Mechanics, Materials Science & Engineering, September 2016 – ISSN 2412-5954
It was seen that the hardness value at 175oC for an ageing time of 10 hours yielded high hardness value of 78 HRB for the heat treatment parameters that was considered throughout this experiment. Beyond this time, over ageing occurred where the hardness values dropped down. The sample surfaces were polished with the use of Silicon carbide paper and diamond paste and then etched by Keller’s reagent for ten seconds. A microstructural analysis was performed using an optical microscope after every ageing cycle so that the growth of the precipitates can be efficiently related to its hardness as shown in Fig. 3. It can be seen that the growth of precipitates is so rapid where they reach a maximum hardness at the time of ten hours. The precipitates might act as a stress raiser where the crack propagation starts to initiate. The microstructures confirmed the rapid growth of precipitates, which supports the experimental hardness values that are obtained. Precipitate formation. The evolution of hardness at the performed trials is directly proportional to the Cu-Al precipitates that are formed. Various research in the past confirms the precipitates to be CuAl2 phase where during the process of quenching, Cu is contained as a super saturated solid solution in the Aluminium rich phase at room temperature[8]. During the aging phase, the combination of copper and aluminium results in the formation of fine crystals of CuAl2 in the solution. The increase in hardness values are a result of the formation of these crystals owing to the solubility of copper in aluminium. From the microstructures obtained in Fig. 3, it is evident that the CuAl2 phase is present by the difference in contrast that is produced. The main elements of the microstructure are characterized as dark, insoluble precipitates composed of complex compounds such as Fe, Mn, Al, Si and also the presence of particles of CuAl2 phase which are the white areas in a matrix of solid solution[8]. It can also be seen that the condition of reduced hardness obtained after the aging time of 11 hours produced a state of over aging as shown in Fig. 2. Hence, for achieving the maximum hardness, the region showing a maximum peak was utilized thereby fixing the aging time to 10 hours at a temperature of 170oC. These parameters yielded hardness values that were higher than the hardness mentioned in the standard[7]. It has to be noted that aging was performed after the samples were bent so that the required shape of the component can be progressed to the desired level of hardness without failure.
80
Hardness (HRB)
75 70 65 60 55 50
45 40 0
2
4
6
8
10
12
14
16
Aging time (Hours)
Fig. 2. Hardness vs Aging time.
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