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Achieving high strength and low cost for hot-forged TiAl based alloy containing β phase
Materials Science and Engineering A 552 (2012) 345–352
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Achieving high strength and low cost for hot-forged TiAl based alloy containing  phase Toshimitsu Tetsui ∗ , Toshiharu Kobayashi, Hiroshi Harada High Temperature Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 6 March 2012 Received in revised form 9 May 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Intermetallics TiAl alloys Forging Ceramic crucible melting Mechanical properties
a b s t r a c t With the objective of achieving high strength and low cost for hot-forged TiAl based alloy containing  phase, the applicability of oxide-crucible-melted ingot was considered for compositions centered on Ti–28.7Al–7.0Mn (wt%). First, as a result of evaluating the influence of crucible contaminants on the hotforgeability of ingots, it was confirmed that, even at the extremely high oxygen content of 0.35 wt%, hotforgeability could be maintained if the amount of  phase was recovered at the forging temperature by reducing the Al content. Next, as a result of verifying the industrial practicality of large ingot, it was found that hot-forging under industrially required conditions was indeed possible for an yttria-crucible-melted ingot corresponding to the above-mentioned center composition with an oxygen content of 0.16 wt%; and in terms of machinability and Charpy impact properties of hot-forged and heat treated material, this ingot was superior to triple-melted VAR ingot having an oxygen content of 0.055 wt%, as well as having substantially better creep rupture strength. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alloys with a primary phase consisting of TiAl intermetallic compounds have seen practical application in turbocharger turbine wheels for passenger vehicles [1–4] and low-pressure turbine blades for jet engines [5,6]. In both cases, the components are small, and precision casting is employed in their manufacture. In order to take further advantage of the superior properties of TiAl alloys, i.e., low weight and excellent high-temperature strength, thereby contributing to fuel savings and the reduction of CO2 emissions, it is necessary to apply to larger sized components. This in turn requires a forging process similar to ordinary metallic materials, given the difficulties involved in using precision casting method in the manufacture of large components. However, the forgeability of conventional TiAl alloys is quite low, and since only special processes such as isothermal forging have been applicable, it has thus far been difficult to produce larger sized parts in an industrially cost-feasible manner. On the other hand, in recent years, a new type of TiAl alloys which contains  phase has garnered attention. A representative example is Ti–28.7Al–7.0Mn (wt%) (or Ti–42Al–5Mn (at%) [7–9], expressed below in wt% terms only), which has already been adopted for specialized applications that are not publicly disclosed. Since this alloy is characterized by the presence of 
∗ Corresponding author. Tel.: +81 029 859 2413; fax: +81 029 859 2501. E-mail address: [email protected] (T. Tetsui). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.05.050
phase having excellent deformability at forging temperature, it can be forged without cracking even when subject to conventional hot-forging with feature of extremely high-speed deformation and rapid cooling. A number of other alloys based on the same principle have also been proposed [10–14]. Because high-temperature strength of this alloy is comparatively low as compensation for excellent high-temperature deformability, lower temperature applications are envisioned than conventional TiAl alloys. Specifically, these would include components such as large blades and flight bodies, currently made from Ni-based superalloys. By making such components lighter, their product performance characteristics could be enhanced. Considering the processes involved in manufacturing these kinds of parts from  containing TiAl alloys, a large ingot would first be produced, which would then be roughly shaped by hot-forging, subjected to heat treatment as required, and finally machined precisely. Accordingly, good manufacturing capability is needed in terms of both hot-forging and machining. The main issues associated with current  containing TiAl alloys are low creep strength and the extremely high cost of large-sized ingots. Although a reduction in creep strength is to be expected with the presence of  phase, current materials are limited in their application due to the substantial differential in creep strength as compared with Ni-based superalloys even in specific strength. One means of addressing this issue would be to eliminate the  phase by minutely adjusting the heat treatment conditions [15,16] such as cooling speed, but it would be extremely difficult to completely eliminate the  phase in the large-sized blocks that would be
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required in the industrial context under discussion. Furthermore, even if the  phase could be completely eliminated, it would be difficult to prevent its subsequent precipitation during long-term use period, given that  phase is stable in the mid-to-low temperature range for the applications being considered. Consequently, it is necessary to achieve elevated creep strength under the assumption that  phase will be present. Next, with respect to ingot production, melting would normally be undertaken using a water-cooled copper crucible, from the standpoint of preventing contamination. Given that the required ingot size (several hundred kg) is difficult to achieve by means of induction skull melting [17] or levitation melting [4], vacuum ark re-melting (hereafter VAR) [18] is the preferred alternative. However, not only are the process costs associated with VAR (i.e., triple melt, etc.) quite high, it is necessary to employ high-precision technology in order to obtain an evenly distributed Mn content in case of the Mn-added alloys, and at present very few manufacturers are capable of such production. The accompanying costs are also prohibitive. In order to resolve the issues currently affecting the  containing TiAl alloys, what is needed is the capability for large-sized ingots that will result in a certain level of creep strength even with the presence of  phase, and at an acceptably low cost. There is potential for this with the use of ingots melted in oxide crucible [19–22] by standard-type induction melting furnace. Since these kinds of ingots tend to have higher amounts of inclusions and greater impurity contents, creep strength can be expected to be higher. Also, because general-use equipment would be used for melting, larger ingot sizes and lower costs could be achieved easily. On the other hand, higher amounts of inclusions and greater impurity contents would tend to negatively affect hot-forging and machining capabilities in manufacturing process, in addition to raising concerns with respect to the reduction of toughness of final products. Accordingly, the objective of the research presented here is to achieve high strength and low cost in a hot-forged TiAl based alloy containing  phase, and the practical realization of oxide-crucible-melted ingots was examined keeping in mind the above-noted issues. Specifically, two types of consideration were undertaken. The first involved using various types of oxide crucibles to melt small-sized ingots, observing the effects of crucible-induced contamination on microstructures, phase compositions, and hotforgeability. The second was in relation to the practical realization of oxide-crucible-melted ingots from an industrial perspective. Thus, a large-sized ingot was test-melted, and was compared with a conventionally produced (VAR triple melt) ingot in terms of manufacturing capability and various mechanical properties.
2. Experimental procedure 2.1. Effects of crucible-induced contamination on the phases, microstructural changes, and hot-forgeability of ˇ containing TiAl alloys Commercially available yttria, calcia, and zirconia crucibles were employed, as well as test-manufactured four types of yttria + alumina mixture crucibles with respective blend ratios of 80:20, 67:33, 60:40, and 38:62 wt%. Crucibles size was approx. 60 mm × h130 mm, and all of the crucibles were porous. Alloys of composition that centered on Ti–28.7Al–7.0Mn (wt%) were melted in these crucibles by means of the induction melting. The Al content was varied in the range of 24–32 wt%, while the Mn content was fixed at 7.0 wt%. The raw materials used for melting were sponge titanium with a purity of 3 N, aluminum pellets with a purity of 4 N, and Mn flakes with a purity of 2 N, with total weight of approx. 700 g per charge. The melting atmosphere was Ar 5.3 × 104 Pa, and
holding time after melting of the entire raw material was varied in the range of 1–6 min so as to change the extent of crucible-induced contamination. Subsequently, the melt was poured into a cast iron mould measuring 40 mm in diameter, thus forming small columnshaped ingots. For the purpose of comparison, similarly sized ingots were also produced using levitation melting in a water-cooled copper crucible. The numbers of ingots with various compositions and melting times were 11 for the yttria, yttria + alumina, and calcia crucibles; 4 for the zirconia crucible; and 2 for the water-cooled copper crucible; making a total of 39. These ingots were evaluated by means of chemical analysis of composition, hot-forging testing after heating to 1225 ◦ C, and optical microstructural observation of the forged material. The hot-forging test was conducted using 40 mm × 60 mm samples obtained from the bottom of the ingots, and subjecting these to upset forging of 4 times. The strain rate was high at 0.45–1.0 s−1 , and the amount of compression strain in each upset forging was approx. 30%. After one forging operation, the sample was returned to the furnace for reheating. This was repeated 4 times, yielding a height of approx. 18 mm. Evaluation of difference in hot-forging capability of respective ingots was performed by comparing the numbers of cracks that developed. In order to estimate microstructure and phase state of the ingots at the forging temperature, small ingot pieces that had been waterquenched from 1225 ◦ C were used and area ratios of inclusions and the respective compositional phases were measured by image processing. This measurement was undertaken for optical micrographs consisting of around 10 views, taking the average thereof. 2.2. Consideration of the practical realization of oxide-crucible-melted large-sized ingot Using an oxide crucible selected based on the work described above, an ingot was produced by means of induction melting, measuring approx. 260 mm × 600 mm, weighing approx. 130 kg, and having a basic composition of Ti–28.7Al–7.0Mn (wt%). For comparison, an ingot produced using triple melt VAR was employed, measuring approx. 270 mm × 550 mm and weighing approx. 130 kg, and having same composition. These ingots were subjected to hot-forging testing under industrially required conditions at 1250 ◦ C. After upset forging to approx. 1/2 the height along the vertical direction of the ingot (reheating after each of several forging operations), the direction of forging on the ingot was changed by 90◦ and extend forging was undertaken. Here, the forging time was approx. 2 min after removing from the furnace, with continuous forging being performed while the material was rotated. With repeated this extend forging operation, the initial cylindrical shape was changed to a square bar, and then the cross-section was gradually thinned to a final size of approx. 70 mm × 70 mm. The presence of any cracking was evaluated with respect to this industrial hotforging test. Next, the two types of forged material were subjected to heat treatment testing in the range of 1200–1300 ◦ C, and appropriate heat treatment conditions were selected. After heat treatment under appropriate conditions, the two types of forged material were further evaluated in terms of machinability, Charpy impact properties, and creep rupture properties. With respect to machinability, tool wear characteristics were evaluated, as production costs in machining process are heavily affected thereby. The cemented carbide cutting tool employed was of K10 material, equilateral triangle shape, and with a relief angle of 0◦ . A lathe was used for this test. The cutting speed was varied among 16, 24, 32, and 40 m/min with a constant feed rate of 0.2 mm per revolution and a constant cutting depth of 0.125 mm, and the relationship between the Frank wear of the tool and the cutting distance was evaluated. In the Charpy impact test, since absorbed energy of TiAl alloys is considerably small [14,23], it was difficult to assess
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Table 1 Representative chemical compositions among 39 small ingots produced by melting in the water-cooled copper crucible and the various types of oxide crucibles. Crucible
Ingot no.
Melting time (min)
Chemical composition (wt%) Al
Mn
O
28.94 32.00
6.76 6.84
0.069 0.057
Note Y
Water-cooled copper crucible
CC-1 CC-2
Y2 O3
YT-1 YT-2 YT-3 YT-4 YT-5 YT-6
1 2 3 4 5 1
28.68 28.76 29.13 28.73 28.82 31.79
6.87 6.92 6.78 6.93 6.86 7.15
0.15 0.10 0.14 0.18 0.20 0.10
0.15 0.19 0.25 0.32 0.35 0.16
80%Y2 O3 + 20%Al2 O3
4Y1A-1
2
28.22
6.99
0.27
0.10
67%Y2 O3 + 33%Al2 O3
2Y1A-1 2Y1A-2
2 2
28.92 27.48
6.91 7.08
0.29 0.22
0.11 0.21
60%Y2 O3 + 40%Al2 O3
3Y2A-1
2
29.21
7.12
0.49
0.05
38%Y2 O3 + 62%Al2 O3
3Y5A-1
2
29.47
6.42
1.20
0.01
CaO
CA-1 CA-2 CA-3 CA-4 CA-5
1 2 4 6 3
28.79 29.16 28.79 28.81 25.94
6.81 7.06 6.73 6.88 6.84
0.11 0.15 0.26 0.27 0.35
ZR-1 ZR-2
2 2
27.77 24.01
6.67 6.52
0.85 1.22
ZrO2
the material difference by using normal notched test pieces, then un-notched test pieces with the cross-section of 10 mm × 10 mm were used instead, and 4 sets of tests were performed at room temperature. Creep rupture testing was undertaken under 3 sets of conditions, consisting of 600 ◦ C × 196 MPa, 700 ◦ C × 147 MPa, and 800 ◦ C × 98 MPa. 3. Results and discussion 3.1. Effects of crucible-induced contamination on the phases, microstructural changes, and hot-forgeability of ˇ containing TiAl alloys Table 1 presents representative chemical compositions among 39 small ingots produced by melting in the water-cooled copper crucible and the various types of oxide crucibles. Ingot number and respective melting time are also shown in Table 1. Ingots targeting a basic composition of Ti–28.7Al–7.0Mn (wt%) are the most prevalent, and notes in Table 1 are shown to indicate where the Al content was varied. Those melted in the oxide crucibles exhibit higher contents of oxygen and ceramic constituents than is the case for the water-cooled copper crucible, and these contents are seen to rise with longer melting times. For similar melting times, oxygen content is lowest for the water-cooled copper crucible, and rises in order of yttria, calcia, yttria + alumina, and zirconia. Hot-forging testing was performed on all 39 ingots. Fig. 1 presents the external appearance of the representative ingots after hot-forging. The chemical compositions for these ingots are indicated in Table 1. Those exhibiting no cracking at all consist of ingot having the basic composition and melted in the water-cooled copper crucible (CC-1), ingots having the basic composition with melting times of 2 min and 5 min respectively in the yttria crucible (YT-2, YT-5), and ingot having a reduced-Al content with a melting time of 3 min in the calcia crucible (CA-5). Cracking occurred in all other ingots in Fig. 1. Thus, hot-forging capability is seen to have been reduced in the cases of ingot melted in the water-cooled copper crucible but having an elevated-Al content (CC-2), and ingots having the basic composition but with high contents of impurities (4Y1A-1, 2Y1A-1 and CA-2). Also, with the exception of ingot ZR-2
Ca
Zr High Al
High Al
Low Al
0.0074 0.0056 0.0074 0.0110 0.0120
Low Al 1.69 2.26
Low Al
having an extremely high level of impurities, reduced-Al content maintains forgeability even with a certain amount of impurities (as in the case of ingot CA-5). Fig. 2 shows the optical microstructures for the as-forged ingots that appeared in Fig. 1. While ingot CC-2 having elevated-Al content and melted in the water-cooled copper crucible exhibits blockshaped ␥ grains, the other ingots all indicate lamellar grains with an intergranular structure composed of ␥ + . However, the compositional ratios of the respective structures vary widely, and it can be seen that there are substantial differences depending on Al and impurity contents. Also, the ingots melted in the yttria and yttria + alumina crucibles exhibit large amounts of inclusions. Results for the test involving water-quenching of small ingot piece from 1225 ◦ C, conducted in order to estimate the microstructure and phase state at the time of hot-forging, are presented below. As an example of microstructure, Fig. 3 shows ingot YT-2. The structure is composed of black inclusions (thought to be Y2 O3 that mixed form the crucible [20]), island-like  phase, and the remaining ␣ phase. Area ratios of these respective phases were obtained by means of image processing for all 39 ingots. Fig. 4 shows the relationship between the area ratio of ␣ phase and oxygen and ceramic constituent content for ingots having nearly the basic composition with Al content in the range of 28.21–29.22 wt%. Given that the Al content is nearly constant, the difference in the ␣ phase area ratio is considered to be induced by impurities. While there is no correlation between ceramic constituent content and ␣ phase area ratio, a definite correlation can be seen between oxygen content and ␣ phase area ratio. For the ingots melted in each of the oxide crucibles, the ␣ phase area ratio increases with higher oxygen content (conversely, the  phase area ratio decreases), and it can be seen that oxygen serves to stabilize the ␣ phase. It was reported that the ratio of the ␣2 phase in the lamellar structure after cooling increases with the oxygen content [24,25], and the result of the present study is similar to it. Comparing among the various crucibles at a similar level of oxygen content, the ␣ phase area ratio for the calcia crucible is greater than for the yttria and yttria + alumina crucibles. The reason for this is considered to be that the latter ingots include a certain amount of oxygen in their respective precipitates, such that the amount of oxygen subject to solid solution in the matrix is
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Fig. 1. External appearance of the representative ingots after hot-forging at 1225 ◦ C. Ingots numbers are shown in Table 1.
less than for the former ingots, and that the influence on ␣ phase stabilization is consequently less. Fig. 5 presents effects of oxygen content, inclusion area ratios and  phase area ratios on the hot-forging capability for all of the ingots. It can thus be seen that a lower  phase area ratio is associated with reduced hot-forgeability. Also, the inclusion area ratio has only a small effect on hot-forgeability. Considering this in more detail, all of the ingots exhibit very high numbers of cracks (5 or more) when the  phase area ratio is within 10%. At a level of 10–20%, the numbers of cracks are lower with lower oxygen content, such that the influence of oxygen content is recognized to a certain degree. On the other hand, no cracking is observed when the  phase area ratio is over 20%, regardless of oxygen content. In an extreme example, an ingot with an extremely high oxygen content of 0.35 wt% (CA-5) exhibits no cracking. Accordingly, in the case of  containing TiAl alloys, the factor that most influences hotforging capability is seen to be the amount of  phase at the forging
temperature, while the effects of oxygen content and inclusion area ratio are seen to be comparatively low. From the foregoing results, it was found that the oxygen content rises in oxide-crucible-melted ingots, and since the amount of ␣ phase at the forging temperature increases ( phase decreases) with oxygen content, hot-forging capability is reduced. However, if the Al content is reduced and the  phase amount is recovered, hot-forging capability is not reduced even when the oxygen content becomes extremely high. 3.2. Consideration of the practical realization of oxide-crucible-melted large-sized ingot As a result of the foregoing evaluation, it was found that hotforgeability can be maintained by reducing the Al content of the alloys under discussion, even if there is substantial contamination from the oxide crucible used for melting. In an industrial context,
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Fig. 2. Optical microstructures for the as-forged ingots that appeared in Fig. 1.
however, changes to the basic alloy composition for which specs have already been determined are likely to have negative impacts. Consequently, at the first stage of this research, the practical realization of oxide-crucible melted ingots were evaluated with respect to the alloy having basic composition, from the standpoints of resulting material hot-forgeability, machinability, and toughness. If the results of such consideration were to suggest that this avenue could not be pursued, it would be necessary to consider reduction in Al content. As can be seen from Fig. 1, etc., in case of the basic composition, forging capability was found to be greatest for the ingots melted
Fig. 3. Optical microstructures for ingot YT-2 after water-quenching from 1225 ◦ C.
in the yttria crucible. Accordingly, it was decided to use an yttria crucible for production of a large-sized ingot. Table 2 presents the chemical compositions for this ingot, together with those corresponding to a triple melted VAR ingot produced for the purpose of comparison. The oxygen content of the latter is 0.055 wt%, while that of the yttria-crucible-melted ingot is 0.16%. As a result of industrial hot-forging testing, no cracking was observed in either ingot. Fig. 6 shows the external appearances of the two hot-forged materials. Neither exhibits any particular defects, and the desired square bar shape was achieved. Thus, even in the case of the basic composition, the yttria-crucible-melted ingot having a relatively higher oxygen content of 0.16 wt% is found to possess a repeated hot-forging capability as required industrially. Next, heat treatment conditions were pursued that would provide similar microstructures for the two forged materials. However, the respective oxygen contents are quite different, and as indicated in Fig. 4, since the compositional phase ratios differ substantially with oxygen content, it proved impossible to identify heat treatment conditions by which similar microstructures could be actually obtained. Accordingly, respective conditions were selected so as to produce microstructures as close as possible, consisting of 1280 ◦ C for the yttria-crucible-melted material, and 1240 ◦ C for the triple melted VAR material. Holding time was 10 h in both cases, with cooling speed of 1000 ◦ C/h. Fig. 7 presents the optical microstructures for the two forged materials after heat treatment, while Table 3 indicates inclusions (thought to be Y2 O3 that mixed form the crucible [20]), and respective phase area ratios. Although the lamellar structure ratio is greater for the yttria-crucible-melted material, the  phase area ratios are similar. Also, a large amount of inclusions of area ratio of 0.26% exists only in the yttria-cruciblemelted material.
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Table 2 Chemical compositions for the large ingots of Ti–28.7Al–7.0Mn (wt%) alloy made by induction melting in the yttria crucible and triple melt VAR. Melting process
Chemical composition (wt%)
Induction melting in the yttria crucible Triple melt VAR
C
Mn
Al
O
N
Y
0.009 0.010
6.98 6.93
28.6 28.4
0.160 0.055
0.006 0.003
0.19 –
100
100
(b)
95
Area ratio of α phase (%)
Area ratio of α phase (%)
(a)
90 85 80 WC-Copper Y2O3 Y2O3+Al2O3
75
WC-Copper Y2O3 Y2O3+Al2O3 CaO
95 90 85 80 75
CaO
70
0
0.1
0.2
0.3
0.4
70
0.5
0.1
0
Oxygen content (wt%)
0.2
0.3
0.4
0.5
Ceramic constituent content (wt%)
Fig. 4. Relationship between the area ratio of ␣ phase in water-quenched ingot piece from 1225 ◦ C and oxygen (a) and ceramic constituent (b) content for ingots having nearly the basic composition with Al content in the range of 28.21–29.22 wt%.
No. of cracks
Crucible 0
1~5
>5
WC copper
60
(b)
Y2O3+Al2O3 CaO
50
Area ratio of β phase (%)
Area ratio of β phase (%)
60
Y2O3
(a)
ZrO2
40 30 20 10 0
0
0.1
0.2
0.3
0.4
0.5
0.6
Oxygen content (wt%)
50 40 30 20 10 0
0
0.1
0.2
0.3
0.4
0.5
0.6
Area ratio of inclusions (%)
Fig. 5. Effects of oxygen content and  phase area ratios on the hot-forging capability (a) and effects of inclusion area ratios and  phase area ratios on the hot-forging capability (b) for all of the ingots.
Fig. 8 shows the machining test results for the two forged materials after heat treatment, indicating the relationship between cutting distance and Frank wear of the tool. In the case of cutting speeds of 16 and 24 m/min, Frank wear is small in both cases, and no appreciable difference can be seen. At cutting speeds of 32 and 40 m/min, Frank wear becomes greater, but the amount of increase is lower for the yttria-crucible-melted material. Comparing the Table 3 Area ratios of inclusions and respective phases for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. Melting process
Induction melting in the yttria crucible Triple melt VAR
Area ratio (%) Lamellar

Inclusions
65.0 35.6
12.1 12.8
0.26 0
microstructural factors associated with the two materials, while the ratio of  phase (which serves to improve machinability) is about the same, the ratio of lamellar structure (which conversely serves to reduce machinability) is actually higher in the yttria-cruciblemelted material. The reason for the superior machining capability of the yttria-crucible-melted material is not clear, but one possibility is the presence of inclusions. As with sulfur free cutting steel [26], the presence of inclusions may reduce cutting resistance and improve separability of chips. Table 4 shows the results of Charpy impact testing at room temperature for the two forged materials after heat treatment. Absorbed energy is slightly greater for the yttria-crucible-melted material. The reason for this may be considered to be as follows. Generally, Charpy absorbed energy is expressed as the sum of the energy required for crack initiation and the energy required for crack propagation, with the former having a close relationship with
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Table 4 Charpy absorbed energy at room temperature for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment using un-notched test pieces with the cross-section of 10 mm × 10 mm. Melting process
Induction melting in the yttria crucible Triple melt VAR
Absorbed energy (J) Test no. 1
Test no. 2
Test no. 3
Test no. 4
Average
4.2 4.2
5.1 5.1
5.1 4.2
5.9 4.2
5.1 4.4
800 Y2O3-16m/min 3VAR-16m/min Y2O3-24m/min 3VAR-24m/min Y2O3-32m/min 3VAR-32m/min Y2O3-40m/min 3VAR-40m/min
700
Frank wear (µm)
600 500 400 300 200 100 Fig. 6. External appearances of the hot-forged Ti–28.7Al–7.0Mn (wt%) alloy under industrially required forging conditions. Ingots were made by induction melting in the yttria crucible (a) and triple melt VAR (b).
material strength [27]. In the case of TiAl alloys, the ratio of the former in overall absorbed energy is quite high. That is, given that the strength of the yttria-crucible-melted material is higher due to increased oxygen content and lamellar structure area ratio, the absorbed energy is also considered to be greater. However, because the absorbed energy at the room temperature of the austempered ductile iron that uses the un-notched specimens of the same shape is about 100 J [28], it is certain that the absorbed energy of these two kinds of forged TiAl alloys is low. Fig. 9 presents results of creep rupture testing of the two forged materials after heat treatment in Larson-Miller parameter comparison. The creep strength of the yttria-crucible-melted material can be seen to be substantially higher. The reason for this is considered to be as noted above, with increased oxygen content contributing to higher creep strength. Summarizing the foregoing results, for an ingot having the basic composition of the alloy under consideration, melted in an yttria crucible and with the oxygen content thus elevated to 0.16 wt%, there were no particular issues in terms of hot-forgeability under
0 0
200
400
600
800
1000
Cutting distance (m) Fig. 8. Relationship between cutting distance and Frank wear of the tool for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. The cutting speed was varied among 16, 24, 32, and 40 m/min with a constant feed rate of 0.2 mm per revolution and a constant cutting depth of 0.125 mm.
industrially required conditions. Also, with respect to machinability and Charpy impact properties to which it is feared to decrease at first, these proved slightly superior to a triple melted VAR ingot used for comparison and having an oxygen content of 0.055 wt%. Furthermore, creep strength was considerably higher than the material used for comparison, as had been predicted. Thus, the results of the research presented here clearly suggest that achieving greater strength and lower cost for hot-forged TiAl based alloy containing  phase is possible by use of large ingot melted in oxide crucibles.
Fig. 7. Optical microstructures for the hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. Ingots were made by induction melting in the yttria crucible (a) and triple melt VAR (b).
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(4) The machinability and Charpy impact properties of above mentioned forged materials after heat treatment are actually superior to triple melted VAR material having an oxygen content of 0.055 wt%, and creep strength is substantially greater.
300
Yttria crucible Triple melt VAR 200
Stress (MPa)
References [1] [2] [3] [4] [5] [6]
100
[7]
90 [8]
80 20
21
22
23
24
25
LMP (C=20)
[9] [10]
Fig. 9. Creep rupture strength in Larson–Miller parameter comparison for two hotforged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment.
[11]
4. Conclusion
[12] [13] [14]
With the objective of achieving greater strength and lower cost for hot-forged TiAl based alloy containing  phase, the effects of contamination from oxide-crucible-melting and the possibility of practical industrial realization of large-sized ingots were considered with respect to alloy compositions centered on Ti–28.7Al–7.0Mn (wt%). Results were obtained as follows:
[15] [16]
[17] [18]
[19]
(1) Oxide-crucibles-melting results in greater oxygen content in the ingot. In this conjunction, the ␣ phase ratio at the forging temperature increases, and the  phase ratio decreases. (2) The  phase ratio has the greatest influence on hot-forging capability, and, even at an extremely high oxygen content of 0.35 wt%, recovery of the  phase amount by reducing the Al content serves to maintain hot-forgeability. (3) A large-sized ingot of the basic composition, melted in an yttria crucible, and having an oxygen content of 0.16 wt%, can be hotforged under industrially required conditions.
[20] [21] [22] [23] [24] [25] [26] [27] [28]
T. Tetsui, Adv. Eng. Mater. 3 (2001) 307–310. T. Tetsui, Mater. Sci. Eng. A 329–331 (2002) 582–588. T. Tetsui, Mater. Sci. Forum 361–363 (2007) 379–382. T. Noda, Intermetallics 6 (1998) 709–713. T. Fujimura, H. Nishikawa, N. Moriya, M. Imamura, IHI Gihou 48 (2008–2009) 153–158 (in Japanese). M. Weimer, T. Kelly, in: Y.-W. Kim, D. Morris, R. Yang, C. Leyens (Eds.), Structural Aluminides for Elevated Temperatures. Gamma Titanium and Other Metallic Aluminides, TMS, Warrendale, PA, 2008, pp. 5–6. T. Tetsui, K. Shindo, S. Kaji, S. Kobayashi, M. Takeyama, Intermetallics 13 (2005) 971–978. T. Tetsui, K. Shindo, S. Kaji, S. Kobayashi, M. Takeyama, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 199–205. T. Tetsui, K. Shindo, S. Kobayashi, M. Takeyama, Scripta Mater. 47 (2002) 399–403. H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, Adv. Eng. Mater. 10 (2008) 707–713. W. Wallgram, T. Schmolzer, L. Cha, G. Das, V. Güther, H. Clemens, Int. J. Mater. Res. 8 (2009) 1021–1030. Y.Y. Chen, F. Yang, F.T. Konga, S.L. Xiao, J. Alloys Compd. 498 (2010) 95–101. H.Z. Niu, F.T. Kong, Y.Y. Chen, F. Yang, J. Alloys Compd. 509 (2011) 10179–10184. T. Tetsui, K. Shindo, S. Kobayashi, M. Takeyama, Intermetallics 11 (2003) 299–306. M. Takeyama, S. Kobayashi, Intermetallics 13 (2005) 993–999. S. Kobayashi, M. Takeyama, T. Motegi, N. Hirota, T. Matsuo, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 165–175. J. Aguilar, A. Schievenbusch, O. Kattlitz, Intermetallics 19 (2011) 757–761. V. Güther, A. Chatterjee, H. Kettner, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 241–247. T. Tetsui, T. Kobayashi, T. Mori, T. Kishimoto, H. Harada, Mater. Trans. 51 (2010) 1656–1662. T. Tetsui, T. Kobayashi, A. Kishimoto, H. Harada, Intermetallics 20 (2012) 16–23. J. Barbosa, C.S. Ribeiro, C. Monteiro, Mater. Sci. Forum 426–432 (2003) 1933–1938. F. Gomes, J. Barbosa, C.S. Ribeiro, Intermetallics 16 (2008) 1292–1297. K. Matsugi, T. Hatayama, O. Yanagisawa, Intermetallics 7 (1999) 1049–1057. S. Takagi, C. Ouchi, Mater. Trans. JIM 38 (1997) 285–291. Y. Ma, Z. Jiang, B. Chen, X. Zhao, M. Gao, K. Liu, Y. Li, J. Mater. Sci. Technol. 26 (2010) 131–135. Y.D. Yashin, S.A. Soldatkin, P.Y. Chesnokov, Metallurgist 49 (2005) 236–241. T. Kobayashi, Int. J. Fract. 23 (1983) R105–R109. O. Eric, D. Rajnovic, S. Zec, L. Sidjanin, M.T. Jovanovic, Mater. Charact. 57 (2006) 211–217.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Achieving high strength and low cost for hot-forged TiAl based alloy containing  phase Toshimitsu Tetsui ∗ , Toshiharu Kobayashi, Hiroshi Harada High Temperature Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 6 March 2012 Received in revised form 9 May 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Intermetallics TiAl alloys Forging Ceramic crucible melting Mechanical properties
a b s t r a c t With the objective of achieving high strength and low cost for hot-forged TiAl based alloy containing  phase, the applicability of oxide-crucible-melted ingot was considered for compositions centered on Ti–28.7Al–7.0Mn (wt%). First, as a result of evaluating the influence of crucible contaminants on the hotforgeability of ingots, it was confirmed that, even at the extremely high oxygen content of 0.35 wt%, hotforgeability could be maintained if the amount of  phase was recovered at the forging temperature by reducing the Al content. Next, as a result of verifying the industrial practicality of large ingot, it was found that hot-forging under industrially required conditions was indeed possible for an yttria-crucible-melted ingot corresponding to the above-mentioned center composition with an oxygen content of 0.16 wt%; and in terms of machinability and Charpy impact properties of hot-forged and heat treated material, this ingot was superior to triple-melted VAR ingot having an oxygen content of 0.055 wt%, as well as having substantially better creep rupture strength. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alloys with a primary phase consisting of TiAl intermetallic compounds have seen practical application in turbocharger turbine wheels for passenger vehicles [1–4] and low-pressure turbine blades for jet engines [5,6]. In both cases, the components are small, and precision casting is employed in their manufacture. In order to take further advantage of the superior properties of TiAl alloys, i.e., low weight and excellent high-temperature strength, thereby contributing to fuel savings and the reduction of CO2 emissions, it is necessary to apply to larger sized components. This in turn requires a forging process similar to ordinary metallic materials, given the difficulties involved in using precision casting method in the manufacture of large components. However, the forgeability of conventional TiAl alloys is quite low, and since only special processes such as isothermal forging have been applicable, it has thus far been difficult to produce larger sized parts in an industrially cost-feasible manner. On the other hand, in recent years, a new type of TiAl alloys which contains  phase has garnered attention. A representative example is Ti–28.7Al–7.0Mn (wt%) (or Ti–42Al–5Mn (at%) [7–9], expressed below in wt% terms only), which has already been adopted for specialized applications that are not publicly disclosed. Since this alloy is characterized by the presence of 
∗ Corresponding author. Tel.: +81 029 859 2413; fax: +81 029 859 2501. E-mail address: [email protected] (T. Tetsui). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.05.050
phase having excellent deformability at forging temperature, it can be forged without cracking even when subject to conventional hot-forging with feature of extremely high-speed deformation and rapid cooling. A number of other alloys based on the same principle have also been proposed [10–14]. Because high-temperature strength of this alloy is comparatively low as compensation for excellent high-temperature deformability, lower temperature applications are envisioned than conventional TiAl alloys. Specifically, these would include components such as large blades and flight bodies, currently made from Ni-based superalloys. By making such components lighter, their product performance characteristics could be enhanced. Considering the processes involved in manufacturing these kinds of parts from  containing TiAl alloys, a large ingot would first be produced, which would then be roughly shaped by hot-forging, subjected to heat treatment as required, and finally machined precisely. Accordingly, good manufacturing capability is needed in terms of both hot-forging and machining. The main issues associated with current  containing TiAl alloys are low creep strength and the extremely high cost of large-sized ingots. Although a reduction in creep strength is to be expected with the presence of  phase, current materials are limited in their application due to the substantial differential in creep strength as compared with Ni-based superalloys even in specific strength. One means of addressing this issue would be to eliminate the  phase by minutely adjusting the heat treatment conditions [15,16] such as cooling speed, but it would be extremely difficult to completely eliminate the  phase in the large-sized blocks that would be
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required in the industrial context under discussion. Furthermore, even if the  phase could be completely eliminated, it would be difficult to prevent its subsequent precipitation during long-term use period, given that  phase is stable in the mid-to-low temperature range for the applications being considered. Consequently, it is necessary to achieve elevated creep strength under the assumption that  phase will be present. Next, with respect to ingot production, melting would normally be undertaken using a water-cooled copper crucible, from the standpoint of preventing contamination. Given that the required ingot size (several hundred kg) is difficult to achieve by means of induction skull melting [17] or levitation melting [4], vacuum ark re-melting (hereafter VAR) [18] is the preferred alternative. However, not only are the process costs associated with VAR (i.e., triple melt, etc.) quite high, it is necessary to employ high-precision technology in order to obtain an evenly distributed Mn content in case of the Mn-added alloys, and at present very few manufacturers are capable of such production. The accompanying costs are also prohibitive. In order to resolve the issues currently affecting the  containing TiAl alloys, what is needed is the capability for large-sized ingots that will result in a certain level of creep strength even with the presence of  phase, and at an acceptably low cost. There is potential for this with the use of ingots melted in oxide crucible [19–22] by standard-type induction melting furnace. Since these kinds of ingots tend to have higher amounts of inclusions and greater impurity contents, creep strength can be expected to be higher. Also, because general-use equipment would be used for melting, larger ingot sizes and lower costs could be achieved easily. On the other hand, higher amounts of inclusions and greater impurity contents would tend to negatively affect hot-forging and machining capabilities in manufacturing process, in addition to raising concerns with respect to the reduction of toughness of final products. Accordingly, the objective of the research presented here is to achieve high strength and low cost in a hot-forged TiAl based alloy containing  phase, and the practical realization of oxide-crucible-melted ingots was examined keeping in mind the above-noted issues. Specifically, two types of consideration were undertaken. The first involved using various types of oxide crucibles to melt small-sized ingots, observing the effects of crucible-induced contamination on microstructures, phase compositions, and hotforgeability. The second was in relation to the practical realization of oxide-crucible-melted ingots from an industrial perspective. Thus, a large-sized ingot was test-melted, and was compared with a conventionally produced (VAR triple melt) ingot in terms of manufacturing capability and various mechanical properties.
2. Experimental procedure 2.1. Effects of crucible-induced contamination on the phases, microstructural changes, and hot-forgeability of ˇ containing TiAl alloys Commercially available yttria, calcia, and zirconia crucibles were employed, as well as test-manufactured four types of yttria + alumina mixture crucibles with respective blend ratios of 80:20, 67:33, 60:40, and 38:62 wt%. Crucibles size was approx. 60 mm × h130 mm, and all of the crucibles were porous. Alloys of composition that centered on Ti–28.7Al–7.0Mn (wt%) were melted in these crucibles by means of the induction melting. The Al content was varied in the range of 24–32 wt%, while the Mn content was fixed at 7.0 wt%. The raw materials used for melting were sponge titanium with a purity of 3 N, aluminum pellets with a purity of 4 N, and Mn flakes with a purity of 2 N, with total weight of approx. 700 g per charge. The melting atmosphere was Ar 5.3 × 104 Pa, and
holding time after melting of the entire raw material was varied in the range of 1–6 min so as to change the extent of crucible-induced contamination. Subsequently, the melt was poured into a cast iron mould measuring 40 mm in diameter, thus forming small columnshaped ingots. For the purpose of comparison, similarly sized ingots were also produced using levitation melting in a water-cooled copper crucible. The numbers of ingots with various compositions and melting times were 11 for the yttria, yttria + alumina, and calcia crucibles; 4 for the zirconia crucible; and 2 for the water-cooled copper crucible; making a total of 39. These ingots were evaluated by means of chemical analysis of composition, hot-forging testing after heating to 1225 ◦ C, and optical microstructural observation of the forged material. The hot-forging test was conducted using 40 mm × 60 mm samples obtained from the bottom of the ingots, and subjecting these to upset forging of 4 times. The strain rate was high at 0.45–1.0 s−1 , and the amount of compression strain in each upset forging was approx. 30%. After one forging operation, the sample was returned to the furnace for reheating. This was repeated 4 times, yielding a height of approx. 18 mm. Evaluation of difference in hot-forging capability of respective ingots was performed by comparing the numbers of cracks that developed. In order to estimate microstructure and phase state of the ingots at the forging temperature, small ingot pieces that had been waterquenched from 1225 ◦ C were used and area ratios of inclusions and the respective compositional phases were measured by image processing. This measurement was undertaken for optical micrographs consisting of around 10 views, taking the average thereof. 2.2. Consideration of the practical realization of oxide-crucible-melted large-sized ingot Using an oxide crucible selected based on the work described above, an ingot was produced by means of induction melting, measuring approx. 260 mm × 600 mm, weighing approx. 130 kg, and having a basic composition of Ti–28.7Al–7.0Mn (wt%). For comparison, an ingot produced using triple melt VAR was employed, measuring approx. 270 mm × 550 mm and weighing approx. 130 kg, and having same composition. These ingots were subjected to hot-forging testing under industrially required conditions at 1250 ◦ C. After upset forging to approx. 1/2 the height along the vertical direction of the ingot (reheating after each of several forging operations), the direction of forging on the ingot was changed by 90◦ and extend forging was undertaken. Here, the forging time was approx. 2 min after removing from the furnace, with continuous forging being performed while the material was rotated. With repeated this extend forging operation, the initial cylindrical shape was changed to a square bar, and then the cross-section was gradually thinned to a final size of approx. 70 mm × 70 mm. The presence of any cracking was evaluated with respect to this industrial hotforging test. Next, the two types of forged material were subjected to heat treatment testing in the range of 1200–1300 ◦ C, and appropriate heat treatment conditions were selected. After heat treatment under appropriate conditions, the two types of forged material were further evaluated in terms of machinability, Charpy impact properties, and creep rupture properties. With respect to machinability, tool wear characteristics were evaluated, as production costs in machining process are heavily affected thereby. The cemented carbide cutting tool employed was of K10 material, equilateral triangle shape, and with a relief angle of 0◦ . A lathe was used for this test. The cutting speed was varied among 16, 24, 32, and 40 m/min with a constant feed rate of 0.2 mm per revolution and a constant cutting depth of 0.125 mm, and the relationship between the Frank wear of the tool and the cutting distance was evaluated. In the Charpy impact test, since absorbed energy of TiAl alloys is considerably small [14,23], it was difficult to assess
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Table 1 Representative chemical compositions among 39 small ingots produced by melting in the water-cooled copper crucible and the various types of oxide crucibles. Crucible
Ingot no.
Melting time (min)
Chemical composition (wt%) Al
Mn
O
28.94 32.00
6.76 6.84
0.069 0.057
Note Y
Water-cooled copper crucible
CC-1 CC-2
Y2 O3
YT-1 YT-2 YT-3 YT-4 YT-5 YT-6
1 2 3 4 5 1
28.68 28.76 29.13 28.73 28.82 31.79
6.87 6.92 6.78 6.93 6.86 7.15
0.15 0.10 0.14 0.18 0.20 0.10
0.15 0.19 0.25 0.32 0.35 0.16
80%Y2 O3 + 20%Al2 O3
4Y1A-1
2
28.22
6.99
0.27
0.10
67%Y2 O3 + 33%Al2 O3
2Y1A-1 2Y1A-2
2 2
28.92 27.48
6.91 7.08
0.29 0.22
0.11 0.21
60%Y2 O3 + 40%Al2 O3
3Y2A-1
2
29.21
7.12
0.49
0.05
38%Y2 O3 + 62%Al2 O3
3Y5A-1
2
29.47
6.42
1.20
0.01
CaO
CA-1 CA-2 CA-3 CA-4 CA-5
1 2 4 6 3
28.79 29.16 28.79 28.81 25.94
6.81 7.06 6.73 6.88 6.84
0.11 0.15 0.26 0.27 0.35
ZR-1 ZR-2
2 2
27.77 24.01
6.67 6.52
0.85 1.22
ZrO2
the material difference by using normal notched test pieces, then un-notched test pieces with the cross-section of 10 mm × 10 mm were used instead, and 4 sets of tests were performed at room temperature. Creep rupture testing was undertaken under 3 sets of conditions, consisting of 600 ◦ C × 196 MPa, 700 ◦ C × 147 MPa, and 800 ◦ C × 98 MPa. 3. Results and discussion 3.1. Effects of crucible-induced contamination on the phases, microstructural changes, and hot-forgeability of ˇ containing TiAl alloys Table 1 presents representative chemical compositions among 39 small ingots produced by melting in the water-cooled copper crucible and the various types of oxide crucibles. Ingot number and respective melting time are also shown in Table 1. Ingots targeting a basic composition of Ti–28.7Al–7.0Mn (wt%) are the most prevalent, and notes in Table 1 are shown to indicate where the Al content was varied. Those melted in the oxide crucibles exhibit higher contents of oxygen and ceramic constituents than is the case for the water-cooled copper crucible, and these contents are seen to rise with longer melting times. For similar melting times, oxygen content is lowest for the water-cooled copper crucible, and rises in order of yttria, calcia, yttria + alumina, and zirconia. Hot-forging testing was performed on all 39 ingots. Fig. 1 presents the external appearance of the representative ingots after hot-forging. The chemical compositions for these ingots are indicated in Table 1. Those exhibiting no cracking at all consist of ingot having the basic composition and melted in the water-cooled copper crucible (CC-1), ingots having the basic composition with melting times of 2 min and 5 min respectively in the yttria crucible (YT-2, YT-5), and ingot having a reduced-Al content with a melting time of 3 min in the calcia crucible (CA-5). Cracking occurred in all other ingots in Fig. 1. Thus, hot-forging capability is seen to have been reduced in the cases of ingot melted in the water-cooled copper crucible but having an elevated-Al content (CC-2), and ingots having the basic composition but with high contents of impurities (4Y1A-1, 2Y1A-1 and CA-2). Also, with the exception of ingot ZR-2
Ca
Zr High Al
High Al
Low Al
0.0074 0.0056 0.0074 0.0110 0.0120
Low Al 1.69 2.26
Low Al
having an extremely high level of impurities, reduced-Al content maintains forgeability even with a certain amount of impurities (as in the case of ingot CA-5). Fig. 2 shows the optical microstructures for the as-forged ingots that appeared in Fig. 1. While ingot CC-2 having elevated-Al content and melted in the water-cooled copper crucible exhibits blockshaped ␥ grains, the other ingots all indicate lamellar grains with an intergranular structure composed of ␥ + . However, the compositional ratios of the respective structures vary widely, and it can be seen that there are substantial differences depending on Al and impurity contents. Also, the ingots melted in the yttria and yttria + alumina crucibles exhibit large amounts of inclusions. Results for the test involving water-quenching of small ingot piece from 1225 ◦ C, conducted in order to estimate the microstructure and phase state at the time of hot-forging, are presented below. As an example of microstructure, Fig. 3 shows ingot YT-2. The structure is composed of black inclusions (thought to be Y2 O3 that mixed form the crucible [20]), island-like  phase, and the remaining ␣ phase. Area ratios of these respective phases were obtained by means of image processing for all 39 ingots. Fig. 4 shows the relationship between the area ratio of ␣ phase and oxygen and ceramic constituent content for ingots having nearly the basic composition with Al content in the range of 28.21–29.22 wt%. Given that the Al content is nearly constant, the difference in the ␣ phase area ratio is considered to be induced by impurities. While there is no correlation between ceramic constituent content and ␣ phase area ratio, a definite correlation can be seen between oxygen content and ␣ phase area ratio. For the ingots melted in each of the oxide crucibles, the ␣ phase area ratio increases with higher oxygen content (conversely, the  phase area ratio decreases), and it can be seen that oxygen serves to stabilize the ␣ phase. It was reported that the ratio of the ␣2 phase in the lamellar structure after cooling increases with the oxygen content [24,25], and the result of the present study is similar to it. Comparing among the various crucibles at a similar level of oxygen content, the ␣ phase area ratio for the calcia crucible is greater than for the yttria and yttria + alumina crucibles. The reason for this is considered to be that the latter ingots include a certain amount of oxygen in their respective precipitates, such that the amount of oxygen subject to solid solution in the matrix is
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Fig. 1. External appearance of the representative ingots after hot-forging at 1225 ◦ C. Ingots numbers are shown in Table 1.
less than for the former ingots, and that the influence on ␣ phase stabilization is consequently less. Fig. 5 presents effects of oxygen content, inclusion area ratios and  phase area ratios on the hot-forging capability for all of the ingots. It can thus be seen that a lower  phase area ratio is associated with reduced hot-forgeability. Also, the inclusion area ratio has only a small effect on hot-forgeability. Considering this in more detail, all of the ingots exhibit very high numbers of cracks (5 or more) when the  phase area ratio is within 10%. At a level of 10–20%, the numbers of cracks are lower with lower oxygen content, such that the influence of oxygen content is recognized to a certain degree. On the other hand, no cracking is observed when the  phase area ratio is over 20%, regardless of oxygen content. In an extreme example, an ingot with an extremely high oxygen content of 0.35 wt% (CA-5) exhibits no cracking. Accordingly, in the case of  containing TiAl alloys, the factor that most influences hotforging capability is seen to be the amount of  phase at the forging
temperature, while the effects of oxygen content and inclusion area ratio are seen to be comparatively low. From the foregoing results, it was found that the oxygen content rises in oxide-crucible-melted ingots, and since the amount of ␣ phase at the forging temperature increases ( phase decreases) with oxygen content, hot-forging capability is reduced. However, if the Al content is reduced and the  phase amount is recovered, hot-forging capability is not reduced even when the oxygen content becomes extremely high. 3.2. Consideration of the practical realization of oxide-crucible-melted large-sized ingot As a result of the foregoing evaluation, it was found that hotforgeability can be maintained by reducing the Al content of the alloys under discussion, even if there is substantial contamination from the oxide crucible used for melting. In an industrial context,
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349
Fig. 2. Optical microstructures for the as-forged ingots that appeared in Fig. 1.
however, changes to the basic alloy composition for which specs have already been determined are likely to have negative impacts. Consequently, at the first stage of this research, the practical realization of oxide-crucible melted ingots were evaluated with respect to the alloy having basic composition, from the standpoints of resulting material hot-forgeability, machinability, and toughness. If the results of such consideration were to suggest that this avenue could not be pursued, it would be necessary to consider reduction in Al content. As can be seen from Fig. 1, etc., in case of the basic composition, forging capability was found to be greatest for the ingots melted
Fig. 3. Optical microstructures for ingot YT-2 after water-quenching from 1225 ◦ C.
in the yttria crucible. Accordingly, it was decided to use an yttria crucible for production of a large-sized ingot. Table 2 presents the chemical compositions for this ingot, together with those corresponding to a triple melted VAR ingot produced for the purpose of comparison. The oxygen content of the latter is 0.055 wt%, while that of the yttria-crucible-melted ingot is 0.16%. As a result of industrial hot-forging testing, no cracking was observed in either ingot. Fig. 6 shows the external appearances of the two hot-forged materials. Neither exhibits any particular defects, and the desired square bar shape was achieved. Thus, even in the case of the basic composition, the yttria-crucible-melted ingot having a relatively higher oxygen content of 0.16 wt% is found to possess a repeated hot-forging capability as required industrially. Next, heat treatment conditions were pursued that would provide similar microstructures for the two forged materials. However, the respective oxygen contents are quite different, and as indicated in Fig. 4, since the compositional phase ratios differ substantially with oxygen content, it proved impossible to identify heat treatment conditions by which similar microstructures could be actually obtained. Accordingly, respective conditions were selected so as to produce microstructures as close as possible, consisting of 1280 ◦ C for the yttria-crucible-melted material, and 1240 ◦ C for the triple melted VAR material. Holding time was 10 h in both cases, with cooling speed of 1000 ◦ C/h. Fig. 7 presents the optical microstructures for the two forged materials after heat treatment, while Table 3 indicates inclusions (thought to be Y2 O3 that mixed form the crucible [20]), and respective phase area ratios. Although the lamellar structure ratio is greater for the yttria-crucible-melted material, the  phase area ratios are similar. Also, a large amount of inclusions of area ratio of 0.26% exists only in the yttria-cruciblemelted material.
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Table 2 Chemical compositions for the large ingots of Ti–28.7Al–7.0Mn (wt%) alloy made by induction melting in the yttria crucible and triple melt VAR. Melting process
Chemical composition (wt%)
Induction melting in the yttria crucible Triple melt VAR
C
Mn
Al
O
N
Y
0.009 0.010
6.98 6.93
28.6 28.4
0.160 0.055
0.006 0.003
0.19 –
100
100
(b)
95
Area ratio of α phase (%)
Area ratio of α phase (%)
(a)
90 85 80 WC-Copper Y2O3 Y2O3+Al2O3
75
WC-Copper Y2O3 Y2O3+Al2O3 CaO
95 90 85 80 75
CaO
70
0
0.1
0.2
0.3
0.4
70
0.5
0.1
0
Oxygen content (wt%)
0.2
0.3
0.4
0.5
Ceramic constituent content (wt%)
Fig. 4. Relationship between the area ratio of ␣ phase in water-quenched ingot piece from 1225 ◦ C and oxygen (a) and ceramic constituent (b) content for ingots having nearly the basic composition with Al content in the range of 28.21–29.22 wt%.
No. of cracks
Crucible 0
1~5
>5
WC copper
60
(b)
Y2O3+Al2O3 CaO
50
Area ratio of β phase (%)
Area ratio of β phase (%)
60
Y2O3
(a)
ZrO2
40 30 20 10 0
0
0.1
0.2
0.3
0.4
0.5
0.6
Oxygen content (wt%)
50 40 30 20 10 0
0
0.1
0.2
0.3
0.4
0.5
0.6
Area ratio of inclusions (%)
Fig. 5. Effects of oxygen content and  phase area ratios on the hot-forging capability (a) and effects of inclusion area ratios and  phase area ratios on the hot-forging capability (b) for all of the ingots.
Fig. 8 shows the machining test results for the two forged materials after heat treatment, indicating the relationship between cutting distance and Frank wear of the tool. In the case of cutting speeds of 16 and 24 m/min, Frank wear is small in both cases, and no appreciable difference can be seen. At cutting speeds of 32 and 40 m/min, Frank wear becomes greater, but the amount of increase is lower for the yttria-crucible-melted material. Comparing the Table 3 Area ratios of inclusions and respective phases for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. Melting process
Induction melting in the yttria crucible Triple melt VAR
Area ratio (%) Lamellar

Inclusions
65.0 35.6
12.1 12.8
0.26 0
microstructural factors associated with the two materials, while the ratio of  phase (which serves to improve machinability) is about the same, the ratio of lamellar structure (which conversely serves to reduce machinability) is actually higher in the yttria-cruciblemelted material. The reason for the superior machining capability of the yttria-crucible-melted material is not clear, but one possibility is the presence of inclusions. As with sulfur free cutting steel [26], the presence of inclusions may reduce cutting resistance and improve separability of chips. Table 4 shows the results of Charpy impact testing at room temperature for the two forged materials after heat treatment. Absorbed energy is slightly greater for the yttria-crucible-melted material. The reason for this may be considered to be as follows. Generally, Charpy absorbed energy is expressed as the sum of the energy required for crack initiation and the energy required for crack propagation, with the former having a close relationship with
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Table 4 Charpy absorbed energy at room temperature for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment using un-notched test pieces with the cross-section of 10 mm × 10 mm. Melting process
Induction melting in the yttria crucible Triple melt VAR
Absorbed energy (J) Test no. 1
Test no. 2
Test no. 3
Test no. 4
Average
4.2 4.2
5.1 5.1
5.1 4.2
5.9 4.2
5.1 4.4
800 Y2O3-16m/min 3VAR-16m/min Y2O3-24m/min 3VAR-24m/min Y2O3-32m/min 3VAR-32m/min Y2O3-40m/min 3VAR-40m/min
700
Frank wear (µm)
600 500 400 300 200 100 Fig. 6. External appearances of the hot-forged Ti–28.7Al–7.0Mn (wt%) alloy under industrially required forging conditions. Ingots were made by induction melting in the yttria crucible (a) and triple melt VAR (b).
material strength [27]. In the case of TiAl alloys, the ratio of the former in overall absorbed energy is quite high. That is, given that the strength of the yttria-crucible-melted material is higher due to increased oxygen content and lamellar structure area ratio, the absorbed energy is also considered to be greater. However, because the absorbed energy at the room temperature of the austempered ductile iron that uses the un-notched specimens of the same shape is about 100 J [28], it is certain that the absorbed energy of these two kinds of forged TiAl alloys is low. Fig. 9 presents results of creep rupture testing of the two forged materials after heat treatment in Larson-Miller parameter comparison. The creep strength of the yttria-crucible-melted material can be seen to be substantially higher. The reason for this is considered to be as noted above, with increased oxygen content contributing to higher creep strength. Summarizing the foregoing results, for an ingot having the basic composition of the alloy under consideration, melted in an yttria crucible and with the oxygen content thus elevated to 0.16 wt%, there were no particular issues in terms of hot-forgeability under
0 0
200
400
600
800
1000
Cutting distance (m) Fig. 8. Relationship between cutting distance and Frank wear of the tool for the two hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. The cutting speed was varied among 16, 24, 32, and 40 m/min with a constant feed rate of 0.2 mm per revolution and a constant cutting depth of 0.125 mm.
industrially required conditions. Also, with respect to machinability and Charpy impact properties to which it is feared to decrease at first, these proved slightly superior to a triple melted VAR ingot used for comparison and having an oxygen content of 0.055 wt%. Furthermore, creep strength was considerably higher than the material used for comparison, as had been predicted. Thus, the results of the research presented here clearly suggest that achieving greater strength and lower cost for hot-forged TiAl based alloy containing  phase is possible by use of large ingot melted in oxide crucibles.
Fig. 7. Optical microstructures for the hot-forged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment. Ingots were made by induction melting in the yttria crucible (a) and triple melt VAR (b).
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(4) The machinability and Charpy impact properties of above mentioned forged materials after heat treatment are actually superior to triple melted VAR material having an oxygen content of 0.055 wt%, and creep strength is substantially greater.
300
Yttria crucible Triple melt VAR 200
Stress (MPa)
References [1] [2] [3] [4] [5] [6]
100
[7]
90 [8]
80 20
21
22
23
24
25
LMP (C=20)
[9] [10]
Fig. 9. Creep rupture strength in Larson–Miller parameter comparison for two hotforged Ti–28.7Al–7.0Mn (wt%) alloy after heat treatment.
[11]
4. Conclusion
[12] [13] [14]
With the objective of achieving greater strength and lower cost for hot-forged TiAl based alloy containing  phase, the effects of contamination from oxide-crucible-melting and the possibility of practical industrial realization of large-sized ingots were considered with respect to alloy compositions centered on Ti–28.7Al–7.0Mn (wt%). Results were obtained as follows:
[15] [16]
[17] [18]
[19]
(1) Oxide-crucibles-melting results in greater oxygen content in the ingot. In this conjunction, the ␣ phase ratio at the forging temperature increases, and the  phase ratio decreases. (2) The  phase ratio has the greatest influence on hot-forging capability, and, even at an extremely high oxygen content of 0.35 wt%, recovery of the  phase amount by reducing the Al content serves to maintain hot-forgeability. (3) A large-sized ingot of the basic composition, melted in an yttria crucible, and having an oxygen content of 0.16 wt%, can be hotforged under industrially required conditions.
[20] [21] [22] [23] [24] [25] [26] [27] [28]
T. Tetsui, Adv. Eng. Mater. 3 (2001) 307–310. T. Tetsui, Mater. Sci. Eng. A 329–331 (2002) 582–588. T. Tetsui, Mater. Sci. Forum 361–363 (2007) 379–382. T. Noda, Intermetallics 6 (1998) 709–713. T. Fujimura, H. Nishikawa, N. Moriya, M. Imamura, IHI Gihou 48 (2008–2009) 153–158 (in Japanese). M. Weimer, T. Kelly, in: Y.-W. Kim, D. Morris, R. Yang, C. Leyens (Eds.), Structural Aluminides for Elevated Temperatures. Gamma Titanium and Other Metallic Aluminides, TMS, Warrendale, PA, 2008, pp. 5–6. T. Tetsui, K. Shindo, S. Kaji, S. Kobayashi, M. Takeyama, Intermetallics 13 (2005) 971–978. T. Tetsui, K. Shindo, S. Kaji, S. Kobayashi, M. Takeyama, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 199–205. T. Tetsui, K. Shindo, S. Kobayashi, M. Takeyama, Scripta Mater. 47 (2002) 399–403. H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, Adv. Eng. Mater. 10 (2008) 707–713. W. Wallgram, T. Schmolzer, L. Cha, G. Das, V. Güther, H. Clemens, Int. J. Mater. Res. 8 (2009) 1021–1030. Y.Y. Chen, F. Yang, F.T. Konga, S.L. Xiao, J. Alloys Compd. 498 (2010) 95–101. H.Z. Niu, F.T. Kong, Y.Y. Chen, F. Yang, J. Alloys Compd. 509 (2011) 10179–10184. T. Tetsui, K. Shindo, S. Kobayashi, M. Takeyama, Intermetallics 11 (2003) 299–306. M. Takeyama, S. Kobayashi, Intermetallics 13 (2005) 993–999. S. Kobayashi, M. Takeyama, T. Motegi, N. Hirota, T. Matsuo, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 165–175. J. Aguilar, A. Schievenbusch, O. Kattlitz, Intermetallics 19 (2011) 757–761. V. Güther, A. Chatterjee, H. Kettner, in: Y.-W. Kim, H. Clemens, A.H. Rosenberger (Eds.), Gamma Titanium Aluminides 2003, TMS, Warrendale, PA, 2004, pp. 241–247. T. Tetsui, T. Kobayashi, T. Mori, T. Kishimoto, H. Harada, Mater. Trans. 51 (2010) 1656–1662. T. Tetsui, T. Kobayashi, A. Kishimoto, H. Harada, Intermetallics 20 (2012) 16–23. J. Barbosa, C.S. Ribeiro, C. Monteiro, Mater. Sci. Forum 426–432 (2003) 1933–1938. F. Gomes, J. Barbosa, C.S. Ribeiro, Intermetallics 16 (2008) 1292–1297. K. Matsugi, T. Hatayama, O. Yanagisawa, Intermetallics 7 (1999) 1049–1057. S. Takagi, C. Ouchi, Mater. Trans. JIM 38 (1997) 285–291. Y. Ma, Z. Jiang, B. Chen, X. Zhao, M. Gao, K. Liu, Y. Li, J. Mater. Sci. Technol. 26 (2010) 131–135. Y.D. Yashin, S.A. Soldatkin, P.Y. Chesnokov, Metallurgist 49 (2005) 236–241. T. Kobayashi, Int. J. Fract. 23 (1983) R105–R109. O. Eric, D. Rajnovic, S. Zec, L. Sidjanin, M.T. Jovanovic, Mater. Charact. 57 (2006) 211–217.