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Influence of different forging processes on the ultrasonic response in titanium alloys TC6 and TC11
Influence of different forging processes.on the ultrasonic response =n titanium alloys TC6 and TC11 J.H. Pan, S.B. Shong and J.W. Li The influence of four different forging processes on the ultrasonic response in T i - 6 A I - 2 . 5 M o - 1 . 5 C r - 0 . 5 F e - 0 . 3 S i (TC6) and Ti-6AI-3.0Mo-1.5Zr-0.3Si (TC11 ) ~-fl alloys is described and analysed. The results show that noise levels and losses of back reflection are lowest after ~-fl forging but increase after forging above the fl transition temperature. These responses are caused by microstructural features, ie the dimensions of ~ colonies and ~ platelets, formed as a result of the different forging processes.
Keywords: ultrasonic response, noise level, loss of back reflection, forging, microstructure, titanium alloys
High noise levels and transmission losses exist generally in ot-fl phase titanium alloys. Granville and Taylor have used the Delta testing technique to improve the signalto-noise ratio (SNR) t12]. Although this is a successful method, it is not used for the ultrasonic testing of titanium alloy products in production sites because the main disadvantage of the technique is difficulty with the accurate sizing and location of defects. At present the noise signal is not avoided by using the pulse-echo method in two-phase titanium alloys. Therefore high noise level is limited in acceptance standards of ultrasonic testing for titanium alloy forgings in order to assure sufficient SNR and prevent the rejected det'ect from being masked. It has been proved that microstructural features have an effect on the noise levelI2- 51. Since these features are determined by hot-working processes, it is important to research further the relationships between forging process, rrlicrostructure and ultrasonic response in order to correctly evaluate the quality of titanium alloy products by ultrasonic testing.
Forging processes The test pieces were reduced by forging to 40-50 mm high cake-shaped specimens with 35 %-55 % deformation, and the surfaces of the specimens were finished at Ra 1.6 #m. The forging processes and post-heat treatments are shown in Table 2.
Ultrasonic testing Testing was carried out using a Krautkramer USIPll flaw detector with probes of 4 MHz, 12 mm diameter and 10 MHz, 5 mm diameter. The directions of the wave beam and the hammering were the same. Ti-6A1-4V annealed alloy standard test blocks with 0.8 mm diameter FBH measured noise levels at 4 MHz and losses of back reflection measured at 4 and 10 MHz were used. The values of noise and loss were measured in decibels.
Micrography examination
Experimental
The transverse section microstructural features were examined with a NEOPHOT 2 microscope. The examined face was perpendicular to the sound testing surface, and the positions of examination and noise signal indicated were uniform.
Materials
Results
Test materials were taken from the nominal two-phase forged alloys Ti-6AI-2.5Mo- 1.5Cr-0.5Fe-0.3Si (TC6) and Ti-6AI-3.0Mo-I.5Zr-0.3Si (TCll'). Their actual compositions are shown in Table 1. The /~ transition temperatures are 970°C (TC6) and 995°C (TC11). Their microstructure was a typical equiaxed structure and the indicated noise level was 12.5% of the response from a 0.8 mm diameter FBH (ie 0.8 FBH - 24 dB).
The results of the tests are shown in Tables 3 and 4 and Figures 1-4.
Noise level As shown in Table 3 and Figure 1, the noise levels are lowest for alloys TC6 and TC11 after =-fl forging. There is an improvement of 4 dB as compared with the original materials.
0308-9126/90/020103-04 © 1990 Butterworth & Co (Publishers) Ltd NDT International Volume 23 Number 2 April 1990
103
Table 1. Compositions (in w t % )
of t i t a n i u m alloys TC6 and TC11
Alloy
AI
Mo
Cr
Zr
Fe
Si
C
N
O
H
TC6 TC11
6.1 9 6.41
2.66 3.45
1.90 --
-1.57
0.30 0.07
0.25 0.29
0.02 <0.1
0.02 0.0063
0.07 0.09
0.01 2 0.01 3
Table 2. Forging processes and p o s t - h e a t t r e a t m e n t s Forging process Test piece number
Type of forging
Heating temperature/time
TC6
1 2 3
c~-/~ /~ Sub-/~
960°C/1 990°C/1 990 °C/1 FC-* 960
TC11
4 5 6
~-/3 /~ SH ~
975°C/1 h 10 min 1015°C/1 h i 0 min 1095°C/1 h 10 min
Alloy
h 20 min h 20 min h 20 min --, °C/45 min
Cool ingb
Post-heat treatmentC
AC AC AC
I I I
AC AC WC
II II II
aSH, super-high-temperature forging bAG, air cooling; WC, water cooling Cl, 920oc/1.5h FCto650°C/2hAC;II, 950 C/1 hAC+530'~C/6hAC Table 3. Ultrasonic response m e a s u r e m e n t results Loss of back reflection (dB)b
Specimen number
Noise level (at 4 MHz) with respect to 0.8 mm FBH ( d B ) a
4 MHz
10 MHz
TC6
1 2 3
-30-+1 -14_+2 -12_+1
0 -1 -1
0 -5 -6
TC11
4 5 6
-28-+1 -22-+2 -19+_1
0 -1 -1
0 -3 0
Alloy
a0.8 mm diameter FBH corresponds to 0 dB bLoss of back reflection is value compared with that of standard test block for same beam path distance
Table 4. A v e r a g e dimensions of m i c r o s t r u c t u r a l features
Alloy TC6
TC11
Specimen number
Microstructural type a
Grain size (#m)
Colony size (/~m)
¢ platelet thickness (#m)
1 2 3
Equiaxial BW BW
-200 + 20 200 -+ 20
30_+5 35-+10
5+1 5_+2
4 5 6
Equiaxial BW BW
-200 _+ 10 210_+ 10
25+5 10+4
4+1 2+1
aBW, basket-weave microstructure
104
NDT International April 1990
rI] -10
-
I
-20
-
Z
-30
-
The noise levels are obviously increased for forging above the fl transition temperature. However, the size of increment is different, the noise levels in TC6 being higher than in TC 11.
TC6
[ ] Tc ,
i
>
i
c¢-13
B
Sub-
D SH
Loss o f back r e f l e c t i o n The results of back reflection loss for the two frequencies are also shown in Table 3. The loss values are negligible at 4 MHz for all forging processes. They are slightly higher at 10 MHz for forging above the fl transition temperature than for ~-fl forging, except for SH processing.
Type of forging
Fig. 1 Comparison of noise levels for titanium alloys TC6 and TC11 after different forging processes
Fig. 2 Typical equiaxed microstructure for titanium alloys TC6 and TC11 after ~-fl forging ( × 250)
Microstructure The microstructural features differ considerably with a change of forging process, as shown in Table 4 and Figures 2-4. After forging in the ~-fl phase region, the microstructures of TC6 and TC11 consist of equiaxed phases with transformed fl between the phases (called an equiaxed structure). The uniformly distributed • phases form ball or polygon structures and their sizes are approximate in every direction (Figure 2). Various basket-weave (BW) structures are obtained after forging above the fl transition temperature in the two alloys. The BW structural feature is very different from the equiaxed structure, in that a platelets are formed along fl grain boundaries, and specifically oriented colonies consisting of :t platelets are banked like a 'basket' and non-uniformly distributed inside the (previously fl) grains (Figures 3 and 4). The dimensions of the BW structure for specimens TC6 and TC11 are given in Table 4. From Table 4 it is seen that the colony sizes and ~ platelet thicknesses are larger in TC6 than in TCll and the platelets are more non-uniformly distributed between colonies. Discussion The results of the tests show that the ultrasonic response is closely related to the microstructural features which are formed by the different forging processes.
Fig. 3 Basket-weave microstructure for titanium alloy TC6 after sub-fl forging ( × 250)
Fig. 4 Basket-weave microstructure for titanium alloy T e l l after super-high-temperature forging ( x 250)
NDT International April 1990
The ultrasound orientation scattering effect is smaller for sound waves travelling in the equiaxed structure tsl. The noise level indicated, therefore, is much lower. Also, since the equiaxed structure is further developed by additional forging in the ~-fl region, it is lower in the test pieces than in the original materials. In various BW structures the specifically oriented colonies and ~ platelets in the grains cause the noise level to increase because more potential scattering faces are formed for sound waves travelling along the ~ colonies and parallel ~ phase boundaries t3-51. The more non-uniformly distributed and the larger the dimensions of colonies and ~ platelets, the higher are the energies of ultrasonic scattering received and the higher are the noise levels. This is why the noise levels in TC6 are higher than in TC1 1 even under the same forging conditions (eg specimens 2 and 5). The results for the loss of back reflection show that higher loss is not related to grain size but to BW microstructural features and their dimensions owing to the higher level of absorption and scattering caused by sound waves travelling along the ~ colonies and ~t phase boundaries. However, the loss decreases when the dimensions are very small, eg specimen 6 (Tables 3 and 4). The wavelength of ultrasound used in this study was approximately 1.5
105
and 0.6 mm respectively. Therefore it can be seen that the effect of colony size on the loss of ultrasound is negligible for colony sizes of 0.006-0.03 of a wavelength and increases for sizes above 0.03 of a wavelength. As discussed above, the ultrasonic response will be changed by different forging processes since it is ultimately decided by the microstructural features. It is possible, however, that a higher noise level caused by an acceptable microstructure being formed by a certain forging process can be unacceptable in accordance with the present inspection standards of ultrasonic testing, eg the case of the sub-fl forging process in TC6. For this reason it is essential to research further the relationships between forging process, microstructure and ultrasonic response and to determine rational acceptance standards of noise level according to different processes for titanium alloy forgings under the precondition of meeting the signal-tonoise ratio requirement.
Conclusions 1. The noise level and loss of back reflection are lowest for titanium alloys TC6 and TC11 after c~-/~ forging. They increase after forging above the /~ transition temperature. The increments of noise level are bigger in TC6 than in TC11. 2. Higher noise level and loss of back reflection are caused by microstructural features formed as a result of the
forging process, ie bigger dimensions of.~ colonies and platelets influence the ultrasonic response.
Acknowledgements The authors wish to thank senior specialists Chao Chunxiao and Zhang Zhifang for their support and valuable suggestions during the course of this work.
References 1 Granville,R.K. and Taylor, J.L 'Improvement in signal-to-noise ratio during the ultrasonic testing of titanium alloys' Br J NDT 4 (1986) 2 Billman, F.R. 'Effects of Ti-6AI-4V alloy metallurgical structures on ultrasonic characteristics' Titanium '72, Science and Technology (1972) pp 693-705 3 Pan, J. 'Effects of microstructural features of titanium alloy bars on ultrasonic response characteristics' Chinese J N D T 9 (1987) 4 Granville,R.K. and Taylor, J.L 'High noise during the ultrasonic testing of titanium alloys' Br J N D T 3 (1985) 5 Ginty,B. 'The influence of microstructure on ultrasonic response in a titanium alloy forging' Titanium '80, Science and Technology 3 (1980) pp 2095-2104
Authors Pan Janhua and Shong Shubo are in the Department of Materials Experiment and Control, Anda Forging Plant, Guizhou, China. Li Jawei is in the Department of Ultrasound Testing, Institute of Aeronautical Materials, Beijing, China.
Paper received 4 August 1989
106
NDT International April 1990
Keywords: ultrasonic response, noise level, loss of back reflection, forging, microstructure, titanium alloys
High noise levels and transmission losses exist generally in ot-fl phase titanium alloys. Granville and Taylor have used the Delta testing technique to improve the signalto-noise ratio (SNR) t12]. Although this is a successful method, it is not used for the ultrasonic testing of titanium alloy products in production sites because the main disadvantage of the technique is difficulty with the accurate sizing and location of defects. At present the noise signal is not avoided by using the pulse-echo method in two-phase titanium alloys. Therefore high noise level is limited in acceptance standards of ultrasonic testing for titanium alloy forgings in order to assure sufficient SNR and prevent the rejected det'ect from being masked. It has been proved that microstructural features have an effect on the noise levelI2- 51. Since these features are determined by hot-working processes, it is important to research further the relationships between forging process, rrlicrostructure and ultrasonic response in order to correctly evaluate the quality of titanium alloy products by ultrasonic testing.
Forging processes The test pieces were reduced by forging to 40-50 mm high cake-shaped specimens with 35 %-55 % deformation, and the surfaces of the specimens were finished at Ra 1.6 #m. The forging processes and post-heat treatments are shown in Table 2.
Ultrasonic testing Testing was carried out using a Krautkramer USIPll flaw detector with probes of 4 MHz, 12 mm diameter and 10 MHz, 5 mm diameter. The directions of the wave beam and the hammering were the same. Ti-6A1-4V annealed alloy standard test blocks with 0.8 mm diameter FBH measured noise levels at 4 MHz and losses of back reflection measured at 4 and 10 MHz were used. The values of noise and loss were measured in decibels.
Micrography examination
Experimental
The transverse section microstructural features were examined with a NEOPHOT 2 microscope. The examined face was perpendicular to the sound testing surface, and the positions of examination and noise signal indicated were uniform.
Materials
Results
Test materials were taken from the nominal two-phase forged alloys Ti-6AI-2.5Mo- 1.5Cr-0.5Fe-0.3Si (TC6) and Ti-6AI-3.0Mo-I.5Zr-0.3Si (TCll'). Their actual compositions are shown in Table 1. The /~ transition temperatures are 970°C (TC6) and 995°C (TC11). Their microstructure was a typical equiaxed structure and the indicated noise level was 12.5% of the response from a 0.8 mm diameter FBH (ie 0.8 FBH - 24 dB).
The results of the tests are shown in Tables 3 and 4 and Figures 1-4.
Noise level As shown in Table 3 and Figure 1, the noise levels are lowest for alloys TC6 and TC11 after =-fl forging. There is an improvement of 4 dB as compared with the original materials.
0308-9126/90/020103-04 © 1990 Butterworth & Co (Publishers) Ltd NDT International Volume 23 Number 2 April 1990
103
Table 1. Compositions (in w t % )
of t i t a n i u m alloys TC6 and TC11
Alloy
AI
Mo
Cr
Zr
Fe
Si
C
N
O
H
TC6 TC11
6.1 9 6.41
2.66 3.45
1.90 --
-1.57
0.30 0.07
0.25 0.29
0.02 <0.1
0.02 0.0063
0.07 0.09
0.01 2 0.01 3
Table 2. Forging processes and p o s t - h e a t t r e a t m e n t s Forging process Test piece number
Type of forging
Heating temperature/time
TC6
1 2 3
c~-/~ /~ Sub-/~
960°C/1 990°C/1 990 °C/1 FC-* 960
TC11
4 5 6
~-/3 /~ SH ~
975°C/1 h 10 min 1015°C/1 h i 0 min 1095°C/1 h 10 min
Alloy
h 20 min h 20 min h 20 min --, °C/45 min
Cool ingb
Post-heat treatmentC
AC AC AC
I I I
AC AC WC
II II II
aSH, super-high-temperature forging bAG, air cooling; WC, water cooling Cl, 920oc/1.5h FCto650°C/2hAC;II, 950 C/1 hAC+530'~C/6hAC Table 3. Ultrasonic response m e a s u r e m e n t results Loss of back reflection (dB)b
Specimen number
Noise level (at 4 MHz) with respect to 0.8 mm FBH ( d B ) a
4 MHz
10 MHz
TC6
1 2 3
-30-+1 -14_+2 -12_+1
0 -1 -1
0 -5 -6
TC11
4 5 6
-28-+1 -22-+2 -19+_1
0 -1 -1
0 -3 0
Alloy
a0.8 mm diameter FBH corresponds to 0 dB bLoss of back reflection is value compared with that of standard test block for same beam path distance
Table 4. A v e r a g e dimensions of m i c r o s t r u c t u r a l features
Alloy TC6
TC11
Specimen number
Microstructural type a
Grain size (#m)
Colony size (/~m)
¢ platelet thickness (#m)
1 2 3
Equiaxial BW BW
-200 + 20 200 -+ 20
30_+5 35-+10
5+1 5_+2
4 5 6
Equiaxial BW BW
-200 _+ 10 210_+ 10
25+5 10+4
4+1 2+1
aBW, basket-weave microstructure
104
NDT International April 1990
rI] -10
-
I
-20
-
Z
-30
-
The noise levels are obviously increased for forging above the fl transition temperature. However, the size of increment is different, the noise levels in TC6 being higher than in TC 11.
TC6
[ ] Tc ,
i
>
i
c¢-13
B
Sub-
D SH
Loss o f back r e f l e c t i o n The results of back reflection loss for the two frequencies are also shown in Table 3. The loss values are negligible at 4 MHz for all forging processes. They are slightly higher at 10 MHz for forging above the fl transition temperature than for ~-fl forging, except for SH processing.
Type of forging
Fig. 1 Comparison of noise levels for titanium alloys TC6 and TC11 after different forging processes
Fig. 2 Typical equiaxed microstructure for titanium alloys TC6 and TC11 after ~-fl forging ( × 250)
Microstructure The microstructural features differ considerably with a change of forging process, as shown in Table 4 and Figures 2-4. After forging in the ~-fl phase region, the microstructures of TC6 and TC11 consist of equiaxed phases with transformed fl between the phases (called an equiaxed structure). The uniformly distributed • phases form ball or polygon structures and their sizes are approximate in every direction (Figure 2). Various basket-weave (BW) structures are obtained after forging above the fl transition temperature in the two alloys. The BW structural feature is very different from the equiaxed structure, in that a platelets are formed along fl grain boundaries, and specifically oriented colonies consisting of :t platelets are banked like a 'basket' and non-uniformly distributed inside the (previously fl) grains (Figures 3 and 4). The dimensions of the BW structure for specimens TC6 and TC11 are given in Table 4. From Table 4 it is seen that the colony sizes and ~ platelet thicknesses are larger in TC6 than in TCll and the platelets are more non-uniformly distributed between colonies. Discussion The results of the tests show that the ultrasonic response is closely related to the microstructural features which are formed by the different forging processes.
Fig. 3 Basket-weave microstructure for titanium alloy TC6 after sub-fl forging ( × 250)
Fig. 4 Basket-weave microstructure for titanium alloy T e l l after super-high-temperature forging ( x 250)
NDT International April 1990
The ultrasound orientation scattering effect is smaller for sound waves travelling in the equiaxed structure tsl. The noise level indicated, therefore, is much lower. Also, since the equiaxed structure is further developed by additional forging in the ~-fl region, it is lower in the test pieces than in the original materials. In various BW structures the specifically oriented colonies and ~ platelets in the grains cause the noise level to increase because more potential scattering faces are formed for sound waves travelling along the ~ colonies and parallel ~ phase boundaries t3-51. The more non-uniformly distributed and the larger the dimensions of colonies and ~ platelets, the higher are the energies of ultrasonic scattering received and the higher are the noise levels. This is why the noise levels in TC6 are higher than in TC1 1 even under the same forging conditions (eg specimens 2 and 5). The results for the loss of back reflection show that higher loss is not related to grain size but to BW microstructural features and their dimensions owing to the higher level of absorption and scattering caused by sound waves travelling along the ~ colonies and ~t phase boundaries. However, the loss decreases when the dimensions are very small, eg specimen 6 (Tables 3 and 4). The wavelength of ultrasound used in this study was approximately 1.5
105
and 0.6 mm respectively. Therefore it can be seen that the effect of colony size on the loss of ultrasound is negligible for colony sizes of 0.006-0.03 of a wavelength and increases for sizes above 0.03 of a wavelength. As discussed above, the ultrasonic response will be changed by different forging processes since it is ultimately decided by the microstructural features. It is possible, however, that a higher noise level caused by an acceptable microstructure being formed by a certain forging process can be unacceptable in accordance with the present inspection standards of ultrasonic testing, eg the case of the sub-fl forging process in TC6. For this reason it is essential to research further the relationships between forging process, microstructure and ultrasonic response and to determine rational acceptance standards of noise level according to different processes for titanium alloy forgings under the precondition of meeting the signal-tonoise ratio requirement.
Conclusions 1. The noise level and loss of back reflection are lowest for titanium alloys TC6 and TC11 after c~-/~ forging. They increase after forging above the /~ transition temperature. The increments of noise level are bigger in TC6 than in TC11. 2. Higher noise level and loss of back reflection are caused by microstructural features formed as a result of the
forging process, ie bigger dimensions of.~ colonies and platelets influence the ultrasonic response.
Acknowledgements The authors wish to thank senior specialists Chao Chunxiao and Zhang Zhifang for their support and valuable suggestions during the course of this work.
References 1 Granville,R.K. and Taylor, J.L 'Improvement in signal-to-noise ratio during the ultrasonic testing of titanium alloys' Br J NDT 4 (1986) 2 Billman, F.R. 'Effects of Ti-6AI-4V alloy metallurgical structures on ultrasonic characteristics' Titanium '72, Science and Technology (1972) pp 693-705 3 Pan, J. 'Effects of microstructural features of titanium alloy bars on ultrasonic response characteristics' Chinese J N D T 9 (1987) 4 Granville,R.K. and Taylor, J.L 'High noise during the ultrasonic testing of titanium alloys' Br J N D T 3 (1985) 5 Ginty,B. 'The influence of microstructure on ultrasonic response in a titanium alloy forging' Titanium '80, Science and Technology 3 (1980) pp 2095-2104
Authors Pan Janhua and Shong Shubo are in the Department of Materials Experiment and Control, Anda Forging Plant, Guizhou, China. Li Jawei is in the Department of Ultrasound Testing, Institute of Aeronautical Materials, Beijing, China.
Paper received 4 August 1989
106
NDT International April 1990