Production Sintering Practices Furnace Loading

Due to reactivity, titanium adheres to most support materials after sintering. The preferred support or racking material is molybdenum, which has a very low solubility in titanium and is able to maintain strength at high temperatures. Ti-6Al-4V plate can also be used by placing sheets of felted ceramic between the work and support. At lower temperatures (1000 to 1080 °C, or 1830 to 1975 °F), high-density graphite, coated with a wash of yttrium oxide, can be used. It must be dried separately and outgassed prior to sintering. To help ensure against contamination of the parts being sintered, some loose titanium powder can be included with each load to act as a "getter" for gases. Molybdenum foil placed on top of the load can be used to contain the powder.

Leak Testing. Prior to heating, the furnace should be leak tested for each load. This can be accomplished by pumping the furnace down at room temperature to diffusion pump levels, then isolating the furnace chamber from the pumping system. The leakage, or drop in vacuum, is read on the tank vacuum gauge. The leakage rate constant should not exceed a value of 40 calculated by multiplying the observed leakage rate (/'m Hg/h) by the furnace chamber volume (ft3). Thus, for a furnace 3 ft in diameter by 3 ft long, with a volume of 21.2 ft3, the maximum allowable leakage rate would be 1.89 /'m Hg/h. This procedure helps to ensure against contamination of the work load, as a high-capacity pumping system can maintain a good vacuum even with a leak. However, continuous introduction of a stream of air over the work rapidly saturates the getter and begins to contaminate the work.

Sintering Cycles. Care must be taken during heating to avoid excessive outgassing of the work, furnace walls, and elements, unless the system pumping capacity is sufficiently large compared to the furnace volume. Heat transfer in vacuum furnaces is primarily by radiation; therefore, a "shadowing" effect occurs with workpieces close to the heating elements—they heat before pieces in the center of the load are heated. Because a heating cycle can be relatively long ( — 8 h), peak temperature should be maintained for 2 to 3 h to ensure equalization of the load temperature before cooling.

The use of load-monitoring thermocouples at several locations in the load is recommended, particularly with large loads. Sintering temperatures of 1000 to 1300 °C (1830 to 2370 °F) are commonly used. Lower temperatures are usually used for porous and commercially pure products, and higher temperatures are used to produce alloy products.

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