An Investigation Of The Effect Of Various Heat Treatment Processes On Microstructure And Stress Corrosion Cracking (SCC) Of Aluminium Alloy 7075

Leng , Siong Cheng (2011) An Investigation Of The Effect Of Various Heat Treatment Processes On Microstructure And Stress Corrosion Cracking (SCC) Of Aluminium Alloy 7075. Project Report. UTeM, Melaka, Malaysia. (Submitted)

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Abstract

Aluminium alloy 7075 has been widely studied, due to its excellent mechanical properties developed by age hardening and their extensive uses in the aircraft structure. Although T6 tempered alloys are known to possess high mechanical strength, however it has poor resistance to SCC. The method of T73 tempering is applied to overcome the SCC problem, but the strength of the material is sacrificed due to over-aging. On the other hand, retrogression and re-aging is done as the method is claimed able to improve SCC resistance of the alloy but retaining the high mechanical strength as of T6 tempered alloy. Material specimens are checked and determined before heat treatments by the means of arc spark analysis. Specimens which are heat treated with the three different methods, namely T6, T73 and RRA, are then subjected to DTSCC test according to ASTM G49-85 for test preparation and ASTM G139-05 as the test model. The SCC test parameter includes the exposure of material specimens to 3.5% sodium chloride solution as the corrosive environment, with duration of 5days after the each heat treatment process. Tensile test is done by the means of Universal Tensile Machine (UTM) according to ASTM E8-04, follows by microstructure observation under scanning electron microscope (SEM). The study shows that T6 specimen possesses of high susceptibility to SCC, whereas T73 tempering is able to lower the SCC susceptibility but its strength is lowered in the same time. RRA is capable to produce the microstructure of mixture of both T6 and T73 specimens, where provides it the combined properties of both T6 and T73.

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