UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

280 IMMC 2018 | 19th International Metallurgy & Materials Congress

Reciprocating Wear Behavior of AA7075/SiCp Composites Fabricated using Powder Metallurgy and Hot Pressing

Kenan Kaynak¹, Ulaş Matik²

¹Department of Manufacturing Engineering, Karabuk University, 78050, Karabuk, Turkey²Vocational School of TOBB Technical Sciences, Karabuk University, 78050, Karabuk, Turkey

Abstract

The influence of silicon carbide particulates (SiCp) on the wear behavior of Al–Zn–Mg–Cu alloy (AA7075)–SiCp composites has been evaluated using a reciprocating wear apparatus. The AA7075 alloy and composites with 10, 20 and 30 wt.% SiC particles were fabricated by powder metallurgy method and hot pressing proces. The effects of the amounts of SiC particles on the microstructure, hardness and reciprocating wear behavior of the AA7075-SiC composites (T6 treated) were investigated. The results indicated that the hardness and wear resistance of composites are increased by increasing the amounts of SiC particles. The increase in hardness by addition of SiCp is due to increases of the hard phase content of the matrix. However, increase in SiCp content of the composites leads to SiCp agglomeration which increased the porosity in the matrix.

1. Introduction

AA7075 alloy (Al–Zn–Mg–Cu) has a superior combination of properties, such as high strength and fracture toughness, low density, good workability and weldability, and remarkable stress corrosion cracking resistance [1]. This alloys have long been regarded as some of the best candidates for demanding structural applications in the aerospace and automotive industries. In fact, 7xxx series Al alloys represent some of the highest strength Al alloys in commercial use [1]. On the other hand, it is well established that introducing a hard particle in an Al-matrix can lead to significant improvements in wear and erosion resistance, stiffness, hardness and strength [2]. Ceramic materials generally used to reinforce Al alloys are SiC, TiC, TiB2, AlN, Si3N4, Al2O3 and SiO2 [3]. In general, SiCp reinforced aluminum alloy composites with enhanced mechanical properties [4].

In this study, SiCp reinforced AA7075 alloy composites were successfully fabricated by the powder metallurgy and hot pressing process. The combined effects of hot pressing temperature and reinforcement weight fraction on the wear behavior of the composites have been studied.

2. Materials and methods

AA7075 powders were blended with different weight (10, 20 and 30 wt.%) SiC powders (Fig. 1). The blended powders were pressed by a two-step hot pressing process. The mixed powders were firstly pressed at 350ºC with the pressure of 200 MPa for 1 min and then were pressed at 475 ºC with the pressure of 125 MPa for 1 min. After that, the compacts were cooled down to ambient temperature under pressure (125 MPa) to 300 ºC and decompressed. The samples had a dimension of 35 mm in diameter and 12 mm in thickness (Fig. 2). All samples were machined into a disc shape with a diameter of 35 mm and a thickness of 8 mm. After machining, the samples were prepared by grinding with SiC abrasive paper and mechanical polishing with diamond paste (3 μm) to obtain a good surface finish.

Figure 1. AA7075 powders and SiC ceramic particles

Figure 2. Hot pressed compact.

For the T6 heat treatment process, the specimens were solidified at temperature 480 °C for 2 h and quenched in water, then aged at a temperature of 120 °C for 24 h in air.

The density of the compacts was measured by the Archimedes’s method, while the theoretical densities calculated by taking the densities of AA7075 aluminium alloy and SiC particles were equal to 2.81 and 3.21 g/cm3, respectively.

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porosity, experimental density, theoreticaldensity,Hardness tests were performed on all samples using a Brinell hardness tester with 2.5 mm diameter ball indenter and 31.25 kgf load. The wear resistance test was performed on a reciprocating ball-on-disk UTS tribometer T10/20 under dry sliding conditions. Wear tests were carried out at normal loads of 10, 20 and 40 N and at the sliding distance of 500 m with sliding velocity of 0.2 m/s. A 6 mm diameter AISI 52100 steel ball was used as the counter body. The wear resistance of the samples was determined by the wear depth. The worn surfaces of samples were studied with Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX).

3. Results and Discussion

Microstructures of alloy and composite samples are shown in Fig. 3. AA7075 alloy sample have a finely dispersed pores (Fig. 3a). Also, in the 10 wt.% SiCpreinforced composite sample, the particles were homogeneously distributed compared to other composite specimens. However, in the 20 and 30 wt.% SiCp reinforced composite samples, the dispersion of the ceramic particles is not uniform and the particles have high tendency for agglomeration and clustering. Increasing of particle content tend to easily agglomerate, and this may probably result in lower strength value [5].

Figure 3. Microstructure of (a) alloy and (b) 10, (c) 20 and 30 wt.% SiC reinforced composite samples.

The variation of teorical ( ) and experimental ( )densities with SiCp weight percent is shown in Fig. 4. The figure shows that the density has increased with increasing SiC content up to 20 wt.% SiC content. However, the density of composite samples with 30 wt.% SiC particles has decreased due to agglomeration at high content of reinforcement (Fig. 5). The porosity of the composite samples increased with increasing SiC content and reached its maximum value in the composite samples with 30 wt.% SiC content (Fig. 5). As a result, the amount of SiC particles increases above the critical level leads to agglomeration and porosity increases [6,7].

Figure 4. The variation of density with SiC particle content.

Figure 5. The variation of porosity with SiC particle content.

The influence of SiCp content on hardness is shown in Fig. 6. Considering this figure, by increasing the amount of SiC, composite hardness increases since its hardness is much higher than that of AA7075 alloy [6,7]. Some other studies report that increasing the amount of hard SiC particles in the matrix would result in more dislocations that increases the hardness of the composite [8]. Also, the effect of the precipitation hardening behavior of the samples on their hardness should not be ignored.

Figure 6. The influence of SiC particles content on hardness.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

282 IMMC 2018 | 19th International Metallurgy & Materials Congress

The wear resistance of the samples was determined by the wear depth. Wear depth of the samples as a function of particle content at loads of 10, 20 and 40 N is shown in Fig. 7. Wear depth of samples increased depending on increased the applied load. This indicated that the amount of wear increased due to the increase in applied load. Ceramic particle reinforced composites shows better wear resistance compared to the AA7075 alloy samples. In the composite samples, the wear depth reduces by the increasing particle content. This is evident from the wear tracks of alloy and composite (30 wt.% SiC) samples (Fig. 8 and Fig. 9). The wear tracks of the AA7075 alloy sample are wider and deeper than that of the composite specimens. The hard reinforcement particles in the composite samples are to carry the applied load, stresses and to avoid plastic deformation which leads to decrease in the wear rate [3].

Figure 7. The variation of wear depth with SiCpcontent and applied load.

Figure 8. Wear tracks of the AA7075 alloy and composite (30 wt.% SiC) samples.

Fig. 9 shows the worn surface morphology of the alloy and composite (30 wt.% SiC) samples against AISI 52100 carbon steel ball at the load of 10 N. It is seen that wear tracks of the samples show severe adhesion, scuffing and deeper grooves generated along the sliding direction. On the worn surface of the alloy sample, plastic deformation was observed to be effective.

Figure 9. Worn surfaces the AA7075 alloy and composite (30 wt.% SiC) samples.

4. Conclusion

The AA7075 alloy and SiCp reinforced composites were successfully fabricated by combining powder metallurgy and hot pressing processes. By increasing the amount of SiC, the hardness and wear resistance of the composites increased. The amount of SiC particles increases above the critical level leads to agglomeration and porosity increases. The main wear mechanism in the samples was identified as abrasive wear.

Acknowledgment

The authors are grateful to the Scientific Research of Karabuk University for the support of this study with project number BAP-16/2-YL-097.

References

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