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Effects of thermomechanical processing on titanium aluminide strip cast by the melt overflow process
Materials Science and Engineering, A179/,4180 (1994) 645-648
645
Effects of thermomechanical processing on titanium aluminide strip cast by the melt overflow process T. A. Gaspar and L. E. Hackman Ribbon Technology Corporation, PO Box 30758, Columbus, OH 43230 (USA)
E. Batawi and J. A. Peters Sulzer-Innotec, Division 1511, PO Box 65, Winterthur 8404 (Switzerland)
Abstract The objective of this research project was to investigate the feasibility of producing titanium aluminide foils from direct cast strip using ribbon technology's plasma melt overflow process. Niobium-modified Ti3AI alloys were melted in a cold copper crucible using a transferred plasma arc and then direct cast into strip on a rotating chill roll. Samples cut from the as-cast Ti3AI-Nb (a2) titanium aluminide strip were encapsulated into a pack. The packs were heated to the rolling temperature and then hot rolled at low strain rates. Foils 70/~m (0.003 in) thick, having a uniform a2-B2 microstructure with oxygen contents as low as 900 wt.ppm were obtained after pack rolling. The strips and foils were characterized in terms of microstructure and chemical composition in the as-received, heat-treated and pack-rolled conditions. The results indicated that it was technically feasible to produce foils from direct cast titanium aluminide strip using pack-rolling technology. The advantage of this technology lies in its cost-effectiveness, since the relatively low cost directcast titanium aluminide strip was thermomechanically processed into foil with the desired microstructure without any intermediate processing steps.
1. Introduction One alternative to rolling foils from ingots is direct casting of strip based on rapid solidification technology (RST). Ribtec developed a single-chill-roll casting technique called melt overflow rapid solidification technology (MORST) to produce fibers, filaments and strips [1-3]. Ribtec first attempted to direct cast foils from Ti aluminide alloys using MORST, but the cast foils exhibited poor mechanical properties when compared with rolled foils. This paper describes the techniques to direct cast a thin Ti aluminide strip and then to pack roll the strip to foil gauge.
2. Experimental procedures Ti aluminide strips were cast in the plasma melt overflow furnace at Ribtec [4]. The plasma melt overflow furnace combined plasma arc melting in a cold copper hearth with MORST by rotating the cold copper hearth about the same axis of rotation as the chill roll to overflow liquid onto the circumference of the chill roll [5]. Using this technique, the distance 0921-5093/94/$7.00 SSDI 0921-5093(93)05582-A
between the cold copper hearth and the chill roll remained constant at 0.6 mm (0.015 in). The plasma arc melting system consisted of a Retech model RP75T plasma arc torch powered by two TAFA model 31"1B dual-voltage d.c. power supplies. A helium plasma (He flow, 0.068 m 3 min -~ at 2.4 bar pressure) was used. The plasma arc voltage was 180 V_+ 5% with currents up to 620 A maximum at a stand-off of 75 mm _+ 10%. The melt overflow caster consisted of a solid molybdenum chill roll. The chill roll casting surface was knurled with a 16 pitch 60 ° diamond pattern to increase the momentum transfer between the substrate and the liquid [6]. The copper pour spout or "lip" was a three-sided notch that measured 102 m m × 19 mm machined on the hearth face plate. The hearth tilt rate and tilt time coupled with the casting speed determined the strip thickness. The direct-cast strip was sent to Sulzer-Innotec for pack rolling. Figure 1 shows a schematic representation and a scanning electron micrograph of a pack containing the Ribtec titanium aluminide foil. The ascast strip was cut into small samples 20 mm (0.8 in) × 20 mm (0.8 mm) and encapsulated in packs © 1994 - Elsevier Sequoia. All rights reserved
646
T A. Gaspar et al.
/
Thermomechanical processing effects on titanium aluminide
containing a release agent. T h e packs were heated at a rate of 8 °C min ~ (14 °F min -1) and soaked for 30 min at 1050 °C (1920 °F) before rolling. T h e rolling parameters were adapted such that the strain rates were low and gradually increased from 2 to 4 s-1. After each pass, the pack was quickly replaced in the furnace to compensate for heat losses due to contact with the cold rolls. Rolling was performed uniaxially, and seven or eight passes were required to achieve foils 70/~m thick.
3. Results and discussion 3.1. A s - c a s t strip Figure 2 shows backscattered electron micrographs of the transverse microstructure of the cast a~ Ti
Fig. 1. (a) Schematic representation of an encapsulated a2 Ti alumnide strip ready for pack rolling. (b) Scanning electron micrograph, secondary-electron contrast, of a sectioned pack. a, b and c represent the capsule, release agent and titanium aluminide strip respectively and correspond to A, B and C in (a).
aluminide strip. T h e cast strip exhibited a Widmanst~itten-type microstructure, with a colony size of 5 - 1 5 #m. No distinct variation in the microstructure was observed from the substrate side (bottom in Fig. 2(a)) to the free side of the ribbon. However, small pores, approximately 2 - 8 /~m in size, were scattered randomly throughout the microstructure. 3.2. Topographic m o r p h o l o ~ ' Figure 3 shows the typical roughness profiles in the longitudinal and transverse directions of both the substrate-cast surface and the free-cast surface of the a2 Ti aluminide strip. T h e strip substrate surface replicated the diamond knurl pattern on the chill roll more closely than the free-cast surface and exhibited a greater peak-to-trough distance. T h e average roughness values R. and R , as well as the maximum peak-totrough heights R m detected over three scans are presented in Table 1.
Fig. 2. Scanning electron micrographs (backscattered electron contrast) of the transverse microstructure of as-cast a 2 Ti aluminide strip. The as-cast strip exhibits a Widmanst~itten microstructure, with no macrosegregation. Pores 2-8/zm in size were detected throughout the ribbon thickness.
T. A. Gaspar et al.
/
Thermomechanical processing effects on titanium aluminide
647
S u b s t r a t e side
,25rnmj
itudinal
lO m I000 ~m
I Transverse
F r e e side
.,.v.
Pack-Rolled Ti3Al-Based 0~2 Titanium AJuminide Foil Ribtec As-Cast Sheet Fig. 4. Optical micrograph of the free-cast surface of the a 2 Ti aluminide strip before and after pack rolling. A 2 crn x 2 cm x 0.5 mm piece of strip in the as-cast condition, without heat treatment (above) and pack rolled at 1050 °C to a final thickness of 70 pm (below). The rolled foil had a darker-grey surface owing to the release agent that had not yet been removed. The metallic surface of the a 2 Ti aluminide can be seen in areas where the release agent debonded near the center of the foil.
1N
Fig. 3. Roughness profiles of the as-cast a 2 Ti aluminide strip (scan length, 15 mm).
TABLE 1. Transverse and longitudinal roughnesses of as-cast strip Side
Direction
R: (/~m)
R, (~m)
(/~m)
Rm
Substrate side
Longitudinal Transverse
30.9 30.4
5.07 5.23
54 55.8
Free side
Longitudinal Transverse
16.2 15.9
2.43 2.33
19.3 21.7
3.3. Pack-rolled foils Figure 4 shows the free-cast surface of the a 2 Ti aluminide strip before and after pack rolling. T h e greycolored release agent was not r e m o v e d f r o m the packrolled foil at b o t t o m of photograph. T h e metallic surface of the strip can be seen near the center of the rolled foil in areas where the release agent d e b o n d e d f r o m the T i aluminide foil. Figure 5 shows a scanning electron micrograph of the rolled foil still in its capsule; the darker-grey region B represents the release agent. T h e surface roughness and internal porosity of the as-cast strip was not found to be deleterious to the quality of the pack-rolled foil. T h e t h e r m o m e c h a n i c a l treatments resulted in the
#
i! ii!!iii
iq ¸ :
:i! i!iG!:iiiiGii!i iiii!ii! iiii!iiiii!!!iiiiiiiil
i :ill i~(!)il iili ! ii/Ji ~ Fig. 5. Scanning electron micrograph of a transverse section of the foil still in its capsule. The roughness and porosity of the ascast strip did not affect its rollability. The release agent appears as a darker-grey region B.
elimination of all internal porosity of the as-cast strip. However, as can be seen in Fig. 6, the surface of the pack-rolled foil remained somewhat rough after rolling. T h e scale of these irregularities was of no severe consequence, since it was standard practice to grind the surfaces of the foils lightly after pack rolling to r e m o v e the release agent f r o m the foil. T h e microstructure of the rolled foils shown in Fig. 6 consisted of a high volume fraction ( 4 0 - 5 0 % ) of globular and equiaxed a2 grains, 5 - 1 0 ktm in size, dispersed within a B2 matrix.
3.4. Gas analysis N o distinct a 2 casing could be distinguished using electron microscopy, suggesting that very limited
648
7~ A. Gaspar et al.
/
Thermomechanical processing e(fects on titanium aluminide
TABLE 2. Carbon and gas contents of (z~Ti aluminidc strips and foils State
C OL tt N (wt.ppm) (wt.ppm)(wt.ppm)(wt.ppm)
a 2ingot 2(10 a 2 as-cast strip 257 a 2as-cast + pack 674 rolled to 70 j~m foil
7(1(t 705 9(19
28 43 80
117 154
ing. This increase in hardness may be explained by the microstructural transformations from the cast Widmansfiitten structure to the fine two-phase mixture of a : grains in a B2 matrix resulting from pack rolling. 4. Conclusions
Fig. 6. Scanning electron micrographs (backscattered electron contrast) of the transverse microstructure of a~ Ti aluminide after pack rolling. The structure consisted of approximately equal volume fractions of globular a2 in a B2 matrix. No distinctive a 2 casing was observed, suggesting little oxygen pick-up.
oxygen contamination occurred. This was confirmed by the gas analysis results shown in Table 2. M o r e specifically, oxygen pick-up due to pack rolling, was restricted to approximately 200 wt.ppm. 3.5. M i c r o h a r d n e s s
Microhardness measurements of as-cast and thermomechanically processed strips were performed to provide a preliminary assessment of the influence of pack rolling on the mechanical properties of the strips. T h e Vickers microhardness of the a2-based strip was found to increase from approximately 416 _+ 15 H V to 471 _+9 H V because of the thermomechanical process-
T h e Plasma Melt Overflow Process is capable of casting strip 500 ~ m (0.020 in) thick from niobiummodified Ti3AI alloys. T h e as-cast strip exhibited a Widmanst/itten microstructure with some internal porosity. T h e carbon and gas contents of the cast strip were low. T h e feasibility of pack rolling direct-case a~ Ti aluminide strips to fabricate thin Ti aluminide foils was demonstrated. Foils 70 ,urn thick, having a uniform a 2 - B 2 microstructure with little oxygen pick-up were produced from the cast strip. Pack rolling of the cast strip resulted in the elimination of internal porosity, as well as a reduction in the scale of roughness of the ascast strip. T h e magnitude of the surface roughness was such that it would be removed during the standard foilgrinding operation for removing the release agent after pack rolling. It should be noted that the advantage of pack rolling direct-cast strip lies in its cost-effectiveness; the cast strip may be thermomechanically processed into foil with the desired microstructure without any intermediate processing steps. References 1 L. E. Hackman, J. Dickson, D. L. Dunlap and M. Handshey, US Pat. 4,930,565, June 5, 1990. 2 L. E. Hackman, J. Dickson, D. L. Dunlap and M. Handshey, US Pat. 4,813,472, March 21, 1989. 3 L.E. Hackman, US Pt. 4, 903, 751, February 2 7, 1990. 4 T. A. Gaspar and L. E. Hackman, Proc. 6th World Conj. Titanium, Cannes, June 6-9, 1988, Vol. I1, Les t~ditions Physique, Les Ulis, p. 739. 5 T. Gaspar, US Pat. 4,907,641, March 13, 1990. 6 T. Gaspar, USPat. 4,705,095, November 10. 1987.
E. E. on
de
645
Effects of thermomechanical processing on titanium aluminide strip cast by the melt overflow process T. A. Gaspar and L. E. Hackman Ribbon Technology Corporation, PO Box 30758, Columbus, OH 43230 (USA)
E. Batawi and J. A. Peters Sulzer-Innotec, Division 1511, PO Box 65, Winterthur 8404 (Switzerland)
Abstract The objective of this research project was to investigate the feasibility of producing titanium aluminide foils from direct cast strip using ribbon technology's plasma melt overflow process. Niobium-modified Ti3AI alloys were melted in a cold copper crucible using a transferred plasma arc and then direct cast into strip on a rotating chill roll. Samples cut from the as-cast Ti3AI-Nb (a2) titanium aluminide strip were encapsulated into a pack. The packs were heated to the rolling temperature and then hot rolled at low strain rates. Foils 70/~m (0.003 in) thick, having a uniform a2-B2 microstructure with oxygen contents as low as 900 wt.ppm were obtained after pack rolling. The strips and foils were characterized in terms of microstructure and chemical composition in the as-received, heat-treated and pack-rolled conditions. The results indicated that it was technically feasible to produce foils from direct cast titanium aluminide strip using pack-rolling technology. The advantage of this technology lies in its cost-effectiveness, since the relatively low cost directcast titanium aluminide strip was thermomechanically processed into foil with the desired microstructure without any intermediate processing steps.
1. Introduction One alternative to rolling foils from ingots is direct casting of strip based on rapid solidification technology (RST). Ribtec developed a single-chill-roll casting technique called melt overflow rapid solidification technology (MORST) to produce fibers, filaments and strips [1-3]. Ribtec first attempted to direct cast foils from Ti aluminide alloys using MORST, but the cast foils exhibited poor mechanical properties when compared with rolled foils. This paper describes the techniques to direct cast a thin Ti aluminide strip and then to pack roll the strip to foil gauge.
2. Experimental procedures Ti aluminide strips were cast in the plasma melt overflow furnace at Ribtec [4]. The plasma melt overflow furnace combined plasma arc melting in a cold copper hearth with MORST by rotating the cold copper hearth about the same axis of rotation as the chill roll to overflow liquid onto the circumference of the chill roll [5]. Using this technique, the distance 0921-5093/94/$7.00 SSDI 0921-5093(93)05582-A
between the cold copper hearth and the chill roll remained constant at 0.6 mm (0.015 in). The plasma arc melting system consisted of a Retech model RP75T plasma arc torch powered by two TAFA model 31"1B dual-voltage d.c. power supplies. A helium plasma (He flow, 0.068 m 3 min -~ at 2.4 bar pressure) was used. The plasma arc voltage was 180 V_+ 5% with currents up to 620 A maximum at a stand-off of 75 mm _+ 10%. The melt overflow caster consisted of a solid molybdenum chill roll. The chill roll casting surface was knurled with a 16 pitch 60 ° diamond pattern to increase the momentum transfer between the substrate and the liquid [6]. The copper pour spout or "lip" was a three-sided notch that measured 102 m m × 19 mm machined on the hearth face plate. The hearth tilt rate and tilt time coupled with the casting speed determined the strip thickness. The direct-cast strip was sent to Sulzer-Innotec for pack rolling. Figure 1 shows a schematic representation and a scanning electron micrograph of a pack containing the Ribtec titanium aluminide foil. The ascast strip was cut into small samples 20 mm (0.8 in) × 20 mm (0.8 mm) and encapsulated in packs © 1994 - Elsevier Sequoia. All rights reserved
646
T A. Gaspar et al.
/
Thermomechanical processing effects on titanium aluminide
containing a release agent. T h e packs were heated at a rate of 8 °C min ~ (14 °F min -1) and soaked for 30 min at 1050 °C (1920 °F) before rolling. T h e rolling parameters were adapted such that the strain rates were low and gradually increased from 2 to 4 s-1. After each pass, the pack was quickly replaced in the furnace to compensate for heat losses due to contact with the cold rolls. Rolling was performed uniaxially, and seven or eight passes were required to achieve foils 70/~m thick.
3. Results and discussion 3.1. A s - c a s t strip Figure 2 shows backscattered electron micrographs of the transverse microstructure of the cast a~ Ti
Fig. 1. (a) Schematic representation of an encapsulated a2 Ti alumnide strip ready for pack rolling. (b) Scanning electron micrograph, secondary-electron contrast, of a sectioned pack. a, b and c represent the capsule, release agent and titanium aluminide strip respectively and correspond to A, B and C in (a).
aluminide strip. T h e cast strip exhibited a Widmanst~itten-type microstructure, with a colony size of 5 - 1 5 #m. No distinct variation in the microstructure was observed from the substrate side (bottom in Fig. 2(a)) to the free side of the ribbon. However, small pores, approximately 2 - 8 /~m in size, were scattered randomly throughout the microstructure. 3.2. Topographic m o r p h o l o ~ ' Figure 3 shows the typical roughness profiles in the longitudinal and transverse directions of both the substrate-cast surface and the free-cast surface of the a2 Ti aluminide strip. T h e strip substrate surface replicated the diamond knurl pattern on the chill roll more closely than the free-cast surface and exhibited a greater peak-to-trough distance. T h e average roughness values R. and R , as well as the maximum peak-totrough heights R m detected over three scans are presented in Table 1.
Fig. 2. Scanning electron micrographs (backscattered electron contrast) of the transverse microstructure of as-cast a 2 Ti aluminide strip. The as-cast strip exhibits a Widmanst~itten microstructure, with no macrosegregation. Pores 2-8/zm in size were detected throughout the ribbon thickness.
T. A. Gaspar et al.
/
Thermomechanical processing effects on titanium aluminide
647
S u b s t r a t e side
,25rnmj
itudinal
lO m I000 ~m
I Transverse
F r e e side
.,.v.
Pack-Rolled Ti3Al-Based 0~2 Titanium AJuminide Foil Ribtec As-Cast Sheet Fig. 4. Optical micrograph of the free-cast surface of the a 2 Ti aluminide strip before and after pack rolling. A 2 crn x 2 cm x 0.5 mm piece of strip in the as-cast condition, without heat treatment (above) and pack rolled at 1050 °C to a final thickness of 70 pm (below). The rolled foil had a darker-grey surface owing to the release agent that had not yet been removed. The metallic surface of the a 2 Ti aluminide can be seen in areas where the release agent debonded near the center of the foil.
1N
Fig. 3. Roughness profiles of the as-cast a 2 Ti aluminide strip (scan length, 15 mm).
TABLE 1. Transverse and longitudinal roughnesses of as-cast strip Side
Direction
R: (/~m)
R, (~m)
(/~m)
Rm
Substrate side
Longitudinal Transverse
30.9 30.4
5.07 5.23
54 55.8
Free side
Longitudinal Transverse
16.2 15.9
2.43 2.33
19.3 21.7
3.3. Pack-rolled foils Figure 4 shows the free-cast surface of the a 2 Ti aluminide strip before and after pack rolling. T h e greycolored release agent was not r e m o v e d f r o m the packrolled foil at b o t t o m of photograph. T h e metallic surface of the strip can be seen near the center of the rolled foil in areas where the release agent d e b o n d e d f r o m the T i aluminide foil. Figure 5 shows a scanning electron micrograph of the rolled foil still in its capsule; the darker-grey region B represents the release agent. T h e surface roughness and internal porosity of the as-cast strip was not found to be deleterious to the quality of the pack-rolled foil. T h e t h e r m o m e c h a n i c a l treatments resulted in the
#
i! ii!!iii
iq ¸ :
:i! i!iG!:iiiiGii!i iiii!ii! iiii!iiiii!!!iiiiiiiil
i :ill i~(!)il iili ! ii/Ji ~ Fig. 5. Scanning electron micrograph of a transverse section of the foil still in its capsule. The roughness and porosity of the ascast strip did not affect its rollability. The release agent appears as a darker-grey region B.
elimination of all internal porosity of the as-cast strip. However, as can be seen in Fig. 6, the surface of the pack-rolled foil remained somewhat rough after rolling. T h e scale of these irregularities was of no severe consequence, since it was standard practice to grind the surfaces of the foils lightly after pack rolling to r e m o v e the release agent f r o m the foil. T h e microstructure of the rolled foils shown in Fig. 6 consisted of a high volume fraction ( 4 0 - 5 0 % ) of globular and equiaxed a2 grains, 5 - 1 0 ktm in size, dispersed within a B2 matrix.
3.4. Gas analysis N o distinct a 2 casing could be distinguished using electron microscopy, suggesting that very limited
648
7~ A. Gaspar et al.
/
Thermomechanical processing e(fects on titanium aluminide
TABLE 2. Carbon and gas contents of (z~Ti aluminidc strips and foils State
C OL tt N (wt.ppm) (wt.ppm)(wt.ppm)(wt.ppm)
a 2ingot 2(10 a 2 as-cast strip 257 a 2as-cast + pack 674 rolled to 70 j~m foil
7(1(t 705 9(19
28 43 80
117 154
ing. This increase in hardness may be explained by the microstructural transformations from the cast Widmansfiitten structure to the fine two-phase mixture of a : grains in a B2 matrix resulting from pack rolling. 4. Conclusions
Fig. 6. Scanning electron micrographs (backscattered electron contrast) of the transverse microstructure of a~ Ti aluminide after pack rolling. The structure consisted of approximately equal volume fractions of globular a2 in a B2 matrix. No distinctive a 2 casing was observed, suggesting little oxygen pick-up.
oxygen contamination occurred. This was confirmed by the gas analysis results shown in Table 2. M o r e specifically, oxygen pick-up due to pack rolling, was restricted to approximately 200 wt.ppm. 3.5. M i c r o h a r d n e s s
Microhardness measurements of as-cast and thermomechanically processed strips were performed to provide a preliminary assessment of the influence of pack rolling on the mechanical properties of the strips. T h e Vickers microhardness of the a2-based strip was found to increase from approximately 416 _+ 15 H V to 471 _+9 H V because of the thermomechanical process-
T h e Plasma Melt Overflow Process is capable of casting strip 500 ~ m (0.020 in) thick from niobiummodified Ti3AI alloys. T h e as-cast strip exhibited a Widmanst/itten microstructure with some internal porosity. T h e carbon and gas contents of the cast strip were low. T h e feasibility of pack rolling direct-case a~ Ti aluminide strips to fabricate thin Ti aluminide foils was demonstrated. Foils 70 ,urn thick, having a uniform a 2 - B 2 microstructure with little oxygen pick-up were produced from the cast strip. Pack rolling of the cast strip resulted in the elimination of internal porosity, as well as a reduction in the scale of roughness of the ascast strip. T h e magnitude of the surface roughness was such that it would be removed during the standard foilgrinding operation for removing the release agent after pack rolling. It should be noted that the advantage of pack rolling direct-cast strip lies in its cost-effectiveness; the cast strip may be thermomechanically processed into foil with the desired microstructure without any intermediate processing steps. References 1 L. E. Hackman, J. Dickson, D. L. Dunlap and M. Handshey, US Pat. 4,930,565, June 5, 1990. 2 L. E. Hackman, J. Dickson, D. L. Dunlap and M. Handshey, US Pat. 4,813,472, March 21, 1989. 3 L.E. Hackman, US Pt. 4, 903, 751, February 2 7, 1990. 4 T. A. Gaspar and L. E. Hackman, Proc. 6th World Conj. Titanium, Cannes, June 6-9, 1988, Vol. I1, Les t~ditions Physique, Les Ulis, p. 739. 5 T. Gaspar, US Pat. 4,907,641, March 13, 1990. 6 T. Gaspar, USPat. 4,705,095, November 10. 1987.
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