Alloy
355 356 357
Grain Fineness (GF)
6.9 6.2 7.5
Table 2: Results of TA Evaluation with Incumbent Procedure
used to provide both cleaning and additional degassing of each melt. The hexachloroethane degassing pill treatments were 10 minutes each and were added at a rate of 0.15% the weight of aluminum to be treated. Elemental spectroscopy of the
Titanium (Ti) levels was historically used to evaluate grain refinement, with a typical target level of 0.15% (+/-0.02%) by weight. A standard reduced pressure test (RPT) was performed to assess degassing effectiveness.
the reduced pressure testing apparatus appears in Figure 1. The RPT specimens were subjected
to the hydrostatic displacement technique to determine each sample’s specific gravity. The minimum threshold specific gravity for each alloy poured appears in Table 1.
Thermal Analysis Testing It’s well established that grain refinement can beneficially affect feeding, fluidity and mechanical properties in aluminum castings. Hence, inadequate grain refinement can yield shrink voids in aluminum castings. Accordingly, it was decided to assess the grain refinement to see if an improved practice was possible. A THERMATEST
5000 NG III
thermal analysis (TA) unit was used to assess the grain refinement (or grain fineness) of the treated melts after they were prepped for pouring.
Thermal
analysis involves collecting data of temperature versus time of a solidifying melt sample and comparing the curve to a set of known curves algorithmically. A
Continued on pg 28
Figure 3: Pictorial Representation of the Grain Refinement Levels as Measured by the TA
May 2020 ❘ 27 ® Figure 2: Photograph of the TA Unit Used in the Melt Assessment
A picture of
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