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Surface topography changes in aluminum alloy sheet during large plastic straining under cyclic pure bending
Journal of Materials Processing Technology 213 (2013) 300–307
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Surface topography changes in aluminum alloy sheet during large plastic straining under cyclic pure bending Glenn A. Lucachick, L. Rafael Sanchez ∗ Mechanical Engineering Department, University of Colorado Denver, 1200 Larimer St, Denver, CO 80217, USA
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 11 August 2012 Accepted 14 September 2012 Available online 21 September 2012 Keywords: Sheet metal Aluminum alloy AA 6022 Surface roughness average Mill finish (MF) texture Electro discharge texture (EDT) Drawbeads Pure bending
a b s t r a c t This study consists of the experimental assessment of the surface topography of aluminum alloy AA 6022 sheet during large cyclic tensile and compressive plastic straining such as those present in the flow through drawbeads and small corner radii. Uniaxial and cyclic bending tests were used to measure the effects of such strains on the substrate layers of the sheet. All testing was performed without tool contact to ensure surface roughness changes were exclusively the result of the straining evolution of the free outer layers. The three-dimensional average surface roughness (Sa ) was measured using optical interferometry. Changes in Sa were evaluated for two sheet textures: mill finish (MF) and electro discharge texture (EDT), along longitudinal and transverse rolling directions. Texture pattern, test direction, and bending/unbending strain are all shown to be significant contributors to variations in Sa . The magnitude of the initial surface roughness was also an important factor of the resultant %Sa variation, ranging from 40% Sa for EDT, to approximately 350% Sa for MF. Unbending the sample to its flat state considerably altered the Sa , disturbing any monotonic Sa trend. This made it unfeasible to assess the effects of strain history on a formed part based only on Sa values. The non-linear relationship between Sa and strain was more significant at medium and large strains. Effects on Sa due to bending/unbending of the substrate layer presented in this study must be taken into account in the modeling of more complex phenomena involving tool radii contact and friction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloy sheet metals have seen an increasing role in many industries including applications in transportation, construction and the food industry. In automotive applications, where weight reduction translates into increased fuel economy, aluminum alloys offer a lightweight alternative as well as excellent mechanical properties, corrosion resistance, and recyclability. However, compared to steel, the formability of aluminum sheet is hindered by its lower ductility and greater springback. The sheet forming process typically involves significant strain due to stretching, bending, unbending, reverse bending, drawing, etc. The outer layers of the sheet undergo particularly large strains at the drawbead and corner radii areas (Fig. 1), where stretching and bending/reverse bending are significant. Large strains result in considerable surface roughness changes. These roughness changes impact the surface quality of the part, the
∗ Corresponding author. Tel.: +1 303 556 2361. E-mail addresses: [email protected], [email protected] (L.R. Sanchez). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.09.011
sheet-tool contact area, and the lubrication effects. Understanding these effects of surface roughness changes on sheet metal is required for the continuous improvement of the forming process. Hou et al. (2009) investigated surface roughness changes on hotdip galvanized and galvannealed steel sheets using a U-channel test. They related tool hardness and surface damage effects to the evolution of the surface roughness of the coatings. Sachtleber et al. (2004) presented experimental results on surface roughening of coated 6022-T4 aluminum sheet. They applied uniaxial and plane strain tensile straining to the sample using tensile tests and bending, respectively. Sachtleber et al. (2004) found that the surface roughness of the coated sheet was a pronounced function of strain. According to their results, different strain regimes and loading modes can lead to different roughening mechanisms. Linear approximations have been proposed for relating surface roughness to through thickness plastic strains for low levels of strain. In their biaxial studies using 70% Cu–30% Zn brass sheets, Wilson et al. (1983) found surface roughness changes proportional to the thickness strain at the earlier stages of straining. These linear approximations were also implied in the work presented by Osakada and Oyane (1971), with the strain mode showing little influence on the surface roughness for constant grain size. In their
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301
Fig. 2. Pure bending moment device. Fig. 1. Typical sheet metal forming modes.
experiments using 70–30 brass sheets, Mahmudi and Mehdizadeh (1998) found that the roughness incremented proportionally to the applied strain for a wide range of parameters. The authors showed a linear dependence between surface roughness and strain for true strains as large as 0.8. In their study of deformation induced surface roughening of annealed 304 stainless steels, Baydogan et al. (2003) also found a linear relation between the mean roughness and uniaxial tensile strain. Conversely, a number of researchers have reported surface roughness to increasingly deviate from linearity at medium and large strains. Wilson et al. (1983) found a non-linear relationship of surface roughness to strain at large strains. Stoudt et al. (2009) were also skeptical of data from models assuming linear relationships between surface roughness and plastic strain. Objecting to the use of linear profiles on the assessment of roughening behavior, they proposed a statistical model based on Weibull distributions instead. Deviations from linearity were also reported by Mizuno and Mulki (1996) in their studies of surface roughness changes with strain and deformation mode. Reported experimental values for surface roughness described in the references above, were most often based on observations of the outer free layer under uniaxial and biaxial tensile tests. Under these conditions, surface roughness changes are influenced by the plastic straining of the substrate under tension. Some authors included bending effects through tests carried out by bending the sheet over fixed cylinder pins (Sachtleber et al., 2004; Hou et al., 2009). During such tests, the convex outer layer is a free surface under in-plane tension. The concave outer layer’s surface, however, is influenced by mechanical contact with the tool, and depending on the test, it may also include friction under the compounded effect of pulling tension during testing. The goal of this research was to determine surface roughness changes exclusively due to cyclic plastic straining of the substrate outer layers under pure bending. In this study, pure bending tests were carried out without tool contact, and roughness changes in the convex and concave surfaces were related to straining due to curvature. The flow through drawbead and corner radii (Fig. 1) is more complex. As studied by Sanchez (2010), it involves normal stresses, transverse shear, contact loading and friction. This paper focuses on surface roughness changes by normal stresses developed under pure bending, unbending and reverse bending. A maximum of three bends and reverse bends were performed, which corresponded to the number of bends on circular drawbeads. The understanding of these roughness changes under cyclic bending is a fundamental
step toward further studies of the more complex tool radii contact and friction phenomena. The scope of this work includes two surface textures, mill finish (MF) and electro discharge textured (EDT) sheets. As reported by Hartfield-Wunsch et al. (2011), these aluminum sheet textures are most common, with mill finish most used in North America and electro discharge (EDT) textures in Europe. On-going interest on the study of these textures is enhanced by differences in formability, friction and paint appearance reported by Bottema et al. (1998) and Hartfield-Wunsch et al. (2011). The sheets were tested along the rolling, or longitudinal (L) and the transverse (T) to the rolling direction. Differences in roughness along these directions have been reported for aluminum alloys. In their tensile testing of aluminum sheets, Sachtleber et al. (2004) found larger roughness differences along the transverse direction. Their study was then conducted with the samples tested along the transverse direction only. The relevance of orthotropic testing directions under cyclic pure bending was assessed in this study. 2. Experimental procedure 2.1. Sheet material Aluminum alloy provided by an international supplier equivalent to the type AA 6022-T43 was chosen in this study. This alloy is used in automotive applications, especially where surface quality is critical. Temper T43 has improved formability, as recommended for automotive panels as well as reinforcement parts. 2.2. Tensile testing Tensile testing properties are shown in Table 1. The mechanical properties were approximately equivalent for both mill finish (MF) and electro discharge (EDT) textures. In both cases, the transverse direction exhibited lower strength than the longitudinal direction. 2.3. Pure bending testing Cyclic plane strain tests were performed using a pure bending moment device developed by Sanchez (2010) to investigate plane strain pure bending, Bauschinger effect and springback phenomena. Fig. 2 shows the working mechanisms of this device. The sheet was held between two clamps: one clamp mounted to a rigid frame and the other to a moving mechanism consisting of a sensor, a pulley, and a moving carriage. The pulley was supported by a ball bearing and rotated freely on the moving carriage. The
302
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Table 1 Tensile test properties for aluminum alloy AA 6022. Test direction
Texture
Yield strength (MPa)
Ultimate strength (MPa)
Strength coefficient (MPa)
Strain hardening exponent
Longitudinal Transverse Longitudinal Transverse
Mill finish Mill finish EDT EDT
152.4 147.1 157.3 147.2
264.3 257.7 268.3 260.5
501 488.6 502.8 484.6
0.259 0.26 0.255 0.251
carriage was supported on linear bearings so that the pulley could rotate and translate in the horizontal plane with minimum friction. Weights hanging on strings were attached to the pulley and applied the couple shown by the arrows in Fig. 2. The couple was opposed to bending by the resistance of the sheet to pure bending and was monitored by the torque sensor. The weights were removed during unloading. Reverse loading was applied by wrapping the strings on the opposite direction and thereby inverting the direction of the load applied to the pulley. The sample was bent to achieve strains similar to those reached by drawbeads and small tool radii. This required clamping the sheet in very narrow regions. A pure bending moment was applied by adding weights until the sample reached curvatures similar to those of drawbeads. The weights were then unloaded and the sample carefully removed from the device, scanned in the interferometer, and returned to the device where the bending process was reversed. In order to replicate the number of bends on circular drawbeads, a total of three bends and reverse bends were performed.
containment. Peak flattening, or a reduction of peak to valley height, would reduce this number. Two magnifications were used, 5.1× and 2.6× with fields of view of 1.28 mm × 0.96 mm, and 2.4 mm × 1.8 mm, respectively. Sa values were approximately equivalent for both settings. However, statistical averages for the radius of curvature taken from profile measurements were slightly improved using the larger field of view at 2.6×. Reported Sa and normalized volume values were the average of seven measurements along the surface of the sheet. The Sa uncertainty on the mean at ±2 varied from 0.02 to 0.05 m for MF and from 0.02 to 0.10 m for EDT textures. Normalized volume uncertainties were around 0.03–0.1 cm3 /m2 for MF and 0.07–0.19 cm3 /m2 for EDT samples. The largest uncertainties in both cases corresponded to the final unbending of the specimen to flat.
3. Experimental results and discussion 2.4. Optical interferometer measurements
3.1. Original sheet sample textures
Three dimensional (3D) surface texture data was collected with a Wyko NT-2000 vertical scanning interferometer, as described by Caber (1993). For this work, the 3D average surface roughness, Sa , and the normalized 3D surface volume (NVOL) where used to quantify the data. These parameters proved useful by Hartfield-Wunsch et al. (2011) in their studies of surface finish on aluminum. Sa is a three dimensional measurement of the roughness average given by: 1 |Zi,j | MN M
Sa =
N
j=1 i=1
Fig. 3 shows surface topographies for MF and EDT textures scanned prior to testing. MF texture (Fig. 3a) was clearly directional, with the peaks and valleys parallel to the longitudinal direction. EDT textures (Fig. 3b) were engineered and applied uniformly along any direction. During the manufacturing of EDT textures (Fig. 3b) the pattern was applied uniformly for each direction. The MF initial texture was significantly finer than EDT. With Sa = 0.37 m and NVOL = 0.73 cm3 /m2 , MF initial values were less than one third those of EDT (Sa = 1.2 m and NVOL = 3.41 cm3 /m2 ).
3.2. Changes to Sa under uniaxial tensile strains
where Zi,j is the point-by-point deviation on height between the measured and the reference surfaces. M and N are the number of data points along two perpendicular directions of the surface of the sheet. The normalized 3D surface volume (NVOL) is the volume above the surface over the lateral area. NVOL represents the amount of fluid per unit area the surface would hold from the lowest valley to the highest peak. NVOL is of interest in evaluating mean lubricant film thickness, deformation of local maxima, and lubricant
As shown in Fig. 4, uniaxial stretching had pronounced effects on Sa . MF textures experienced much larger changes than EDT for the strain magnitudes reached for this study (around ε = 0.14). Sa increased 2.9× along the longitudinal (L) direction, and 3.3× along the transverse (T). The relationship between Sa and uniaxial strain was approximately linear, with Sa values along T systematically higher (slope 5.81 m/strain) than along L (slope 4.81 m/strain).
Fig. 3. 3D surface roughness scans of (a) MF and (b) EDT.
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
0.20
1.8
True Outer Strains ε
EDT-T 1.4
EDT- L Sa roughness (μm)
Side B
0.15
1.6
1.2 1.0 0.8
0.10
V
0.05 U 0.00
-0.05
0.00
0.26
0.01
-0.25 -0.02
0.38
0.03
-0.10
MF-T
Side A -0.15
0.6
MF-L
Inner Curvature (1/mm)
0.4
Fig. 6. Outer true strains under cyclic pure bending for MF-L sample.
0.2 0.00
0.02
0.05
0.07
0.10
0.12
0.14
True Strain ε Fig. 4. Changes on Sa under uniaxial tensile strain.
Roughness changes on EDT textures were less sensitive to small strains, tending to depart from a linear fit in this zone (Fig. 4). This is represented by the smaller increase in Sa ; 0.4 m for EDT, compared to MF Sa ∼ 0.65 m at 0.14 true strain. However, EDT samples were initially coarser (Sa = 1.2 m) than MF. These results showed that the coarser EDT texture was less influenced by substrate uniaxial strains. EDT texture was significantly more isotropic than MF, with equivalent Sa values along L and T to strains ε = 0.07, and with a small increase along T at larger strains. 3.3. Changes to NVOL under uniaxial tensile strains The NVOL (normalized surface volume) values are shown in Fig. 5. NVOL was approximately linear to uniaxial strain for all
5.00 4.50
Normalized Surface Volume (cm3/m2)
303
EDT-T
4.00
EDT-L
3.50 3.00 2.50
MF-T
2.00 1.50
MF-L
cases. Under equivalent straining, the volume between valleys and peaks increased more for MF textures. This is shown by larger slopes for MF (15.6, 13.9 cm3 /m2 along T and L) than for EDT (10.9, 10.4 cm3 /m2 along T and L). The close values between EDT slopes along L and T were related to its more isotropic behavior. For EDT textures, NVOL behaved more linear than Sa , specifically for strains ε ≤ 0.07. This relation depends on the nature of the topographic changes as the outer layers stretch and new surfaces form. Valley creation would increase NVOL without necessarily increasing Sa . 3.4. Strain sequence under cyclic pure bending The sheet was subjected to the following pure bending test sequence: (i) from original flat state, to first bend, then unbent back to flat, (ii) reverse bend (second bend), and unbent to flat, (iii) third bend, and unbent to flat. The concave side at first bend was labeled “side A”, with “side B” on the convex side. The radii of the bent sides were determined from interferometer measurements of the profiles. The outer plastic strains were calculated from radii and thickness changes, and by applying constancy of volume between the final and the initial sections. Since the sample width was approximately unchanged during pure bending, plane strain conditions were operative. The inner curvature is defined as the curvature at the concave side of the sample. Typical strains on the outer layers of the sheet are shown in Fig. 6 as a function of inner curvature. Due to practical limitations of the test, the specimens were not unbent to a perfect flat, and a small remaining curvature (large radius of curvature) was measured. As a result, strains between sides A and B (such as points U, V, in Fig. 6) were not equal at flat. Thickness measurements at the flat condition after unbending are shown in Table 2 for the MF-L sheet. The thickness systematically decreased, and an increasing longitudinal strain was determined with each additional cycle. These strains were particularly noticeable in this study due to the repeated cycling to small radii. Table 2 MF-L thickness at flat after cyclic unbending.
1.00
Description
Thickness (mm)
Net true strain Longitudinal
Original thickness Flat after first bend Flat after reverse bend Flat after third bend
0.978 0.973 0.955 0.949
0.0000 0.0052 0.0237 0.0303
0.50 0.00
0.02
0.05
0.07
0.10
0.12
True Strain ε Fig. 5. Changes on NVOL under uniaxial tensile strain.
0.14
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2
R M
Sa Roughness ( μm)
1.6
H
1.4
I 1.2 1
O
Side A
R
P
J
V M
H
Side B
0.4
Q
0.08
I
0.67
0.06 0.57 0.04 0.47
0.37
O 0.00
P
0.02
K 0.26
0.00
0.01
First bend Inner Curvature (1/mm)
K
O
0.10
J
N J
U
S
Side A
I
0.6
U
Q
H
0.77
V
S
K
MF-L
0.8
N
Sa Side A Sa side B
a)
1.8
Sa Roughness ( μm)
EDT-L
True Strain
304
b) 0.97
0.15
0.2
M 0.00
0.25
0.02
-0.27
0.01
0.40
0.04
0.87
0.13 0.10
N
0.67
0.08
0.57
Q P
0.47 0.37
J K 0.01
0.05
True Strain
Cyclic straining resulted in considerable changes of Sa roughness. As seen in Figs. 7–9, bending and reverse bending increased the roughness while unbending the sample to flat typically decreased it. The surface of the sheet, particularly at the convex (tensile) sides, became increasingly rough at successive bending cycles. Changes on Sa where severe at some locations. Point R of MF-L in Fig. 7 for instance, reached 1.9 m under tensile strains. This represented a fivefold increase of the initial roughness of the sheet. Fig. 8 relates the evolution of the Sa values of the MF-L sample to its substrate strains. The strain scale was initially normalized to the Sa scale as shown, with side A as a reference. Zero strain corresponded to the initial Sa roughness of 0.37 m. Sa values on the convex side (side B) were lower, but approximately equivalent to those on the concave side (side A), and both roughly doubled as the tensile and compressive substrates strained during bending. Upon unbending to flat, the resultant outer strains returned to near the initially unstrained condition. Changes in Sa were fairly proportional to changes on the outer strains and also returned to near their initial values upon unbending. At the unbent condition (points J, K), Sa showed a small, but noticeable departure from proportionality to strain. The strain at side A located close to Sa at side B and vice versa. These discrepancies increased for the next cycles, and can be traced to surface effects during the thinning of the substrate layers upon unbending to flat. The second bending cycle (reverse bending), is shown in Fig. 8b. Side A became convex, and the tensile strain increased from 0.01 to approximately 0.11. Upon straining, Sa increased from point J (0.43 m) to M (0.94 m). A linear relationship between Sa and strain would have required and increase to approximately 0.8 m. At the concave side B in Fig. 8b, Sa changed from K to N, showing a closer linear relationship to compressive strain. In terms of their effect on roughness, the compressive and the tensile substrate strains were not equivalent.
0.77
0.03 0.00
-0.25
-0.02
Second Bend Inner Curvature (1/mm)
c)
1.27
0.23
R
1.17
0.21
1.07
V
0.97
0.15
0.87
U
0.77
0.47
0.13 0.10
S
0.67 0.57
0.18
0.08
Q
True Strain
3.5. Sa changes under cyclic bending for MF texture along L
Sa Roughness ( μm)
Fig. 7. Evolution of Sa under cyclic pure bending along L.
Sa Roughness ( μm)
Inner Curvature (1/mm)
0.05
P
0.03
0.37
0.00 -0.02
0.38
0.03
Third Bend Inner Curvature (1/mm) Fig. 8. MF-L Outer strains and Sa values at (a) first, (b) second and (c) third bending cycle.
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
roughness of MF samples resulted in larger changes in roughness. This fact must be considered when defining the initial roughness values needed for optimal forming performance.
2
R
EDT-T 1.8
N
Sa Roughness ( μm)
1.6
I 1.2 1
Q
M
1.4
O
K
V
S
H Side A
U R
P
J
U V
M MF-T S
Side A
H, I
0.8
P
N 0.6
Side B
Q
0.4 0.2
J, K
O 0.00
0.28
0.03
-0.27
0.03
305
0.41
0.05
Inner Curvature (1/mm)
3.7. Changes on Sa along the transverse testing direction As shown in Fig. 9, cyclic bending of MF samples along T followed Sa trends similar to the MF-L samples (Fig. 7). In both cases, Sa became progressively larger for each additional cycle after each unbending to flat at points JK, PQ, and UV. Roughness Sa reached its largest value at the third, convex (tensile) bend for side B, represented by point R in Figs. 7 and 9. For MF samples, Sa values along the transverse direction at point R were around 20% higher than those along L. This difference was traced to the unbending to flat at the second cycle (from N to P). Up to points M and N, roughness values along L and T were similar, and Sa between points P and R at the third bend were equivalent (Sa ∼ 0.71 m along L and T). Fig. 9 shows Sa values for EDT samples bent along the transverse direction (EDT-T). Compared to EDT-L, EDT-T points H and I were further apart, with Sa significantly higher at the compressive layer (point H). Sa values were also higher for side B at the reverse bend (point N). As with all other bending cases, the largest Sa values occurred at the tensile outer layer at point R.
Fig. 9. Changes on Sa under cyclic pure bending along T.
3.8. Substrate strain effects on Sa During unbending side A changed from convex (point M) to flat (point Q), and side B from N to P. For both sides Sa values decreased, but remained 50% higher for side A and 35% higher for side B. The third bending cycle continued the increase of Sa roughness. Consistent with the previous trends, the largest Sa values were under tensile straining. In Fig. 8c, side B endured the largest tensile strains as it was bent from P (flat) to R (convex). Correspondingly, Sa increased by 2.4 times to around 1.2 m. Although the Sa increase was promoted by the substrate tensile strain, it can be seen in Fig. 8c that the increase of Sa was significantly larger. 3.6. Sa changes under cyclic bending for EDT texture along L The general Sa trend for EDT textures along L showed similarities to MF-L samples (Fig. 7). Both textures exhibited an increasing Sa trend with increasing cycles, shown by points H, M, and R. However, cyclic roughness differences between the concave and convex sides A and B were smallest for EDT-L samples. A most significant difference between EDT and MF textures was the much smaller Sa variability for EDT textures. From Fig. 7, at maximum point R, Sa increased ∼50% for EDT, which compared to over 400% for MF. The Sa between the initial (point O) and the maximum value at R were significantly closer; Sa ∼ 0.6 m for EDT, and Sa ∼ 0.8 m for MF. The smaller percent variability for EDT texture resided in the magnitude of the initial Sa roughness (∼1.2 m for EDT and ∼0.35 m for MF). The much finer initial
Sa values correlated with uniaxial strains from the tensile test and tensile plane strains from pure bending. Trends of Sa under uniaxial tensile strains were then used to support an explanation of Sa changes under bending. Since the tensile test was monotonic, it correlated with plane strains at point I, at the first bending cycle. From Fig. 6, the tensile plane strain at the convex side B was about 10% for the first bend. The Sa values under tensile bending and uniaxial tests at 10% strain are summarized in Table 3. From the table, higher Sa values for MF were linked to the transverse direction, while lower values correlated with the longitudinal. At 10% strain, Sa values for EDT textures were equivalent and isotropic under both tests. From Table 1, the mechanical properties of the substrate are identical for both textures, with the yield strength lower along T. For MF-longitudinal samples in Fig. 7, the Sa value at point H, which was under compressive strains, was larger than point I, the tensile side. According to uniaxial data in Table 3, Sa was expected to increase when testing along the transverse direction. That increase was in agreement to bending data shown in Fig. 9. Also in correlation with uniaxial data, point I remained at around Sa = 1.4 m when bending EDT textures along both, longitudinal (Fig. 7), and transverse (Fig. 9) directions. From Fig. 7, point H (Sa = 1.49 m) was higher than point I. At the first bend, compressive strains increased EDT roughness more than tensile strains. Along the transverse direction (Fig. 9) compressive strains further increased EDT roughness (Sa = 1.53 m), and points H and
Table 3 Sa values at approximately 10% tensile strain. Test Uniaxial
Strain mode
Texture
Test direction
Sa (m)
Uniaxial tension
MF
L T L T
0.78 0.87 1.36 1.4
L T L T
0.72 0.83 1.4 1.38
EDT
Pure bending (point I)
Plane strain tension
MF EDT
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
a) 1.27
3.91
R
1.17
3.56
1.07
VR M
0.97 0.87 0.77 0.67
N
0.57
2.50
1.79
Q
1.44
J
P
0.47 0.37
U
2.85
2.14
I
O
S
VM
H
3.20 V
1.09
K
Normalized Surface Volume (cm3/m2)
Fig. 10 compares changes of NVOL and Sa under cyclic bending. At the first bending-unbending to flat cycle, changes in NVOL were proportional to changes in Sa . This indicated no peak flattening, and no reduction in peak to valley height, which might change the rate of NVOL during bending. The proportionality between changes on Sa and normalized volume was approximately maintained for the sides undergoing compressive straining throughout further testing. However, the layers subjected to tensile straining showed variations. For example, Sa at the reverse bend point M (Fig. 10a) was higher than the corresponding normalized volume value (at point VM ). At the third bend this effect between points R (maximum Sa ) and VR (normalized volume) was even more pronounced. Since there was no external surface contact during testing, changes on Sa and normalized volume were related exclusively to the effects on the texture due to the straining of the substrate layer. As shown previously in Table 2, the sheet progressively stretched and new surface was exposed with each cycle of unbending to flat. Under tensile straining, the new surfaces resulted on increased roughness, but peak to valley heights did not increase in the same proportion and NVOL decreased. Fig. 10b shows the evolution of the normalized volume and Sa for EDT-L. The normalized volume values for both outer layers departed from the Sa trend at the tensile side (points I, M and R), while followed Sa changes more closely at the compressive side (points H, N and S). Changes on normalized surface volume and Sa roughness may impact the formability, friction and paint appearance, as observed by Hartfield-Wunsch et al. (2011). For the EDT material, the normalized volume values increased between 25% and 50% during the cyclic bending (Fig. 10b). The NVOL percent increases for MF textures were significantly larger, ranging from 300% to 400% (Fig. 10a). In comparison to EDT textures, MF texture was at disadvantage in terms of lubricant containment. The NVOL values, initially lower for MF textures, was required to increase several times during bending and unbending. These increases would result in additional loss of performance in lubrication and impact surface finish quality and paint appearance.
Sa side B NVOL Side B
0.73
0.00 0.26 0.01 -0.25 -0.02 0.38 0.03 MF-L Inner Curvature (1/mm)
R
b) 1.80
S
1.70
5.09
VR
U V 4.71
M 1.60 H
1.50
N
1.40 1.30 O
VM
4.34 Q
I
1.20
5.46
K J
P
3.96 3.59
Normalized Surface Volume (cm3/m2)
3.9. Relationships between NVOL and Sa
Sa Side A NVOL Side A
Sa Roughness ( μm)
I showed farther apart. Since the T direction had smaller yield strength (Table 1), Sa was more sensitive to compressive strains along T. As the sheet was bent to a second and third cycle, tensile straining of the substrate progressively became the dominant mechanism for roughness increase. The resultant stretching after unbending to flat (Table 2) was the onset of an increasingly larger non-linear relationship between Sa and strain.
Sa Roughness ( μm)
306
3.22
0.00 0.25 0.02 -0.28 0.02 0.40 0.05 EDT-L Inner Curvature (1/mm) Fig. 10. Sa and normalized volume values: (a) MF-L and (b) EDT-L.
A visual inspection of the final MF texture (Fig. 11a) showed a predominantly irregular pattern over a background showing traces of the original texture (compare with Fig. 3a). The final EDT texture (Fig. 11b) still preserved some of the main features of the original EDT texture (Fig. 3b). The changes in texture were significant, and
Fig. 11. Texture at final flat after three cycles: (a) MF and (b) EDT.
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prone to affect friction and other contact phenomena, such as wear, galling and scoring. 4. Conclusions Substrate strains which developed during bending and unbending had significant impact on the texture of aluminum sheet alloy. The three dimensional surface roughness Sa , and the normalized surface volume (NVOL) were useful quantifiers of the changes in surface texture which may affect properties such as friction and paint finish. For the aluminum alloy studied, Sa and NVOL changes were approximately linear to substrate strain at the first bend, and became increasingly non-linear under the medium and large strains developed by successive bending and unbending. Texture changes during straining were more significant for MF samples, which initial roughness Sa was less than one third the EDT value. Percent changes were smaller for EDT textures, which have a higher initial NVOL and Sa values. Research toward optimizing surface textures must take into account the initial values and the variability of these parameters upon deformation. The effect on texture by tensile strains correlated well for uniaxial and plane strain modes. However, the effects on texture under compressive strains at the concave side of the bend were not generally equivalent to tensile strains. Compressive strains were more significant at the initial stages, while changes under tensile strains were more significant at successive bending. Roughness changes were larger along the transverse direction. Changes in texture were initially isotropic for EDT samples, but became larger along the transverse direction as the deformation became increasingly nonlinear. Sa roughness and NVOL changed significantly upon unbending the sample to its flat condition. This operation disturbed any monotonic Sa trend. The Sa values typically decreased as the magnitude of the net strain became smallest at the flat condition. The relationship between Sa and strain history became complex, and, as shown by this study, became unfeasible to determine strain history effects upon unbending based only on measurements of Sa roughness values.
307
Under drawbead flow, the sheet unbends to flat in a very small area at the point of inflexion between the bent and reverse bend. This study showed that texture changes at that section are significant and must be measured. The understanding of texture changes under bending strains is a contribution to further research on friction and contact phenomena in forming areas such as drawbeads and corner radii. References Baydogan, M., Akoy, M.A., Kayali, E.S., Cimenoglu, H., 2003. Deformation induced surface roughening of austenitic stainless steels. ISIJ 43 (11), 1795–1798. Bottema, J., Lahaye, C., Baartman, R., Zhuang, L., De Smet, P., Schoepen, F., 1998. Recent developments in AA6016 aluminum type body sheet product. SAE Paper 981007. doi:10.4271/981007. Caber, P.J., 1993. Interferometric profiler for rough surfaces. Applied Optics 32 (19), 3438–3441, http://dx.doi.org/10.1364/AO.32.003438. Hartfield-Wunsch, S., Cohen, D., Sanchez, L.R., Brattstrom, L.E., 2011. The effect of surface finish on aluminum sheet friction behavior. SAE International Journal of Materials and Manufacturing 4, 818–825, http://dx.doi.org/10.4271/2011-01-0534. Hou, Y., Yu, Z., Zhang, W., Jiang, H., Lin, Z., 2009. Surface topography evolvement of galvanized steels in sheet metal forming. Transactions of Nonferrous Metals Society of China 19, 305–310, http://dx.doi.org/10.1016/S1003-6326(08)60269-1. Mahmudi, R., Mehdizadeh, M., 1998. Surface roughening during uniaxial and equi-biaxial stretching of 70–30 brass sheets. Journal of Materials Processing Technology 80–81, 707–712. Mizuno, T., Mulki, H., 1996. Changes in surface texture of zinc-coated steel sheets under plastic deformation. Wear 198, 176–184. Osakada, K., Oyane, M., 1971. On the roughening of free surface in deformation processes. JSME 14 (68), 171–177. Sachtleber, M., Raabe, D., Weiland, H., 2004. Surface roughening and color changes of coated aluminum sheets during plastic straining. Journal of Materials Processing Technology 148, 68–76. Sanchez, L.R., 2010. Modeling of springback, strain rate, and Bauschinger effects during a two-dimensional steady state cyclic flow of sheet metal subjected to bending under tension. International Journal of Mechanical Sciences 52, 429–439. Stoudt, M.R., Hubbard, J.B., Iadicola, M.A., Banovic, S.W., 2009. A study of the fundamental relationships between deformation-induced surface roughness and strain localization in AA5754. Metallurgical and Materials Transactions A 40 (7), 1611–1622, http://dx.doi.org/10.1007/s11661-009-9881-6. Wilson, D.V., Mirshams, A.R., Roberts, W.T., 1983. An experimental study of the effect of sheet thickness and grain size on limit-strains in biaxial stretching. International Journal of Mechanical Sciences 25 (12), 859–870.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Surface topography changes in aluminum alloy sheet during large plastic straining under cyclic pure bending Glenn A. Lucachick, L. Rafael Sanchez ∗ Mechanical Engineering Department, University of Colorado Denver, 1200 Larimer St, Denver, CO 80217, USA
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 11 August 2012 Accepted 14 September 2012 Available online 21 September 2012 Keywords: Sheet metal Aluminum alloy AA 6022 Surface roughness average Mill finish (MF) texture Electro discharge texture (EDT) Drawbeads Pure bending
a b s t r a c t This study consists of the experimental assessment of the surface topography of aluminum alloy AA 6022 sheet during large cyclic tensile and compressive plastic straining such as those present in the flow through drawbeads and small corner radii. Uniaxial and cyclic bending tests were used to measure the effects of such strains on the substrate layers of the sheet. All testing was performed without tool contact to ensure surface roughness changes were exclusively the result of the straining evolution of the free outer layers. The three-dimensional average surface roughness (Sa ) was measured using optical interferometry. Changes in Sa were evaluated for two sheet textures: mill finish (MF) and electro discharge texture (EDT), along longitudinal and transverse rolling directions. Texture pattern, test direction, and bending/unbending strain are all shown to be significant contributors to variations in Sa . The magnitude of the initial surface roughness was also an important factor of the resultant %Sa variation, ranging from 40% Sa for EDT, to approximately 350% Sa for MF. Unbending the sample to its flat state considerably altered the Sa , disturbing any monotonic Sa trend. This made it unfeasible to assess the effects of strain history on a formed part based only on Sa values. The non-linear relationship between Sa and strain was more significant at medium and large strains. Effects on Sa due to bending/unbending of the substrate layer presented in this study must be taken into account in the modeling of more complex phenomena involving tool radii contact and friction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloy sheet metals have seen an increasing role in many industries including applications in transportation, construction and the food industry. In automotive applications, where weight reduction translates into increased fuel economy, aluminum alloys offer a lightweight alternative as well as excellent mechanical properties, corrosion resistance, and recyclability. However, compared to steel, the formability of aluminum sheet is hindered by its lower ductility and greater springback. The sheet forming process typically involves significant strain due to stretching, bending, unbending, reverse bending, drawing, etc. The outer layers of the sheet undergo particularly large strains at the drawbead and corner radii areas (Fig. 1), where stretching and bending/reverse bending are significant. Large strains result in considerable surface roughness changes. These roughness changes impact the surface quality of the part, the
∗ Corresponding author. Tel.: +1 303 556 2361. E-mail addresses: [email protected], [email protected] (L.R. Sanchez). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.09.011
sheet-tool contact area, and the lubrication effects. Understanding these effects of surface roughness changes on sheet metal is required for the continuous improvement of the forming process. Hou et al. (2009) investigated surface roughness changes on hotdip galvanized and galvannealed steel sheets using a U-channel test. They related tool hardness and surface damage effects to the evolution of the surface roughness of the coatings. Sachtleber et al. (2004) presented experimental results on surface roughening of coated 6022-T4 aluminum sheet. They applied uniaxial and plane strain tensile straining to the sample using tensile tests and bending, respectively. Sachtleber et al. (2004) found that the surface roughness of the coated sheet was a pronounced function of strain. According to their results, different strain regimes and loading modes can lead to different roughening mechanisms. Linear approximations have been proposed for relating surface roughness to through thickness plastic strains for low levels of strain. In their biaxial studies using 70% Cu–30% Zn brass sheets, Wilson et al. (1983) found surface roughness changes proportional to the thickness strain at the earlier stages of straining. These linear approximations were also implied in the work presented by Osakada and Oyane (1971), with the strain mode showing little influence on the surface roughness for constant grain size. In their
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301
Fig. 2. Pure bending moment device. Fig. 1. Typical sheet metal forming modes.
experiments using 70–30 brass sheets, Mahmudi and Mehdizadeh (1998) found that the roughness incremented proportionally to the applied strain for a wide range of parameters. The authors showed a linear dependence between surface roughness and strain for true strains as large as 0.8. In their study of deformation induced surface roughening of annealed 304 stainless steels, Baydogan et al. (2003) also found a linear relation between the mean roughness and uniaxial tensile strain. Conversely, a number of researchers have reported surface roughness to increasingly deviate from linearity at medium and large strains. Wilson et al. (1983) found a non-linear relationship of surface roughness to strain at large strains. Stoudt et al. (2009) were also skeptical of data from models assuming linear relationships between surface roughness and plastic strain. Objecting to the use of linear profiles on the assessment of roughening behavior, they proposed a statistical model based on Weibull distributions instead. Deviations from linearity were also reported by Mizuno and Mulki (1996) in their studies of surface roughness changes with strain and deformation mode. Reported experimental values for surface roughness described in the references above, were most often based on observations of the outer free layer under uniaxial and biaxial tensile tests. Under these conditions, surface roughness changes are influenced by the plastic straining of the substrate under tension. Some authors included bending effects through tests carried out by bending the sheet over fixed cylinder pins (Sachtleber et al., 2004; Hou et al., 2009). During such tests, the convex outer layer is a free surface under in-plane tension. The concave outer layer’s surface, however, is influenced by mechanical contact with the tool, and depending on the test, it may also include friction under the compounded effect of pulling tension during testing. The goal of this research was to determine surface roughness changes exclusively due to cyclic plastic straining of the substrate outer layers under pure bending. In this study, pure bending tests were carried out without tool contact, and roughness changes in the convex and concave surfaces were related to straining due to curvature. The flow through drawbead and corner radii (Fig. 1) is more complex. As studied by Sanchez (2010), it involves normal stresses, transverse shear, contact loading and friction. This paper focuses on surface roughness changes by normal stresses developed under pure bending, unbending and reverse bending. A maximum of three bends and reverse bends were performed, which corresponded to the number of bends on circular drawbeads. The understanding of these roughness changes under cyclic bending is a fundamental
step toward further studies of the more complex tool radii contact and friction phenomena. The scope of this work includes two surface textures, mill finish (MF) and electro discharge textured (EDT) sheets. As reported by Hartfield-Wunsch et al. (2011), these aluminum sheet textures are most common, with mill finish most used in North America and electro discharge (EDT) textures in Europe. On-going interest on the study of these textures is enhanced by differences in formability, friction and paint appearance reported by Bottema et al. (1998) and Hartfield-Wunsch et al. (2011). The sheets were tested along the rolling, or longitudinal (L) and the transverse (T) to the rolling direction. Differences in roughness along these directions have been reported for aluminum alloys. In their tensile testing of aluminum sheets, Sachtleber et al. (2004) found larger roughness differences along the transverse direction. Their study was then conducted with the samples tested along the transverse direction only. The relevance of orthotropic testing directions under cyclic pure bending was assessed in this study. 2. Experimental procedure 2.1. Sheet material Aluminum alloy provided by an international supplier equivalent to the type AA 6022-T43 was chosen in this study. This alloy is used in automotive applications, especially where surface quality is critical. Temper T43 has improved formability, as recommended for automotive panels as well as reinforcement parts. 2.2. Tensile testing Tensile testing properties are shown in Table 1. The mechanical properties were approximately equivalent for both mill finish (MF) and electro discharge (EDT) textures. In both cases, the transverse direction exhibited lower strength than the longitudinal direction. 2.3. Pure bending testing Cyclic plane strain tests were performed using a pure bending moment device developed by Sanchez (2010) to investigate plane strain pure bending, Bauschinger effect and springback phenomena. Fig. 2 shows the working mechanisms of this device. The sheet was held between two clamps: one clamp mounted to a rigid frame and the other to a moving mechanism consisting of a sensor, a pulley, and a moving carriage. The pulley was supported by a ball bearing and rotated freely on the moving carriage. The
302
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Table 1 Tensile test properties for aluminum alloy AA 6022. Test direction
Texture
Yield strength (MPa)
Ultimate strength (MPa)
Strength coefficient (MPa)
Strain hardening exponent
Longitudinal Transverse Longitudinal Transverse
Mill finish Mill finish EDT EDT
152.4 147.1 157.3 147.2
264.3 257.7 268.3 260.5
501 488.6 502.8 484.6
0.259 0.26 0.255 0.251
carriage was supported on linear bearings so that the pulley could rotate and translate in the horizontal plane with minimum friction. Weights hanging on strings were attached to the pulley and applied the couple shown by the arrows in Fig. 2. The couple was opposed to bending by the resistance of the sheet to pure bending and was monitored by the torque sensor. The weights were removed during unloading. Reverse loading was applied by wrapping the strings on the opposite direction and thereby inverting the direction of the load applied to the pulley. The sample was bent to achieve strains similar to those reached by drawbeads and small tool radii. This required clamping the sheet in very narrow regions. A pure bending moment was applied by adding weights until the sample reached curvatures similar to those of drawbeads. The weights were then unloaded and the sample carefully removed from the device, scanned in the interferometer, and returned to the device where the bending process was reversed. In order to replicate the number of bends on circular drawbeads, a total of three bends and reverse bends were performed.
containment. Peak flattening, or a reduction of peak to valley height, would reduce this number. Two magnifications were used, 5.1× and 2.6× with fields of view of 1.28 mm × 0.96 mm, and 2.4 mm × 1.8 mm, respectively. Sa values were approximately equivalent for both settings. However, statistical averages for the radius of curvature taken from profile measurements were slightly improved using the larger field of view at 2.6×. Reported Sa and normalized volume values were the average of seven measurements along the surface of the sheet. The Sa uncertainty on the mean at ±2 varied from 0.02 to 0.05 m for MF and from 0.02 to 0.10 m for EDT textures. Normalized volume uncertainties were around 0.03–0.1 cm3 /m2 for MF and 0.07–0.19 cm3 /m2 for EDT samples. The largest uncertainties in both cases corresponded to the final unbending of the specimen to flat.
3. Experimental results and discussion 2.4. Optical interferometer measurements
3.1. Original sheet sample textures
Three dimensional (3D) surface texture data was collected with a Wyko NT-2000 vertical scanning interferometer, as described by Caber (1993). For this work, the 3D average surface roughness, Sa , and the normalized 3D surface volume (NVOL) where used to quantify the data. These parameters proved useful by Hartfield-Wunsch et al. (2011) in their studies of surface finish on aluminum. Sa is a three dimensional measurement of the roughness average given by: 1 |Zi,j | MN M
Sa =
N
j=1 i=1
Fig. 3 shows surface topographies for MF and EDT textures scanned prior to testing. MF texture (Fig. 3a) was clearly directional, with the peaks and valleys parallel to the longitudinal direction. EDT textures (Fig. 3b) were engineered and applied uniformly along any direction. During the manufacturing of EDT textures (Fig. 3b) the pattern was applied uniformly for each direction. The MF initial texture was significantly finer than EDT. With Sa = 0.37 m and NVOL = 0.73 cm3 /m2 , MF initial values were less than one third those of EDT (Sa = 1.2 m and NVOL = 3.41 cm3 /m2 ).
3.2. Changes to Sa under uniaxial tensile strains
where Zi,j is the point-by-point deviation on height between the measured and the reference surfaces. M and N are the number of data points along two perpendicular directions of the surface of the sheet. The normalized 3D surface volume (NVOL) is the volume above the surface over the lateral area. NVOL represents the amount of fluid per unit area the surface would hold from the lowest valley to the highest peak. NVOL is of interest in evaluating mean lubricant film thickness, deformation of local maxima, and lubricant
As shown in Fig. 4, uniaxial stretching had pronounced effects on Sa . MF textures experienced much larger changes than EDT for the strain magnitudes reached for this study (around ε = 0.14). Sa increased 2.9× along the longitudinal (L) direction, and 3.3× along the transverse (T). The relationship between Sa and uniaxial strain was approximately linear, with Sa values along T systematically higher (slope 5.81 m/strain) than along L (slope 4.81 m/strain).
Fig. 3. 3D surface roughness scans of (a) MF and (b) EDT.
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
0.20
1.8
True Outer Strains ε
EDT-T 1.4
EDT- L Sa roughness (μm)
Side B
0.15
1.6
1.2 1.0 0.8
0.10
V
0.05 U 0.00
-0.05
0.00
0.26
0.01
-0.25 -0.02
0.38
0.03
-0.10
MF-T
Side A -0.15
0.6
MF-L
Inner Curvature (1/mm)
0.4
Fig. 6. Outer true strains under cyclic pure bending for MF-L sample.
0.2 0.00
0.02
0.05
0.07
0.10
0.12
0.14
True Strain ε Fig. 4. Changes on Sa under uniaxial tensile strain.
Roughness changes on EDT textures were less sensitive to small strains, tending to depart from a linear fit in this zone (Fig. 4). This is represented by the smaller increase in Sa ; 0.4 m for EDT, compared to MF Sa ∼ 0.65 m at 0.14 true strain. However, EDT samples were initially coarser (Sa = 1.2 m) than MF. These results showed that the coarser EDT texture was less influenced by substrate uniaxial strains. EDT texture was significantly more isotropic than MF, with equivalent Sa values along L and T to strains ε = 0.07, and with a small increase along T at larger strains. 3.3. Changes to NVOL under uniaxial tensile strains The NVOL (normalized surface volume) values are shown in Fig. 5. NVOL was approximately linear to uniaxial strain for all
5.00 4.50
Normalized Surface Volume (cm3/m2)
303
EDT-T
4.00
EDT-L
3.50 3.00 2.50
MF-T
2.00 1.50
MF-L
cases. Under equivalent straining, the volume between valleys and peaks increased more for MF textures. This is shown by larger slopes for MF (15.6, 13.9 cm3 /m2 along T and L) than for EDT (10.9, 10.4 cm3 /m2 along T and L). The close values between EDT slopes along L and T were related to its more isotropic behavior. For EDT textures, NVOL behaved more linear than Sa , specifically for strains ε ≤ 0.07. This relation depends on the nature of the topographic changes as the outer layers stretch and new surfaces form. Valley creation would increase NVOL without necessarily increasing Sa . 3.4. Strain sequence under cyclic pure bending The sheet was subjected to the following pure bending test sequence: (i) from original flat state, to first bend, then unbent back to flat, (ii) reverse bend (second bend), and unbent to flat, (iii) third bend, and unbent to flat. The concave side at first bend was labeled “side A”, with “side B” on the convex side. The radii of the bent sides were determined from interferometer measurements of the profiles. The outer plastic strains were calculated from radii and thickness changes, and by applying constancy of volume between the final and the initial sections. Since the sample width was approximately unchanged during pure bending, plane strain conditions were operative. The inner curvature is defined as the curvature at the concave side of the sample. Typical strains on the outer layers of the sheet are shown in Fig. 6 as a function of inner curvature. Due to practical limitations of the test, the specimens were not unbent to a perfect flat, and a small remaining curvature (large radius of curvature) was measured. As a result, strains between sides A and B (such as points U, V, in Fig. 6) were not equal at flat. Thickness measurements at the flat condition after unbending are shown in Table 2 for the MF-L sheet. The thickness systematically decreased, and an increasing longitudinal strain was determined with each additional cycle. These strains were particularly noticeable in this study due to the repeated cycling to small radii. Table 2 MF-L thickness at flat after cyclic unbending.
1.00
Description
Thickness (mm)
Net true strain Longitudinal
Original thickness Flat after first bend Flat after reverse bend Flat after third bend
0.978 0.973 0.955 0.949
0.0000 0.0052 0.0237 0.0303
0.50 0.00
0.02
0.05
0.07
0.10
0.12
True Strain ε Fig. 5. Changes on NVOL under uniaxial tensile strain.
0.14
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2
R M
Sa Roughness ( μm)
1.6
H
1.4
I 1.2 1
O
Side A
R
P
J
V M
H
Side B
0.4
Q
0.08
I
0.67
0.06 0.57 0.04 0.47
0.37
O 0.00
P
0.02
K 0.26
0.00
0.01
First bend Inner Curvature (1/mm)
K
O
0.10
J
N J
U
S
Side A
I
0.6
U
Q
H
0.77
V
S
K
MF-L
0.8
N
Sa Side A Sa side B
a)
1.8
Sa Roughness ( μm)
EDT-L
True Strain
304
b) 0.97
0.15
0.2
M 0.00
0.25
0.02
-0.27
0.01
0.40
0.04
0.87
0.13 0.10
N
0.67
0.08
0.57
Q P
0.47 0.37
J K 0.01
0.05
True Strain
Cyclic straining resulted in considerable changes of Sa roughness. As seen in Figs. 7–9, bending and reverse bending increased the roughness while unbending the sample to flat typically decreased it. The surface of the sheet, particularly at the convex (tensile) sides, became increasingly rough at successive bending cycles. Changes on Sa where severe at some locations. Point R of MF-L in Fig. 7 for instance, reached 1.9 m under tensile strains. This represented a fivefold increase of the initial roughness of the sheet. Fig. 8 relates the evolution of the Sa values of the MF-L sample to its substrate strains. The strain scale was initially normalized to the Sa scale as shown, with side A as a reference. Zero strain corresponded to the initial Sa roughness of 0.37 m. Sa values on the convex side (side B) were lower, but approximately equivalent to those on the concave side (side A), and both roughly doubled as the tensile and compressive substrates strained during bending. Upon unbending to flat, the resultant outer strains returned to near the initially unstrained condition. Changes in Sa were fairly proportional to changes on the outer strains and also returned to near their initial values upon unbending. At the unbent condition (points J, K), Sa showed a small, but noticeable departure from proportionality to strain. The strain at side A located close to Sa at side B and vice versa. These discrepancies increased for the next cycles, and can be traced to surface effects during the thinning of the substrate layers upon unbending to flat. The second bending cycle (reverse bending), is shown in Fig. 8b. Side A became convex, and the tensile strain increased from 0.01 to approximately 0.11. Upon straining, Sa increased from point J (0.43 m) to M (0.94 m). A linear relationship between Sa and strain would have required and increase to approximately 0.8 m. At the concave side B in Fig. 8b, Sa changed from K to N, showing a closer linear relationship to compressive strain. In terms of their effect on roughness, the compressive and the tensile substrate strains were not equivalent.
0.77
0.03 0.00
-0.25
-0.02
Second Bend Inner Curvature (1/mm)
c)
1.27
0.23
R
1.17
0.21
1.07
V
0.97
0.15
0.87
U
0.77
0.47
0.13 0.10
S
0.67 0.57
0.18
0.08
Q
True Strain
3.5. Sa changes under cyclic bending for MF texture along L
Sa Roughness ( μm)
Fig. 7. Evolution of Sa under cyclic pure bending along L.
Sa Roughness ( μm)
Inner Curvature (1/mm)
0.05
P
0.03
0.37
0.00 -0.02
0.38
0.03
Third Bend Inner Curvature (1/mm) Fig. 8. MF-L Outer strains and Sa values at (a) first, (b) second and (c) third bending cycle.
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
roughness of MF samples resulted in larger changes in roughness. This fact must be considered when defining the initial roughness values needed for optimal forming performance.
2
R
EDT-T 1.8
N
Sa Roughness ( μm)
1.6
I 1.2 1
Q
M
1.4
O
K
V
S
H Side A
U R
P
J
U V
M MF-T S
Side A
H, I
0.8
P
N 0.6
Side B
Q
0.4 0.2
J, K
O 0.00
0.28
0.03
-0.27
0.03
305
0.41
0.05
Inner Curvature (1/mm)
3.7. Changes on Sa along the transverse testing direction As shown in Fig. 9, cyclic bending of MF samples along T followed Sa trends similar to the MF-L samples (Fig. 7). In both cases, Sa became progressively larger for each additional cycle after each unbending to flat at points JK, PQ, and UV. Roughness Sa reached its largest value at the third, convex (tensile) bend for side B, represented by point R in Figs. 7 and 9. For MF samples, Sa values along the transverse direction at point R were around 20% higher than those along L. This difference was traced to the unbending to flat at the second cycle (from N to P). Up to points M and N, roughness values along L and T were similar, and Sa between points P and R at the third bend were equivalent (Sa ∼ 0.71 m along L and T). Fig. 9 shows Sa values for EDT samples bent along the transverse direction (EDT-T). Compared to EDT-L, EDT-T points H and I were further apart, with Sa significantly higher at the compressive layer (point H). Sa values were also higher for side B at the reverse bend (point N). As with all other bending cases, the largest Sa values occurred at the tensile outer layer at point R.
Fig. 9. Changes on Sa under cyclic pure bending along T.
3.8. Substrate strain effects on Sa During unbending side A changed from convex (point M) to flat (point Q), and side B from N to P. For both sides Sa values decreased, but remained 50% higher for side A and 35% higher for side B. The third bending cycle continued the increase of Sa roughness. Consistent with the previous trends, the largest Sa values were under tensile straining. In Fig. 8c, side B endured the largest tensile strains as it was bent from P (flat) to R (convex). Correspondingly, Sa increased by 2.4 times to around 1.2 m. Although the Sa increase was promoted by the substrate tensile strain, it can be seen in Fig. 8c that the increase of Sa was significantly larger. 3.6. Sa changes under cyclic bending for EDT texture along L The general Sa trend for EDT textures along L showed similarities to MF-L samples (Fig. 7). Both textures exhibited an increasing Sa trend with increasing cycles, shown by points H, M, and R. However, cyclic roughness differences between the concave and convex sides A and B were smallest for EDT-L samples. A most significant difference between EDT and MF textures was the much smaller Sa variability for EDT textures. From Fig. 7, at maximum point R, Sa increased ∼50% for EDT, which compared to over 400% for MF. The Sa between the initial (point O) and the maximum value at R were significantly closer; Sa ∼ 0.6 m for EDT, and Sa ∼ 0.8 m for MF. The smaller percent variability for EDT texture resided in the magnitude of the initial Sa roughness (∼1.2 m for EDT and ∼0.35 m for MF). The much finer initial
Sa values correlated with uniaxial strains from the tensile test and tensile plane strains from pure bending. Trends of Sa under uniaxial tensile strains were then used to support an explanation of Sa changes under bending. Since the tensile test was monotonic, it correlated with plane strains at point I, at the first bending cycle. From Fig. 6, the tensile plane strain at the convex side B was about 10% for the first bend. The Sa values under tensile bending and uniaxial tests at 10% strain are summarized in Table 3. From the table, higher Sa values for MF were linked to the transverse direction, while lower values correlated with the longitudinal. At 10% strain, Sa values for EDT textures were equivalent and isotropic under both tests. From Table 1, the mechanical properties of the substrate are identical for both textures, with the yield strength lower along T. For MF-longitudinal samples in Fig. 7, the Sa value at point H, which was under compressive strains, was larger than point I, the tensile side. According to uniaxial data in Table 3, Sa was expected to increase when testing along the transverse direction. That increase was in agreement to bending data shown in Fig. 9. Also in correlation with uniaxial data, point I remained at around Sa = 1.4 m when bending EDT textures along both, longitudinal (Fig. 7), and transverse (Fig. 9) directions. From Fig. 7, point H (Sa = 1.49 m) was higher than point I. At the first bend, compressive strains increased EDT roughness more than tensile strains. Along the transverse direction (Fig. 9) compressive strains further increased EDT roughness (Sa = 1.53 m), and points H and
Table 3 Sa values at approximately 10% tensile strain. Test Uniaxial
Strain mode
Texture
Test direction
Sa (m)
Uniaxial tension
MF
L T L T
0.78 0.87 1.36 1.4
L T L T
0.72 0.83 1.4 1.38
EDT
Pure bending (point I)
Plane strain tension
MF EDT
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a) 1.27
3.91
R
1.17
3.56
1.07
VR M
0.97 0.87 0.77 0.67
N
0.57
2.50
1.79
Q
1.44
J
P
0.47 0.37
U
2.85
2.14
I
O
S
VM
H
3.20 V
1.09
K
Normalized Surface Volume (cm3/m2)
Fig. 10 compares changes of NVOL and Sa under cyclic bending. At the first bending-unbending to flat cycle, changes in NVOL were proportional to changes in Sa . This indicated no peak flattening, and no reduction in peak to valley height, which might change the rate of NVOL during bending. The proportionality between changes on Sa and normalized volume was approximately maintained for the sides undergoing compressive straining throughout further testing. However, the layers subjected to tensile straining showed variations. For example, Sa at the reverse bend point M (Fig. 10a) was higher than the corresponding normalized volume value (at point VM ). At the third bend this effect between points R (maximum Sa ) and VR (normalized volume) was even more pronounced. Since there was no external surface contact during testing, changes on Sa and normalized volume were related exclusively to the effects on the texture due to the straining of the substrate layer. As shown previously in Table 2, the sheet progressively stretched and new surface was exposed with each cycle of unbending to flat. Under tensile straining, the new surfaces resulted on increased roughness, but peak to valley heights did not increase in the same proportion and NVOL decreased. Fig. 10b shows the evolution of the normalized volume and Sa for EDT-L. The normalized volume values for both outer layers departed from the Sa trend at the tensile side (points I, M and R), while followed Sa changes more closely at the compressive side (points H, N and S). Changes on normalized surface volume and Sa roughness may impact the formability, friction and paint appearance, as observed by Hartfield-Wunsch et al. (2011). For the EDT material, the normalized volume values increased between 25% and 50% during the cyclic bending (Fig. 10b). The NVOL percent increases for MF textures were significantly larger, ranging from 300% to 400% (Fig. 10a). In comparison to EDT textures, MF texture was at disadvantage in terms of lubricant containment. The NVOL values, initially lower for MF textures, was required to increase several times during bending and unbending. These increases would result in additional loss of performance in lubrication and impact surface finish quality and paint appearance.
Sa side B NVOL Side B
0.73
0.00 0.26 0.01 -0.25 -0.02 0.38 0.03 MF-L Inner Curvature (1/mm)
R
b) 1.80
S
1.70
5.09
VR
U V 4.71
M 1.60 H
1.50
N
1.40 1.30 O
VM
4.34 Q
I
1.20
5.46
K J
P
3.96 3.59
Normalized Surface Volume (cm3/m2)
3.9. Relationships between NVOL and Sa
Sa Side A NVOL Side A
Sa Roughness ( μm)
I showed farther apart. Since the T direction had smaller yield strength (Table 1), Sa was more sensitive to compressive strains along T. As the sheet was bent to a second and third cycle, tensile straining of the substrate progressively became the dominant mechanism for roughness increase. The resultant stretching after unbending to flat (Table 2) was the onset of an increasingly larger non-linear relationship between Sa and strain.
Sa Roughness ( μm)
306
3.22
0.00 0.25 0.02 -0.28 0.02 0.40 0.05 EDT-L Inner Curvature (1/mm) Fig. 10. Sa and normalized volume values: (a) MF-L and (b) EDT-L.
A visual inspection of the final MF texture (Fig. 11a) showed a predominantly irregular pattern over a background showing traces of the original texture (compare with Fig. 3a). The final EDT texture (Fig. 11b) still preserved some of the main features of the original EDT texture (Fig. 3b). The changes in texture were significant, and
Fig. 11. Texture at final flat after three cycles: (a) MF and (b) EDT.
G.A. Lucachick, L.R. Sanchez / Journal of Materials Processing Technology 213 (2013) 300–307
prone to affect friction and other contact phenomena, such as wear, galling and scoring. 4. Conclusions Substrate strains which developed during bending and unbending had significant impact on the texture of aluminum sheet alloy. The three dimensional surface roughness Sa , and the normalized surface volume (NVOL) were useful quantifiers of the changes in surface texture which may affect properties such as friction and paint finish. For the aluminum alloy studied, Sa and NVOL changes were approximately linear to substrate strain at the first bend, and became increasingly non-linear under the medium and large strains developed by successive bending and unbending. Texture changes during straining were more significant for MF samples, which initial roughness Sa was less than one third the EDT value. Percent changes were smaller for EDT textures, which have a higher initial NVOL and Sa values. Research toward optimizing surface textures must take into account the initial values and the variability of these parameters upon deformation. The effect on texture by tensile strains correlated well for uniaxial and plane strain modes. However, the effects on texture under compressive strains at the concave side of the bend were not generally equivalent to tensile strains. Compressive strains were more significant at the initial stages, while changes under tensile strains were more significant at successive bending. Roughness changes were larger along the transverse direction. Changes in texture were initially isotropic for EDT samples, but became larger along the transverse direction as the deformation became increasingly nonlinear. Sa roughness and NVOL changed significantly upon unbending the sample to its flat condition. This operation disturbed any monotonic Sa trend. The Sa values typically decreased as the magnitude of the net strain became smallest at the flat condition. The relationship between Sa and strain history became complex, and, as shown by this study, became unfeasible to determine strain history effects upon unbending based only on measurements of Sa roughness values.
307
Under drawbead flow, the sheet unbends to flat in a very small area at the point of inflexion between the bent and reverse bend. This study showed that texture changes at that section are significant and must be measured. The understanding of texture changes under bending strains is a contribution to further research on friction and contact phenomena in forming areas such as drawbeads and corner radii. References Baydogan, M., Akoy, M.A., Kayali, E.S., Cimenoglu, H., 2003. Deformation induced surface roughening of austenitic stainless steels. ISIJ 43 (11), 1795–1798. Bottema, J., Lahaye, C., Baartman, R., Zhuang, L., De Smet, P., Schoepen, F., 1998. Recent developments in AA6016 aluminum type body sheet product. SAE Paper 981007. doi:10.4271/981007. Caber, P.J., 1993. Interferometric profiler for rough surfaces. Applied Optics 32 (19), 3438–3441, http://dx.doi.org/10.1364/AO.32.003438. Hartfield-Wunsch, S., Cohen, D., Sanchez, L.R., Brattstrom, L.E., 2011. The effect of surface finish on aluminum sheet friction behavior. SAE International Journal of Materials and Manufacturing 4, 818–825, http://dx.doi.org/10.4271/2011-01-0534. Hou, Y., Yu, Z., Zhang, W., Jiang, H., Lin, Z., 2009. Surface topography evolvement of galvanized steels in sheet metal forming. Transactions of Nonferrous Metals Society of China 19, 305–310, http://dx.doi.org/10.1016/S1003-6326(08)60269-1. Mahmudi, R., Mehdizadeh, M., 1998. Surface roughening during uniaxial and equi-biaxial stretching of 70–30 brass sheets. Journal of Materials Processing Technology 80–81, 707–712. Mizuno, T., Mulki, H., 1996. Changes in surface texture of zinc-coated steel sheets under plastic deformation. Wear 198, 176–184. Osakada, K., Oyane, M., 1971. On the roughening of free surface in deformation processes. JSME 14 (68), 171–177. Sachtleber, M., Raabe, D., Weiland, H., 2004. Surface roughening and color changes of coated aluminum sheets during plastic straining. Journal of Materials Processing Technology 148, 68–76. Sanchez, L.R., 2010. Modeling of springback, strain rate, and Bauschinger effects during a two-dimensional steady state cyclic flow of sheet metal subjected to bending under tension. International Journal of Mechanical Sciences 52, 429–439. Stoudt, M.R., Hubbard, J.B., Iadicola, M.A., Banovic, S.W., 2009. A study of the fundamental relationships between deformation-induced surface roughness and strain localization in AA5754. Metallurgical and Materials Transactions A 40 (7), 1611–1622, http://dx.doi.org/10.1007/s11661-009-9881-6. Wilson, D.V., Mirshams, A.R., Roberts, W.T., 1983. An experimental study of the effect of sheet thickness and grain size on limit-strains in biaxial stretching. International Journal of Mechanical Sciences 25 (12), 859–870.