METASTABLE BETA-TITANIUM ALLOY

Description

Field of the invention

[0001] This invention relates to the non-ferrous metallurgy, and more particularly to the development of new titanium-base alloys combining high strength and ductility properties using relatively low-cost alloying elements. The alloys of this invention can be applied in a wide range of products especially fasteners and different coil springs.

Prior state of art

[0002] One of the known titanium alloys is the alloy containing (mass %): 2-6 Al; 6-9 Mo; 1-3 V; 0.5-2 Cr; 0-1.5 Fe; Ti balance. Ref: USSR Inventor's Certificate No. 180351, Class C22C 14/00, 1966.

[0003] However, this alloy has insufficient ductility due to the high content of Al and the presence of Cr. Besides this alloy is rather expensive.

[0004] The other known titanium alloy contains (mass %): 4-6.3 Al; 4.5-5.9 V; 4.5-5.9 Mo; 2.0-3.6 Cr; 0.2-0.5 Fe; Ti balance. Ref: RF Patent No. 2169204, Class C22C 14/00, published 2001.

[0005] The said alloy as-heat treated has high strength properties in heavy section forgings, but its ductility is insufficient and so the alloy cannot be used for production of such parts as coil springs.

[0006] The most close to the claimed invention is the metastable β-titanium alloy containing (mass %): 4-5 Fe; 4-7 Mo; 1-2 Al; O₂ up to 0.25; Ti balance. Ref: US Patent No. 5,294,267,2 Class C22C 14/00, published 1994. This alloy will be the prototype.

[0007] This alloy has high machinability, demands relatively low costs and is widely used for production of coil cylindrical springs in automotive industry.

[0008] However, the said alloy has low ductility properties, especially elongation, which reduces the application of this alloy and is of importance during manufacture of some types of coil springs and fastener components.

Disclosure of the invention

[0009] The object of this invention is to provide a titanium alloy with combination of high ductility and strength properties in as-heat treated condition, which can be produced using low cost alloying elements.

[0010] In accordance with the invention this is achieved by addition of vanadium to the metastable β-titanium alloy containing aluminum, molybdenum and iron at the following content of components (mass %):

Aluminum	1.5-3.5
Molybdenum	4.5 - 8.0
Vanadium	1.0-3.5
Iron	1.5-3.8
Titanium	balance

[0011] The addition of 1.0-3.5% vanadium increases the alloy ductility as required.

[0012] To achieve high strength properties this alloy has higher content of aluminum than the prototype. The content of aluminum of less than 3.5% does not significantly influence on the alloy ductility. The contents of aluminum greater than 3.5% and iron greater than 3.8% increases the α-phase quantity, causes hardening and reduces ductility lower than desired. The lower content of iron (< 4%) than in the prototype ensures greater phase stability during thermal cycles (deformation and heat treatment). The desired strength properties cannot be achieved with aluminum below 1.5%. The content of molybdenum below 4.5% and iron below 1.5% reduces β-phase quantity and does not allow to achieve high strength of the as-heat treated alloy.

[0013] The increase in the content of such β-stabilizers as molybdenum and vanadium exceeding the specified limits reduces the alloy stability in hardened and aged conditions and increases the grain size during heat treatment, which significantly reduces the alloy ductility (β < 4%; Ψ < 7%).

[0014] Molybdenum is added as ferromolybdenum with 55-75% of molybdenum and iron balance.

[0015] Vanadium is added as ferrovanadium with 65-85 % of vanadium and iron balance or Ti-Al-V system scrap.

Embodiments of the invention

[0016] To study the properties of the alloy, ingots with the composition shown in Table 1 were melted in a vacuum arc furnace and 20 mm diameter bars were made from these ingots. The bars were heat treated under the following conditions: heating to temperature of 30 °C below beta transus temperature, water cooling, heating to 480 °C for 8 hours, air cooling. Then tensile specimens were tested according to ASTM E 8.
Mechanical properties of the produced bars from the evaluated alloys are shown in Table 2.

Table 1

Example	Element Content (wt %)
	Al	Mo	V	Fe	Ti
1	1.5	4.5	1.0	1.5	balance
2	2.5	5.5	2.0	2.5	balance
3	3.0	6.5	3.0	3.5	balance
4	3.5	8.0	3.5	3.8	balance

Table 2

Example	Mechanical Properties
	Ultimate Strength, σB, MPa	Yield Strength, σ0.2, MPa	Elongation δ, %	Reduction of Area Ψ, %
1	1100	1020	25.1	58.7
2	1250	1190	20.2	46.4
3	1440	1390	6.1	10.2
4	1520	1480	4.9	7.3

Commercial practicability

[0017] The claimed metastable β-titanium alloy as compared to the known alloys has the specified optimal combination of beta and alpha stabilizing alloying elements, which ensure high strength and ductility of the as-heat treated alloy. It is low cost and can be used for production of a wide range of critical parts, especially fastener components and different coil springs.

Claims

1. Metastable β-titanium alloy containing aluminum, molybdenum and iron, wherein it additionally contains vanadium, at the following content of components (mass %):

Aluminum	1.5 - 3.5
Molybdenum	4.5 - 8.0
Vanadium	1.0 - 3.5
Iron	1.5 - 3.8
Titanium	balance