ing the quality of net-shaped castings was first proposed by French metalcast- ers. The researchers studied the effects of casting conditions, metal composi- tion and aging time on the mechanical
in tensile properties, especially elonga- tion. So, how good can the quality be, and how can it be measured? At a respected metalcasting facility,
a heat of A356 alloy with 0.07 % Fe, 0.36 % Mg and 0.08 % Ti was melted in a double chamber dry hearth furnace, degassed and maintained at a low gas level. The metal temperature inside the crucible was held between 1,350- 1,380F (730 to 750C). The metal was modified with strontium, grain refined and filtered. The resulting castings were solution heat treated for six hours at 1,000F (538C) and water quenched. Af- ter a 24 hour hold at room temperature, the castings were aged for six hours at 310F (160C). Table 3 shows the resulting mechanical property data (average of 10 samples for each location). The tensile properties were considerably improved over those found in the Aluminum As- sociation study, proving the importance of proper degassing and metal filtration.
Standard Mold 2—ASTM Another standard mold commonly
used in North America is specified by ASTM: B108. This gravity-fed perma- nent mold casting (Fig. B) can be used to evaluate metal quality, as shown in the following example. An A356 alloy containing 7.0% Si, 0.03 % Fe, 0.36% Mg, 0.02% Zn, 0.08% Ti and 0.0002% P was melted in a reverbatory furnace, degassed and filtered. The Cu and Mn in the alloy were below the limits of detection. The
properties of Al-Si-Mg (356-type) alloys. As they analyzed the aging process, they noticed that for a given “quality” of casting—as determined by freezing rate, porosity and iron content—the T6
alloy was modified with 0.012% strontium and grain refined with a 5Ti-1B master alloy. Duplicate heats were made by adding small amounts of Fe to the base alloy. All test bar castings were given a T4 solution heat treatment (8 hours at 1,000F [538C]), water quenched, pre-aged 24 hours at room temperature, and then aged between two and 18 hours at 310F (155C). The tensile properties obtained
Table 3. Tensile Properties of A356-T6 Alloy Location
UTS
3 (7/8”) 5 (3/8”) 4 (1/2”)
270 269 292 290 308
Yield
(Thickness) (MPa) (MPa) 1 (1-3/8”) 2 (2”)
193 197 197 198 210
aging process produced tensile proper- ties that followed a straight line when the ultimate tensile strength was plotted versus the logarithm of the elongation to fracture. The following equation was
were plotted (Fig. C), as well as the con- stant quality index (Q) and yield strength (YS) of the material. Fig. C shows how aging time deter-
mines the trade-off between strength and elongation, and how heat treat- ment may be changed to produce desired properties in a casting. The loss of elongation and strength with increased iron is also clear.
METAL
Elongation DAS (%)
6.2 6.2
12.3 10.1 14.3
(microns) 51 46 38 33 25
Quality Index (MPa)
389 388 455 440 481
Fig. B. The ASTM standard mold is for a gravity-fed permanent mold casting.
March/april 2011 Metal casting Design anD purchasing 39 39
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