Tantalum has a high melting point temperature. Therefore, the alloying of tantalum and other elements such as niobium or tungsten (which also have high melting point temperatures) usually requires the use of an electron beam furnace to melt the compendium of a hot pressed sintering mixture containing tantalum powder and alloying element powder. Tantalum is also relatively ductile. Thus, for example, non-alloyed tantalum waste or primary metals produced by sodium reduction methods must usually be embrittleed by hydrogenation before the tantalum can be crushed into a powder form. Tantalum hydride powder must also usually be dehydrogenated prior to hot pressing and sintering with other alloying element powder to produce input compacts for use in electron beam melting furnaces. This hydrogenation-dehydrogenation (HDH) process requires significant capital and operational infrastructure (including hydrogenation furnaces, crushers, compactors, vacuum furnaces, and compression/sintering equipment), adding significant additional costs to the already high cost of primary sodium-reduced tantalum input materials.
Downstream electron beam melting of pressed sintered powder compacts containing tantalum and other alloying elements may involve other problems. On a macroscopic scale, tantalum powder and other alloying element powder are uniformly mixed before pressing and sintering. However, the resulting compactor does not contain a homogeneous solid solution, which contains alloying elements completely dissolved in the tantalum matrix. Instead, the pressed block contains discrete and separated regions or inclusions of alloying elements, such as tungsten, distributed in relatively continuous regions or phases of the tantalum metal. The discrete alloy-element region and tantalum region of this multiphase microstructure correspond to the corresponding powder particles that are metallurgically bound together to form a compacted block.
The purpose of electron beam melting is to homogenize and refine the alloy composition and to produce ingots with uniform microstructure, low content of relatively volatile inclusion elements and specified alloying elements that are completely dissolved in solid solution and uniformly distributed in the tantalum matrix. In practice, however, liquid-phase mixing of materials with high melting points (e.g., tantalum, niobium, and tungsten) may be difficult to achieve using electron beam melting. For example, the smaller molten pool and the lack of overheating in the molten pool may prevent thorough liquid-phase mixing. In addition, the dripping of molten material from the press into the molten pool of the electron beam melting furnace may reduce the dispersion of alloy components. Currently, industrial-scale electron beam melting furnaces lack the ability to induce the complementary physical agitation of molten pools that can improve the alloying dispersion and homogenization of alloy components.
