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the metalcasting industry to either induce a tridymite transition which leads to a secondary expansion or induce the cristobalite tran- sition at a lower temperature which causes a large second- ary expansion. Tese addi- tives caused large changes in the volume of bonded sand. Certain additives promote the sintering of the surface of the core and forms a partially melted surface. Tis increases the rigidity of the surface due to the increase in the viscosity of the sintered surface. Te increase in viscosity at higher tempera- tures leads to higher strengths on the surface of the core, thereby resulting in reduced core distortion. Previous research has shown the


Fig. 2. This is a high temperature aggregate dilatometer used for measur- ing thermal expansion of sand samples.


Tese cracks then are filled by the liquid metal, which forms veins on the surface of the casting. Specialty aggregates have a lower


veining defect in cores and molds is a result of a tensile stress exerted at the mold metal interface by a combina- tion of contracting sand and subsurface expanding sand. Tis tensile stress is created by the loss of volume observed in silica sand after the alpha-beta phase transition at 1,063 F (573C). Tis leads to a network of cracks formed due to the high thermal expansion of silica sand.


thermal expansion when compared to silica sand, which will prevent cracks being formed on the surface of cores and reduce veining defects. A major limitation is their high cost. For most applications, a small improvement in the high temperature physical proper- ties of silica sand will be more viable for good casting quality. Research was conducted to evalu-


ate the effect of the addition of specialty


aggregates to silica sand. Te addition of two types of zircon and chromite were studied. Te zircon sand utilized was a special domestic zircon sand which exhibited unique chemi- cal properties not seen in other zircon sands. Te use of this special zircon sand emulated the thermal expansion effects of commercial engineered sand additives while improving the sands refractory value. High temperature physical proper- ties of the various blends were evaluated and test step-cone castings were poured and analyzed for most blends.


Discussion of Results Figure 3 shows the thermal expan-


sion results for the baseline silica sand sample. Te sample can be observed to have a steady expansion leading to the alpha-beta phase transition at 1,063F (573C). A peak expansion of 0.01228 in./in. was observed at this temperature. After this transition, a steady contraction is seen leading to the cristobalite phase transition at higher temperature, where a secondary expansion can be observed. Te beta quartz-beta cristobalite phase transi- tion occurs at approximately 2,642F (1,450C). A graphical representation of the


temperature profile of a step-cone core at 5% solid fraction can be seen in Fig. 4. The temperature of the core surface at the thicker metal sections is already at 2,372-2,462F (1,300-1,350C). At this temperature range, the baseline silica sand sam- ple is contracting after the alpha- beta phase transition. However, the subsurface, is still at a temperature range of 752-1,112F (400-600C) where the silica sand is expand- ing leading to the alpha-beta phase transition. This combination of contracting sand on the surface with expanding sand directly beneath the surface creates tensile failures that fill with liquid metal to form the defect classified as veining. Figure 5 shows the expansion


Fig. 3. Shown is the baseline silica thermal expansion curve. 32 | MODERN CASTING February 2017


results for the silica with zircon blends samples. Te peak expansion


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