Authors: Kuprunin Dmitry Gennadievich; Gavze Arkady Lvovich, Candidate of Technical Sciences, Chusov Sergey Yurievich JSC Research Institute of Steel, Moscow, Russia
Titanium alloys, widely used as structural materials in modern technology, are distinguished by a number of unique physical and mechanical properties that provide them with a significant advantage over traditional materials based on iron and aluminum. These properties include: high specific strength, corrosion resistance in the atmosphere, seawater and a number of aggressive environments, reduced elastic modulus, thermal strength, non-magnetism, satisfactory manufacturability. Comparative characteristics of some physical and mechanical properties of structural steels and titanium alloys are given in Table 1.
At the same time, titanium alloys have a number of qualities that prevent their widespread use for the manufacture of loaded parts of ground-based transport vehicles. In this series, it should be noted the insufficient wear resistance and low antifriction characteristics of titanium parts operating under friction contact conditions, the reduced endurance characteristics of titanium parts under cyclic loading, high sensitivity to stress concentrators and low contact strength under static and cyclic loads.
JSC Research Institute of Steel, together with a number of scientific organizations and industrial enterprises, conducted systematic studies of various methods of volumetric and surface hardening of titanium alloys. Thermal, deformation-thermal, chemical-thermal processing methods were used, as well as hardening methods based on the use of highly concentrated energy flows (electron beams, radiation from optical quantum generators, high-frequency currents and low-voltage and high-voltage electrical discharges).
Comparison of physical and mechanical properties of structural steels and titanium alloys
notes: * Isothermal hardening[1, 2]
** High temperature thermomechanical treatment
As a result of research and development work, fundamental technical solutions were found, technological processes were developed and implemented that made it possible to largely overcome the disadvantages of titanium alloys noted above and recommend their use in the manufacture of a number of critical parts of the chassis of special-purpose vehicles of light weight category (Table 2).
Structural parts made of titanium alloys
Already in the 1950s, it was noted in the works of US research laboratories that titanium alloys are promising as protection against small arms bullets. By the early 1960s, American researchers presented the armor characteristics of the Ti-6Al-4V alloy, which demonstrated the possibility of significantly reducing the weight of obstacles in comparison with homogeneous steel armor.
The advantage of Ti-6Al-4V titanium alloy over steel and aluminum armor is clearly shown in Fig. 1-4, which shows comparative data of US standards for the resistance of barriers made of medium and high hardness armored steel, armored aluminum alloys and the specified titanium alloy.
The graphs below show the dependences of the impact velocities corresponding to a 50% probability of through penetration (V50) on the surface density of the obstacles (mass of 1m2 of the protective barrier) when fired by various means of destruction at a 90° angle of encounter with the surface of the barrier.
The study of the armor characteristics of titanium alloys in relation to armored vehicles and means of individual armor protection of the Institute of Steel (then the Institute of Steel) was started in the late 1950s and continues to the present day with varying degrees of intensity. As a result of the work carried out, it was found that of the serial titanium alloys, the optimal complex of armor properties (durability and survivability) and technological characteristics (deformability and weldability) are possessed by alloys OT4-1, VT6 (analog of the American-6Al-4V) and VT23. Some properties of titanium alloys used in protective structures are given in Table 3.
For a long time, the main obstacles to the widespread use of titanium alloys for ground-based military equipment were the scarcity and high cost of semi-finished products. Currently, the scarcity factor of titanium alloys has completely lost its significance, since the production of titanium semi-finished products in the Russian Federation is loaded by less than 60%.
Fig.1. The dependence of the impact velocity V 50 of the AR M2 bullet of 7.62 mm caliber on the surface density of the barrier from:
Row 1 - Titanium alloy Ti-6Al-4V (ϬB=880-1050 MPa)
Row 2 - Aluminum alloy 7085 (ϬB=537 MPa)
Row 3 - steel (hardness 420-470H B)
Row 4 - Aluminum alloy 7017 (ϬB=434 MPa)
Row 5 - Aluminum alloy 5059 (ϬB=365 MPa)
Fig 2. The dependence of the impact velocity V 50 of the 12.7 mm AR M2 bullet on the surface density of the barrier from:
Row 3 - steel (420-470H B)
Fig. 3. The dependence of the impact velocity V 50 of the 20 mm FSP fragment simulator on the surface density of the barrier from:
Fig. 4. The dependence of the impact velocity V50c of the API-T M602 caliber 20mm
from the surface density of the barrier from:
Row 1 -Titanium alloy Ti-6Al-4V (ϬB=880-1050 MPa)
Row 2 - steel (hardness 477-534HB).
Physical and mechanical properties of titanium alloys used in protective structures
At the same time, the cost factor has become subjectively less significant due to the comparison of prices of titanium semi-finished products with the cost of composite materials and ceramic armor used recently, as well as a sharp decrease in the share of the cost of the hull and chassis parts in relation to the cost of aggregates and devices of modern AIBTV products.
In addition, research aimed at reducing the cost of titanium blanks by using cheaper charge materials and optimizing technological methods for obtaining ingots has not stopped.
The result of the work in this direction was the creation of new economically alloyed alloys with an increased content of impurities of copper and iron, which, with the loss of some properties (thermal strength, thermal stability during reheating), mandatory for long-term operation of parts at temperatures above 400℃, have the necessary margin of structural strength and reliability during their operation at climatic temperatures in conditions of intensive shock and fatigue loading.
It should be noted that this is a worldwide trend associated with an increase in the use of titanium alloys for armor protection. In addition to the main alloy-6AL-4V, its Low-cost analogues are actively used abroad:ATI-Ti-425TM,TimetTi-62STM (with an oxygen content of up to 0.3%) with a reduced manufacturing cost (due to the involvement of scrap, replacement of expensive alloying elements with gold).
An example of similar Russian alloys is the titanium two-phase (α+β)-alloy VST 2 (VST-2B), developed by PJSC VSMPO-AVISMA Corporation (Verkhnyaya Salda) with the participation of JSC Research Institute of Steel.VST-2B belongs to the martensitic class alloys, the content of β-stabilizers is close to the VT23 alloy. When smelting its ingots, a low-grade titanium sponge is used, 30% of lump titanium waste and 10% of waste in the form of chips are involved. Due to this, the cost of products made from this alloy can be reduced.
The results of the determination of the complex of mechanical properties and the assessment of armor characteristics carried out with the participation of traditional and modern means of destruction indicate the prospects of using the alloy VST-2B:
- for the manufacture of structural parts for the developed products instead of those made from more expensive serial titanium alloys with a significant economic effect;
- to replace parts, usually made of structural steels, with a reduction in the weight of parts by 40%;
- for the manufacture of elements of personal protective equipment (SIB) for various purposes;
- for the manufacture of armor covers, hatches, hinged parts of the booking of hulls. 
Some preliminary test results of the VST-2B plates to assess their protective characteristics are shown in Table 4.
In addition to the production of homogeneous sheets and plates of VST-2B alloy, VSMPO-AVISMA Corporation has developed a technology for surface treatment of sheets and plates made of VST-2B alloy, which allows to obtain on their surface a thick layer of metal-ceramic composition with a hardness of up to 72 HRC, firmly connected with the bulk of the sheet (plate), while maintaining high strength and toughness of the back side of the composite obstacles at the level of the initial blank. The specified surface layer causes the destruction of high-hard cores of armor-piercing bullets and leads to a sharp increase in the resistance of armor barriers with their satisfactory survivability (the permissible distance between lesions is from 25 to 60-90 mm, depending on the means of destruction).
Comparison of the protective characteristics of homogeneous titanium rolled products from the VST-2B alloy and from serial titanium alloys of the Ti-6Al-4V type
Plates made in this way can be used both as a mono barrier and as part of spaced or combined armor.
Some preliminary test results of VST-2 B (VST 2) plates with surface hardening and compositions with their application are shown in Table 5.
Weight characteristics of composite titanium-ceramic armor panels and protective structures with their use in comparison with protective structures based on armored steels and alumina ceramics
In addition, the advantages of composite titanium-ceramic armor panels in comparison with ceramic-polymer ones based on aluminum oxide ceramics are:
-lower labor intensity of manufacturing;
-using only domestic materials;
-preservation of protective properties when heated to a temperature of 8500C;
-the possibility of manufacturing welded structures;
- reduction of the thickness of the armor panels and the magnitude of the dynamic deflection during firing;
- preservation of the cost of composite titanium-ceramic armor panels at the level of products made of traditional titanium alloys.
Based on the above, the following areas are promising for the use of the VST-2B alloy in the protection structures of AIBTV products and personal armor protection equipment (SIB):
Replacement of parts from serial titanium alloys in serial SIB with parts from VST-2B alloy: shields of combined titanium armor helmets (with an increase in the protection class from Br 1 to Br 2), armor elements and armor plates in bulletproof vests and protective suits of the sapper (reduction in the cost of these parts).
Replacement of steel armor elements in serial bulletproof vests with armor elements made of VST-2B alloy, including heterogeneous (surface hardening), with a reduction in the weight of these bulletproof vests by 15-25%.
Development of technology for manufacturing titanium caps made of VST-2B alloy for combined titanium armor helmets instead of using less durable serial titanium alloys (reducing the weight of helmets and the consumption of aramid fabrics for the manufacture of fabric-polymer supports).
The use of heterogeneous materials based on the VST-2B alloy for the manufacture of armor elements and armor plates of SIB products: weight reduction; increasing the protection class to Br 3-Br 5.
Replacement of attachments made of armored steels and ceramic-polymer composites with attachments made of heterogeneous materials based on VST-2B alloy for ground armored vehicles, ships, launcher protection, etc. (weight reduction up to 40% compared to steel attachments, cost reduction compared to attachments made of ceramic-polymer composites).
The use of VST-2B alloy for the manufacture of covers and hatches of armored vehicles.
list of literature
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The article was published in the magazine "HERE" series 16 No.7-8, 2018
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The use of titanium alloys for structural and armor parts of AIBTV products and means of individual armor protection (SIB)
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