ISSN 00360295, Russian Metallurgy (Metally), Vol. 2014, No. 1, pp. 66–70. © Pleiades Publishing, Ltd., 2014. Original Russian Text © K.S. Senkevich, S.V. Skvortsova, I.M. Kudelina, M.I. Knyazev, V.V. Zasypkin, 2014, published in Metally, 2014, No. 1, pp. 77–82.
Effect of a Microstructure and Surface Hydrogen Alloying of a VT6 Alloy on Diffusion Welding K. S. Senkevich, S. V. Skvortsova, I. M. Kudelina, M. I. Knyazev, and V. V. Zasypkin Tsiolkovskii Russian State Technological University MATI, Moscow, Russia email:
[email protected] Received May 23, 2012
Abstract—The effect of a structural type (lamellar, fine, gradient) and additional surface alloying with hydro gen on the diffusion bonding of titanium alloy VT6 samples is studied. It is shown that the surface alloying of VT6 alloy parts with hydrogen allows one to decrease the diffusion welding temperature by 50–100°C, to obtain highquality porefree bonding, and to remove the “structural” boundary between materials to be welded that usually forms during welding of titanium alloys with a lamellar structure. DOI: 10.1134/S0036029514010121
problem of the low diffusion weldability of titanium alloys with a lamellar structure is solved by increasing the degree of deformation in the weld zone [9] and/or the temperature and also by combination of semiprod ucts of like titanium alloys of various structural types (globular, fine, lamellar) in the same welded pair. As a result, the diffusion activity and the plasticity required for the formation of physical contact increase; in addi tion, the strain resistance and the structural stability (due to a lamellar structure) also increase [8]. It is noted in [1] that an efficient method for increasing dif fusion weldability of titanium alloys with a lamellar structure is the formation of a layer with a fine equiax ial structure characterized by high hightemperature creep rates on their surfaces. The possibility of the for mation of such a structure using reversible hydrogen alloying was demonstrated in [7]. Changing the hydro genation conditions (hydrogen concentration, heating temperature, exposure time, cooling temperature) and the vacuum annealing temperature, we can form a structure with various degrees of dispersion in the sur face layers, retain an unchanged lamellar structure in the volume, and (hence) change the physicomechani cal properties of the structure as a whole.
INTRODUCTION During diffusion welding of twophase (α + β) tita nium alloys under pressure, the process of plastic deformation is substantially dependent on their microstructure and phase composition [1]. The struc tural parameters form at the stage of producing semi products during thermomechanical treatment or sub sequent heat treatments. Varying the percentage of the α and β phases, the type of microstructure (lamellar, globular, mixed, and other), and the size of structural components, one can substantially optimize the diffu sion welding conditions (time, pressure, temperature) and the properties of welded structures. Apart from various thermomechanical treatment methods, ther mal hydrogen treatment (THT) is an effective method for controlling the structural state of titanium alloys and, thus, the process of diffusion welding. THT is based on reversible alloying of titanium and its alloys with hydrogen and makes it possible to obtain the required size and morphology of the structural com ponents [2, 3]. A positive effect of additional hydrogen alloying, which made it possible to decrease the tem perature and/or the deformation force during diffu sion welding, was demonstrated in [4–7].
The formation of such a gradient structure can increase the quality of diffusion welding of semiprod ucts from titanium alloys with a lamellar structure.
There are problems to obtain highquality welded joints during diffusion welding of semiproducts from titanium alloys having a lamellar structure. They are related to the fact that these alloys exhibit lower plas ticity than that of alloys with a globular structure. The welded joints of alloys with a lamellar structure have an interface between the bonded materials formed by the elements of microstructures of these materials. How ever, in some cases, the use of semiproducts with a lamellar structure is necessary. For example, this structure is promising for designing precision thin wall cellular and layered titanium structures used in aviation [8] owing to its high strain resistance. The
The aim of this work is to study the effect of various structural types and surface alloying with hydrogen on the formation of diffusion bonding of VT6 alloy sam ples. For this purpose, we formed various types of structure (coarselamellar, disperse, and gradient) using heat treatment or THT and also surface hydro gen alloying in order to change the quantitative pro portion of the α and β phases and their chemical com positions along with changes in the microstructure of the surface layer. 66
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Fig. 1. (a) Coarse lamellar and (b) intragranular fine bulk twophase (α + β) microstructures of VT6 alloy samples.
EXPERIMENTAL The studies were performed on samples cut from a VT6 alloy plate produced by an industrial technology. The alloy chemical composition corresponded to State Standard GOST 19807–91. The samples were saturated with hydrogen in a Siverts setup in the medium of highpure gaseous hydrogen; and subse quent cooling was performed at rate of 1 K/s. The quantity of the introduced hydrogen was determined by a change in the pressure in the system with the known volume and controlled by an increase in the sample weight using an Adventure AR2140 electron laboratory balance. Vacuum annealing of the samples was performed in an SVNE1.3.1/16I3 furnace. Met allographic studies were carried out on an AXIO Observer.A1m optical microscope at a magnification up to 1000 using the NEXSYS ImageExpert Pro 3 image analysis program. Diffusion welding was carried out in an SDVU40 unit at temperatures of 850, 900, and 950°C and pressures of 10, 20, and 30 MPa for 1 h. RESULTS AND DISCUSSION A coarse lamellar structure was obtained after annealing in the β region and subsequent cooling to room temperature (Fig. 1a). During treatment, a homogeneous microstructure was formed in the alloy over the entire cross section of the sample. An intra granular fine twophase (α + β) structure (Fig. 1b) was obtained from a coarse lamellar structure during THT including hydrogenation at 850°C to obtain 0.6 wt % H2 and subsequent vacuum annealing at 750°C for 5 h. An almost homogeneous β state of the surface was obtained in VT6 alloy samples after hydrogen alloying at 800°C for 3 min and accelerated cooling to normal temperature (Fig. 2a; end of sample). A gradient structure in the alloy was obtained using the technique described in [7], which includes vacuum annealing along with the described surface saturation of the alloy with hydrogen (Fig. 2b). At the first stage of this work, we studied the possi bility of transformation of a bulk coarse lamellar struc ture (Fig. 1a) to a bulk fine structure (Fig. 1b) using RUSSIAN METALLURGY (METALLY)
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THT. In addition, we studied the effect of the obtained fine structure on the structure formation in the diffu sion bonding of VT6 alloy samples. After diffusion welding of the samples at a temperature of 850°C and a pressure of 10 MPa for 1 h, we observed individual elongated pores in the bonding zone (Fig. 3). The dif ference in the microstructures of diffusion bonding of the alloy is due to the structure formation in the con tact zone. In the samples with a coarse lamellar α + β structure, there is a clear boundary between the bonded surfaces and there are no common structural elements (Fig. 3a). In the case of diffusion welding of the samples with a fine structure, the interface is less pronounced and common structural elements are observed in some places. When the welding temperature is increased to 900°C, the number and the size of pores decrease; however, in this case, the difference in the specific fea tures of the welding bond structure is retained. A fur ther increase in the welding temperature to 950°C allowed us to completely remove pores in the welded samples with an initial fine structure, but individual spherical pores were observed in the samples with a lamellar structure. Thus, it is found that the samples with an initial fine structure exhibit higher diffusion weldability, which manifests itself in the possibility of obtaining diffusion bonding with common structural elements and with out pores in the bonding zone. At the second stage of this work, we studied the possibility of obtaining qualitative diffusion bonding of VT6 alloy samples only due to surface hydrogen alloy ing with a retained coarse lamellar microstructure at the center. Some samples were welded in a hydroge nated state (Fig. 2a), and the others were welded after vacuum annealing (Fig. 2b). It is known from [4–6] that additional hydrogen alloying of a VT6 alloy provides highquality diffusion bonding even at 850°C. However, the studies performed showed that surface hydrogen alloying does not lead to the formation of porefree diffusion bonding. After diffu sion welding at a temperature of 850°C and a pressure of 10 MPa for 1 h, the welded joint zone contains individual
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Fig. 2. Gradient microstructures of VT6 alloy samples after (a) surface hydrogenation and (b) subsequent vacuum annealing.
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Fig. 3. Microstructures in the welding zone (WZ) formed during diffusion welding of VT6 alloy samples with an initial (a) coarse lamellar or (b) fine lamellar structure at temperatures of 850–950°C and a pressure of 10 MPa for 1 h.
small pores (Fig. 4). An increase in the pressure to 20 and then to 30 MPa does not improve the quality of diffusion bonding. The same result was obtained during diffusion welding of the samples with a gradient structure. In all the cases, individual small spherical pores were observed in the bonding zone.
The insufficient quality of the welded joint of the samples that were additionally hydrogen alloyed is likely to be related to the fact that the hydrogen con tent in the surface layer is much higher than the opti mal level (0.3–0.4 wt %) [6–10], at which the alloy plasticity is maximal due to the socalled hydrogen
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plasticization effect [11]. In addition, the hydrogen plasticization effect in the case under consideration occurs only in the nearsurface layers of the samples and does not take place in the entire volume. As a result, its contribution to the process of diffusion weld ing differs from the case of “bulk” hydrogenation. A high quality of diffusion bonding was obtained at a welding temperature of 900°C and a pressure of 10 MPa. In the welded joint zone of the VT6 alloy samples, no pores were observed and a structural inter face between bonded samples was practically absent (Fig. 5a). At the same time, individual small pores were observed in the welding zone in the samples with a gradient structure (Fig. 5b). Surface hydrogen alloying of the VT6 alloy samples provides the possibility of obtaining highquality dif fusion bonding at a temperature of 900°C (Fig. 6). We failed to obtain a defectless welded joint by diffusion
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Fig. 4. Microstructure in the welding zone (WZ) of VT6 alloy samples preliminarily alloyed with hydrogen after diffusion welding at a temperature of 850°C and pressure of 10 MPa for 1 h.
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Fig. 5. Microstructures of welded joints obtained by diffusion welding of VT6 alloy samples at a temperature of 900°C and pressure 10 MPa for 1 h after (a) hydrogenation and (b) subsequent vacuum annealing.
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Fig. 6. (a) Macro and (b)–(d) microstructures of preliminarily hydrogenated VT6 alloy samples after diffusion welding at a tem perature of 900°C and a pressure of 10 MPa for 1 h. RUSSIAN METALLURGY (METALLY)
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welding, which indicates that the welding conditions should be corrected relative to an increase in the weld ing time, temperature, or pressure. However, the application of semiproducts with a gradient structure preliminarily formed by THT for diffusion welding is likely to be ineffective, since it is related to a substantial increase in the process time. In the case of surface hydrogen alloying, vacuum anneal ing used for removing hydrogen can be combined with diffusion welding in the same technological cycle. CONCUSIONS (1) We studied the effect of various structures (lamellar, fine, gradient) and additional surface alloy ing with hydrogen on the process of diffusion bonding of titanium alloy VT6 samples. (2) It was shown that surface hydrogen alloying of VT6 alloy samples decreased the diffusion welding temperature by 50–100°C and formed highquality porefree welded joints. Moreover, we succeeded in removing the structural interface of the materials to be joined, which is characteristic of the welded joints of titanium alloys with a lamellar structure. ACKNOWLEDGMENTS The studies were performed using the equipment of the Center for Joint Use “Aviation and Space Materi als and Technologies” of MATI.
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Translated by Yu. Ryzhkov
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