool under high energy input conditions.
Figure 4. The microstructure of Al2O3/Al6Ti2O13 composite ceramics prepared under different energy inputs: (a) 0.27 W*min2 g-1mm-1, (b) 0.36 W*min2 g-1mm-1, (c) 0.45 W*min2 g-1mm-1, (d) 0.54 W*min2 g-1mm-1, and (e) 0.63 W*min2 g-1mm-1, respectively (1 stands for longitudinal section, and 2 stands for cross section).
4. Mechanical Properties
Solidification defects and microstructure, which are highly sensitive to energy input, are important factors to determine the mechanical properties of MGCs. On the one hand, the improved fluidity and prolonged duration of the molten pool reduced solidification defects, which improved the mechanical properties. On the other hand, the phase size of α-Al2O3 increases with the increase of energy input, which had a negative effect on the mechanical properties. Since the microstructure was relatively fine and the solidification defects almost disappeared, A/AT MGCs exhibited excellent comprehensive properties when the energy input was between 0.36-0.54 W*min2 g-1mm-1.
5. About the Authors
Dong-Jiang Wu is a full professor and doctoral supervisor at Dalian University of Technology, where he is also a member of the National Standard Committee for Additive Manufacturing. Prof. Wu’s research focuses on the interaction mechanism of laser beam and materials, laser precision/micro machining and additive manufacturing (3D printing) technology, especially in the laser welding of Hastelloy thin plate, composite welding of aluminum alloy thick plate, additive manufacturing of ceramic matrix materials, and metal/ceramic composite functionally gradient materials. In recent years, he has been responsible for a number of national key projects. More than 150 papers have been published in authoritative journals such as Scripta Materials, Journal of the European Ceramic Society, Journal of Materials Processing Technology and other academic journals, and 30 national invention patents have been authorized.