Mechanical properties and structure of welded joints of VT6 alloy through nanocrystalline interlayer of VT22

Experimental studies of the solid-phase weldability of the VT6 alloy through an interlayer of VT22 alloy in the temperature range of the low-temperature superplasticity (T=820 °C) of one of the elements to be joined have been carried out. Welding of dissimilar titanium alloys allows us to significantly reduce the temperature of pressure welding due to the use of alloys alloyed with different elements. The VT22 with nanocrystalline structure interlayer allows localizing deformation at the junction zone due to a significant difference between the values of the interlayer flow stress and welded billets. To improve the mechanical properties, the welds were heat-treated at T=900 °C. The optimal properties of welded joints are increased after the heat treatment of researched titanium alloys. This leads to the activation of diffusion processes and the disappearance of micropores. Thus, it can be concluded, that getting dissimilar titanium alloys high-quality solid-phase joint depends not only on the size of the interlayer grains but also on the material in the joint zone chemical and phase composition.


Introduction
An increase in demand for new technology and the development of modern mechanical engineering are associated with the search for new cost-effective technologies for forming parts of complex configuration from traditionally difficult materials. Pressure welding combined with superplastic forming (SPF / SD) is one of the perspective technological areas of mechanical engineering [1,2]. The use of the phenomenon of low-temperature superplasticity [3] opens up a way for lowering the temperature of the thermal deformation treatment of metals. The problem of reducing the treatment temperature is especially urgent in the production of articles from titanium alloys.
The solution to this problem is possible by using a process of solid-phase deformation welding of bulk billets in a nanostructured state [4] based on the effect of low-temperature superplasticity. Nanostructuring of titanium alloy VT6 allows to significantly reduce the temperature of superplastic deformation [5,6] and observe the phenomenon of "low-temperature superplasticity". The lower temperature boundary of manifestation of superplasticity depends on the grain size of the treated alloy and decreases upon its decease. The creation of nanocrystalline structure in the titanium alloys can be accompanied by the manifestation of low-temperature superplasticity [7,8]. Isothermal rolling of twophase titanium alloy VT22 in low-temperature superplasticity ensured nanostructuring directly during sheet production without preliminary deformation treatment [9,10]. In this connection, the creation of high mechanical properties in a joint obtained by pressure welding in the state of superplasticity (SP), which depends both on the temperature and on the structural state of the treated alloy, is an interesting metal science problem due to the experimental results.
The aim of the present work is considered in evaluating the mechanical properties of specimens obtained by joining a typical two-phase titanium alloy VT6 through the alloy interlayer VT22 in the temperature range of the low-temperature superplasticity.

Materials and experimental procedure
The rod, 30 mm in diameter out of two-phase titanium micro-grain alloy VT6, with an average grain size of α-phase 3-5 μm, standard chemical composition was used in the experiment. Sheets of nanostructured VT22 alloy with a thickness of 2 mm, containing structural elements with dimensions of not more than 0.3-0.5 μm [11] were obtained by isothermal rolling. The chemical composition of alloys is given in table 1. Table 1. Chemical composition of the studied alloys. The mechanical properties of allows in initial conditions at room temperature are presented in table 2. The microstructure of the analyzed titanium alloys in initial states is shown in figure 1.
Pressure welding of specimens with a total length of 58 mm from alloy VT6 through interlayer VT22 was carried out in a vacuum furnace under conditions of low-temperature SP (figure 2a). Welding was carried out at T=820 °C for 120 minutes with constant pressure (P = 3 MPa) and followed by heat treatment at T=900 °C. The vacuum depth during the experiment was not less than P = 2.0 × 10 -3 Pa. The method of welding of the studied materials is described in [12].
The quality of the joint was estimated metallographically with the help of a TESCAN MIRA3 LMU scanning electron microscope in terms of the relative volume fraction of pores in the crosssection of the weld zone and using the results of tensile mechanical tests.
The surface of the joint was oriented perpendicularly to the tension axis in the central part of the specimen. The strength characteristics of the welded specimens were evaluated not only after pressure welding but also after heat treatment. Specimens with a diameter of 3 mm and a length of 10 mm were a b Figure 1. VT6 alloy microstructure in the initial state (a) and VT22 alloy microstructure after isothermal rolling (b).  2b). The mechanical properties of the alloy were determined by tensile tests at least three specimens in the "Instron 5982" testing machine at room temperature and deformation rate 1 mm/min.

Results and discussions
As a result of pressure welding, a solid-phase joint was formed, two interfaces can be distinguished,between the titanium alloys VT6 and VT22 (zone VT6 + VT22) and between the titanium alloys VT22 and VT6 (zone VT22 + VT6) ( figure 3). The interface can be identified due to the different size of the grains in the rod and the interlayer.
Metallographically studies in the solid-state zones of the specimen after pressure welding revealed pores, at the degree of deformation ε = 1-2%. At the α-grain boundaries, separate pores and a chain of pores are visible (figure 3a). The relative long-range areas of the pores are 0.32. As a result of heat treatment at T=900 °C of these specimens, the relatively long-range areas of the pores decreased to 0.26 (figure 3b). The decrease in porosity in the bonding zone is probably due to the activation of diffusion, in particular caused by the structural gradient at the interface of the welded joints. The intensity of diffusion processes is evidenced by the significant growth of grains in the interlayer. The results of mechanical tensile welded specimens through a nanolayer are given in table 3. As the results of mechanical tensile tests after pressure welding at temperature T = 820 °C indicates, the strength of the solid-phase joint was 975%, and after heat treatment increasing to 1108%. After tensile tests, neck formation is practically not observed, as confirmed by the small value of relative necking.
The fractograms of the central part of the failure are presented in figure 4.
A fractographic study of specimen fractures indicates, after pressure welding at a temperature of 820°C, the crack initiation zone is an intergranular fracture with small pits on the fracture facets. The size of the pits in the fractures correlates with the sizes of the structural components in the samples. After heat treatment, the nature of the fracture changes sharply (intergranular -intragranular): the general view of the fracture has a mixed structure, including areas with flat fragments -quasi-cleavage facets and viscous fracture areas. And this leads to an increase in the strength of the welded joint.
Thus, the possibility of obtaining an equal strength solid-state of specimens from a bulk industrial titanium alloy VT6 through a layer of alloy VT22 at a relatively low temperature (T = 820 °C) and further heat treatment at 900 °C was shown.

Conclusions
The possibility of obtaining of equal hardness solid-state joints of the VT6 specimens obtained by pressure welding through VT22 interlayer in temperature -strain rate regime of low-temperature superplasticity is experimentally shown. To improve the complex of mechanical properties of solidstate joints, specimens were heat-treated at T=900 °C. This leads to the activation of diffusion processes and the disappearance of micropores. a b Figure 4. Fraktogramma of the specimens obtained by pressure welding at T=820 °C (a) and the further heat treatment during 2 hours at T=900 °C (b) after mechanical tensile tests. The work was carried out within the framework of the state assignments of the IMSP RAS No. АААА-А17-117041310221-5.