Effects of Punch Configuration on the AHSS Edge Stretchability.
Conventional automotive mild and high strength steels are being replaced by advanced high strength steels (AHSS) due to the demands for vehicle weight reduction and safety performance improvement. However, several local formability issues have been raised in stamping processes such as edge cracking and shear fracture in small radius stretch bending. It has been found that the edge cracking issue is related to sheet metal shearing processes such as blanking, trimming and piercing. Nakata et el.  has studied the shear deformation properties and the damage behavior on both low and high strength steels using the conventional shearing die. They found that the thickness clearance is critical in trimming high strength steels. The experimental comparison of the edge stretchability of AHSS between standard punched hole, drilled hole and laser cutting hole were conducted by Konieczny  and Karelova  et el. at different punching clearances. Results showed a better edge stretchability was directly associated with a better shearing process. Golovashchenko  modified the conventional shearing process by adding an elastic pad underneath the blank, which would reduce burrs in a wide variety of clearances without deteriorating the total elongation or edge stretchability. However, the value of the elongation was lower than that observed from the conventional shearing process. The shearing process was also modeled using different Finite Element Analysis (FEA) models [1,5,6], but the simulation results were in limited agreement with experiments. Although some limited data or guidelines might be available on the optimal shearing variables set up, they are often based on conventional lower strength, higher ductility sheet steels. They may not be applicable for shearing AHSS. To optimize the shearing variables for AHSS, Shih et el. [7,8] developed a bevel shear hole piercing process to improve the quality of the sheared edge based on the stretchability or flangeability of the sheared edge. The purpose of this study is to further investigate the effects of punch configurations on the AHSS sheared edge stretchability. Punches with different geometries and surface treatments are fabricated to a production piercing condition to study the as-pierced edge stretchability of AHSS in this study. The computer controlled punch system was used for the hole piercing test and hole expansion test was used to evaluate the edge stretchability.
HOLE PIERCING TEST
A computer controlled hole piercing process was developed on a lab hydraulic press and the setup is shown in Figure 1-1. A general-purpose punch die set was inverted and mounted to the computer controlled hydraulic press. A schematic diagram of the punch die set is shown in Figure 1-2. The punch diameter (Pd) was 10 mm, and various die with different inner diameters (Dd) were used to achieve different die clearances (CL). The punch speed was kept at 1 mm/sec and the die clearance per side was fixed at 15% of metal thickness (Shih et el. [7,8]) regardless of the test materials. For each material, five specimens were tested on each punch condition.
There are a total of seven different punch configurations included in this study. The conventional flat punch (FP), flat punch with in-line grinding (ILG), 4 micron fine polish (4M), flat punch with radius on the corner (Edge Hone, EH), edge hone punch with 4 micron fine polish (4MEH), 6 degrees conical shear and 6 degrees bevel shear (6D). All punches are made of M2 steel except for the flat punch (FP) and 6 degrees bevel shear (6D) are made of D2 steel with hardness of 60 HRC for all punches. The 6 degrees bevel shear punch was developed from previous studies [7,8] for optimal piercing AHSS with combining 15% die clearance and the shearing direction parallel to the material rolling direction. The 6 degrees is also implemented to make the conical shear punch, and the detail of each punch is discussed below.
6 degrees Bevel Shear (6D)
The 6 degrees bevel shear punch was previously developed to achieve optimal piercing condition for AHSS as shown in Figure 2. Due to the beveled angle, the tip of the punch will first contact and pierce the material at one side of the hole, and the trailing edge will eventually contact with the material and complete the piercing.
In the production piercing condition, it is a challenge to ensure the material rolling direction is aligned parallel as the bevel shearing and the horizontal force against the beveling shear face could deflect the punch tip when high speed piercing AHSS. This could lead to an unbalanced die clearance condition. To prevent these situations, a 6 degrees fully symmetrical conical shear punch, as shown in Figure 3, was fabricated for this study.
The tip of the conical shear (Figure 3) can be used to locate the center of the hole on the sample to ensure the even die clearance condition throughout the piercing process regardless of the gauge and strength of the material. The sheet metal within the punch hole area is pre-bent and stretched before the edge of the punch begins to contact and shear off the material, which is different from that of the bevel shear.
Flat Punch (FP), In-Line Grinding Punch (ILG) and 4 Micron Polish Punch (4M)
In addition to the geometry and shear angle altering, different surface polishing conditions were applied to the conventional flat punch. Figures 4-1 through 4-3 show the conventional flat punch (FP) with typical tangential direction polish, in-line (axial direction) grinding and 4 micron 45 degrees fine polish (4M). The differences among these three punches are the polish direction and surface smoothness.
Edge Hone (EH) and Edge Hone with 4 Micron Polish (4MEH)
The sharpness of a fresh built punch would be slowly reduced after production break-in. For the application of the AHSS, the pierced hole edge quality could be affected by the reduced sharpness of the punch. To address the concern, a 0.14 mm die corner radius was applied to the flat punch, usually called edge hone, and the configuration is illustrated in Figure 5. Two edge hone punches with conventional polish and 4 micron fine polish (EH and 4MEH), as shown in the Figures 4-1 and 4-3 respectively, are compared in this study.
Three commercially produced galvanneal sheet steels with different thicknesses - DP600 1.5 mm, DP780 1.0 mm, and DP980 1.2 mm and 2.0 mm - were included in this study. The mechanical properties for those materials are given in Table 1.
The hole piercing test was carried out on a computer controlled hydraulic press. The punch speed was kept at 1 mm/sec and the die clearance per side was 15% of metal thickness (Shih et el. [7,8]). The load displacement data among different punch conditions are compared in Figure 6 for piercing DP780. It is observed that the edge hone with 4 micron polish (4MEH) punch results in the highest peak load and steeper load profile, indicating a higher friction and contact pressure between punch and pierced edge of sample. The bevel shear (6D) punch exhibits a much lower punch force due to less contact area in piercing and two peak loads associated with the bevel shear angle (leading punch tip and trailing edge) as discussed by Shih et al. , which is as expected, For the conical shear punch, regardless having a similar peak load as the conventional flat punch (same length of contact), the force profile is reduced and increases progressively due to the angle shear and metal thinning associated with pre-bent and stretching. Those punches with special surface polish (4M, ILG) and die corner radius (EH) tend to have slightly higher peak load due to higher friction in piercing.
The same trend was observed for piercing DP600 and DP980 steels, where the peak load comparison are illustrated in Figures 7 and 8, respectively. The 4MEH punch consistently shows the highest peak load regardless of the test materials. This is associated with the combining effect of the die corner radius and fine surface polishing. The die corner radius tends to compress the material before the shearing process begins. This increases the contact pressure and friction force locally around the punch corner, where the piercing force was increased accordingly. The individual peak load comparison between the edge hone (EH) and 4 micron (4M) punches indicates that the die corner radius and fine surface polish have similar effect in increasing peak load from the flat punch. The 6 degrees bevel shear punch can easily reduce the punch force more than 50% from the conventional flat punch, which should be considered when the load capacity of the piercing equipment is limited and to extend the productive punch life and increase production rates in between punch sharpening.
Edge Stretchability Evaluation
The hole expansion test, as shown in Figure 9, is used to evaluate the edge stretchability and flangeability of sheet metal. A detailed description of the hole expansion test can be found in publications [2,9]. A conical punch was used in this study and a minimum of three replicates were tested for each test condition. All of the specimens were tested with the burr up condition  in the hole expansion test.
The hole expansion ratio (HER) is calculated based on the initial and final diameters of the hole:
HER = (D - d)/d x 100(%)
where D represents the diameter of the expanded hole and d represents the initial diameter of the punched hole. Previous studies [7,8] showed the effects of material blanking orientation on the edge stretchability, which was accomplished by aligning samples both parallel (L) and perpendicular (T) to the bevel shear direction when piercing with 6D punch. Figures 10 and 11 show the HER value for DP600 (1.6 mm) and DP980 (2.0 mm) under different punch conditions. Results in both Figures indicate the conical shear punch has the average highest and consistent HER values among all punch conditions. It is due to the angle shear and proper pre-bent and stretching effects in the piercing process. The pre-bent and stretching concept was also proposed by Takahashi et al.  using a hump bottom punch as shown in Figure 12. It was found that the specimen hardness near the pierced edge from the hump bottom punch was lower than that of the flat punch. The same hardness reduction behavior in the shear affect zone was discovered by Chiriac et al.  using the bevel shear (6D) punch, as shown in Figure 13, which resulted in a better edge stretchability. As expected, the HER value for the bevel shear punch (6D) is comparable to the conical shear punch, especially when it is pierced along the material rolling direction. The flat punch and edge hone punch generated the average lowest HER value, while those punches applied with nonconventional polish condition; ILG and 4M, tended to have better HER value. Although higher friction forces and peak loads were identified for these punches, the as-polish surface topography of the punch serves as a micro surface grinder, which helps to remove some imperfection on the pierced edge and results in the increase of the HER value from the FP punch.
The HER values for thin gauge DP780 and DP980 are compared in Figure 14, where a similar trend as the thicker gauge was observed. The results validate the advantage of the non-conventional polish methodology.
To examine the tool wear of the punch visually, a 20X macrograph was taken from the worn edge on each punch after the experiments. The photographs are displayed in order from the most severe to the least severe wear condition, as shown from Figures 15 to 18. Each punch was applied to pierce four different materials with five duplicates, which results in twenty piercing test. The only difference is for the bevel shear (6D) punch that the piercing test number was double due to bevel shearing in both material rolling and transverse directions.
As illustrated in Figure 15, the D2 flat punch has the most severe wear among test punches including prominent abrasive wear, material chip-out and tear conditions. The poor performance is partially due to its relative lower alloy grade tool steel property of D2 comparing to M2, and the flat head geometry with conventional tangential direction polish. During the piercing of the AHSS, the edge of the punch needs to withstand a much higher cutting, bending and friction force associated with the sheet steel property and the severer contact condition. The corner of the edge serves as the stress concentration point, where the tip tends to be worn out easier than other parts of the punch. A decent heat treatment and better tool steel property are critical for the longevity of the flat punch.
On the wear-severity ranking next to the flat punch are the ILG and 4M punches as shown in Figures 16-1 and 2. Only the prominent abrasive wear is observed for both punches. Although a higher friction force induced by the axial and 45 degrees polishing was identified in Figures 6 to 8 for these punches, the higher alloy content of M2 steel prevents the punch from chipping out and tearing.
When a radius was implemented on the punch corner (edge hone), the stress concentration condition in piercing was reduced and the edge of the punch can be preserved more with no chipping and less abrasive wear as identified in Figures 17-1 and 2 for both EH and 4MEH punches. The light reflection area illustrated a larger surface grinding area (Figure 17-1) for EH as compared to 4MEH punch, while the 4MEH punch has a slightly more abrasive wear area due to its higher punch and friction force (Figures 7 and 8).
Figure 18 shows relatively no wear for the 6D bevel shear punch and a very minor wear for conical shear punch. This is as expected since the geometry of angle shear results in much lower punch and friction force in piercing. The conical shear tends to have slightly more wear than 6D bevel shear, which is associated with its higher punch and friction force.
* The conical shear punch produced the pierced edge with the most consistent and best edge stretchability among all test punches. It is recommended for production piercing AHSS when the better edge stretchability is required.
* The bevel shear punch can reduce the punch force more than 50% from the conventional flat punch, which should be considered when the load capacity of the piercing equipment is limited and to extend the productive punch life and increase production rates in between punch sharpening.
* The punch force has no direct correlation with pierced edge stretchability.
* The new polish methodologies (ILG and 4M) can improve the pierced edge stretchability, but not the tool wear reduction due to higher friction and punch force in piercing.
* The angle shear punches (conical shear and bevel shear) resulted in the least tool wear, and the punches with die corner radius (edge hone) were also found to be effective in reducing the tool wear.
[1.] Nakata, M, Uematsu, K. and Koseki, S., "Shear Deformation Properties of Ultra High Strength Steel Sheet," IDDRG, pp. 527-534, 2006.
[2.] Konieczny, A. and Henderson, T., "On Formability Limitations in Stamping Involving Sheared Edge Stretching," SAE Technical Paper 2007-01-0340, 2007, doi:10.4271/2007-01-0340.
[3.] Karelova, A. and Krempaszky, C., "Influence of the Edge Conditions on the Hole Expansion Property of Dual-Phase and Complex-Phase Steels," MS&T, pp. 159-169, 2007.
[4.] Golovashchenko, S. F., and Ilinich, A. M., "Trimming of Advanced High Strength Steels," IMECE 2005-79983, 2005.
[5.] Dalloz, A., Gourgues, A-F., Pineau, A. and Besson, J., "Influence of the Shear Cutting Process on Damage in Laboratory Dual Phase Steels Developed for Automotive Application," MS&T, pp. 171-181, 2007.
[6.] Widenmann, R., Sartkulvanich, P. and Altan, T., "Finite Element Analysis on the Effect of Sheared Edge Quality in Blanking Upon Hole Expansion of Advanced High Strength Steel," IDDRG, pp. 559-570, 2009.
[7.] Shih, H-C, Chiriac, C. and Shi, M., "The Effects of AHSS Shear Edge Conditions on Edge Fracture," MSEC2010-34062, 2010.
[8.] Shih, H-C and Shi, M., "An Innovative Shearing Process for AHSS Edge Stretchability Improvements," JMSAE-061018, 2011.
[9.] Chiriac, C., and Shih, H-C., "Investigations of Shear Edge Image of Dual Phase 780 Steel," MS&T 2011.
[10.] Takahashi, Y., Kawano, O., Horioka, S., and Ushioda, K., "Improvement of Stretch Flangeability of High-Tensile-Strength Steel Sheets by Piercing under Tension Using Humped Bottom Punch," SAE Technical Paper 2013-01-0609, 2013, doi:10.4271/2013-01-0609.
The material in this paper is intended for general information only. Any use of this material in relation to any specific application should be based on independent examination and verification of its unrestricted availability for such use and a determination of suitability for the application by professionally qualified personnel. No license under any patents or other proprietary interests is implied by the publication of this paper. Those making use of or relying upon the material assume all risks and liability arising from such use or reliance
The author would like to thank the Dayton Lamina of Misumi for fabricating the punches and United States Steel Corporation for permission to publish this paper.
United States Steel Corp.
FCA US LLC
Table 1. Mechanical properties of test materials. Mat. T(mm) YS(MPa) TS(MPa) TE(%) N DP600 1.5 401 618 26.4 0.155 DP780 1.0 511 876 16.9 0.118 DP980 1.2 760 1038 12.4 0.096 DP980 2.0 718 1013 11.1 0.101
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|Author:||Shih, Hua-Chu; Zhou, Dajun; Konopinski, Bruce|
|Publication:||SAE International Journal of Engines|
|Date:||Oct 1, 2017|
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