New vacuum plasma spray-forming technology combined with post-spray heat treating produces near-net-shape refractory metal components with very efficient material utilization and the elimination of post-fabrication machining.

Vacuum plasma spray (VPS) chamber used to make the high temperature, corrosion resistant cartridges. Courtesy Plasma Processes Inc.

High temperature furnace components used for microgravity processing in space are made of refractory metals, taking advantage of their high melting temperatures and inherent chemical stability. Techniques were developed to produce near-net-shape refractory metal components using vacuum plasma spraying (VPS), a process wherein material utilization is very high, and laborious machining can be avoided. As-spray-formed components performed adequately, but higher mechanical and thermal properties were needed. Properties were improved via post-processing thermal treatments such as hydrogen sintering and vacuum annealing. Components were made of tungsten, rhenium, tantalum, niobium and molybdenum refractory metal alloys.

VPS forming of a refractory metal cartridge tube with a corrosion resistant ceramic liner. Courtesy Plasma Processes Inc.

The problem

Microgravity experiments conducted in space (on the International Space Station, for example) are expected to involve processing a wider range of materials than in the past. Improved furnace cartridge materials are essential for further development in the field of materials science because current materials are reaching limits to their chemical inertness and process temperatures. Refractory metals and alloys offer the necessary high melting temperatures and an inherent chemical stability in these nonoxidizing environments. However, difficulty in forming these materials into complex shapes has limited their application in the past.

New VPS forming techniques were used to fabricate high-temperature containment cartridges for use in microgravity research in furnaces such as the Crystal Growth Furnace (CGF) and the High Gradient Furnace with Quench (HGFQ), which operate at a maximum temperature of 1400 to 1600C (2550 to 2910F). These furnaces are directional-solidification furnaces used in orbit to determine what effects microgravity has on the solidification of different materials.

Cartridge material must have a high melting temperature and resist attack from the material being processed, which are the general characteristics of the refractory metals. Previous research in this area has shown that cartridges can be fabricated from several refractory metals. However, compression testing of as-sprayed components resulted in relatively low strengths with little ductility. Therefore, research was conducted to fabricate cartridges from several refractory metals by VPS forming and then to perform post-spray thermal treatments on these materials to improve ductility and to evaluate the effect of the thermal treatments on the microstructures.

VPS formed Mo-41Re sample ampoule containment cartridge comprised of an alumina liner, Mo-41Re structural wall and MoSi2 exterior for use in NASA-MSFC’s GEDS Experiment (dimensions: ~20-in. length x 1 in. diameter). Courtesy Plasma Processes Inc.; Alumina liner functionally graded to the Mo-41Re structural wall to minimize stresses due to coefficient of thermal expansion (CTE) mismatch between the two materials. Courtesy Plasma Processes Inc.

Technical description

Refractory metal alloys investigated included W-3.5Ni-1Fe, Mo-40Re, Ta-10W, W-25Re, and Nb-1Zr. The Nb-1Zr material was the only truly alloyed powder, while the other compositions were composed of elemental powders. Compatibility and compression specimens were made for each material. The compatibility specimens were small open-ended tubes, measuring 8 mm long by 10 mm ID (0.313 in. by 0.375 in.) by 0.9 to 1.27 mm (0.035 to 0.050 in.) wall thickness. The compression specimens were also small open-ended tubes, measuring 25.4 mm long by 25.4 mm ID (1 by 1 in.) by 0.8 to 1.27 mm (0.030 to 0.050 in.) wall thickness.

Vacuum plasma spray forming

A revolutionary step in the fabrication process was VPS forming onto preformed graphite mandrels measuring 305 mm (12 in.) long with 25.4 mm and 10 mm diameters for the compression and compatibility samples, respectively. Prior to spraying, the vacuum chamber was evacuated and backfilled with a partial pressure of argon. Powder (-45 to +10 µm) was delivered to the spray gun by an argon carrier gas, and an argon/hydrogen plasma was used to melt the powder and project it toward the mandrel. The mandrel was rotated during spraying to allow the formation of the tube. Approximately 254 mm (10 in.) of each mandrel was coated, resulting in an open-ended 254 mm long tube by 0.7-1.2 mm (0.03-0.05 in.) wall thickness. The as-sprayed cylinder is removed from the graphite mandrel.

Thermal treatments

The thermal treatment for each material was selected based on current heat treatments for sintering and annealing conventional powder metallurgy components. Each tube was packed with high purity alumina sand to prevent slumping of the thin-walled tubes during heating. Hydrogen was used for heat treating Mo-40Re, W-25Re and W-3.5Ni-1.0Fe to aid in densification and reduce oxides. Both a liquid-phase sinter (LPS) and a solid-state sinter (SSS) were used on the W-Ni-Fe alloy. Hydrogen was not used to heat treat Ta-10W and the Nb-1Zr due to the formation of brittle hydrides. Instead, they were only given a vacuum anneal.

Testing

As-sprayed and heat treated samples were prepared using standard metallurgical polishing techniques, and examined in the as-polished and etched conditions using an optical microscope. Density was determined using quantitative microscopy. Helium leak tests also were performed to determine if any interconnected porosity was open to the surface. A limited number of compression tests were performed on the heat-treated materials to determine any improvements in mechanical properties. The segments were laid on their side and compressed at room temperature until failure at a crosshead speed of 0.025 cm/min (0.010 in./min).

Material characteristics

New parameters and techniques for fabricating spray-formed tubes from several refractory metal alloys were developed. Post-spray thermal treatments were performed to determine the effect on the microstructure and mechanical properties of the materials investigated. Changes in the microstructure and the mechanical properties are related.

The SSS and LPS heat treatments significantly improved the toughness and ductility of the W-Ni-Fe alloys and resulted in acceptable leak rates (1 x 10 He cc/sec). Heat treating the Mo-40Re alloy reduced the amount of interconnected porosity to acceptable levels, but decreased the ductility. The Nb-1Zr alloy in the as-sprayed condition had an acceptable leak rate and recrystallized microstructure. The 1,500 C (2,730 F) heat treatment was insufficient for alloying of the two constituents in the Ta-10W samples. The 1,730 C hydrogen sinter of as-sprayed W-25Re deposits resulted in a recrystallized, homogeneously alloyed microstructure.

The new vacuum plasma spray-forming technology, in conjunction with post-spray heat treating, is a viable method for fabricating new and improved materials for high-temperature furnace cartridges. IH