SCI SINTERING 56 04 2024 05pdf
SCI SINTERING 56 04 2024 05pdf
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  1. Science of Sintering, 56 (2024) 455-474________________________________________________________________________ _____________________________ *) Corresponding author:hakanada@gazi.edu.tr https://doi.org/10.2298/SOS240208011AUDK: 691.73; 692.533.1 Optimization of Bonding Parameters in Diffusion Bonding of Cu10Sn - B4C Composites Produced by P/M Method Hakan Ada1,2*), Serkan Özsoy3,41Gazi University, Faculty of Technology, Department of Metallurgical and Materials Engineering, Ankara, 06560, Türkiye. 2Kastamonu University, Faculty of Engineering and Architecture, Department of Mechanical Engineering, Kastamonu, 37150, Türkiye. 3Kastamonu University, Institue of Science, Department of Mechanical Engineering, Kastamonu, 37150, Türkiye. 4Kastamonu Taşmektep Vocational and Technical High School, Türkiye. Abstract: In this study, bronze matrix and B4C reinforced composite materials were produced by the P/M method, and diffusion bonding processes were applied to these materials in the experimental setup created by the Taguchi method. In the bonding processes, it is assumed that the temperature required for diffusion will also be sufficient for the sintering of the samples, so an additional sintering process has not been performed on the specimens. Shear and microhardness tests and microstructural examinations were carried out to determine the material characterization. Microstructure examinations indicated that the powder metal specimens were properly sintered. In experimental studies, a decrease in microhardness and an increase in shear strength were observed as the temperature increased. With the increase in reinforcement rate, an increase in the shear strength of bonds and microhardness was observed. After the optimization process, the optimum result was detected in the bonding at 820°C temperature, 20 kg load, and 40 minutes duration in unreinforced bonding. At the end of the verification experiments, it was observed that the resistance value obtained in the experimental studies and the estimated value were negligible at 3.71%, and there was a 21.92% progress in shear strength compared to the initial parameter value. Keywords: P/M; Bronze composite; Cu10Sn; Diffusion bonding; Taguchi optimization. 1. Introduction Bronze is an essential industrial alloy with high strength, excellent corrosion resistance, and high thermal and electrical conductivity. [1-5]. Bronze, used as a bearing material, is also an alloy suitable for use under large and impact loads and at high temperatures that pose a risk of corrosion. Due to these properties, bronze alloys are preferred in electronics, aviation, maritime, and defence industries [5-9]. Powder Metallurgy (P/M) is a widely used industrial manufacturing method for bronze materials. However, in materials produced by P/M, porosity within the structure is one of the factors that reduce the mechanical properties of bronze [10]. Adding reinforcement particles to the bronze matrix is one of the steps to increase the mechanical properties of bronze composites. One of the most important of these reinforcement particles is Boron Carbide (B4C), along with Alumina
  2. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________456(Al2O3) and Silicon Carbide (SiC), and it is one of the reinforcement materials frequently used in metal matrix composites [11-15]. B4C is an essential ceramic material with physical, mechanical, and chemical properties such as high hardness, elastic modulus, and thermal andchemical stability. These properties have made B4C an essential material for various engineering applications. B4C is widely used in many fields, such as abrasive powders and coatings due to its high wear resistance, ballistic performance due to its high hardness and lowdensity, and nuclear applications as a neutron radiation absorber. Composite materials are one of the engineering materials generally used in optical, electronic, refractory, high temperature, and tribological properties. Composite materials can be produced as metal-ceramic, metal-polymer, ceramic-polymer, or metal-ceramic-polymer using different manufacturing methods and combined with bonding techniques. In the literature, it has been observed that studies have been carried out using methods such as diffusion bonding [16-19], brazing [20-22], and ultrasonic welding [23,24] to combine metal matrix and ceramic-added composite materials. Among these, diffusion bonding with high structural integrity is one of the frequently used joining methods in which metallurgical bonding is achieved by applying temperature, duration, and axial forces on solid surfaces [16-19]. Determining the optimum parameters is essential for the welded joints [25-28]. In recent years, the Taguchi method has been an essential optimization method that reveals the best levels and factors, especially in determining optimum parameters [29-31]. Taguchi method consists of planning, execution, analysis, and verification stages. Unlike many optimization methods based on advanced statistical methods, the Taguchi method focuses on applying the most effective strategy in solving an engineering problem with less time and cost. To determine mechanical performance, researchers conduct many experiments that consider different parameters, which takes too much time. However, since process parameters and speed are vital today, obtaining the optimum experimental design is crucial to saving time and cost with different optimization methods. For this reason, methods such as Taguchi, Artificial Neural Networks, and Anova for experimental design and optimization are popular in recent literature studies. Among these methods, the Taguchi method is mainly preferred because it provides optimum design by simultaneously solving the connection between multiple factors and variables [32]. Various studies on parametric optimization of welding processes are reported in the literature. In studies using the Taguchi method and optimization, Ada and Çetinkaya [31] reported the bonding performance of steel pipes using covered electrode arc welding.Ogbonna et al. [33] optimized the multiple performance properties of MIG welded butt joints of AISI 1008 and AISI 316 steels. Niranjan et al. [34] optimized the effect of the tool pin profile in the friction stir welding process. Ramarao et al. [35] optimized welding parameters such as welding current, voltage, and slope angle to obtain better impact strength in joining SA387 and SS304 steels. Saeheaw [36] optimized the multi-criteria process in Nd:YAG laserwelding. Santosh et al. [37] optimized the effects of welding current, gas flow rate, and filler diameter on the mechanical properties of gas tungsten arc welded joints of AA2024 and AA6061 Aluminium (Al) alloy. Arunkumar et al. [38] optimized critical welding parameters such as welding voltage, current value, and welding slope angle to achieve higher impact strength when joining SA387 and SS316 steels. Vishwakarma and Dwivedi [39] optimizedthe process parameters in Submerged Arc Welding (SAW). In the reported studies on optimizing bonding parameters with the Taguchi method, Venugopal and Mahendran [40] investigated the effect of diffusion bonding parameters on the strength of AA5083 Al alloys. Sharma et al. [41] investigated the impact pressure-assisted diffusion bonding process of low-carbon steel using a silver interlayer. Sittaramane and Mahendran [42,43] investigated the effect of diffusion bonding parameters such as temperature, pressure, and duration on the bond strength in joining AA6061 Al matrix and SiC and AA6061 Al matrix and fly ash-reinforced Al composites by diffusion bonding. Mei et al. [44] investigated the uniaxial
  3. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________457tensile strength and deformation rate of connections in diffusion-bonded joining experiments. Safarian et al. [45] investigated the effect of the sintering temperature, dwell duration, and heating rate of the insert made of 316L stainless steel and the injected area on the diffusion bonding process. Cooke et al. [46] investigated the transient liquid phase diffusion bondingprocess. Many studies are in the literature on diffusion bonding and optimization of diffusion bonding parameters. However, no studies have been found on the diffusion bonding of copper or bronze matrix composite materials produced by the P/M method and the optimization of bonding parameters in diffusion-bonded joints of these materials. Therefore, this study focused on the production of Cu10Sn matrix and B4C reinforced composite materials, diffusion bonding and optimization of bonding parameters, and the closeness of the predictionresults of the Taguchi method. Thus, the results in the shear strength of bonding bronze matrix and B4C reinforced composite materials with diffusion bonding were interpreted comparatively. 2. Materials and Experimental Procedures 2.1. Determining the Experimental Setup Primarily, the experimental setup was determined for the experimental studies in which diffusion welding processes were carried out. Since there would be a need to conduct hundreds of experiments when all factors are considered to determine the accuracy of the parameters, the experimental setup was determined using the Taguchi method to create an effective and adequate number of experiments. The inputs include 4 factors at 4 different levels suggested by the Taguchi method's L16 (4*4) orthogonal array. Material type, temperature, load, and duration were determined as input parameters on the Minitab19 software program, where the Taguchi method was applied, and experiments were carried out by grading each factor with 4 levels. The materials produced from B4C reinforced combinations of bronze, which is widely preferred in industry, were selected in different proportions in accordance with the literature (unreinforced, reinforced 4 wt.%, 8 wt.% and 12 wt.%) [2,47,48]. As the type of material used, materials produced from B4C reinforced combinations of bronze in different proportions (unreinforced, 4 wt.%, 8wt.%, 12wt.%), which is widely preferred in the industry, were chosen. The temperature in experiments was determined as 700, 740, 780, and 820°C; load was determined as 10, 15, 20, and 25 kgs; and duration was determined as 20, 40, 60, and 80 minutes. The levels and factors used in the Taguchi experimental setup are given in Table I, and the operating parameters determined in the L16 (4*4) orthogonal array are given in Table II. Tab. I Experiment Factors and Levels. Symbol Factors Levels 1 2 3 4 A Material Type %100 Cu10Sn %96 Cu10Sn %4 B4C %92 Cu10Sn %8B4C %88 Cu10Sn %12 B4C B Temperature (°C) 700 740 780 820 C Load (kg) 10 15 20 25 D Duration (Min.) 20 40 60 80
  4. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________458Tab. II Diffusion bonding parameters determined by Taguchi Method in the L16 (4*4) orthogonal array. Experiment Number Material –Reinforcement Rate (%) Temperature (°C) Load (Kg) Duration (minute) E1%100 Cu10Sn 700 10 20 E2%100 Cu10Sn 740 15 40 E3%100 Cu10Sn 780 20 60 E4%100 Cu10Sn 820 25 80 E5%96 Cu10Sn + % 4 B4C 700 15 60 E6% 96 Cu10Sn + % 4 B4C 740 10 80 E7% 96 Cu10Sn + % 4 B4C 780 25 20 E8% 96 Cu10Sn + % 4 B4C 820 20 40 E9%92Cu10Sn + % 8 B4C 700 20 80 E10 %92 Cu10Sn + % 8 B4C 740 25 60 E11 %92 Cu10Sn + % 8 B4C 780 10 40 E12 %92 Cu10Sn + % 8 B4C 820 15 20 E13 %88 Cu10Sn + % 12 B4C 700 25 40 E14 %88 Cu10Sn + % 12 B4C 740 20 20 E15 %88 Cu10Sn + % 12 B4C 780 15 80 E16 %88 Cu10Sn + % 12 B4C 820 10 60 A graphical abstract expressing the steps followed during the experimental studies and optimization process is given in the schematic representation in Fig. 1. Fig. 1. Process steps in experimental studies (Graphical abstract of the study). 2.2. Preparation of composite materials For the second phase of this study, the execution phase, primarily unreinforced Cu10Sn alloys, and Cu10Sn matrix and B4C reinforced composite materials (4 wt.%, 8 wt%, 12 wt.%) were produced. In the production of composite materials, pre-alloyed Cu10Sn powders with a size of <44 μm and 85% purity used as the main matrix material, and B4C
  5. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________459powders with a size of <45 μm and 99.5% purity used as reinforcement material were supplied from Nanokar. SEM images of the powders used in the study are given in Fig. 2. Fig. 2. SEM images of the powders used in the study: a) Cu10Sn powders, b) B4C powders. The powders, which were added with 4 wt. %, 8 wt.%, and 12 wt.% B4C into the Cu10Sn matrix by weight along with 100% Cu10Sn alloy, were prepared by weighing by using a 10-4precision balance. Mechanical Alloying (MA) was done on the prepared powders with a Retsch brand PM100 model planetary ball mill device at a speed of 400 rpm and 2 hours duration. In the MA process carried out with 10 mm diameter stainless steel balls in a stainless steel container, the powder/ball ratio was determined as 1:10. Cylindrical specimens with dimensions of Ø10 x 10 mm were produced by compressing the mechanically alloyed powders in a steel mould under 400 MPa pressure. Considering that the temperature required for diffusion in bonding processes will meet the temperature requirement for the sintering of the specimens, no additional sintering process was performed at this stage, as the specimens will be both sintered and bonded together in solid form. 2.3. Diffusion bonding of the specimens Fig. 3. Atmosphere-controlled diffusion bonding machine and a macro view of the bonded specimen. All the samples were prepared for microstructural characterization. A total of 128 specimens were produced for the tests and examinations in each experiment, and the produced specimens were then subjected to bonding processes. The temperature in experiments was determined as 700, 740, 780, and 820°C; load was determined as 10, 15, 20, and 25 kgs; and
  6. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________460duration was determined as 20, 40, 60, and 80 minutes. The bonding processes were carried out in a machine specially designed for diffusion bonding under 3 lt/min pressure and atmosphere control with high purity Argon gas. The image of a specimen assembled with a diffusion bonding machine and a diffusion bonded specimen is given in Fig. 3. 2.4. Microstructural Characterization The microstructural features of the bonding areas were analyzed by Optical Microscope (OM), Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), and X-Ray Diffraction (XRD) examinations. Diffusion-bonded specimens were cut in half longitudinally for sectional analysis. All the samples were prepared for standard microstructural characterization. The specimens were prepared for microstructural examinations by being etched with an etching reagent consisting of 10 ml HF + 30 ml HNO3and 50 ml H2O. OM images were taken with the help of a Nikon brand device; SEM images and element distribution spectroscopy were taken with the help of a Fei Quanta 250 Feg brand device; and XRD analyses were carried out with the help of a Bruker D8 Advance brand device. 2.5. Mechanical Tests Microhardness measurements of the specimens, of which surfaces were prepared for metallographic applications, were carried out using the Vickers hardness measurementmethod by applying a force of 0.98 N on the Shimadzu HVM-2 brand microhardness measuring device. The shear test method was used to determine the planar sliding properties of materials bonded by diffusion bonding along with microhardness tests to determine mechanical properties. The shear strengths of the specimens bonded by diffusion bonding were determined in a Shimadzu brand tensile-compression device at a speed of 0.5 mm/min. For this experiment, a special design was prepared and produced, the schematic of which is given in Fig. 1. 3. Results and Discussion 3.1. Macrostructural Investigations Materials produced from Cu10Sn matrix alloys that were unreinforced and containing 4 wt.%, 8 wt.%, and 12 wt.% B4C were bonded by the diffusion bonding method by the parameters specified in Table II. Macro images of the combined specimens are given in Fig. 4. Fig. 4. Macro images of the bonded specimens. The macro images of the diffusion-bonded specimens in Fig. 4 show that bonds occurred in all bindings. As a result of plastic deformation occurring with increasing temperature, load,
  7. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________461and duration, the changes in the dimensions of the bonded specimens (such as E4, E5, E7, E8, E12 and E15) were observed. It is understood from the macro images that while the lengths of the specimens in question shorten depending on load, temperature, and duration, there is an expansion in their diameters to some extent. It can be observed in the visual examinations that some blackening and scale layers occur on the surfaces of the bonded specimens due to the effect of temperature and heat despite the protective Argon atmosphere. 3.2. Microstructural Investigations In the specimens bonded by the diffusion bonding method, OM and SEM images were taken to support one another for microstructure examinations to examine the effects of temperature, load, and duration on the bonding area (Table III). To see the effects of the experiment parameters, OM images of the specimens taken from the bonds of E1, E6, E11,and E16 are given in Fig. 5, and SEM images are given in Fig. 6. Fig. 5. OM images of the diffusion bonds a) E1 (Unreinforced, 700°C, 10 kg, 20 minutes), b) E6 (4 wt.% B4C reinforced, 740°C, 10 kg, 80 minutes),c) E11 (8 wt.% B4C reinforced, 780°C, 10 kg, 40 minutes), d) E16 (12 wt.% B4C reinforced, 820°C, 10 kg, 60 minutes). Fig. 6. SEM images of diffusion bonds a) E1 (Unreinforced, 700°C, 10 kg, 20 minutes), b) E6 (4 wt.% B4C reinforced, 740°C, 10 kg, 80 minutes), c) E11 (8 wt.% B4C reinforced, 780°C, 10 kg, 40 minutes), d) E16 (12 wt.% B4C reinforced, 820°C, 10 kg, 60 minutes).
  8. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________462Tab. III Shear test results. Experiment Number Material - Reinforcement Rate (%) T (°C) Load (Kg) Duration (min.) Average Shear Strength (N/mm2) E1%100 Cu10Sn 700 10 20 142.77 E2%100 Cu10Sn 740 15 40 147.31 E3%100 Cu10Sn 780 20 60 221.07 E4%100 Cu10Sn 820 25 80 262.89 E5%96 Cu10Sn + % 4 B4C 700 15 60 67.23 E6% 96 Cu10Sn + % 4 B4C 740 10 80 105.37 E7% 96 Cu10Sn + % 4 B4C 780 25 20 163.43 E8% 96 Cu10Sn + % 4 B4C 820 20 40 345.21 E9%92Cu10Sn + % 8 B4C 700 20 80 65.04 E10 %92 Cu10Sn + % 8 B4C 740 25 60 122.27 E11 %92 Cu10Sn + % 8 B4C 780 10 40 162.77 E12 %92 Cu10Sn + % 8 B4C 820 15 20 209.42 E13 %88 Cu10Sn + % 12 B4C 700 25 40 67.23 E14 %88 Cu10Sn + % 12 B4C 740 20 20 111.16 E15 %88 Cu10Sn + % 12 B4C 780 15 80 152.81 E16 %88 Cu10Sn + % 12 B4C 820 10 60 217.31 In the microstructure images in Fig. 5 and Fig. 6, OM and SEM images of B4C reinforced composite materials at various rates (E6, E11, and E16) and the unreinforced alloy (E1) were given. In all images, bonding regions were observed to occur along a line. It was understoodthat the B4C particles, which are reinforcing elements, in the bonding specimens in E6, E11, and E16 show a homogeneous distribution in the Cu10Sn matrix. Lamellar structures were noticeable in the microstructures. Lamellar structures are known to be formed due to MA triggered by B4C reinforcement and provide homogeneous distribution [49]. The changes in microstructure images with increasing temperature, load, and duration are also noticeable in all specimens bonded under different conditions. It is known that atomic transmigration between the bonding areas of bonded specimens increases with increasing temperature andduration under load [50]. In the light of information obtained from the literature, in specimens with diffusion bonding, in general, in the first stage of bonding, contact was achieved in a large area at the interface with the flow, and creep mechanisms in the surface roughness and bonds generally occurred at the grain boundaries [51]. Under the influence of pressure, the interlayers on the surface were broken, and atom flow started. In the second stage of bonding, diffusion was more effective than deformation. At this stage, the pores disappeared due to grain boundary diffusion, and some formed within the grain. In the third bonding stage, it was observed that the atomic bond between the specimens to be bonded was completed. At this stage, the pores were eliminated largely by volume diffusion [52]. In the microstructures in Fig. 5, lamellar structures are noticeable. It is known that lamellar structures are formed due to mechanical alloying triggered by B4C reinforcement and provide homogeneous distribution. As observed from both OM and SEM images, the non-porous structures formed indicate that the diffusion bonding process has been carried out well. In most of the bonds, interfaces are formed depending on the Cu10Sn/B4C alloy ratios, and contact is achieved. In some bonds (E1), it was observed that the interface and contact could not be fully achieved;
  9. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________463therefore, diffusion could not occur as desired, and the interlayer in these bonds was formed along a line in OM and SEM images. Also, in the specimens produced using reinforcement, it was observed that the reinforcement material B4C consisted mainly of dark grey, angular, and complex-shaped grains in the microstructure images. Positive results were obtained for all parameters in all experiments due to diffusion bonding processes performed at different temperatures, loads, and durations. It can be thought that the temperature (700°C) andduration (20 min) applied in bonding the unreinforced Cu10Sn alloy in E1 were insufficient for diffusion bonding. It is understood from the OM and SEM images taken for E1 that the interlayer has not entirely disappeared. In the E6, E11, and E16, it can be justified that the B4C reinforcement caused the interlayer to tear and disintegrate. While the interlayer is seen along a line in the diffusion bonding of unreinforced alloys, it has been determined that there is no interlayer in the bonds using B4C reinforcement. It is understood that the interlayer is disintegrated or destroyed. It can be said that reinforcement materials positively affect the elimination of the interlayer in the diffusion source of composite materials, and the B4C reinforcement used as reinforcement initiates the diffusion event early by eliminating the interlayer [53,54]. Therefore, it is understood that the reinforcing element used in joining B4C reinforced composite materials by diffusion welding tears the interlayer under pressure, resulting in sound diffusion.When OM and SEM examinations were evaluated in general, it was observed that the diffusion welding processes in the Taguchi experimental setup were realised, and porosity did not occur in the structure. In addition, it was determined that the powder metal specimens produced by the P/M method were sintered correctly. It wasunderstood that the temperatures and durations used for diffusion bonding can be used for the sintering process of specimens produced by the P/M method. 3.3. EDS Investigations The SEM image taken from the diffusion zone of a test specimen is given in Fig. 7, and the graphs obtained by EDS analyses applied for E1, E6, E11, and E16 in the example are given in Fig. 8. When EDS analyses were examined, Cu, Sn, B and C elements were present in the EDS peaks, and it was also observed that there was an increase in the peaks of the elements depending on the increase in the reinforcement ratio. Fig. 7. SEM image of the zones measured by EDS.
  10. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________464Fig. 8. EDS graphic and results a) Unreinforced alloy, b) 4 wt.% B4C reinforced alloy, c) 8 wt.% B4C reinforced alloy, d) 12 wt.% B4C reinforced alloy. 3.4. XRD Analysis As in the microstructure and EDS analyses, XRD examinations were carried out on one specimen from each experimental group to serve as an example depending on the content. XRD graphs obtained from measurements are given in Fig. 9. Fig. 9 shows the XRD analysis of Cu10Sn metal matrix composites containing different proportions of B4C by weight. XRD analyses were performed at the 2θ angle between 10° and 90° on the specimens taken fromthe bonds. Fig. 9. XRD graphics a) Unreinforced alloy, b) 4 wt.% B4C reinforced alloy, c) 8 wt.% B4C reinforced alloy, d) 12 wt.% B4C reinforced alloy.
  11. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________465 The identities of the patterns were determined by correlating the peaks in the graph given in Fig. 9 with the literature. In the XRD pattern analysis obtained, CuSn and B4C peaks were determined by the effects of temperature, duration, and axial force. This situation reveals that diffusion bonding parameters do not cause the formation of different intermetallic phases in the bonds of bronze matrix composite materials. The highest CuSn peaks obtained from the graph occur at 42.75°, 49.60°, 73.00° and 88,40° 2θ angle positions, respectively, while the peaks of the B4C compound occur at 31.55° and 38.85° 2θ angle positions, respectively. XRD analysis shows that CuSn alloy has more significant peaks and B4C has weaker peaks. With the increase of reinforcement content, the peak intensity of B4C increased, while the intensity of CuSn peaks partially decreased. In the specimens taken, it was observed that there was no copper oxide phase, and the dominant phase consisted of CuSn. It was also determined that there was no interfacial reaction between the Cu10Sn matrix and B4C in the spectrum. The XRD analysis has shown no interaction between the particles in the Cu10Sn-B4C composites. XRD peaks obtained in similar studies were compatible with those obtained in our study [55-57]. In addition, the XRD graph shows intensity decreases in XRD peaks due to the decrease in the crystallite size and increase in the internal strain of the Cu10Sn matrix due to the amount of B4C reinforced and the effect of MA [58,59]. 3.5. Microhardness Investigations Fig. 10. Microhardness results obtained from experiments a) 100 wt.% Cu10Sn (unreinforced), b) 96 wt.% Cu10Sn – 4 wt.% B4C, c) 92 wt.% Cu10Sn – 8 wt.% B4C, d) 88 wt.% Cu10Sn - 12 wt.% B4C.
  12. H. Ada et al.,/Science of Sintering, 56(2024)455-474 ___________________________________________________________________________466 Specimens bonded by diffusion bonding were subjected to microhardness tests. Microhardness tests were carried out using the Vickers hardness measurement method, as seen in Fig. 10, by taking the diffusion line as the centre and making nine measurements at 50 μm intervals to the right and left of the diffusion line. In the measurements, a load of 0.98 N force was applied; the average of the diagonal lengths of the resulting trace was taken, and the microhardness was determined in GPa using the formula given in Eq. 1.: =1854,4 (2) (GPa) (1)The results obtained from microhardness tests are given graphically in Fig. 10, with groupings based on reinforcement ratios. Since the temperature affects and distributes equally in all regions throughout the diffusion bonding, hardness values are expected to be measured close to each other in the hardness examinations of the materials bonded by diffusion bonding, and the graph gives a linear image with close values. The microhardness graphs in Fig. 10 reveal measurements consistent with this expected situation. The graphs generally show small fluctuations in measurement values, which have been determined to be higher in some regions. The higher increase and decrease in microhardness measurements of B4C reinforced composite materials in these regions than in other regions may be due to the hardness difference between the matrix material and the reinforcement material. Fig. 10a shows the microhardness results of specimens containing 100 wt.% Cu10Sn (E1, E2, E3, and E4). When the microhardness results were examined, it was determined thatE1 and E3 were close to each other and contained the most average results. The highest microhardness results were seen in E2, and the lowest microhardness results were seen in E4. Microhardness measurements with an average value of 1.0 GPa were determined in this experimental group. Considering these results, temperature is thought to be an essential factor. Decreases in microhardness measurements were observed due to the effect of temperature and duration. The microhardness measurements performed for the specimens between E5 and E8 in Fig. 10b showed a significant increase in microhardness compared to the unreinforced specimens between E1 and E4 (Fig. 10a). This condition appears to be caused by B4C supplementation, a harsh compound. From the graph in Fig. 10b, it was determined that the microhardness results between E5 and E8 were close to each other and approximately 1.5 GPa. When the results were examined, it was seen that, on average, the highest microhardness result occurred in E5, and the lowest microhardness result occurred in E8. When the microhardness results were examined in Figure 10.c, it is noteworthy in the measurements that the microhardness increases with the increasing amount of B4C. On average, the microhardness was found to be approximately 1.8 GPa here. The graphs obtained from microhardness tests performed on diffusion bonds of composite materials containing 12 wt.% B4C are given in Figure 10.d. The results obtained from the microhardness tests here showed that with the increase in the reinforcement ratio, there was no significant increase compared to the 8 wt.% ratio and even a slight decrease. It is known from many studies that B4C reinforcements used in composite materials above a specific rate do not have any positive effect on mechanical properties and may even cause a decrease [60,61]. When microhardness data were examined, it was observed that microhardness values of approximately 1.7 GPareached 12 wt.% reinforcement rates. In general, in microhardness measurements, it can be said that microhardness increases with the inclusion of 4 wt.% and 8 wt.% B4C particles by weight into the bronze matrix, and 12 wt.% B4C does not have very different results from the 8 wt.% reinforcement ratio. Therefore, microhardness can stabilise at this rate and can even show a slight decrease.
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