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Simulation and numerical analysis of warm stamping (AA6061)
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 16 (2019) 598–603
www.materialstoday.com/proceedings
ICAMMAS17
Simulation and numerical analysis of warm stamping (AA6061) K S Dhinesha,T SubhaSankaria,T Paneerselvama, Venkatesan Ma,* A
School of Mechanical Engineering, SASTRA University, Tirumalaisamudram, Thanajvur – 613401, Tamilnadu, India
Abstract
Stamping process is an important sheet metal forming process used in domestic and automotive components. In this process, blank material is converted into the desired complicated shape by punching process.Stamping includes a process such as punching using a punch or ram, blanking, embossing, bending, flanging, and coining.Cost minimisation ,less time consumption, increase in efficiency are very important in the recent manufacturing industry. Today simulation techniques are used to analyse the performing of dies, processes and billets prior to real timeimplementation. Simulations enable us to find out the common defects such as tears, wrinkles, and material thinning. Finite element analysis (FEA) is the method of simulating the process of sheet metal forming to determine various behaviour of the sheet metal and helps us to find out whether it will produce parts free of defects such as fracture or wrinkling. Sheet metal forming is a popularly used metal working process. In this paper stamping process is analysed numerically using a simulation software package LSTC – LSDYNA. A three dimensional thermomechanically coupled stamping model is analysed and its validations are presented. The analysis data from the numerical simulation helps in understanding the real-time mechanism of sheet metal forming process.There areonly few works were reported on numerical analysis of warm stamping of AA6061 such as load requirement, blank holder pressure, stress-strain analysis.This work is based on study the deformation behaviour of sheet metal, flow stress analysis, contact pressure and punch velocity. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
Keywords: Thermo-mechanical finite element analysis; Ls-dyna 4.1; hot stamping; AA6061 sheet metal;
1. Introduction Aluminium and its alloy are the prime material of construction of aircrafts because of its light weight.Now-a-days, the ultimate aim of automobile industries is to reduce the weight.SoAluminium and its alloy are widely used in building due to their properties such as light weight, corrosion resistance and excellent formability. Formability of aluminium increases with increase in temperature. Thus, forming of sheet metals at the temperature close to its recrystallization temperature is called warm forming.AA6061 shows more formability when it is heated to its recrystallization temperature.
*Correspondingauthor.Tel.:+91 9841309048. E-mailaddress:[email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
599
Stamping is used for converting flat sheets into cup-shaped products such as utensils, cook wares, engine casing, ladders and automobile body panels. In the stamping process, the metal either in the form of sheet or coil is held between the die and punch. The punch is pushed into the die cavity, there bysheet metal will get shape.Increasing the ductility of the material when the process is done above the re- crystallization temperaturethere by prevents the material from strain hardening [1]. Effect of Blank-holder pressure (BHP) on the deep drawn product, the effect of BHP variation on failure limits and the limiting drawing ratio in axisymmetric deep drawing operations were studied in [2]. Warm forming of series of aluminium alloys sheets by thermo- mechanical simulation and its validation were discussed in [3]. Warm forming of Al Mg alloys described variation of material properties with increasing temperature. Stress reduction and the formability is improvement due to increased strain hardening rate were analysed in [4]. Effect of various parameters that cause the wrinkling in deep drawing process of cylindrical cup and various factors like BHF, punch round radius, and die fillet radius on the wrinkling were studied [5]. Significant improvement in the deformability by means of warm forming at temperature around recrystallization temperature investigated in [6]. Reduction of transmittable force, increased depth of drawing due to uniform increase in temperature and formability improvement were discussed in [7]. Advantage of metal forming process at elevated temperature and the factors that reduces the wrinkling tendency were studied [8].Numerical simulation of the 3-D stamping process with elastic strains were presented [9]. The above literature works show that very few works have been done on simulation of stamping for various materials. Only a few simulations of aluminium warm forming have been reported so far. The present work is on numerical simulation of stamping process and the stress-strain behaviour, velocity requirement, energy analysis of AA6061are presented. 2. Validation For validation, previous work done by the author on warm deep drawing of AZ31 magnesium alloy sheet is used. The ultimate stress value during the forming process is found to be 204 MPa [10]. In this present paper, we got the ultimate stress around that value and the true stress vs strain curve follow the same pattern. These give the validation of present paper. 3. Finite element modeling 3.1Material: The sheet metal material for this study is Aluminium alloy 6061. Table: 1the Composition of AA6061
ElementAlMgSiCuCr Composition (%)97.91.00.60.80.2 Table:2 Properties of AA6061:
Poisson's ratio Elastic modulus(GPa) Density(kg/m3) Heat capacity (J/Kg K) Heat conduction (W/m K)
0.33 70-80 2700 920 121
3.2Three-dimensionalmodeling: Punch diameter: 110mm; Fillet radius: 10mm Die outer diameter: 125mm; Die inner diameter: 113mm; Fillet radius: 15mm. For the 3-D modeling, LS DYNA version 4.1 is used. The punch and die does not undergo any deformation it is assumed as rigid material using the command (MAT_20_RIGID). The material of the die is die steel (S50C). Punch
600
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
and die are shown fig. 1. The sheet metal is assigned to (MAT_24_PIECEWISE_LINEAR_PLASTICITY). The punch is constrained in x and z direction. The lower die is constrained in all direction. The composition and properties of the sheet metal are given in Table: 1and 2. The sheet metal is meshed with 61500 no of elements. The sheet metal is assigned with an initial temperature 400°C (Based on recrystallization temperature). No preheating of the die was made. The punch was given with the velocity of 500m/s to 750 m/s.
Fig. 1 punch and die
Fig. 2 End product of stamping
4. Results and Discussion
Fig. 3 stress-strain of AA6061
Fig.3 shows the stress-strain behavior of AA6061.Generally formability of aluminium increases with increase in temperature. The flow stress was observed to be more at low temperature rangebut it decreases with increase in temperature there by increasing the formability. When the punch hit the sheet metal thermal softening and strain hardening occur and the specimen deforms due to sudden load. Since the sheet metal temperature is based on recrystallization temperature of aluminium alloy minimal flow stress is observed. More flow stress level is associated with more strain rate. Increase in the strain-hardening is due to the short annihilation or recovery time available during the test. The lowest strain rate leading to low flow stress values.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
601
Fig. 4 Internal energy – time plot
Fig. 4 show the graph between internal energy and the time. It can be observed that the internal energy of the sheet metal gradually increases as punch hits the sheet metal. It is because of the sudden impact given by hammer on the sheet metal. The instant drop in the graph is because of resistance offered by the sheet metal to the sudden impact of the punch. It is called as hammering effect. It causes external work on the sheet metal there by gradually increasing the internal energy of sheet metal.
Fig.5.1 (V=500 m/s) Fig. 5.2 (V=750 m/s)
Fig. 5(Effect of velocity of punch in stamping)
Fig. 5 gives the effect of punch velocity on deformation (Depth of cup) of sheet metal. The variation in the deformation of sheet metal at different velocities ranging from 500 to 750 m/s were studied by means of this simulation. Figures 5.1 and 5.2 shows the final deformation of sheet metal blank at 150°C and at different punch velocities. It was observed that the deformation increasing with increase in velocity of punch at constant temperature range. Fig. 5.1 shows minimal deformation because of low velocity (500 m/s). Whereas Fig. 5.2 shows more deformation because of high velocity (750 m/s). Fig.5.3 shows that the velocity of punch gradually increases as the time step increase. Here the punch reaches as the maximum velocity at (t=0.03 sec) and becomes constant till the end of the process. The above cases shows for the further deformation more velocity is required it’s because of strain hardening effect of the sheet metal. It is nothing but as the plastic deformation increases the yield stress also increase. It is because of dislocation movements and dislocation generation within the crystal lattice of sheet metal.
602
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
Fig. 5.3 (velocity vs. time plot)
Fig. 6 (stress plot) Fig. 7(Von Mises stress)
Fig. 6 shows the developed stress in the stamping process. The analysis of Von Mises stress is important to predict the crack propagation and yielding of materials during the metal forming process. Distortion energy failure theory gives the concepts of Vonmises stress. Distortion energy is energy required for shape deformation of a material. According to this theory, the chances of crack propagation will be less if the distortion energy in stamping process is less than that of distorted energy in the testing of material using tensile force (At the failure in tension test). So, to prevent crack in the component the Von mises stress developed during the process should be less than or equal to the yield strength of the component. Fig 7 shows the maximum Von mises stress developed in the component Maximum Von mises stress of 99.38Mpa can be observed at the end of sheet metal formation.As Von mises stress developed in the component is equal to the yield stress of the component then there is a chance for cracks formation. The red color region shows maximum stress region. The blue color region shows minimum stresses in the structure. 5. Conclusion Thus the stamping process is simulated by using ls dyna and numerical analysis are made. Flow stress, effect of punch velocity, strain hardening, Von-Mises stress are discussed in this paper. The internal energy changes in the stamping process are also presented successfully.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
603
6. References [1] Queen City Forging Co (www.cforge.net). [2] Ajay Kumar Choubey, GeetaAgnihotri, C. Sasikumar, Numerical Validation of Experimental Result in Deep-Drawing. [3] JohannesWinklhofer, GernotTrattnig, Christoph Lind, Christ Of Sommitsch and Hannes Feuerhuber, Simulation of aluminium sheet deep drawing at elevated temperatures using ls-dyna. [4]R.A. Ayres, M.L. Wenner, Strain and strain rate hardening effects in punch stretching of 5182-0 aluminium at elevated temperature, Metallurgical transaction A 10A,1979 ,41-46. [5] R. Venkat Reddy, Dr. T.A. Janardhan Reddy, Dr. G.C.M. Reddy, Effect of Various Parameters on the Wrinkling In Deep Drawing Cylindrical Cups, International Journal of Engineering Trends and Technology, 3 (2012) 53- 58. [6] D. M. Finch, S.P. Wilson, J.E. Dorn, Deep drawing aluminium alloys at elevated temperatures,1946, ASM Trans 36: 254-289. [7] G. Venkateswarlu, M. J. Davidson and G. R. N. Tagore,Finite element simulation of deep drawing of aluminium alloy sheets at elevated temperatures, Department of Mechanical Engineering, National Institute of Technology, Warangal, A. P., India. [8] A.M. Szacinski, P.F. Thomson, Wrinkling behaviour of aluminium sheet during formation at elevated temperature, Materials science and technology 7,1991,37-41. [9] L.F. Menezes and C. Teodosiu, Three-dimensional numerical simulation of the deep-drawing process using solid finite elements, ELSEVIER, Journal of material processing technology 97(2000), 2000, pp 100-106 [10] Qun-Feng Chang, Da-Yong li, Ying-Hong Peng, Xiao-Qin Zeng, Experimental and numerical study of warm deep drawing pa AZ31 magnesium Alloy sheet, International journal of machine tool and manufacture 47(2007)436-443.
ScienceDirect Materials Today: Proceedings 16 (2019) 598–603
www.materialstoday.com/proceedings
ICAMMAS17
Simulation and numerical analysis of warm stamping (AA6061) K S Dhinesha,T SubhaSankaria,T Paneerselvama, Venkatesan Ma,* A
School of Mechanical Engineering, SASTRA University, Tirumalaisamudram, Thanajvur – 613401, Tamilnadu, India
Abstract
Stamping process is an important sheet metal forming process used in domestic and automotive components. In this process, blank material is converted into the desired complicated shape by punching process.Stamping includes a process such as punching using a punch or ram, blanking, embossing, bending, flanging, and coining.Cost minimisation ,less time consumption, increase in efficiency are very important in the recent manufacturing industry. Today simulation techniques are used to analyse the performing of dies, processes and billets prior to real timeimplementation. Simulations enable us to find out the common defects such as tears, wrinkles, and material thinning. Finite element analysis (FEA) is the method of simulating the process of sheet metal forming to determine various behaviour of the sheet metal and helps us to find out whether it will produce parts free of defects such as fracture or wrinkling. Sheet metal forming is a popularly used metal working process. In this paper stamping process is analysed numerically using a simulation software package LSTC – LSDYNA. A three dimensional thermomechanically coupled stamping model is analysed and its validations are presented. The analysis data from the numerical simulation helps in understanding the real-time mechanism of sheet metal forming process.There areonly few works were reported on numerical analysis of warm stamping of AA6061 such as load requirement, blank holder pressure, stress-strain analysis.This work is based on study the deformation behaviour of sheet metal, flow stress analysis, contact pressure and punch velocity. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
Keywords: Thermo-mechanical finite element analysis; Ls-dyna 4.1; hot stamping; AA6061 sheet metal;
1. Introduction Aluminium and its alloy are the prime material of construction of aircrafts because of its light weight.Now-a-days, the ultimate aim of automobile industries is to reduce the weight.SoAluminium and its alloy are widely used in building due to their properties such as light weight, corrosion resistance and excellent formability. Formability of aluminium increases with increase in temperature. Thus, forming of sheet metals at the temperature close to its recrystallization temperature is called warm forming.AA6061 shows more formability when it is heated to its recrystallization temperature.
*Correspondingauthor.Tel.:+91 9841309048. E-mailaddress:[email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
599
Stamping is used for converting flat sheets into cup-shaped products such as utensils, cook wares, engine casing, ladders and automobile body panels. In the stamping process, the metal either in the form of sheet or coil is held between the die and punch. The punch is pushed into the die cavity, there bysheet metal will get shape.Increasing the ductility of the material when the process is done above the re- crystallization temperaturethere by prevents the material from strain hardening [1]. Effect of Blank-holder pressure (BHP) on the deep drawn product, the effect of BHP variation on failure limits and the limiting drawing ratio in axisymmetric deep drawing operations were studied in [2]. Warm forming of series of aluminium alloys sheets by thermo- mechanical simulation and its validation were discussed in [3]. Warm forming of Al Mg alloys described variation of material properties with increasing temperature. Stress reduction and the formability is improvement due to increased strain hardening rate were analysed in [4]. Effect of various parameters that cause the wrinkling in deep drawing process of cylindrical cup and various factors like BHF, punch round radius, and die fillet radius on the wrinkling were studied [5]. Significant improvement in the deformability by means of warm forming at temperature around recrystallization temperature investigated in [6]. Reduction of transmittable force, increased depth of drawing due to uniform increase in temperature and formability improvement were discussed in [7]. Advantage of metal forming process at elevated temperature and the factors that reduces the wrinkling tendency were studied [8].Numerical simulation of the 3-D stamping process with elastic strains were presented [9]. The above literature works show that very few works have been done on simulation of stamping for various materials. Only a few simulations of aluminium warm forming have been reported so far. The present work is on numerical simulation of stamping process and the stress-strain behaviour, velocity requirement, energy analysis of AA6061are presented. 2. Validation For validation, previous work done by the author on warm deep drawing of AZ31 magnesium alloy sheet is used. The ultimate stress value during the forming process is found to be 204 MPa [10]. In this present paper, we got the ultimate stress around that value and the true stress vs strain curve follow the same pattern. These give the validation of present paper. 3. Finite element modeling 3.1Material: The sheet metal material for this study is Aluminium alloy 6061. Table: 1the Composition of AA6061
ElementAlMgSiCuCr Composition (%)97.91.00.60.80.2 Table:2 Properties of AA6061:
Poisson's ratio Elastic modulus(GPa) Density(kg/m3) Heat capacity (J/Kg K) Heat conduction (W/m K)
0.33 70-80 2700 920 121
3.2Three-dimensionalmodeling: Punch diameter: 110mm; Fillet radius: 10mm Die outer diameter: 125mm; Die inner diameter: 113mm; Fillet radius: 15mm. For the 3-D modeling, LS DYNA version 4.1 is used. The punch and die does not undergo any deformation it is assumed as rigid material using the command (MAT_20_RIGID). The material of the die is die steel (S50C). Punch
600
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
and die are shown fig. 1. The sheet metal is assigned to (MAT_24_PIECEWISE_LINEAR_PLASTICITY). The punch is constrained in x and z direction. The lower die is constrained in all direction. The composition and properties of the sheet metal are given in Table: 1and 2. The sheet metal is meshed with 61500 no of elements. The sheet metal is assigned with an initial temperature 400°C (Based on recrystallization temperature). No preheating of the die was made. The punch was given with the velocity of 500m/s to 750 m/s.
Fig. 1 punch and die
Fig. 2 End product of stamping
4. Results and Discussion
Fig. 3 stress-strain of AA6061
Fig.3 shows the stress-strain behavior of AA6061.Generally formability of aluminium increases with increase in temperature. The flow stress was observed to be more at low temperature rangebut it decreases with increase in temperature there by increasing the formability. When the punch hit the sheet metal thermal softening and strain hardening occur and the specimen deforms due to sudden load. Since the sheet metal temperature is based on recrystallization temperature of aluminium alloy minimal flow stress is observed. More flow stress level is associated with more strain rate. Increase in the strain-hardening is due to the short annihilation or recovery time available during the test. The lowest strain rate leading to low flow stress values.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
601
Fig. 4 Internal energy – time plot
Fig. 4 show the graph between internal energy and the time. It can be observed that the internal energy of the sheet metal gradually increases as punch hits the sheet metal. It is because of the sudden impact given by hammer on the sheet metal. The instant drop in the graph is because of resistance offered by the sheet metal to the sudden impact of the punch. It is called as hammering effect. It causes external work on the sheet metal there by gradually increasing the internal energy of sheet metal.
Fig.5.1 (V=500 m/s) Fig. 5.2 (V=750 m/s)
Fig. 5(Effect of velocity of punch in stamping)
Fig. 5 gives the effect of punch velocity on deformation (Depth of cup) of sheet metal. The variation in the deformation of sheet metal at different velocities ranging from 500 to 750 m/s were studied by means of this simulation. Figures 5.1 and 5.2 shows the final deformation of sheet metal blank at 150°C and at different punch velocities. It was observed that the deformation increasing with increase in velocity of punch at constant temperature range. Fig. 5.1 shows minimal deformation because of low velocity (500 m/s). Whereas Fig. 5.2 shows more deformation because of high velocity (750 m/s). Fig.5.3 shows that the velocity of punch gradually increases as the time step increase. Here the punch reaches as the maximum velocity at (t=0.03 sec) and becomes constant till the end of the process. The above cases shows for the further deformation more velocity is required it’s because of strain hardening effect of the sheet metal. It is nothing but as the plastic deformation increases the yield stress also increase. It is because of dislocation movements and dislocation generation within the crystal lattice of sheet metal.
602
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
Fig. 5.3 (velocity vs. time plot)
Fig. 6 (stress plot) Fig. 7(Von Mises stress)
Fig. 6 shows the developed stress in the stamping process. The analysis of Von Mises stress is important to predict the crack propagation and yielding of materials during the metal forming process. Distortion energy failure theory gives the concepts of Vonmises stress. Distortion energy is energy required for shape deformation of a material. According to this theory, the chances of crack propagation will be less if the distortion energy in stamping process is less than that of distorted energy in the testing of material using tensile force (At the failure in tension test). So, to prevent crack in the component the Von mises stress developed during the process should be less than or equal to the yield strength of the component. Fig 7 shows the maximum Von mises stress developed in the component Maximum Von mises stress of 99.38Mpa can be observed at the end of sheet metal formation.As Von mises stress developed in the component is equal to the yield stress of the component then there is a chance for cracks formation. The red color region shows maximum stress region. The blue color region shows minimum stresses in the structure. 5. Conclusion Thus the stamping process is simulated by using ls dyna and numerical analysis are made. Flow stress, effect of punch velocity, strain hardening, Von-Mises stress are discussed in this paper. The internal energy changes in the stamping process are also presented successfully.
K S Dhinesh et al. / Materials Today: Proceedings 16 (2019) 598–603
603
6. References [1] Queen City Forging Co (www.cforge.net). [2] Ajay Kumar Choubey, GeetaAgnihotri, C. Sasikumar, Numerical Validation of Experimental Result in Deep-Drawing. [3] JohannesWinklhofer, GernotTrattnig, Christoph Lind, Christ Of Sommitsch and Hannes Feuerhuber, Simulation of aluminium sheet deep drawing at elevated temperatures using ls-dyna. [4]R.A. Ayres, M.L. Wenner, Strain and strain rate hardening effects in punch stretching of 5182-0 aluminium at elevated temperature, Metallurgical transaction A 10A,1979 ,41-46. [5] R. Venkat Reddy, Dr. T.A. Janardhan Reddy, Dr. G.C.M. Reddy, Effect of Various Parameters on the Wrinkling In Deep Drawing Cylindrical Cups, International Journal of Engineering Trends and Technology, 3 (2012) 53- 58. [6] D. M. Finch, S.P. Wilson, J.E. Dorn, Deep drawing aluminium alloys at elevated temperatures,1946, ASM Trans 36: 254-289. [7] G. Venkateswarlu, M. J. Davidson and G. R. N. Tagore,Finite element simulation of deep drawing of aluminium alloy sheets at elevated temperatures, Department of Mechanical Engineering, National Institute of Technology, Warangal, A. P., India. [8] A.M. Szacinski, P.F. Thomson, Wrinkling behaviour of aluminium sheet during formation at elevated temperature, Materials science and technology 7,1991,37-41. [9] L.F. Menezes and C. Teodosiu, Three-dimensional numerical simulation of the deep-drawing process using solid finite elements, ELSEVIER, Journal of material processing technology 97(2000), 2000, pp 100-106 [10] Qun-Feng Chang, Da-Yong li, Ying-Hong Peng, Xiao-Qin Zeng, Experimental and numerical study of warm deep drawing pa AZ31 magnesium Alloy sheet, International journal of machine tool and manufacture 47(2007)436-443.