GB2190100A - A titanium alloy and machine parts made therefrom - Google Patents

Abstract

A titanium alloy forged, cast or sintered to form a machine part has a tensile strength of at least 640 N/mm<2>. The alloy contains between 3 and 28% of one or more of the elements aluminium, chromium, iron, hafnium, cobalt, copper, manganese, molybdenum, nickel, niobium, palladium, silver, silicon, tantalum, vanadium, tungsten, tin, zirconium, beryllium, boron, carbon, oxygen, rare earths and yttrium,the remainder titanium together with unavoidable impurities. The surface layers of the alloy part are treated at over 700 DEG C in glow-discharge plasma, in order to improve resistance to abrasion, including erosion and cavitation and/or in order to increase the permissible surface pressure, the treatment gas containing small quantities (partial pressures 0.1 to 500 mbar) of nitrogen.

Description

SPECIFICATION A titanium alloy and machine parts made therefrom The invention relates two a forged, cast orsintered technical titanium alloy and to machine parts made therefrom, the alloy being of either a, ora + ss. or sstype and being for use in machine parts whose surface layers are treated at above 70000 in glow-discharge plasma for improving abrasion resistance, including resistance against erosion and cavitation and/or in order to increase the permissible surface pressure; and in which the elements necessaryforforming the surface layers are derived from a treatment gas containing small quantities (partial pressure 0.1 to 0.4 mbar) of nitrogen and, if necessary, carbon and/or oxygen.

Machine parts are nowadays usually made from steels of certain composition groups. Particularly interesting for high-value parts in stationary motors and turbines and in motors and transmissions for motor vehicles and aircraft and, in general, for machine parts which reciprocate rapidly, are titanium alloys, due to their high strength-to-density ratios. Nevertheless, quite apart from the cost of these alloys, their poor wear-resistance has hitherto prevented them from enjoying a wide use.

Many methods have been proposed for remedying the tendancy of parts made from tita nium and titanium alloys to weld together where they slide in contact with each other, by giving the parts abrasion-resistant coatings. But all the hitherto known methods have disadvantages. Gaivanic coatings do not adhere well.

Sprayed-on anti-abrasion layers have limited uses dueto the inhomogeneous transition atthe phase-boundary surface ofthe substrate material. This also applies to the titanium nitride layers of micrometre thicknesses applied by the PVD and CVD processes.

In the salt bath, or in the gas phase, it is possible by diffusing elements low on the Periodic Table into the surface of a part made oftitanium oratitanium alloyto form zones of mixed crystals containing continuously changing concentrations of foreign atoms. But in large-scale manufacture, for various reasons, the diffusion zones, and the resulting hard layers, are limited in depth to only,forexample, 0.03 to 0.06 mm.

This also applies to gas nitriding in nitrogen orammonia, and to carburising in pure wood charcoal. Even the Tiduran process, which is the most used in industrial practice, gives only the hardness depths shown in Figure 1.

The intention in the present invention isto provide a titanium alloy for machine parts which is capable of acquiring a diffusion zone with a greater hardness depth than has hitherto been attainable.

To solve this problem it is proposed, according to the invention, to use in machine parts a technical titanium alloy with atensilestrength of at least 640 N/mm2 and the alloy containing between 3 and 28% of one or more of the elements aluminium, chromium, iron, hafnium,-cobalt, copper, manganese, molybdenum, nickel, niobium, palladium, silver, silicon, tantalum, vanadium, tungsten, tin, zirconium, beryllium, boron, carbon, oxygen, rare earths and yttrium, the remainder titanium together with unavoidable impurities; such an alloy being used for making machine parts whose surface layers are treated at over 70000 in glow-discharge plasma, in order to improve resistance to abrasion, including erosion and cavitation and/or in orderto increase the permissible surface pressure, the treatment gas containing small quantities (partiai pressures 0.1 to 500 mbar) of nitrogen.

Preferably the surface treatment includes the use of a gas which can contain, besides nitrogen, also small quantities of carbon and/or oxygen, for the purpose described above.

The treatment time should be at least one hour and the hardened depth at leat 0.02 mm, preferably 0.25 mm.

Thea4raction, particularly in (a + ss )- alloys, is advantageously being diminished by solution heat treatment.

Preferably also, the glow-discharge plasma treatment takes place at a temperature which, in order to increase diffusion velocity, is not more than 200"C underthe P -transition temperature for the alloy.

In the titanium alloy as claimed herein, the internal stresses resulting from preliminary treatments, orthe structural changes associated with volume changes, can be largely nuetralised at the treatmenttemperature by a low-stress annealing of the pre-worked parts, the temperature of the stress-relief annealing being ofthe same order, if necessary up to 50"C higher, than the subsequent treatmenttemperature.

The machine parts being in a practically finished state before treatment be glow-discharge plasma, any subsequent machining, such as polishing, honing or lapping,forcorrecting small increases in volume or roughness, is advantageously controlled to take less than 0.020 mm ofsurface away.

The alloy now provided may be used for spindles, transmission shafts, toothed wheels, toothed rods, rolls, rollers, pinions, synchronisation rings, chain links, plain, ball, rollerand needle bearings, crankshaftsand camshafts, connecting rods, piston rings, valve rockers, valves, the leading edges of steam turbine blades, conveyor worms, cylinders, nozzles, sonotrodes and the cutting edges of parting tools.

The hardness at distances of 0.05 and 0.1 mm from the surface, i.e. the 0.05 and 0.1 mm hardnesses, and the depths from the surface at which hardness values of at least 600 HV can still be detected, are distinctly improved, in the titanium alloy ofthe present invention, afterthe parts have been given the specified surface treatment in the plasma of a glow discharge. Whereas in the Tiduran-treated alloy the 0.05 hardness is approximatelythe core hardness, i.e. practically no hardening has taken place, the plasma-treated sample shows an 0.05 hardness of 500 HV. The hardened depth ofthis sample is therefore about three times greater than it is aftertheTiduran treatment.

Figure 1 demonstrates this improvement.

Afurtherprocess advantage is that the greater hardened depth obtainable in principle by the plasma treatment can in practice be fully utilised because, in contrast to the surfaces attacked in the salt bath, the plasma-treated surfaces require little or no subsequentfinishing. This makes it possible to utilisefullythe different properties ofthethree layers which are typical oftitanium alloys.

The micrographs ofthreetitanium alloys of this invention, treated in a glow-discharge plasma, whose hardness curves are shown in Figure 2, all show the same compound-layered structure.

The outermost layer is a titanium nitride layer 1 to 2 micrometres thick and of greater density. This layer has a yellowish appearance in the metallographic polished specimen. Underthatthere is a white #-stabilised layer about 5 micrometres thick, which merges into the 0.1 5 to 0.40 mm diffusion layer showing inwards-decreasing concentrations of interstitial elements (nitrogen, carbon, oxygen).

Figure2shows clearly the advantages oftreating titanium alloys in glow-discharge plasma, compared with other diffusion methods in the gas phase, in the salt bath orworking with powders, these advantages resulting from the greater numberof adjustable process variables. It will be seen that the greater hardened depth of the plasma-treated alloy Ti- 6AI- 4V isfurtherconsiderably increased in the alloy Ti- 6AI- 2Sn4Zr- 2Mo by deepening the diffusion layer.

The increased alloy content and the decreased amount of a-phase in the crystalline structure now makes it possible, with a core hardness of 400 HV, to obtain 0.05 and 0.1 hardnesses of 640 and 540 HV. The nitride hardness depth defined as the sum of the core hardness plus 50 HVunitscan be as high as 0.27 mm NHT.

The invention makes it possible, for the first time, to obtain in titanium alloys, hardened depths and hardness values which are comparable with the properties of nitrided steels. The process now described increases the depth of the diffusion layer, whose hardness increases continually, and this in turn increases its abilityto support the outer, very hard layers.

This opens up the possibility of using the titanium alloys in highly stressed toothed driving wheels in the transmission systems of fixed wing and rotary wing aircraft, giving a weight saving of up to 40%. The increased hardened depth should also make it possible to make titanium alloy bearings. The use of connecting rods, piston rings, valve rockers and valves made of titanium alloys, which are about 40% less densethan steel, can reduce inertial forces and improve the efficiency of machines.

Machine parts such as conveying worms, cylinders, nozzles and sonotrodes made oftitanium alloys and surface-hardened by a glow-discharge plasma treatment operate at improved efficiency in the processing of products which are subjected to corrosive and/or abrasive attack. The coating method proposed in the present invention should also increase the use of lightweight and/or rapidly moved parting tools made of titanium alloys.

Using treatment times of at least one hour, the upper limit being set only by economic considerations, hardened depths of 0.02 mm and more, preferably 0.25 mm, can be achieved. Products suitable for making from titanium alloys and treating in glow-discharge plasma include spindles, gearbox shafts,toothed wheels, toothed racks, rolls, pinions, synchronisation rings, plain, ball, roller and needle bearings, crankshafts and camshafts, connecting rods, piston rings, valve rockers, valves, the leading edges of steam turbine blades, conveying worms, cylinders, nozzles, sonotrodes and the cutting edges of parting tools.

Claims (12)

1. Atechnical titanium alloy forged, cast orsinteredto form a machine part, the alloy having atensile strength of at least 640 N/mm2,the alloy containing between 3 and 28% of one or more of the elements aluminium, chromium iron, hafnium, cobalt, copper, manganese, molybdenum, nickel, niobium, palladium, silver, silicon, tantalum, vanadium, tungsten, tin, zirconium, beryllium, boron, carbon, oxygen, rare earths and yttrium,the remaindertitanium together with unavoidable impurities, the surface layers of the alloy part being treated at over 700"C in glow-discharge plasma, in order to improve resistance to abrasion, including erosion and cavitation and/or in order to increase the permissible surface pressure, the treatment gas containing small quantities (partial pressures 0.1 to 500 mbar) of nitrogen.

2. Atitanium alloy partas claimed in Claim 1,whose surface treatment includes the useofa gaswhich can contain, besides nitrogen, also small quantities of carbon and/or oxygen, forthe purpose described in Claim 1.

3. Atitanium alloy part as described in Claims 1 and 2, the treatment time being at least one hourandthe hardened depth at least 0.02 mm, preferably 0.25 mm.

4. Atitanium alloy as claimed in Claim 1, and in which the thea4raction, particularly in (a + ss )- alloys, is diminished by solution heat treatment.

5. Atitanium alloy part as claimed in Claim 1, and in which the glow-discharge plasmatreatmentis conducted at a temperature which, in order to increase diffusion velocity, is not more than 200"C underthe ss -transitiontemperatureforthe alloy.

6. Atitanium alloy part as claimed in Claim 1, and in which any internal stresses resulting from preliminary treatments, or any structural changes associated with volume changes, are largely nuetralised at thetreatmenttemperature by low-stress annealing of the pre-worked parts, the temperature ofthe stress-relief annealing being of the same order, or if necessary up to 500C higher, than the subsequent treatment temperature.

7. Use of a titanium alloy as claimed in Claim 1 forthe purpose described in Claim 1,the machine parts being in a practically finished state before treatment by glow-discharge plasma, any subsequent machining, such as polishing, honing or lapping, for correcting small increases in volume or roughness, taking less than 0.020 mm of surface away.

8. Use of a technical titanium alloy as claimed in Claim 1 forthe purpose described in Claim 1,for spindles, transmission shafts, toothed wheels, toothed rods, rolls, rollers, pinions, synchronisation rings, chain links, plain, ball, rollerand needle bearings, crankshafts and camshafts, connecting rods, piston rings, valve rockers, valves, the leading edges of steam turbine blades, conveyor worms, cylinders, nozzles, sonotrodes and the cutting edges of parting tools.

9. Use of a titanium alloyfora machine part as claimed in Claim 1,the alloy being ofthetype: Ti-6AI-4V, and having the composition:

4.5to6.75%aluminium

3.5 to 4.5% vanadium the remaindertitanium and unavoidable impurities, forthe purpose described in Claim 1.

10. Use of a titanium alloyfora machine part as claimed in Claim 1,the alloy being ofthetype: T1 - 6Al- 2Sn-4Zr- 2Mo-Si, and having the composition:

5.5 to 6.5% aluminium 3.6to 4.4% zirconium 1 .8to 2.2% molybdenum 1.8to 2.2%tin at most 0.1% silicon the remainder titanium and unavoidable impurities,forthe purpose described in Claim 1.

11. Use ofatitanium alloyfora machine part as claimed in Claim 1,the alloy being ofthetype: Ti- 8AI - 1 Mo- 1 V, and having the composition:

7.35 to 8.35% aluminium 0.75 to 1.25% molybdenum 0.75 to 1.25% vanadium the remaindertitanium and unavoidable impurities, forthe purpose described in Claim 1.

12. Amachine part made from atitanium alloy and substantially as hereinbefore described.