Mechanochemical processing for metals and metal alloys

Details Mechanochemical processing for metals and metal alloys Mechanochemical processing for metals and metal alloys

1999-02-03

2001-05-15

Wyszomierski, George (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or purifying free metal powder or producing or...

C075S354000, C075S619000, C075S710000, C075S711000, C075S745000, C423S084000, C423S645000

Reexamination Certificate

active

06231636

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to powder metallurgy and, more particularly, to the application of mechanical alloying techniques to chemical refining through sold state reactions.
BACKGROUND OF THE INVENTION
Mechanical alloying is a powder metallurgy process consisting of repeatedly welding, fracturing and rewelding powder particles through high energy mechanical milling. Mechanochemical processing is the application of mechanical alloying techniques to chemical refining through sold state reactions. The energy of impact of the milling media, the balls in a ball mill for example, on the reactants is effectively substituted for high temperature so that solid state reactions can be carried out at room temperature. Although a number of elemental and alloy powders have been easily produced using mechanochemical processing techniques, the production of titanium has been problematic due to long milling times and the contamination associated with the long milling times.
Titanium and its alloys are attractive materials for use in aerospace and terrestrial systems. There are impediments, however, to wide spread use of titanium based materials in, for example, the cost conscious automobile industry. The titanium based materials that are commercially available now and conventional techniques for fabricating components that use these materials are very expensive. Titanium powder metallurgy, however, offers a cost effective alternative for the manufacture of titanium components if low cost titanium powder and titanium alloy powders were available. The use of titanium and its alloys will increase significantly if they can be inexpensively produced in powder form.
Currently, titanium powder and titanium alloy powders are produced by reducing titanium chloride through the Kroll or Hunter processes and hydrogenating, crushing and dehydrogenating ingot material (the HDH process). The cost of production by these processes is much higher than is desireable for most commercial uses of titanium powders. In the case of titanium alloy powders, especially multi-component alloys and intermetallics, the cost of HDH production escalates because the alloys must generally be melted and homogenized prior to HDH processing.
Presently, the production of titanium by reducing titanium chloride is a multi-step process. First titanium oxide is converted to titanium chloride in the presence of carbon at high temperature, as shown in Eq. 1.
TiO
2
+2Cl
2
(in the presence of carbon at high temperature)→TiCl
4
  (1)
Then, the titanium chloride is reduced by magnesium at a temperature above 800° C. Magnesium chloride MgCl
2
is a by-product of the reaction in this process, which is shown in Eq. 2.
TiCl
4
+2Mg→Ti+2MgCl
2
  (2)
The magnesium chloride MgCl
2
is removed by leaching or vacuum distilling to low levels to get sponge titanium. The powder or “sponge fines” is the small size faction of the sponge. Leaching is carried out by dissolving the unreacted magnesium using a mixture of hydrochloric HCl and 10% nitric HNO
3
acids followed by several washings with water. The cost of producing titanium powder this way is high because of the large consumption of energy, problems associated with the high temperatures and the difficulties in removing magnesium chloride MgCl
2
.
A number of attempts have been made in the past to reduce the cost of producing titanium sponge. These include continuous injection of titanium chloride into a molten alloy system consisting of titanium, zinc and magnesium, vapor phase reduction and aerosol reduction. Although cost reductions as high as 40% have been estimated for some of these techniques, a common feature of all of these processes is the use of high temperatures to reduce titanium chloride or titanium oxide.
Apart from cost, production of titanium base alloys present another important problem with regard to their brittleness. The use of high temperature titanium aluminides prepared by conventional techniques is limited by low ductility. Recent work on aluminides has shown that their ductility can be increased considerably by producing the material in nanocrystalline form.
SUMMARY OF THE INVENTION
The present invention is directed to a set of processes for preparing metal powders, including metal alloy powders, by ambient temperature reduction of a reducible metal compound by a reactive metal or metal hydride through mechanochemical processing. The reduction process includes milling reactants to induce and complete the reduction reaction. The preferred reducing agents include magnesium and calcium hydride powders. A process of pre-milling magnesium as a reducing agent to increase the activity of the magnesium has been established as one part of the invention.
One objective of the invention and the research efforts through which the invention was achieved is the development of a cost affordable process for the production of titanium and titanium alloy powders. The objective was approached through the reduction of titanium chloride by calcium hydride to synthesize hydrided titanium powder. Co-reduction of two or more chlorides of titanium, aluminum and vanadium has been employed to synthesize binary intermetallic compounds and the ternary work-horse alloy Ti-6Al-4V, also in hydrided powder form. Cost may be reduced by partially substituting magnesium for calcium hydride. Such substitution also reduces hydrogen pressure build- up during milling. The distinction between the use of a metallic reductant, magnesium for example, and a metal hydride, calcium hydride for example, is the production of titanium with the metal and titanium hydride with the metal hydride. In the case of hydride reducing agents, the titanium and titanium alloys formed by this process are hydrides and hence passivated against oxidation. The hydrides are readily converted to the metal by vacuum annealing.


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patent: 3301494 (1967-01-01), Tornquist
patent: 3376107 (1968-04-01), Oka
patent: 4300946 (1981-11-01), Simons
patent: 4902341 (1990-02-01), Okudaira et al.
patent: 90/07012 (1990-06-01), None

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