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Multiple passes of friction stir processing for the creation of superplastic 7075 aluminum
Materials Science and Engineering A 464 (2007) 255–260
Multiple passes of friction stir processing for the creation of superplastic 7075 aluminum L.B. Johannes, R.S. Mishra ∗ Department of Materials Science and Engineering, University of Missouri-Rolla, Rolla, MO 65401, United States Received 15 November 2006; received in revised form 30 January 2007; accepted 31 January 2007
Abstract A staggered pass sample of friction stir processed (FSP) 7075 aluminum was created to make samples with one through four passes of FSP under identical conditions. The tensile testing temperatures ranged from 673 to 763 K with initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1 . Materials processed by single as well as multiple passes exhibited superplasticity across various testing temperatures and strain rates while the as received materials exhibited elongations below 200%. This study demonstrated the effectiveness of four consecutive FSP passes in creating large areas of superplastic material. However, the largest elongations were observed for the single pass material. © 2007 Elsevier B.V. All rights reserved. Keywords: Friction stir processing; Aluminum alloy; Superplasticity
1. Introduction Friction stir processing (FSP) is a new solid state technique which uses the principles of friction stir welding [1] to process materials in a variety of other ways besides joining them. In FSP, a tool containing a shoulder and a pin provides frictional heating and mechanical mixing in the area covered by the tool. In addition, the large processing strain results in microstructural refinement and homogenization. FSP has developed into a broad field covering microforming [2], casting modification [3], powder processing [4], and channelling [5]. Another area in which FSP shows a lot of promise is in the creation of superplastic materials [6]. FSP creates a region called the ‘nugget’ where the microstructural refinement occurs with equiaxed grains containing high angle grain boundaries. The resultant microstructure in the nugget region can present the ideal conditions for superplasticity in some materials. There are many advantages to superplastic forming as it is a cost effective, near net shape forming process that can easily be adapted into commercial applications. As a result, there has been extensive research in the field for ways to deform metals to elongations greater than 200%, particularly if this can be
∗
Corresponding author. Tel.: +1 573 341 6361; fax: +1 573 341 6934. E-mail address: [email protected] (R.S. Mishra).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.141
done at commercially important high strain rates (>10−2 s−1 ). A significant amount of the research being performed in the area of superplastic forming involves thermo-mechanical processing (TMP) of standard alloys in order to make the material superplastic. TMP involves controlled recovery, recrsytallization, and grain growth of alloys through processes such as forging, rolling, and extrusion [7]. Complex rolling processes and equal channel angular extrusion are common methods for creating superplastic material. Mishra et al. [8] were the first to use FSP as TMP for superplasticity where they demonstrated FSP as a simple technique to enhance grain size dependent properties such as superplasticity in 7075 Al where elongations >1000% were seen in the FSPed alloy at 736 K at a strain rate of 1 × 10−2 s−1 . This work was followed by a similar study which also showed that FSP significantly enhances the superplastic properties of 7075 Al for two different grain sizes [9]. In the sample with the 3.8 m grain size, a maximum elongation of 1440% was achieved at 753 K and 3 × 10−3 s−1 . When the grain size was 7.5 m, the elongation was found to be 1040% at 773 K and 3 × 10−3 s−1 . After it was well established that using a single pass of FSP on 7075 could produce superplastic material, multiple passes began being performed to create greater areas of superplastic material in order to demonstrate the usefulness of the process on a larger scale [10] followed by punch forming studies to determine the effectiveness of multiple passes on the macroscale [11].
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Table 1 Chemical composition of the 7075 Al alloy used in this study Element
Measured Nominal
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
0.11 < 0.40
0.23 <0.50
1.47 1.2–2.0
0.03 <0.30
2.54 2.1–2.9
0.19 0.18–0.28
5.48 5.1–6.1
0.03 <0.20
The current study examines the effect of multiple passes on 7075 Al by adding one pass at a time to a total of four passes. In doing so, the properties of single pass FSP can be compared with FSP for up to four passes in order to help determine the effectiveness of multiple passes in creating superplastic material. 2. Experimental Plates of 7075 Al with the composition listed in Table 1 and 6.4 mm in thickness were used in the as-rolled condition. Fig. 1 shows the configuration used to create samples of one through four passes of FSP. To create this sample, a pass of FSP was run the entire length of the 50 cm plate. The second pass was run three quarters of the length of the plate and the tool was moved towards the advancing side. The third pass was run half the length of the plate and the fourth pass a quarter of the length, both towards the advancing side. In doing this, the effect of multiple passes on the previous and subsequent passes could be determined by looking at samples of one to four passes created under identical conditions. All of the separations between passes were 6.4 mm which resulted in a 42% nugget overlap. The FSP conditions were 400 rpm and 50 mm/min in all passes. The FSP was performed using a high-strength cobalt alloy (MP159) tool with a pin 6.4 mm in diameter and length. The shoulder on the
tool was flat and 25 mm in diameter. The tool pin was threaded with a right-hand screw and the rotation was counterclockwise. Metallographic cross-sections etched with Keller’s reagent were examined using an optical microscope to determine the grain size of the various passes by the mean linear intercept technique (average spatial grain size = 1.78 × mean linear intercept). The microstructure was also observed by using a Philips EM430T (Mahwah, NJ) transmission electron microscope (TEM) at 300 kV using jet-polished foils from the centers of passes in the samples. To evaluate superplastic properties, samples were taken at the center of each nugget on the staggered pass samples. From these areas, dog-bone shaped tensile specimens with a gage length of 1.3 mm and a gage width of 1.0 mm were electro discharge machined (EDM) from the FSP material in the transverse direction. The tensile specimens were polished to a final thickness of 0.5 mm and 1 m surface finish. Testing was conducted using a custom-built, computer-controlled mini tensile tester operable at constant cross-head speeds. The tensile testing temperatures ranged from 673 to 763 K with initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1 . 3. Results and discussion 3.1. Microstructure The average grain size was determined for the various passes, and Table 2 shows the results for nugget centers for each of the passes. From this table it can be seen that the grain sizes of the various passes are all within the same range, between 3.6 and 5.4 m with a majority of the grain sizes being around 4.6 m. The microstructures seen in the last pass of each of the one through four pass samples can be seen in Fig. 2. The last pass in all four samples appears to have similar grain sizes, as shown in Table 2, although some of the grains in the four-pass sample appear to be a little larger. TEM images of the grains at three locations in the multiple pass samples can be seen in Fig. 3. The grains shown again are all similar in size although the number of precipitates observed in the images varies. Thus, from the analysis of the multiple passes from the four passes in Table 2 Summary of average grain sizes at various locations Pass 1 (m)
Fig. 1. Configuration used to create one through four pass samples under identical conditions. Each pass consumes the advancing side of the nugget of the previous pass.
One pass sample Two-pass sample Three-pass sample Four-pass sample
4.7 3.6 4.1 4.5
± ± ± ±
2.3 1.9 2.2 2.3
Pass 2 (m)
Pass 3 (m)
Pass 4 (m)
– 4.6 ± 2.1 4.6 ± 2.4 4.8 ± 2.5
– – 4.6 ± 2.6 4.9 ± 2.2
– – – 5.4 ± 2.7
L.B. Johannes, R.S. Mishra / Materials Science and Engineering A 464 (2007) 255–260
257
Fig. 2. Microstructures found in the last pass in the one through four pass samples (a)–(d). Their locations are denoted by the corresponding letter in the adjacent image.
this study, the grain sizes achieved seem to be in the same range for the various passes. Dutta et al. [11] found that in a nine-pass sample, the average grain size was 6.1 m. This suggests that if multiple passes are going to be used there may be some grain coarsening due to the additional thermal cycles on the plate. 3.2. Tensile behavior Fig. 4 shows the elongations found for various regions in the sample compared to the elongations found in the unprocessed material for the temperature range of 673–763 K with an initial strain rate of 1 × 10−2 s−1 . From this graph it can be noted that the unprocessed material was not superplastic (>200% elongation) at any of the temperatures tested but the FSPed material was superplastic across all of the temperatures for all the nugget regions in the samples. The highest elongation of 1255% was found in the one pass sample at 743 K. A comparison of various single pass samples of FSP 7075 with the current single pass portion of the one through four pass
Fig. 4. Tensile elongations for FSPed 7075 at various locations for different temperatures at 1 × 10−2 s−1 .
Fig. 3. Individual grains from the single pass (a), first of four pass (b), and fourth of four pass (c) samples. The grains all appear similar in size but the number of precipitates seen varies.
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Table 3 Comparison of elongations in single pass studies for FSP 7075 Al Grain size (m)
Temperature (K)
Strain rate (s−1 )
3.3 3.8 7.5 4.7
763 753 773 743
1 × 10−2 3 × 10−3 3 × 10−3 1 × 10−2
Elongation (%) >1000 1440 1040 1225
Reference [8] [9] [9] Current
samples can be seen in Table 3. From this table it can be seen that the single pass portion of this sample is a good representation of the one pass work that has been previously performed and tensile elongation of over 1000% were easily achieved. This study, however, went beyond the previous studies in that the effects of multiple passes were examined. Since multiple passes will be needed for the implementation of selective superplasticity, they were further examined and compared to the single pass, also referred to as “Pass 1 of 1” which is also the first and last pass of the one pass sample. The strain rate of 1 × 10−2 s−1 is commercially important because high strain rate superplasticity occurs in the 1 × 10−2 to 1 × 10−1 s−1 range. An interesting thing to note in Fig. 4 is that the single pass sample has the highest elongation at every temperature. This shows that while single pass studies may have good results, the multiple pass samples will have slightly decreased superplasticity. However, the elongations achieved in the multiple pass samples are still all in the superplastic regime and much better than the elongations found in the unprocessed material which showed no superplasticity at any of the temperatures tested. From these results it can be inferred that using multiple passes of friction stir processing will enhance the superplastic properties of the area. This was examined in a study by Dutta et al. [11] in which nine pass samples were created and punch formed to show the enhanced formability of the multiple pass FSP samples in 7075 on a macroscopic scale using the overall area of the nine passes. The temperature at which each sample achieved the highest elongation was used to test samples at various strain rates. Fig. 5 shows the samples that were taken from the first pass of each of the samples. Fig. 6 shows the elongations in the last pass of each of the one through four pass samples. It should be noted that in the one pass sample the first and last pass are the same. From these graphs it can be seen that in the one to three pass samples, the greatest elongations were achieved at 1 × 10−2 s−1 . It is interesting to note that in both the first and the last pass of the four-pass sample, the elongation was higher at lower strain rates than at 1 × 10−2 s−1 . These graphs also show that at these optimum testing temperatures the multiple pass samples are all superplastic (elongations >200%) for the range of strain rates at which they were tested, 1 × 10−3 to 1 × 10−1 s−1 . Again, the single pass samples typically have the highest elongations. Using the stress–strain rate plots, the strain rate sensitivity (m) was calculated. Figs. 7 and 8 show the stress–strain rate plots of the first and the last passes of the one through four pass samples, respectively. The strain rate sensitivities of the various samples are summarized in Table 4. Strain rate sensitivity is an
Fig. 5. Elongations achieved for the various strain rates tested for the first pass in each of the one through four pass samples.
Fig. 6. Elongations achieved for the various strain rates tested for the last pass in each of the one through four pass samples.
important indicator for the level of elongation a material can achieve. Low m values lead to strain localization in the neck, resulting in necking in the material being tested which results in lower total strain to failure [12]. From Table 4, it can seen that the strain rate sensitivities for the various locations tested are all around m = 0.5, the condition for grain boundary sliding dominating as the flow process. It has been determined that the constitutive relationship for superplasticity, specifically in fine-grained aluminium, can be Table 4 Summary of strain rate sensitivities for the one through four pass samples
First pass Last pass
One pass sample
Two-pass sample
Three-pass sample
Four-pass sample
0.51 0.51
0.63 0.55
0.55 0.56
0.53 0.58
L.B. Johannes, R.S. Mishra / Materials Science and Engineering A 464 (2007) 255–260
Fig. 7. Stress vs. strain rate of the first pass of each of the one through four pass samples used to determine strain rate sensitivity.
expressed as [13]: 2 D0 Eb 84000 σ − σ0 2 b ε˙ = 40 exp − kT RT d E
(1)
where D0 is the pre-exponential constant for diffusivity, E the Young’s modulus, b the Burger’s vector, k Boltzmann’s constant, T the absolute temperature, R the gas constant, d the grain diameter, σ the applied stress, and σ 0 is the threshold stress. In examining this equation, the importance of fine grain size, one of the prerequisites for superplasticity, can be noted. As the grain size is decreased, the strain rate at which superplastic forming can occur is increased. It can also be seen that as the grain size is decreased the forming temperature at which superplastic forming occurs can also be decreased. Eq. (1) also predicts that as the grain size is decreased the flow stress will be decreased which is also quite advantageous for superplas-
Fig. 8. Strain rate sensitivities of the last pass of each of the one through four pass samples.
259
Fig. 9. The variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the single pass FSPed 7075 Al from this study. The solid line represents the best fit for the data while the dashed and dotted line represent the relationships found in previous studies by Ma et al. [9] and Mishra et al. [13], respectively.
tic forming. Therefore, obtaining smaller grain size is always advantageous for superplasticity. In this equation, a stress exponent of 2 is associated with grain boundary sliding (GBS) related mechanisms for superplastic deformation [12]. For the various locations in the multiple pass samples that were tested, a stress exponent of ∼2 was observed indicating that the primary mechanism of deformation is GBS throughout the passes. Ma et al. [9] examined the superplasticity of FSPed 7075 in material with two different grain sizes, 3.8 and 7.5 m. In this analysis, it was found that the dimensionless constant that fit the data for the FSP alloys was much larger than that found in Eq. (1). Rather that it being 40, it was found that the constant was 790 by plotting (˙εkTd 2 /Dg Eb3 ) versus (σ/E). In the current study, the single pass data had a grain size of 4.7 m. When this data is plotted as (˙εkTd 2 /Dg Eb3 ) versus ((σ − σ 0 )/E), the constant becomes 1396. Fig. 9 shows a comparison of the single pass data from this study and compared the constant found with this data compared to that of Ma et al. [9] and Eq. (1) [13]. To expand this analysis into the multiple pass regime, two plots were created—one using the data from the first pass of the one through four pass samples and one using the data from the last pass of the one through four pass samples. Fig. 10 shows the data from the first passes and Fig. 11 shows the data from the last passes. When the data was plotted, new values of the dimensionless constant were calculated for the two sets of data. The line that fits the data in Fig. 10 uses a dimensionless constant of 1134. Eq. (1) is also plotted on this graph for comparison using a dotted line. The constant for Fig. 11, the last passes of the one through four pass samples, was found to be 1334 and can be compared to Eq. (1) which is represented by the dotted line. The data is found to have the same parametric dependencies as previous studies but with the constants being similar to the one found in the current study for the single pass data. All these values of the dimensionless constant suggest that the kinetics
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4. Conclusions 1. The effectiveness of friction stir processing in creating superplastic material can be extended to multiple passes to create larger areas of material with superplastic properties. 2. The elongations achieved in multiple passes of 7075 Al are superplastic, although the single pass material exhibits slightly greater elongations. 3. Grain boundary sliding is the primary mechanism for superplastic deformation of multiple pass samples of 7075 Al. Acknowledgements The authors gratefully acknowledge the support of the National Science Foundation through the grant DMI – 0323725, Mary Lynn Realff, program manager. Fig. 10. The variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the first pass of each of the four pass samples. The solid line represents the best fit for the date while the dotted line represents Eq. (1).
Fig. 11. Variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the last pass of each of the four pass samples. The solid line represents the best fit for the date while the dotted line represents Eq. (1).
of grain boundary sliding dominant deformation in the current material is higher than that shown in previous studies. It is likely that the differences in kinetics for the GBS are due to differences in tooling, as the previous study was performed using two different processing conditions with two different grain sizes with the same result.
References [1] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. TempleSmith, C.G. Dawes, Friction Stir Butt Welding, G.B. Patent Application No. 9125978.8 (December 6, 1991); W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Temple-Smith, C.G. Dawes, Friction Stir Butt Welding, US Patent 5,460,317 (October 24, 1995). [2] R.S. Mishra, Superplastic forming of micro components, US Patent 6,655,575 (December 2, 2003). [3] S.R. Sharma, Z.Y. Ma, R.S. Mishra, Scripta Mater. 51 (2004) 237– 241. [4] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Scripta Mater. 44 (2001) 61–66. [5] R.S. Mishra, Integral channels in metal components and fabrication thereof, US Patent 6,923,362 (August 2, 2005). [6] R.S. Mishra, M.W. Mahoney, Metal superplasticity enhancement and forming process, US Patent 6,712,916 (March 30, 2004). [7] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, 2nd ed., Elsevier, 2004. [8] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukerjee, Scripta Mater. 42 (2000) 163–168. [9] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419– 4430. [10] L.B. Johannes, I. Charit, R.S. Mishra, Proceedings of Friction Stir Welding and Processing III, 2005 TMS Annual Meeting, 2005, p. 123. [11] A. Dutta, I. Charit, L.B. Johannes, R.S. Mishra, Mater. Sci. Eng. A (2005) 173. [12] T.G. Nieh, J. Wadsworth, O.D. Sherby, Superplasticity in Metals and Ceramics, Cambridge University Press, 2005. [13] R.S. Mishra, T.R. Bieler, A.K. Mukherjee, Acta Metal. Mater. 43 (1995) 877.
Multiple passes of friction stir processing for the creation of superplastic 7075 aluminum L.B. Johannes, R.S. Mishra ∗ Department of Materials Science and Engineering, University of Missouri-Rolla, Rolla, MO 65401, United States Received 15 November 2006; received in revised form 30 January 2007; accepted 31 January 2007
Abstract A staggered pass sample of friction stir processed (FSP) 7075 aluminum was created to make samples with one through four passes of FSP under identical conditions. The tensile testing temperatures ranged from 673 to 763 K with initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1 . Materials processed by single as well as multiple passes exhibited superplasticity across various testing temperatures and strain rates while the as received materials exhibited elongations below 200%. This study demonstrated the effectiveness of four consecutive FSP passes in creating large areas of superplastic material. However, the largest elongations were observed for the single pass material. © 2007 Elsevier B.V. All rights reserved. Keywords: Friction stir processing; Aluminum alloy; Superplasticity
1. Introduction Friction stir processing (FSP) is a new solid state technique which uses the principles of friction stir welding [1] to process materials in a variety of other ways besides joining them. In FSP, a tool containing a shoulder and a pin provides frictional heating and mechanical mixing in the area covered by the tool. In addition, the large processing strain results in microstructural refinement and homogenization. FSP has developed into a broad field covering microforming [2], casting modification [3], powder processing [4], and channelling [5]. Another area in which FSP shows a lot of promise is in the creation of superplastic materials [6]. FSP creates a region called the ‘nugget’ where the microstructural refinement occurs with equiaxed grains containing high angle grain boundaries. The resultant microstructure in the nugget region can present the ideal conditions for superplasticity in some materials. There are many advantages to superplastic forming as it is a cost effective, near net shape forming process that can easily be adapted into commercial applications. As a result, there has been extensive research in the field for ways to deform metals to elongations greater than 200%, particularly if this can be
∗
Corresponding author. Tel.: +1 573 341 6361; fax: +1 573 341 6934. E-mail address: [email protected] (R.S. Mishra).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.141
done at commercially important high strain rates (>10−2 s−1 ). A significant amount of the research being performed in the area of superplastic forming involves thermo-mechanical processing (TMP) of standard alloys in order to make the material superplastic. TMP involves controlled recovery, recrsytallization, and grain growth of alloys through processes such as forging, rolling, and extrusion [7]. Complex rolling processes and equal channel angular extrusion are common methods for creating superplastic material. Mishra et al. [8] were the first to use FSP as TMP for superplasticity where they demonstrated FSP as a simple technique to enhance grain size dependent properties such as superplasticity in 7075 Al where elongations >1000% were seen in the FSPed alloy at 736 K at a strain rate of 1 × 10−2 s−1 . This work was followed by a similar study which also showed that FSP significantly enhances the superplastic properties of 7075 Al for two different grain sizes [9]. In the sample with the 3.8 m grain size, a maximum elongation of 1440% was achieved at 753 K and 3 × 10−3 s−1 . When the grain size was 7.5 m, the elongation was found to be 1040% at 773 K and 3 × 10−3 s−1 . After it was well established that using a single pass of FSP on 7075 could produce superplastic material, multiple passes began being performed to create greater areas of superplastic material in order to demonstrate the usefulness of the process on a larger scale [10] followed by punch forming studies to determine the effectiveness of multiple passes on the macroscale [11].
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Table 1 Chemical composition of the 7075 Al alloy used in this study Element
Measured Nominal
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
0.11 < 0.40
0.23 <0.50
1.47 1.2–2.0
0.03 <0.30
2.54 2.1–2.9
0.19 0.18–0.28
5.48 5.1–6.1
0.03 <0.20
The current study examines the effect of multiple passes on 7075 Al by adding one pass at a time to a total of four passes. In doing so, the properties of single pass FSP can be compared with FSP for up to four passes in order to help determine the effectiveness of multiple passes in creating superplastic material. 2. Experimental Plates of 7075 Al with the composition listed in Table 1 and 6.4 mm in thickness were used in the as-rolled condition. Fig. 1 shows the configuration used to create samples of one through four passes of FSP. To create this sample, a pass of FSP was run the entire length of the 50 cm plate. The second pass was run three quarters of the length of the plate and the tool was moved towards the advancing side. The third pass was run half the length of the plate and the fourth pass a quarter of the length, both towards the advancing side. In doing this, the effect of multiple passes on the previous and subsequent passes could be determined by looking at samples of one to four passes created under identical conditions. All of the separations between passes were 6.4 mm which resulted in a 42% nugget overlap. The FSP conditions were 400 rpm and 50 mm/min in all passes. The FSP was performed using a high-strength cobalt alloy (MP159) tool with a pin 6.4 mm in diameter and length. The shoulder on the
tool was flat and 25 mm in diameter. The tool pin was threaded with a right-hand screw and the rotation was counterclockwise. Metallographic cross-sections etched with Keller’s reagent were examined using an optical microscope to determine the grain size of the various passes by the mean linear intercept technique (average spatial grain size = 1.78 × mean linear intercept). The microstructure was also observed by using a Philips EM430T (Mahwah, NJ) transmission electron microscope (TEM) at 300 kV using jet-polished foils from the centers of passes in the samples. To evaluate superplastic properties, samples were taken at the center of each nugget on the staggered pass samples. From these areas, dog-bone shaped tensile specimens with a gage length of 1.3 mm and a gage width of 1.0 mm were electro discharge machined (EDM) from the FSP material in the transverse direction. The tensile specimens were polished to a final thickness of 0.5 mm and 1 m surface finish. Testing was conducted using a custom-built, computer-controlled mini tensile tester operable at constant cross-head speeds. The tensile testing temperatures ranged from 673 to 763 K with initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1 . 3. Results and discussion 3.1. Microstructure The average grain size was determined for the various passes, and Table 2 shows the results for nugget centers for each of the passes. From this table it can be seen that the grain sizes of the various passes are all within the same range, between 3.6 and 5.4 m with a majority of the grain sizes being around 4.6 m. The microstructures seen in the last pass of each of the one through four pass samples can be seen in Fig. 2. The last pass in all four samples appears to have similar grain sizes, as shown in Table 2, although some of the grains in the four-pass sample appear to be a little larger. TEM images of the grains at three locations in the multiple pass samples can be seen in Fig. 3. The grains shown again are all similar in size although the number of precipitates observed in the images varies. Thus, from the analysis of the multiple passes from the four passes in Table 2 Summary of average grain sizes at various locations Pass 1 (m)
Fig. 1. Configuration used to create one through four pass samples under identical conditions. Each pass consumes the advancing side of the nugget of the previous pass.
One pass sample Two-pass sample Three-pass sample Four-pass sample
4.7 3.6 4.1 4.5
± ± ± ±
2.3 1.9 2.2 2.3
Pass 2 (m)
Pass 3 (m)
Pass 4 (m)
– 4.6 ± 2.1 4.6 ± 2.4 4.8 ± 2.5
– – 4.6 ± 2.6 4.9 ± 2.2
– – – 5.4 ± 2.7
L.B. Johannes, R.S. Mishra / Materials Science and Engineering A 464 (2007) 255–260
257
Fig. 2. Microstructures found in the last pass in the one through four pass samples (a)–(d). Their locations are denoted by the corresponding letter in the adjacent image.
this study, the grain sizes achieved seem to be in the same range for the various passes. Dutta et al. [11] found that in a nine-pass sample, the average grain size was 6.1 m. This suggests that if multiple passes are going to be used there may be some grain coarsening due to the additional thermal cycles on the plate. 3.2. Tensile behavior Fig. 4 shows the elongations found for various regions in the sample compared to the elongations found in the unprocessed material for the temperature range of 673–763 K with an initial strain rate of 1 × 10−2 s−1 . From this graph it can be noted that the unprocessed material was not superplastic (>200% elongation) at any of the temperatures tested but the FSPed material was superplastic across all of the temperatures for all the nugget regions in the samples. The highest elongation of 1255% was found in the one pass sample at 743 K. A comparison of various single pass samples of FSP 7075 with the current single pass portion of the one through four pass
Fig. 4. Tensile elongations for FSPed 7075 at various locations for different temperatures at 1 × 10−2 s−1 .
Fig. 3. Individual grains from the single pass (a), first of four pass (b), and fourth of four pass (c) samples. The grains all appear similar in size but the number of precipitates seen varies.
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Table 3 Comparison of elongations in single pass studies for FSP 7075 Al Grain size (m)
Temperature (K)
Strain rate (s−1 )
3.3 3.8 7.5 4.7
763 753 773 743
1 × 10−2 3 × 10−3 3 × 10−3 1 × 10−2
Elongation (%) >1000 1440 1040 1225
Reference [8] [9] [9] Current
samples can be seen in Table 3. From this table it can be seen that the single pass portion of this sample is a good representation of the one pass work that has been previously performed and tensile elongation of over 1000% were easily achieved. This study, however, went beyond the previous studies in that the effects of multiple passes were examined. Since multiple passes will be needed for the implementation of selective superplasticity, they were further examined and compared to the single pass, also referred to as “Pass 1 of 1” which is also the first and last pass of the one pass sample. The strain rate of 1 × 10−2 s−1 is commercially important because high strain rate superplasticity occurs in the 1 × 10−2 to 1 × 10−1 s−1 range. An interesting thing to note in Fig. 4 is that the single pass sample has the highest elongation at every temperature. This shows that while single pass studies may have good results, the multiple pass samples will have slightly decreased superplasticity. However, the elongations achieved in the multiple pass samples are still all in the superplastic regime and much better than the elongations found in the unprocessed material which showed no superplasticity at any of the temperatures tested. From these results it can be inferred that using multiple passes of friction stir processing will enhance the superplastic properties of the area. This was examined in a study by Dutta et al. [11] in which nine pass samples were created and punch formed to show the enhanced formability of the multiple pass FSP samples in 7075 on a macroscopic scale using the overall area of the nine passes. The temperature at which each sample achieved the highest elongation was used to test samples at various strain rates. Fig. 5 shows the samples that were taken from the first pass of each of the samples. Fig. 6 shows the elongations in the last pass of each of the one through four pass samples. It should be noted that in the one pass sample the first and last pass are the same. From these graphs it can be seen that in the one to three pass samples, the greatest elongations were achieved at 1 × 10−2 s−1 . It is interesting to note that in both the first and the last pass of the four-pass sample, the elongation was higher at lower strain rates than at 1 × 10−2 s−1 . These graphs also show that at these optimum testing temperatures the multiple pass samples are all superplastic (elongations >200%) for the range of strain rates at which they were tested, 1 × 10−3 to 1 × 10−1 s−1 . Again, the single pass samples typically have the highest elongations. Using the stress–strain rate plots, the strain rate sensitivity (m) was calculated. Figs. 7 and 8 show the stress–strain rate plots of the first and the last passes of the one through four pass samples, respectively. The strain rate sensitivities of the various samples are summarized in Table 4. Strain rate sensitivity is an
Fig. 5. Elongations achieved for the various strain rates tested for the first pass in each of the one through four pass samples.
Fig. 6. Elongations achieved for the various strain rates tested for the last pass in each of the one through four pass samples.
important indicator for the level of elongation a material can achieve. Low m values lead to strain localization in the neck, resulting in necking in the material being tested which results in lower total strain to failure [12]. From Table 4, it can seen that the strain rate sensitivities for the various locations tested are all around m = 0.5, the condition for grain boundary sliding dominating as the flow process. It has been determined that the constitutive relationship for superplasticity, specifically in fine-grained aluminium, can be Table 4 Summary of strain rate sensitivities for the one through four pass samples
First pass Last pass
One pass sample
Two-pass sample
Three-pass sample
Four-pass sample
0.51 0.51
0.63 0.55
0.55 0.56
0.53 0.58
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Fig. 7. Stress vs. strain rate of the first pass of each of the one through four pass samples used to determine strain rate sensitivity.
expressed as [13]: 2 D0 Eb 84000 σ − σ0 2 b ε˙ = 40 exp − kT RT d E
(1)
where D0 is the pre-exponential constant for diffusivity, E the Young’s modulus, b the Burger’s vector, k Boltzmann’s constant, T the absolute temperature, R the gas constant, d the grain diameter, σ the applied stress, and σ 0 is the threshold stress. In examining this equation, the importance of fine grain size, one of the prerequisites for superplasticity, can be noted. As the grain size is decreased, the strain rate at which superplastic forming can occur is increased. It can also be seen that as the grain size is decreased the forming temperature at which superplastic forming occurs can also be decreased. Eq. (1) also predicts that as the grain size is decreased the flow stress will be decreased which is also quite advantageous for superplas-
Fig. 8. Strain rate sensitivities of the last pass of each of the one through four pass samples.
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Fig. 9. The variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the single pass FSPed 7075 Al from this study. The solid line represents the best fit for the data while the dashed and dotted line represent the relationships found in previous studies by Ma et al. [9] and Mishra et al. [13], respectively.
tic forming. Therefore, obtaining smaller grain size is always advantageous for superplasticity. In this equation, a stress exponent of 2 is associated with grain boundary sliding (GBS) related mechanisms for superplastic deformation [12]. For the various locations in the multiple pass samples that were tested, a stress exponent of ∼2 was observed indicating that the primary mechanism of deformation is GBS throughout the passes. Ma et al. [9] examined the superplasticity of FSPed 7075 in material with two different grain sizes, 3.8 and 7.5 m. In this analysis, it was found that the dimensionless constant that fit the data for the FSP alloys was much larger than that found in Eq. (1). Rather that it being 40, it was found that the constant was 790 by plotting (˙εkTd 2 /Dg Eb3 ) versus (σ/E). In the current study, the single pass data had a grain size of 4.7 m. When this data is plotted as (˙εkTd 2 /Dg Eb3 ) versus ((σ − σ 0 )/E), the constant becomes 1396. Fig. 9 shows a comparison of the single pass data from this study and compared the constant found with this data compared to that of Ma et al. [9] and Eq. (1) [13]. To expand this analysis into the multiple pass regime, two plots were created—one using the data from the first pass of the one through four pass samples and one using the data from the last pass of the one through four pass samples. Fig. 10 shows the data from the first passes and Fig. 11 shows the data from the last passes. When the data was plotted, new values of the dimensionless constant were calculated for the two sets of data. The line that fits the data in Fig. 10 uses a dimensionless constant of 1134. Eq. (1) is also plotted on this graph for comparison using a dotted line. The constant for Fig. 11, the last passes of the one through four pass samples, was found to be 1334 and can be compared to Eq. (1) which is represented by the dotted line. The data is found to have the same parametric dependencies as previous studies but with the constants being similar to the one found in the current study for the single pass data. All these values of the dimensionless constant suggest that the kinetics
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4. Conclusions 1. The effectiveness of friction stir processing in creating superplastic material can be extended to multiple passes to create larger areas of material with superplastic properties. 2. The elongations achieved in multiple passes of 7075 Al are superplastic, although the single pass material exhibits slightly greater elongations. 3. Grain boundary sliding is the primary mechanism for superplastic deformation of multiple pass samples of 7075 Al. Acknowledgements The authors gratefully acknowledge the support of the National Science Foundation through the grant DMI – 0323725, Mary Lynn Realff, program manager. Fig. 10. The variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the first pass of each of the four pass samples. The solid line represents the best fit for the date while the dotted line represents Eq. (1).
Fig. 11. Variation of (˙εkTd 2 /Dg Eb3 ) with the normalized effective stress for the last pass of each of the four pass samples. The solid line represents the best fit for the date while the dotted line represents Eq. (1).
of grain boundary sliding dominant deformation in the current material is higher than that shown in previous studies. It is likely that the differences in kinetics for the GBS are due to differences in tooling, as the previous study was performed using two different processing conditions with two different grain sizes with the same result.
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