Materials Science, Vol. 42, No. 3, 2006
DEVELOPMENT OF A NEW SYSTEM OF ALLOYING OF HIGH-STRENGTH TITANIUM ALLOYS FOR WELDED STRUCTURES V. M. Zamkov,1 V. P. Topol’s’kyi,2 S. L. Antonyuk,3 and O. H. Molyar4
UDC 669.295
On the basis of the accumulated experimental data on the redistribution of alloying elements between the α- and β-phases in the process of phase transformation and the well-known concept of necessity of guaranteeing equal molybdenum equivalents in the cast state in different parts of the grains in the β-phase, which is attained by complex alloying of titanium with β-stabilizing elements whose distribution coefficients are greater and lower than one, we choose an alloying complex and specify its limits ensuring the high strength and good weldability of the material. On the basis of the Ti – Al – Mo – V – Nb – Fe– Zr system, we propose the composition of a new high-strength titanium alloy with good weldability and develop new modes of its hot deformation, welding, and thermal treatment.
In recent decades, the major world trends in the development of titanium industry are characterized by the extensive search of the ways of reducing the costs of manufacturing of semifinished and finished products made of titanium alloys and improvement of their basic characteristics, i.e., strength, plasticity, fracture toughness, and fatigue. This can be realized by the extensive application of medium and high titanium alloys. The optimization of the system of alloying and creation of special methods of complex thermomechanical (TMT) and thermal treatment (TT) enables one to get the ultimate strength of the metal σu ≥ 1400 MPa. These alloys are used in various branches of industry: from heavy-duty welded units of the landing gears of aircrafts to high-strength springs of the cars. An important factor contributing to the extensive application of titanium alloys is their weldability. Indeed, the welded structures are highly technological in manufacturing and guarantee high coefficients of consumption of the material. This is why the level of mechanical properties of welded joints is the key point determining whether the analyzed structural titanium material is promising [1]. The required weight efficiency of welded structures can be attained by increasing either the strength of the base material for σu1 / σu0 = 0.9, where σu1 is the strength of the welded joint and σu0 is the strength of the base material, or the ratio σu1 / σ u0 . Note that commercially pure titanium and low alloys are well welded and the strength of their welded joints is on the same level as the strength of the base material. At the same time, the corresponding characteristics of high-strength (α + β ) -alloys (guaranteeing the maximum weight efficiency) are much worse than the characteristics of these and other structural materials. The VT22 high-strength alloy is extensively used in heavy-duty elements of aircrafts, including welded structures. Under the influence of the thermal cycle of welding, the phase composition and structure formed in the weld metal and the zone of thermal influence (ZTI) are characterized by extremely low characteristics of 1 Deceased. 2 Paton Institute of Electric Welding, Ukrainian Academy of Sciences, Kyiv. 3 Antonov ANTK, Kyiv. 4 Kurdyumov Institute of Physics of Metals, Ukrainian Academy of Sciences, Kyiv.
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 65–70, May–June, 2006. Original article submitted March 28, 2006. 1068–820X/06/4203–0353
© 2006
Springer Science+Business Media, Inc.
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plasticity and impact toughness. This is connected with the distinctive features of chemical composition of the alloy, its chemical inhomogeneity, and precipitation of intermetallic compounds in welded joints [2, 7–10]. To guarantee satisfactory levels of plasticity of the weld metal, the procedure of welding is performed by using SPT-2 (Ti–Al–V–Zr) admixtures, which decreases the strength of the joint to 900 MPa. To compensate the decrease in the strength of the weld and its operating reliability, the welded joints are technologically made with larger regular cross sections, which increases not only the weight of the structure but also the costs of its manufacturing. Therefore, the development of high-strength alloys ensuring high levels of strength not only of the base material but also of the welded joints is an extremely urgent scientific and practical problem. In recent years, a complex of research works into the creation of new compositions of titanium alloys satisfying the indicated requirements to welded aircraft structures has been performed at the Antonov Aviation Scientific-Engineering Concern. These works resulted in getting a Patent of the Ukraine (C22C14/00) for a structural (α + β)-alloy of the Ti – Al – Fe – V – Nb – Mo – Zr system in 2003. After traditional procedures of thermal and thermomechanical treatment, this alloy has the level of strength not lower than 1200 MPa with preservation of high plasticity and impact toughness [3]. Experiment In choosing the alloying system, we followed the recommendations given by Prof. A. I. Khorev [4 – 6]. The alloying system and the quantitative ratio of isomorphic and eutectoid-forming alloying elements were selected experimentally. For the first time, it was proposed to use β-isomorphic alloying elements readily soluble in the α-phase. The procedure of complex alloying with β-stabilizers whose distribution coefficients are higher (Mo and Nb) and lower (Fe) than one enables one to get constant amounts of these elements in different regions of the βsolution and guarantee the possibility of more uniform hardening of both the base and weld metals. We also take into account the fact that this type of alloying hardens the α- and β-solid solutions and decreases the gradient of strength between the phases, which positively affects the characteristics of the alloy under the operating conditions. In this case, the ratio of the isomorphic and eutectoid-forming elements in the alloy must vary within the range from 2 / 1 to 1 / 1, which not only guarantees their uniform distributions but also prevents the eutectoid decomposition. It is well known that the introduction of 4% Nb positively affects the weldability of (α + β)-alloys [11] and abruptly increases the plasticity and, especially, the impact toughness of the weld metal. It is assumed that the indicated influence of niobium is explained by its high solubility in the α-phase. Parallel with hardening of the β-solid solution, it stabilizes the α-phase, increases the solubility of aluminum in this phase, and prevents the precipitation of the α2 -phase, which also must promote the increase in the impact toughness of the weld metal. To enhance the positive influence of niobium on the properties of welded joints, we studied the solubility of metals in different phases in the process of complex alloying of titanium alloys. Special attention was given to the influence of β-isomorphic stabilizers on the solubility of iron and to the evaluation of the ranges of concentrations of alloying elements guaranteeing not only high levels of the solubility of iron in the α-phase but also the uniformity of distributions of these elements between the phases required to decrease the strength gradient. In analyzing the solubility of alloying elements, we use a CAMECA scanning microscope and a JEM2000FX11 electron microscope equipped with a standard Si (Li) detector and an AN-10000/95S EDS spectrometer. It is shown that, in the regions with elevated concentration of niobium (inside the α-phase), the concentration of iron also increases (Fig. 1). Moreover, in the presence of admixtures of tungsten, the solubility of iron in the α-phase decreases. In the process of complex alloying of titanium with Al, Fe, Nb, and Mo, the solubilities of Nb and Mo in the α-phase are higher than the solubility of vanadium.
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Fig. 1. Schematic diagrams of analysis (a, b) and the diagram of distribution (c) of alloying elements over the specimen surface inside a single grain: (1) scanning path in the CAMECA analysis, (2) α-plate, (3) β-interlayer, (4) sites of local micro-X-ray spectral analysis (JEM-2000FX11).
Fig. 2. Influence of complex alloying on the solubility of alloying elements in the phase components: ( 䉬 ) α-phase, ( 䊏 ) β-phase. In the course of alloying with Nb and Mo, the solubility of Fe in this phase increases. Niobium promotes the uniform distribution of aluminum in both phases. It is also demonstrated that in the presence of 5–6% Al, niobium and iron are more uniformly distributed between the phases.
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Fig. 3. Structures of the metal in the as-cast state in the course of complex alloying with Al, Fe, Nb, V, and Zr; × 500: (a) Ti –6.1Al– 2.59Fe – 1.51Zr, (b) Ti – 6Al – 2.51Fe – 0.9Zr – 3.43Nb – 6.01V, (c) Ti – 6Al – 2.62Fe – 0.88Zr – 2.22Nb – 4.93V, (d) Ti – 6Al – 2.54Fe – 0.9Zr – 3.87Nb – 4.9V.
If the concentration of iron is lower than 2%, then the solubility of Al in the α- and β-phases increases. However, if the concentration of iron is ≥ 2%, then the solubility of niobium in the phase components decreases (Fig. 2). It should be emphasized that, as the Al and Fe contents of the alloy increase (in the investigated range), the structure does not undergo significant changes inside the grains. Only as a result of complex alloying with ≥ 10% Al, Fe, Nb, and V, we observe changes in the morphology of precipitates of the α- and β-phases inside the grains (Fig. 3). It is shown that the Nb + Fe system of elements can be used as an alloying complex provided that the basic regularities of complex alloying are obeyed. It is known that iron is one of the strongest hardeners of the βphase. Moreover, the data of investigations available from the literature and the experience of its application in commercial alloys demonstrate that these alloys have sufficiently high operating and technological plasticity if the concentration of iron varies within the range 1.5–2.5%. The analysis of welded joints of these alloys does not reveal the presence of the Ti Fe intermetallic compound in the weld metal [7–10]. Thus, as the main hardening element for the developed alloy, we use iron. Further, by introducing 2% Fe and 4% Nb, we get a molybdenum equivalent of ≈ 6%. In view of the fact that the hardening effect of niobium is insignificant, according to the basic regularities of complex alloying, we additionally introduce in the alloy molybdenum and vanadium. It is known that, by adding ≈ 1.5% of these elements, one can increase the strength of the welds by 15% with preservation of high levels of plasticity, toughness, and crack-initiation resistance. To harden the α-phase, we introduce aluminum, which also increases the heat resistance and the modulus of elasticity of the alloy. Its maximum content never exceeds 6% to prevent the possibility of formation of the brittle α2 -phase in the structure of the metal and the appearance of coarse lamellar structures with low impact toughness in the welds. These structures appear as soon as the aluminum content becomes ≥ 5.5%.
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Fig. 4. Influence of the temperature of annealing on the mechanical properties of joints made by electron-beam welding for various procedures of cooling: (a) cooling in the furnace, (b) cooling in air; ( 䉬 ) α-phase, ( 䊏 ) β-phase.
Table 1. Chemical Composition of Technological Ingots
Contents of alloying elements and admixtures, wt.% Al
Mo
V
Nb
Fe
Zr
O
N
H
4.9
0.9
1.25
4.74
1.61
0.1
0.102
0.011
0.003
Moreover, to improve the weldability of the material, it is additionally alloyed with zirconium. The experience of application of this element, both in additions and in alloys, reveals its positive influence on the mechanical characteristics of the ( α + β )-alloys as a result of the decrease in grain sizes and the concentrations of admixtures on grain boundaries. Moreover, if the zirconium content does not exceed 1.5%, then the susceptibility of the weld metal to delayed fracture decreases. As a result of the analysis of the experimental and literature data, we choose an alloy of the Ti – Al – Mo – Nb – Fe – Zr system for subsequent investigations. In view of the requirements to the level of strength of the alloy (≥ 1100 MPa) an its weldability, we establish the following ranges of concentrations of alloying elements (wt.%): 5–6 Al, 0.8–1.8 Mo, 0.8–2.0 V, 3.5–4.8 Nb, 1.5–2.5 Fe, and 0.3–0.8 Zr [3]. The molybdenum equivalents for the minimum and maximum contents of alloying elements are equal to 6.2 and 10.9%, respectively. For the development of the technology of thermomechanical treatment of the alloy and evaluation of its mechanical characteristics and weldability, we used special technological ingots cast according to the technology of electron-beam remelting with intermediate vessel (Table 1). The ingots were forged into rods, rolled into plates, and annealed in the following mode: heating to 750°C, holding for 1 h, and cooling in air. As a result of treatment, we obtained the following parameters (for specimens cut out in the longitudinal direction): σ u = 1100 MPa, σ0.2 = 1080 MPa, δ = 20%, ψ = 55%, and K C V = 43 J / cm3 . In the development of the technology of manufacturing of rolled semifinished products, it is taken into account that, in industry, round ingots are first forged into slab and then rolled. Since it is possible to get slabs with rectangular cross sections in the course electron-beam melting, we used two versions of rolling, namely, from the cast slab and from the preliminarily forged metal. Two schemes of rolling, namely, in the β- and ( α + β )-temperature ranges were investigated. The analyses of microstructures and mechanical characteristics of the alloy after rolling both from the preliminarily forged metal and from the cast slab show that the difference is insignificant. The commercial technology of rolling of blanks directly from the cast slab was elaborated for the first time.
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Fig. 5. Macro- and microstructures of welded joints of T110 alloy made by using different procedures of welding: (a–c) electronbeam welding, (d–f) tungsten argon-arc welding; (b, e) weld, × 500, (c, d) zone of thermal influence, × 500.
The weldability of the alloy and the modes of annealing of welded joints were studied on rolled plates. Welding was performed by using either the procedure of electron-beam welding (EBW) or the procedure of tungsten argon-arc welding (TAAW) with through fusion for a single pass but without admixtures and beveling. This is why the properties of welded joints depend on the chemical composition of the base metal (i.e., on the amounts of alloying elements and impurities), the procedure of welding, and the conditions of postwelding thermal treatment. It is shown that the optimal temperature of annealing of welded joints lies within the range 800 –850°C. As the indicated temperature becomes higher than 870°C, the grains start to grow and we observe the precipitation of coarser phase components inside the grains (Fig. 4). Similar data were obtained for argon-arc welding. The microstructural examinations of the EBW and TAAW welded joints do not reveal defects in the form of pores, microcracks, exfoliations, etc. Lamellar precipitates are typical of microstructures observed for both types of the welds. However, in the case of TAAW, these precipitates are somewhat coarser (Fig. 5). The EBW joints have a much smaller structurally changed zone. It is shown that the proposed alloy is well welded independently of the procedure of welding. The mechanical tests demonstrate that the strength of electron-beam joints is on the same level as the strength of the base metal with satisfactory plasticity. At the same time, the strength of the argon-arc joints is not lower than 90% of the strength of the base material. CONCLUSIONS The alloying complex and the limits of alloying (guaranteeing high strength and good weldability) are chosen for a new domestic T110 titanium alloy of the Ti – Al – Mo – V – Nb – Fe – Zr system. The alloy is recommended for manufacturing heavy-duty welded aircraft structures. The procedure of additional alloying of Ti – Al – Mo – V – Fe alloys with β-isomorphic elements characterized by high solubility in the α-phase (up to 5% Nb) significantly improves their weldability and makes the impact toughness of welded joints more than 1.5 times higher. After introducing 5 –6% Al in Ti – Al – Mo – V – Nb – Fe – Zr alloys, the distributions of Nb and Fe between the phases become more uniform. If the concentration of iron does not exceed 2%, then the solubility of aluminum in the α- and β-phases increases. However, as the concentration of iron becomes ≥ 2%, the solubility of niobium in the phase components decreases. It is shown that the procedure of alloying of titanium with Nb and Mo in the Ti – Al – Mo – V – Nb – Fe – Zr system increases the solubility of the eutectoid-forming β-stabilizer (Fe) in the α-phase, which decreases the gradient of strength between the phases. The T110 alloy exhibits
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good weldability for different procedures of welding: electron-beam welding, tungsten argon-arc welding with through fusion, and tungsten argon-arc welding over a layer of flux. After annealing, the EBW welded joints have the same strength as the base material. At the same time, the welds made by using argon-arc methods have the strength not smaller than 90% of the strength of the base material with high impact toughness and plasticity of the weld metal and the zone of thermal influence. REFERENCES 1. G. A. Krivov, V. R. Ryabov, A. Ya. Ishchenko, et al., Welding in the Aircraft Industry [in Russian], MIIVTs, Kiev (1998). 2. Yu. G. Kirillov, F. R. Kulikov, V. N. Moiseev, et al., “Properties of welded joints of VT22 high-strength titanium alloy,” in: Alloying and Thermal Treatment of Titanium Alloys [in Russian] ONTI VIAM, Moscow (1977), pp. 159–170. 3. A High-Strength Titanium Alloy [in Ukrainian], Patent of the Ukraine UA 40087 C2, 16.06.2003. 4. A. I. Khorev, Complex Alloying and Thermomechanical Treatment of Titanium Alloys [in Russian], Mashinostroenie, Moscow (1979). 5. A. I. Khorev, Contemporary Methods Aimed at Increasing the Structural Strength of Titanium Alloys [in Russian], Voenizdat, Moscow (1979). 6. A. I. Khorev and M. A. Khorev, “Contemporary titanium alloys in the aircraft and spacecraft engineering,” in: Aviakosmich. Tekhn. Tekhnol., No. 1, 15–22 (1977). 7. V. N. Moiseev, R. F. Kulikov, Yu. G. Kirilov, et al., Welded Joints of Titanium Alloys [in Russian], Metallurgiya, Moscow (1979). 8. S. M. Gurevich, V. N. Zamkov, N. A. Kushnirenko, et al., “Welding and thermal treatment of VT22 titanium alloy,” Avtomat. Svarka, No. 5, 54–56 (1982). 9. S. M. Gurevich, F. R. Kulikov, V. N. Zamkov, et al., Welding of High-Strength Titanium Alloys [in Russian], Mashinostroenie, Moscow (1975). 10. V. N. Zamkov (editor), Metallurgy and Technology of Welding of Titanium and Its Alloys [in Russian], Naukova Dumka, Kiev (1986). 11. S. M. Gurevich, V. N. Zamkov, N. A. Kushnirenko, et al., “Additives for welding ( α + β )-titanium alloys,” in: Urgent Problems of Welding of Nonferrous Metals [in Russian], Naukova Dumka, Kiev (1980), pp. 314–320.
