a .hosseinimonazzah

Slurry casting method is a novel process to produce metal forms, which makes it possible to produce a porous structure with open cell. In the present study, the microstructure and compressive behavior of aluminum foams produced by slurry casting method, under different number of immersion times were investigated. For the production of aluminum foams with different cell sizes, polyurethane preforms with characteristics of 45, 55 and 65 ppi were selected, and after immersing in a slurry having a solid mass of 88% and removing the excess semiliquid mixture, the samples were sintered at 630˚C. The size of polyurethane perform cell as well as the number of immersion times control the microstructure and compression performance of porous structures. The results of the study showed that the portability of porous aluminum increases by decreasing the size of preform cell or increasing the number of immersion times, which leads to thicker strut. In addition, the probability of crack existence, exactly at the corner of structures, decrease via enhancing the thickness of strut. Meanwhile, excessive increase in the number of immersion, i.e. third times, was associated with some closed-cells which results in strain localization and stress concentration. Therefore, the maximum plateau stress as well as the superior energy absorption capacity was observed in the sample having the minimum preform pore sizes which was immersed for two times in the aluminum slurry.

The effect of artificial ageing on tensile properties and fracture behavior of 2024 Al alloy in the presence and absence of notch was investigated. The tensile samples having two V notches, as well as, common tensile specimens were prepared according to ASTM E8. T8 heat treatment consisting of solutionizing, cold rolling, and aging was selected to establish precipitation hardening in Al2024 alloy. Considering T8 condition, following solutionizing of Al sheet at 5000C, samples were cold rolled with 30% reduction. Finally artificial aging was applied at 2000C via different times. The results of Brinell test shows that the maximum value of hardness was achieved at 1 hour, which indicated the peak aging condition. Moreover, soaking samples in aging condition at 2000C for 15 minutes and 4 hours were considered as under-aging and over-aging states, respectively. Similar to hardness results, the superior amount of yield strength and ultimate tensile strength of un-notched specimens were observed at peak aging time, while in notched samples, the role of aging time was not prominent due to the presence of V shaped flaws. By applying notches on specimens, the yield and ultimate stress were found to increase with simultaneously decrease in elongation at fracture. It was found that due to localizing plastic deformation around flaws, the presence of notch increases the strength of 2024 Al alloy, while elastic behavior controls deformability far from notches. The energy which was absorbed till fracture, was indicated as toughness and it was found to be absolutely dominated by the elongation at fracture and significantly decreased in notched specimens. The peak-ageing condition of notched tensile specimens had the lowest toughness compare to under-ageing and over-ageing conditions. Although three fracture mechanisms, such as matrix deformation, particle cracking and matrix/particle debonding observed in all samples, the distribution of these mechanisms were more homogeneous in un-notched specimens. In the other words, one may claim that the presence of flaws enhanced the cracked particles, which accompanied with lowering matrix deformability lessened the elongation and toughness.