An Overview: Different Manufacturing Techniques used for Fabricating Functionally Graded Material

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ScienceDirect Materials Today: Proceedings 18 (2019) 2942–2951

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ICMPC-2019

An Overview: Different Manufacturing Techniques used for Fabricating Functionally Graded Material Sarada Prasad Paridaa, Pankaj Charan Jenab* a, b

Department of Production Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India-768018

Abstract The industrial applications of functionally graded material (FGM) have increased with its specific capability. This makes curiosity among the researchers how to optimize the design and manufacturing technique as well as the numerical analysis methodology in the priority basis. Contrast to composite material, the properties of the FG material can be varied through the dimension. Usually, the properties are varied along the thickness direction by power distribution formula. However, it is possible to vary the properties of FG material along longitudinal and transverse direction with better improvised technique. Present study is an overview of FGM modelling, design and various manufacturing techniques used by past researchers as well as achieved current stage industrial applications. Further, this study can show a road map to current authors for improvising fabrication techniques in their research on FGM. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: functionally graded material; manufacturing methods; property; etc.

1. Introduction A functionally graded material is nothing but a high class of composite material initially developed by the researchers to meet the need of variation of thermal environments where a temperature difference of 1000°C is to maintain within a few millimeter of thickness. Now days, the on-going process of advancement of material development FG materials are designed according to the need. Starting from the application of parts of aerospace to electronic appliances FG materials have taken a wide range of popularity and acceptance. A diversified variety of FG materials are elaborated along with different process techniques and applications in the literature. In this work an

* Corresponding author. Tel.: +918895100075; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019

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effort is given to give a throughput on the concept of FG material along with the processing techniques and formulae of material properties. The first development of FG material is reported in 1972 by Bever and Duwez [1, 2] . However it got the wide range of popularity and the call of wakeup for study to the researchers when a group of scientists from Japan developed a high class of FG materials to act as a thermal insulation barrier of 1000°C in 10mm thickness in late of 1980[3,4]. Since then it becomes a perspective research area of for thousands of researchers and engineers. The volume fractions of constituent materials are varied according to the need. Generally it is varied along the thickness direction. Based on this a FG may be classified as continuously graded FG material or stepwise graded FG structure [5].

Figure1. (a) Continuously graded FGM; (b) Stepwise graded FGM. In a continuously graded structure the composition and microstructure varies continuously with the dimension whereas in stepwise graded structure the FG material is assumed as stack of piles of layers of varied composition and microstructure. The properties of FG material are uniform in each layer. The schematic diagram is shown in Figure 1(a-b). Generally, the FG material is prepared in two steps. First the whole material is spatially divided to inhomogeneous structure known as gradation and then it is transformed to a bulk structure known as consolidation. Recent days developments in automation technique enable the smart use of gradation. In gradation usually the concentration or density of constituent materials are varied in microscopic level. Usually the constituent material is mixed in powder form. The gradation of composition and microstructure of FG material during manufacturing processes can be done either any of following rules [6]: (a) Constitutive principle; (b) Homogenous; (c) Segregated method. In constitutive gradation the constituent materials are graded in layer wise manner similar to stepwise graded structure. According to the concentration or density of microstructure a stepwise or constitutive graded FG can be also further divided. Yang et al. [7] studied the Buckling and post buckling of functionally graded multilayer graphene platelet reinforced composite beams. For the purpose they presented 4 models of GPLRC beam is of 4 types: (i). U – GPLRC; (ii). X – GPLRC; (iii). O – GPLRC; (iv) A – GPLRC. In U- GPLRC material each layer is assumed to have isotropic property. In the case of X-GPLRC, the surface layers are GPL rich while this is inversed in O-GPLRC where the middle layers are GPL rich. For the A-GPLRC, the GPL content gradually increases from the top layer to the bottom layer. 2. Fabrication Methods FGMs are manufactured by spatially distributing the constituent materials; the constituent materials may be metals, ceramics and polymer with continuous subtle variation in composition makeup to obtain the required specified property [8, 9]. There are several preparation techniques of FG materials depending upon its physical state of matter and conditional specific applications. Naebe et al. [10] explained the various processing techniques used for the preparation of FG materials extensively. Sasaki [11] explained that there are mainly three approaches to get the desired compositional variation. The constituent materials can be mixed in solid, liquid or gaseous phases of material and accordingly processing operations can be done as depicted in Figure 2.

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Figure 2: Hierarchical classification of FGM. 2.1 Gaseous phase processing Generally in gaseous phase processing FGM the constituent materials are boiled to their vapor state and then condensed after a thorough mixing to form a solid FG material of required microstructure and property [12, 13]. The commonly used gas-based methods to fabricate FGM Chemical vapour deposition (CVD) method, chemical vapour infiltration (CVI) method, physical vapour deposition (PVD), ion plating, plasma spraying and ion mixing [14]. In a broad sense the gas based techniques can be subdivided under three subheadings namely CVD/ CVI, thermal spray and surface coating reaction process. In 1983, the first use of CVD came into notice by De Lodyguine [15] to produce a carbon lamp filament from tungsten by reduction process of WCl6 by H2 on it. Later on this technique is used to produce semiconductors, electronic circuits, advanced ceramics, turbine blades and solar cells [16]. Bromide, hydride and chloride are used as source of gas. Rate of flow, pressure and composition of gas influences the properties and microstructure of FGM produced by CVD and CVI process [17-23]. The Vapour deposition technique is energy intensive but it produces poisonous gases as their by-products [24, 25]. Application of thermal spray technique came into existence when M.U. Schoop prepared a corrosion resistant material from the flame spray of tin and lead in the year 1911. Since then thermal spray technique is treated as one of the widely accepted technique to produce super alloys, cutting tools, coatings etc.[26]. In traditionally in thermal spraying method, a mixture of dry powder is injected in a hot gas jet; the jets melt the particles and accelerate them towards the substrate. When solidified it assumes the shape of substrate with the required property. Different methods of thermal spray like flame spray, detonation, high velocity oxy fuel (HVOF) us combustion and wire ARC are used according to the need and working temperature [27- 36]. It is observed by different researchers that the

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heating temperature, number of layer, carrier gas, cooling system, plasma gas composition, pressure and the distance of spray coating affects the microstructure of coating and mechanical property i.e. rolling contact fatigue(RCF) of the FGM [32- 36]. The particle size of the substrate matter has a greater influence on the microstructure of FGM as reported [37-47]. To overcome this difficulty now a day’s an improved technique of thermal spraying called as innovative thermal spraying technique is developed. Suspension Plasma Spraying (SPS) and High-Velocity Suspension Flame Spraying (HVSFS) used simultaneously to process very fine particles of micrometre to nanometre scale to obtain compact microstructure of FGM [48-50]. Surface treatment or reaction processing are the gas based techniques where the surfaces of metals are carburized or nitridized, Ion-nitriding, plasma nitriding, laser and gas nitriding techniques are commonly used techniques [51-56]. 2.2. Liquid phase processing 2.2.1 Centrifugal method It is similar to centrifugal casting process. Here the constituent materials are mixed in liquid state and poured in a casting cylinder. Due to differences in their densities they form graded layers when rotated. This process is only capable of making cylindrical parts with heavier constituent materials at outer periphery and the lighter materials at interior. [57, 58] Generally this process is preferred for metal based FGM [59-61]. Depending upon the processing technique there are two types of centrifugal casting. If processing temperature is higher than alloy temperature then it is call as centrifugal insitu-technique else it is termed as centrifugal solid particle technique [62]. The quality of the casted part mainly depends of the rotation speed, alloy temperature, processing temperature, cooling provision etc.[63-68]. 2.2.2 Combustion method This process is somehow similar to thermit welding. In this process, the added constituent materials react exothermically and produce high amount of heat energy which is capable to melt the constituent material. Hence this method sometimes called as self-propagating high temperature synthesis (SHS) method [69-75]. The commonly used combustion techniques reported are reactive combustion forging, rolling, pressing, extrusion, and casting. 2.2.3 Casting method The other commonly used methods of manufacturing FGM in liquid phase are termed to be casting method. In this process the constituent materials in powder form are mixed thoroughly in a solution of solvent and plasticizer. Then it is casted or solidified to required geometry. When the thickness of casting is within the range of 100-300µm then it is called as tape casting [76-77]. Slip casting and gel casting methods are other two types of commonly used casting methods. In both the methods slurry is prepared and powders of constituent materials are pored and mixed. Then the slurry is casted in a predesigned mould. In slip casting method the slurry is prepared by water and the part casted is removed after it is fully dried [3-4]. However, in gel casting, the slurry is prepared from monomers and the part is removed from the mould in wet condition and then followed by drying and sintering [78-81]. 2.2.4. Deposition method In deposition method a solution of powders of constituent materials in a medium is prepared. Then the deposition of these materials is triggered by electric, chemical or by laser field. Accordingly the methods are termed as Electro deposition method, Chemical solution deposition method or Laser deposition method. Electro-coating, e-coating, electro-phoretic coating, anodic electrode position, cathodic-electrode position and electro-phoretic painting are commonly used methods in industrial practices [82-85]. Chemical solution deposition technique is used produce thin substrates of FGM on a parent material preferably a metal. A solution is prepared from inorganic or metal-organic metals salt a dielectric organic fluid or water. When the parent metal or the substrate is dipped inside, a thin layer of metal oxides is coated as a result pyrolysis and crystallization followed by heat treatment process. A numerous study on this technique is reported [86-90] In laser decomposition technique, a high intensity of laser beam is used to deposit metallic or ceramic powders on the substrate material. Generally this process is used for making FGM materials with significance difference in melting points of constituent material [91-95]. The other commonly used process of liquid phase processing includes Sedimentation Electrochemical gradation, directional solidification etc.[96-102 ]

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2.3 Solid Phase Processing The solid phase processing of FGM generally involves the processing of constituent materials in solid state. The constituent materials may be processed in powder form. Generally, powder metallurgy method is assumed as the simplest method of preparation of FGM. In this method mixtures of powders of constituent materials are kept in a die in desired composition, compacted and then sintered to get the final product. Bulk processing methods or PM methods may be formally classified into powder (materials) stacking and stack consolidation processes [103,104]. The commonly used powder stacking method are Single powder stacking, Mixed powders or layers stacking, fiber/sheets stacking, normal gravity powder stacking, microgravity powder stacking , Injection molding and slip casting. The commonly used stack consolidation methods include: Solid state sintering, Microwave and hybrid sintering, hot pressing and HIP, Spark Plasma Sintering (SPS), Combustion/reactive sintering (SHS) and Shock compression (explosion consolidation). The other methods of PM methods used are dry pressing, centrifugal forming, sedimentation, sintering, HP, HIP, electrophoresis, electrolysis, plasma activated sintering diffusion and reaction [104-111]. 2.3.1 3D printing technique It is powder based technique First the powders are prepared by crushing and pulverizing the constituent material. Then the powders are coated with a catalyst to insure rapid chemical reaction. Polymer or resin is then sprayed or impinged to the powder to react and to form the solid model. The solid model is retrieved and pyrolised [112-117]. 2.3.2 Solid state foaming It is a two-stage batch processing method; the polymer sample is impregnated first with an inert gas such as CO2 by saturation process inside a pressure vessel and then by changing the pressure or temperature it is converted to a thermodynamically unstable state where the absorbed CO2 nucleates and forms bubbles in the polymer matrix. [118-120]. 2.3.3 Solid Freeform (SFF) Fabrication Method Laser engineered net Shaping is a solid freeform fabrication process, which involves laser processing fine powders into fully dense three-dimensional shapes directly from a computer-aided design model. The LENS process is able to fabricate complex prototypes in near-net shape, leading to time and machining cost savings. A variety of metals and alloys have been deposited by the LENS process, such as H13 steel, 316 stainless steel, nickel-base super alloys and titanium alloys [121–124]. The other methods includes: laminated object manufacturing, sterio-lithography, fused deposition modeling, ultrasonic consolidation. 2.3.4 Laminate / stack processing methods This process is used to process the laminates. Generally thin sheets of different compositions made by tape casting or slip casting are piled up and joined to form a step gradient of FGM [125,126] 3. Types of FGMS According to their applications FGMs can be classified to different categories [127]. According to the combination of materials FGM divided into metal-ceramic, ceramic-ceramic, and ceramic-plastic groups. Similarly depending upon the composition of constituent materials FGMs may be gradient type, functional gradient coating type or functional gradient connection type. On the same way depending upon the density gradients FGM s may be changing nature of FGM, composition FGM, optical FGM, fine FGM etc.[6]. According to their uses FGMs can also be categorized as heat resisting FGM biology FGM, chemical engineering FGM, electronic engineering FGM [128]. According to the continuous or non-continuous change of the content or structure along a specific direction, functionally graded materials (FGMs) are classified into two categories, which are continuous FGMs and noncontinuous FGM [129]. 4. Material properties of FGMs Thus the effective material properties such as elastic moduli, shear moduli, density, poison’s ratio etc. of FGM depends upon the volume fraction distribution of constituent materials. There are various idealization schemes to evaluate the material properties. The exponential law, power law and Mori Tanka scheme are widely accepted formulations used for the purposes. According to exponential law the effective properties of FGM can be calculated as reported by various researchers [130].

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P ( z ) = Ut exp(−η(1 −

2z 1 U  )) where η = ln  t  h 2  Ub 

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(1)

The power law is preferably used to calculate stresses in the FGM [107]. According to this the stress can be defined as a function of height and moduli of constituent materials.

P ( z ) = (Pt − Pb )(

Z 1 k + ) + Pb H 2

(2)

The Mori-Tanka scheme is more appropriated for composites with particulate reinforcement. However this can be used

to calculate the mechanical properties like density, bulks modulus, shear modulus and volume fraction where a small amount of substrate materials are added to form FGM [131,132]. The general formulation as reported is given by

P = PV t t + PV b b

(3)

5. Application of FGM From the first day of invention FGMs are first used as thermal barrier and insulation material [1] .Since then FGM treated as a special material of interest. Due to the superior feature and application required tailored properties FGMs have asserted a wide range of applications. The day by day newly developed processing methods eased the making of FGMs for different uses. Now days the most promising application of FGMs are found in aerospace to manufacture ceramic tiles, turbine blades and etc.[2-10]. Biomedical applications such as dental implants, artificial joint, bone , knee joint , artificial grafts femur and hip prosthesis, inter vertebral discs, orbits and bone plates are widely used now[133-138]. 6. Conclusion A functionally graded material is nothing but a high class of composite material initially developed by the researchers to meet the need of variation of thermal environments where a temperature difference of 1000°C is to maintain within a few millimetre of thickness. Starting from the application of parts of aerospace to electronic appliances FG materials have taken a wide range of popularity and acceptance. A diversified variety of FG materials are elaborated along with different process techniques and applications in the literature. In this work an effort is given to give a throughput on the concept of FG material along with the processing techniques and formulae of material properties. In the current research, the study is made on functionally graded materials. The study is also focused how the fabrication methods and properties of FGM are different from the fabrication methods and properties difference traditional engineering materials. The detail studies on FGM fabrication techniques are discussed. It is also observed from the past literatures that the FG materials can be designed in such way that the properties are varied through the dimension as per the distribution of the constituent material over the volume. Usually, the properties are varied along the thickness direction by power distribution formula. However, it is possible to vary the properties of FG material along longitudinal and transverse direction with better improvised technique. Present study is an overview of FGM modelling, design and various manufacturing techniques used by past researchers as well as achieved current stage industrial applications. Further, this study can show a road map to current authors for improvising fabrication techniques in their research on FGM. Acknowledgement: The authors do hereby acknowledge the Department of Production Engineering, VSSUT, Burla and TEQIP-III for providing the infrastructure facilities and financial support for carrying out this research work. References [1]. [2]. [3]. [4].

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