Manufacture of near-net shape titanium alloy articles from metal powders by sintering with presence of atomic hydrogen

Abstract

Disclosed herein is a process that includes:(a) forming a powder blend by mixing Commercially Pure (C.P.) titanium powder, one or more hydrogenated titanium powders containing around 3.4 to around 3.9 weight % of hydrogen (e.g., hydrogenated titanium powders available or referred to nominally as “titanium hydride” or TiH2), and one or more hydrogenated titanium powders containing around 0.2 to around 3.4 weight % of hydrogen, or a mixture of the hydrogenated titanium powders without the C.P. titanium powder,(b) consolidating the powder blend by either compacting the powder blend using die pressing, direct powder rolling, cold isostatic pressing, impulse pressing, metal injection molding, other room temperature consolidation method, or combination thereof, at a pressure in the range of around 400 to around 960 MPa, or loose sintering, to provide a green compact having a density lower than that of a green compact formed from only C.P. titanium powder, such that the subsequent sintering of said green compacts is promoted by an increased hydrogen content retained in the green compact which provides emission of hydrogen and a high partial pressure during subsequent cleaning and sintering steps,(c) heating the green compact to a temperature ranging from around 100° C. to around 250° C. at a heating rate≦around 15° C. / min, thereby releasing absorbed water from the titanium powder, and holding the green compact at this temperature for a holding time ranging from around 10 to around 360 min, wherein the holding time and a thickness of the green compact are such that there is around 20 to around 24 min of holding time per every 6 mm of the thickness of the green compact,(d) forming β-phase titanium and releasing atomic hydrogen from the hydrogenated titanium by heating the green compact to a temperature of around 400 to around 600° C. in an atmosphere of hydrogen emitted by the hydrogenated titanium and holding the green compact at this temperature for around 5 to around 30 min thereby forming and releasing reaction water from the hydrogenated titanium powder,(e) reducing surface oxides on particles of the titanium powder by contact with atomic hydrogen released by heating of the green compact to a temperature of around 600 to around 700° C. and holding at this temperature for a holding time of around 30 to around 60 min sufficient to transform β-phase titanium into α-phase titanium while preventing dissolution of oxygen in the metallic body of the titanium particles and simultaneously providing maximum cleaning of titanium powders before forming closed pores,(f) diffusion-controlled chemical homogenizing of the green compact and densification of the green compact by heating to around 800 to around 850° C. at a heating rate of around 6 to around 8° C. / min, followed by holding at this temperature for 30-40 min resulting in complete or partial dehydrogenation and more active shrinkage of titanium powder formed from the initial hydrogenated titanium powder to form a cleaned and refined compact,(g) heating the cleaned and refined green compact in vacuum at a temperature in the range of around 1000 to around 1350° C., and holding the cleaned and refined green compact at such temperature for at least around 30 minutes, thereby sintering titanium to form a sintered dense compact, and(h) cooling the sintered dense compact to form a sintered near-net shaped article.

Description

[0001]This application is a continuation-in-part of U.S. Ser. No. 11 / 811,578, filed Jun. 11, 2007, the entire contents of which are incorporated herein by reference.BACKGROUND[0002]1. Field[0003]Disclosed herein are methods and compositions related to powder metallurgy of titanium and titanium alloys, as well as methods of using these compositions in aircraft, automotive, Naval applications, oil equipment, chemical apparatus, and other industries. More particularly, there is disclosed herein methods for the manufacture of near-net shape titanium articles from sintered elemental and alloyed powders.[0004]2. Description of Related Art[0005]Titanium alloys are known to exhibit light weight, high resistance to oxidation or corrosion, and the highest specific strength (the strength-to-weight ratio) of all metals except beryllium. Articles of titanium alloys have been produced by melting, forming, and machining processes, or by certain powder metallurgy techniques. However, the first meth...

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Benefits of technology

[0013]As a result of the drawbacks of the techniques described above, there remains a need in the art for processes that will increase the mechanical properties, particularly strength and plasticity, of near-net shape articles manufactured by sintering titanium alloys from elemental and / or alloyed metal powders. In order to obtain a high level of mechanical properties, any oxidation or contamination of powdered components must be prevented during heating and sintering.

[0016]Finally, there remains a need in the art for processes of sintering titanium and titanium alloy powders mixed and compacted with a titanium hydride (TiH2 containing over 3.4 weight % hydrogen) powder and hydrogenated titanium powders containing less than 3.4 weight % of hydrogen) which provide both low content of all impurities and improved mechanical properties of the final product in order to meet requirements of such industrial specs as AMS and ASTM.

[0024](e) reducing surface oxides on particles of the titanium powder by contact with atomic hydrogen released by heating of the green compact to a temperature of around 600 to around 700° C. and holding at this temperature for a holding time of around 30 to around 60 min sufficient to transform β-phase titanium into α-phase titanium while preventing dissolution of oxygen in the metallic body of the titanium particles and simultaneously providing maximum cleaning of titanium powders before forming closed pores,

[0033](e) reducing surface oxides on particles of the titanium powder by contact with atomic hydrogen released by heating of the green compact to a temperature of around 600 to around 700° C. and holding at this temperature for a holding time of around 30 to around 60 min sufficient to transform β-phase titanium into α-phase titanium while preventing dissolution of oxygen in the metallic body of the titanium particles and simultaneously providing maximum cleaning of titanium powders before forming closed pores,

[0040]In an alternative embodiment, formation of the β-phase titanium and releasing of atomic hydrogen from the hydrogenated titanium powder is carried out by slow heating, i.e., heating the green compact to a temperature ranging from about 250° C. to about 600° C. in an atmosphere of emitted hydrogen at the heating rate≦around 15° C. / min to enhance the chemical reduction and cleaning effect of the emitted hydrogen and to release reaction water from titanium hydride and hydrogenated titanium powders.

[0042]As indicated above, in a particular embodiment, consolidating of the powder blend can result from compaction, or from loose sintering. Loose sintering can be used without use of room temperature consolidation. In this case, a 40% to 90% dense sintered preform is further processed by high temperature deformation (forging, rolling, extrusion, etc.) to reach the required full theoretical density, which can be followed by the appropriate annealing or other stress relief operations. Cleaning of titanium particles by emitted atomic hydrogen is facilitated in the loose-sintered green compact due to the developed porosity of the material.

Problems solved by technology

However, the first method is not cost effective (although it provides high levels of desired properties of titanium alloys).

The second method is cost effective but as previously implemented cannot completely realize all of the desirable advantages of titanium alloys.

But all of these processes, as well as conventional powder metallurgy techniques, impose certain limitations with respect to the characteristics of the produced titanium alloys.

However, the resulting alloy, contaminated by oxygen, iron, and other impurities, also exhibits insufficient mechanical properties.

This method cannot completely prevent the oxidation of highly-reactive titanium powders during the second heating, because hydrogen is permanently outgassing from the working chamber.

Also, the method does not provide sufficient cleaning of titanium powder that resulted in deviations of final products from AMS and ASTM specifications.

In addition, this method is not suitable for powdered mixtures containing low-melting metal and phases.

While the preliminary sintering partially resolves one technical problem (how to improve uniform distribution of alloying components), the process generates another problem (oxidation of the “mother” powder during pulverization).

As a result, the “cleaning effect” of hydrogen is not fully obtained, and partial oxidation reoccurs after the removal of hydrogen from the vacuum chamber.

Thus, the method does not provide an effective improvement of mechanical properties of sintered alloys, in spite of any sintering that may be promoted by thermal dissociation of titanium hydride.

However, this publication does not describe a process wherein Commercially Pure (C.P.) titanium powder can be used.

Other known processes for making near-net shape titanium alloys from metal powders have the same drawbacks: (a) insufficient purity and low mechanical properties of sintered titanium alloys, (b) irregular porosity and insufficient density of sintered titanium alloys, and (c) low reproduction of mechanical properties that depend on the purity of raw materials.

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