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the formulation and characterization of a suitable nonreactive coating for titanium casting applications. Te coating was evaluated with steel Gertsman castings, which are designed to test for metal penetration into the mold. Trial cylindrical mold metal reactivity


Fig. 1. The glove box assembly shows a purification system (lower right), water-chiller for the hearth (lower center), melting hearth (in box), 700-amp welding and arc-melt power supply (left) and motion control system (far left, outside box).

molds were produced using a ceramic bonded with the furan and sodium sili- cate binder systems to evaluate the two binder systems. Molds were also made using alumina and zirconia aggregates to compare the specialty aggregates.Pre- liminary titanium casting trials were held at Iowa State University, Ames, Iowa, where the molds were poured. Te alpha case layer formed was measured. Te final component of the testing

involved titanium casting trials at the Rock Island Arsenal, Rock Island, Ill. Silica sand molds and the newly devel- oped refractory coating were used. Te alpha case layer of titanium was correlated to the preliminary trials. To evaluate the aggregates, binder

Fig. 2. The cold-copper melting hearth and tilt-pour system with attached mold and alloy charge are pictured.

tions, such as pumps and valves. Tis technology will be critical in the next generation of nuclear power genera- tion facilities. Titanium melts between 3,000 to 3,050F (1,649 to 1,677C). Casting the alloy always has been problematic because of the reactivity of the liquid metal with materials that contain it. Any refractory used for titanium casting must have minimal reactivity. Te more negative the free energy of formation, the greater the stability of the oxide and the less tendency of the oxide to give up its oxygen to the titanium. Oxygen obtained from molding materials combines with molten titanium to form a hard brittle

surface that is termed alpha case. Tis alpha case must be removed by machin- ing or chemical etching before the cast- ing can be used. Tis additional process step can increase the cost of a cast titanium component significantly. Te most common refractory flours

used in casting aluminum and steel, zir- con (ZrSiO4) and fused silica (SiO2), are not used as the face coat refractory when casting titanium as these contain some amount of silica, which is readily reduced by the molten titanium. But, alumina exhibits a highly negative free energy of formation while providing a high melting point and good availabil- ity at a moderate cost.

systems and coating, a lab-scale glove- box-based melting system was designed and fabricated at Iowa State University. Te system was used for autogenous weld passes for systematic investigation of cooling rate effects on solidification structure; cold-crucible melting with direct-chill solidification; and inert/ vacuum arc-melting and tilt-pour casting to study solidification in shape castings and the performance of various mold materials’ chemical compatibility, cooling behavior and interface heat transfer. Te overall system is shown in

Fig. 1. A water-cooled copper hearth was integrated into a tilt-pour system to operate within the glove box contain- ment system. Te hearth and pouring assembly were then adapted to accom- modate the molds. Tis is shown in Fig. 2. Vickers microhardness was measured for all castings starting from the alpha- case layer down to the base metal, where the hardness flattens out. Te depth was recorded at the same time. Te titanium alloy used at Rock Island Arsenal was titanium with 6% aluminum and 4% vanadium (TiAl6V4). Te solidification behavior of this alloy was provided by the University of Iowa.

April 2015 MODERN CASTING | 33

Procedure Te experimental method-

ology in this research work was divided into three components. First, the study investigated

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