10.20535/2521-1943.2020.89.211400 Structural regularities of welded seam between Ti-Tib and vanadium with 12X18H10T interlayer by using electron beam

2020 / Accepted: 25 May 2020 Abstract. Possibilities of obtaining and using an intermediate layer for joining of Ti-TiB and austenitic steel 12X18H10T (analog of AISI 321; 1.4541; X10CrNiTi18-10) were studied. An alloyed interlayer with TiB reinforcing fibers and the main alloying elements of 12X18H10T steel was formed on the surface of the Ti-TiB sample. Such interlayer was formed during electron beam welding of Ti-TiB with vanadium using foil made of 12X18H10T steel. The microstructure of the interlayer consists of a metal matrix reinforced with microfibers. It’s composition corresponds to vanadium-alloyed titanium boride. In comparison to the initial Ti-TiB alloy, significant grinding of reinforcing boride microfibers in the interlayer microstructure was observed. Besides, the phase formations close to Ti2B in zone of primary titanium borides decomposition in the interlayer, initiated by electron beam were determined. As a result, application of Ti (63–68 at. %) and V (18–25 at. %) based interlayer with alloying additives (Fe, Cr, Ni, B, C) has great potential for utilization in electron-beam welding of 12 Х 18 Н


Introduction
In the present article some basic regularities and optimization directions of Ti-TiB n alloys welded joints using both with titanium alloys, and other structural materials, were investigated. The Ti-TiB alloys are characterized by composition type structure, such as titanium matrix reinforced by TiB microfibers [1][2] with high mechanical properties [3][4]. Therefore they can be used for joining of main materials used in mechanical engineering. The possibilities of Ti-TiB alloy welded joints using the same alloys as well as other titanium alloys for electron-beam welding are shown in [5]. Optimization of welding parameters allowed to obtain welded joints with strength characteristics comparable to the based materials.
At the same time, for formation of welded joint of Ti-TiB alloy with 12Х18Н10Т steel (analog of AISI 321; 1.4541; X10CrNiTi18-10) the use of niobium and low-carbon steel interlayers were needed [6]. It ensured the obtaining of permanent joint, but the level of mechanical characteristics was considerably less than properties of materials subjected to welding.

Problem statement
Utilization of interlayers from materials with high solubility in welded materials is a well-known processing method [7][8][9]. In case of welding of 12Х18Н10Т austenite steel with such materials as niobium or vanadium, the creation of high thermal stresses in heat affected zone is stipulated by large difference in coefficients of thermal expansion. Perhaps, the easiest way to solve the problem of forming an intermediate layer in a welded joint is to form an alloyed layer on the surface of the sample of Ti-TiB alloy, containing reinforcing fibers TiB and the basic alloying elements of steel 12X18H10T.

Materials and Methods
In order to solve the this problem, the Ti-TiB alloy specimens (see the Fig. 1) of 50×50×10 mm size were used. There were connected by 50×10 mm edge with vanadium plate of 2×50×10 mm size through the foil of 12Х18Н10Т steel. The manufacturing method of Ti-TiB alloy and its specimens is described in [5][6]. The foil of 12Х18Н10Т steel has 0.1×50×10 mm sizes according to the GOST 4986-79 state standard; The vanadium plates had 99.9 % purity (the material contains near 0.98 % of C in form of vanadium carbide, see the Fig. 2) from HMW Hauner GmbH&Co.KG. Results of carbide inclusions composition analyses are presented in the Table 1.  Electron-beam welding was carried out on UL-144 (УЛ-144, Pilot Paton Plant, Ukraine) welding machine. The welding was carried out using the following mode of operation: U acc = 60 kV, I eb = 90 mA, electron beam movement velocity was v eb = 13 mm·s -1 , beam sweep during titanium alloys welding with refractory metals and steels was circular, 0.5 mm in diameter. Distance from the electron beam to the welded joint was 70 mm. Beam spot was displaced towards Ti-TiB alloy by the distance ~ 0.3 mm.
Metallographic investigations were carried out with utilization of the JAMP-9500F (JEOL Ltd., Japan) scanning electron microscope with the OXFORD EDS INCA Energy 350 (OXFORD INSTRUMENTS INDUSTRIAL PRODUCTS LIMITED, Abingdon, Oxfordshire, United Kingdom) energy-dispersive Auger-spectrometer, as well with the JSM-840 scanning electron microscope (JEOL Ltd., Japan) with probe for X-ray microanalysis. Different modes of image obtaining were used for metallographic analysis: in secondary electrons (SEI), in backscattered electrons (BSE) and with characteristic X-ray emission. At that, surface image in backscattered electrons was formed in the following modes: topographic (BSE TOPO), representing the relief, and COMPO, representing the phase components contrast.

Results and Discussion
In the Fig. 3 the metallographic structure and distribution character of main elements, in direction perpendicular to the surface of Ті-TiB -12Х18Н10Т -V connection joint, is presented.
In order to evaluate the utilization of interlayer obtained for Ti-TiB alloy welding with stainless austenite steel (particularly 12Х18Н10Т), it was necessary to analyze its structure. In the Fig. 4 the microstructure of Ti (63-68 at. %) and V (18-25 at. %) based interlayer with alloying additions is presented. Revealed microstructure of formed Ti (63-68 at. %) V (18-25 at. %) bases interlayer with alloying additions is characterized with pattern of metal matrix reinforced with microfibers, which composition (see the Table 2) corresponds to titanium boride alloyed with vanadium [5]. Distribution pattern of main elements, revealed in Ti (63-68 at. %) V (18-25 at. %) based interlayer with alloying additions from vanadium plate, Ti-TiB alloy and 12Х18Н10Т steel foil, demonstrates that only at the boundary of interlayer with vanadium and Ti-TiB alloy diffusive character of main elements distribution were observed (see the Fig. 2). At that, the concentration profiles at the boundary with vanadium are characterized with more sharp decreasing character than that at the boundary with Ti-TiB alloy. This is an evidence of considerably more intensive diffusion processes proceeding in Ti-TiB alloy area. It is well-known that in liquid phase the diffusive processes have such intensity which are characterized with diffusion coefficient ~ 10 -5 сm 2 ·s -1 [10]. Intensity of such processes depends considerably less from temperature and type of migrating atoms, than in solid state. Probably, the considerable differences in concentration profiles at the boundary between interlayer with vanadium and Ti-TiB alloy are witness of momentary presence of material in liquid state in these boundary areas.
Distribution pattern of main elements by interlayer thickness is even, which is usually observed at maximum duration of diffusion processes proceeding. At the electron beam movement velocity v eb = 13 mm·s -1 the duration of diffusion processes is determined mainly by residual warming up. In area of formation of Ti (63-68 at. %) V (18-25 at. %) based interlayer with alloying additions the elements redistribution is determined, seemingly, by convection processes caused by electron beam movement.
The microstructure of contact area of Ti (63-68 at. %) V (18-25 at. %) based interlayer with alloying additions and Ti-TiB alloy with distinctive distribution of main chemical elements is presented in the Fig. 5. For more precise determination of distribution by boron, the analysis was carried out by rectangular areas (see the Fig. 5). It should be noted, that the appearance of microstructure of transient zone of the Ti (63-68 at. %) V (18-25 at. %) based interlayer with alloying additions and Ti-TiB alloy is very close to the microstructure of "welded seam" -"basic material" transient zone in welded joint of Ti-TiB alloy obtained by electron-beam welding [5]. The formation of molten metal borides can be observed in the melted areas of materials. The size of molten metal borides is considerable less, than in the initial Ti-TiB alloy. Their location is random and any predominant orientation of fibers was not observed.
Analysis of the main distinctive areas of transient zone was carried out. In the Fig. 6 the microstructure of such area with elemental analysis points 1 -10 selected is presented. Results of analysis are in the Table 3.  Table 3. Elemental composition of Auger spectral analysis points (Fig. 6) Elongated bright phases (points 1, 2), according to the iron -titanium phase diagram [11], correspond to α-hard solution. Darker phase, which in appearance is close to the titanium matrix of Ti-TiB alloy (see point 10), is not typical for main alloy Ti-TiB. Elemental composition of such discharge (see. the Fig. 7) is presented in the Table 4.  Submicron clusters of carbide phase of titanium form the eutectic type areas presented in the Fig. 8.  Essential distinction of titanium matrix in area of such microstructure formations of eutectic-like type is the presence of oxygen in it from 1 to 5 at. %. The presence of oxygen in some zones of Ti-TiB alloy, seems to be connected with technology of its manufacture from the powder materials [1][2]5].
Primary titanium borides in molten zone, formed by thermal action of electron beam, are dissolved and are the source of boron delivery for crystallization of microfibers, which composition fit with titanium boride doped with vanadium (see the Fig. 4). At the boundary of the melt and Ti-TiB alloy the part of TiB fibers is retained (see the Fig. 9), dissolving partly only.  Near partially dissolved TiB microfibers the formation of secondary boron-containing fibers were observed (see the Fig. 10), which composition is close to the Ti 2 B -meta-stable phase and which existence in welded seam heat affected zone was supposed in [5]. Elemental composition in 1 -6 points of analysis is presented in the Table 7.  Thus, the electron-beam welding of specimens made of Ti-TiB alloy, joined with vanadium plates through the foil of 12Х18Н10Т steel, permits to obtain the Ti (63-68 at. %) V (18-25 at. %) based interlayer with alloying additions (Fe, Cr, Ni, B) reinforced with crushed titanium boride microfibers with increased length/thickness ratio (in comparison with primary borides). In the interlayer formed, which thickness was ~1.7 mm, the structural defects and phase formations, capable to initiate brittle fracture in this zone, are not revealed. Adding of such vanadium and iron interlayer in the material ensures the reduction of iron linear expansive coefficient [11], which results in lowering of thermal stresses level in heat affected zone of Ti-TiB alloy/12Х18Н10Т steel joint, which are critically influenced on mechanical properties of such joint [6].

Conclusions
1. Electron-beam welding joint of Ti-TiB alloy with vanadium through foil of 12Х18Н10Т steel permits to ensure the formation of doped interlayer on Ti-TiB alloy surface, which contains TiB reinforcing fibers, as well as the main alloying elements of 12Х18Н10Т steel and vanadium.