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melted and held in an electric resistance furnace; the alloy was degassed with ultra high purity argon gas in a rotary degasser at about 5 l/min and 300 RPM rotation for about 30 min. The melt pouring temperature of the casting process was 680C (1256F) and the mold pre-heat temperature was 250C (482F).


Micro-Hardness Measurements


The Vickers micro-indentation hardness measurements were carried out on the samples according to the ASTM Standard E384-09 using a Clemax™ Intelligent micro- hardness system with a load of 25 gf and a dwell time of 10 seconds for each loading. The diamond indenter used was a Vickers indenter in the shape of a square based pyramid with the angle of 136° between the faces. The size of the indentation was measured with a micrometer integrated in the microscope at 400X.


A special metallic holder was designed and manufactured to hold the cylindrical samples for the micro-hardness measurements. Equation 1 was used to evaluate the micro- hardness from the measurements of the indentation on the primary Al phase.


Micro-Hardness (HV) = 1854 (F/d2 ) Equation 1


In this equation, F is the applied load in grams and d is the mean of two diagonals of the diamond shaped indentation, in µm.


Heat Treatment


All heat treatment was carried out in an electric resistance furnace with a maximum temperature deviation of ±1C (±2F). The time between end of solution treatment and quenching was less than 5 seconds. Quenching was carried out such that the temperature of the quenching medium in- creased to less than 1C (±2F) above the initial temperature.


Figure 1 shows the temperature versus time (thermal) profile for the T6 heat treatment carried out on all the A356.2 alloy samples in this study. The quenching rate shown in Figure 1 is for quenching in water at 80C (176F). A typical incubation time of 6 hours at room temperature and artificial aging time of 6 hours at 155C (311F) are shown in Figure 1. Samples were air cooled to room temperature after artificial aging.


Figure 2 (a), (b) and (c) show temperature and quenching rate profile as a function of time obtained from the sam- ple during quenching in three different media maintained at -40C (-40F), 25C (73F) and 80C (176F), respectively. The rate of quenching increased as the temperature of the quenching medium decreased. Further, quenching in water


Table 2. Average Elemental Composition (wt %) of the A356.2 Al Alloy


(a)


(b)


Figure 1. Typical temperature versus time profile obtained during T6 heat treatment of A356.2 alloy components (a) full heat treatment cycle and (b) magnified view of the quenching period from Figure (a).


International Journal of Metalcasting/Fall 2011 21


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