New high-strength titanium alloy
A new high-strength titanium alloy of increased manufacturability for the aerospace and defense industry
Titanium alloys occupy an increasingly strong position in aircraft and rocket engineering due to the unique combination of physical, mechanical and operational properties: high specific strength, non-magnetism, low coefficient of thermal expansion and thermal conductivity, relatively low modulus of elasticity, high corrosion resistance in almost all natural aggressive environments.
A special place is occupied by high-strength titanium alloys, which at temperatures up to 5000 C have the highest specific strength characteristics among all structural alloys. One of the disadvantages of modern high-strength titanium alloys is their low manufacturability in thermo mechanical and mechanical processing.
In this regard, we made an attempt to create a new high-strength complex alloyed welded titanium alloy and develop technological processes for obtaining ingots and various semi-finished products from it – forgings, rolled rods, hot-and cold-rolled sheets, hot-pressed and cold-rolled pipes, we tested the manufacturing technology of some parts, including welded (hydraulic cylinders, bolts, springs, armored doors of the aircraft).
The choice of the system of alloying, smelting ingots.
For the DT-01 alloy (working name – titanium of the future), a multicomponent doping system was chosen, taking into account the balance between α -, β-isomorphic and β - eutectoid-forming alloying elements in order to obtain maximum technological plasticity while maintaining high operational strength and durability. Alloy alloying system - - Ti-Al-V-Nb-Mo-Zr-Fe . To obtain ingots with a uniform chemical composition for DT-01 alloy, a complex multicomponent ligature (master alloys) was developed and smelted. The complex ligature was prepared in such a way that its melting point was as close as possible to the melting temperature of titanium. Electrodes for the first vacuum-arc remeltings were pressed from the prepared charges (Fig. 1), which were melted into a mold with a diameter of 220mm.
Fig.1.
Pressed electrodes for the first remeltings of the ingot.
Production of semi-finished products-approaches to thermo mechanical processing, mastered semi-finished products.
The properties of titanium alloys, in particular two-phase high-strength, are largely determined by the structure. The dependence of the strength of two-phase titanium alloys on the grain size is quite well described by the Petch-Hall law:
σв = σ0 + KD - 1/2
where:
σв - is the strength of the poly-crystalline;
σ0 - strength independent of grain size (single crystal strength);
K - constant of grain boundary braking;
D - value (given diameter) of the grain.
It is known that when the strength of any alloy increases, its plastic characteristics decrease. Nevertheless, academician O. Ivasishin and his colleagues showed that for high-strength titanium alloys there is a boundary value of the microstructure grain, below which it is possible to obtain quite high plastic characteristics of alloys regardless of alloying systems (Fig.2)
Fig.2.
Dependence of elongation at break on grain size for different alloys.
These known regularities were the basis for the technology of semi-finished products from DT-01 alloy. It is obvious that in order to obtain a combination of good strength and plastic properties, it is necessary to ensure that the grain size of the microstructure is less than D1 (Fig.2). In addition, the morphology of the α-phase separation is important, which is determined by the thermo power parameters of the thermo mechanical treatment of the ingot (Fig.3). In our opinion, the globular microstructure is optimal to provide a complex of operational properties of aircraft parts.
a b
c
Fig.3
Variants of separation of α-phase in two-phase titanium alloys ( a - plate, b - bimodal, c - globular structure)
Obtaining a globular fine-grained structure is possible only if certain thermo power parameters of hot deformation of ingots are observed. Special attention was paid to this stage of semi-finished products manufacturing technology. Modes of press forging of ingots with obtaining of the necessary sizes of forgings under further processing in final semi-finished products and the set microstructure were fulfilled.(Fig.4)
Fig.4.
Forging ingots on a hydraulic press.
In the process of mastering the thermo mechanical processing of DT-01 alloy ingots, a number of semi-finished products were mastered, the properties of which are presented in table 2.
From the mastered semi-finished products by now experimental testing of production of the following details is carried out:
Plates and sheets.
Plates and sheets in a wide range of thicknesses from 0.5 to 60 mm with a fine grained structure were obtained (Fig.5)
Fig.5
The microstructure of sheet of alloy DT-01 along rolling.
Currently, aircraft armor elements for the cabin door of a passenger aircraft are made of sheets with a thickness of 3 mm, which have passed ballistic tests in Ukraine (Fig. 6)
a b
Fig.6
Samples of aircraft armor after ballistic tests in Ukraine
The use of DT-01 alloy Instead of the TC4 used today will reduce the thickness of the crew door from 4.5 to 3 mm.
Pipes
Production of pipes from high-strength alloys is very difficult and currently only TC4 alloy pipes are commercially produced. Taking into account the high manufacturability of DT-01 alloy, the production of pressed pipes with a diameter of 45 to160 mm with a wall thickness of 3 to 20mm was mastered. Hydraulic cylinders were manufactured from the pipes, which successfully passed technological tests, including the application of wear-resistant coatings. Research continues. In parallel, the manufacture of pipe barrels of mortars was tested (Fig.7)
Fig.7 pipe for mortar
Bars
Currently, the production of rods with a diameter of 8 to 20 mm has been mastered. Given that small sections of semi-finished products can achieve a super fine-grained structure with a grain size of 1-5 microns, the technology of manufacturing high-strength bolts with a head obtained by hot planting has been tested (Fig.8)
a b
Fig.8
High-strength (Ϭв =1350-1500MPa) docking bolt from an alloy DT-01 :
a – after the hot forging of bolt heads; b – finally made, after the cold rolling thread.
Currently, AN experimental batch of bolts is made of DT-01 alloy, which is undergoing fatigue tests at ANTONOV Company.
Given the possibility of obtaining a super-strong state of DT-01 alloy while providing ultra-fine grain, it is of interest to use this alloy for the manufacture of titanium springs. The issue is being studied.
In the process of creating the DT-01 alloy, its ability to weld was taken into account. In this regard, tests were carried out on welded samples, the properties of which are presented in table 3
As can be seen from table 3, the strength of the weld is almost equal to the base material. The tests carried out without Stripping the weld showed destruction of the base material outside the zone of thermal influence, which indicates good properties of the alloy to welding (Fig.9)
Fig.9
DT-01 alloy welded specimen after tensile test.
The work performed on the new alloy DT-01 showed a number of advantages over the traditional widely used high-strength alloy TC18, in particular:
- higher technological properties on weldability and hot deformation. It is possible to produce extruded tubes and thin sheet;
- higher plastic properties in the heat-strengthened state;
- higher strength at elevated temperatures ;
- higher fatigue properties on smooth samples.
Work has begun on DT-02 alloy (composite material), additionally alloyed with boron. The first results are obtained, indicating the possibility of achieving the strength of the alloy to the level Ϭв ≥1500 MPa. However, there are problems and work continues.
Thank you for your attention.