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The effect of temperature on formability of EDD steel
Materials Today: Proceedings xxx (xxxx) xxx
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Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
The effect of temperature on formability of EDD steel R. Raman Goud a,⇑, Aryan Rachala b a b
Dept of Mechanical Engineering, GRIET, Hyderabad 500090, India Dept of Computer Science and Engineering, OUCE, Hyderabad 500044, India
a r t i c l e
i n f o
Article history: Received 1 August 2019 Accepted 13 August 2019 Available online xxxx Keywords: Sheet metal forming Stretchforming Formability EDD steel L-S Dyna
a b s t r a c t In the present decades according to the literature the forming process is most trustworthy technique in metal forming process in which flat sheet is converted into desired components. Formability analysis has to be done in order to get the successful forming components. In this paper the formability of EDD steel sheets was estimated by conducting stretchforming operations on 1 mm EDD steel sheets at different temperatures. The wide range of specimen sizes considered for the experimentation is from 110 mm 110 mm to 110 mm 20 mm of EDD steel. Experiments were conducted at various temperatures RT, 150 °C, 300 °C and 450 °C on stretchforming setup. The exact forming setup has been modelled and executed the finite element simulation. The data like load vs. Displacement, elongation, strain, stress and other parameters were found and differentiated with the experimentation results. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 1st International Conference on Manufacturing, Material Science and Engineering.
1. Introduction Now-a-days the metal forming process became the prominent area of research in manufacturing because of the increased importance of the product quality. Formability can defined as the ability of a material to be deformed into a specific shape and size. It is a property of material to manufacture the qualitative and complex parts with utmost accuracy. Day by day the trend has been a continuing towards development of materials with improved formability. A punch of shape hemispherical is forced over the sheet metal to form the appropriate shape and size. The punch can be controlled by either Computer or manual. The methodology involved in forming process is creating the desired shape by progressive movement of the hemispherical punch towards sheet metal. Since localized deformation is developed during forming, more stretching occurs than in conventional forming. This process is mostly implemented due to more flexibility and reducing tooling cost. This process is capable of forming sheet metal which is used in aerospace, biomedical and automotive industries [1]. To improve the formability of sheet metal, many researchers have studied the process of mechanics [2–5]. Deformation in stretchforming is due to combined shear and stretching phenomena [4]. In some cases, bending of sheet metal also involved in addition to shear and stretching. The stretching and shearing of sheet metal ⇑ Corresponding author. E-mail address: [email protected] (R. Raman Goud).
leads to wall thinning in parts, which can be predicted by finite element analysis [6–10]. The wall thickness is affected by the forming wall angle which the sheet metal can withstand without fracture or necking. The formability of the sheet metal can be anticipated by constructing forming limit diagrams (FLD) which divides the forming region from failure region. It is represented in terms of major strain and minor strain subjected to plane stress conditions [11–15]. FLD is regulated by the sheet thickness, forming speed, punch diameter etc. [16–21]. Tool path determines the formability and surface finish of sheet metal, which were on-par with forming limit diagrams by using finite element analysis [22–33]. This study aims to compose the forming limit diagrams of EDD steel material after stretchforming at elevated temperatures. Finally the experimental results were simulated with FE analysis (Fig. 1). 2. Experimentation 2.1. Material and chemical composition In the present research Extra deep drawn (EDD) steels sheets were used because of their vital importance in aerospace, automotive and other sheet metal industry applications. Present experimentation was carried out in two stages. First one is characterization of Extra deep drawn (EDD) steels sheets for finding the mechanical properties and formability parameters from room temperature to 450 °C in steps of 25 °C. And second one is conducting
https://doi.org/10.1016/j.matpr.2019.08.231 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 1st International Conference on Manufacturing, Material Science and Engineering.
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
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1. Grid markings on the specimen sheet 2. Punch stretching the grid marked samples to failure or onset of localized necking or safe and 3. Measurement of strains
Fig 1. Hydraulic Press.
formability tests on EDD steel sheets to construct the forming limit diagrams at various temperatures. To eliminate the variation in formability due to the properties of the base material, sheets of same base metal grade and thickness, which were processed under identical conditions, were obtained. However, some minor differences in the chemistry of these sheets are unavoidable because of the micro-variations in the processing from one batch to another batch. These sheets were cold rolled. The chemical composition of the EDD steel sheet used in the present investigations was analyzed by a spectrometer. 2.2. Mechanical properties and anisotropy For tensile tests, specimens were prepared as per ASTEM standard specifications and conducted tensile tests on electronic universal testing machine [24]. The experiments were conducted on EDD steel specimens along the three different directions, namely with the tensile axis being parallel (0°), diagonal (45°) and perpendicular (90°) to the rolling direction of the sheet on a 5 tonne capacity universal testing machine. The standard tensile properties yield strength (YS), ultimate tensile strength (UTS), elongation, strain hardening coefficient (n), strength coefficient (K) were determined from the load-elongation data obtained from these tests. The strain-hardening exponent was determined from the tensile test using the Hollomon equation [3]:
r ¼ Ken
Grid marking on the sheet samples of extra deep drawing (EDD) steels sheets was done by using etching machine and noncontacting grid stencil of 2.5 mm radius circles. Stretchforming experiments were conducted (up to necking/fracture/safe of the cup/dome) using appropriately designed and fabricated die and punch tools on a 20-tonne capacity hydraulic press. The velocity of punch was kept constant at 5 mm/min. Minor fluctuations in punch speed triggers negligible changes in the strain rate, which will not influence the flow behaviour of low carbon steel as it is known that many materials including low carbon steel have less sensitivity to changes in strain rate at room temperature [6]. The strain rate sensitivity index (m) of low carbon steel at room temperature is less than 0.01. The amount of biaxiality during punch stretching was varied by the width of samples (70–110 mm in steps of 10 mm). A draw bead of 36 mm radius was given on the dies to constrict the material flow from outside. Adequate blank holding pressure was applied using the upper die to hold the material in the draw bead. For each blank width, 5 specimens were examined to acquire maximum number of data points. The circles on the tested specimen transformed into ellipses after deformation falling into safe, necked and failed zone. The major and minor strains e1 and e2 of the transformed ellipses lying in safe, necked and fractured regions were calculated by measuring the major and minor axes of the ellipse using digital microscope. Thus measured major and minor strains were plotted against each other. The forming limit diagrams were drawn clearly distinguishing the safe limiting strains from the unsafe zone containing the necked and fractured ellipses. The EDD steel specimen for each width ranges from 110 mm 20 mm to 110 mm 110 mm at various temperatures 25 °C, 150 °C, 300 °C and 450 °C were obtained from the experimentation and for one temperature it is shown below Fig. 3. 3. Finite element analysis Finite element analysis (FEA) was executed using the commercial explicit finite element code LS-Dyna to comprehend the formability of the EDD steel at elevated temperatures and to compare with experiment results. ETA/pre-processor was utilised to generate the finite element mesh, assign the boundary conditions and to construct the LS-Dyna deck for the analysis. The finite element model was used for the simulation as shown in Figs. 2 & 3.
The plastic strain ratio (r) was determined by using the specimens made as per ASTEM standards [24]. The r value was evaluated in three directions as in the tensile tests mentioned above. The normal anisotropy –r and planar anisotropy Dr were calculated by using the standard formulae [5].
r ¼ ðr0 þ 2r45 þ r90 Þ=4
Dr ¼ ðr0 2r45 þ r90 Þ=2 where the r0, r45, r90 indicates strain ratio of the specimen with respect to the orientation axis of rolling direction. 2.3. Forming limit diagrams The forming limit diagrams for uniaxial and biaxial stretching modes were determined by following the modified Hecker’s simplified technique [11]. In this method, the experimental procedure mainly involves three stages
Fig 2. Stretching Die.
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
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Fig 3. EDD steel specimens stretched at temperature of 150 °C.
4.2. Mechanical properties and anisotropy
Fig 4. Finite Element Model of the Forming setup.
The blank, punch, die and blank holder were created using 2D shell elements. Mid surface of the blank is considered for the finite element meshing. A complete integrated shell element with five integration points through the thickness was used for the blank. Adaptive refinement mesh is taken for the blank to automatically subdivide the elements and to create refined mesh in areas of high curvature. Punch, die and blank holder are created as rigid parts. The contact between the blank and the rigid punch, the rigid die, and the rigid blank holder were created with the CONTACT_FORM ING_ONE_WAY_SURFACE_TO_SURFACE option (Fig. 4). 4. Results and discussions 4.1. Material and chemical composition From the chemical composition analysis, it was found that EDD steel sheets have less content of carbon (C < 0.048). EDD steel sheets contain Mn in the scale 0.3–0.39%, Si of 0.83, P of 0.019%, Sulphur in the scale 0.022–0.024%, Sn of 0.04%, Cu of 0.019 and Mb of 0.028. Cr and Ni elements are also alloyed in this material to improve the mechanical properties. Since the chemical composition of the base metal is identical for all the sheets used in the entire experimentation. The variation shown in mechanical properties in the final form could be expected only from the effect of temperature but not because of the chemical composition or the prior to the deformation history.
The common tensile properties and anisotropy parameters of EDD steel sheet are given in Table 1. few scattered values were observed in the results. So the average values are reported in Table 1. It is keenly examined that the sheets at room temperature and at 450 °C exhibited higher yield strength values. But notable differences were found in the UTS values and total elongation between 400 °C and 450 °C for EDD steel sheets. However the mean values of UTS for the EDD steel sheet at 450 °C is greater than the 150 °C and 300 °C. The EDD steel at room temperature and at 450 °C exhibited higher total elongation. The strain hardening capacity is greater for 450 °C, when compared to 150 °C and 300 °C. This is indicated by higher value of strain hardening coefficient (n) of the material. This could be correlated with higher uniform elongation of EDD steel. Higher strain hardening exponent increases the ability of the metal to undergo uniform plastic deformation before localized necking/excessive thinning occurs and hence enhances the stretchability of a material. In stretch forming, failure occurs by localized necking/excessive thinning caused due to tensile instability. The values of n and K obtained from log r log e plots shows in Table 1. 4.3. Forming limit diagrams The FLDs have shown that formability of sheet metal is mainly affected by the value of strain hardening exponent (n). The n values of EDD steel sheets are illustrated in Table 1. From the table it can be perceived that the values acquired from true uniform strain in tensile test go well with the FLDs acquired at RT, 150 °C, 300 °C and 450 °C mostly in the plain strain region. The formability of EDD steel at RT, 150 °C, 300 °C and 450 °C temperatures are unvarying with expectations based on the uniaxial tensile characteristics. The influence of temperature on EDD steel sheets is detected and the height of the FLD is clearly noticed, mainly at the plain strain condition. The height of the FLD is reduced with increase in sheet temperature, which is precisely consistent with the strain hardening exponent (n) at every particular temperature. But irrespective of rise in temperature, at 450 °C the FLD values were elevated. It is because by increasing the temperature further there was the effect of sensitivity index and also dynamic strain regime begins to occur in the material near this temperature. It can be observed from the Table 1 that at 450 °C there is a rise in strength, strength coefficient and relative elevation in the work hardening exponent.
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
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Table 1 Mechanical properties of EDD steel. Temp (°C)
YS (MPa)
UTS (MPa)
% elongation
K
n
25 150 300 450
202 188 184 216
337 304 294 329
44 35 29 39
677 577 548 684
0.304 0.274 0.261 0.289
It can be noticed from these FLDs that as the temperature rises, strain data points in the neck and fracture regions, show falling trend in data points towards biaxial stress line. It is essentially due to the rise in temperature and there will be a drop in the mean flow stresses and due to that lower amount of load will be needed to deform the material. This occurrence is observed in the load-displacement plots. As temperature of specimen rises, there is a drop
in the load requirement in the specimen. There is a minute rise in the load at 450 °C as compared to 300 °C. This is due to the dynamic strain aging where there is a rise in work hardening coefficient. Due to this strain data points in the biaxial stress region are slightly elevated trend. It can be perceived from load-displacement plots that by reducing the width of the strip, analogous trend in the data is noticed (Figs. 5–7).
Fig 5. Forming limit diagram of EDD steel sheet at 150 °C.
Fig 6. Forming limit diagram of EDD steel sheet at 300 °C.
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
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Fig 7. Forming limit diagram of EDD steel sheet at 450 °C.
1. The above FLDs illustrate that formability of EDD steel is largely affected by the strain hardening exponent (n). The n values of EDD steel sheets are shown in Table 1. From the table it can be seen that the values recorded from uniform strain in tensile test well matched with the FLDs obtained at room temperature, 150 °C, 300 °C and 450 °C especially in plain strain region. The formability of EDD steel at room temperature, 150 °C, 300 °C and 450 °C temperatures are compatible with expectations based on the uniaxial tensile properties. The strain hardening exponent value of EDD material (0.3), which is in the specific range. It is well established that strain hardening exponent indicates the formability of a material with high n value is desirable for better formability.
2. The influence of temperature on EDD steel is noticed and the height of the FLD is clearly perceived, especially at the plain strain state. The height of the FLD fell with rise in sheet temperature, which is consistent with strain hardening exponent (n) at each temperature. But the height of FLD rose at 450 °C irrespective of rise in temperature. It is due to rise in temperature that there was an impact on sensitivity index and also DSA begins occurring in the material. It is observed that at 450 °C there is rise in strength, strength coefficient and relative rise in the work hardening exponent. The limit strains of extra deep drawing steel are greater at 450 °C in contrast with 300 °C. This is demonstrated by the rise in forming limit diagram. This change could be due to the presence of a DSA region of the EDD steel
Fig 8. Forming limit diagram of EDD steel sheet at 150 °C.
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from 350 °C to 450 °C. This regional surface transforms a material from ductile to brittle site. The cleavage generally originates within the brittle inter-metallic layer. It propagates to the next surface but the normal ductile failure occurs in steel due to exhaustion of ductility followed by severe localized necking. In FLDs of the extra deep drawing steel which are determined experimentally, the limiting strains are reasonably higher, the maximum major strain being approximately 60% in contrast with all other temperature steel sheets (54–56%). Approximately 48% the maximum major safe strain was observed in EDD steel in plain strain condition at 450 °C. 5. Finite element analysis Simulated and experimental data was compiled. In the simulation it is noticed that the displacement at necking or fracture was higher for smaller width specimen. In biaxial tension the dome height was very small for the specimen 50–60 mm. It is because forming a portion on the material the sides is subjected to compressible hoop stresses. In the deep drawing operation normally we apply hoop stresses but in stretching operation when width of the specimen is somewhere in the middle. Due to this uncontrolled hoop stresses fracture will be appear in the material. The forming limit diagrams were well matched with experimental ones (Fig. 8).
6. Conclusions The formability of EDD steel at various temperatures was measured by plotting the formability limit diagrams at various temperatures. EDD steels are naturally having good formability at room temperature but their formability improved by increasing the temperature primarily because of drop in the mean flow stresses. Specially at 450 °C this material exhibits highest formability due to the dynamic strain regime and same is shown in the formability limit diagrams. It was observed from FLDs that adequate portion of plain strains were observed at all the temperatures. References [1] D. Ravi Kumar, K. Swaminathan, Formability of two Al alloys, Mater. Sci. Technol. 15 (1999) 1241–1252. [2] D. Ravi Kumar, Formability analysis of extra-deep drawing steel, J. Mater. Process. Technol. 130–131 (2002) 31–41. [3] Kumar Singh Swadesh, K. Mahesh, K. Apurv, M. Swathi, Understanding formability of extra-deep drawing steel at elevated temperature using finite element simulation, J. Mater. Des. 31 (2010) 4478–4484. [4] R. Raman Goud, K.Eswar Prasad, Swadesh Kumar Singh, Formability limit diagrams of extra-deep-drawing steel at elevated temperatures. [5] G.E. Dieter, Mechanical Metallurgy, McGraw Hill, 1988, pp. 241–272. [6] J. Hiam, A. Lee, Factors influencing the forming limit curves of sheet steel, Sheet Metal Ind. 5 (1978) 631–643. [7] S.P. Keeler, Understanding sheet metal formability, Sheet Metal Ind. 3 (1971) 352–357.
[8] K. Swaminathan, K.A. Padmanabhan, Some investigations on the forming behaviour of an indigenous extra-deep drawing low carbon steel-Part-I: Experimental results, Trans. Indian Inst. Met. 44 (1991) 231–247. [9] R. Raman Goud, K. Eshwara Prasad, Swadesh Kumar Singh, George Varghese, Thickness distribution of extra deep drawn steel in stretchforming at elevated temperatures, Mater. Today: Proc. (2017). [10] A.K. Ghosh, S.S. Hecker, Failure in thin sheets stretched over rigid punches, Metall. Trans. A 6A (1975) 1065–1074. [11] B. Sarkar, B.K. Jha, N. Banerjee, D. Mukherjee, Formability assessment of EDD steels for automotive application, Steel India 22 (1999) 10–19. [12] X.H. Zhu, S.A. Majlessi, E.C. Aifantis, Predicting the FLD for sheet metals with planar anisotropy, Soc. Autom. Eng. 107 (1998) 56–61. [13] G.J. Coubrough, D.K. Matlock, C.J. Von Tyne, Formability of Type 304 Stainless steel sheet, Soc. Autom. Eng. 102 (1993) 968–978. [14] F. Barlat, Forming Limit Diagrams: Predictions Based on Some Microstructural Aspects of Materials, The Minerals, Metals and Materials Society, 1989. [15] Tyng-Bin Huang, Yung-An Tsai, Fuh-Kuo Chen, Finite element analysis and formability of non-isothermal deep drawing of AZ31B sheets, J. Mater. Process. Technol. 177 (2006) 142–145. [16] Joji Satoh, Manabu Gotoh, Yasuharu Maeda, Stretch-drawing of titanium sheets, J. Mater. Process. Technol. 139 (2003) 201–207. [17] F. Toussaint, L. Tabourot, F. Ducher, Experimental and numerical analysis of the forming process of a CP titanium scoliotic instrumentation, J. Mater. Process. Technol. 197 (2008) 10–16. [18] Hu. Weilong, Z.R. Wang, Anisotropic characteristic of materials and basic selecting rules with different sheet metal forming processes, J. Mater. Process. Technol. 127 (2002) 374–381. [19] Thomas B. Stoughton, Jeong Whan Yoon, Sheet metal formability analysis for anisotropic materials under non-proportional loading, Int. J. Mech. Sci. 47 (2005) 1972–2002. [20] Julian M. Allwood, Daniel R. Shouler, Generalized forming limit diagrams showing increased forming limits with non-planar stress states, Int. J. Plast. 25 (2009) 1207–1230. [21] Philip Eyckens, Albert Van Bael, Paul Van Houtte, Marciniak–Kuczynski type modeling of the effect of Through-Thickness Shear on the forming limits of sheet metal, Int. J. Plast. 25 (2009) 2249–2268. [22] S. Mahabunphachai, M. Koc, Investigations on forming of aluminum 5052 and 6061 sheet alloys at warm temperatures, Mater. Des. 31 (2010) 2422–2434. [23] M. Firat, Computer aided analysis and design of sheet metal forming processes: Part II – Deformation response modeling, Mater. Des. 28 (2007) 1304–1310. [24] G. Palumbo, D. Sorgente, L. Tricarico, The design of a formability test in warm conditions for an AZ31 magnesium alloy avoiding friction and strain rate effects, Int. J. Mach. Tools Manuf. 48 (2008) 1535–1545. [25] M. Jie, C.H. Cheng, L.C. Chan, C.L. Chow, Forming limit diagrams of strain-ratedependent sheet metals, Int. J. Mech. Sci. 51 (2009) 269–275. [26] F. Djavanroodi, A. Derogar, Experimental and numerical evaluation of forming limit diagram for Ti6Al4V titanium and Al6061-T6 aluminum alloys sheets, Mater. Des. 31 (2010) 4866–4875. [27] S.B. Kim, H. Huh, H.H. Bok, M.B. Moon, Forming limit diagram of auto-body steel sheets for high-speed sheet metal forming, J. Mater. Process. Technol. 211 (2011) 851–862. [28] Ming-Yao Li, Xinhai Zhu, E. Chu, Effect of strain rate sensitivity on FLDs-An instability approach, Int. J. Mech. Sci. 64 (2012) 273–279. [29] Hyuk Jong Bong, Barlat Frederic, Myoung-Gyu Lee, Deok Chan Ahn, The forming limit diagram of ferritic stainless steel sheets: experiments and modelling, Int. J. Mech. Sci. 64 (2012) 1–10. [30] Yanshan Lou, Hoon Huh, Sungjun Lim, Keunhwan Pack, New ductile fracture criterion for prediction of fracture forming limit diagrams of sheet metals, Int. J. Solids Struct. 49 (2012) 3605–3615. [31] D. Banabic, L. Lazarescu, L. Paraianu, I. Ciobanu, I. Nicodim, D.S. Comsa, Development of a new procedure for the experimental determination of the forming limit curves, CIRP Ann. Manuf. Technol. 62 (2013) 255–258. [32] Gupta, A.K.D. Ravi Kumar, Formability of galvanized interstitial-free steel sheets, J. Mater. Process. Technl. 172 (2006) 225–237. [33] Chadaram Srinivasu, Limbadri Vishnu, R. Raman Goud, et al., Finite element simulation of stretching operation of EDD steel at different temperatures, Mater. Today: Proc. (2015).
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
The effect of temperature on formability of EDD steel R. Raman Goud a,⇑, Aryan Rachala b a b
Dept of Mechanical Engineering, GRIET, Hyderabad 500090, India Dept of Computer Science and Engineering, OUCE, Hyderabad 500044, India
a r t i c l e
i n f o
Article history: Received 1 August 2019 Accepted 13 August 2019 Available online xxxx Keywords: Sheet metal forming Stretchforming Formability EDD steel L-S Dyna
a b s t r a c t In the present decades according to the literature the forming process is most trustworthy technique in metal forming process in which flat sheet is converted into desired components. Formability analysis has to be done in order to get the successful forming components. In this paper the formability of EDD steel sheets was estimated by conducting stretchforming operations on 1 mm EDD steel sheets at different temperatures. The wide range of specimen sizes considered for the experimentation is from 110 mm 110 mm to 110 mm 20 mm of EDD steel. Experiments were conducted at various temperatures RT, 150 °C, 300 °C and 450 °C on stretchforming setup. The exact forming setup has been modelled and executed the finite element simulation. The data like load vs. Displacement, elongation, strain, stress and other parameters were found and differentiated with the experimentation results. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 1st International Conference on Manufacturing, Material Science and Engineering.
1. Introduction Now-a-days the metal forming process became the prominent area of research in manufacturing because of the increased importance of the product quality. Formability can defined as the ability of a material to be deformed into a specific shape and size. It is a property of material to manufacture the qualitative and complex parts with utmost accuracy. Day by day the trend has been a continuing towards development of materials with improved formability. A punch of shape hemispherical is forced over the sheet metal to form the appropriate shape and size. The punch can be controlled by either Computer or manual. The methodology involved in forming process is creating the desired shape by progressive movement of the hemispherical punch towards sheet metal. Since localized deformation is developed during forming, more stretching occurs than in conventional forming. This process is mostly implemented due to more flexibility and reducing tooling cost. This process is capable of forming sheet metal which is used in aerospace, biomedical and automotive industries [1]. To improve the formability of sheet metal, many researchers have studied the process of mechanics [2–5]. Deformation in stretchforming is due to combined shear and stretching phenomena [4]. In some cases, bending of sheet metal also involved in addition to shear and stretching. The stretching and shearing of sheet metal ⇑ Corresponding author. E-mail address: [email protected] (R. Raman Goud).
leads to wall thinning in parts, which can be predicted by finite element analysis [6–10]. The wall thickness is affected by the forming wall angle which the sheet metal can withstand without fracture or necking. The formability of the sheet metal can be anticipated by constructing forming limit diagrams (FLD) which divides the forming region from failure region. It is represented in terms of major strain and minor strain subjected to plane stress conditions [11–15]. FLD is regulated by the sheet thickness, forming speed, punch diameter etc. [16–21]. Tool path determines the formability and surface finish of sheet metal, which were on-par with forming limit diagrams by using finite element analysis [22–33]. This study aims to compose the forming limit diagrams of EDD steel material after stretchforming at elevated temperatures. Finally the experimental results were simulated with FE analysis (Fig. 1). 2. Experimentation 2.1. Material and chemical composition In the present research Extra deep drawn (EDD) steels sheets were used because of their vital importance in aerospace, automotive and other sheet metal industry applications. Present experimentation was carried out in two stages. First one is characterization of Extra deep drawn (EDD) steels sheets for finding the mechanical properties and formability parameters from room temperature to 450 °C in steps of 25 °C. And second one is conducting
https://doi.org/10.1016/j.matpr.2019.08.231 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 1st International Conference on Manufacturing, Material Science and Engineering.
Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231
2
R. Raman Goud, A. Rachala / Materials Today: Proceedings xxx (xxxx) xxx
1. Grid markings on the specimen sheet 2. Punch stretching the grid marked samples to failure or onset of localized necking or safe and 3. Measurement of strains
Fig 1. Hydraulic Press.
formability tests on EDD steel sheets to construct the forming limit diagrams at various temperatures. To eliminate the variation in formability due to the properties of the base material, sheets of same base metal grade and thickness, which were processed under identical conditions, were obtained. However, some minor differences in the chemistry of these sheets are unavoidable because of the micro-variations in the processing from one batch to another batch. These sheets were cold rolled. The chemical composition of the EDD steel sheet used in the present investigations was analyzed by a spectrometer. 2.2. Mechanical properties and anisotropy For tensile tests, specimens were prepared as per ASTEM standard specifications and conducted tensile tests on electronic universal testing machine [24]. The experiments were conducted on EDD steel specimens along the three different directions, namely with the tensile axis being parallel (0°), diagonal (45°) and perpendicular (90°) to the rolling direction of the sheet on a 5 tonne capacity universal testing machine. The standard tensile properties yield strength (YS), ultimate tensile strength (UTS), elongation, strain hardening coefficient (n), strength coefficient (K) were determined from the load-elongation data obtained from these tests. The strain-hardening exponent was determined from the tensile test using the Hollomon equation [3]:
r ¼ Ken
Grid marking on the sheet samples of extra deep drawing (EDD) steels sheets was done by using etching machine and noncontacting grid stencil of 2.5 mm radius circles. Stretchforming experiments were conducted (up to necking/fracture/safe of the cup/dome) using appropriately designed and fabricated die and punch tools on a 20-tonne capacity hydraulic press. The velocity of punch was kept constant at 5 mm/min. Minor fluctuations in punch speed triggers negligible changes in the strain rate, which will not influence the flow behaviour of low carbon steel as it is known that many materials including low carbon steel have less sensitivity to changes in strain rate at room temperature [6]. The strain rate sensitivity index (m) of low carbon steel at room temperature is less than 0.01. The amount of biaxiality during punch stretching was varied by the width of samples (70–110 mm in steps of 10 mm). A draw bead of 36 mm radius was given on the dies to constrict the material flow from outside. Adequate blank holding pressure was applied using the upper die to hold the material in the draw bead. For each blank width, 5 specimens were examined to acquire maximum number of data points. The circles on the tested specimen transformed into ellipses after deformation falling into safe, necked and failed zone. The major and minor strains e1 and e2 of the transformed ellipses lying in safe, necked and fractured regions were calculated by measuring the major and minor axes of the ellipse using digital microscope. Thus measured major and minor strains were plotted against each other. The forming limit diagrams were drawn clearly distinguishing the safe limiting strains from the unsafe zone containing the necked and fractured ellipses. The EDD steel specimen for each width ranges from 110 mm 20 mm to 110 mm 110 mm at various temperatures 25 °C, 150 °C, 300 °C and 450 °C were obtained from the experimentation and for one temperature it is shown below Fig. 3. 3. Finite element analysis Finite element analysis (FEA) was executed using the commercial explicit finite element code LS-Dyna to comprehend the formability of the EDD steel at elevated temperatures and to compare with experiment results. ETA/pre-processor was utilised to generate the finite element mesh, assign the boundary conditions and to construct the LS-Dyna deck for the analysis. The finite element model was used for the simulation as shown in Figs. 2 & 3.
The plastic strain ratio (r) was determined by using the specimens made as per ASTEM standards [24]. The r value was evaluated in three directions as in the tensile tests mentioned above. The normal anisotropy –r and planar anisotropy Dr were calculated by using the standard formulae [5].
r ¼ ðr0 þ 2r45 þ r90 Þ=4
Dr ¼ ðr0 2r45 þ r90 Þ=2 where the r0, r45, r90 indicates strain ratio of the specimen with respect to the orientation axis of rolling direction. 2.3. Forming limit diagrams The forming limit diagrams for uniaxial and biaxial stretching modes were determined by following the modified Hecker’s simplified technique [11]. In this method, the experimental procedure mainly involves three stages
Fig 2. Stretching Die.
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Fig 3. EDD steel specimens stretched at temperature of 150 °C.
4.2. Mechanical properties and anisotropy
Fig 4. Finite Element Model of the Forming setup.
The blank, punch, die and blank holder were created using 2D shell elements. Mid surface of the blank is considered for the finite element meshing. A complete integrated shell element with five integration points through the thickness was used for the blank. Adaptive refinement mesh is taken for the blank to automatically subdivide the elements and to create refined mesh in areas of high curvature. Punch, die and blank holder are created as rigid parts. The contact between the blank and the rigid punch, the rigid die, and the rigid blank holder were created with the CONTACT_FORM ING_ONE_WAY_SURFACE_TO_SURFACE option (Fig. 4). 4. Results and discussions 4.1. Material and chemical composition From the chemical composition analysis, it was found that EDD steel sheets have less content of carbon (C < 0.048). EDD steel sheets contain Mn in the scale 0.3–0.39%, Si of 0.83, P of 0.019%, Sulphur in the scale 0.022–0.024%, Sn of 0.04%, Cu of 0.019 and Mb of 0.028. Cr and Ni elements are also alloyed in this material to improve the mechanical properties. Since the chemical composition of the base metal is identical for all the sheets used in the entire experimentation. The variation shown in mechanical properties in the final form could be expected only from the effect of temperature but not because of the chemical composition or the prior to the deformation history.
The common tensile properties and anisotropy parameters of EDD steel sheet are given in Table 1. few scattered values were observed in the results. So the average values are reported in Table 1. It is keenly examined that the sheets at room temperature and at 450 °C exhibited higher yield strength values. But notable differences were found in the UTS values and total elongation between 400 °C and 450 °C for EDD steel sheets. However the mean values of UTS for the EDD steel sheet at 450 °C is greater than the 150 °C and 300 °C. The EDD steel at room temperature and at 450 °C exhibited higher total elongation. The strain hardening capacity is greater for 450 °C, when compared to 150 °C and 300 °C. This is indicated by higher value of strain hardening coefficient (n) of the material. This could be correlated with higher uniform elongation of EDD steel. Higher strain hardening exponent increases the ability of the metal to undergo uniform plastic deformation before localized necking/excessive thinning occurs and hence enhances the stretchability of a material. In stretch forming, failure occurs by localized necking/excessive thinning caused due to tensile instability. The values of n and K obtained from log r log e plots shows in Table 1. 4.3. Forming limit diagrams The FLDs have shown that formability of sheet metal is mainly affected by the value of strain hardening exponent (n). The n values of EDD steel sheets are illustrated in Table 1. From the table it can be perceived that the values acquired from true uniform strain in tensile test go well with the FLDs acquired at RT, 150 °C, 300 °C and 450 °C mostly in the plain strain region. The formability of EDD steel at RT, 150 °C, 300 °C and 450 °C temperatures are unvarying with expectations based on the uniaxial tensile characteristics. The influence of temperature on EDD steel sheets is detected and the height of the FLD is clearly noticed, mainly at the plain strain condition. The height of the FLD is reduced with increase in sheet temperature, which is precisely consistent with the strain hardening exponent (n) at every particular temperature. But irrespective of rise in temperature, at 450 °C the FLD values were elevated. It is because by increasing the temperature further there was the effect of sensitivity index and also dynamic strain regime begins to occur in the material near this temperature. It can be observed from the Table 1 that at 450 °C there is a rise in strength, strength coefficient and relative elevation in the work hardening exponent.
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Table 1 Mechanical properties of EDD steel. Temp (°C)
YS (MPa)
UTS (MPa)
% elongation
K
n
25 150 300 450
202 188 184 216
337 304 294 329
44 35 29 39
677 577 548 684
0.304 0.274 0.261 0.289
It can be noticed from these FLDs that as the temperature rises, strain data points in the neck and fracture regions, show falling trend in data points towards biaxial stress line. It is essentially due to the rise in temperature and there will be a drop in the mean flow stresses and due to that lower amount of load will be needed to deform the material. This occurrence is observed in the load-displacement plots. As temperature of specimen rises, there is a drop
in the load requirement in the specimen. There is a minute rise in the load at 450 °C as compared to 300 °C. This is due to the dynamic strain aging where there is a rise in work hardening coefficient. Due to this strain data points in the biaxial stress region are slightly elevated trend. It can be perceived from load-displacement plots that by reducing the width of the strip, analogous trend in the data is noticed (Figs. 5–7).
Fig 5. Forming limit diagram of EDD steel sheet at 150 °C.
Fig 6. Forming limit diagram of EDD steel sheet at 300 °C.
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Fig 7. Forming limit diagram of EDD steel sheet at 450 °C.
1. The above FLDs illustrate that formability of EDD steel is largely affected by the strain hardening exponent (n). The n values of EDD steel sheets are shown in Table 1. From the table it can be seen that the values recorded from uniform strain in tensile test well matched with the FLDs obtained at room temperature, 150 °C, 300 °C and 450 °C especially in plain strain region. The formability of EDD steel at room temperature, 150 °C, 300 °C and 450 °C temperatures are compatible with expectations based on the uniaxial tensile properties. The strain hardening exponent value of EDD material (0.3), which is in the specific range. It is well established that strain hardening exponent indicates the formability of a material with high n value is desirable for better formability.
2. The influence of temperature on EDD steel is noticed and the height of the FLD is clearly perceived, especially at the plain strain state. The height of the FLD fell with rise in sheet temperature, which is consistent with strain hardening exponent (n) at each temperature. But the height of FLD rose at 450 °C irrespective of rise in temperature. It is due to rise in temperature that there was an impact on sensitivity index and also DSA begins occurring in the material. It is observed that at 450 °C there is rise in strength, strength coefficient and relative rise in the work hardening exponent. The limit strains of extra deep drawing steel are greater at 450 °C in contrast with 300 °C. This is demonstrated by the rise in forming limit diagram. This change could be due to the presence of a DSA region of the EDD steel
Fig 8. Forming limit diagram of EDD steel sheet at 150 °C.
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from 350 °C to 450 °C. This regional surface transforms a material from ductile to brittle site. The cleavage generally originates within the brittle inter-metallic layer. It propagates to the next surface but the normal ductile failure occurs in steel due to exhaustion of ductility followed by severe localized necking. In FLDs of the extra deep drawing steel which are determined experimentally, the limiting strains are reasonably higher, the maximum major strain being approximately 60% in contrast with all other temperature steel sheets (54–56%). Approximately 48% the maximum major safe strain was observed in EDD steel in plain strain condition at 450 °C. 5. Finite element analysis Simulated and experimental data was compiled. In the simulation it is noticed that the displacement at necking or fracture was higher for smaller width specimen. In biaxial tension the dome height was very small for the specimen 50–60 mm. It is because forming a portion on the material the sides is subjected to compressible hoop stresses. In the deep drawing operation normally we apply hoop stresses but in stretching operation when width of the specimen is somewhere in the middle. Due to this uncontrolled hoop stresses fracture will be appear in the material. The forming limit diagrams were well matched with experimental ones (Fig. 8).
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Please cite this article as: R. Raman Goud and A. Rachala, The effect of temperature on formability of EDD steel, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.231