SEM Analysis on the Affected Zones
Each amorphous electrical steel sheet of the pierced stack was analyzed by SEM to measure its affected zones by piercing.
Figure 3 shows the SEM images on the right-side surface of the first to fifth perforated sheets in the pierced stack. When using punch #2, two types of defects were induced to the first sheet in the pierced stack, e.g., a shear droop at the edge of the pierced hole, and a circumferential crack along the hole. The cracked zone width varies from one sheet to the other in
Figure 3. The shear droop was commonly detected in the whole sheets and its width was smaller than 10 µm. The total affected zone width, including the shear droop and cracked zone, was 25 µm for each pierced sheet. When using punch #3, a wavy distortion was also noticed near the hole of the first sheet in addition to these two defects, while no wavy distortions were seen in the third sheet. On the other hand, these three defects were seen in every sheet of the stack when using punch #1. Later, we explain how the circumferential cracks and the wavy distortion are induced by the piercing process when using the punches with different edge conditions.
Figure 4 compares the measured punching affected zone widths of five pierced sheets when using punch #1, #2 and #3, respectively. The zone width of the fifth sheet is strongly affected by the die edge. The first to fourth pierced sheets are representative of the affected zone width by punching. When using punch #1, a small width of 9 μm for the first sheet jumped up in the second to fourth sheets. In the case of punch #2, the affected zone width was nearly constant by 25 μm from the first to the fifth sheet. When using punch #3, the highest affected zone width decreased monotonously from the first to the fourth sheet, e.g., it reached down to 7 μm at the fourth sheet. This significant difference in the affected zone width among the three punches, reveals that the micro-/nano-textures on the side surface of the punch have a significant influence on the piercing behavior. Let us evaluate this difference among the three punches in the third work piece. When using punches #2 and #3, the affected zone widths are nearly the same as 20 μm. While it became 90 μm, more than the original amorphous sheet thickness by three times when using punch #1. The affected zone width was saved by 74.5% only by changing the normal ground WC (Co) punch to the nitrided SKD11 #2 and #3 punches with micro-/nano-textures.
Let us measure the three-dimensional profile at the vicinity of the pierced hole on the right-side surface of the third sheet. As depicted in
Figure 5a,b, the amorphous sheet is sheared and compressed to deform the flat surface to this convex profile. This convex profile by piercing punches #2 and #3, differs from each other. Two cross-sections were selected to analyze two convex profiles for punch #2 and #3, respectively. The A–A’ cross-section represents the surface profile in the circumferential direction. The B–B’ cross-section describes the height elevation in the lateral direction. When using punch #2, many deep peaks and valleys are noticed in
Figure 5c while a few shallow valleys with the maximum depth of 6 μm were only seen in
Figure 5d. These pairs of peaks and valleys are caused by wrinkling distortion. In comparison, severe distortions were seen when using punch #1, the damage by the wrinkling distortion is reduced when using punches #2 and #3. Nearly flat A–A’ cross-sectional profile in
Figure 5d reveals that the piercing process by punch #3 is free from the wrinkling distortion. The B–B’ cross-sectional profiles in
Figure 5e,f include the geometrical change by the shear droop near the hole edge and the discontinuous peaks and valleys. This total height change reaches 60 μm in
Figure 5e and 70 μm in
Figure 5f, respectively. There is little difference of induced shear droop by piercing with the use of punch #2 and #3.
The transient surface profile of each sheet in the stack was measured by controlling the applied stroke (δ). The punch stroke was applied by
δ < δ
final, which is a final stroke to make a complete perforation. In particular, the stroke was terminated to be δ = δ
final − 30 μm and δ = δ
final − 10 μm to measure each sheet surface profile of stack by using the three-dimensional profilometer.
Figure 6 compares the three-dimensional profile of the first to fifth sheets in the stack at a stroke of δ = δ
final − 30 μm and δ = δ
final − 10 μm, respectively, by using two micro-/nano-textured punches. In common, the first and second sheets both at δ = δ
final − 30 μm and δ = δ
final − 10 μm, have a W-lettered profile. During the piercing process, the punch edge indents into each sheet and causes the stress concentration to generate the fixed moment at the indented sheet by punch edge. When unloading from the pierced state, this moment is released to spring back the sheet to this W-lettered profile.
In this
Figure 6, the broken lines denote the position on each sheet surface to be stressed by the punch edge. This distance (D
H) between two positions represents the hole diameter to be punched out. Since the first to second sheets are tensiled by indentation of punch, their D
H-distances are less than the punch diameter (D
p) due to the elastic spring-back. The third to fifth sheet deformation is also affected by the die edge. Different from the first to second sheets, the third to fifth sheets do not deform in the W-lettered shape but in the U-lettered shape seen in
Figure 6. This reveals that shearing takes place earlier in them than the first to second sheets far from the die edge. Let us compare this shearing behavior when using punch #2 and #3. When comparing this U-lettered deformation of the third sheet at
δfinal − 30 μm, this U-letter deformation is accelerated by using punch #3. The deeper U-shape is formed by using punch #3; the shearing process is enhanced when piercing with the use of punch #3.
The whole measured D
H-distances in
Figure 6 are summarized in
Figure 7 together with the punch diameters (D
p’s) for punches #2 and #3. When using punch #2, D
H < D
p for the first to fourth sheets at δ = δ
final − 30 μm. D
H < D
P is only for the first and second sheet, but D
H > D
p for the third and fourth sheet is at δ = δ
final − 10 μm. This implies that the shearing changes local bending within the clearance between the punch and the die edge. In fact, at the fifth sheet in contact to the die edge, D
H > D
p in every stage, irrespective of the punch indentation. The local bending process governs the sheet deformation near the die edge. When changing punch #2 to #3, the variation of D
H for the first to fifth sheets becomes insensitive to the stroke in the punch indentation. D
H < D
p for the first to third sheets, and D
H > D
p for the fourth to fifth sheets. In correspondence to the difference in U-shaping for the third to fifth sheets in
Figure 6 between two punches, the shearing deformation by the punch indentation governs the whole piercing process when using punch #3. As seen in
Figure 6 and
Figure 7, the transient state in punching out the stack work before perforation, reveals that the micro-grooves with nanotextures on the punch side surface play an essential role to control the shearing behavior in the piercing process. The shearing process is more enhanced when using punch #3. This proves that the nanotextured punch side surface in parallel to the piercing direction might well be suitable for the fine piercing process.
The product quality of the pierced stack work is also determined by the perforated hole diameter. Let us measure each hole diameter in the pierced sheet after the punching test. The pierced hole diameter by using punch #1 was also measured as a reference. The pierced hole diameters by punch #2 and #3 were measured for five samples to deduce their average and deviation. The measured hole diameters (D
N for 1 < N < 5) and the punch diameters (D
p) are compared in
Figure 8 among three punches #1 to #3. D
N is always larger than Dp irrespective of N when using punch #1. This proves that perforation through the five-layered stack work is propelled by local bending in the clearance, and largely affected zones are induced in each sheet. When using the micro-/nanotextured punch #2 and #3, D
N is always smaller than D
p irrespective of N. If the piercing process were completely governed by simple shearing, the hole diameter could be equal to D
p, or, be shortened to be D
N < D
p by the elastic spring-back. When using the punches with D
p ~ 2 mm, this spring-back displacement (Ds) is estimated at maximum by D
s = 0.01 × D
p = 20 μm. Since D
s < 15 μm and D
s < 11 μm when using punch #2 and #3, the whole sheet in the stack is sheared by indentation of the punch with less dependency of clearance between the punch and die edges.
This difference between D
N and Dp reflects on the load–stroke relations in piercing with the use of three punches. As shown in
Figure 9, the piercing load increases non-linearly to the maximum load when using punch #1. While this load to stroke curve becomes semi-linear relation up to the maximum load when using punches #2 and #3. The maximum piercing load is reduced by 4.0% when using punch #2, while it decreases by 2.5% in case of the punch #3.
Figure 10 shows the multi-scale SEM images of the punch edge after punching. When using punch #2, its surface is almost covered by the adhesion of particle debris. On the other hand, from
Figure 10b, some adhesion was seen on the top of longitudinal micro-textures. Most of the longitudinal nanotextures are seen on the micro-grooves of punch #3.