Fig. 6. SDAS vs. solidification time in aluminum casting alloys is graphed.
Many of the early papers reported
cell size in their studies. However, it is now known that a better measure is the secondary dendrite arm spacing (SDAS). Te easiest way to measure SDAS is to use the linear intercept method. Tis is illustrated in Fig. 5 for a modified Al-7% Si alloy. Lines are drawn on a micrograph where well
Fig. 7. This is a SEM micrograph of secondary dendrite arms in a large pore (A356 alloy.)
defined dendrite arms can be observed, and the average spacing between the centers of adjoining arms is measured. Typically, a number of measurements are made and the results averaged. Te SDAS can be used to determine
the local solidification time at any point in a casting. Te results of many com- mercial and laboratory measurements
on Al-Cu alloys have been reviewed. Results from castings made from 356 and 319 alloys are also shown in Fig. 6, where measurements of SDAS are plotted versus the local solidification time (as measured by thermocouples in the casting). It can be seen that, for a given freez-
ing rate, the copper-containing 319 alloy has a somewhat smaller SDAS than the 356 alloy. Te correlation for most other foundry alloys would prob- ably lie somewhere between these two curves. Te ability to measure SDAS, and the correlations shown in Fig. 6, represents a useful tool. It can help in learning about the thermal history of a sample from an “unknown” casting (e.g., a competitor’s product) or from one’s own castings. It may not always be convenient to place thermocouples in the mold, but the solidification time at various points in the casting can be estimated from the SDAS. Te dendritic structure is often
visible if you look carefully into pores on the fracture surface of tensile bars. An example is shown in Fig. 7. Te rounded ends of the secondary dendrite arms are sticking out from the left hand-side of this picture. Te SDAS in the sample appears to be between 40 and 50 microns, which corresponds to a local solidification time of about two minutes (for an A356 alloy).
Tis article was based on Paper 13-1224 which was presented at the 117th Met- alcasting Congress.
Next month this series continues by examining how elements dissolved in an alloy segregate during solidifi- cation and determine a casting’s mechanical properties.
42 | MODERN CASTING May 2014
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