Experimental methods
The alloy used for all experiments was Al-9Si-3.1Cu- 0.1Mg-0.53Zn-0.86Fe-0.16Mn-<0.2(others total). HPDC alloy specimens for tensile testing were produced using a Toshiba cold chamber die-casting machine with a 250 ton locking force, a shot sleeve with an internal diameter of 50 mm (~2 in.) and a stroke of 280 mm (~11 in.). The calculated metal velocity at the gate was 82 m/s (3228 in./s) for all tensile test samples. The die provided two cylindrical tensile specimens and one flat tensile speci- men from each shot, conforming to specification AS1391. The flat tensile specimen produced was not used. The cy- lindrical tensile test bars had a total length of 100 mm (~4 in.), with a central parallel gauge section 33 mm (1.3 in.) long and a diameter of 5.55±0.1 mm (0.218±0.004 in.). Gage length was 25mm (1 in.). Tensile testing was conducted following standard procedures, at a crosshead speed of 5 mm/min (0.2in./min). Samples for each batch were selected randomly from a larger set of cast samples. As-cast data was reported previously.1
For heat treatment,
the alloys were solution treated for 15 minutes immersion at 490C (914F), water-quenched and either naturally aged 14 days at ~25C (77F) to achieve the T4 temper or aged to peak strength and hardness at 150C (302F) to achieve the T6 temper. For each of the heat treated conditions at least 40 samples were used, and, for the T4 temper, 100 sam- ples were used. For determination of the tensile properties in the solution treated condition, batches of ten samples at a time were solution treated, quenched, and then immedi- ately tested. The time between solution treatment and ten- sile testing was under 1 hour for these samples. Scanning electron microscopy was conducted using a Leica S440 SEM microscope. Quantitative analysis was conducted on optical microstructures using Image Pro-Plus software across a total area of ~122063 μm2
.
Table 1. As-Cast Tensile Properties, High/Low Values and μ-3σ(sd)
results and Discussion mechanical Properties
The average mechanical properties, one standard deviation (σ(sd)
3.8% to 8.2%. Subsequent ageing at 25C (77F) to the T4 tem- per, caused the 0.2% proof strength and tensile strength to in- crease to 217 and 386 MPa respectively, but the mean value of Ef
three standard deviations, from the statistical mean, µ) are shown for the four conditions examined in Tables 1-4. For the solution treated condition, when compared to the as-cast tensile properties, there was a decrease in mean 0.2% proof stress from 172 MPa to 126 MPa, a decrease in mean tensile strength from 351 MPa to 327 MPa, but a substantial increase in mean elongation at failure Ef
), high and low values, as well as µ-3σ(sd) , of more than double from now decreased to 5.4%. All tensile properties were how-
ever above those recorded for the as-cast condition. For the T6 treated samples, the 0.2% proof strength was raised further to an average value of 351 MPa, the tensile strength was raised to 434 MPa, and Ef
was decreased to 3.1%.
of data. Solution treatment (Table 2) caused the value of σ(sd) (for Ef
is important to note as well, that the lowest value of the solu- tion treated condition (5.1%) was still higher than the high- est value of the as-cast condition (5%) despite the increase in value of σ(sd)
crease in the spread of data occurred simultaneously to the increases in average, high, and low values of Ef
.
For the T4 temper achieved by natural ageing (Table 3), the standard deviation, σ(sd)
for Ef decreased from 1.6 in the so-
Table 3. T4 Tensile Properties, High/Low Values and μ-3σ(sd)
This provides a rapid evaluation of the change in the spread ) to be increased, from 0.52 (as-cast) to 1.6. This in- observed. It
It is interesting firstly to consider the value of one standard deviation (σ(sd)
) in Ef for the four different conditions tested. (i.e. minus
Table 2. Solution Treated Tensile Properties, High/Low Values and μ-3σ(sd)
Table 4. T6 Tensile Properties, High/Low Values and μ-3σ(sd)
50
International Journal of Metalcasting/Fall 2011
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