is the use of a quality index. Another approach is to use a test casting to measure the metal’s quality index. The test casting may be a production cast- ing with a good historical pedigree or
a standardized casting produced using a standard, as specified by one of two governing bodies. In order to assess the relative qual- ity of a production or test casting, one
The quality of the metal sold to
you by a metalcaster can be deter- mined by examining a test casting produced in a standard mold. A quality index can then be assigned to the test casting to compare it to other materials. Two governing bod- ies have developed standard molds for test castings.
Standard Mold 1: The Alumi- num Association
The Aluminum Association stan-
dard casting mold (Fig. A) was estab- lished and used by the association to develop property data in six differ- ent alloys. Tensile test samples were taken from five areas, each having a different solidification rate and sec- tion thickness. The data tabulated for A356-T62 alloy castings is given with properties for separately cast test bars in Table 1. (The full listing of available tensile property data may be found in a report published by the association.) The dendrite arm spacing ob-
served at each of the five locations also is given. This spacing has been used to estimate the local solidifica- tion times from published correla- tions. According to the data, elonga- tion and ultimate tensile strength de- crease as solidification time increases. The data published
by the Aluminum As- sociation for its test casting are average val- ues obtained when the same mold was sent to a number of different metalcasting facilities. When examining the average values, con- sider the range of al- loy compositions used and tensile properties obtained (Table 2). Some facilities used for the test implemented better practices than others, resulting in a significant difference
38 Fig. A. Tensile tests are taken from the test casting at the locations shown on the right.
may use a quality index developed by the industry.
Theoretical Basis for Quality Index The most common index for defin-
Examining the Standard Molds
Fig. C. Shown are the properties of 356 alloy with various iron levels and aging times.
Table 1. Average Tensile Properties of A356-T62 Alloy in the AA Mold Location UTS Yield Elongation
Dendrite arm spacing
(Thickness) (MPa) (MPa) 1 (1-3/8”) 246 216 2 (2”)
252 221
3 (7/8”) 274 230 5 (3/8”) 274 229 4 (1/2”) 288 233 test bars 294 242
(%) 2.7 3.0 4.6 4.9 6.5 4.8
inches 0.0020 0.0018 0.0015 0.0013 0.0010 0.0012
Solidification
microns time (sec) 51 46 38 33 25 30
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
0.28/0.40
166/242 166/242 173/162 173/162 166/269
UTS (MPa) YS (MPa) Elongation (%) 235/276 231/283 252/297 248/293 259/314
1.8/4
1.5/4.5 3/7.7 3/7.5
3.5/9.5 Metal Casting Design anD PurChasing Quality
Index (MPa) 311 324 373 378 410 396
Location 1 2 3 5 4
MarCh/aPril 2011
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