Microsoft Word - ETASR_V12_N4_pp8785-8790 Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8785 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … Functionally Graded Material Production and Characterization using the Vertical Separator Molding Technique and the Powder Metallurgy Method I. Kayabasi Motor Vehicles and Transportation Technology Department Küre Vocational School of Higher Education Kastamonu University Kastamonu, Turkey ikayabasi@kastamonu.edu.tr G. Sur Mechanical Engineering Department Engineering Faculty Karabuk University Karabuk, Turkey gokhansur@karabuk.edu.tr H. Gokkaya Mechanical Engineering Department Engineering Faculty Karabuk University Karabuk, Turkey hgokkaya@karabuk.edu.tr Y. Sun Metallurgy and Materials Engineering Department Engineering Faculty Karabuk University Karabuk, Turkey ysun@karabuk.edu.tr Received: 28 April 2022 | Revised: 12 May 2022 | Accepted: 15 May 2022 Abstract-Functionally Graded Materials (FGMs) are advanced customized engineering materials that gradually and continuously change their composition. The current study investigated the production feasibility and some post-production mechanical/physical properties of B4C particle-reinforced (avg. 40µm) AA7075 matrix (avg. 60µm) FGM composites with the vertical separator molding technique using the high-temperature isostatic pressing powder metallurgy method. FGMs produced consist of three (0 – 30 – 60 wt. % B4C) and four (0 – 20 – 40 – 60 wt. % B4C) layers. The powders were mixed in a power blender mixer for 2h and were placed in the mold sections with a vertical separator. The lid was closed, and a pre-pressure of 10Mpa was applied. The FGM green sheet was transferred from the vertical separator mold to the hot work tool steel with a press. In this mold, FGMs were sintered at 560°C for 30 min under a pressure of 325MPa. Microstructural examinations did not reveal any separation or crack formation in the layer transition regions of the FGMs. In addition, a relatively homogeneous B4C reinforcing distribution was observed in the layers with a low reinforcement ratio (wt. 20% and 30%) compared to the other layers. The highest hardness was 170 HBN in one layer of the four-layer FGM containing 40% by weight B4C reinforcement. The highest transverse rupture strength was measured in the test performed from the region with the most reinforcement of the four-layer FGM at 482MPa. Keywords-functionally graded material; powder metallurgy; hot pressing; transverse rupture strength I. INTRODUCTION FGMs are widely used in various fields such as aerospace, automotive, electronics, defense industry, gas turbine engines, and engineering applications. They have emerged as a relatively new material class [1, 2]. The FGMs are materials that change their composition and structure and seriously alter their material properties gradually along with their height [3, 4]. Functionally Graded Metal Matrix Composites (FGMMCs) with metal and ceramic content have great potential and importance for the fabrication and design of components and structures. FGMMCs have superior features to bring advanced engineering components to a better level along with material design. Specific properties such as high-temperature surface wear resistance, thermal mismatch correction, reduction of interfacial stresses, minimization of thermal stresses, increased metal-ceramic interfacial adhesion, delayed crack formation, and increased fracture toughness can be obtained by utilizing FGMMCs [1]. Aluminum Matrix Composite (AMC) is considered one of the most promising materials due to its lightweight and high specific strength values [5-7]. Among the existing AMCs, AA7075 is a matrix material with high strength, sufficient toughness, and corrosion resistance [8]. The vast-majority of reinforcing materials of AMCs are non-metallic ceramic materials. Al2O3, SiC, and B4C are the most commonly used ceramic reinforcements in AMCs [9]. B4C reinforcement material stands out among ceramics due to its good chemical inertness, high hardness, high elastic modulus, and high melting properties [10]. One of the most important areas in FGM research is the production method [1]. Many different fabrication methods have been developed to fabricate FGMs, such as gas-based Corresponding author: I.Kayabasi Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8786 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … methods, liquid-phase methods, solid-phase methods, and biopolymeric-based functionally graded structures. Powder metallurgy, one of the solid phase methods, is a good method for obtaining a homogeneous and graded structure in FGM production [11, 12]. The powder metallurgy method offers advantages such as low processing temperature, ease of obtaining the final product, continuity in product quality, and low cost with its suitability for mass production compared to melting methods [13, 14]. In FGM production by powder metallurgy method, powders are gradually laid into the mold according to the reinforcement ratio [15]. This process is mostly carried out based on one's dexterity. It is difficult to obtain a smooth transition surface between layers in such manual procedures. In FGM production with another powder metallurgy method, the powder is laid in the mold for each stage and subjected to pre-pressing [16, 17]. This adversely affects the formation of the desired bond structure on the layer transition surfaces during the sintering phase. Vertical separator molding technique has been developed in order to obtain a smoother transition zone, to create the desired bond structure in the layer transition zones, and to provide the layer transition zones in the desired two-dimensional geometric structures. In this study, a two-stage mold consisting of a vertical separator, a filling chamber, and a hot-pressing mold was designed and manufactured for FGM production under the high temperature isostatic pressing technique of the powder metallurgy method. The availability of planeness of transition surfaces of the B4C reinforced AA7075 matrix FGMs with 3 (0-30-60% wt.) and 4 (0-20-40-60% wt.) layers of the same thickness as the manufactured mold was examined, along with the post-production metallurgical and mechanical properties. II. MATERIALS AND METHODS This study was carried out for FGM production. The chemical composition of AA7075 matrix (avg. 60μm) is presented in Table I. Reinforcement material B4C (avg. 40μm) powder was used. TABLE I. CHEMICAL COMPOSITION OF AA7075 Element Zn Mg Cr Cu Fe Si Mn Ti Al %Avg. 5.5 2.5 2.5 1.6 0.5 0.4 0.3 0.2 Bal. The Scanning Electron Microscopy (SEM) images of matrix and reinforcement are shown in Figure 1. SEM investigations showed that AA7075 powder was relatively spherical in shape and variable in grain size. It has been observed that the reinforcement B4C powder was composed of particles with complex geometry and sharp edges, whose aspect ratio is relatively close to each other. In determining the percent weight ratio of the reinforcement of the FGM layers, one side is designed in a brittle structure with a high reinforcement ratio (high hardness, high wear resistance, high compressive strength), and the other side is designed in a tensile structure without reinforcement (high toughness, high % elongation). Information about the number of layers and reinforcement content of the produced FGMs are presented in Table II. (a) (b) Fig. 1. SEM images of the powders used in FGM production: (a) AA7075, (b) B4C. TABLE II. LAYER AND REINFORCEMENT RATIOS OF THE PRODUCED FGMs Number of layers 3 4 Layer reinforcement rates (% wt.) 0 – 30 – 60 0 – 20 – 40 – 60 Under the ratios in Table II, matrix and reinforcement powders were weighed on a precision balance at suitable values for the mixing ratio of each layer. In order to obtain a homogeneous powder mixture, it was mixed in a power mix blender for 2 hours. The mixed powders were filled into mold chambers (3 layers 60×60×8mm, 4 layers 60×60×6mm), whose layer areas were separated in equal volume by vertical steel separators. After the powder filling process, the steel separators were removed, the upper surface of the mold was closed, and a pre-compression force of 10MPa was applied under uniaxial hydraulic press. The pre-compression force was applied from the surface of the layer with the highest reinforcement ratio (60% wt. B4C). Pre-compressed green sheet FGM was transferred into a separate mold made of hot work tool steel using a hydraulic press to carry out high-temperature isostatic pressing and sintering processes. Green sheet FGM samples were heated up to 560°C in a hot work tool steel mold and waited for 30min. Then, sintering was carried out at this temperature for 30min under a pressure of 325MPa and the samples were left to cool in the mold at room temperature. At Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8787 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … least two FGM samples were produced from each layer number. The samples were 60×60×12.7 ±0.13mm in size. The produced FGM samples and the cross-sectional macro views of the 3 and 4 layered samples are shown in Figure 2. The microstructure characterization of the produced FGM samples was investigated using the FEI QUANTA FEG 250 SEM located in Kastamonu University Central Research Laboratories and Research Center. The hardness values of the layers of the FGMs were determined by the Brinell hardness measurement method using the QNESS Q250M type hardness measuring device at Karabuk University’s Iron and Steel Institute. Fig. 2. FGM sample images: (a) After sintering, (b) 3-layer cross-sectional view, (c) 4-layer cross-sectional view. FGM hardness measurements were carried out using 2.5mm diameter steel balls under a 62.5kg load. At least 5 measurements were made for each layer of the FGM, and the average determined the hardness value for the general layer. The produced FGMs were cut according to the ASTM B528-05 standard (31.9×12.7×12.7±0.13mm) and turned into transverse rupture strength test specimens. Transverse rupture strength tests were performed on the cut samples in a 50kN capacity INSTRON 3369 tension/compression device. Transverse rupture strength tests were performed at a deformation rate of 2mm/min. Transverse rupture strength load was applied separately to the layer surface of the FGM with the highest reinforcement and to the layer surface without reinforcement. The transverse rupture strength test results were determined by the average of the measurements after at least 4 measurements in each direction. III. RESULTS AND DISCUSSION A. Microstructure Results SEM images of each layer and the interlayer transition zones were taken to determine the microstructure properties of the produced FGMs. SEM images are given in Figure 3 for the 3-layer FGM and Figure 4 for the 4-layer FGM. (a) (b) Fig. 3. SEM microstructure images of 3-layer FGMs: (a) 0% to 30% wt. B4C transition surface image, (b) 30% to 60% wt. B4C transition surface image. (a) (b) (c) Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8788 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … It was determined that B4C reinforcement in FGMs settled at grain boundaries and showed a relatively homogeneous distribution in low-rate layers. It was observed that B4C reinforcement accumulated at the grain boundary with increasing reinforcement ratios. It is not easy to obtain a homogeneous distribution while producing composite materials with the powder metallurgy technique because the reinforcement can settle at the grain boundaries. (a) (b) (c) Fig. 4. SEM microstructure images of 4-layer FGMs: (a) 0% to 20% wt. B4C transition surface image, (b) 20% to 40% wt. B4C transition surface image, (c) 40% to 60% wt. B4C transition surface image. In addition, it has been reported that the porosity increases due to the increasing reinforcement ratio [1, 18]. It was determined that agglomeration and micropores increased as the amount of reinforcement increased in the layers of the produced FGMs. When the transition zones between the layers of the produced FGMs were examined, no delamination was observed. It has been observed that the transition of FGM between layers is continuous and has structural integrity. B. Hardness Results It has been reported that the hardness value increases with the increase of the ratio in the reinforcement structure in composite materials up to certain values [19-22]. The hardness measurements performed on the layers of the produced FGM samples showed that the hardness value increased up to 40% wt. B4C, and after this value, it decreased depending on the decreasing matrix amount and increasing porosity. Similarly, in [15], the measured hardness value was 40% wt. Graphs of change in the hardness (Brinell; HBN) values of each layer depending on the number of layers of FGMs are presented in Figure 5. (a) (b) Fig. 5. Layer hardness of FGMs: (a) 3-layer, (b) 4-layer. It can be seen in Figure 5 that the hardness increases to a certain extent (30% wt. B4C in 3 layers and 40% wt. B4C in 4 layers) depending on the increase of layer reinforcement. Suppose a composite is produced with success within the desired limits (homogeneous distribution property, porosity amount, etc.) of composite materials. In that case, it is expected Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8789 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … that the mechanical properties and hardness values will increase according to the mixture theory as the reinforcement ratio increases up to a certain value. Equation (1) presents the mathematical expression used to theoretically calculate the hardness of the composites according to the mixture theory. �� � ���� � ���� (1) where Hk is the hardness of the composite, Hm is the hardness of the matrix, fm is the weight ratio of the matrix, Ht is the reinforcement hardness, and ft is the reinforcement by weight ratio. It was determined that the layer with the highest hardness value of the produced FGMs was the third layer of the 4-layer FGM reinforced with 40%wt. B4C. The hardness of this layer was measured as 170HBN. The lowest hardness value was 94HBN in the layer with the highest reinforcement ratio (60% wt.) of the 3-layer composite. When the SEM images of the 3- and 4-layer FGMs in Figure 3 and Figure 4 are examined, it was seen that the 60% wt. B4C reinforced layer had the highest porosity. Therefore, Brinell's hardness decreased in layers containing 60% wt. B4C reinforcement. C. Transverse Rupture Strength Tests Result In order to determine the breaking strength of FGMs, transverse rupture strength test was performed on reinforced and non-reinforced layer surfaces under test conditions prepared under the ASTM B528-05 standard. The flexure stress-strain graphs obtained as a result of the experiments are presented in Figure 6. It can be seen in Figure 6 that the transverse rupture strength of FGMs differs according to the region where the load is applied. The measured transverse rupture strength of the FGM was higher (~153% in the 3-layer, ~65% in the 4-layer composite) with the application of the fracture load from the dense region of the reinforcement, because composites are more resistant to compressive stresses [23]. The highest obtained transverse rupture strength was 482MPa, from a 4-layer FGM applied to the load region with the highest number of reinforcement. In Figure 6(b), 4-layer FGM transverse ruptured from the region with less reinforcement showed brittle fracture than the 3-layer FGM due to the decreased amount of matrix due to the layer increase. The increase in strength in composites can be explained according to the law of mixtures. Equation (2) shows the strength increase in composite materials according to the law of mixtures [24, 25]. �� � ���� � ���� (2) where σK refers to the mechanical strength of the composite, σm refers to the mechanical strength of the matrix, fm refers to the weight ratio of the matrix, σt refers to the mechanical strength of the reinforcement, and ft refers to the weight ratio of the reinforcement. This equation predicts the strength values when the composite is produced within the desired limits without exceeding the ideal production conditions. As the ratio of the side with a higher strength value compared to others increases, the strength of the composite also increases. It was observed that the porosity increased as the amount of reinforcement increased, in accordance with [1, 15]. The highest Brinell hardness in FGM was obtained in the 40% wt. B4C layer. As the reinforcement ratio increased, the hardness of the composite decreased. Similar results were found in [1]. The best results in the transverse rupture strength test were obtained on the reinforced layer surface. Similar results were obtained in [15]. (a) (b) Fig. 6. FGM transverse rupture strength graphics: (a) 3-layer, (b) 4-layer. IV. CONCLUSIONS This study produced 3 (0-30-60% wt.) and 4 (0-20-40-60 wt.%) layers of B4C reinforced AA7075 matrix FGM with the vertical separator moulding technique. The following results were obtained in this study, in which the microstructure characterization, hardness and transverse rupture strength of the produced FGMs were examined. • With the vertical separator moulding technique, FGMs with smooth surface transition were successfully produced without any delamination in the transition areas. • It was determined that the distribution of B4C reinforcement in the layers of FGMs with a low reinforcement ratio (wt. 20% and 30%) was relatively homogeneous compared to the other layers. • No delamination was observed between the layers of FGM. Engineering, Technology & Applied Science Research Vol. 12, No. 4, 2022, 8785-8790 8790 www.etasr.com Kayabasi et al.: Functionally Graded Material Production and Characterization using the Vertical … • The highest porosity was observed in the layer containing 60 wt.% B4C. The lowest measured Brinell hardness was 92HBN in the 3-layer FGM containing 60% wt. B4C reinforcement. • The highest measured Brinell hardness was 170HBN in the 4-layer FGM containing 40% wt. B4C reinforcement. • In terms of transverse rupture strength, the tests applied to the reinforcement layer surface gave better results than the tests applied to the non-reinforcement layer surface (~153% in three layers, ~65% in four layers). • The greatest measured transverse rupture strength was 482MPa due to the application of the load from the region of the 4-layer FGM with the highest reinforcement. REFERENCES [1] F. Erdemir, A. Canakci, and T. 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