Materials Science and Engineering A329– 331 (2002) 99 – 105 www.elsevier.com/locate/msea
Spray forming and subsequent forging of g-titanium aluminide alloys Gerhard Wegmann a,*, Rainer Gerling a, Frank-Peter Schimansky a, Jin-Xu Zhang b a
Institute for Materials Research, GKSS Research Centre, Max-Planck-Str, Geesthacht 21502, Germany b Shanghai Jiao Tong Uni6ersity, Shanghai 200030, People’s Republic of China
Abstract Spray forming experiments with a binary Ti–48.9Al (at.%) alloy and an advanced g-TiAl alloy with the composition Ti–47Al–4(Nb, Mn, Cr, Si, B) (at.%), designated as g-TAB, were carried out. Subsequently, the spray formed materials were forged. The sprayed and forged conditions were characterized in terms of microstructure, porosity, and impurity content. Tensile properties were evaluated at room and elevated temperatures. Upon forging the microstructures turned from nearly lamellar to near g with a grain size of 4.9 mm (Ti–48.9Al) and from duplex to near g with a grain size of 2.2 mm (g-TAB) owing to dynamic recrystallization. The porosity of the spray formed materials almost vanished after forging. The room temperature (RT) tensile strength was improved due to the significant microstructural refinement. The sprayed and forged g-TAB alloy sustains an elongation of 120% at 800 °C indicating the possibility of superplastic forming. The results are discussed in comparison with conventionally P/M-processed and hot isostatically pressed materials of the same composition. © 2002 Elsevier Science B.V. All rights reserved. Keywords: g-Titanium aluminide alloys; Spray forming; Forging; Hot isostatic pressing; Porosity; Tensile test
1. Introduction In order to increase the efficiency of jet engines and land-based gas turbines and to reduce the fuel consumption and exhaust gas emission, engineers demand light-weight structural materials with sufficient temperature capability. In this light, g-titanium aluminide based materials, and in particular two-phase g-TiAl/a2Ti3Al alloys, are gaining increasing interest as candidates to fill the temperature gap between the usage of Ti alloys and heavy-weight Ni based superalloys. Twophase titanium aluminide alloys exhibit an attractive combination of properties, i.e. a low density, relatively high melting temperature, strength and stiffness retention with temperatures up to around 700 °C, and resistance against titanium fire [1,2]. Casting of these alloys is usually encountered with macrosegregation and a coarse grained microstructure. Subsequent thermomechanical treatments are required to refine the microstructure and reduce the segregation-related inhomogeneities [3]. P/M processing generally yields * Corresponding author.
fine and homogeneous microstructures [4,5]. The disadvantages of P/M processing are the susceptibility for impurity contamination and the additional consolidation by hot isostatic pressing (HIP). The present research efforts are aimed at employing an alternative approach to overcome the outlined difficulties on processing these alloys, namely spray forming. This technique offers the possibility to avoid macrosegregation, to minimize microsegregation, to achieve compositional homogeneity and to ultimately improve the mechanical performance of g-TiAl alloys [6–10]. In spray forming or spray deposition, a molten stream of metal is gas atomized and the partly solidified droplets are collected on a substrate, where they coalesce to form a deposit. On the one hand, the highly efficient heat extraction during atomization and deposition ensures high cooling rates, which limit large-scale segregation and coarsening phenomena. On the other hand, spray forming allows the build-up of near net shape deposits and thus reduces the multiple steps of conventional P/M processing, i.e. powder production, sieving, degassing and consolidation to a single step process. A comprehensive description of the spray forming process is given in [11].
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Up to now only a limited knowledge of the mechanical properties of spray formed g-TiAl alloys has been compiled [6–10]. In a recent article, we showed that optimized conditions lead to a residual average porosity in the deposits of around 1– 2% [8]. The ductility for g-TAB in the as sprayed condition ranges around 0.5% plastic elongation in a tensile test [6]. Pores are considered to reduce the ductility of brittle materials. Thus elimination of pores should result in an improved ductility. Two approaches for pore closure were investigated, the first is to apply an additional HIP-procedure [12,13]; the second is to apply a single step hot forging on the sprayed deposit [13]. The objective of this study is to characterize the microstructure, the porosity and the tensile properties at room and elevated temperature of two spray formed and single-step-forged g-TiAl alloys. This work focuses on the binary alloy Ti– 48.9Al (at.%) and the advanced alloy Ti– 47Al – 4(Nb, Mn, Cr, Si, B) (at.%), which are designated as Ti– 48.9Al and g-TAB, respectively, throughout this article. The tensile performance of the present sprayed and forged alloys will be compared with that obtained on conventionally P/M-processed alloys of the same composition under different HIP conditions.
2. Experimental procedures The alloys, Ti– 48.9Al and g-TAB, were prepared by skull melting with a plasma torch in a water-cooled copper crucible using the PIGA facility at GKSS. The melt was poured by means of an induction heated water-cooled copper funnel into a water-cooled cylindrical mould. Low contamination with interstitials (O2 and N2 and C) are obtained by this technique, which is partly due to the fact that the melt never gets in contact to ceramic material. The resulting cast rods (¥ 40 mm) were used as the starting material for the spray forming process. The process technology is based on the ‘electrode induction melting gas atomization’ (EIGA) technique. The rod dips into an induction coil and is melted at its lower end. The melt drops into the center of a ring-slit nozzle, where it is atomized with argon at a gas pressure of 0.5 MPa. The atomized droplets are collected on a specially designed Ti substrate, which rotates and can be moved in horizontal and vertical directions. A deposit is build up with a shape resembling a volcano and the mass ranges around 1.2 kg. Melting as well as spray forming was conducted in argon atmosphere of normal pressure. The processing parameters, i.e. gas pressure, melt flow rate, offset distance and distance between atomizer and the substrate surface, were chosen according to optimization experiments carried out in the forehand [8]. Cylindrical samples (height, 50 mm; diameter, 45 mm) were machined out of the sprayed deposits. Hot forging
of the cylinders was conducted under near isothermal conditions at : 1100 °C (deformation rate, 0.002 s − 1; degree of deformation, 77%). The resulting pancakes have a thickness of :12 mm and a diameter of :90 mm. The forged pancakes were subjected to a stress relieving treatment (designated as stress relieved (SR) throughout this article) at 1030 °C for 2 h in air. Microstructural characterization was carried out by optical microscopy (OM) and by scanning electron microscopy (SEM) operating with back-scattered electrons. The porosity was measured in an optical microscope with the aid of an image processing system. The smallest detectable pore has a diameter of 1 mm. Impurity concentrations (O2, N2 and Ar) were analyzed on samples taken from deposits fabricated under identical conditions and from the forged and SR pancakes. All tensile tests on sprayed and forged material were carried out in air in the SR condition. Room temperature (RT) tensile tests were performed on flat samples with a gauge length of 9 mm and a rectangular cross section of 1× 2 mm2. The RT tensile tests were conducted on an Instron 1195 testing apparatus with an initial strain rate of 9× 10 − 5 s − 1, the tests at elevated temperatures on a Zwick 1474 with constant strain rate control at 1× 10 − 4 s − 1. For tests at RT and at 500 °C the fracture stress was taken for UTS because of the brittle response to tensile loading. For the higher testing temperatures a ductile stress–strain behavior was observed with a lower fracture stress compared with UTS.
3. Results
3.1. Microstructure, porosity and impurity content The microstructures of the sprayed and forged alloys are presented in Fig. 1 before (Fig. 1a and b) and after (Fig. 1c and d) a stress relieving heat treatment for 2 h at 1030 °C. The microstructures of the forged pancakes were found to appear very homogenous throughout the thickness and also in the radial directions for both investigated alloys. Upon forging the coarse nearly lamellar microstructure of the sprayed Ti–48.9Al deposit (cf. [6]) was broken down into a near g-microstructure with a mean g-grain size of 2.9 mm (Fig. 1a). In the SEM the a2-phase becomes discernible as small bright dots situated mainly on g/g-grain boundaries. The a2-volume fraction was determined as less than 0.1%. On forging the originally duplex microstructure of the sprayed alloy g-TAB (cf. [6]) also turned into a near g-microstructure with a g-grain size of 1.9 mm (Fig. 1b). The a2-fraction appears to be higher in the alloy g-TAB with respect to Ti–48.9Al. This is understood by the lower aluminum concentration.
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Fig. 1. Microstructure of spray formed and forged g-TiAl alloys, imaged in the SEM with backscattered electrons. (a) Ti – 48.9Al, sprayed + forged; (b) g-TAB, sprayed + forged; (c) Ti – 48.9Al, sprayed +forged + SR at 1030 °C, 2 h; (d) g-TAB, sprayed + forged +SR at 1030 °C, 2 h.
By the SR treatment at 1030 °C for 2 h the g-grain size was increased to 4.9 mm in the case of Ti– 48.9Al (Fig. 1c). Single g-grains experienced extraordinary grain growth. Whereas the largest observed g-grains range up to 12 mm in the sprayed+ forged condition, some g-grains have grown to 44 mm upon the SR treatment. In addition, a2-particles have been precipitated and coarsened and the volume fraction increased to 0.7%. In the case of the alloy g-TAB, however, no significant g-grain growth was noticed after the SR treatment (Fig. 1d). The grain size was evaluated to 2.2 mm. The largest observed g-grains before and after the SR treatment show a comparable size with 5.5 and 7.5 mm, respectively. The a2-volume fraction is not noticeably affected by the SR treatment. The porosity was reduced from originally 1.0% (cf. [6]) in the sprayed Ti– 48.9Al deposit to 0.04% after forging. The porosity remained constant upon the stress relieving heat treatment at 1030 °C, and no significant growth of pores was noticed. In the case of the alloy g-TAB the porosity turned from 2.0% in the sprayed condition (cf. [6]) to 0.03% upon forging. Again no significant change of the porosity level and the size of
the largest pores (: 15 mm) was detected after the SR treatment. The contamination with oxygen and nitrogen in the as sprayed deposits and in the sprayed+ forged+SR condition is comparable with ingot metallurgically produced alloys (Table 1). The oxygen and nitrogen concentrations are only slightly above the corresponding impurity levels of the mixture of elements prior to alloying. The argon concentrations in the as sprayed and sprayed+ forged+ SR condition range from 3.5 to 4.5 mg g − 1 (Table 1).
Table 1 Impurity content of spray formed TiAl-alloys in the as sprayed condition and after forging+SR Alloy
Condition
Ti–48.9Al As sprayed Sprayed+forged+SR g-TAB As sprayed Sprayed+forged+SR
N2 O2 Ar (mg g−1) (mg g−1) (mg g−1) 370 320 460 390
40 35 30 20
3.5 4.5 3.8 4.3
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Table 2 Effect of an additional single-step forging+SR on the RT tensile properties Alloy
Processing
Yield strength |0.2 (MPa)
Fracture stress |f (MPa)
Plastic elongation (%)
Ti–48.9Al
As sprayed Sprayed+forged+SR
378
466
1.3
g-TAB
As sprayed Sprayed+forged+SR
488 609
517 658
2.0 0.7
–
691
0.1
Data for as sprayed conditions taken from reference [6].
3.2. Tensile properties The additional forging treatment enhances the RT tensile strength of sprayed g-TiAl alloys (Table 2). In the case of Ti– 48.9Al an increased yield strength (378 [ 488 MPa) is noticed after the forging and SR treatment accompanied with slightly increased ductility, even though the two tested samples of the sprayed+forged+ SR condition exhibited a large scatter with 0.9 and 3.1% plastic elongation. In the case of g-TAB the tensile strength, reflected by the measured fracture stress, is also raised (658 [ 691 MPa) upon forging +SR, even though the effect is much less pronounced with respect to Ti– 48.9Al. However, in contrast to Ti– 48.9Al, the alloy g-TAB is far from being ductilized upon the forging treatment, the plastic elongation drops from 0.7% in the as sprayed condition down to 0.14% after forging. The evolution of the tensile properties, UTS and plastic elongation, with temperature is displayed in Fig. 2a and b, respectively. For comparison purpose the corresponding data of g-TAB processed by conventional HIP of gas atomized powders are included. At RT the UTS of g-TAB is about 170 MPa higher than that of Ti– 48.9Al, but at elevated temperatures this difference diminishes. At 700 °C the UTS of both alloys is comparable around 560 MPa. Between 500 and 700 °C a brittle to ductile transition was recognized for both alloys (Fig. 2b). The samples tested at 800 °C (Ti – 48.9Al and g-TAB) and 700 °C (g-TAB) show necking. Above 700 °C sprayed+forged+ SR g-TAB exhibits the highest elongation with 120% at 800 °C compared with 65% for Ti–48.9Al at the same temperature.
with a g-grain size of 2.2 mm. After a true strain of around 1.4, inferred from the local reduction of sample width in the gauge, the microstructure is substantially refined with a g-grain size of only 1 mm but the g-grains are still equi-axed (Fig. 3b). A few round-
3.3. Microstructural e6olution upon plastic deformation at 800 °C in sprayed + forged + SR k-TAB The sequence of pictures taken from one sample of g-TAB tested at 800 °C in Fig. 3a– c illustrates the evolution of the microstructure and the pore appearance with increasing true strain. The microstructure in the sample head (Fig. 3a) compares to the microstructure before the tensile experiment in Fig. 1d
Fig. 2. Tensile properties as a function of the testing temperature. (a) UTS; (b) plastic elongation.
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4. Discussion
Fig. 3. Microstructural evolution with true strain observed in the necking region of a single tensile specimen of g-TAB tested at 800 °C with a strain rate of 10 − 4 s − 1. (a) m =0.0, Sample head; (b) m= 1.4; (c) m =3.8.
shaped micropores have developed. When increasing the true strain to around 3.8 in the necking region of the sample the microstructure appears still equi-axed, but more refined and a high number of pores elongated in the tensile direction have evolved.
The main result of this study is that an additional forging of spray formed g-TiAl alloys is capable to refine the as sprayed microstructure, to significantly reduce the apparent porosity and to improve, in most cases, the tensile performance. Very homogeneous microstructures resulted upon forging of spray formed deposits with no segregation and inhomogeneities detectable up to the surface of forged pancakes for both alloys. The porosity of 1–2% in the as sprayed condition was significantly reduced to less than 0.04% coming along with the majority of pores being a few micrometers in size, usually considered as micropores. Micropores with an expansion of less than 3 mm are known not to have detrimental influence on the tensile performance [14–16]. Another important implication of this work is the result that the SR treatment at a high temperature of 1030 °C does not result in a considerable increase of the porosity. A number of materials processed by P/M routes suffer from so-called thermally induced porosity, i.e. micropores are coarsened upon a high temperature exposure because of the material softening and expansion of the enclosed process gas [17]. This result deserves even more attention when considering the relative high argon content of the materials with around 4 mg g − 1. It has recently been shown that the argon content of HIP-compacted g-TAB alloy powder (4 h, 1000 °C, 200 MPa) ranges up to only 0.45 mg g − 1 [18]. It is of further importance for g-TiAl alloys processed via spray forming with additional compaction treatments that a subsequent stress relieving treatment at high temperatures does not alter the microstructure significantly. In this respect the binary alloy is more susceptible to some coarsening of the g-grains upon a heat treatment at 1030 °C. The alloy g-TAB responds in a more stable fashion to a high temperature exposure due to the presence of insoluble borides, which stabilize the interfaces. The additional forging of the sprayed g-TiAl deposits is seen to lead to an improvement of the tensile performance. The improvement of the tensile strength grounds basically on the significant microstructural refinement accomplished by forging. Dynamic recrystallization takes place during forging resulting in a refined near g-microstructure with equi-axed g-grains. The response of sprayed+ forged+ SR materials to tensile loading adheres to the Hall–Petch type strengthening mechanism. In an attempt to correlate the RT strength with the g-grain size the measured fracture stress of Ti –48.9Al and g-TAB in the sprayed+ forged+ SR condition is plotted versus d − 0.5 (d=g-grain size) in Fig. 4, together with fracture stress data of comparable fine-grained near g-microstructures produced via conventional HIP of gas atomized g-TAB powders, HIP’ed
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at various temperatures. Originally the dislocation pileup mechanism responsible for Hall– Petch-type strengthening by grain refining applies to the yield strength [19]. In this study not always yield strength data could be derived from the tensile experiments because of the brittle behavior of the investigated materials, especially of g-TAB. Apparently, the experimental results (Fig. 4) are well represented by the linear function |f = |0 +ky/d 0.5, with the parameters |0 =126.5 MPa and ky =0.91 MPa m and the correlation coefficient equal to 0.94. The evaluated parameters are almost identical to |0 =133 MPa and ky = 0.91 MPa m derived for the yield strength— ggrain size relationship of HIP’ed g-TAB alloy powders in compression experiments [20]. The Hall–Petch parameters for g-TAB, fabricated by spray forming+ forging+ SR or by conventional HIP coincide well with literature data on g-TiAl alloys with slightly different compositions and different grain sizes [21–25]. The higher RT strength of sprayed+ forged + SR gTAB with respect to Ti– 48.9Al can thus be ascribed to the finer g-grain size. Obviously, as soon as the g-grain size of g-TAB becomes finer than around 4 mm the RT plasticity approaches zero, possibly caused by local fracture phenomena at heterogeneities or grain boundaries. In the light of these results, 0.7% plastic elongation at a tensile strength of 658 MPa of g-TAB in the as sprayed condition turns out as an excellent performance, which can hardly be topped by additional forging. As shown in [6] the UTS of spray formed g-TAB without any additional consolidation treatment outperforms that of investment cast and HIP processed g-
TAB, showing a UTS of 540 MPa at a slightly higher ductility (1.1% for cast+ HIP’ed compared with 0.7% for sprayed material). The reason for the brittleness of fine grained (g-grain size less than : 4 mm) g-TAB processed by P/M routes or by spray forming is still a matter of intense research efforts. It is well known that the tetragonal ordered g-phase suffers from limited number of slip systems at RT and hence responds intrinsically brittle to tensile loading. The higher the strength of a material the more local fracture phenomena at inhomogeneities or weak boundaries instead of homogeneous deformation determine the ductility measured in a tensile experiment [26]. Imayev et al. [27] attributed the drop of tensile elongation from 3.1% (g-grain size, 10 mm) to 1.1% (g-grain size, 2 mm) with decreasing g-grain size in a wrought binary alloy Ti– 49.7at.%Al to a change of the fracture mechanism from transgranular to intergranular due to insufficient cohesive strength of the grain boundaries. An interesting implication of forging a spray formed deposit of g-TAB is the greatly enhanced deformability at 800 °C up to a true strain of around 1.4. This effect is of practical importance since it may be exploited to reduce the working temperatures for further deformation processes, e.g. rolling or superplastic forming, when taking the sprayed+ forged+ SR pancake as a prematerial for a subsequent forming process. The microstructural observations of the deformed gauge area of the tensile sample tested at 800 °C show that the g-grains remained equi-axed even at high true strains of 3.8. This indicates that possibly grain boundary sliding plays a main role as deformation mechanism at 800 °C. The failure, finally, occurs due to the evolution of pores elongated in tensile direction, which were observed in the necking region close to the fracture surface. Similar observations were made on nanocrystalline g-TiAl alloys [25].
5. Conclusions
Fig. 4. Hall – Petch-type plot of the RT fracture stress as a function of d − 0.5 for equi-axed near g-microstructures of spray formed + forged+ SR (S+F) conditions and conventionally P/M processed gas atomized g-TAB powders HIP’ed at various temperatures indicated in the diagram.
Spray formed g-TiAl alloys were forged and show a very homogeneous and refined microstructure. The apparent porosity of spray formed deposits of g-TiAl was significantly reduced upon forging. The tensile strength of both investigated alloys is improved upon forging owing to the microstructural refinement, the ductility is slightly enhanced in the case of Ti –48.9Al, but is deteriorated in the case of g-TAB. The sprayed+ forged+SR g-TAB exhibits some prospect for superplastic forming at relatively low temperatures. Because of the fine and homogeneous near g-microstructure spray formed and forged g-TiAl can serve as an ideal prematerial for subsequent forming processes.
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Acknowledgements The authors would like to thank U. Lorenz for forging and Dr A. Bartels for the high temperature tests.
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