This is best seen by considering data tabulated in an ex- tensive study of aerospace castings by Oswalt and Misra.10 They cut test bar samples from A356 (0.35% Mg and 0.11% Fe) and A357 (0.56% Mg, 0.14% Fe and 0.04% Be) alloy castings. Sample locations were selected from areas having different section thickness to determine the effect of freezing rate on tensile properties. The DAS was used to characterize the solidification rate of each sample. Different aging times (2, 4 and 8 hours) were employed to obtain different strength levels in each alloy. The resulting tensile properties are plot- ted in Figure 5.


There are several features of this plot worth noting. Follow- ing Drouzy, Richard and Jacob, a logarithmic scale has been used for the elongation. As a consequence, the values of both constant quality index and yield strength appear as straight lines. This type of plot is extremely valuable. It shows all of the important tensile properties (yield strength, ultimate tensile strength, and elongation) as well as the Quality In- dex. This figure also shows the combined effect of three im- portant process variables: heat treatment, alloy composition and solidification rate. Once this plot is understood, you will know how the alloy used, the heat treatment, and the casting process can be controlled to provide the mechanical proper- ties desired for any particular application.


In practice, one may strengthen a casting by increasing the aging time or adding magnesium; or by doing both. The primary difference between these two alloys is the fact that A357 has more magnesium. By adjusting the magnesium level and the aging time it is possible obtain a yield strength anywhere between 170 and 320 MPa (24-46 ksi) and a UTS (for rapid solidification rates) between 260 and 400 MPa (38-58 ksi).


The solidification rate is indicated in Figure 5 by the numeri- cal values for the secondary DAS. For A356 alloy the small- est DAS was 25 microns, which corresponds to a local so- lidification time of about 20 seconds and produces a quality index of 475-500 MPa. The largest DAS was 125 microns, which corresponds to a freezing time of 20 minutes. This produced a casting quality just under 300 MPa. The impor- tant effect of freezing rate on elongation and tensile strength is clearly seen from these results.


It may be worth noting again the data shown in Figure 2 for the AA test casting. The freezing rates in this casting varied from 20 seconds to 2 minutes. Better degassing practices (in ‘LHF’) helped to limit the loss of quality observed in the thick sections of this casting. The data in Figure 5 was for aerospace castings produced nearly 30 years ago. It would be interesting to repeat this study now, to see if improved de- gassing practices reduce the loss in casting quality observed with slow solidification.


We now consider some of the theoretical underpinnings of casting quality.


International Journal of Metalcasting/Winter 11


Figure 5. Tensile strength and quality of A356-T6 and A357-T6 alloy castings.10


11


Theoretical Basis for the Quality Index


There are a number of ways to define the quality of net- shaped castings, but the one most commonly used was first proposed by French foundrymen.5


They studied the effects


of casting conditions, metal composition, and aging time on the mechanical properties of Al-Si-Mg (356 type) al- loys. As they analyzed the aging process, they noticed that for a given ‘quality’ of casting; as determined by freezing rate (DAS), porosity and iron content; the T6 aging process produced tensile properties that followed a straight line on a certain type of plot. Figures 4 and 5 give examples of the French quality plot, where the ultimate tensile strength was plotted versus the logarithm of the elongation to frac- ture. Equation [1] is the formula developed empirically by Drouzy, Jacob and Richard to calculate the Quality Index. It can be seen that the change of tensile properties versus aging time in Figures 4 and 5 follow closely lines of con- stant quality index.


This result has important ramifications: • The Quality Index describes the relative trade-off between strength and elongation. If we need to in- crease the strength or the elongation in a casting, this behavior allows us to change the heat treatment ac- cordingly. (The same effect may also be obtained by increasing or decreasing the Mg content.)


• The Quality Index allows us to compare two dif- ferent castings, which may have received different heat treatments, or whose chemical composition (especially %Mg) are different.


• The Quality Index gives an indication of improve- ments that might be made in any particular casting.


• The iso-quality and iso-yield strength contours ap- pear as straight lines in the quality plots developed by the French. Thus, the quality plot shows all im- portant tensile properties.


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