Sıcak ekstrüze edilmiş AA7075/SiCp kompozitlerin sertlik ve korozyon direncine T6 ve T73 ısıl işlemlerinin etkisi

In this study, the influence of T6 and T73 tempers on the microstructure, hardness and corrosion resistance of Al7075 matrix composites reinforced with SiC particle fabricated by extrusion of powder metal compacts were investigated. For this purpose, powder mixtures obtained by addition of average 32 μm sized SiC particles with various ratio (5, 10, 15 and 20 wt%) to AA7075 aluminum alloy powders (wt.% composition: 4.78% Zn, 1.84% Mg, 1.45% Cu, 0.24% Si, 0.25% Cr, 0.26% Mn, 0.23% Fe and Al bal.) were cold pressed under uniaxial pressure (350 MPa). The pressed compacts were heated at 480 °C for 1 hour and then extruded into 12 mm diameter bars. After extrusion, all the samples were solution treated at 480 °C with a soaking duration of 60 min and then water quenched. For T6 temper, the solution treated samples were aged at 120 °C for 24 h. For T73 temper, the solution treated samples were aged at 110 °C for 8 h and then at 175 °C for 8 h in two stages. Characterization of the produced alloy and composite samples included metallographic examination, hardness measurement and density determination. Chemical composition of the AA7075 alloy was analyzed by XRF. The phases and morphology of samples were examined by XRD (Rigaku-Ultima IV) and SEM/EDX (Carl Zeiss Ultra Plus Gemini Fesem). The relative densities of the extruded AA7075 alloy and AA7075/SiC composites were measured by Archimedes’ principle. The hardness values of the samples were measured on the polished sections of the samples by Brinell indentation (QNESS Q250M) using a ball with diameter of 2.5 mm under a load of 31.25 kg. For preparation of samples for exfoliation corrosion tests, the sample surfaces were ground with silicon carbide papers and then polished 6 μm alumina paste. To determine of the corrosion resistance, the alloy and composite samples were exposed to EXCO corrosion solution for 4, 8 and 16 hours. The corrosion tests were immersion tests where the weight loss method was used. The samples were first weighed before being immersed in EXCO corrosion solution and later taken out after 4, 8 and 16 h respectively. SiC particle addition caused a significant increase in the density of the composite samples, and the theoretical density was much closer to the 10% particulate additive composites. In addition, further increase in the amount of SiC particle cause the formations of clustering and finally initiates the formation of pores and weak regions in the composite. SiC particle addition caused a significant increase in the hardness of alloy and composite samples compared with that alloy. The results reveal that the highest hardness values were achieved with T6 heat treated samples while the hardness increases as the weight fraction of the reinforcement increases. Corrosion tests shows that the highest corrosion resistance in both heat treatment types with 10% SiC particle reinforced specimens. T73 temper improved the corrosion resistance of samples, while decreased the hardness about 15%-20% compared to T6 temper.