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A Review of Super plastic forming
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 4452–4459
www.materialstoday.com/proceedings
ICMPC 2017
A Review of Super plastic forming Ritam Chatterjeea, Jyoti Mukhopadhyayb* a
b*
Discipline of Mechanical Engineering, Indian Institute of Technology Gandhinagar-382355, India Discipline of Material Science and Engineering, Indian Institute of Technology Gandhinagar-382355, India
Abstract Super plasticity is the property exhibited by a few metals and alloys which involves, under tensile loading, very high elongation without necking until failure. This was first closely studied by Back ofen, Turner and Avery at MIT in 1964 [1]. Their pioneering work has since given rise to the multi-million dollar superplastic forming industry which has been highly successful in fabrication of complex shaped components mainly for the aerospace and automobile industries. The present paper reviews the developments related to superplastic forming that has taken place mainly over the past two decades. The process parameters, underlying mechanisms, constitutive equations and the process economics have been discussed in details. Furthermore, there is a brief discussion about quick-plastic forming which is an upcoming metal forming technique that promises to replace superplastic forming in the near future. This is largely due to the faster forming times associated with quick-plastic forming as compared to superplastic forming. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization. Keywords:Superplasticity; superplastic forming (SPF);constitutive equations;strain rate sensitivity; thinning factor; Quick-plastic forming (QPF)
1. Introduction In 1960’s, Zn-Al alloys were the first materials to exhibit significant superplastic deformation. The formation of a superplastic bubble as shown in Fig.1 was a landmark achievement. This was superseded in 1969 by the introduction of the Al-6%Cu-0.5%Zr alloy i.e. SUPRAL 100 by Tube Investments Research Labs (TIRL) at Cambridge, U.K. The major reason for this change was the significant weight reduction for SUPRAL as well as inferior creep performance of Zn-Al alloys due to which they couldn’t be used for making automobile body panels [1].
* Correponding author E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.
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Figure 1. The first superplastically formed bubble (1964) [1]
The major players in the SPF industry during those days were TIRL, ISC Alloys Ltd. and Superform Company all from U.K and Rockwell International, U.S. The latter organization played a pioneering role in the early 1980’s for the development of SPF Ti alloys, especially products formed using Ti-6Al-4V alloy. Under the leadership of C.H.Hamilton et. al [2], several remarkable structures were fabricated using T-6Al-4V alloy as shown in Fig 2.
Figure 2. Complex structures fabricated by Rockwell Intl., U.S.A [1]
There are three primary requirements for superplasticity: 1. Ultra fine grain size less than 10µm. 2. Forming temperatures greater than 0.5Tm where Tm is the melting temperature of the material. This is to allow reasonably high diffusion processes while the specimen is under tensile load. 3. High value of strain rate sensitivity (m) around 0.5. Ideal superplastically formed alloys should have [1]: 1. Fast forming cycles. 2. Excellent mechanical properties achieved from superplastic forming process. 3. Uniform thinning of the material across all cross sections 4. No requirement for expensive post forming processing.
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Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
The major advantages of SPF components are: • Close Dimensional accuracy. • Excellent surface finish. • Ability for large sized components to be formed in one operation, thereby avoiding the need for numerous sub-assemblies. Hence, there are no interfaces and no defects which normally occur at interfaces. Saves a lot of manufacturing effort and time. • Excellent mechanical properties due to having been formed from ultra-fine equi-axed grains. Some of the commercially available SP alloys are enlisted below in Table 1: Table 1. Major commercially available SP alloys [1]
Titanium Aluminum Magnesium Others
Ti-6Al-4V, Ti6Al2Sn4Zr2Mo, Ti3Al-2.5V, SP700 SP2004 (Supral100), SP7475, SP5083, SP2195 AZ31B, ZK10 INCO718, IN744, NAS65
The only major limitation of SPF is the high forming cycle times. This problem has been alleviated using an intermediate deep drawing step for the alloy sheet prior to the SPF process. This process is known as Quick-plastic forming. Both the manufacturing costs and the forming times have been reduced here due to which it is likely to replace SPF in the near future. A detailed discussion of QPF has been presented in a separate section in the present work. 2. The SPF Process 2.1. The Equipment and process steps
Figure 3. The Traditional Superplastic forming process [3]
Fig.3 demonstrates that there is an upper die with a hole in it. The purpose of the hole is to allow the gas pressure to be introduced into the chamber. The lower die has the profile of the component which is to be formed. The superplastic alloy sheet is kept in between the upper and the lower dies and gas pressure is introduced as shown. There is no punch for this process and gas pressure alone is responsible for shaping the sheet into the desired shape. The pressure is increased slowly so as to maintain optimum strain rate on the sheet so as to have high value of strain rate sensitivity ‘m’. This is important in order to achieve the optimum superplastic elongation and thereby, form complex shaped products having excellent dimensional accuracy and mechanical properties. 2.2. Constitutive Equations Till todate, there is no one equation which encapsulates accurately all the characteristics of a superplastic deformation process. The challenge is to develop constitutive equations that accurately model SP behaviour over a range of states i.e. under: • Slower strain rates and lower temperatures where creep and grain boundary sliding is prevalent. • Faster strain rates, higher temperatures where slip deformation is dominant.
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The gas pressure is the most important variable to be optimized. This is important in order to impose uniform strain all across cross sections and minimize the forming cycle time. By properly controlling the gas forming pressure, we can achieve uniform material thinning across all cross sections and thereby superior mechanical properties. The critical requirements from any constitutive equation are [4]: • The range of validity of the operating parameters say temperature and strain rate. • How the different coefficients and parameters were experimentally obtained. • How robust is the calibration of instruments used in experimental testing which is carried out to arrive at the values of process parameters. Bylya et al. [4] carried out the numerical simulation of a deep drawing + SPF process using metal forming software QForm8. The material chosen was Ti-6Al-4V since there were many constitutive models available for the SPF process of this alloy. Equi-axed grain size ~ 8µm was chosen and the forming temperature was calibrated to 900oc. A comparative simulation of the material behavior was carried out using processing parameters from different models viz. Backofen model (B), Backofen-Avery model (BA), Smirnov model (S) and the Bylya-Blackwell-Valiev model (BBV).
Figure 4. (a) True Stress vs True Strain curves obtained for different constitutive models b) the shape and thickness of the part after preforming, calculated using different models. [4]
It can be observed from Fig 4 (a) and (b) that although the true stress vs true strain curves match closely for the BA, S and the BBV models, there is still some difference in the material thickness especially at point G and region EF. Hence, it is clear from the results that different constitutive models can accurately model the SP behavior in different regions of the stress-strain space. 2.3. Primary Mechanism
Figure 5. Grain Boundary Sliding in (a) coarse grained material (d>λ) and (b) in SP fine grained material (d<λ) [5]
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Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
It is now widely accepted that grain boundary sliding is the underlying mechanism responsible for superplasticity. Here, the grains slide over each other via intragranular slip. In Fig.6, d is the grain size and λ is the equilibrium subgrain size associated with applied stress. For d>λ,the deformation proceeds via general creep. As observed in Fig.5 (a) (b), initially the dislocations pile up at the grain boundary triple point A. Via slip, dislocations move into the adjacent grain and then again pile up at point B which is the immediate sub-grain boundary. Here, dislocation glide from A to B is the rate controlling step. For d<λ, there is no sub-grain boundary encountered after point C. Hence, dislocations directly pile up at the next grain boundary D and the dislocation climb is the rate controlling step [5]. 2.4. Thinning factor for Dome Forming Test Horita et.al. [6] subjected cylindrical rods of an Al-3% Mg-0.2% Sc alloy to ECA pressing. The resultant grain size achieved was 0.2µm. Disks were cut from these rods and then subjected to superplastic biaxial gas forming at 10atm, 673K and up to a forming time of at max 60s to achieve smooth domes as shown in Fig.6. The strain rates were applied so as to maintain strain rate sensitivity ~ 0.5 which is important for SP deformation.
Figure 6. ECA pressed disk (left) being subjected to SPF using Argon gas for, 30s (center) 60s (right) [6]
As mentioned earlier, an ideal superplastic alloy sheet would have uniform thinning of the material across all cross sections. To measure the thinning at different points of the dome, a ‘thinning factor’ has been defined by Cornfield and Johnson [6]. Thinning factor = Measured thickness at any point of the dome Average thickness which would be achieved if the volume stays constant and the sheet is deformed uniformly. The denominator in the above formula is the ratio of the height of the dome at that point to the dimension of the base of hemisphere.
Figure 7. (a) Schematic diagram of angular increments to measure dome thickness at different points (b) Variation of the local thickness with the angular position around the dome for the two domes as shown in Fig. 6. [6]
At the pole, the thinning factor was measured to be 0.89. The values at all other points were found to be between 0.89 and 1. Hence, the thinning at all points was nearly uniform. Also as shown in Fig.7 (a), the dome was
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divided into angular intervals such that the pole was marked 0 degree and the ends were marked 90 degree. In Fig.7 (b), the measured thicknesses were plotted with these angular values and it can be clearly observed that there is nearly uniform thinning across all cross sections of the dome, especially after 60s. Hence, it is proved that for high value of strain rate sensitivity, the material thinning during SP deformation is uniform across all cross sections [6]. 2.5. The Economics of SPF Ceschini and Afrikantov [8] have studied the economic feasibility of SPF with respect to other manufacturing techniques as well as many influential process parameters such as SPF times, material prices, press prices, die prices and the die life values. Their findings are well encapsulated in Fig.8. (a) to (f).
Figure 8. (a) Economic Competitiveness of SPF with respect to other processes. Variation between Production efficiency and production quantity with different (b) SPF times (c) Press prices (d) Material prices (e) Die prices (f) Die Life Values [7]
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Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
From Fig.8 (a), it can be observed that on comparison with machining and drawing processes, SPF is an economically viable option mainly if the production quantity is roughly in the range of 100 to 10000 [8]. In this range, the cost incurred per product i.e. unit cost is lower as compared to the other manufacturing processes. From Fig.8 (b), it can be inferred that as the SPF cycle time increases, less number of products can be formed at high process efficiency. From Fig 8.(c), (d) and (e), it can be inferred that an increase in the prices of presses, materials and dies respectively always leads to a reduction in product quantity while maintaining the same level of process efficiency. And finally, it can be concluded from Fig 8. (f) that there is no significant change in product quantity while maintaining the same level of process efficiency for different die life values. 3. The QPF Process Quick Plastic Forming is a relatively new technique which includes an intermediate hot drawing step prior to SPF using gas pressure. It takes advantage of the high speed of hot drawing and the excellent formability of superplastic materials. The result is that there is decrease in the overall forming cycle time while maintaining the excellent product quality as achieved using SPF. A schematic diagram is shown in Fig.9 (a) (b) and (c).
Figure 9. A schematic diagram of QPF with (a) Heating and Clamping (b) Hot drawing to a certain depth (c) SPF using Argon gas [9]
QPF has larger advantages over SPF: • Hot drawing does not require temperatures as high as SPF. This greatly brings down the processing cost. • There is no requirement for superplastic grade alloy sheet. Ordinary alloy sheet can be used since the first step is hot drawing and not SP deformation. Hence, further reduction in processing cost. [3] • There is uniform material thinning across all cross sections of the component due to the pre-forming step i.e. hot drawing. [3] • The forming times are reduced by about 40% due to the intermediate hot drawing step. 3.1. Micro-structural Evolution across different steps of QPF Mei-Ling Guo et. al. [10] carried out Electron Backscatter Diffraction (EBSD) analysis to observe the microstructural evolution across different QPF steps for a non-superplastic grade Ti-6Al-4V sheet.
It can be inferred that although the grain size marginally decreases after the hot drawing step, there is little variation in the percentage of HAB’s (High Angle Grain Boundaries). Thereafter, the percentage of HAB’s
Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
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increases across the gas forming steps and the aspect ratio decreases, thereby signifying the formation of equiaxed grains and also, recrystallization being the primary deformation mechanism. 4. Conclusions The following conclusions are hereby drawn: • Superplastic forming is an excellent technique for forming complex shaped components with smooth curvature, high dimensional accuracy and excellent mechanical properties. • The major limitation of SPF is the slow forming time due to gradual increase in forming pressure. This is alleviated by including an intermediate hot drawing step prior to SPF using gas pressure. This process is known as Quick Plastic forming and offers tremendous advantages over Superplastic forming. • Different constitutive equations are valid over certain ranges of applied strain rates and flow stresses. There is no universal equation to accurately predict the superplastic behaviour across different process parameters. • It is critical to control the applied strain rate on the material in order to have high values of strain rate sensitivity in order to achieve uniform thinning across different cross sections of the product. • Superplastic forming is economically viable only in a certain range of production quantity. Increase in prices of dies, materials and presses adversely affects the economic viability. 5. Acknowledgement I would like to thank my mentor Dr. Jyoti Mukhopadhyay of Indian Institute of Technology, Gandhinagar for his valuable inputs and able guidance without which this review paper could not have been satisfactorily written. 6. References [1] Barnes A.J, Journal of Materials Engineering and Performance, Volume 16, Issue 4 (2007) 440-454. [2] Ghosh. Ak, Hamilton. Ch, Net Shape Technology in Aerospace, Vol 36, No 2, April 1986, pp 153-177 [3] W. Guofeng, S. Chao, L. Shufen,Y. Mo, Materialwissenschaft und Werkstofftechnik, Volume 45, Issue 9 (2014) 854-859 [4] O.I. Bylya, R.A. Vasin & P.L. Blackwell, 10.4028/www.scientific.net/MSF.838-839.468 [5] Langdon, Terence G., Materials Science Forum, Volume 838-839 (2016) 3-12. [6] Z. Horita, M. Furukawa, M. Nemoto, A. J. Barnes and T. G. Langdon, Acta Materialia, Volume 48, Issue 9 (2000) 3633-3640 [7] Ceschini. L, Afrikantov. A, Metallurgical Science and technology, Volume 10, Issue 3 (1992) 41-55 [8] Yan Ma, Minoru Furukawa, Z. Horita, M. Nemoto, Ruslan J.Valiev, T. G. Langdon, Material Transactions, JIM, Vol.37,No.3 (1996) pp. 336 to 339 [9] Mei-Ling Guo, Jun Liu, Ming-Jen Tan, Beng-Wah Chua, Procedia Engineering, Volume 81 (2014) 1090-1095 [10] Jun Liu, Ming-Jen Tan, Chao-Voon S. Lim, Beng-Wah Chua, International Journal of Advanced Manufacturing Technology, Volume 69, Issue 9-12 (2013) 2415-2422
ScienceDirect Materials Today: Proceedings 5 (2018) 4452–4459
www.materialstoday.com/proceedings
ICMPC 2017
A Review of Super plastic forming Ritam Chatterjeea, Jyoti Mukhopadhyayb* a
b*
Discipline of Mechanical Engineering, Indian Institute of Technology Gandhinagar-382355, India Discipline of Material Science and Engineering, Indian Institute of Technology Gandhinagar-382355, India
Abstract Super plasticity is the property exhibited by a few metals and alloys which involves, under tensile loading, very high elongation without necking until failure. This was first closely studied by Back ofen, Turner and Avery at MIT in 1964 [1]. Their pioneering work has since given rise to the multi-million dollar superplastic forming industry which has been highly successful in fabrication of complex shaped components mainly for the aerospace and automobile industries. The present paper reviews the developments related to superplastic forming that has taken place mainly over the past two decades. The process parameters, underlying mechanisms, constitutive equations and the process economics have been discussed in details. Furthermore, there is a brief discussion about quick-plastic forming which is an upcoming metal forming technique that promises to replace superplastic forming in the near future. This is largely due to the faster forming times associated with quick-plastic forming as compared to superplastic forming. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization. Keywords:Superplasticity; superplastic forming (SPF);constitutive equations;strain rate sensitivity; thinning factor; Quick-plastic forming (QPF)
1. Introduction In 1960’s, Zn-Al alloys were the first materials to exhibit significant superplastic deformation. The formation of a superplastic bubble as shown in Fig.1 was a landmark achievement. This was superseded in 1969 by the introduction of the Al-6%Cu-0.5%Zr alloy i.e. SUPRAL 100 by Tube Investments Research Labs (TIRL) at Cambridge, U.K. The major reason for this change was the significant weight reduction for SUPRAL as well as inferior creep performance of Zn-Al alloys due to which they couldn’t be used for making automobile body panels [1].
* Correponding author E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.
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Figure 1. The first superplastically formed bubble (1964) [1]
The major players in the SPF industry during those days were TIRL, ISC Alloys Ltd. and Superform Company all from U.K and Rockwell International, U.S. The latter organization played a pioneering role in the early 1980’s for the development of SPF Ti alloys, especially products formed using Ti-6Al-4V alloy. Under the leadership of C.H.Hamilton et. al [2], several remarkable structures were fabricated using T-6Al-4V alloy as shown in Fig 2.
Figure 2. Complex structures fabricated by Rockwell Intl., U.S.A [1]
There are three primary requirements for superplasticity: 1. Ultra fine grain size less than 10µm. 2. Forming temperatures greater than 0.5Tm where Tm is the melting temperature of the material. This is to allow reasonably high diffusion processes while the specimen is under tensile load. 3. High value of strain rate sensitivity (m) around 0.5. Ideal superplastically formed alloys should have [1]: 1. Fast forming cycles. 2. Excellent mechanical properties achieved from superplastic forming process. 3. Uniform thinning of the material across all cross sections 4. No requirement for expensive post forming processing.
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Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
The major advantages of SPF components are: • Close Dimensional accuracy. • Excellent surface finish. • Ability for large sized components to be formed in one operation, thereby avoiding the need for numerous sub-assemblies. Hence, there are no interfaces and no defects which normally occur at interfaces. Saves a lot of manufacturing effort and time. • Excellent mechanical properties due to having been formed from ultra-fine equi-axed grains. Some of the commercially available SP alloys are enlisted below in Table 1: Table 1. Major commercially available SP alloys [1]
Titanium Aluminum Magnesium Others
Ti-6Al-4V, Ti6Al2Sn4Zr2Mo, Ti3Al-2.5V, SP700 SP2004 (Supral100), SP7475, SP5083, SP2195 AZ31B, ZK10 INCO718, IN744, NAS65
The only major limitation of SPF is the high forming cycle times. This problem has been alleviated using an intermediate deep drawing step for the alloy sheet prior to the SPF process. This process is known as Quick-plastic forming. Both the manufacturing costs and the forming times have been reduced here due to which it is likely to replace SPF in the near future. A detailed discussion of QPF has been presented in a separate section in the present work. 2. The SPF Process 2.1. The Equipment and process steps
Figure 3. The Traditional Superplastic forming process [3]
Fig.3 demonstrates that there is an upper die with a hole in it. The purpose of the hole is to allow the gas pressure to be introduced into the chamber. The lower die has the profile of the component which is to be formed. The superplastic alloy sheet is kept in between the upper and the lower dies and gas pressure is introduced as shown. There is no punch for this process and gas pressure alone is responsible for shaping the sheet into the desired shape. The pressure is increased slowly so as to maintain optimum strain rate on the sheet so as to have high value of strain rate sensitivity ‘m’. This is important in order to achieve the optimum superplastic elongation and thereby, form complex shaped products having excellent dimensional accuracy and mechanical properties. 2.2. Constitutive Equations Till todate, there is no one equation which encapsulates accurately all the characteristics of a superplastic deformation process. The challenge is to develop constitutive equations that accurately model SP behaviour over a range of states i.e. under: • Slower strain rates and lower temperatures where creep and grain boundary sliding is prevalent. • Faster strain rates, higher temperatures where slip deformation is dominant.
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The gas pressure is the most important variable to be optimized. This is important in order to impose uniform strain all across cross sections and minimize the forming cycle time. By properly controlling the gas forming pressure, we can achieve uniform material thinning across all cross sections and thereby superior mechanical properties. The critical requirements from any constitutive equation are [4]: • The range of validity of the operating parameters say temperature and strain rate. • How the different coefficients and parameters were experimentally obtained. • How robust is the calibration of instruments used in experimental testing which is carried out to arrive at the values of process parameters. Bylya et al. [4] carried out the numerical simulation of a deep drawing + SPF process using metal forming software QForm8. The material chosen was Ti-6Al-4V since there were many constitutive models available for the SPF process of this alloy. Equi-axed grain size ~ 8µm was chosen and the forming temperature was calibrated to 900oc. A comparative simulation of the material behavior was carried out using processing parameters from different models viz. Backofen model (B), Backofen-Avery model (BA), Smirnov model (S) and the Bylya-Blackwell-Valiev model (BBV).
Figure 4. (a) True Stress vs True Strain curves obtained for different constitutive models b) the shape and thickness of the part after preforming, calculated using different models. [4]
It can be observed from Fig 4 (a) and (b) that although the true stress vs true strain curves match closely for the BA, S and the BBV models, there is still some difference in the material thickness especially at point G and region EF. Hence, it is clear from the results that different constitutive models can accurately model the SP behavior in different regions of the stress-strain space. 2.3. Primary Mechanism
Figure 5. Grain Boundary Sliding in (a) coarse grained material (d>λ) and (b) in SP fine grained material (d<λ) [5]
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Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
It is now widely accepted that grain boundary sliding is the underlying mechanism responsible for superplasticity. Here, the grains slide over each other via intragranular slip. In Fig.6, d is the grain size and λ is the equilibrium subgrain size associated with applied stress. For d>λ,the deformation proceeds via general creep. As observed in Fig.5 (a) (b), initially the dislocations pile up at the grain boundary triple point A. Via slip, dislocations move into the adjacent grain and then again pile up at point B which is the immediate sub-grain boundary. Here, dislocation glide from A to B is the rate controlling step. For d<λ, there is no sub-grain boundary encountered after point C. Hence, dislocations directly pile up at the next grain boundary D and the dislocation climb is the rate controlling step [5]. 2.4. Thinning factor for Dome Forming Test Horita et.al. [6] subjected cylindrical rods of an Al-3% Mg-0.2% Sc alloy to ECA pressing. The resultant grain size achieved was 0.2µm. Disks were cut from these rods and then subjected to superplastic biaxial gas forming at 10atm, 673K and up to a forming time of at max 60s to achieve smooth domes as shown in Fig.6. The strain rates were applied so as to maintain strain rate sensitivity ~ 0.5 which is important for SP deformation.
Figure 6. ECA pressed disk (left) being subjected to SPF using Argon gas for, 30s (center) 60s (right) [6]
As mentioned earlier, an ideal superplastic alloy sheet would have uniform thinning of the material across all cross sections. To measure the thinning at different points of the dome, a ‘thinning factor’ has been defined by Cornfield and Johnson [6]. Thinning factor = Measured thickness at any point of the dome Average thickness which would be achieved if the volume stays constant and the sheet is deformed uniformly. The denominator in the above formula is the ratio of the height of the dome at that point to the dimension of the base of hemisphere.
Figure 7. (a) Schematic diagram of angular increments to measure dome thickness at different points (b) Variation of the local thickness with the angular position around the dome for the two domes as shown in Fig. 6. [6]
At the pole, the thinning factor was measured to be 0.89. The values at all other points were found to be between 0.89 and 1. Hence, the thinning at all points was nearly uniform. Also as shown in Fig.7 (a), the dome was
Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
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divided into angular intervals such that the pole was marked 0 degree and the ends were marked 90 degree. In Fig.7 (b), the measured thicknesses were plotted with these angular values and it can be clearly observed that there is nearly uniform thinning across all cross sections of the dome, especially after 60s. Hence, it is proved that for high value of strain rate sensitivity, the material thinning during SP deformation is uniform across all cross sections [6]. 2.5. The Economics of SPF Ceschini and Afrikantov [8] have studied the economic feasibility of SPF with respect to other manufacturing techniques as well as many influential process parameters such as SPF times, material prices, press prices, die prices and the die life values. Their findings are well encapsulated in Fig.8. (a) to (f).
Figure 8. (a) Economic Competitiveness of SPF with respect to other processes. Variation between Production efficiency and production quantity with different (b) SPF times (c) Press prices (d) Material prices (e) Die prices (f) Die Life Values [7]
4458
Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
From Fig.8 (a), it can be observed that on comparison with machining and drawing processes, SPF is an economically viable option mainly if the production quantity is roughly in the range of 100 to 10000 [8]. In this range, the cost incurred per product i.e. unit cost is lower as compared to the other manufacturing processes. From Fig.8 (b), it can be inferred that as the SPF cycle time increases, less number of products can be formed at high process efficiency. From Fig 8.(c), (d) and (e), it can be inferred that an increase in the prices of presses, materials and dies respectively always leads to a reduction in product quantity while maintaining the same level of process efficiency. And finally, it can be concluded from Fig 8. (f) that there is no significant change in product quantity while maintaining the same level of process efficiency for different die life values. 3. The QPF Process Quick Plastic Forming is a relatively new technique which includes an intermediate hot drawing step prior to SPF using gas pressure. It takes advantage of the high speed of hot drawing and the excellent formability of superplastic materials. The result is that there is decrease in the overall forming cycle time while maintaining the excellent product quality as achieved using SPF. A schematic diagram is shown in Fig.9 (a) (b) and (c).
Figure 9. A schematic diagram of QPF with (a) Heating and Clamping (b) Hot drawing to a certain depth (c) SPF using Argon gas [9]
QPF has larger advantages over SPF: • Hot drawing does not require temperatures as high as SPF. This greatly brings down the processing cost. • There is no requirement for superplastic grade alloy sheet. Ordinary alloy sheet can be used since the first step is hot drawing and not SP deformation. Hence, further reduction in processing cost. [3] • There is uniform material thinning across all cross sections of the component due to the pre-forming step i.e. hot drawing. [3] • The forming times are reduced by about 40% due to the intermediate hot drawing step. 3.1. Micro-structural Evolution across different steps of QPF Mei-Ling Guo et. al. [10] carried out Electron Backscatter Diffraction (EBSD) analysis to observe the microstructural evolution across different QPF steps for a non-superplastic grade Ti-6Al-4V sheet.
It can be inferred that although the grain size marginally decreases after the hot drawing step, there is little variation in the percentage of HAB’s (High Angle Grain Boundaries). Thereafter, the percentage of HAB’s
Ritam Chatterjee et al./ Materials Today: Proceedings 5 (2018) 4452–4459
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increases across the gas forming steps and the aspect ratio decreases, thereby signifying the formation of equiaxed grains and also, recrystallization being the primary deformation mechanism. 4. Conclusions The following conclusions are hereby drawn: • Superplastic forming is an excellent technique for forming complex shaped components with smooth curvature, high dimensional accuracy and excellent mechanical properties. • The major limitation of SPF is the slow forming time due to gradual increase in forming pressure. This is alleviated by including an intermediate hot drawing step prior to SPF using gas pressure. This process is known as Quick Plastic forming and offers tremendous advantages over Superplastic forming. • Different constitutive equations are valid over certain ranges of applied strain rates and flow stresses. There is no universal equation to accurately predict the superplastic behaviour across different process parameters. • It is critical to control the applied strain rate on the material in order to have high values of strain rate sensitivity in order to achieve uniform thinning across different cross sections of the product. • Superplastic forming is economically viable only in a certain range of production quantity. Increase in prices of dies, materials and presses adversely affects the economic viability. 5. Acknowledgement I would like to thank my mentor Dr. Jyoti Mukhopadhyay of Indian Institute of Technology, Gandhinagar for his valuable inputs and able guidance without which this review paper could not have been satisfactorily written. 6. References [1] Barnes A.J, Journal of Materials Engineering and Performance, Volume 16, Issue 4 (2007) 440-454. [2] Ghosh. Ak, Hamilton. Ch, Net Shape Technology in Aerospace, Vol 36, No 2, April 1986, pp 153-177 [3] W. Guofeng, S. Chao, L. Shufen,Y. Mo, Materialwissenschaft und Werkstofftechnik, Volume 45, Issue 9 (2014) 854-859 [4] O.I. Bylya, R.A. Vasin & P.L. Blackwell, 10.4028/www.scientific.net/MSF.838-839.468 [5] Langdon, Terence G., Materials Science Forum, Volume 838-839 (2016) 3-12. [6] Z. Horita, M. Furukawa, M. Nemoto, A. J. Barnes and T. G. Langdon, Acta Materialia, Volume 48, Issue 9 (2000) 3633-3640 [7] Ceschini. L, Afrikantov. A, Metallurgical Science and technology, Volume 10, Issue 3 (1992) 41-55 [8] Yan Ma, Minoru Furukawa, Z. Horita, M. Nemoto, Ruslan J.Valiev, T. G. Langdon, Material Transactions, JIM, Vol.37,No.3 (1996) pp. 336 to 339 [9] Mei-Ling Guo, Jun Liu, Ming-Jen Tan, Beng-Wah Chua, Procedia Engineering, Volume 81 (2014) 1090-1095 [10] Jun Liu, Ming-Jen Tan, Chao-Voon S. Lim, Beng-Wah Chua, International Journal of Advanced Manufacturing Technology, Volume 69, Issue 9-12 (2013) 2415-2422