bars for mechanical property assessment using the tilt-pour gravity assisted permanent mould casting process. The aim is to show that CDS is a viable technology for shape casting Al wrought alloys and also demonstrate a valid management scheme for the foundry returns (runners, risers, in-gates and defective castings) of these alloys. The process conditions such as alloy design and temperature control have been op- timized to obtain a casting with reasonable integrity. How- ever, the various process parameters such as tilt casting, melt handling/cleanliness, mixing process, heat treatment of cast parts and die surface/thermal management were not investi- gated or optimized to attain maximum mechanical proper- ties and this work is on-going.
The metal mould used to shape cast in the tilt-pour process was designed, developed and validated in this study.3
In conventional casting processes, one alloy is melted, cast and re-melted so managing the recycling of foundry returns is not a grave issue. However, in CDS technology the initial pre-cursor alloys are of a different composition than that of the final cast part and hence a viable recycling strategy for foundry returns would have to be conceived and formulated. This paper presents such a strategy for CDS casting of 2024 and 7075 Al wrought alloys.
Methodology
In this study, three wrought alloys from 2xxx, 6xxx and 7xxx series Al alloys, namely 2024, 6082 and 7075 were prepared and cast by CDS. Figure 2 shows the flow chart describing the steps involved in designing the CDS process for any particular Al alloy composition.
would be initially chosen followed by the com- positions of Alloy 1 and Alloy 2 which when mixed would yield the required alloy. Various isopleths of multi-compo- nent phase diagram(s) containing the elemental additions in Alloy 3 would have to be investigated to evaluate the liqui- dus temperatures, TL1 TL1
is less than 55C (99F), a different (typically higher) mass ratio would have to be chosen and the optimiza- tion of the alloy compositions would be carried out again. In the optimization of the precursor alloy temperatures, suitable values of T1
of the individual alloy temperatures, T1 ried out.4
and TL2 , ∆TL If ∆TL
peated. It has been previously observed2, 4 tion in T2
and T2 morphology of the primary Al phase. Hence, it is recommend- however, marginal variations in T1 International Journal of Metalcasting/Spring 11
be carried out with the setup shown in Figure 3 to obtain a CDS casting. The microstructure of the resulting casting alloy would be investigated to confirm a non-dendritic morphology of the primary Al phase. If this is not achieved, a different set of T1
and T2
values would be assumed and experiments re- that marginal varia-
does not affect the result of the CDS experiments, significantly influences the
would be selected and experiments would
is greater than 55C (99F) the optimization and T2
and TL2
. When the difference between could be car-
For a specific alloy to be cast using the CDS technology, a mass ratio, mr
ed that T1
precursor alloy temperatures. A predominantly non-dendritic morphology of the primary phase in the microstructure would be considered as favorable. The above mentioned procedure as shown in Figure 2 was carried out for each of the three al- loys in this study.
be changed rather than T2
Figure 3 shows a schematic and photograph of the setup used in the experiments carried out to optimize the precur- sor alloy temperatures as shown in Figure 2. In Figure 3 (a), Alloy 2 with the lower thermal mass was melted in a cru- cible to the required temperature T2
in an electric furnace. A second empty crucible (with a 6 mm hole fitted with a spout
was poured into the top crucible and the stopper was lifted so that this alloy could fill and solidify in the bottom crucible to yield a conventional casting sample. The conventional cast sample provided comparable microstructure data for a low superheat casting of Alloy 3.
and the bottom at 5C (9F) above TL2
in the bottom) was heated to the required temperature T1 along with Alloy 1 contained in a third crucible. The empty crucible (with hole) was fitted above the crucible contain- ing Alloy 2 and the hole in the top crucible was plugged shut with a graphite stopper. Alloy 1 was poured into the top crucible and the stopper was released so it can mix into Alloy 2 at a controlled rate to produce a CDS casting of the resultant Alloy 3 in the bottom crucible, shown in Figure 3. K-type thermocouples were inserted in both the top and bottom crucibles to monitor and record the temperature of Alloy 1 and Alloy 2, respectively. The thermocouple in the bottom crucible recorded the thermal events during the mix- ing process as well. Temperature data was recorded at a rate of 100 Hertz with data acquisition software and hardware. An additional experiment using the setup shown in Figure 3 was carried out to obtain a sample of Alloy 3 solidified from a low superheat melt of Alloy 3. In this experiment, both the top and bottom crucibles were initially empty; wherein the top crucible was maintained at a temperature of about 5C (9F) above TL3
. Alloy 3
in the optimization of the
The optimized value of the precursor alloy compositions and temperatures were subsequently used in the shape casting of the respective alloys into test bars for mechanical property assessment with a tilt-pour casting process.
Figure 4 shows the metallic mould used in the tilt-pour casting process. Each casting comprised of two tensile test bars (for testing in accordance with ASTM B557-06) and a fatigue test bar (for testing in accordance with ASTM E446-96 [2002]). In the tilt-pour casting, the precursor alloys, Alloy 1 and Alloy 2 were melted in two separate electric crucible furnaces and maintained at temperatures T1
and T2
, respectively. The mould was preheated to 350C (662F). The desired amount of Alloy 2 was poured by a ladle into the pouring cup followed by the desired amount of Alloy 1 onto the Alloy 2 in the pouring cup. The tilting process was commenced immediately after Alloy 1 was poured in the pouring cup.
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