FGM Fabrication Techniques: A Review on Powder Metallurgical Processes 1. Introduction A concept of functionally graded materials (FGMs) was proposed in 1984 by Japanese, as a means of preparing thermal barrier materials [1]. FGMs are made by combining two or more materials and posses position-dependent microstructure and chemical composition resulting in smooth gradients in the properties like mechanical strength and thermal conductivity. This concept of continuous changes in the composition, microstructure, porosity, etc., of these materials, has spread world-wide during the past years through the research [2]. Important advances in fabrication and processing techniques have made it possible to produce FGM using processes that allow for favorable latitude in the tailoring of microstructure and material composition. The manufacturing process of a FGM can usually be divided in building the spatially inhomogeneous structure (“gradation”) and transformation of this structure into a bulk material (“consolidation”). Gradation process [2] comprises powder consolidation processes (i.e. powder metallurgy) and coating processes (i.e. plasma spraying). Whereas the consolidation is transport based process which includes processes based on transport of heat, mass, or fluid. Moreover modern FGM fabrication techniques may be divided into four basic types namely bulk, layer, perform and melt processing [2-4] as shown in Figure 1. A brief discussion on these techniques had been done by Mortensen and Suresh [2]. 2. Powder Metallurgy for FGM Fabrication Various manufacturing methods are reviewed to select most appropriate one (in terms of cost and relevance) to manufacture metal/ceramic FGM plate in this project.
Figure 1 Classification of FGMs fabrication techniques It is found that Powder metallurgy is popular technology for the FGMs fabrication and is increasingly being used to create gradients on metal/ceramic FGMs. This method is appropriate for FGMs fabrication using solid materials. It includes the elementary steps illustrated in Fig. 2: the selection of a required material combination of metals and ceramics; design of the optimum compositional distribution; step-wise or continuous stacking of powder premixes according to the pre-designed compositional profile; die compaction and subsequent cold isostatic pressing of the stacked powder and sintering of the pressed compact. The shown steps incorporates two routes in which one is to produce stepwise FGM structure and other is for continuously graded FGM structure. Fabrication of each type of FGM structure requires specific procedure which is illustrated here in successive paragraphs. 2.1 Stepwise Compositional Control Concerning this type of FGM structure, several methods have been proposed and demonstrated, the simplest one is step-by-step stacking of mixed powders of different compositions. Other methods which enable to make continuous stacking with changing composition include the wet filtration process, is the vibrating stacking process, the centrifugal process, the wet powder spray
Figure 2 Flow chart of powder metallurgical fabrication of FGMs [8]. forming process, sequential slip-casting and the slurry dipping process. Among them, the powder spray forming process is one of the popular methods for continuous stacking of mixed powders in thin layers as well as for manipulating the three-dimensional gradient structure. 2.1.1
Die compaction of layers
In this simple and well established method stepwise gradients are formed by the deposition of powder layers with changing compositions in the compacting die [4-5]. Kapuria et al.[11] used this technique in which four stocks consisting of Aluminium and silicon carbide powders in the required proportions were mixed, dried and blended for 12 h in ball milling machine with rotation speed of 150– 250 rpm. Four stocks were made with ceramic volume fraction of 10%, 20%, 30% and 40%.
Figure 3 Schematic of powder metallurgy technique to fabricate layered functionally graded beams made by Al/SiC
Layers of different stocks were put in the die successively, ensuring uniform thickness and distribution of powders in each layer. The specimen was then subjected to unidirectional green compaction at a maximum force of 490.4 kN (50 tonnes) in the thickness direction. The abovementioned procedure of Metal-Ceramic (Al/SiC) FGM fabrication is illustrated in Fig. 3. The powder-stacking method allows effective laboratory studies of functionally graded systems [5] by fabricating Metal-Ceramic FGM easily hence here this powder stacking method is selected to prepare metal/ceramic FGM plate.
2.1.2
Sheet Lamination
In this method of FGM fabrication the thin sheets of desired compositions are formed by dry or wet powder techniques [5]. These possessed sheets are joined to form a stepwise gradient. Tape casting and Powder rolling may be used to form and join these thin sheets [6, 7]. 2.1.3
Solid Freeform Fabrication
Solid Freeform Fabrication (SFF) refers to a class of manufacturing processes which provides a faster alternative to the conventional methods to optimize the whole process of FGM fabrication by computer simulation. In this method the FGM composition profile, distribution of green density and particle size are obtained by computer simulation. Optimisation of symmetric FGM plates and disks during sintering by SFF was illustrated by Gasik et. al.[8]. 2.2 2.2.1
Continuous Composition Control Centrifugal Powder Forming
In CPF (Centrifugal Powder Forming), the continuous composition of constituent materials in form of powder mixtures are controlled with the aid off computer and these mixtures are fed onto a rotating distributor plate, which accelerates towards the inner wall of a rotating cylinder. A green body of sufficient strength is formed by simultaneously spraying an organic binder onto the wall. This method is limited to cylindrical parts however various graded structures can be formed using CPF. This methodology of FGM fabrication was used by Kieback et. al. for the production of W/Cu FGMs [5]. 2.2.2
Electrophoretic Deposition
Electrophoretic deposition from suspensions containing more than one component can be used to produce graded bodies. In the simplest case an external mixing system supplies suspensions with the variable concentrations of the components or the second component is added with time in
calculated proportions. Functionally graded WC–Co materials were fabricated using electrophoretic deposition from a suspension of hard metal powder in acetone, with variable cobalt content. The deposits were sintered to closed porosity at 1290 and 1340 °C [9]. 2.2.3
Pressure Filtration/ Vacuum Slip Casting
By continuously changing the powder composition supplied to the filtration system, a defined one-dimensional gradient in the deposit it is obtained. Sequential slip casting is proposed as an alternative route for the future family of dense functionally gradient ceramics (FGCs) with complex shapes and tailored microarchitectures. The basic principle behind this method is the utilization of the density difference among the species used and the size difference among the particles within the powders used to influence the sedimentation velocity of particles in a slip. A continuous functionally gradient material was fabricated using the slip casting method [10]. 3. Concluding remarks Processing of FGMs has reached at substantial level of maturity. Wide range of FGM fabrication methods is available and selection of appropriate process is dependent on parent materials involved as well as on the type of the gradient, and the required scale of the geometry. The powder metallurgical process is a very viable and suitable route for FGM fabrication. However the processing of such materials by powder metallurgical methods has often faced undesirable excessive bending or warping of the component and a better control on fabrication parameters are required to avoid these problems.
References
1. Shiota, I., & Miyamoto, Y. (Eds.). (1997). Functionally graded materials 1996. Elsevier.. 2. Mortensen, A., & Suresh, S. (1995). Functionally graded metals and metal-ceramic composites: Part 1 Processing. International Materials Reviews, 40(6), 239-265. 3. Watanabe, Y., & Sato, H. (2011). Review fabrication of functionally graded materials under a centrifugal force. 4. Kawasaki, A., & Watanabe, R. (1997). Concept and P/M fabrication of functionally gradient materials. Ceramics International, 23(1), 73-83. 5. Kieback, B., Neubrand, A., & Riedel, H. (2003). Processing techniques for functionally graded materials. Materials Science and Engineering: A, 362(1), 81-106. 6. Jung, Y. G., Ha, C. G., Shin, J. H., Hur, S. K., & Paik, U. (2002). Fabrication of functionally graded ZrO< sub> 2/NiCrAlY composites by plasma activated sintering using tape casting and it's thermal barrier property. Materials Science and Engineering: A, 323(1), 110-118. 7. Rahaman, M. N., Dutton, R. E., & Semiatin, S. L. (2003). Fabrication of dense thin sheets of γ-TiAl by hot isostatic pressing of tape-cast monotapes. Materials Science and Engineering: A, 360(1), 169-175. 8. Gasik, M. M., Zhang, B. S., Van der Biest, O., Vleugels, J., Anné, G., & Put, S. (2003, January). Design and fabrication of symmetric FGM plates. InMaterials Science Forum (Vol. 423, pp. 23-28). Aedermannsdorf, Switzerland: Trans Tech Publications, 1984.
9. Put, S., Vleugels, J., & Van der Biest, O. (2001). Functionally graded WC–Co materials produced by electrophoretic deposition. Scripta materialia, 45(10), 1139-1145. 10. Chu, J., Ishibashi, H., Hayashi, K., Takebe, H., & Morinaga, K. (1993). Slip casting of continuous functionally gradient material. Nippon seramikkusu kyokai gakujutsu ronbunshi, 101(7), 841-844. 11. Kapuria, S., Bhattacharyya, M., & Kumar, A. N. (2008). Bending and free vibration response of layered functionally graded beams: a theoretical model and its experimental validation. Composite Structures, 82(3), 390-402.
