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Crystallographic texture development in extruded AA 2195 and AA 7075
Materials Characterization 160 (2020) 110121
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Crystallographic texture development in extruded AA 2195 and AA 7075 ⁎
T
Judith M. Dickson , Thomas H. Sanders Jr. Georgia Institute of Technology, School of Materials Science and Engineering, 771 Ferst Drive, Atlanta, GA 30332-0245, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystallographic texture Aluminum lithium alloy Thermomechanical deformation Extrusion Rolled plate
The addition of lithium to high strength aluminum alloys significantly improves specific strength as is the case with AA 2195. However, unlike non-lithium containing alloys, such as AA 7075, lithium containing alloys display anisotropic mechanical properties in low aspect ratio extruded sections. The goal of this study is to elucidate the origins of anisotropy in AA 2195 and to investigate processing methods to control mechanical properties. To this end, both AA 2195 and AA 7075 were systematically extruded over a range of aspect ratios from 2 to 15 while maintaining a constant extrusion ratio. Through crystallographic texture component analysis and the use of a newly proposed parameter termed the “Plate-Like Number”, this study indicates that the Copper texture is primarily responsible for previously observed anisotropic properties of AA 2195. A series of rolling studies analyzed using the Plate-Like Number suggest that a higher initial billet temperature will minimize the Copper texture component and thus reduce the anisotropy in extruded AA 2195.
1. Introduction Material development in the aerospace industry is driven by the need for low density materials that are easy and affordable to manufacture that display high strength, fracture toughness, and favorable corrosion properties. The addition of lithium to Al-Cu alloys, denoted as 2xxx-series, results in densities and properties competitive with polymer matrix composite materials [1]. AA 2195 is a commercially relevant alloy that can be manufactured through the established thermo-mechanical methods of extrusion and rolling. Al-Cu-Li extrusions exhibit different properties in low and high aspect ratio regions: low aspect ratio regions have greater longitudinal strength. Furthermore in low aspect ratio profiles there is a small difference between the longitudinal and transverse yield strengths, a small spread between the yield and ultimate tensile strengths (a difference of an order of magnitude between lithium containing and non-lithium alloys is typical) and a low transverse ductility in lithium containing alloys [2–7]. In combination, these anisotropic mechanical properties are typically not observed in non Li-containing alloys such as AA 7075 [3,8]. There is currently no consensus in the literature as to the source of these anisotropic mechanical properties. Some authors attribute the anisotropy to crystallographic texture development, primarily of the Brass texture component [3,9,10]. Other authors attribute it to a complex interaction between the crystallographic texture and the strengthening phases: T1 (Al2CuLi) has a {111} habit plane and θ′
⁎
(Al2Cu) has a {100} habit plane [11,12]. Still another group of authors state that there is no discernable effect of the strengthening precipitates on the anisotropy [9,13–15]. Although the grain morphology varies between regions of high and low aspect ratio, the literature indicates that microstructural variations observed across all alloy systems are unlikely to cause the anisotropy observed uniquely in Li-containing alloys [9,16,17]. Many of the aforementioned studies were performed on complex extrusion profiles where the effects of aspect ratio, extrusion ratio, and transition regions were convoluted. (e.g. regions of varying aspect ratio also varied in extrusion ratio and transition zones were labeled as low aspect ratio regions). Overall, the studies did not provide direct comparison to non Li-containing extrusions, nor did they suggest a solution for controlling texture development. This study seeks to overcome these limitations by the systematic investigation of the effects of extrusion aspect ratio and processing variables on microstructure and crystallographic texture development in both AA 2195 and AA 7075. Additionally, a mechanism for controlling texture development in AA 2195 is proposed. 2. Materials and methods Homogenized billets of AA 2195 and AA 7075 were supplied by Vista Metals. Billets were industrially extruded on a direct press through flat faced dies by Universal Alloy Corporation in Canton, GA. In this manuscript, the profile aspect ratio (AR) is defined as profile width
Corresponding author. E-mail address: [email protected] (J.M. Dickson).
https://doi.org/10.1016/j.matchar.2020.110121 Received 3 September 2019; Received in revised form 9 December 2019; Accepted 4 January 2020 Available online 10 January 2020 1044-5803/ © 2020 Elsevier Inc. All rights reserved.
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
AR2
AR3 AR5 (a)
AR7
(b)
Fig. 3. Representative (a) microstructure of the AA 2195 ingot and (b) associated binary image used for volume fraction analysis.
AR8
polarized light optical microscopy were cut from the center of the L-ST (longitudinal-short transverse), T-ST (transverse-short transverse), and L-LT (longitudinal-longitudinal transverse) extrusion planes. After hot mounting, each sample was mechanically ground with silicon carbide paper and polished using diamond suspension and colloidal silica. Samples for bright field optical microscopy were etched with Keller's reagent while samples for polarized light microscopy were electropolished and etched with Barker's reagent. Texture analysis was performed on samples taken from the L-LT plane, midway through the thickness of the sample. X-ray diffraction was performed on a PANalytical Materials Research Diffractometer with a point detector using an operating voltage of 45 kV and a current of 40 mA. MTEX 3.5.0 was used to convert the raw data into calculated pole figures and orientation distribution functions (ODFs), these were then used to determine the volume fractions of common deformation textures (i.e. Brass, S, and Copper) [18].
AR15 Fig. 1. Representative extrusion profile cross sections demonstrating variation in aspect ratio (AR) from 2 to 15 while maintaining a constant extrusion ratio of 20.
divided by profile height. The extrusion ratio is defined as billet cross sectional area divided by profile area. The profile aspect ratio was systematically varied from AR2-AR15 while maintaining a constant extrusion ratio of 20 (Fig. 1). All other extrusion parameters within an alloy system were held relatively constant (i.e. initial billet temperature, container temperature, die temperature, ram speed, billet diameter, etc.) This design of experiment methodology allowed for an isolated study on the effects of extrusion aspect ratio on crystallographic texture without the confounding effects of extrusion ratio, transition zones, or variation in press parameters. Systematic variation of extrusion press parameters is cost prohibitive in an industrial setting. Although an industrial extrusion press was used to evaluate the effects of extrusion aspect ratio on texture, a laboratory scale rolling mill was used to investigate the effects of press parameters on AA 2195. Through three passes, samples were rolled to a 75% reduction (Fig. 2). To determine the effect of temperature and dell time, three different temperatures (343 °C, 454 °C, and 515 °C) and four dwell times (1, 4, 8, and 24 h) were considered for a total of 12 plate samples. The aspect ratio for the plates was approximately 20. The processing temperatures were chosen because differential scanning calorimetry measurements indicated that the onset of melting for the as-received AA 2195 billet occurred at 516 °C. Dwell times were based on quantitative characterization of the evolution of the billet microstructure at these temperatures. Thirty bright field optical micrographs at each time and temperature were converted to binary in MATLAB and used to find volume fractions of secondary phases (representative example in Fig. 3, results in Fig. 12). After extrusion, samples were allowed to air cool. After rolling, samples were quenched in water. All characterization was performed on as-extruded, F-temper material (i.e. as-quenched). Samples for
3. Results 3.1. Extrusions: effects of aspect ratio 3.1.1. Grain morphology Although each profile had the same extrusion ratio, the grain morphologies evolved as a function of aspect ratio for both AA 2195 and AA 7075. For both alloys, grains evolved from long, cylindrical “cigar-like” grains at low aspect ratios (AR2 and AR3) to flatter, “pancake-like” grains at higher aspect ratios (Figs. 4 and 5). 3.1.2. Crystallographic texture Based on observation of the {111} pole figures and orientation distribution functions (ODFs), the macrotextures that developed in extruded AA 2195 (Figs. 6 and 8) and AA 7075 (Figs. 7 and 9) evolved similarly with aspect ratio. The three primary deformation components in rolled metals are the Brass, S, and Copper crystallographic textures, the locations of which are given in Table 1. In plates, Brass is typically found in the highest volume fractions, S in the second highest, and Copper in the lowest volume fractions. The volume percent of each
25%
50%
75% Water Quench
Ingot
Dwell
Dwell
Dwell
Fig. 2. Schematic representing the rolling procedure used to investigate the effects of processing parameters on texture development. 2
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
Fig. 5. Polarized light optical micrographs at 50× from the center of the Ftemper AA 7075 (a) AR2 and (b) AR15 extrusions.
Fig. 4. Polarized light optical micrographs at 50× from the center of the Ftemper AA 2195 (a) AR2 and (b) AR15 extrusions.
Plate− Like Number for prediction of relative strength anisotropy in
component at mid-thickness was plotted as a function of aspect ratio for the AA 2195 (Fig. 10a) and AA 7075 (Fig. 10b) extrusions. In both alloy systems, the Brass and S texture components increased with increasing aspect ratio, while the Copper texture component decreased with increasing aspect ratio. The primary difference between the two alloy systems occurred at AR2 and AR3. In the AR2 and AR3 extrusions of AA 2195, the volume percent of the Copper component was greater than that of both the Brass and S components. The other AA 2195 extrusions and all of the AA 7075 extrusions had the typical rolled plate percent volume fraction ordering: Brass, S, Copper. To capture the variation in the Brass, S, and Copper textures, the authors propose a “Plate-Like Number” equal to the difference between the volume fraction of each of the primary deformation components divided by the sum of the components (Eq. (1)). Use of the Plate-Like Number allows for prediction of anisotropy. Negative values for the Plate-Like Number are correlated with the anisotropic properties observed in low aspect ratio AA 2195 extrusions (Fig. 11). The high aspect ratio AA 2195 extrusions and all of the AA 7075 extrusions have positive values for the Plate-Like Number.
aluminum alloys. Plate − Like Number = =
(Brass − S ) + (S − Copper ) + (Brass − Copper ) Brass + S + Copper 2(Brass − Copper ) Brass + S + Copper
(1)
3.2. Plates: effects of processing variables on AA 2195 Prediction of anisotropy based on the volume fraction of deformation textures is useful, however, the ability to control texture and tailor the alloy's associated properties is the primary goal of this study. Given that there is less of a need to control the crystallographic texture in AA 7075, the effects of processing variables on texture was only studied in AA 2195. The area fraction of secondary phases was determined for the as-received AA 2195 ingot and for the ingot after time at temperature as described in Section 2. The as-received ingot contained 54% second phase particles. Based on the literature, the particles forming a Widmanstätten pattern are likely T1 (Al2CuLi) while the particles at the grain boundaries are likely T2 and Al20Cu2Mn3 [19]. The second phase particles dissolved to < 1% within the first hour at 515 °C (Fig. 12). At 454 °C, the microstructure consisted of 9% second phase particles after 24 h (Figs. 12 and 13). At 343 °C, the microstructure consisted of 37% 3
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
AR2
AR3
AR5
AR7
AR8
AR15
Fig. 6. {111} Pole figures as a function of aspect ratio (AR) from rectangular extrusions of AA 2195.
secondary phases after 24 h. As expected, based on this microstructural development with time at temperature, the crystallographic texture of the final plate was found to vary with dwell time and temperature. When the percent of each deformation texture was considered independently, there was no correlation between dwell time and rolling temperature (Fig. 14a–c). However, when the Plate-Like number was considered, a clearer picture emerged (Fig. 14d). The greatest change in the Plate-Like Number (0.0 to 0.4) was associated with the samples processed at 454 °C, which also showed the greatest change in volume fraction of second phase particles from 1 to 24 h (34 to 9%). This indicates that the volume fraction of second phase particles present during deformation likely plays a role in the final texture developed. Both the 515 °C and 454 °C plates exhibited positive values for the Plate-Like Number while the 343 °C plates all had negative values. Extrapolating from the extrusion results, the 343 °C plates may exhibit more anisotropy than the plates processed at higher rolling temperatures.
Table 1 Commonly observed crystallographic deformation components and the locations used to calculate volume percent. Plane
Direction
Euler angles (ϕ1,Φ,ϕ2)
Brass S Copper
{011} {123} {112}
〈211〉 〈634〉 〈111〉
35°,45°,0° 59°,33°,65° 90°,30°,45°
similar conclusion [9,16,17]. It is important to note that variation in the grain boundary area per unit volume would likely lead to variation in the electro-chemical performance as a function of aspect ratio. The pole figures and ODFs in this study evolved similarly as a function of aspect ratio for 2195 and 7075. The anisotropic mechanical properties in low aspect ratio Li-containing extrusions were termed the “Al-Li fiber texture problem” by Denzer et al. [3]. Similar to the results reported in the present study, Denzer reported that AA 7050 developed the fiber component as strong as that of the Li-containing alloy. However, Denzer's 7xxx-series alloy did not exhibit the same levels of anisotropy as the Li-containing alloy. Since both alloys developed similar fiber textures, but different levels of strength anisotropy, development of the fiber texture alone was not responsible for the difference in mechanical behavior and the “Al-Li fiber texture problem” is a misnomer. Denzer et al. [3] did not perform crystallographic texture component analysis. Based on the crystallographic texture analysis performed in the present study, the primary difference between the AA 2195 and AA 7075 extrusions was in the volume fraction of the Copper texture
4. Discussion As described in the introduction, many authors observed strength anisotropy in low aspect ratio Al-Cu-Li extrusions. However, the origin of this anisotropy is disputed in the literature. In the studies presented herein, the grain morphology in the AA 2195 and AA 7075 extrusions evolved from cigar-like to pancake-like with increasing aspect ratio for both alloy systems. Because it was present in both alloy systems, the change in grain morphology is likely not responsible for the anisotropy observed in the Li-containing systems; other authors have come to a
AR2
Name
AR8
AR15
Fig. 7. {111} Pole figures as a function of aspect ratio (AR) from rectangular extrusions of AA 7075. 4
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
AR2
AR8
AR15
Fig. 8. Orientation distribution functions (ODFs) as a function of aspect ratio from rectangular extrusions of AA 2195.
AR2
AR8
AR15
Fig. 9. Orientation distribution functions (ODFs) as a function of aspect ratio from rectangular extrusions of AA 7075.
AA2195
12 Brass
10
Volume Percent (%)
Volume Percent (%)
12
8 S
6 4
Copper
2 0
2 3
5
7 8 Aspect Rao
AA7075
Brass
10 8 6
S
4 2 0
15
Copper 2 3
5
7 8 Aspect Rao
15
(b)
(a)
Fig. 10. Volume percent of common deformation crystallographic texture components as a function of aspect ratio for (a) AA 2195 and (b) AA 7075 extrusions.
results also indicate that lowering the Copper texture in Li-containing alloys to fractions seen in a 7xxx-series alloy resulted in strength anisotropy equivalent to that observed in the 7xxx-series alloy. This conclusion is in agreement with the present study wherein the strength anisotropy in Li-containing low aspect ratio extrusions is attributed to the relatively high volume percent of the Copper texture. The Copper texture is equivalent to grains orientated with the {112} plane parallel to the rolling/L-LT plane and the ⟨111⟩ direction in the rolling/extrusion direction as illustrated within an extrusion reference frame in Fig. 15. Using a theoretical approach, Barlat and Richmond [20] modeled the yield surfaces for an isotropic material, a material comprised of 50% Brass texture, a material comprised of 50% S texture, and a material comprised of 50% Copper texture. Of the three, the
component at low aspect ratios. For the AR2 and AR3 extrusions of AA 2195, the volume fraction of the Copper texture was greater than the Brass and the S components (resulting in a negative Plate-Like Number). In all other extrusions of AA 2195 and all extrusions of AA 7075, the highest volume fraction percent was Brass, followed by S and then by Copper. In addition, the Brass and S components were lower in the low AR AA 2195 extrusions than they were in the low AR AA 7075 extrusions. Jata, Hopkins, and Rioja [10] similarly reported lower fractions of the Brass texture in a Li-containing alloy and higher fractions of the Brass texture in a 7xxx-series alloy. They conclude that lowering the Brass component in a Li-containing alloy results in a decrease in strength anisotropy to that of levels observed in 7xxx-series alloys (AA 7150 and AA 7055). Though not explicitly stated, their 5
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
low aspect ratio parts. 5. Conclusions This study presents a systematic investigation of the origins of and methods of controlling anisotropy in AA 2195. The results of this study highlight the importance of considering aspect ratio in addition to extrusion ratio. Although each profile in this study had an extrusion ratio of 20, the crystallographic texture and grain morphology varied with aspect ratio. Most notably, the increased fraction of the Copper component in low aspect ratio lithium containing extrusions was associated with strength anisotropy reported for these extrusions. The Plate-Like Number was introduced as a novel way to understand the effects of processing conditions on the Brass, S, and Copper crystallographic texture components and resulting properties. A negative Plate-Like Number is associated with increased anisotropy. Based on results using rolled plates, higher processing temperatures are recommended to reduce anisotropy and chunkiness in extruded AA 2195. The effects of aspect ratio on grain morphology and crystallographic texture in thermo-mechanically processed AA 2195 and AA 7075 were characterized. The significant conclusions are listed below:
Fig. 11. Variation in the Plate-Like Number with aspect ratio for AA 2195 and AA 7075 extrusions.
1. Aspect ratio should be considered as an extrusion variable in addition to extrusion ratio. 2. A single parameter, the Plate-Like Number, was developed to quantify the similarity of a sample's crystallographic texture to that typically observed in rolled plate and can be used to more clearly assess the effects of processing conditions on properties. 3. Extrusions with an aspect ratio of 5 and higher have positive values of the Plate-Like Number and more isotropic, plate-like crystallographic textures. 4. The high volume percent of the Copper crystallographic texture component (associated with a negative Plate-Like Number) is likely the primary cause of strength anisotropy in low aspect ratio AA 2195 extrusions. 5. The Plate-Like Number systematically increases with increasing processing temperature. Increasing the processing temperature will likely lead to a reduction in the Copper texture and thus the anisotropy in extruded AA 2195.
Fig. 12. Evolution of the area fraction of secondary phases in the AA 2195 ingot with time at temperature.
model material with 50% Copper texture resulted in the most deformed yield surface. Compared to the isotropic material, the 50% Copper material had a yield surface with a contracted transverse direction and a flat region near biaxial stretching [20]. Barlat and Richmond's result supports the theory that increasing the Copper texture in a sample leads to more anisotropic mechanical properties than equivalent increases in the Brass or S texture. If the Copper texture is primarily responsible for anisotropy, then reduction of the Copper texture should reduce the mechanical anisotropy. The plates rolled at 515 °C had the highest Plate-Like Numbers and the lowest volume fractions of the Copper texture; whereas, the low temperature 343 °C plates had the lowest Plate-Like numbers and the highest volume fractions of the Copper texture. Therefore, this study suggests that higher processing temperatures will likely reduce anisotropy in AA 2195. It is important to note that these results are drawn from high aspect ratio plates and may not present the same influence on
(a) 454°C 1 hr.
(b) 454°C 2 hrs.
Acknowledgements The authors would like to offer their sincere gratitude to Victor Dangerfield of Universal Alloy Corporation for his support and guidance throughout this effort. Additional thanks are extended to Justin Lamb of Universal Alloy Corporation for his assistance with the extrusions, David Tavakoli of Georgia Institute of Technology for providing training and technical expertise on X-ray diffraction, and Dr. Jordan Ciciliano for graphic assistance in creating Figure 15. This work was supported by the Department of Defense (DoD) through the National Science & Engineering Graduate Fellowship (NDSEG) Program. Due to the sensitive nature of exact processing parameters, the extrusion data is considered proprietary and is not reported in this manuscript. Supporting texture data can be found in the related
(c) 454°C 4 hrs.
(d) 454°C 8 hrs.
(e) 454°C 24 hrs.
Fig. 13. Optical micrographs of the AA 2195 ingot microstructure after (a) 1, (b) 2, (c) 4, (d) 8, and (e) 24 h dwells at 454 °C. 6
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
Fig. 14. Volume percent of the (a) Brass, (b) S, and (c) Copper crystallographic textures and (d) the evolution of the Plate-Like Number as a function of dwell time for AA 2195 plates. [4] A. Bois-Brochu, F. Goma, C. Blais, D. Larouche, R. Gauvin, J. Boselli, Adv. Mater. Res. 409 (2012) 29–34. [5] J. J. Witters, B. A. Cheney, and R. J. Rioja, US Patent 5151136, 1992. [6] G. Tempus, W. Calles, G. Scharf, Mater. Sci. Technol. 7 (1991) 937–946. [7] S.J. Hales, R.A. Hafley, Mater. Sci. Eng. A 257 (1998) 153–164 (1998). [8] T.H. Sanders Jr., Development of Structure and Properties of 7050 and 7475 Alloys During Direct Extrusion, Alum. Co. of Amer. Alcoa Center PA Alcoa Labs, 1981. [9] G. Tempus, G. Scharf, W. Calles, J. Phys. 48 (1987) C3–187. [10] K.V. Jata, aK Hopkins, R.J. Rioja, Mater. Sci. Forum 217–222 (1996) 647–652. [11] A.K. Vasudevan, M.A. Przystupa, J. Fricke, G. W., Scr. Metall. Mater. 24 (1990) 1429–1434. [12] N.J. Kim, E.W. Lee, Acta Met. Mater. 41 (1993) 941–948. [13] R. Crooks, Z. Wang, V.I. Levit, R.N. Shenoy, Mater. Sci. Eng. A257 (1998) 145–152. [14] K.V. Jata, S. Panchanadeeswaran, A.K. Vasudevan, Mater. Sci. Eng. A257 (1998) 37–46. [15] A. Bois-Brochu, C. Blais, F.A. Tchitembo Goma, D. Larouche, Mater. Sci. Eng. A 673 (2016) 581–586. [16] A. Bois-Brochu, C. Blais, F.A.T. Goma, D. Larouche, J. Boselli, M. Brochu, Mater. Sci. Eng. A 597 (2014) 62–69. [17] R.C. Dorward, Metall. Trans. A. 18 (10) (1987) 1820–1823. [18] F. Bachmann, R. Hielscher, H. Schaeben, Sol. State Phen. 160 (2010) 63–68. [19] R.J. Rioja, J. Liu, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 43 (2012) 3325–3337. [20] F. Barlat, O. Richmond, Mater. Sci. Eng. 95 (1987) 15–29.
Fig. 15. A schematic of the Copper crystallographic texture within an extrusion reference frame.
dissertation: J.M. Dickson, “Development and control of strength anisotropy and crystallographic texture during extrusion of aluminum 2195 and 7075,” 2017. References [1] E. Starke, J. Staley, Prog. Aerosp. Sci. 32 (1996) 131–172. [2] K. P. Armanie, R. J. Rioja, D. K. Denzer, C. E. Brooks, D. K. Gadbery, and R. Newell, US Patent 6113711, 2000. [3] D.K. Denzer, P.A. Hollingshead, J. Liu, K.P. Armanie, R.J. Rioja, AluminiumLithium 2 (1992) 903–908 (1992).
7
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Crystallographic texture development in extruded AA 2195 and AA 7075 ⁎
T
Judith M. Dickson , Thomas H. Sanders Jr. Georgia Institute of Technology, School of Materials Science and Engineering, 771 Ferst Drive, Atlanta, GA 30332-0245, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystallographic texture Aluminum lithium alloy Thermomechanical deformation Extrusion Rolled plate
The addition of lithium to high strength aluminum alloys significantly improves specific strength as is the case with AA 2195. However, unlike non-lithium containing alloys, such as AA 7075, lithium containing alloys display anisotropic mechanical properties in low aspect ratio extruded sections. The goal of this study is to elucidate the origins of anisotropy in AA 2195 and to investigate processing methods to control mechanical properties. To this end, both AA 2195 and AA 7075 were systematically extruded over a range of aspect ratios from 2 to 15 while maintaining a constant extrusion ratio. Through crystallographic texture component analysis and the use of a newly proposed parameter termed the “Plate-Like Number”, this study indicates that the Copper texture is primarily responsible for previously observed anisotropic properties of AA 2195. A series of rolling studies analyzed using the Plate-Like Number suggest that a higher initial billet temperature will minimize the Copper texture component and thus reduce the anisotropy in extruded AA 2195.
1. Introduction Material development in the aerospace industry is driven by the need for low density materials that are easy and affordable to manufacture that display high strength, fracture toughness, and favorable corrosion properties. The addition of lithium to Al-Cu alloys, denoted as 2xxx-series, results in densities and properties competitive with polymer matrix composite materials [1]. AA 2195 is a commercially relevant alloy that can be manufactured through the established thermo-mechanical methods of extrusion and rolling. Al-Cu-Li extrusions exhibit different properties in low and high aspect ratio regions: low aspect ratio regions have greater longitudinal strength. Furthermore in low aspect ratio profiles there is a small difference between the longitudinal and transverse yield strengths, a small spread between the yield and ultimate tensile strengths (a difference of an order of magnitude between lithium containing and non-lithium alloys is typical) and a low transverse ductility in lithium containing alloys [2–7]. In combination, these anisotropic mechanical properties are typically not observed in non Li-containing alloys such as AA 7075 [3,8]. There is currently no consensus in the literature as to the source of these anisotropic mechanical properties. Some authors attribute the anisotropy to crystallographic texture development, primarily of the Brass texture component [3,9,10]. Other authors attribute it to a complex interaction between the crystallographic texture and the strengthening phases: T1 (Al2CuLi) has a {111} habit plane and θ′
⁎
(Al2Cu) has a {100} habit plane [11,12]. Still another group of authors state that there is no discernable effect of the strengthening precipitates on the anisotropy [9,13–15]. Although the grain morphology varies between regions of high and low aspect ratio, the literature indicates that microstructural variations observed across all alloy systems are unlikely to cause the anisotropy observed uniquely in Li-containing alloys [9,16,17]. Many of the aforementioned studies were performed on complex extrusion profiles where the effects of aspect ratio, extrusion ratio, and transition regions were convoluted. (e.g. regions of varying aspect ratio also varied in extrusion ratio and transition zones were labeled as low aspect ratio regions). Overall, the studies did not provide direct comparison to non Li-containing extrusions, nor did they suggest a solution for controlling texture development. This study seeks to overcome these limitations by the systematic investigation of the effects of extrusion aspect ratio and processing variables on microstructure and crystallographic texture development in both AA 2195 and AA 7075. Additionally, a mechanism for controlling texture development in AA 2195 is proposed. 2. Materials and methods Homogenized billets of AA 2195 and AA 7075 were supplied by Vista Metals. Billets were industrially extruded on a direct press through flat faced dies by Universal Alloy Corporation in Canton, GA. In this manuscript, the profile aspect ratio (AR) is defined as profile width
Corresponding author. E-mail address: [email protected] (J.M. Dickson).
https://doi.org/10.1016/j.matchar.2020.110121 Received 3 September 2019; Received in revised form 9 December 2019; Accepted 4 January 2020 Available online 10 January 2020 1044-5803/ © 2020 Elsevier Inc. All rights reserved.
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
AR2
AR3 AR5 (a)
AR7
(b)
Fig. 3. Representative (a) microstructure of the AA 2195 ingot and (b) associated binary image used for volume fraction analysis.
AR8
polarized light optical microscopy were cut from the center of the L-ST (longitudinal-short transverse), T-ST (transverse-short transverse), and L-LT (longitudinal-longitudinal transverse) extrusion planes. After hot mounting, each sample was mechanically ground with silicon carbide paper and polished using diamond suspension and colloidal silica. Samples for bright field optical microscopy were etched with Keller's reagent while samples for polarized light microscopy were electropolished and etched with Barker's reagent. Texture analysis was performed on samples taken from the L-LT plane, midway through the thickness of the sample. X-ray diffraction was performed on a PANalytical Materials Research Diffractometer with a point detector using an operating voltage of 45 kV and a current of 40 mA. MTEX 3.5.0 was used to convert the raw data into calculated pole figures and orientation distribution functions (ODFs), these were then used to determine the volume fractions of common deformation textures (i.e. Brass, S, and Copper) [18].
AR15 Fig. 1. Representative extrusion profile cross sections demonstrating variation in aspect ratio (AR) from 2 to 15 while maintaining a constant extrusion ratio of 20.
divided by profile height. The extrusion ratio is defined as billet cross sectional area divided by profile area. The profile aspect ratio was systematically varied from AR2-AR15 while maintaining a constant extrusion ratio of 20 (Fig. 1). All other extrusion parameters within an alloy system were held relatively constant (i.e. initial billet temperature, container temperature, die temperature, ram speed, billet diameter, etc.) This design of experiment methodology allowed for an isolated study on the effects of extrusion aspect ratio on crystallographic texture without the confounding effects of extrusion ratio, transition zones, or variation in press parameters. Systematic variation of extrusion press parameters is cost prohibitive in an industrial setting. Although an industrial extrusion press was used to evaluate the effects of extrusion aspect ratio on texture, a laboratory scale rolling mill was used to investigate the effects of press parameters on AA 2195. Through three passes, samples were rolled to a 75% reduction (Fig. 2). To determine the effect of temperature and dell time, three different temperatures (343 °C, 454 °C, and 515 °C) and four dwell times (1, 4, 8, and 24 h) were considered for a total of 12 plate samples. The aspect ratio for the plates was approximately 20. The processing temperatures were chosen because differential scanning calorimetry measurements indicated that the onset of melting for the as-received AA 2195 billet occurred at 516 °C. Dwell times were based on quantitative characterization of the evolution of the billet microstructure at these temperatures. Thirty bright field optical micrographs at each time and temperature were converted to binary in MATLAB and used to find volume fractions of secondary phases (representative example in Fig. 3, results in Fig. 12). After extrusion, samples were allowed to air cool. After rolling, samples were quenched in water. All characterization was performed on as-extruded, F-temper material (i.e. as-quenched). Samples for
3. Results 3.1. Extrusions: effects of aspect ratio 3.1.1. Grain morphology Although each profile had the same extrusion ratio, the grain morphologies evolved as a function of aspect ratio for both AA 2195 and AA 7075. For both alloys, grains evolved from long, cylindrical “cigar-like” grains at low aspect ratios (AR2 and AR3) to flatter, “pancake-like” grains at higher aspect ratios (Figs. 4 and 5). 3.1.2. Crystallographic texture Based on observation of the {111} pole figures and orientation distribution functions (ODFs), the macrotextures that developed in extruded AA 2195 (Figs. 6 and 8) and AA 7075 (Figs. 7 and 9) evolved similarly with aspect ratio. The three primary deformation components in rolled metals are the Brass, S, and Copper crystallographic textures, the locations of which are given in Table 1. In plates, Brass is typically found in the highest volume fractions, S in the second highest, and Copper in the lowest volume fractions. The volume percent of each
25%
50%
75% Water Quench
Ingot
Dwell
Dwell
Dwell
Fig. 2. Schematic representing the rolling procedure used to investigate the effects of processing parameters on texture development. 2
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Fig. 5. Polarized light optical micrographs at 50× from the center of the Ftemper AA 7075 (a) AR2 and (b) AR15 extrusions.
Fig. 4. Polarized light optical micrographs at 50× from the center of the Ftemper AA 2195 (a) AR2 and (b) AR15 extrusions.
Plate− Like Number for prediction of relative strength anisotropy in
component at mid-thickness was plotted as a function of aspect ratio for the AA 2195 (Fig. 10a) and AA 7075 (Fig. 10b) extrusions. In both alloy systems, the Brass and S texture components increased with increasing aspect ratio, while the Copper texture component decreased with increasing aspect ratio. The primary difference between the two alloy systems occurred at AR2 and AR3. In the AR2 and AR3 extrusions of AA 2195, the volume percent of the Copper component was greater than that of both the Brass and S components. The other AA 2195 extrusions and all of the AA 7075 extrusions had the typical rolled plate percent volume fraction ordering: Brass, S, Copper. To capture the variation in the Brass, S, and Copper textures, the authors propose a “Plate-Like Number” equal to the difference between the volume fraction of each of the primary deformation components divided by the sum of the components (Eq. (1)). Use of the Plate-Like Number allows for prediction of anisotropy. Negative values for the Plate-Like Number are correlated with the anisotropic properties observed in low aspect ratio AA 2195 extrusions (Fig. 11). The high aspect ratio AA 2195 extrusions and all of the AA 7075 extrusions have positive values for the Plate-Like Number.
aluminum alloys. Plate − Like Number = =
(Brass − S ) + (S − Copper ) + (Brass − Copper ) Brass + S + Copper 2(Brass − Copper ) Brass + S + Copper
(1)
3.2. Plates: effects of processing variables on AA 2195 Prediction of anisotropy based on the volume fraction of deformation textures is useful, however, the ability to control texture and tailor the alloy's associated properties is the primary goal of this study. Given that there is less of a need to control the crystallographic texture in AA 7075, the effects of processing variables on texture was only studied in AA 2195. The area fraction of secondary phases was determined for the as-received AA 2195 ingot and for the ingot after time at temperature as described in Section 2. The as-received ingot contained 54% second phase particles. Based on the literature, the particles forming a Widmanstätten pattern are likely T1 (Al2CuLi) while the particles at the grain boundaries are likely T2 and Al20Cu2Mn3 [19]. The second phase particles dissolved to < 1% within the first hour at 515 °C (Fig. 12). At 454 °C, the microstructure consisted of 9% second phase particles after 24 h (Figs. 12 and 13). At 343 °C, the microstructure consisted of 37% 3
Materials Characterization 160 (2020) 110121
J.M. Dickson and T.H. Sanders
AR2
AR3
AR5
AR7
AR8
AR15
Fig. 6. {111} Pole figures as a function of aspect ratio (AR) from rectangular extrusions of AA 2195.
secondary phases after 24 h. As expected, based on this microstructural development with time at temperature, the crystallographic texture of the final plate was found to vary with dwell time and temperature. When the percent of each deformation texture was considered independently, there was no correlation between dwell time and rolling temperature (Fig. 14a–c). However, when the Plate-Like number was considered, a clearer picture emerged (Fig. 14d). The greatest change in the Plate-Like Number (0.0 to 0.4) was associated with the samples processed at 454 °C, which also showed the greatest change in volume fraction of second phase particles from 1 to 24 h (34 to 9%). This indicates that the volume fraction of second phase particles present during deformation likely plays a role in the final texture developed. Both the 515 °C and 454 °C plates exhibited positive values for the Plate-Like Number while the 343 °C plates all had negative values. Extrapolating from the extrusion results, the 343 °C plates may exhibit more anisotropy than the plates processed at higher rolling temperatures.
Table 1 Commonly observed crystallographic deformation components and the locations used to calculate volume percent. Plane
Direction
Euler angles (ϕ1,Φ,ϕ2)
Brass S Copper
{011} {123} {112}
〈211〉 〈634〉 〈111〉
35°,45°,0° 59°,33°,65° 90°,30°,45°
similar conclusion [9,16,17]. It is important to note that variation in the grain boundary area per unit volume would likely lead to variation in the electro-chemical performance as a function of aspect ratio. The pole figures and ODFs in this study evolved similarly as a function of aspect ratio for 2195 and 7075. The anisotropic mechanical properties in low aspect ratio Li-containing extrusions were termed the “Al-Li fiber texture problem” by Denzer et al. [3]. Similar to the results reported in the present study, Denzer reported that AA 7050 developed the fiber component as strong as that of the Li-containing alloy. However, Denzer's 7xxx-series alloy did not exhibit the same levels of anisotropy as the Li-containing alloy. Since both alloys developed similar fiber textures, but different levels of strength anisotropy, development of the fiber texture alone was not responsible for the difference in mechanical behavior and the “Al-Li fiber texture problem” is a misnomer. Denzer et al. [3] did not perform crystallographic texture component analysis. Based on the crystallographic texture analysis performed in the present study, the primary difference between the AA 2195 and AA 7075 extrusions was in the volume fraction of the Copper texture
4. Discussion As described in the introduction, many authors observed strength anisotropy in low aspect ratio Al-Cu-Li extrusions. However, the origin of this anisotropy is disputed in the literature. In the studies presented herein, the grain morphology in the AA 2195 and AA 7075 extrusions evolved from cigar-like to pancake-like with increasing aspect ratio for both alloy systems. Because it was present in both alloy systems, the change in grain morphology is likely not responsible for the anisotropy observed in the Li-containing systems; other authors have come to a
AR2
Name
AR8
AR15
Fig. 7. {111} Pole figures as a function of aspect ratio (AR) from rectangular extrusions of AA 7075. 4
Materials Characterization 160 (2020) 110121
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AR2
AR8
AR15
Fig. 8. Orientation distribution functions (ODFs) as a function of aspect ratio from rectangular extrusions of AA 2195.
AR2
AR8
AR15
Fig. 9. Orientation distribution functions (ODFs) as a function of aspect ratio from rectangular extrusions of AA 7075.
AA2195
12 Brass
10
Volume Percent (%)
Volume Percent (%)
12
8 S
6 4
Copper
2 0
2 3
5
7 8 Aspect Rao
AA7075
Brass
10 8 6
S
4 2 0
15
Copper 2 3
5
7 8 Aspect Rao
15
(b)
(a)
Fig. 10. Volume percent of common deformation crystallographic texture components as a function of aspect ratio for (a) AA 2195 and (b) AA 7075 extrusions.
results also indicate that lowering the Copper texture in Li-containing alloys to fractions seen in a 7xxx-series alloy resulted in strength anisotropy equivalent to that observed in the 7xxx-series alloy. This conclusion is in agreement with the present study wherein the strength anisotropy in Li-containing low aspect ratio extrusions is attributed to the relatively high volume percent of the Copper texture. The Copper texture is equivalent to grains orientated with the {112} plane parallel to the rolling/L-LT plane and the ⟨111⟩ direction in the rolling/extrusion direction as illustrated within an extrusion reference frame in Fig. 15. Using a theoretical approach, Barlat and Richmond [20] modeled the yield surfaces for an isotropic material, a material comprised of 50% Brass texture, a material comprised of 50% S texture, and a material comprised of 50% Copper texture. Of the three, the
component at low aspect ratios. For the AR2 and AR3 extrusions of AA 2195, the volume fraction of the Copper texture was greater than the Brass and the S components (resulting in a negative Plate-Like Number). In all other extrusions of AA 2195 and all extrusions of AA 7075, the highest volume fraction percent was Brass, followed by S and then by Copper. In addition, the Brass and S components were lower in the low AR AA 2195 extrusions than they were in the low AR AA 7075 extrusions. Jata, Hopkins, and Rioja [10] similarly reported lower fractions of the Brass texture in a Li-containing alloy and higher fractions of the Brass texture in a 7xxx-series alloy. They conclude that lowering the Brass component in a Li-containing alloy results in a decrease in strength anisotropy to that of levels observed in 7xxx-series alloys (AA 7150 and AA 7055). Though not explicitly stated, their 5
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J.M. Dickson and T.H. Sanders
low aspect ratio parts. 5. Conclusions This study presents a systematic investigation of the origins of and methods of controlling anisotropy in AA 2195. The results of this study highlight the importance of considering aspect ratio in addition to extrusion ratio. Although each profile in this study had an extrusion ratio of 20, the crystallographic texture and grain morphology varied with aspect ratio. Most notably, the increased fraction of the Copper component in low aspect ratio lithium containing extrusions was associated with strength anisotropy reported for these extrusions. The Plate-Like Number was introduced as a novel way to understand the effects of processing conditions on the Brass, S, and Copper crystallographic texture components and resulting properties. A negative Plate-Like Number is associated with increased anisotropy. Based on results using rolled plates, higher processing temperatures are recommended to reduce anisotropy and chunkiness in extruded AA 2195. The effects of aspect ratio on grain morphology and crystallographic texture in thermo-mechanically processed AA 2195 and AA 7075 were characterized. The significant conclusions are listed below:
Fig. 11. Variation in the Plate-Like Number with aspect ratio for AA 2195 and AA 7075 extrusions.
1. Aspect ratio should be considered as an extrusion variable in addition to extrusion ratio. 2. A single parameter, the Plate-Like Number, was developed to quantify the similarity of a sample's crystallographic texture to that typically observed in rolled plate and can be used to more clearly assess the effects of processing conditions on properties. 3. Extrusions with an aspect ratio of 5 and higher have positive values of the Plate-Like Number and more isotropic, plate-like crystallographic textures. 4. The high volume percent of the Copper crystallographic texture component (associated with a negative Plate-Like Number) is likely the primary cause of strength anisotropy in low aspect ratio AA 2195 extrusions. 5. The Plate-Like Number systematically increases with increasing processing temperature. Increasing the processing temperature will likely lead to a reduction in the Copper texture and thus the anisotropy in extruded AA 2195.
Fig. 12. Evolution of the area fraction of secondary phases in the AA 2195 ingot with time at temperature.
model material with 50% Copper texture resulted in the most deformed yield surface. Compared to the isotropic material, the 50% Copper material had a yield surface with a contracted transverse direction and a flat region near biaxial stretching [20]. Barlat and Richmond's result supports the theory that increasing the Copper texture in a sample leads to more anisotropic mechanical properties than equivalent increases in the Brass or S texture. If the Copper texture is primarily responsible for anisotropy, then reduction of the Copper texture should reduce the mechanical anisotropy. The plates rolled at 515 °C had the highest Plate-Like Numbers and the lowest volume fractions of the Copper texture; whereas, the low temperature 343 °C plates had the lowest Plate-Like numbers and the highest volume fractions of the Copper texture. Therefore, this study suggests that higher processing temperatures will likely reduce anisotropy in AA 2195. It is important to note that these results are drawn from high aspect ratio plates and may not present the same influence on
(a) 454°C 1 hr.
(b) 454°C 2 hrs.
Acknowledgements The authors would like to offer their sincere gratitude to Victor Dangerfield of Universal Alloy Corporation for his support and guidance throughout this effort. Additional thanks are extended to Justin Lamb of Universal Alloy Corporation for his assistance with the extrusions, David Tavakoli of Georgia Institute of Technology for providing training and technical expertise on X-ray diffraction, and Dr. Jordan Ciciliano for graphic assistance in creating Figure 15. This work was supported by the Department of Defense (DoD) through the National Science & Engineering Graduate Fellowship (NDSEG) Program. Due to the sensitive nature of exact processing parameters, the extrusion data is considered proprietary and is not reported in this manuscript. Supporting texture data can be found in the related
(c) 454°C 4 hrs.
(d) 454°C 8 hrs.
(e) 454°C 24 hrs.
Fig. 13. Optical micrographs of the AA 2195 ingot microstructure after (a) 1, (b) 2, (c) 4, (d) 8, and (e) 24 h dwells at 454 °C. 6
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Fig. 14. Volume percent of the (a) Brass, (b) S, and (c) Copper crystallographic textures and (d) the evolution of the Plate-Like Number as a function of dwell time for AA 2195 plates. [4] A. Bois-Brochu, F. Goma, C. Blais, D. Larouche, R. Gauvin, J. Boselli, Adv. Mater. Res. 409 (2012) 29–34. [5] J. J. Witters, B. A. Cheney, and R. J. Rioja, US Patent 5151136, 1992. [6] G. Tempus, W. Calles, G. Scharf, Mater. Sci. Technol. 7 (1991) 937–946. [7] S.J. Hales, R.A. Hafley, Mater. Sci. Eng. A 257 (1998) 153–164 (1998). [8] T.H. Sanders Jr., Development of Structure and Properties of 7050 and 7475 Alloys During Direct Extrusion, Alum. Co. of Amer. Alcoa Center PA Alcoa Labs, 1981. [9] G. Tempus, G. Scharf, W. Calles, J. Phys. 48 (1987) C3–187. [10] K.V. Jata, aK Hopkins, R.J. Rioja, Mater. Sci. Forum 217–222 (1996) 647–652. [11] A.K. Vasudevan, M.A. Przystupa, J. Fricke, G. W., Scr. Metall. Mater. 24 (1990) 1429–1434. [12] N.J. Kim, E.W. Lee, Acta Met. Mater. 41 (1993) 941–948. [13] R. Crooks, Z. Wang, V.I. Levit, R.N. Shenoy, Mater. Sci. Eng. A257 (1998) 145–152. [14] K.V. Jata, S. Panchanadeeswaran, A.K. Vasudevan, Mater. Sci. Eng. A257 (1998) 37–46. [15] A. Bois-Brochu, C. Blais, F.A. Tchitembo Goma, D. Larouche, Mater. Sci. Eng. A 673 (2016) 581–586. [16] A. Bois-Brochu, C. Blais, F.A.T. Goma, D. Larouche, J. Boselli, M. Brochu, Mater. Sci. Eng. A 597 (2014) 62–69. [17] R.C. Dorward, Metall. Trans. A. 18 (10) (1987) 1820–1823. [18] F. Bachmann, R. Hielscher, H. Schaeben, Sol. State Phen. 160 (2010) 63–68. [19] R.J. Rioja, J. Liu, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 43 (2012) 3325–3337. [20] F. Barlat, O. Richmond, Mater. Sci. Eng. 95 (1987) 15–29.
Fig. 15. A schematic of the Copper crystallographic texture within an extrusion reference frame.
dissertation: J.M. Dickson, “Development and control of strength anisotropy and crystallographic texture during extrusion of aluminum 2195 and 7075,” 2017. References [1] E. Starke, J. Staley, Prog. Aerosp. Sci. 32 (1996) 131–172. [2] K. P. Armanie, R. J. Rioja, D. K. Denzer, C. E. Brooks, D. K. Gadbery, and R. Newell, US Patent 6113711, 2000. [3] D.K. Denzer, P.A. Hollingshead, J. Liu, K.P. Armanie, R.J. Rioja, AluminiumLithium 2 (1992) 903–908 (1992).
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