#698 Estimation of Cutting Parameters in Two-Stage Piercing to Reduce Edge Strain Hardening

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1 #698 Estimation of Cutting Parameters in Two-Stage Piercing to Reduce Edge Strain Hardening Abstract Diaz-Infante D., Narayanan, A. and Altan T. The Ohio State University Edge fracture is a common problem when forming advanced high strength steels (AHSS). A particular case of edge fracture occurs during a collar forming/hole extrusion process, which is widely used in the sheet metal forming industry. This study attempts to relate the edge stretchability in collar forming to the strain hardening along the pierced edge; thus, Finite Element (FE) simulations can be used to reduce the number of experiments required to improve cutting settings for a given material and thickness. Using a complex-phase steel, CP-W 800 with thickness of 4.0 mm, a single-stage piercing operation is compared with a two-stage piercing operation, so called shaving, in terms of strains along the pierced edge, calculated by FE simulation. Results indicated that strains were reduced along the pierced edge by shaving. These results are correlated with experimental observations made at the Technical University of Munich (TUM) where better hole expansion ratios (HERs) were obtained using shaving as opposed to single-stage piercing. Moreover, in this study, the combination of cutting parameters that would generate the least edge strain hardening is evaluated based on simulation results. This approach could reduce the need for hardness measurements along the pierced edge as well as the number of experiments required to improve the shaving process. Keywords: Collar Forming, Hole Extrusion, Edge Stretchability, Advanced High Strength Steel, Shaving. 1 Introduction Typically, the formability of the material decreases with increase in its strength. Therefore, fracture issues are more commonly observed when using Advanced High Strength Steels (AHSS). A specific case of this phenomenon is edge fracture. This type of fracture originates from the edge, due to tensile stresses. This fracture occurrence can be related to the damage on the edge created by a given cutting process. A common representation of a cut edge and its damaged area is shown in Figure 1. stretchability [1-3]. Nevertheless, the practicality of each of these cutting methods depends on factors such as the production speed rate required, the hardness of the material to be cut and the volume of the production. This study only encompasses conventional blanking/piercing operations since this is, currently, the most used method in the sheet metal forming industry. Methods such as abrasive waterjet (AWJ), laser cutting or electrical discharge machining (EDM) could reduce the damage at the edge when compared to conventional blanking/piercing processes and thus improve the edge 1

2 using single-stage piercing or two-stage piercing (shaving), Figure 2. Figure 1 Typical sheared/blanked edge characteristics according to Pätzold et al. [8] Fracture initiation has been related to factors such as residual stresses at the edge, burr height, microcracks or hardness near the edge. Mori et al. [4] have related compressive stresses at the edge to a delay in the edge fracture. Chintamani & Sriram [5] and Wang and & Golovashchenko [6] have related the edge cracking to burr height on DP500 and AA6111-T4 respectively. Hasegawa et al. [7] conducted a hole expansion study focusing steels with different microstructures. They found that the microcracks is the cause for material failure. Finally, Pätzold et al. [8] reported the hardness near the sheared edge as the main factor affecting edge fracture for a DP800/1.5 mm steel. To establish a relation between edge fracture and any of the aforementioned factors, a considerable amount of experiments is required for each given material and thickness. Typically, at least two stages are required to conduct edge fracture experiments. The first stage consists of a cutting process for selected conditions such as material, sheet thickness, tool geometry, cutting clearance, cutting speed, etc. Later, in a second stage, the cut edge is stretched by using a selected testing method. Some examples of the testing methods are the Half Dome Test (HDT) [9], the Edge Fracture Tensile Test (EFTT) [10], the Hole Expansion Test (HET) [11] or the Collar Forming Test (CFT) [12]. Finally, the fracture occurrence is related to the initial cutting conditions as a guideline. Other researchers have focused on developing new cutting techniques that may help to achieve edges with better stretchability. Feistle et al. [12] pierced holes of 50 mm diameter, on a CP-W 800 steel with 4.0 mm thickness, either Figure 2 Schematic of a two stage piercing (shaving) of a round hole. The edge quality of the blanked edges was quantified using Collar Forming Tests (CFT), represented in Figure 3. In the CFT, a hole with initial diameter d is fully expanded to a collar shape of diameter D, using a cylindrical punch. After the collar is fully formed, the edge is evaluated for fractures. If the edge of the collar is free of fracture then the relation between the collar diameter D and the initial hole diameter d is increased until the edge fracture is observed. Figure 3 Schematic of a collar forming process. d =pierced hole diameter, D = formed collar diameter. In the CFT, the ratio between the maximum collar diameter that can be achieved without fracture and initial hole diameter is called Hole Expansion Ratio (HER) and it is determined by Equation 1. It is assumed that a larger HER leads to a better stretchability of the edge. Where, HER = (D d) 100 % (1) d 2

3 D=Maximum collar diameter that can be obtained without fracture. d=initial blanked hole diameter. A schematic of the main geometric parameters involved in collar forming is shown in Figure 4, where T is the initial thickness of the sheet, T is the thickness of the sheet at edge of the collar, D is the final collar diameter, d is the initial hole diameter, H is the height of the collar and R d is the corner radius of the collar. Figure 4 Geometric parameters involved in a collar forming operation. Feistle et al. [12] reported that, the holes created by shaving reached a maximum HER of 40%, while the holes produced by single-stage blanking reached only a HER of 5%. These results, summarized for all tested configurations in Table 1, indicate significant HER improvements due to the use of shaving. Table 1 Hole Expansion Ratio for single stage cutting (z 1 = 0 mm) versus two-stage cutting, shaving (z 1 > 0 mm). Each tested case is considered successful when the 10 samples are formed (F) without crack (C) [12]. Significant efforts have been done towards the representation of edge fracture using FE simulations. Two major trends can be observed in this regard. On one hand, the main goal is to predict the edge fracture occurrence based on given cutting conditions [13]. On the other hand, the FE simulations are focused on material modeling for effective process design [14]. There are still mixed opinions about the type of fracture criteria, material model and other parameters that should be considered in the simulations to obtain an accurate representation of the cutting process. Due to the necessity of reducing experimentation time, this study intends use FE simulations to correlate the edge hardness near the edge to the strains produced by the blanking/piercing process. It is hypothesized that the hardness increases with increasing the strains and thus, the edge stretchability is reduced. This methodology could be used in the future not only for evaluation of edge fracture potential, but also for development of new cutting techniques. The aforementioned work done by Feistle et al. [12], at the Technical University of Munich (TUM), is used as an experimental reference to compare the FE simulation results obtained in the present study. 2 Approach 2.1 Finite Element (FE) Model The piercing conditions used by Feistle et al. [12], listed in Table 1, were replicated by an FE model developed using DEFORM 2D software. The tooling set up shown in Figure 5 was considered axisymmetric with brick elements for the FE simulations; dimensions for this set up are listed in Table 2. The mesh was refined to a size of around 0.03 mm near the cutting region. The punch corner radius (r p ), was selected as 0.05 mm to match the one used by Feistle et al [12]. However, the die corner radius used in experiments was not reported and thus it was assumed similar to the punch corner radius, 0.05 mm, for the FE simulations. The coefficient of friction (CoF) was assumed to be 0.1. Mechanical and chemical properties of CP- W mm thick are listed in Table 3 and Table 4. 3

4 Therefore, the blanking speed used in the experiments can be assumed without having an impact on the material behavior. Table 3 Chemical composition for CP-W 800, 4.0 mm thick [17] Table 4 Mechanical properties for CP-W 800, 4.0 mm thick [17] Figure 5 Tooling set up used for the axisymmetric FE simulations using 2D Deform software. Table 2 Geometric parameters used for the piercing tooling set up used in FE simulations. Figure 6 Flow stress curve used in piercing FE simulation for CP-W 800/ 4.0 mm material. Obtained using method described by Kardes et al. [15-16] The flow stress curve for CP-W 800 in Figure 6 was used as an input for the simulation. It was generated, as described by Kardes et al. [15-16], using its minimum Yield Strength (Y s = 680 MPa), its minimum Tensile Strength (UTS = 780 MPa), and its E modulus, assumed as 210 GPa. A power law strain hardening behavior was considered for the flow stress curve extrapolation. The strain rate and temperature dependency of the material were neglected in order to simplify the problem. A Critical Damage Value (CDV), following the Adapted Rice & Tracey criterion described in Equation 2, was used to define the initiation of the crack in FE simulation. This criterion was selected based on observations by Widenmann et al. [18] where several other fracture criteria were also investigated. FE results using these criteria were compared with edge geometry from experiments, for the same geometric parameters. According to this study, [18], Adapted Rice & Tracey criterion matched the experimental results in a closer way. Where, ε f 0 e ασ m σ dε = CDV (2) 4

5 σ m is the Hydrostatic Stress, σ is the Effective Stress, ε is the Effective Strain and α = 2.9 for Adapted Rice & Tracey criterion. The CDV was calculated using inverse engineering. A CDV value of 1.25 was found for CP-W 800, after matching the length of the shear zone of the cut edge in simulations with experiments. compared for different cutting clearances. The average strain near the shear zone was computed as shown in Figure 7 below. A similar procedure was adopted for computing the average effective strain at fracture zone. 2.2 Simulation Matrix The first set of the conducted FE simulations is listed in Table 5. After relating the calculated strains to edge fracture occurrence in the CFTs, a second set of simulations, listed in Table 6, were conducted in order to improve the shaving process by further adjusting the cutting clearance (u) and the cutting offset (z) shown in Figure 2. Table 5 Simulation matrix used to replicate experiments by Feistle et al. [12] Figure 7 Nodes used for the average strain calculation along the shear zone length. All average calculated values are presented and compared with experimental results [12] in the next section. 3 Results The average strains computed at shear and fracture zones for FE simulations of single-stage piercing with various cutting clearances are shown in Table 7 below. Table 7 Strains calculated along the shear and fracture zone for the pierced edges using a single stage cutting operation. Table 6 Simulation matrix used to determine individual effect of the cutting clearance during the initial cutting stage (u 1 ). In all cases the cutting offset was z = 4 mm. 2.3 Strain calculations To reduce numerical errors due to singularities in the simulation, the average strains at shear zone and fracture were For a single stage cutting operation, the calculated average effective strains along the shear zone of the pierced edge remain approximately constant with increasing the cutting clearance between 10% and 25% of sheet thickness. However, these strains are larger when using 5% for the cutting clearance. On the other hand, the average effective strains along the fracture zone increase with decreasing cutting clearance. 5

6 Therefore, from simulation, it is inferred that the edge stretchability increases with increasing cutting clearance for the tested cases. (cutting offset z= 4 mm, in Table 8) are shown in Figure 9 and Figure 10. These graphs are evaluated at both the shear and fracture zones. These simulation results are in agreement with the HERs measured using collar forming tests [12]. Pierced holes using 10% to 25% cutting clearance achieved a 10% HER while the holes pierced with a 5% cutting clearance did not even achieve a 5% HER. Furthermore, a few samples were successfully formed using a 15% HER for 25% cutting clearance followed by 20% cutting clearance. The average strains after shaving with various cutting offset values, 2 mm, 3 mm and 4 mm, are shown in Table 8 below. Table 8 Strains calculated along the shear and fracture zone for the pierced edges using a two-stage cutting operation. Figure 8 Location for strain calculation perpendicularly to the cutting direction. It is seen that the damage accumulated in the area near the deformed or cut edge is lower for shaving as compared to single stage piercing. The average effective strains at fracture zone are very similar for all the cutting offsets considered. The average effective strain at shear zone decreases slightly with increasing cutting offset for CP-W 800 of 4.0 mm thickness. The experimental observations [12] indicated that all specimens cut by shaving with cutting offset (z) of 4 mm could form upto a HER of 40 %, followed by holes shaved with 3 mm cutting offset (35 % HER) and 2 mm cutting offset (25 % HER). Figure 9 Effective strains measured along line 1, shear zone, in Figure 8. A reduction in average effective strains of 25% to 35% was observed between shaving and single stage piercing for the tested configurations, when computed both at the shear and fracture zones. This can also be correlated to the increase in HER values for samples pierced using shaving. Furthermore, using the points illustrated in Figure 8, graphs comparing the variation of effective strains with distance from the pierced edge for the best ranked single stage operation (i.e. 25 % cutting clearance, in Table 7) and shaving Figure 10 Effective strains measured along line 2, fracture zone, in Figure 8. 6

7 Figure 11 shows another representation of the lower strains, assumed as lower damage, along and near the edge for the shaving operation when compared with those obtained from the single stage operation. Figure 11 Effective strain distribution near the pierced edge for a) single stage operation (u = 25%) and b) shaving operation (u 1 = 10%, u 2 = 15%, and z = 4 mm). Areas in red indicate strains larger than 0.5. Refer to Table 7and Table 8 for actual average values. The effect of first stage cutting clearance (u 1 ), keeping the second stage cutting clearance fixed (u 2 = 15%), on the average effective strains at the end of shaving process is shown in Table 9 below. The cutting offset was kept as 4 mm. Table 9 Effect of the initial cutting clearance (u 1 ), used in shaving, on the average effective strains along the pierced edge. The average effective strains at both the shear and fracture zones increased with increase in cutting clearance (u 1 ). Therefore, from FE simulation it is inferred that choosing a smaller cutting clearance (u 1 ) will lead to better edge stretchability of the shaved hole. 4 Conclusions/Summary In this study, a relation between the average effective strains along a pierced edge and the fracture occurrence in collar forming tests for CP-W 800 with 4.0 mm of thickness [12] was established using FE simulations. The principal conclusions of this study, for the tested conditions are listed below. For a single stage cutting operation, the average effective strains along the complete pierced edge decrease with increasing cutting clearance. However, when considering only the shear zone, these strains remain approximately constant when using cutting clearances between 10 and 25% of the sheet thickness. For a two stage cutting operation, also known as shaving, the largest cutting offset used (4 mm) leads to lower average effective strains along the pierced edge. In a shaving operation, for a given cutting clearance (u 2 ), the initial cutting clearance (u 1 ) also affects the average effective strains calculated after the second cutting operation. Lower average effective strains along the shear zone of the pierced edge could be related to higher HER during a CFT. This hypothesis could be used to improve existing piercing/blanking techniques and to propose new ones using FE simulations. 5 References 1. Shih, H.C., Hsiung, C.K., and Wendt, B., Optimal Production Trimming Process for AHSS Sheared Edge Stretchability Improvement, SAE Technical Paper , 2014, doi: / Schneider, M., Geffert, A., Peshekhodov, I., Bouguecha, A. et al., Overview and Comparison of Various Test Methods to Determine Formability of a Sheet Metal Cut-Edge and Approaches to the Test Results Application in Forming Analysis, Materialwissenschaft und Werkstofftechnik 46(12): , Krajcarz, D., Comparison Metal Water Jet Cutting with Laser and Plasma Cutting, Procedia Engineering 69: , Mori, K., Abe, Y., Kidoma, Y., and Kadarno, P., "Slight clearance punching of ultra-high strength steel 7

8 sheets using punch having small round edge," International Journal of Machine Tools and Manufacture 65, pp.41-46, Chintamani, J., & Sriram, S., Sheared edge characterization of steel products used for closure panel applications, SAE Technical Paper , Wang, N., & Golovashchenko, S. F., Mechanism of fracture of aluminum blanks subjected to stretching along the sheared edge, Journal of Materials Processing Technology, 233, , Hasegawa, K., Kawamura, K., Urabe, T., & Hosoya, Y., Effects of microstructure on stretch-flangeformability of 980 MPa grade cold-rolled ultra-high strength steel sheets, ISIJ international, 44(3), , Paetzold, I., Dittmann, F., Feistle, M., Golle, R., Haefele, P., Hoffmann, H., & Volk, W. Influence of shear cutting parameters on the fatigue behavior of a dual-phase steel. In Journal of Physics: Conference Series (Vol. 896, No. 1, p ). IOP Publishing, Shih, H. C., Hsiung, C. K., & Wendt, B., Optimal Production Trimming Process for AHSS Sheared Edge Stretchability Improvement, SAE Technical Paper, , Golovashchenko, S. and Ilinich, A., Analysis of Trimming Processes for Stamped Body Panels, Presented at Great Design in Steel (GDIS), The International Organization for Standardization (ISO), Metallic materials-sheet and strip-hole expanding test, ISO 16630:2009 (E). 12. Feistle, M., Pätzold, I., Golle, R., Volk, W., Frehn, A., & Ilskens, R., Maximizing the Expansion Ratio through Multi-stage Shear-cutting Process during Collar Forming, In IOP Conference Series: Materials Science and Engineering (Vol. 418, No. 1, p ). IOP Publishing, Braun, M., Atzema, E., Beier, T., Bülter, M., Brockmann, S., Larour, P., Müller, T., Neuhaus, R., Richter, A. and Schneider, M., Forming Simulation Involving a Reduced Formability of Shear Cut Steel Sheet Edge, Presented within the Steel Institute VDEh. September Personal Communication. 14. Krinninger, M., Opritescu, D., Golle, R., & Volk, W., On the opportunities of problem-and process-adapted shear cutting simulations for effective process design, Procedia Engineering, 207, , Kardes, N., Choi, C., Yang, X. and Altan, T., "Determining the Flow Stress Curve with Yield and Ultimate Tensile Strengths - Part I", Stamping Journal, p , May/June, Kardes, N., et al "Determining the Flow Stress Curve with Yield and Ultimate Tensile Strengths - Part II", Stamping Journal, p , July/August, Thyssenkrupp, Steel CP-W and CP-K: Product information for complex-phase steels. (accessed 15 October 2018) 18. Widenmann, R., Sartkulvanich, P., & Altan, T., Finite element analysis on the effect of sheared edge quality in blanking upon hole expansion of advanced high strength steel. Presented in IDDRG International Conference, June, Acknowledgements The authors would like to thank the Metal Forming and Casting group at Technische Universität München (TUM) for sharing their knowledge related to edge fracture issues. 8