index possible in Al-Si-Mg (A356/357) alloy castings is 470-500 Mpa. Tis value is obtained in castings that solidify in about 30 seconds or less.
Exploring Fatigue Failure Aluminum castings are often used
in structural components subject to cycles of applied stress. Over their commercial lifetime, millions of stress cycles may occur. In these applications, it is important to characterize the com-
ponents’ fatigue life. Tis is especially true for safety critical applications, such as automotive suspension components. Te commercial importance of fatigue has provided the motivation for extensive studies by automo- tive and university researchers. A simplified approach to the problem of fatigue failure can offer practical guidelines for measuring quality in a casting. Consider a piece of mate- rial subjected to tensile stress in
the vertical direction. In the center is a circular-shaped crack. Because the crack provides no mechanical strength, stress accumulates there. When the load is applied cyclically to the material, the crack grows slightly each time the load is applied. We want to know how fast the crack grows and how long it will take for failure to occur. Te growth of the crack with each cycle is related to the number of times the stress has been applied.
ties were considerably improved over those found in the Aluminum Association 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. Tis gravity-fed permanent 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 detec- tion. The alloy was modified with 0.012% strontium and grain refined with a 5Ti-1B master alloy. Dupli- cate heats were made by adding small amounts of Fe to the base alloy.
Table 3. Tensile Properties of A356-T6 Alloy UTS
Grade (inches)
1 (1-3/8) 2 (2)
3 (7/8) 5 (3/8) 4 (1/2)
(MPa) 270 269 292 290 308
Yield (MPa)
193 197 197 198 210
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 tempera- ture, and then aged between two and 18 hours at 310F (155C). The tensile properties obtained
were plotted (Fig. C), as well as the constant quality index (Q) and yield
Elongation %
6.2 6.2
12.3 10.1 14.3
DAS
(microns) 51 46 38 33 25
Quality Index (MPa)
389 388 455 440 481
strength (YS) of the material. Fig. C shows how aging time
determines the trade-off between strength and elongation, and how heat treatment may be changed to produce desired properties in a casting. The loss of elongation and strength with increased iron also is clear.
Fig. B. The ASTM standard mold is for a gravity- fed permanent mold casting.
Fig. C. Shown are the properties of 356 alloy with various iron levels and aging times. June 2011 MODERN CASTING | 33
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