that followed a straight line when the ultimate tensile strength was plotted versus the logarithm of the elongation to fracture. Te following equation was developed empirically to represent the resulting quality index: Q=UTS + 150 log E
where Q=Quality Index (MPa),
UTS=Ultimate Tensile Strength (MPa) and E=Elongation to Fracture (%) Te index has the following qualities: • It describes the relative tradeoff
between strength and elongation. If you need increased strength or elongation in a casting, the heat treatment can be changed accord- ingly. The same effect may also be obtained by increasing or decreasing magnesium content in Al-Si-Mg alloys.
• It allows buyers of castings to compare two different castings that may have received different heat treatments, or whose chemi-
EXAMINING THE STANDARD MOLDS The quality of the metal you
are casting can be determined by examining a test casting produced in a standard mold. A quality index is then assigned to the test casting to compare it to other materials. Two governing bodies have developed standard molds for test castings.
Standard Mold 1:
The Aluminum Association The Aluminum Association
standard casting mold (Fig. A) was established and used by the association to develop property data in six different alloys. Tensile test samples were taken from five areas, each having a differ- ent solidification rate and section thickness. The data tabulated for A356-T62 alloy castings is given with properties for separately cast test bars in Table 1. (The full list- ing of available tensile property data may be found in a report published by the association.) The dendrite arm spacing
observed at each of the five loca- tions also is given. This spacing has been used to estimate the local solidification times from published correlations. Accord- ing to the data, elongation and ultimate tensile strength decrease as solidification time increases. The data published by the Aluminum Association for its test casting are average values obtained when the same mold was sent to a number of different
32 | MODERN CASTING June 2011 32
cal composition is different.
• It gives an indication of improve- ments that might be made in any particular casting.
• Because the iso-quality and iso- yield strength contours appear as straight lines in the quality plots, the relationship indicates all impor- tant tensile properties. Experimental measurements and
theoretical considerations have shown that the maximum value of the quality
metalcasting facili- ties. When examining the average values, consider the range of alloy compositions used and tensile properties obtained (Table 2). Some facilities used for the test implemented better practices than others, resulting in a significant difference in tensile properties, espe- cially elongation. So, how good can the qual- ity 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
Location (Thickness
3 (7/8”) 5 (3/8”) 4 (1/2”)
(MPa)
1 (1-3/8”) 246 2 (2”)
252 274 274 288
test bars 294
Yield (MPa)
216 221 230 229 233 242
Elonga- tion (%)
2.7 3.0 4.6 4.9 6.5 4.8
0.28/0.40
UTS (MPa) 235/276
231/283 252/297 248/293 259/314
Fig. A. This test casting was pro- duced in an Aluminum Association standard mold.
maintained at a low gas level. The metal temperature inside the crucible was held between 1,350- 1,380F (730-750C). The metal was modi- fied with strontium, grain refined and filtered. The resulting castings were solu- tion heat treated for six hours at 1,000F (538C) and water quenched. After 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 with an average of 10 samples for each location. The tensile proper-
Table 1. Average Tensile Properties of A356-T62 Alloy in the AA Mold UTS
Dendrite arm spacing (inches)
0.0020 0.0018 0.0015 0.0013 0.0010 0.0012
46 38 33 25 30
YS (MPa) 166/242
166/242 173/162 173/162 166/269
(microns) 51
Solidification Quality
100 63 36 25 11 20
Table 2. Minimum/Maximum Values for A356-T62 Alloy in AA Study % Si % Fe % Mg 6.8/7.45 0.12/0.18
Elongation (%) 1.8/4
1.5/4.5 3/7.7 3/7.5
3.5/9.5
311 324 373 378 410 396
Location 1
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