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Surface asperity evolution and microstructure analysis of Al 6061T5 alloy in a quasi-static cold uniaxial planar compression (CUPC)
Applied Surface Science 347 (2015) 193–201
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface asperity evolution and microstructure analysis of Al 6061T5 alloy in a quasi-static cold uniaxial planar compression (CUPC) Hejie Li a,∗ , Zhengyi Jiang a , Dongbin Wei a,b , Xingjian Gao a , Jianzhong Xu c , Xiaoming Zhang c a
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia School of Electrical, Mechanical and Mechatronic Systems, University of Technology, Sydney, NSW 2007, Australia c State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, Liaoning 110004, China b
a r t i c l e
i n f o
Article history: Received 1 November 2014 Received in revised form 9 March 2015 Accepted 6 April 2015 Available online 16 April 2015 Keywords: Strain rate Surface asperity Cold uniaxial planar compression (CUPC) Microstructure Texture
a b s t r a c t In a quasi-static cold uniaxial planar compression, surface asperity evolution and microstructure analysis of Al 6061T5 alloy are carried out by employing Atomic Force Microscope (AFM) and Electron Backscattered Diffraction (EBSD) methods. Strain rate affects the surface asperity evolution obviously. While lubrication can hinder the surface asperity flattening by constraining the surface localized deformation. Lubrication can accelerate the crystallization in CUPC process. It also impedes the activation of some orientation components by hindering the activation of related slip systems in light metal Al alloy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In metal forming process, one of the most important parameters of metal products is the surface quality. Manufacturers always intend to improve the surface quality of metal products by employing all kinds of new technologies. As a main parameter of surface quality, surface roughness is always a focused factor. In metal forming process, the surface roughness evolution depends on many parameters such as original surface microstructures (grain size and texture) [1–5], original surface roughness [6], friction between the sample and the tool [7], tool surface roughness and hardness and deformation conditions (loading path and deformation rate) [8–11]. In general, the workpiece surface transfer in metal plastic forming process includes two typical cases: one is the free surface evolution, in which the workpiece surface does not receive any constraint from the tool such as the tensile and stretching processes [12–14]; the other is the constraint surface evolution, in this process, workpiece always contacts with the tool, therefore the surface is not free, but constrained by the tool. Previous research has investigated free surface evolution [15,16]. However, there are few literatures focusing on constrained surface
∗ Corresponding author. Tel.: +61 7 3217 2753. E-mail addresses: [email protected] (H. Li), [email protected] (Z. Jiang). http://dx.doi.org/10.1016/j.apsusc.2015.04.043 0169-4332/© 2015 Elsevier B.V. All rights reserved.
roughness evolution. Practically, in order to improve the surface quality of metal products, the deforming tool will be manufactured much flatter and smoother according to the requirement of produced products’ surface quality. So the surface roughness evolution in most constrained deformation is really a surface asperity flattening process [17]. In recent years, Li et al. [18–23] employed crystal plasticity theory to develop a 3D surface asperity flattening model to simulate the practical constrained deformation process, analyzed the relationship between the surface asperity and reduction, friction and texture. It was found that during the normal CUPC process surface roughness evolution of workpiece is proportional to gauged reduction, friction plays an important role in surface asperity flattening by decreasing the surface roughness and increasing micro hardness and flow stress. However, in the previous study, our research only focused on surface asperity evolution under a high strain rate. We did not investigate the influence of low strain rate on surface asperity flattening process. Therefore, in order to study the relationship between the surface asperity evolution and quasi-static deformation, and to investigate the influence of quasi-static deformation on the surface microstructure and surface quality, a low strain rate was employed to analyze the evolution of surface asperity and microstructure in a quasi-static cold uniaxial planar compression. In this study, the behavior of surface asperity evolution in a quasi-static CUPC process will be investigated by analysing the surface topology from AFM and microstructures from
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Fig. 3. Schematic of EBSD sample. Fig. 1. Microstructure after annealing heat-treatment.
2.2. Experimental procedure FEG-SEM-EBSD. The reason is because AFM can track the surface characters accurately and FEG-SEM EBSD can observe the evolution of microstructure and texture in surface areas obviously.
2. Experimental 2.1. Material In this study, 6061 Al alloy (face centered crystal, FCC) has been chosen as the research material. Originally, all workpieces have been put in a furnace, heated to 500 ◦ C, holding for 2 h, and then cooled in furnace. In order to generate the required surface roughness, samples were ground by the sand paper of No. P220. Surface roughness of all samples ranges as: Ra is about 0.72 m, Rq is about 0.84 m, and the minimum error is about 0.02 m. In a single FCC aluminum crystal there are 12 slip systems {1 1 1} 1 1 0: 4 slip planes A, B, C and D, and three slip directions in each slip plane. The slip systems are shown in Table 1.
Table 1 Slip planes and directions of FCC aluminum [21]. Slip plane
Slip direction
A (1 1 1) B (−1 1 1) C (1 −1 1) D (1 1 −1)
[0 1 −1] [1 0 1] [1 0 −1] [1 −1 0]
[1 0 −1] [1 1 0] [0 1 1] [1 0 1]
[1 −1 0] [0 1 −1] [1 1 0] [0 1 1]
After a fully annealed heat treatment (450 ◦ C, 2 h, see the Fig. 1), aluminum alloy 6061 samples were compressed on a channel die [19,21], while samples were constrained in the transverse direction. In order to reduce the influence of tool, the compressing tool is polished smoothly (roughness is about 10 nm) and flat. The deformation ranges from 0 to 60%. In further study, the larger deformation will be considered. The compressing channel die includes two parts: compressing mold and tool (see Fig. 2) [18,20,21]. The compression test was carried out by the INSTRON servo-hydraulic testing machine. Strain rate is about 0.001 s−1 . The experimental results with strain rate of 0.01 s−1 are used to compare with the experimental results with 0.001 s−1 , further show the influence of low strain rate of 0.001 s−1 on surface asperity flattening process. The experimental condition of strain rate of 0.01 s−1 is the same as the experimental condition of strain rate of 0.001 s−1 . In the present study, molly bond is chosen as the lubricant. The friction coefficient between the molly bond and aluminum is 0.2. 2.3. Cold Uniaxial Planar Compression (CUPC) process The compression was carried out in INSTRON material testing machine. The testing schedule is shown in Table 2. On the basis of static deformation, strain rates are chosen as 0.001 s−1 . Normally, it is difficult to control the strain rate on the INSTRON material testing machine, and in metal forming process, the strain rate does not keep the same value. There are two solutions employed: firstly in very short deformation process, the variation of strain rate is not significant, and in some cases, the strain rate in this kind process can be considered as a constant; the other case
Fig. 2. Compressing equipment and sample: (a) compressing device, (b) sample [18,21]. Unit: mm.
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Fig. 4. Development of surface roughness under different deformation rates: 0.001 and 0.01: (a) Ra ; (b) Rq . Table 2 Compression schedule. Samples
Height (mm)
Reduction (%)
Strain rate (s−1 )
Displacement rate (mm/min)
Displacement (mm)
Height after compression (mm)
N1/L1 N2/L2 N3/L3 N4/L4
6.30 6.30 6.30 6.30
0 20 40 60
0.001 0.001 0.001 0.001
0 −3.39 −2.96 −2.48
0 1.26 2.51 3.78
6.30 5.04 3.79 2.52
is to use certain parameters to control the real strain rate. In this study, the strain rate is low and the deformation is a quasi-static process, therefore the strain rate can be considered as a constant. In the INSTRON material testing machine, the displacement rate is used to replace the strain rate [21–23]. The relationship between the strain rate and displacement rate is H0 = H + H,
ε = ln
H , H0
t =
ε , ε˙
U˙ =
H t
(1)
where H0 , H, H are the original height, the height after compression and the height increment respectively. ε is the true strain. t is the time of deformation. U˙ is the displacement rate of sample in the compression. 2.4. Surface measurement by atomic force microscopy Before and after the compressions, sample surface morphology and roughness were measured by the atomic force microscopy (AFM). The AFM maps are scanned by a contact mode with the following set of parameters: scan size of 50.00 m, scan rate of 1.5 Hz and resolution of 512. The distance between mid-section and surface is only about 400 m. 2.5. Grain measurement by electron back-scattering diffraction The samples used for Electron Back-Scattering Diffraction (EBSD) were shown in Fig. 3. AFM and EBSD are combined to analyze the relationship between the surface asperity feature and surface texture. The measuring map of cold-planar compressed sample is analyzed by the method shown in Ref. [21,24]. Low-angle grain boundary (LAGB) is defined as 2◦ ≤ < 15◦ , where is the spreading angle of grain orientation. High-angle grain boundary (HAGB) is defined as 15◦ ≤ ≤ 62.8◦ . The threshold of sub-grain is 2◦ . EBSD
test was conducted at the mid-section of the samples in the normal direction (ND)-rolling direction (RD), step sizes of 0.5 and 0.25 m were employed for the samples with the different reductions. The EBSD acquisition is detailed in Ref. [24]. Before and after the compression, the sample surface morphology and roughness were measured in the same area as by AFM.
3. Results and discussion 3.1. Surface asperity In the normal cold uniaxial planar compression, lubrication plays a role in hindering the surface asperity flattening process [19–22]. However, there are no concerns about the effect of the deformation rate on the surface asperity flattening process. Fig. 4 shows the previous experimental results with high strain rate (0.01 s−1 ) are compared with the current results with the low strain rate (0.001 s−1 ). From Fig. 4, it can be seen that the deformation rate plays an obvious role in the evolution of surface asperity (surface roughness, Ra and Rq ). It is common that with the increase of gauged reduction, surface roughness tends to decrease. However, under the same gauged reduction, the workpiece with a low deformation rate will have a much rougher surface than that of the one with large deformation rate. The reason for this is probably because when the deformation takes place slowly, even in a quasi-static deformation process, the surface strain is very compatible due to the reason of time. On the other hand, if the deformation happens in a very short period, strain inhomogeneity in surface layer will be significant due to the low compatibility of deformation. So compared to the case in a low deformation rate, large deformation rate may lead to an obvious surface inhomogeneous deformation. The inhomogeneous deformation may come from the different strain gradients of the
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Fig. 5. Development of surface asperity under different strain rate: 0.001 and 0.01.
ridge and valley areas. If the deformation rate is large, the ridge areas will contact the tool firstly, take place a large scale deformation and make the ridge height become lower. However, owing to the short deformation time and no contact with the tool, the valley areas will have a small scale deformation. This will lead to the inhomogeneity of deformation and then decrease the surface roughness of workpiece. Fig. 5 shows the effect of strain rate on the surface morphology. Under the same gauged reduction, workpiece deformed under a large deformation rate will have an obviously scratched surface with a low surface roughness. This may be led by the
contact between the tool and the workpiece, the inconsistency of deformation in the short time deformation. In grain-scale surface roughness evolution, friction coefficient might become a function of the strain [25]. Surface roughness evolution is also a function of strain. With an increase of strain rate, the relative surface slide in certain deformation period will go up significantly. This will also result in an increase of friction coefficient. Due to the influence of friction coefficient, the friction will also increase. This is the reason why surface with large deformation rate has a low surface roughness but with obvious surface scratches.
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Fig. 6. Comparison of surface asperity under 40% reduction with different lubrication conditions: (a) with lubrication, (b) without lubrication.
Fig. 7. Development of microstructures of surface layer under different strain rates: 0.001 and 0.01: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
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Fig. 8. Microstructure components under different gauged reductions and lubrication conditions: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
Fig. 6 compares the surface asperity evolution under the same reduction (40%) and different lubrication conditions. In Fig. 6(b), surface roughness of workpiece is high, but it has a smooth surface without any surface scratches. However, in Fig. 6 (a), workpiece surface is flat, but has some obvious surface scratches. It is very clear that under the same reduction, the sample compressed with lubrication (molly bond) was rougher than the sample compressed without lubrication. Therefore, in cold uniaxial planar compression, the lubrication hinder the surface asperity flattening process obviously.
not work well. When the gauged reduction exceeds 40%, influence of lubrication becomes obvious: under the same gauged reduction, workpieces compressed without lubrication has much flattened surface microstructures. When the reduction reaches to 60%, the influence becomes much more obvious. Workpiece compressed without lubrication obtains a fiber-like microstructure with some in-granular shear bands. While the workpiece compressed without lubrication has a wider microstructure with less shear bands. Friction between the tool and the workpiece can be a new drive to intensify the shear deformation in surface layer.
3.2. Analysis of microstructure
3.2.2. (b) Contents in microstructure In a metal forming process, the contents of microstructure normally includes three components (Fig. 8): crystallized microstructure, substructure and deformed microstructure. When the deformation takes place in an environment of lubrication, substructured component tends to rise with an increase of gauged reduction, and recrystallised component also increases to some extent (Fig. 8(a), (c) and (e)). On the other hand, if the deformation happens in a no-lubrication condition, the two previous
3.2.1. (a) Surface microstructure In a quasi-static cold uniaxial planar compression, lubrication can affect the evolution of surface microstructure obviously. Fig. 7 shows the development of surface microstructure under different reductions and friction conditions: when the reduction is about 20%, the influence of lubrication is not obvious. Both workpieces have the similar microstructures. In this process, lubrication does
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components do not show any obvious change. However, the ratio of deformed microstructure tends to increase. Normally, the substructure comes with the energy change. So in the condition of lubrication, the external work rate promotes the substructure of surface microstructure. While the friction between the tool and the workpiece contributes more to the deformation of surface microstructure. 3.3. Development of orientation factor (Schmid factor) 3.3.1. (a) Orientation factor If the external load is applied on the workpiece (Fig. 9), the shear stress which acts on the slip system along the slip direction can be expressed as nd = y =
Fig. 9. Relationship between the stress axis and slip plane and direction [20,21].
P cos cos A
(2)
where P is the load, A is the loaded section, is the angle between the loading axis and normal to the slip plane, is the angle between the loading axis and slip direction and nd is the shear stress of slip system. nd = ˛
(3)
Fig. 10. Development of Schmid factor under different gauged reduction and strain rate: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
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Fig. 11. Development of ODF (Orientation Distribution Function) map under different gauged reductions and strain rates: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%; (g) ideal orientation components.
where ˛ represents the critical resolved shear stress. The Schmid factor is defined as M = cos cos
plastic deformation process, the orientation factor (Schmid factor) M can be obtained by Eq. (4).
(4)
When a crystal takes plastic deformation, stresses on different slip systems will reach a critical resolved shear stress ˛ . This is the key point of plastic deformation led by the slip. If the stresses of different slip systems do not reach the critical shear stress, the stress on different slip systems can be calculated by Eq. (5). In the
i = s Mi
(5)
where i is the stress of the ith slip system. s is the material yield strength, and in this study, it is 276 MPa [21]. Mi is the orientation factor (Schmid factor) of ith slip system.
H. Li et al. / Applied Surface Science 347 (2015) 193–201
3.3.2. (b) Development of orientation factor The Schmid factor is also called orientation hardness, which shows the deformation can take place easy or not. It is shown in Fig. 10 that when the gauged reduction increases, the orientation will distribute according to a certain rule: increasing slip systems orientate to a soft orientation (about 0.5) in which slip can take place easily. If a slip system is in soft orientation, it means the stress in it reaches the critical resolved shear stress ˛ . Lubrication can hinder the soft orientation. Under the influence of lubrication, the slip systems of the workpiece cannot shift to soft orientation easily. 3.4. Development of orientation distribution function (ODF) In most cases, the analysis does not need all the data of ODF. The popular way is to choose several typical sections for analysis. In the analysis of cubic system, normally two sections are used for analysis: 2 = 0◦ and 2 = 45◦ [26]. Here we also use these two sections for orientation distribution analysis. After annealed heat treatment, the surface microstructure of workpiece has two main orientation components: cubic orientation {0 0 1} 1 0 0 and Goss orientation {1 1 0} 0 0 1. In quasi-static CUPC process, the development of orientation components follows a certain rule and lubrication also affects the orientation components obviously. If workpiece is compressed with lubrication, an increase of gauged reduction can lead to an obvious decrease of cubic orientation and Goss, an increase of brass orientation. If the workpiece contacts with the deforming tool directly, the increase of gauged reduction can result in a significant drop of Goss orientation, an obvious increase of Brass orientation R {1 1 1} 1 1 2 and a few copper orientation {1 1 2} 1 1 1. The reason for this is because lubrication condition can affect the stress state in the surface layer of workpiece. While the orientation variation depends on the related stress state. The different lubrication conditions lead to different stress states in the surface layers, then result in the difference in orientation (Fig. 11). 4. Conclusions In quasi-static cold uniaxial planar compression, the development of surface asperity shows the following results: (a) Deformation rate (strain rate) plays a significant role in the surface asperity flattening: low strain rate can hinder the surface asperity flattening by restraining the localized surface strain hardening. (b) Lubrication also affects the microstructure in surface layer obviously: lubrication can constrain the surface asperity flattening process, while friction between the tool and workpiece become a drive to intensify the localized surface deformation.
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(c) Lubrication can increase the recrystallisation in CUPC by less constrained deformation. (d) Lubrication can hinder the activation of slip systems in the light metal Al by slowing down the orientation softening. (e) In the quasi-static cold uniaxial planar compression, with an increase of gauged reduction, lubrication can promote the generation of some orientation components such as brass orientation, but decrease the orientation components for example Goss orientation {1 1 0} 0 0 1 and cubic orientation {0 0 1} 1 0 0. Under the same reduction, friction can accelerate the forming of Brass orientation R {1 1 1} 1 1 2 and a few copper orientation {1 1 2} 1 1 1. Acknowledgements The authors would like to thank UOW Electron Microscopy Centre (EMC) for EBSD mapping and Mr. Cameron Neilson for the operation of CUPU process. References [1] R. Becker, Acta Mater. 46 (1998) 1385–1401. [2] D. Raabe, M. Sachtleber, H. Weiland, G. Scheele, Z. Zhao, Acta Mater. 51 (2003) 1539–1560. [3] M. Jain, D.J. Lloyd, S.R. Macewen, Int. J. Mech. Sci. 38 (1996) 219–232. [4] Z. Zhao, M. Ramesh, D. Raabe, A.M. Cuitino, R. Radovitzky, Int. J. Plasticity 24 (2008) 2278–2297. [5] Z. Zhao, R. Radovitzky, A. Cuitino, Acta Mater. 52 (2004) 5791–5804. [6] K. Osakada, M. Oyane, Bull. JSME 14 (1971) 171. [7] M. Tokizawa, Y. Yosikawa, J. Japan. Inst. Met. 37 (1973) 19–25. [8] G. Chen, H. Shen, S. Hu, B. Baudelet, Mater. Sci. Eng. A 128 (1990) 33–38. [9] M.R. Stoudt, R.E. Ricker, Metall. Mater. Trans. A 33 (2002) 2883–2889. [10] W.R.D. Wilson, J. Manuf. Sci. 119 (1997) 695–698. [11] W.R.D. Wilson, W.M. Lee, J. Manuf. Sci. Eng. 123 (2001) 279–283. [12] A. Makinouchi, H. Ike, M. Murakawa, N. Koga, Wear 128 (1988) 109–122. [13] S. Sheu, W.R.D. Wilson, Proc. 11. N.A.M.R.C., 1983, pp. 172–178. [14] M.P.F. Sutcliffe:, Int. J. Mech. Sci. 11 (1988) 847–868. [15] G.E. Dieter, Mech. Metall., 3rd edition, McGraw-Hill, New York, NY, 1986. [16] P. Groche, R. Schafer, H. Justinger, M. Ludwig, Int. J. Mech. Sci. 52 (2010) 523–530. [17] P. Groche, R. Schafer, M. Henning, Proceedings of the Third International Conference on Tribology in Manufacturing Processes, ICTMP2007, Yokohama National University, Yokohama, 2007, pp. 219–226. [18] H.J. Li, Z.Y. Jiang, D.B. Wei, Wear 301 (2013) 11–18. [19] H.J. Li, Z.Y. Jiang, D.B. Wei:, Tribol. Int. 66 (2013) 282–288. [20] H.J. Li, Z.Y. Jiang, D.B. Wei, Tribol. Lett. 46 (2012) 101–112. [21] H.J. Li, A study of surface roughness in the metal forming process, Ph.D Thesis, UOW, 2012. [22] H.J. Li, Z.Y. Jiang, D.B. Wei, P.J. Yan, A.K. Tieu, J.T. Han, International Conference on Tribology in Manufacturing Processes, 2010, pp. 397–406. [23] H.J. Li, Z.Y. Jiang, D.B. Wei, J.T. Han, A.K. Tieu, Wear 271 (2011) 1778–1784. [24] A. Gazder, M. Sanchez-Araiza, J. Johnas, E. Pereloma, Acta Mater. 59 (2011) 4847–4865. [25] M. Sachtleber, Z. Zhao, D. Raabe, Mater. Sci. Eng. A 336 (2002) 81–87. [26] W.M. Mao, P. Yang, L. Cheng. Analysing Principles and Test Techniques of Material Texture. Metallurgical Industry Press.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface asperity evolution and microstructure analysis of Al 6061T5 alloy in a quasi-static cold uniaxial planar compression (CUPC) Hejie Li a,∗ , Zhengyi Jiang a , Dongbin Wei a,b , Xingjian Gao a , Jianzhong Xu c , Xiaoming Zhang c a
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia School of Electrical, Mechanical and Mechatronic Systems, University of Technology, Sydney, NSW 2007, Australia c State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, Liaoning 110004, China b
a r t i c l e
i n f o
Article history: Received 1 November 2014 Received in revised form 9 March 2015 Accepted 6 April 2015 Available online 16 April 2015 Keywords: Strain rate Surface asperity Cold uniaxial planar compression (CUPC) Microstructure Texture
a b s t r a c t In a quasi-static cold uniaxial planar compression, surface asperity evolution and microstructure analysis of Al 6061T5 alloy are carried out by employing Atomic Force Microscope (AFM) and Electron Backscattered Diffraction (EBSD) methods. Strain rate affects the surface asperity evolution obviously. While lubrication can hinder the surface asperity flattening by constraining the surface localized deformation. Lubrication can accelerate the crystallization in CUPC process. It also impedes the activation of some orientation components by hindering the activation of related slip systems in light metal Al alloy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In metal forming process, one of the most important parameters of metal products is the surface quality. Manufacturers always intend to improve the surface quality of metal products by employing all kinds of new technologies. As a main parameter of surface quality, surface roughness is always a focused factor. In metal forming process, the surface roughness evolution depends on many parameters such as original surface microstructures (grain size and texture) [1–5], original surface roughness [6], friction between the sample and the tool [7], tool surface roughness and hardness and deformation conditions (loading path and deformation rate) [8–11]. In general, the workpiece surface transfer in metal plastic forming process includes two typical cases: one is the free surface evolution, in which the workpiece surface does not receive any constraint from the tool such as the tensile and stretching processes [12–14]; the other is the constraint surface evolution, in this process, workpiece always contacts with the tool, therefore the surface is not free, but constrained by the tool. Previous research has investigated free surface evolution [15,16]. However, there are few literatures focusing on constrained surface
∗ Corresponding author. Tel.: +61 7 3217 2753. E-mail addresses: [email protected] (H. Li), [email protected] (Z. Jiang). http://dx.doi.org/10.1016/j.apsusc.2015.04.043 0169-4332/© 2015 Elsevier B.V. All rights reserved.
roughness evolution. Practically, in order to improve the surface quality of metal products, the deforming tool will be manufactured much flatter and smoother according to the requirement of produced products’ surface quality. So the surface roughness evolution in most constrained deformation is really a surface asperity flattening process [17]. In recent years, Li et al. [18–23] employed crystal plasticity theory to develop a 3D surface asperity flattening model to simulate the practical constrained deformation process, analyzed the relationship between the surface asperity and reduction, friction and texture. It was found that during the normal CUPC process surface roughness evolution of workpiece is proportional to gauged reduction, friction plays an important role in surface asperity flattening by decreasing the surface roughness and increasing micro hardness and flow stress. However, in the previous study, our research only focused on surface asperity evolution under a high strain rate. We did not investigate the influence of low strain rate on surface asperity flattening process. Therefore, in order to study the relationship between the surface asperity evolution and quasi-static deformation, and to investigate the influence of quasi-static deformation on the surface microstructure and surface quality, a low strain rate was employed to analyze the evolution of surface asperity and microstructure in a quasi-static cold uniaxial planar compression. In this study, the behavior of surface asperity evolution in a quasi-static CUPC process will be investigated by analysing the surface topology from AFM and microstructures from
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Fig. 3. Schematic of EBSD sample. Fig. 1. Microstructure after annealing heat-treatment.
2.2. Experimental procedure FEG-SEM-EBSD. The reason is because AFM can track the surface characters accurately and FEG-SEM EBSD can observe the evolution of microstructure and texture in surface areas obviously.
2. Experimental 2.1. Material In this study, 6061 Al alloy (face centered crystal, FCC) has been chosen as the research material. Originally, all workpieces have been put in a furnace, heated to 500 ◦ C, holding for 2 h, and then cooled in furnace. In order to generate the required surface roughness, samples were ground by the sand paper of No. P220. Surface roughness of all samples ranges as: Ra is about 0.72 m, Rq is about 0.84 m, and the minimum error is about 0.02 m. In a single FCC aluminum crystal there are 12 slip systems {1 1 1} 1 1 0: 4 slip planes A, B, C and D, and three slip directions in each slip plane. The slip systems are shown in Table 1.
Table 1 Slip planes and directions of FCC aluminum [21]. Slip plane
Slip direction
A (1 1 1) B (−1 1 1) C (1 −1 1) D (1 1 −1)
[0 1 −1] [1 0 1] [1 0 −1] [1 −1 0]
[1 0 −1] [1 1 0] [0 1 1] [1 0 1]
[1 −1 0] [0 1 −1] [1 1 0] [0 1 1]
After a fully annealed heat treatment (450 ◦ C, 2 h, see the Fig. 1), aluminum alloy 6061 samples were compressed on a channel die [19,21], while samples were constrained in the transverse direction. In order to reduce the influence of tool, the compressing tool is polished smoothly (roughness is about 10 nm) and flat. The deformation ranges from 0 to 60%. In further study, the larger deformation will be considered. The compressing channel die includes two parts: compressing mold and tool (see Fig. 2) [18,20,21]. The compression test was carried out by the INSTRON servo-hydraulic testing machine. Strain rate is about 0.001 s−1 . The experimental results with strain rate of 0.01 s−1 are used to compare with the experimental results with 0.001 s−1 , further show the influence of low strain rate of 0.001 s−1 on surface asperity flattening process. The experimental condition of strain rate of 0.01 s−1 is the same as the experimental condition of strain rate of 0.001 s−1 . In the present study, molly bond is chosen as the lubricant. The friction coefficient between the molly bond and aluminum is 0.2. 2.3. Cold Uniaxial Planar Compression (CUPC) process The compression was carried out in INSTRON material testing machine. The testing schedule is shown in Table 2. On the basis of static deformation, strain rates are chosen as 0.001 s−1 . Normally, it is difficult to control the strain rate on the INSTRON material testing machine, and in metal forming process, the strain rate does not keep the same value. There are two solutions employed: firstly in very short deformation process, the variation of strain rate is not significant, and in some cases, the strain rate in this kind process can be considered as a constant; the other case
Fig. 2. Compressing equipment and sample: (a) compressing device, (b) sample [18,21]. Unit: mm.
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Fig. 4. Development of surface roughness under different deformation rates: 0.001 and 0.01: (a) Ra ; (b) Rq . Table 2 Compression schedule. Samples
Height (mm)
Reduction (%)
Strain rate (s−1 )
Displacement rate (mm/min)
Displacement (mm)
Height after compression (mm)
N1/L1 N2/L2 N3/L3 N4/L4
6.30 6.30 6.30 6.30
0 20 40 60
0.001 0.001 0.001 0.001
0 −3.39 −2.96 −2.48
0 1.26 2.51 3.78
6.30 5.04 3.79 2.52
is to use certain parameters to control the real strain rate. In this study, the strain rate is low and the deformation is a quasi-static process, therefore the strain rate can be considered as a constant. In the INSTRON material testing machine, the displacement rate is used to replace the strain rate [21–23]. The relationship between the strain rate and displacement rate is H0 = H + H,
ε = ln
H , H0
t =
ε , ε˙
U˙ =
H t
(1)
where H0 , H, H are the original height, the height after compression and the height increment respectively. ε is the true strain. t is the time of deformation. U˙ is the displacement rate of sample in the compression. 2.4. Surface measurement by atomic force microscopy Before and after the compressions, sample surface morphology and roughness were measured by the atomic force microscopy (AFM). The AFM maps are scanned by a contact mode with the following set of parameters: scan size of 50.00 m, scan rate of 1.5 Hz and resolution of 512. The distance between mid-section and surface is only about 400 m. 2.5. Grain measurement by electron back-scattering diffraction The samples used for Electron Back-Scattering Diffraction (EBSD) were shown in Fig. 3. AFM and EBSD are combined to analyze the relationship between the surface asperity feature and surface texture. The measuring map of cold-planar compressed sample is analyzed by the method shown in Ref. [21,24]. Low-angle grain boundary (LAGB) is defined as 2◦ ≤ < 15◦ , where is the spreading angle of grain orientation. High-angle grain boundary (HAGB) is defined as 15◦ ≤ ≤ 62.8◦ . The threshold of sub-grain is 2◦ . EBSD
test was conducted at the mid-section of the samples in the normal direction (ND)-rolling direction (RD), step sizes of 0.5 and 0.25 m were employed for the samples with the different reductions. The EBSD acquisition is detailed in Ref. [24]. Before and after the compression, the sample surface morphology and roughness were measured in the same area as by AFM.
3. Results and discussion 3.1. Surface asperity In the normal cold uniaxial planar compression, lubrication plays a role in hindering the surface asperity flattening process [19–22]. However, there are no concerns about the effect of the deformation rate on the surface asperity flattening process. Fig. 4 shows the previous experimental results with high strain rate (0.01 s−1 ) are compared with the current results with the low strain rate (0.001 s−1 ). From Fig. 4, it can be seen that the deformation rate plays an obvious role in the evolution of surface asperity (surface roughness, Ra and Rq ). It is common that with the increase of gauged reduction, surface roughness tends to decrease. However, under the same gauged reduction, the workpiece with a low deformation rate will have a much rougher surface than that of the one with large deformation rate. The reason for this is probably because when the deformation takes place slowly, even in a quasi-static deformation process, the surface strain is very compatible due to the reason of time. On the other hand, if the deformation happens in a very short period, strain inhomogeneity in surface layer will be significant due to the low compatibility of deformation. So compared to the case in a low deformation rate, large deformation rate may lead to an obvious surface inhomogeneous deformation. The inhomogeneous deformation may come from the different strain gradients of the
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Fig. 5. Development of surface asperity under different strain rate: 0.001 and 0.01.
ridge and valley areas. If the deformation rate is large, the ridge areas will contact the tool firstly, take place a large scale deformation and make the ridge height become lower. However, owing to the short deformation time and no contact with the tool, the valley areas will have a small scale deformation. This will lead to the inhomogeneity of deformation and then decrease the surface roughness of workpiece. Fig. 5 shows the effect of strain rate on the surface morphology. Under the same gauged reduction, workpiece deformed under a large deformation rate will have an obviously scratched surface with a low surface roughness. This may be led by the
contact between the tool and the workpiece, the inconsistency of deformation in the short time deformation. In grain-scale surface roughness evolution, friction coefficient might become a function of the strain [25]. Surface roughness evolution is also a function of strain. With an increase of strain rate, the relative surface slide in certain deformation period will go up significantly. This will also result in an increase of friction coefficient. Due to the influence of friction coefficient, the friction will also increase. This is the reason why surface with large deformation rate has a low surface roughness but with obvious surface scratches.
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Fig. 6. Comparison of surface asperity under 40% reduction with different lubrication conditions: (a) with lubrication, (b) without lubrication.
Fig. 7. Development of microstructures of surface layer under different strain rates: 0.001 and 0.01: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
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Fig. 8. Microstructure components under different gauged reductions and lubrication conditions: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
Fig. 6 compares the surface asperity evolution under the same reduction (40%) and different lubrication conditions. In Fig. 6(b), surface roughness of workpiece is high, but it has a smooth surface without any surface scratches. However, in Fig. 6 (a), workpiece surface is flat, but has some obvious surface scratches. It is very clear that under the same reduction, the sample compressed with lubrication (molly bond) was rougher than the sample compressed without lubrication. Therefore, in cold uniaxial planar compression, the lubrication hinder the surface asperity flattening process obviously.
not work well. When the gauged reduction exceeds 40%, influence of lubrication becomes obvious: under the same gauged reduction, workpieces compressed without lubrication has much flattened surface microstructures. When the reduction reaches to 60%, the influence becomes much more obvious. Workpiece compressed without lubrication obtains a fiber-like microstructure with some in-granular shear bands. While the workpiece compressed without lubrication has a wider microstructure with less shear bands. Friction between the tool and the workpiece can be a new drive to intensify the shear deformation in surface layer.
3.2. Analysis of microstructure
3.2.2. (b) Contents in microstructure In a metal forming process, the contents of microstructure normally includes three components (Fig. 8): crystallized microstructure, substructure and deformed microstructure. When the deformation takes place in an environment of lubrication, substructured component tends to rise with an increase of gauged reduction, and recrystallised component also increases to some extent (Fig. 8(a), (c) and (e)). On the other hand, if the deformation happens in a no-lubrication condition, the two previous
3.2.1. (a) Surface microstructure In a quasi-static cold uniaxial planar compression, lubrication can affect the evolution of surface microstructure obviously. Fig. 7 shows the development of surface microstructure under different reductions and friction conditions: when the reduction is about 20%, the influence of lubrication is not obvious. Both workpieces have the similar microstructures. In this process, lubrication does
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components do not show any obvious change. However, the ratio of deformed microstructure tends to increase. Normally, the substructure comes with the energy change. So in the condition of lubrication, the external work rate promotes the substructure of surface microstructure. While the friction between the tool and the workpiece contributes more to the deformation of surface microstructure. 3.3. Development of orientation factor (Schmid factor) 3.3.1. (a) Orientation factor If the external load is applied on the workpiece (Fig. 9), the shear stress which acts on the slip system along the slip direction can be expressed as nd = y =
Fig. 9. Relationship between the stress axis and slip plane and direction [20,21].
P cos cos A
(2)
where P is the load, A is the loaded section, is the angle between the loading axis and normal to the slip plane, is the angle between the loading axis and slip direction and nd is the shear stress of slip system. nd = ˛
(3)
Fig. 10. Development of Schmid factor under different gauged reduction and strain rate: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%.
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Fig. 11. Development of ODF (Orientation Distribution Function) map under different gauged reductions and strain rates: (a) with lubrication, reduction 20%; (b) without lubrication, reduction 20%; (c) with lubrication 40%; (d) without lubrication 40%; (e) with lubrication, reduction 60%; (f) without lubrication, reduction 60%; (g) ideal orientation components.
where ˛ represents the critical resolved shear stress. The Schmid factor is defined as M = cos cos
plastic deformation process, the orientation factor (Schmid factor) M can be obtained by Eq. (4).
(4)
When a crystal takes plastic deformation, stresses on different slip systems will reach a critical resolved shear stress ˛ . This is the key point of plastic deformation led by the slip. If the stresses of different slip systems do not reach the critical shear stress, the stress on different slip systems can be calculated by Eq. (5). In the
i = s Mi
(5)
where i is the stress of the ith slip system. s is the material yield strength, and in this study, it is 276 MPa [21]. Mi is the orientation factor (Schmid factor) of ith slip system.
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3.3.2. (b) Development of orientation factor The Schmid factor is also called orientation hardness, which shows the deformation can take place easy or not. It is shown in Fig. 10 that when the gauged reduction increases, the orientation will distribute according to a certain rule: increasing slip systems orientate to a soft orientation (about 0.5) in which slip can take place easily. If a slip system is in soft orientation, it means the stress in it reaches the critical resolved shear stress ˛ . Lubrication can hinder the soft orientation. Under the influence of lubrication, the slip systems of the workpiece cannot shift to soft orientation easily. 3.4. Development of orientation distribution function (ODF) In most cases, the analysis does not need all the data of ODF. The popular way is to choose several typical sections for analysis. In the analysis of cubic system, normally two sections are used for analysis: 2 = 0◦ and 2 = 45◦ [26]. Here we also use these two sections for orientation distribution analysis. After annealed heat treatment, the surface microstructure of workpiece has two main orientation components: cubic orientation {0 0 1} 1 0 0 and Goss orientation {1 1 0} 0 0 1. In quasi-static CUPC process, the development of orientation components follows a certain rule and lubrication also affects the orientation components obviously. If workpiece is compressed with lubrication, an increase of gauged reduction can lead to an obvious decrease of cubic orientation and Goss, an increase of brass orientation. If the workpiece contacts with the deforming tool directly, the increase of gauged reduction can result in a significant drop of Goss orientation, an obvious increase of Brass orientation R {1 1 1} 1 1 2 and a few copper orientation {1 1 2} 1 1 1. The reason for this is because lubrication condition can affect the stress state in the surface layer of workpiece. While the orientation variation depends on the related stress state. The different lubrication conditions lead to different stress states in the surface layers, then result in the difference in orientation (Fig. 11). 4. Conclusions In quasi-static cold uniaxial planar compression, the development of surface asperity shows the following results: (a) Deformation rate (strain rate) plays a significant role in the surface asperity flattening: low strain rate can hinder the surface asperity flattening by restraining the localized surface strain hardening. (b) Lubrication also affects the microstructure in surface layer obviously: lubrication can constrain the surface asperity flattening process, while friction between the tool and workpiece become a drive to intensify the localized surface deformation.
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(c) Lubrication can increase the recrystallisation in CUPC by less constrained deformation. (d) Lubrication can hinder the activation of slip systems in the light metal Al by slowing down the orientation softening. (e) In the quasi-static cold uniaxial planar compression, with an increase of gauged reduction, lubrication can promote the generation of some orientation components such as brass orientation, but decrease the orientation components for example Goss orientation {1 1 0} 0 0 1 and cubic orientation {0 0 1} 1 0 0. Under the same reduction, friction can accelerate the forming of Brass orientation R {1 1 1} 1 1 2 and a few copper orientation {1 1 2} 1 1 1. Acknowledgements The authors would like to thank UOW Electron Microscopy Centre (EMC) for EBSD mapping and Mr. Cameron Neilson for the operation of CUPU process. References [1] R. Becker, Acta Mater. 46 (1998) 1385–1401. [2] D. Raabe, M. Sachtleber, H. Weiland, G. Scheele, Z. Zhao, Acta Mater. 51 (2003) 1539–1560. [3] M. Jain, D.J. Lloyd, S.R. Macewen, Int. J. Mech. Sci. 38 (1996) 219–232. [4] Z. Zhao, M. Ramesh, D. Raabe, A.M. Cuitino, R. Radovitzky, Int. J. Plasticity 24 (2008) 2278–2297. [5] Z. Zhao, R. Radovitzky, A. Cuitino, Acta Mater. 52 (2004) 5791–5804. [6] K. Osakada, M. Oyane, Bull. JSME 14 (1971) 171. [7] M. Tokizawa, Y. Yosikawa, J. Japan. Inst. Met. 37 (1973) 19–25. [8] G. Chen, H. Shen, S. Hu, B. Baudelet, Mater. Sci. Eng. A 128 (1990) 33–38. [9] M.R. Stoudt, R.E. Ricker, Metall. Mater. Trans. A 33 (2002) 2883–2889. [10] W.R.D. Wilson, J. Manuf. Sci. 119 (1997) 695–698. [11] W.R.D. Wilson, W.M. Lee, J. Manuf. Sci. Eng. 123 (2001) 279–283. [12] A. Makinouchi, H. Ike, M. Murakawa, N. Koga, Wear 128 (1988) 109–122. [13] S. Sheu, W.R.D. Wilson, Proc. 11. N.A.M.R.C., 1983, pp. 172–178. [14] M.P.F. Sutcliffe:, Int. J. Mech. Sci. 11 (1988) 847–868. [15] G.E. Dieter, Mech. Metall., 3rd edition, McGraw-Hill, New York, NY, 1986. [16] P. Groche, R. Schafer, H. Justinger, M. Ludwig, Int. J. Mech. Sci. 52 (2010) 523–530. [17] P. Groche, R. Schafer, M. Henning, Proceedings of the Third International Conference on Tribology in Manufacturing Processes, ICTMP2007, Yokohama National University, Yokohama, 2007, pp. 219–226. [18] H.J. Li, Z.Y. Jiang, D.B. Wei, Wear 301 (2013) 11–18. [19] H.J. Li, Z.Y. Jiang, D.B. Wei:, Tribol. Int. 66 (2013) 282–288. [20] H.J. Li, Z.Y. Jiang, D.B. Wei, Tribol. Lett. 46 (2012) 101–112. [21] H.J. Li, A study of surface roughness in the metal forming process, Ph.D Thesis, UOW, 2012. [22] H.J. Li, Z.Y. Jiang, D.B. Wei, P.J. Yan, A.K. Tieu, J.T. Han, International Conference on Tribology in Manufacturing Processes, 2010, pp. 397–406. [23] H.J. Li, Z.Y. Jiang, D.B. Wei, J.T. Han, A.K. Tieu, Wear 271 (2011) 1778–1784. [24] A. Gazder, M. Sanchez-Araiza, J. Johnas, E. Pereloma, Acta Mater. 59 (2011) 4847–4865. [25] M. Sachtleber, Z. Zhao, D. Raabe, Mater. Sci. Eng. A 336 (2002) 81–87. [26] W.M. Mao, P. Yang, L. Cheng. Analysing Principles and Test Techniques of Material Texture. Metallurgical Industry Press.