15.1 Introduction

The design policy of the new composite materials should improve the mechanical and physical properties of the pieces used in new technical applications, considerably better than the current ones. Therefore, in the his stage of the research project that is going on, we proposed a new design aluminium matrix composites for the mechanical joints such as the pin – joint very often known as knuckle components, as rod-couplings, recently a wind shield wipers for automotive industries as transmitting pieces. Here, the basic idea is to create a heterogeneous structure actually for the pin joint. In fact some of the industrial partners use extensively knuckle component as pin joint, etc. As well known, heterogeneous and anisotropic materials are useful structures for decreasing damage zone by creating dynamic instabilities such as local plastic deformation, buckling, etc., it means that the behaviour of the materials can be changed by the heterogeneous structures, [1,2,3,4,5,6,7,8].

Again, the Powder Metallurgy (PM) route is known as most commonly used method for the preparation of reinforced MMCs. This method is generally used as low – medium cost to produce small objects (especially round), tough, the high strength and resistant materials. Since no melting is involved, there is no reaction zone developed, showing high strength properties. For this reason, in the present work, a simple idea was developed on the production of pin joint pieces from the high resistance aluminium based composites [9,10,11,12,13].

The composites developed in the frame of the research that is going on were based on aluminum matrix originally from the recycled chips of the aluminium series of AA7075 (90 wt %) and AA1050 (10 wt %) given by French aeronautic company. We aimed for the application for the connection link in a mechanism to transfer motion, for example; between the two railways wagons etc. and also some connecting link in aeronautical pieces as an alternative replacement for conventional alloys used in this area. As an alternative composite was produces through combined method of powder metallurgy followed by Sintering + Forging [1, 4, 6,7,8, 10, 13,14,15,16,17,18]. This composite contains basically zinc, silicon, magnesium and further reinforcements and auxiliary elements, where the Zn and Mg and Mn contents are 8, 0, 15 and 3 wt % respectively. For the sake of the mutual diffusion of certain reinforcements in the matrix, a few percentages of nickel was also added in the structure. This structure can be accepted as hypoeutectic structure [1, 4, 17].

15.2 Experimental Conditions

15.2.1 Materials Processing

In the present work, an alternative aluminum matrix composite (AMCs) was designed from the recycled chips of the aluminium series of AA7075 (90 wt %) and AA1050 (10 wt %) in the atomized form given by French aeronautic company through combined method of powder metallurgy (Sintering) followed by Forging. First of all, A typical Al-Zn-Mg-Si-Ni matrix was developed and reinforced basically with B2O3 (5, 10%). Chip milling was performed using high energy milling in a planetary ball mill with an inert argon atmosphere to prevent oxidation of the powders. Two compositions were prepared with two different percentages of B2O3, here called after J1 (5%) and J2 (10%) also one composition was kept without reinforcement (JB) for comparison with the reinforced ones. For easy wettability of the reinforcement of B2O3, certain amount of pure nano aluminium was added in the mixture. The composites produced with this novel combined method have certain advantages regarding to conventional manufacturing processes such as low cost, capability of manufacturing of the pieces with complex shapes, and processing simplicity, etc. The compositions of the three groups developed in the present work were given in Table 15.1.

Table 15.1 Composition of the three groups developed
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This composite contains basically zinc, silicon, magnesium and further reinforcements and auxiliary elements, where the Zn and Mg and Mn contents are 8, 0, 15 and 3 wt % respectively. For the sake of the mutual diffusion of certain reinforcements in the matrix, a few percentages of nickel was also added in the structure. This structure can be accepted as hypoeutectic structure.

Microstructural characterization and Mapping analyses was done by means of scanning electron microscope (SEM). The dispersion of reinforcement particles in the matrix and interface at matrix/reinforcements was evaluated. Micro hardness tests (HV0.1) have been carried out on the polished and etched specimens.

All the density measurements of the specimens were carried out by using Archimedes method. These values change between 2.72, 2.88 and 2.90 ± 3 for the JB, J1 and J2 respectively. Three cylindrical specimens (H/D ≥ 1.5) for each composition were tested under quasi-static compression conditions according the DIN 50106 norm by using a constant test speed as 0.0067 1/s.

Dynamic test known as drop weight tests have been carried out on a universal drop weight test device (Dynatup Model 8200 machine) with a total weight of 10.5 kg, punch height of 600 mm and with an impact velocity of 3 m/s.

Wear resistance was measured by scratch wear tests at a frequency of 15 Hz. All of the compositions were tested in two different numbers of cycles, 50*103, 100*103 cycles. After scratch test, damaged zone was investigated by 3D optical roughness meter. Surface and volume loss/time and maximum depth were evaluated for damage characterization.

15.3 Results and Discussions

15.3.1 Microstructure and Mapping Analyses of the Three Compositions Produced by “Sintering” and “Sinter+ Forging Process”

General microstructures of the three compositions (JB, J1 and J2) were presented in the Fig. 15.1. It is noted that the main structure of JB without reinforcement is quasi hypoeutectic structure with high Zn (8%) and Mg (3%) contents. Distribution of auxiliary elements is more and less homogenous. It seems that some of the certain elements are precipitated around the grain boundaries. “EDS” analyses for the three compositions were also given in the Fig. 15.1.

Fig. 15.1
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General microstructure of the compositions produced by only sintering process, JB, JI and J2 respectively

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However, local agglomeration is also observed. This case is directly related of mixture conditions. All of the specimens were prepared under our laboratory conditions, therefore, it should be improved operational conditions. As for the reinforcements, boron oxide and boron and also nickel were appeared on the microstructure. Major reinforcement elements such as B2O3, was well seen that its distribution is homogenous. However, the particles size of the boron oxide (black color) is stayed big may be found agglomeration locally like a small island. This is related to the sintering process. It is not sufficient to prevent this type of agglomeration, etc.

In the Fig. 15.2, the microstructure obtained thorough the combine method sinter + Forging. All of the reinforcement particles and microstructure have been improved well. Distribution and the size of the reinforcements have been changed successfully. Small size and homogenous distribution of the reinforcements and also the grain size of the matrix have been carried out by application of the combined methods; sintering + forging. At the second stage of the process, it means that hot forging influences considerably. For this reason, this method has been accepted as more advantageous than the conventional methods. In comparison of this method as cost effect point of view, the combined method is seen much more quality, simple and economic manufacturing method.

Fig. 15.2
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General microstructure of the compositions produced by double process; sintering + Forging), JI and J2 respectively

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15.3.2 Mapping Analyses of the Three Compositions Produced by Sinter+ Forging Process

As obtain vey homogenous structure with the combined method, the microstructure and mapping quantitative analyses have been carried out by SEM to see the distribution of the reinforcement and further auxiliary elements. Figures 15.3 and 15.4 present the general distribution of the micro and nano size particles in the microstructures of J1 and J2 compositions, respectively. It is noted that the distribution of certain auxiliary elements, for example, Fe, Mn, and Ni show small agglomeration in the matrix but the main particles are very smoothly distributed. This is main advantage of the combined method; sinter+forging. In any case, these structures can be improved by means of operating parameters it means that experimental parameters such as milling conditions, sintering and forging temperature, etc. should be controlled for obtaining ideal microstructures.

Fig. 15.3
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Mapping analyses of the composite for J1 produced by combined process; Sinter + Forging

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Fig. 15.4
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Mapping analyses of the composite for J2 produced by combined process; Sinter + Forging

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Micro hardness evaluation has been carried out on the mounted and polished specimens obtained only by the combined method; Sinter + Forging. It is noted that the hardness values have shown an increase but these are not so considerable increase as shown in the Table 15.2. This increase was observed directly with the percentage of the reinforcement elements. In fact, the reinforcements used here, such as boron oxide B2O3, Mn and Mg have a considerable effect on the mechanical properties, among them, vibrational fatigue properties can be improved by using these composites.

Table 15.2 Micro hardness (HVN) measurements for three compositions as mean values
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15.3.3 Static Compression Test Results

The idea for novel composite design can be applied very well on the industrial parts in an economic way. Actually, sintered forging process is a novel process mainly called as near-net shape process for the manufacturing of the pieces [13,14,15,16]. Mainly, this process is used for bulk materials in industrial applications. In fact, low-cost sinter-forging approach to processing of particle-reinforced metal matrix composites gives always high performance applications of the industrial pieces (fatigue-creep, static and impact compression, etc.).

In the frame of this present work, a simple static compression test was given in the laboratory scales for mechanical behaviour of this novel composite produced with sinter+forging processes. This idea should be developed very well on the many other composites in the industrial scale. It means that very tough and strong pieces can be obtained with this combined process but cheaper than the other manufacturing processes.

Figure 15.5 gives static compression test results obtained on the specimens produced by sinter + forging processes. These values are mean values obtained from three tests for each composition. Evidently standard deviation is variable around ±20–25 MPa. One may observe that the higher reinforcement element gives always higher resistance regarding to the simple composition without reinforcement (as compared with JB, J1 and J2). Ultimate tensile strength values in static compression tests for the specimens of J1 and J2 have been found around 220–250 MPa whereas the specimens of JB stayed at the level of 100 MPa.

Fig. 15.5
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Static compression test for the test specimens containing three compositions, J1, J2 and J3

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15.3.4 Low Velocity or Dynamic Compression (Drop Weight) Test Results

Low velocity or dynamic compression tests results were given in the Fig. 15.6. Maximum force was evaluated there by the values from both support data points. Here, a series of impact tests were performed at room temperature at the center of cylindrical specimens using the instrumented drop weight test device as explained in detail in 2nd the section (experimental conditions). Three specimens were tested for each composition (Fig. 15.5).

Fig. 15.6
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Impact behaviour for three different specimens obtained by dynamic compression test, JB, J1 and J2 respectively

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First of all, the effect of combined effect of sinter+forging processing on the impact resistance of the specimens (J1, and J2) can be detected very clearly from these graphs. The specimens of JB (without reinforcements) have shown relatively low values regarding to the compositions with the reinforcements.

It seems from these graphs, impact resistance is related to absorbed energy. In fact, all of the specimens have shown that the most part of the impact force is used to maintain the balance with the inertia force, and only a small portion of the impact force is actually used to deform and fracture of the specimen. Absorbed energy should be related with the process used here that this energy increases considerably in the structure obtained with sinter+forging. These results are only obtained in laboratory scales and should be improved with detail analyses.

15.3.5 Wear (Scratch) Test Results

Reduction of friction rests the key task for wear-resistant composites. Evaluation of the wear resistance of this composite developed in this work have been carried out in two different numbers of cycles, 50*103, and 100*103 cycles. Influence of reinforcement elements and essentially influence of the manufacturing processes, sinter + Forging are observed for three different specimens in the Fig. 15.7 and also wear (scratch) test results were given in Table 15.3 for the specimens of JB, J1 and J2 respectively.

Fig. 15.7
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Images of surface damage zones produced by wear – scratch test results for the three different composition in the cycle of 50*103 and 100.103 cycles for the comparison of damage zone defined with surface and volume lost

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Table 15.3 Wear (macro scratch) test results for the specimens of JB, J1 and J2 respectively
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The surface damage (mm2), volume lost (μm3) and the depth (μm) values are presented in the same figures for each test condition. Effects of reinforcements and obviously, the effects of the sinter + Forging process are observed as an advantage over others.

As shown in the results of scratch tests, the composites processed with sinter+forging has shown always higher wear resistance regarding to the simple structures. In this particular case, the size of the reinforcements and their dispersion on the matrix should have contributed to improve the wear resistance under experimental conditions carried out in the present work.

Total evaluations of the microstructure and mechanical properties (static compression, dynamic- drop weight and also wear-scratch tests) give a clear idea about the combined process (sintered + forging). This process applied for these types of composites is caused by bonding diffusion at the interface between matrix and reinforcement and some of the particles were forced into the grains during the forging (second) stage of this process. For this reason, very tough, solid and homogeneous structure could be obtained. Porosity and other structural – micro defects were quasi eliminated.

15.4 Conclusions

A novel composite was designed design aluminium matrix composites for the mechanical joints such as the pin – joint very often known as knuckle components as alternative composite against the conventional pieces used for example as rod-couplings, for automotive industries as transmitting pieces. In the frame of this common project, novel composites have been developed from aluminium AA7075 powder obtained with atomization of fresh scrap-chips as the initial form reinforced with B2O3, Mn, Ni, Mg etc. particles as main reinforcements in an economic way. Low cost manufacturing of these composites have been successfully managed through the combined method of sinter + forging.

Microstructural analysis has shown that a good bonding at interface of matrix-reinforcement essentially in the specimens manufactured with combined process sinter + forging; a tough and complete microstructure was obtained without porosity. Wear resistance and ductility of these structures should be improved with doping process and good pretreatment conditions; ball milling in longer time is needed for helping the fine and homogeneous distribution of the particles in the matrix. Mechanical behaviour of these specimens produced with sinter + forging process are better than the specimens produced without reinforcement. This process seems very confident values for future work of the production of alternative pieces used in joint parts and also for other tribological applications. Optimizations of the certain parameters such as processing parameters, reinforcement content, etc. need much more experimental work to create real parts in the industrial scales. Here, only limited measurements at room temperature were presented as they are indicative parameters for better understanding the effect of the reinforcements on the optimization of the mechanical, (static and dynamic) and wear properties of the composites produced in the present work.