Method for production of alloyed titanium welding wire

Abstract

A method for producing a weldable titanium alloy and/or composite wire. The method includes: a) forming a green object by blending particulates of titanium sponge with one or more powdered alloying additions and cold compacting the blended mixture and subjecting the blended mixture including lubricant to pressure; b) forming a work body of alloyed titanium by heating the green object in a protected atmosphere and holding the temperature for a period of at least 4 hours, and then hot working the green object at a temperature of less than 200° C. apart from the beta transition temperature of the titanium alloy and shaping the green object to obtain an elongated profile; and c) forming the welding wire by placing the elongated profile of the work body in a rolling mill having one or more rolls disposed in series.

Description

[0001]This application is a continuation application of U.S. patent application Ser. No. 14 / 006,412, filed Nov. 4, 2013, which was the National Stage of International Patent Application No.: PCT / IB2012 / 051346, filed Mar. 21, 2012, which claims priority to United Kingdom Patent Application No.: 1104764.4, filed Mar. 22, 2011, all of which are incorporated herein by reference. This invention relates to a method for producing a weldable titanium alloy and / or composite wire by cold compaction, extrusion and rolling of a blended mixture of titanium sponge and alloying additions and / or reinforcing particles whereby consolidation and forming is performed wholly in the solid state without melting or encapsulation of said mixture of titanium sponge and alloying additions and / or reinforcing particles.BACKGROUND[0002]Titanium alloys and composites possess superior structural efficiencies owing to their high specific stiffness and strength. However, the present production process for making tit...

AI Technical Summary

Benefits of technology

[0029]One advantage of the present invention is that it allows forming homogenous titanium alloys from relatively coarse particulates of crushed titanium sponge which are processed solely in the solid state without melting the titanium at any time during processing. One of the vital features allowing this advantage is the cold compacting of the blended mixture of titanium sponge particulates, alloying powder additions, and optionally other additives such as reinforcing particulates, lubricants etc. to be pressed, without any additional processing or treatments such as spherodizing or cleaning, to form a billet up to a density in the range from about 80 to about 90% of theoretical maximum density of the titanium alloy. One expected problem with cold compacting coarse particulates of titanium with arbitrary shapes and sharp edges is galling or other form of tearing damages on the pressing tools. A solution to this problem is to employ uniaxial presses operating with the floating die principle where the stress ring is allowed to float along with the press ram. An example of a suitable compaction tool is the stripwinding tool of Strecon in Denmark which employs a pre-stressed stress ring made up of multiple layers of a high-strength steel strip material which is wound around a core of hardened tool steel or tungsten carbide. This tool has the advantage of behaving fully elastic even under very high loads, and is thus able to withstand the high shear forces under compaction of coarse particles of titanium. A surprising discovery made by the applicant is that the problem with galling becomes worse when the average particle size of the titanium particulates becomes smaller than 1 mm, such that a practical lower limit of the mean particle size of the titanium particulates becomes 0.5 mm. By the term “cold compaction” as used herein, means that the temperature of the particulates of crushed titanium sponge is below 200° C. when being subject to the pressing forces. The cold pressing may be performed at room temperature.

[0030]The problem with galling may be alleviated by coating a lubricant onto the wall of the press ring before compaction of the blended mixture. The lubricant may also be added and mixed into the blended mixture to increase the lubrication effect. The lubricant should be driven off the pressed billet after compaction in order to avoid pollution of the titanium alloy during the sintering. This may be obtained by subjecting the billet to a moderate heat treatment up to a temperature of about 400° C. The present invention may apply any known or conceivable lubricant employed in powder metallurgical compaction which may be driven off the compact at temperatures below 400° C., since this is the maximum temperature at which titanium is resistant towards oxygen in the ambient air. Thus, the driving off of the lubricant should thus be obtained by a relatively moderate heating to a temperature range from about 200 to 400° C., and hold the billet at this temperature until the lubricant has stopped gassing off, typically giving a holding period from 0.5 to 10 hours. If the lubricant is mixed into the blended mixture of particulates and powder, it will reduce internal friction between the particles during compacting and thus alleviate the compacting process up to a point where the lubricant becomes a steric hindrance for further compaction. Another function of the lubricant is to reduce external friction which is friction between the compact and die wall. External friction is a source of galling and wear of the pressing tools, and is especially a problem during compaction of particulate titanium sponge due to the hardness of the titanium particulates. Lubricants for powder metallurgical compaction are commercially available as three types; metal stearates, amide waxes, and composite lubricants. Examples of suitable lubricants includes, but is not limited to; zinc stearate, N,N′ ethylene bisstearamide.

Problems solved by technology

However, the present production process for making titanium alloys, whereby titanium sponge particles are blended with alloying additions, such as Al, V, Fe, TiO2, Mo, and Zr, severely restricts the range of titanium alloys that are commercially feasible.

The production of titanium composites is additionally complicated by the fact that many desirable reinforcing particles are either rapidly dissolved (e.g. carbon fibers, SiC, Al2O3) or coarsen (TiB) when immersed in molten titanium.

The former leads to loss of the reinforcing particles, while the latter will be a failure initiation site under mechanical loading.

The beneficial properties imbued by the elements and reinforcing particles listed above, either alone or in combination, have not been realized, due to the processing complications inherent in ingot metallurgy that are described above.

Taking into account yield losses during subsequent handling, the cost increase in converting Ti sponge and alloying additions to bar and subsequently a weldable alloy wire represents the majority of the total cost of wire production.

The plurality of processing steps is accompanied by multiple manual handling operations and the production cost for titanium alloy weld wire renders its use as an additive manufacturing feedstock unattractive except for niche applications.

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