Procedia Engineering 97 ( 2014 ) 1975 – 1982

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1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Organizing Committee of GCMM 2014doi: 10.1016/j.proeng.2014.12.352

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12th GLOBAL CONGRESS ON MANUFACTURING AND MANAGEMENT, GCMM 2014

Numerical Analysis on Formability of AZ61A Magnesium Alloy by Incremental Forming

R.Senthil a and A.Gnanavelbabu b*

aResearch Scholar, Department of Mechanical Engineering, Arunai Engineering College, Tiruvannamalai- 606 603, TamilNadu, India bAssociate Professor, Department of Industrial Engineering, CEG Campus, Anna University, Chennai- 600 025, TamilNadu, India

Abstract

The objective of this paper is to discuss the numerical solution of the formability of AZ61A magnesium alloy. Magnesium alloys are the improving materials with their better mechanical properties. The analysis was performed by using explicit version of ABAQUS Finite Element Modules. For achieving better and more realistic solution Incremental Sheet Forming (ISF) technique was used. In the present investigation, a Single Point Incremental Forming (SPIF) tool containing hemispherical end was used to characterize the formability of AZ61A magnesium alloy sheet. For the simulations the punch and dies were assumed to be rigid bodies. The forming loads and details of formability for AZ61A were obtained using contour plots of stress and strain. From the analysis, the formability of the material has been obtained. Stress, strain developed were also found and the maximum stressed area are noted which helps in determining the failure of the sheet metal during forming operation.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Organizing Committee of GCMM 2014.

Keywords:AZ61A Mg alloy, incremental forming, sheet metal, formability, ABAQUS, SPIF

1. Introduction

Magnesium (Mg) alloys are very attractive as structural materials in order to achieve high performance and energy saving of machines and structures, because of their advantages such as light weight, high specific strength, stiffness, good machinability and workability [1-2]. Magnesium alloys are generally classified into two categories; cast alloys and wrought alloys. Cast alloys have advantages such as cost saving and flexibility in fabrication compared with wrought alloys. Therefore, cast alloys have increasing applications in transportation industries, particularly in automobile industry. On the other hand, wrought alloys possess much better mechanical properties than cast alloys; they are expected to be applied to high loading components for which fatigue is critical.

The material used in the present study is extruded AZ61A Magnesium alloy. Optical microstructure

observations reveals that the extruded AZ61A Mg alloy consists of equated grains with an almost identical average grain size of 20 microns, measured using the Mean Linear Intercept Method [3].

* Corresponding author : Tel.: +91 98943 88442. E-mail: agbabu@annauniv.edu

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Organizing Committee of GCMM 2014

1976 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

2. Material

Extruded AZ61A magnesium alloy is used for the investigation. Extruded sheet metal with diameter of 100

mm and a thickness of 1 mm is used for the study. The chemical composition of the extruded AZ61A magnesium alloy is summarized in Table 1. The geometry and the dimensions of the circular testing specimen for investigations are shown in Fig. 1.

Table 1: Chemical Composition of AZ61A Magnesium Alloy, Wt%.

Al Mn Si Cu Zn Fe Ni Other impurities Mg

6.5 0.325 0.1 0.05 0.95 0.005 0.005 0.3 Balance

Fig. 1: Circular Specimen Sheet (AZ61A)

The tensile strength is of 220-260MN/m2, when the alloy is in sheet form. Cast magnesium alloys dominate 85-90% of all magnesium alloys products, with Mg-Al-Zn system being the most widely used. Not readily plastically deformed at room temperature due to HCP structure [4]. Specific strength, specific stiffness of materials and structure are important for the design of weight saving components. Weight saving is particularly important for automotive bodies, components and other products where energy consumption and power limitations are a major concern.

Table 2: Static Material Properties of AZ61A

Density E G σyt(Min) σyt(Max) Sut Poisson’s ratio Melting point Boiling point

1.718g/cm3 43.3 GPa 16.4 GPa 192 MPa 120 MPa 279MPa 0.35 650°C 1107°C

Fig. 2: Yield Stress Vs Plastic Strain Curve

1977 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

This plasticity curve of the material AZ61A is used to assign the plasticity values in Abaqus/Explicit.

3. Incremental Sheet Forming

The incremental sheet forming process (ISF) has been developed in the context of sheet metal forming to increase the flexibility of industrial process. ISF is mainly used to produce small batch size or as a rapid prototyping process in different industries, from transportation to medical field. ISF allows a significant reduction of the tooling cost for small production of sheet parts since traditional, expensive and complex tools are replaced by a simple hemispherical punch moving on a controlled tool path [5-6]. To increase the quality of the final geometry, it is still possible to use the die. The die can be manufactured in a cheap material because the applied forces are low. Another advantage of this process is the high limit of formability, compared to classical limit forming observed in stamping. In the industrial context, the numerical simulation must efficiently predict the geometry of the part, the thickness, strains and stresses in the sheet throughout the forming process. In ISF the contact zone between the tool and the sheet is limited and is always changing with the movement of the tool along its path. Each material point of the sheet is then subjected to elasto plastic loading and unloading depending if the tool is far or not from that point. In an attempt to reduce the CPU time we studied the computational aspects involved to represent the elasto-plastic material behaviour. The classical elasto-plastic integration scheme based on the flow rule requires several iterations [7]. The incremental deformation theory of plasticity is considered as an alternative to reduce the CPU time. The first part of this paper deals with these two aspects of the plasticity (i.e. flow rule and incremental deformation theory). After that, the implementation of CAM tool path into ABAQUS is explained. Finally, results obtained for a benchmark are presented and discussed.

Fig. 3: Single Point Incremental Forming (SPIF)

4. Modelling and Numerical Analysis

FEM simulation of SPIF process is a complicated task due to many modelling and long movement of forming tool. A full model should be used for simulation of SPIF process because this process is not symmetrical [8]. The AZ61A magnesium alloy sheet which was analysed has the diameter of 100mm and thickness of 1mm. The blank is taken as deformable type. The die, punch, blank holder are taken as discrete rigid.

4.1 Simulation Procedures

Forming processes simulation consists of several steps are as follows,

(i) Building Part models (sheet, tool, holder, and part with desired shape) (ii) Tool path generation for the movement of tool

(iii) Building finite element model, applying boundary conditions, defining material properties, contact parameters etc,

(iv) Solving model, post processing.

1978 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

Tool path generation is a step that is usually not needed in simulation. However, for simulation of incremental forming it is used to make tool moving along predefined trajectory. In simple cases the coordinates for tool paths can be calculated by spreadsheet program, but in work by Pohlak, the Computer Automated Manufacturing system was used. Researchers claimed that FEM simulation of SPIF process took a huge computational time because of large model and non-linear conditions in the system. With ABAQUS/Explicit solver, there are two approaches that can be applied to decrease the computational time such as mass scaling and time scaling [9-10]. Depending on applications, a proper approach should be considered to have an accurate result. Therefore, mass scaling approach is the first choice for decreasing the computational time, the kinetic energy of deforming material is less than 4%-10% of total energy of the system. The software used is ls-dyna, the dynamic effect may affect the simulation results. The process may be speed up with methods like mass scaling or time scaling. Normally rigid tool move around at a maximum speed of 0.6m/s [11]. The second option is mass scaling where minimum time step is fixed to lower bound.

4.2 Adaptive Meshing The final shape of the product in SPIF process is drastically different from the original shape of sheet. A

mesh considered optimally in the original sheet geometry can become unsuitable in later stages of the SPIF process. A large material deformation in this process led to a severe element distortion and entanglement. These can lead to decrease in the size of the stable time increment and accuracy of simulated results [10]. Therefore, the adaptive meshing tool should be used in the simulation of SPIF process to increase meshing quality.

Fig. 4: Generated Mesh with ABAQUS

The parts, tool, blank holder, die are taken as discret rigid element and geometric order as linear. The element type is quad R3D4-4 Nodes 3-Bilinear. The sheet is taken as deformable type and linear Hex C3D8R. 8-node linear brick with global size of the sheet given for meshing is 2.

4.3 Tool Path Generation The movement of forming tool in SPIF process is very complex path in long distance. To obtain the

quasi-static simulation and to avoid an extreme accelerate condition, the forming tool is moved by using a smooth step definition method in ABAQUS [11-13]. Total number of steps used for this analysis is 29. The Dynamic/Explicit method is used to solve the problem. 4.4 Boundary Conditions

There are four parts, so every part has to be constrained properly [14]. That is why boundary conditions

in every FEM analysis are essential. Boundary conditions implementation is divided in twenty nine steps Table.3. In first step constraints are implemented, and from the second step onwards the force are applied on the tool.

1979 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

Table 3: Boundary Conditions

Parts Translations Rotations

X Y Z X Y Z

Sheet 0 0 0 0 0

Die 0 0 0 0 0 0

Tool 0 0 0

Blank Holder 0 0 0 0 0 0

5. Results The contour plots shown in Figures 5-9, illustrate the evolution of the damage field and the propagation

cracks in the field. It is evident in both plots that the stress and strain in the damage zone increases towards the shear line. The figure shows the final output of the analysis. The deformed shape is in the form of a truncated pyramid. The dies and the blank holder hold the sheet and it is restricted to move and to reduce wrinkling.

Fig. 5: Deformed shape of the sheet

1980 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

Fig. 6: Stress distribution

Fig. 7: Maximum logarithmic Principal strain

1981 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

Fig. 8: Minimum logarithmic Principal strain

Fig. 9: Nodal displacement

1982 R. Senthil and A. Gnanavelbabu / Procedia Engineering 97 ( 2014 ) 1975 – 1982

Fig. 10: Discplacement vs Time

6. Conclusion

The simulations provide reasonable prediction of work piece deformation and strain distribution. The models were able to predict the areas in work piece that would experience thinning. In this work, the incremental deformation theory of plasticity has been implemented in ABAQUS. The deformability of AZ61A magnesium alloy was observed by the analysis. It’s found that the numerical errors are acceptable while a reduction of CPU time was observed. The tool is made as rigid material and the tool path was created and the maximal velocity of the tool is controlled by smooth STEP function. The stress, strain and deformation developed on the sheet at various areas are calculated. These results will help to solve experimental analysis.

References

[1] R. Senthil, A. GnanavelBabu, Effect of heat treatment on tensile strength and microstructure of AZ61A Mg Alloy, Applied Mechanics and Materials (2014). Vol.606, pp 55 -59 .

[2] E. Giner, N. Sukumar, F.D. Denia, F.J. Fuenmayor ,Extended finite element method for fretting fatigue crack propagation, International Journal of Solids and Structures (2008). Vol.45, pp 234-239.

[3] K. Tokaji, M. Kamakura, Y. Ishiizumi , N. Hasegawa, Fatigue behaviour and fracture mechanism of a rolled AZ31 magnesium alloy, International Journal of Fatigue (2004). Vol.26, pp 485-492.

[4] Keiro Tokaji, Masaki Nakajima, Yoshihiko Uematsu, Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments, International Journal of Fatigue (2009). Vol.31,pp 865-872.

[5] Qin Yu, Jixi Zhang, Yanyao Jiang, Qizhen Li, Multiaxial fatigue of extruded AZ61A magnesium alloy, International Journal of Fatigue (2011). Vol.33,pp 312-318.

[6] Jixi Zhang, Qin Yu, Yanyao Jiang, Qizhen Li, An experimental study of cyclic deformation of extruded AZ61A magnesium alloy, International Journal of Plasticity (2011). Vol27,pp 155-160.

[7] H. Iseki, An approximation deformation analysis and FEM analysis for the incremental bulging of sheet metal using a spherical roller, Journal of Materials Processing Technology (2011).Vol.111,pp 492-497.

[8] Myoung-Sup, Shinm, Jong-Jing Park, The formability of aluminium sheet in incremental forming, Journal of Materials and processing technology (2012) .Vol.113,pp 673-678.

[9] I. Cerro, E. Maidagan, J. Arana, A. Riveroa, P.P. Rodrıguez, Theoretical and experimental analysis of the die less incremental sheet forming process, Journal of Materials Processing Technology (2006) .Vol.177,pp 816-821.

[10] C. Robert, P. Dal Santo, A. Delamezier, A. Potiron1, L. Batoz, On some computational aspects for incremental sheet metal forming Simulations (2010).Vol.20,pp 158-163.

[11] D. H. Nimbalkar,V. M. Nandedkar, Review of Incremental Forming of Sheet Metal Components, Int. Journal of Engineering Research and Applications ( 2013).Vol.175,pp 678-683.

[12] R. Aerens, P. Eyckens, A. Van Bael, J. R. Duflou, Force prediction for single point incremental forming deduced from experimental and FEM observations, International Journal of Advanced Manufacturing Technology.(2009).Vol.345,pp 763-769.

[13] L. Taylor, J.Cao, A. P. Karafillis, M. C. Boyce, Numerical simulation of sheet metal forming, Journal of and processing technology (1995).Vol.50,pp 582-589.

[14] T. Trzepiecinski, 3D elasto-plastic FEM analysis of the sheet drawing of anisotropic steelSheet, Archives of Civil and Mechanical Engineering (2008).Vol.12,pp 352-359.