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________________

Corresponding author: Pankaj K. Bhoyar,

E-mail address: pankajbhoyar318@gmail.com

Doi:http://dx.doi.org/10.11127/ijammc.2013.02.043

Copyright@GRIET Publications. All rights reserved.

Advanced Materials Manufacturing & Characterization Vol3 Issue 1 (2013)

Advanced Materials Manufacturing & Characterization

journal home page: www.ijammc-griet.com

FEA of Superplastically formed Front Fender Car Panel

Pankaj K. Bhoyar1, C.M. Sedani2 , Monika S. Agrawal

1Asst. Professor, Mechanical Engineering Department. 2Professor, Mechanical Engineering Department. Jawaharlal Darda Institute of Engineering & Technology, Yavatmal-445201, Maharashtra. India.

A R T I C L E I N F O Article history: Received 10 Dec 2012 Accepted 26 Dec 2012 Keywords: Aluminum Alloy 5182 & 8090, Superplastic forming, Front fender, Finite Element Simulation

A B S T R A C T

Superplastic forming (SPF) is a near net-shape forming process which offers many advantages over conventional forming operations including low forming pressure under low flow stress, low die cost, more design flexibility, and the ability to shape hard metals to form complex shapes. However, low production rate due to slow forming process and limited predictive capabilities provides lack of accurate constitutive models for superplastic deformation, treated as an obstacle to the widespread use of SPF. Recent advancements in finite element tools have shown while analyzing the complex superplastic forming operations. These tools can be utilized successfully in order to develop optimized superplastic forming techniques to develop the future materials.To present the discussion mentioned above an analysis of superplastically formed front Fender car panel using HYPERFORM 9.0 software is elaborated here. Present work consist of a finite element simulations of superplastic forming of aluminum 5182 & 8090 alloy sheet in to the front fender panel of car is carried out at 4600C-5000C temperature to estimate the pressure tonnage, % thinning & major & minor strain in terms of FLD curve. Further analysis is carried out by increasing the blank holder pressure & compares the results from different pressure levels (low, medium & high), by considering better pressure level comparison of Al alloy 5182 sheet to the Al-Li alloy 8090 sheet has been explained. The major objective of present paper is to introduce the future material as a substitute in automobile industries.

Introduction

Aluminum automotive components made using a hot blow forming process are reducing vehicle weight and increasing the fuel efficiency of today’s cars. However, before General Motors (GM) and the Department of Energy (DOE) sponsored research in this technology, blow forming of aluminum was not a viable process for automakers. The prior blow forming process called superplastic forming (SPF), was not suitable for the industry’s high-production-rate demands, and the materials required for SPF were too expensive three times the cost of standard, non-SPF, aluminum sheet metal. Therefore, bringing SPF to the automotive industry required developing low-cost SPF alloys and faster forming cycles. Reducing the cost of SPF alloys also required demonstrating the viability of SPF to both the automotive and aluminum industries. DOE initiated a program of joint-funded research between GM and Kaiser Aluminum, with the Pacific Northwest National Laboratory (PNNL) providing a catalyst to encourage GM to develop aluminum blow forming

processes. Based on early research, General Motors recognized the inherent limitations of superplastic forming and moved forward with the commercialization of an advanced blow forming process, called Quick Plastic Forming. The result is cost effective, higher-volume manufacturing technology that is producing lightweight components for today’s automobiles. [1]

With the recognition of environment, all the governments have the strict standard of the letting epilogue. So, reducing the heavy is becoming the key of automobile development. One of the means of reducing the heavy of automobile is using the aluminum alloy and magnesium alloy. Aluminium, for its low density compared to steel, is the most promising material for achieving weight savings in future automotive body structure. Aluminum alloy has larger springback and the lower plasticity; all of characters are difficulty for the stamping of complex shape of aluminum alloy car. As well known, aluminum alloy is one of the superplastic materials which have longer elongation and no spring back under the superplastic state [2]. These advantages of superplasticforming (SPF) process are the possibility to realize very complex part in one single

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operation and the low tooling costs [3]. According to the capability of aluminum alloy and the advantages of SPF, a study of Finite Element Analysis (FEA) of superplastically formed front fender of aluminum alloy has been carried out. Considering the cost of the formed part, a commercial aluminum alloy sheet 5182 i.e. its material properties, which are used in FEA of stamping process of car panels, is selected as analytical material. The front fender of an all aluminum micro car is shown in the Fig.1. The dimension of the front fender is 916mm×181mm×710mm. [4]

Figure 1: The Front Fender Model [4]

Superplastic Behaviors of 5182 Commercial Aluminum Alloy Sheet

Aluminium sheet for automobile body-in-white application have been used since the early days of car and aluminum production. In the time of increasing mass production and low cost priorities, however, steel has taken over the lead. But increasing fuel prices, CO2 regulations and additional comfort and equipment loads lead to a strong tendency for light weighting. But an aspects of comfort and sportive driving helps to promote innovations in light weight design and engineering, predominantly in Western Europe. [5]

The main aluminum alloy classes for automotive sheet application are the non-heat treatable Al-Mg (EN 5xxx series) and the heat treatable Al-Mg-Si(EN 6xxx series) alloy system, some especially tailored by variations inchemical composition and processing, e.g. Al-Mg alloys optimized for strength and corrosion resistance for use in chassis or Al-Mg-Si alloys applied for autobody sheets have been improved for formability, surface appearance and age hardening response. The specific properties and principal differences are illustrated in Figure 3[5]. The

effects of varying alloy additions and process parameters are well developed for enhanced performance and efficient manufacturing. [5]

Figure 3: 5xxx and 6xxx Alloys and their Competition for Car Body Sheets [5]

Superplastic Blow Forming

Blow forming is a pressurized forming process which is widely used to produce complex shapes using superplastic alloys. The various steps involved in the blow forming process are shown in Figure 4[6]. In this process the sheet is tightly clamped around its periphery and gas pressure is applied on its surface. An inert atmosphere is required in the forming chamber, and argon gas is generally used for both pressurization and maintenance of protective atmosphere. Predetermined pressure-time profile is used to achieve complete adaptation of the metal sheet to the die surface at a controlled rate of deformation. [6]

During the initial stage of deformation the sheet is not in contact with the die. Deformation in this stage is concentrated at the pole. Consequently greatest strain occurs in this region during this stage. Once the pole comes in contact with the surface of the die, the material is locked due to friction. This prevents further deformation. The remaining free region continues to deform until complete contact with the die occurs. The corners of the die are usually the last to be filled, causing greater strain to occur in these regions, consequently theses regions are more prone to failures. [6]

Figure 4: Superplastic blow forming process [6]

Numerical Simulation of SPF Process

Finite element method is a numerical procedure for analyzing a wide range of problems that are too complicated to be solved satisfactorily by classical analytical methods & tools. Since the early 1950’s to present, enormous advances have been made in the application of the finite element method to solve engineering problems. Finite element method models a structure as an assemblage of small parts (elements). Each element is of simple geometry and therefore is much easier to analyze than the actual structure. [7]

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The steps involved during finite element method with respect to Superplastic forming analysis using Hyperform 9.0, are listed below.

1. As the first step of processing is design, a geometric design of the die surface geometry has been designed with the help of CATIA V5 17 is shown in Fig.5. Based on the feasibility and the shape of the formed product, choice has to be made between a 3-D solid model & 3-D Surface model. This is essential to obtain accurate results with low computational cost. Complex assemblies & geometries can be generated using specialized modeling software’s and these models can be imported into the finite element analysis software as an IGES file format i.e., in the hypermesh software.

Figure5: CAD model of fender die

Figure6: Finite Element Model of Fender die

2. After importing the geometry in Hyperform, clean up the geometry and save it. Then unsuppress the lines of edges & suppress the fillets. Divide the body into an equivalent system of finite elements with associated nodes and choosing the most appropriate element type to model most closely the actual physical behavior. Fig.6 shows the finite element mesh with most appropriate element type i.e., Quad4 & Tria3 elements with size of element are 2. The accuracy of the results is greatly dependent on the size of the elements. The finer the mesh, greater is the accuracy; however this increases the computational time. Commercial computer programs, called preprocessors, help in generating a mesh. Generated number of elements & nodes are as follows.

Nodes . . . . . . . . . . 4034

Elements. . . . . . . . . 4080

QUAD4 Element . . . . . . . 3783

TRIA3 Element . . . . . . . 297

3. Create the material first, by applying the material properties of Aluminum alloy 5182. Assign the material to the component elements i.e., car fender die & also give the thickness of the aluminum sheet 1mm. The material properties of aluminum alloy 5182 are as shown in table 1. For generating the material i.e., 8090 aluminum-li alloy we need to apply the material properties of it. The material properties of 8090 Al-Li alloy is as shown in table no.2.

Table 1: Material properties of 5182 Al-Mg alloy

Sr.No.

Symbols Parameters Values

1 E Young’s modulus 70000Mpa 2 nu Possion’s ratio 0.3 3 TS Tensile strength 348MPa 4 ε0 Pre-strain coefficient 0.029 5 n Strain-hardening

exponent 0.24

6 K Strength coefficient (Power law equation)

623.1Mpa

7 YS Yield strength 265.5Mpa

Table 2: Material properties of 8090 Al-Li alloy

Sr.No.

Symbols Parameters Values

1 E Young’s modulus 77000Mpa 2 nu Possion’s ratio 0.34 3 TS Tensile strength 480MPa 4 ε0 Pre-strain coefficient 0.017

5 n Strain-hardening

exponent

0.24

6 K Strength coefficient

(Power law equation)

859.45cm

Mpa

7 YS Yield strength 323Mpa

K=Strength Coefficient=n

n

n

eTS =623.10MPa

o =Strain Coefficient=

n

K

YS1

=0.0289

Where,

TS=Tensile strength

YS=Yield strength

4. Assign the initial boundary conditions. This involves constraining all the degrees of freedom of the nodes associated with the clamped portion of the sheet.

5. Apply the loads associated with the complete finite element model. In the case of Hyperform specify the blankholder, friction

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&blankholder pressure level (high, medium & low) on the elements of die where the sheet is clamped. Also apply temperature load of 4500C [4] to the all elements of component.

6. Before running the problem save it properly with .hm file The advantages associated with finite element analysis are its ability to model & analyze the irregular shaped bodies, handle general load conditions, handle unlimited number and kinds of boundary conditions, alter the finite element model relatively easily and cheaply, include dynamic effects and handle nonlinear behavior existing with large deformation and nonlinear materials. These advantages make it an ideal Hyperform tool for the analysis of superplastic metal forming process.

Results & Discussion

In order to estimate rationality of the designed surface geometry, a numerical simulation of superplastic air bulging of the front fender is carried out. One of the main results in a forming analysis is the percent thinning of a component after it has been superplastically formed. Usually if a part exceeds 30% thinning it can start to split resulting in failure of the component. This is one of the main problems in the forming of sheet metal components. A simulation can be used to determine the percent thinning of components. Fig. 7 shows the results of the percent thinning of a front fender that was analyzed using RADIOSS and HyperForm. Element percentage thinning values are positive when material thins out. The simulated result of the wall thickness distribution is shown in Fig. 8. From the figure it can be seen that the maximum wall thickness of the formed part is 0.704mm (initial thickness =1.0 mm). This result shows that the deformation capability of the provided Al alloy sheet can satisfy the requirement of the superplastic air bulging of the fender. In order to get the better thickness distribution of the formed part the die surface modification is required. The Effective stress is the average normal force per unit area transmitted directly from particle to particle also known as Effective Pressure. From the Table no. 4 the results & comparison between two alloys from analyzed contours, there is a only change in themaximum and minimum effective stress. The Effective pressure of 5182 Al-Mg alloy gives 591MPa and 8090 Al-Li alloy gives 818MPa. Al-Li alloys need more pressure for achieving this shape. Therefore effective pressure required is more.

After comparing the analyzed results of two materials, the estimated applied pressure for 8090 Al-Li alloy at high blankholder pressure is 0.325E+02. This is greater than the pressure of 5182 Al-Mg alloy. Hence the Al-Mg alloy is more superplastic than 8090 Al-Li alloy as shown in table no.3. From the FLD (Forming Limit Diagram) curve there are two elements are above the FLD limit line means two elements are failed at these three blankholder pressure levels is shown in fig no.9. This

gives a measure of the relative tension & compression at each element on the blank surface. Forming limit curves (FLC) to be used for the evolution of formability at the end of analysis. Fig.10 shows formability contour it shows that maximum elements are safe which are in green color. Hyperform shows required pressure for superplastic forming process by giving the material properties, boundary conditions, constraints & other details required for it.

Figure 7: % Thinning (29.6%)

Figure 8: Thickness Distribution (0.704mm)

Figure 9: FLD at High Blankholder pressure

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Figure 10: FomabilityahighBlankholder pressure

Table 3: Results & comparison between three BH pressure levels from analyzed contours

Table 4. Results & comparison between two alloys from analyzed contours

Conclusions

The Finite Element Analysis is carried out to understand and optimize the superplastic behavior of 5182 & 8090 aluminum alloy. FEA shows that the commercial 5182 aluminum alloy sheet has certain extent of superplasticity and can be used for forming of car panels. Estimated pressure at high

blankholder pressure is 0.128e+02 tons. High blankholder pressure shows better results. The maximum wall thickness of the formed part is 0.704mm which is (29.6%) well below 30% of thinning. High blankholder pressure level is good at effective constant strain rate is 3.32E-03.

The 5182 Al-Mg alloy is more superplastic than 8090 Al-Li alloy. The effect of superplastic forming at various blankholder pressures of the formed product is observed. For weight reduction of automotive component aluminum can be a major candidate, however superplastic forming (SPF) should be used to

enhance the productivity.

Sr.

No

Blankholder pressure Estimated pressure of

5182 Al-Mg Alloy (Tons)

Estimated pressure of

8090 Al-Li Alloy (Tons)

1 Low 0.464 tons 0.125E+02 0.318E+02

2 Medium 1.159 tons 0.126E+02 0.321E+02

3 High 2.318 tons 0.128E+02 0.325E+02

Sr.

No.

Out-put Results 5182 Al-Mg

alloy results

8090 Al-Li

alloy results

Comparison

between these

two alloys High BH pressure

1 Min. Thickness 0.704mm 0.702mm reliable

Max. Thickness 1mm 1mm reliable

2 Max. Major Strain 0.568 0.575 reliable

Min. Major Strain 2.55e-03 2.59e-03 reliable

3 Max. Minor Strain 0.119 0.108 reliable

Min. Minor Strain -0.350 -0.346 reliable

4 Max. Effective Strain 0.804 0.817 reliable

Min. Effective Strain 3.39e-03 3.32e-03 reliable

5 Max. Effective Stress 591MPa 818MPa Changed

Min. Effective Stress 70.1MPa 94.4MPa Changed

6 Max. Deformation Mode 41.0 40.7 reliable

Min. Deformation Mode -72.8 -72.8 reliable

7 Max. Displacement 17.5cm 17.4cm reliable

Min. Displacement 3.75cm 3.79cm reliable

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References

1. Joseph Carpenter, Mark T. Smith, November 2002. “Superplastic forming of aluminum sheet metal for automotive applications”, Bulletin of Transportation for the 21st Century.

2. V.Pancholi, B.P.Kashyap, March 2003, “Effect of local strain distribution on concurrent microstructural evolution during superplastic deformation of Al-Li 8090 alloy”, Material Science & Engineering Conference, 1 pp.

3. B. Davisand J. Hryn, 15 December 2007, “Innovative Forming and Fabrication Technologies: New Opportunities”, Argonne National Laboratory U.S., pp: 2-41.

4. ZhipengZeng, Yanshu Zhang, Yi Zhou, 2005, “Superplastic Forming of Aluminum Alloy Car Body panels”, Material science forum, Trans Tech Publications, pp 1- 4.

5. J. Hirsch, 2004, “Automotive Trends in Aluminium - The European Perspective”, Materials Forum, Institute of Materials Engineering Australasia Ltd, vol 28, pp 19-20.

6. PushkrajVasantDeshmukh, 2003 “Study of Superplastic Forming Process using Finite Element Analysis”, PhD Thesis, University of Kentuky, pp: 1-18.

7. V.Pancholi, B.P.Kashyap, 2007, “Effect of layered microstructure on superplastic forming property of AA8090 Al-Li alloy”, Journal of Material Processing Technology, pp 1.

8. Wang Zhongjun*, Wang Zhaojing, Zhu Jing, 2011 “Superplastic deformation of a relatively coarse-grained AZ80 magnesium alloy”, Advance Materials Letters, vol 2, pp 113-117.