The three sets of tensile property data (from Tables 1, 3 and 5) have been plotted in Figure 2. The elongation is shown on a logarithmic scale, so that constant values of quality (in MPa, as defined by Equation 1) appear as diagonal, straight lines. The results from the Aluminum Association report (Table 1) are labeled ‘AA’. The Stahl Specialty data (Table 3) is labeled ‘SS’ and the most recent results from castings poured at Littlestown Hardware and Foundry (Table 5) are labeled ‘LHF’.


The important effect of melt treatment on casting quality should be obvious from this plot. The ‘AA’ data is for cast- ings produced 25-30 years ago, when degassing was a hap- hazard affair and the need for melt treatment was not widely known. At this time many foundrymen melted ingot and poured the metal into the mold without any treatment. Many shops did not even do a chemical analysis. The castings pro- duced at Stahl Specialty (‘SS’) were filtered and degassed, and showed a significant improvement in properties. How- ever, Stahl Specialty used only a short degassing treatment with a porous plug. The most recent castings (‘LHF’) used a 30 minute treatment with a rotary impeller degasser. Very low gas contents were found in this metal and the highest mechanical properties were obtained.


Standard Molds--ASTM B108 Test Bar


Another standard mold commonly used in North America is specified in ASTM B108. This is a gravity-fed permanent mold casting. (Figure 3.) An example will show how the mold may be used to evaluate metal quality.4


An A356 alloy contain-


ing 7.0% Si, 0.03 % Fe, 0.36% Mg, 0.02% Zn, 0.08% Ti and 0.0002% P was melted in a reverbatory furnace, degassed, and filtered. The Cu and Mn in this alloy were below the limits of detection. The alloy was modified with 0.012% strontium and grain refined with a 5Ti-1B master alloy. Duplicate heats were made by adding small amounts of Fe to the base alloy.


All test bar castings were given a T4 solution heat treatment (8 hours at 1000F, [538C]), water quenched, pre-aged 24 hours at room temperature, and then aged for times between 2 and 18 hours at 310F (155C). The tensile properties ob- tained are plotted in Figure 4. The iron content of the alloy and the aging time used are shown in black numbers. The lines of constant quality index (Q) are indicated in red, and blue lines show the yield strength (YS) of the material.


Figure 4 shows how aging time determines the trade-off between strength and elongation, and how heat treatment may be changed to produce desired properties in a casting. The loss of elongation and strength with increased iron is also clear.


In general, there are four factors which have an important influence on the tensile properties of a casting:


• melt treatment and pouring procedures • heat treatment • alloy composition • solidification time, or freezing rate


The first three were considered in the results shown above for the two ‘standard’ molds. We now consider the effect of solidification rate.


Effect of Solidification Time


Many studies have shown that the rate of solidification has a significant influence on the properties of castings. Castings that freeze quickly can tolerate quite high contents of gas. Slowly cooled castings, however, easily form significant amounts of porosity.7


relative size of silicon particles8


The freezing rate also determines the and iron intermetallics9


that


form during freezing. The practical implication is simple: Casting quality depends strongly on solidification time.


Figure 3. ASTM B108 test bar mold. 10


Figure 4. Mechanical properties of 356 alloy containing various iron levels and aged 2, 6 and 18 hours at 310F (155C). (Data supplied by Stahl Specialty.4


) International Journal of Metalcasting/Winter 11


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