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EAC for the preliminary casting trials of titanium at Iowa State University and evaluate the performance of the coating in titanium castings. Te coupled expansion and viscosity

results for uncoated silica sand bonded with the furan binder follow an expan- sion pattern typical of silica sand. An Alpha-Beta transition is seen at approxi- mately 1,063F (573C). A peak at higher temperatures, which corresponds to the cristobalite transition in silica sand, also denotes the sintering temperature of the sample, when sand grains partially melt and start fusing to each other. Te tem- porary increase in viscosity at this stage depicts an increase in the strength of the core or mold due to partial fusion of the sand grains. Te viscosity measurement can be used as an indication to the extent of the same. From the expansion and viscosity

results for the coated silica sand sample, it was determined that the peak expansion at the alpha-beta phase transition was reduced. A peak expansion of 0.007 in./ in. (0.018 cm/cm) was measured. Rather than a contraction after 1,112F (600C), a steady expansion was observed until the cristobalite phase transition. Tis behav- ior prevented the coated core or mold from cracking from high strain caused by contraction of the sample. An increase in viscosity vs. the uncoated sample lead to higher strength, enabling the core to withstand higher temperatures. As mentioned earlier, four molds

were cast with titanium at the Iowa State University. All molds were coated with the EAC. Te resulting mold metal reactivity molds castings were evaluated for alpha-case depth. Vickers hardness was measured for all castings and the extent and depth of the alpha layer was measured. A chart was plotted with the depth in microns on X axis and Vick- ers hardness on Y axis. In all cases, it was seen that the hardness was initially high at a lower depth and decreased with depth till the base layer of titanium was reached. At this point, the hardness stabilizes and is representative of the unaffected base metal. Te base layer of titanium is reached

at approximately 350 VHN. With the exception of ceramic with sodium silicate binder system, the other three samples have similar results. Zirconia with sodium silicate, alumina with sodium

silicate and ceramic with furan recorded an alpha case layer depth of 115 microns, 129 microns and 160 microns, respec- tively. Ceramic with sodium silicate recorded an alpha case layer depth of 237 microns. Te results are in Table 2. Te performance of the ceramic with

furan resin compares well with those of specialty aggregates such as Alumina or Zirconia. Te alpha case layer depths recorded for all samples in the pre- liminary casting trials is comparable to investment casting of titanium. Te EAC is believed to have played a major role in protecting the base layer of the core from being reduced by titanium. Tis can be explained from the similarity seen in the results for the four samples. Titanium castings were poured using 3D printer molds at Rock Island Arsenal. Te mold was coated with EAC and dried before pouring. Te resulting castings were sectioned and evaluated for microhardness (Vickers). Te wedge casting had different sec-

tion thicknesses to evaluate the effect of cooling rate on the formation of alpha

case layer defects. Te results were similar across the sections. Tis shows the dif- ferent cooling rates in the casting did not play an active part in the formation of the alpha-case layer defect of titanium. Te base layer was measured to be at approximately 350 VHN (see Table 3). Te results from the experiments at

Rock Island Arsenal were compared with previous results from Iowa State University to check for variances. Te results were similar between the pour at the two facilities. Te silica sand casting has a maximum hardness of 544 VHN compared to a hardness of 587.8 VHN for the ceramic with sodium silicate. For the same section thickness, a

hardness of 510.89 was measured with ceramic sand and furan resin com- pared to 544 for the silica sand casting. Hence, similar alpha case layer depth was measured with the 3D printed silica sand mold. Te results are comparable to the specialty aggregates tested in the preliminary trials. In conclusion, the research indicated

Fig. 5. This is a photo of the ceramic with experimental alumina coating (EAC).

that by using a specially designed refrac- tory coating, titanium castings could be produced to an acceptable quality level in silica sand molds. Te refractory coating effectively prevented the molten titanium from reacting with the base molding material. Tis was evidenced by the depth of the oxygen rich alpha-case layer exhibited in the experimental castings. Large-scale experimental castings agreed well with laboratory scale castings. From all the testing done and look- ing at the final casting results, it can be concluded that using rapid prototyp- ing technology to produce silica sand molds and using an effective refractory coating such as a water-based alumina coating, the alpha-case layer defect in titanium castings can be reduced to a large extent in sand casting. In addition, 3-D printed molds

provided the quality and integrity required to pour titanium castings, as long as the surface was protected with a nonreactive refractory coating. Tis technology has the potential

to reduce the cost of titanium castings and increase their use in land-based applications.

Fig. 6. Silica sand with experimental alumina coating (EAC) is pictured.

Tis article is based on a paper (14-029) that was presented at the 2014 AFS Metalcasting Congress.

April 2015 MODERN CASTING | 35

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