Manufacture of near-net shape titanium alloy articles from metal powders by sintering at variable pressure

Abstract

The process includes (a) mixing a titanium hydride powder having a particle size of <=150 mum with alloying metal powders (master alloys or elemental metal powders) having a particle size in the range of {fraction (1 / 15-2 / 5 of the maximal particle size of titanium hydride powder, (b) compacting the resulting powder mixture by molding at the pressures of 400-1000 MPa, (c) heating up to the sintering temperature of the predetermined alloy composition at variable pressures in a furnace chamber: initial heating to 400° C. in vacuum of less than 10<-2 >Pa, then, heating to a temperature range of 400-900° C. with the pressures up to 10<4 >Pa, which is controlled by hydrogen being emitted by the decomposition of titanium hydride contained in the compacted powdered alloy, and finally, heating to over 900° C. to the sintering temperature at the pressure continually decreasing to the starting vacuum level, and (d) sintering. Heating to the sintering temperature is performed at the rate of 10-15 grad / min. The new technology allows the purity and mechanical properties of sintered titanium alloys and the manufacture of near-net shape sintered titanium articles to be controlled by a cost-effective method.

Description

[0001] The present invention relates to powder metallurgy of titanium alloys, and can be used in aircraft, automotive, Naval applications, oil equipment, chemical apparatus, and other industries. More particularly, the invention is directed at the manufacture of near-net shape titanium articles from sintered elemental and alloyed powders.[0002] Titanium alloys are well known to exhibit lightweight, high resisdence to oxidation or corrosion, as well as the highest specific strength (the strength-to-weight ratio) amid all metals except beryllium. Previously, articles of titanium alloys have been produced by melting, forming and machining processes, or by powder metallurgy techniques. The first method is not cost effective but provides high levels of all properties of titanium alloys. The second method is cost effective but cannot completely realize all advantages of titanium alloys.[0003] Various processes have been developed during the last three decades for the fabrication of near-n...

AI Technical Summary

Benefits of technology

[0011] The object of the invention is to 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.

[0012] In order to obtain a high level of mechanical properties, any oxidation or contamination of powdered components must be prevented during heating and sintering.

[0013] Another objective of the present invention is to provide low porosity and high-density structures of sintered titanium alloys to achieve the densities close to the theoretical value.

Problems solved by technology

The first method is not cost effective but provides high levels of all properties of titanium alloys.

The second method is cost effective but cannot completely realize all 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.

The irregular porosity in the interior portion of the sintered articles is the drawback of this method, which decreases mechanical properties, especially the strength.

The oxidation of resulting articles during the hot pressing results in the loss of mechanical properties.

This method allows the manufacture of the alloy having a density close to the theoretical value but the resulting alloy, contaminated by oxygen, iron, and other impurities, also exhibits low 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.

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

So, the "cleaning effect" of hydrogen is not used properly, and partial oxidation reoccurs after the removal of hydrogen from the vacuum chamber.

Thus, these methods do not provide an effective improvement of mechanical properties of sintered alloys, in spite of the sintering promoted by thermal dissociation of titanium hydride.

These methods cannot prevent the contamination of sintered metals as well as the methods mentioned above: after the replacement of a hydrogen-containing atmosphere by an inert gas, the oxidation of reactive powders reoccurs.

All 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.