Standard Molds¯Aluminum Association Casting


The Aluminum Association (AA) mold was introduced al- most thirty years ago.1


The mold and the resulting casting


are shown in Figure 1. This mold was used by the AA 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. Section 4 has the fastest solidification rate. The available tensile property data may be found in a report published by the AA.2


The


data tabulated for A356-T62 alloy castings is given in Table 1, together with properties for separately cast test bars. The dendrite arm spacing (DAS) observed at each of the five lo- cations is also given. This DAS has been used to estimate the local solidification times from published correlations.3


It is


readily seen that the elongation and ultimate tensile strength (UTS) decrease as the solidification time increases.


It should be emphasized that these are average values ob- tained when the same mold was sent to a number of different foundries. It is useful to consider the range of alloy composi- tions used and tensile properties obtained. Table 2 shows the minimum and maximum values recorded.


We first consider castings produced at Stahl Specialty Com- pany.4


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. The metal in the dip out well was degassed by po- rous plugs and maintained at a low gas level, as determined by reduced pressure samples taken every thirty minutes. A filter crucible was placed in the furnace dip out well and all metal was ladled from this crucible. The metal temperature


Table 1. Average Tensile Properties of A356-T62 Alloy in the AA Mold2


Some foundries evidently had much better practices than others. There was a significant difference in tensile proper- ties, especially elongation. This variability in quality is a good example of the problem one faces when mechanical property data lacks the proper ‘pedigree’ regarding melt- ing and casting practices. There is no information about the degassing process used, if any, in the AA report. Likewise, other important treatments are unknown, such as filtration, grain refinement and modification. These all may have an influence on mechanical properties.


The above information is interesting, but what does it really mean? Is it possible to do better? To find out it will be useful to compare these results with castings having a better ‘pedigree’.


Table 2. Minimum/Maximum Values for A356-T62 Alloy in AA Study2


Table 3. Tensile Properties of A356-T6 Alloy (Stahl Specialty)4


8


International Journal of Metalcasting/Winter 11


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