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Effect of Forming Parameters in sheet Hydro Mechanical Deep Drawing Process

Vplyv parametrov tvrnenia pri hydromechanickom procese hlbokho ahania plch

D. Rajenthirakumar, G. Chandramohan

Abstract Advanced manufacturing technology, which is the basis

of modern industries, is very important. The technology of hydro mechanical deep drawing combines the features of conventional deep drawing and hydroforming and widely used in many industries, especially automobile and aircraft industry. In this process, the punch deforms the blank to its final shape by moving against a controlled pressurized fluid. The objective of this work is to develop a method to analyze the effects of the forming parameters on the quality of the part formability and determine the most significant forming parameter affecting the hydroformability. Experimental ana-lysis and Taguchi method are used to study the effect of for-ming parameters. Experiments have been carried out on In-conel 625, a nickel-based super alloy sheets by varying the process parameters such as chamber internal pressure, dra-wing ratio, punch geometry and friction coefficient. To de-sign an orthogonal experimental array in order to study the effect of each forming parameter Taguchi method is used.

In this work Inconel 625 sheet cups with drawing ratios of 2.5 and 2.6 are obtained to study the effect of forming parameters for the lowest thinning ratio. The results obtain-ned from this work are useful for forming quality parts in terms of uniform thickness distribution.

Keywords: hydro mechanical deep drawing, thinning ra-tio, Taguchi method Abstrakt

Vyspel technolgia vroby, ktor je zkladom moder-nho priemyslu, je vemi dleit. Technolgia hydrome-chanickho hlbokho ahania kombinuje prvky konvenn-ho hlbokho ahania a hydrotvrnenia a je iroko vyvanou technolgiou hlavne v automobilovom a leteckom priemy-sle. Pri tomto procese ank deformuje materil do jeho v-slednho tvaru pohybom proti kontrolovanej stlaenej kva-paline. Cieom tejto prce je vyvin metdu na analyzo-vanie vplyvom parametrov formovania na kvalitu formova-tenosti objektu a stanovi parameter, ktor najvznam-nejie vplva na hydrotvrnenie. Na vyhodnotenie parame-trov tvrnenia boli pouit Taguchiho metda a experimen-tlna analza. Pokus sa realizoval na tabuli Inconel 625, su-per zliatina na bze niklu, spsobom varicie procesnch parametrov akmi boli naprklad vntorn tlak komory, po-mer ahania, geometria tanka a koeficiente trenia. Pre n-vrh ortogonlnho experimentlneho poa za elom tdia jednotlivch parametrov ahania bola pouit Taguchiho metda.

V tomto prspevku bolo realizovan tglikov ahanie s pomerom vytiahnutia s pomerom vytiahnutia 2,5 a 2,6, na ktorch nsledne prebehla analza vplyvu parametrov aha-nia pre najni pomer hrbky. Vsledky zskan z tejto prce s poit pre tvorbu kvalitatvnych dielov v zmysle rovnomernej distribcie hrbky.

Kov slov: hydromechanick hlbok ahanie, po-

mer ztenenia, Taguchiho metda

1 Introduction Special deep drawing processes have been developed in

the past years in order to overcome some problems inherent in the conventional deep drawing process. The objective of such processes is to increases the drawing ratio; minimize thickness variation of the drawn cups, reducing the tooling cost. In the special drawing processes only one half rigid tool of either the punch or the die is used for forming the sheet metal to the required shape. The shape of this rigid tool should be similar to the shape of the drawing component [1].

The hydro mechanical deep drawing process is one of the special drawing processes in which the bottom of the sheet is supported with a bed of pressurized viscous fluid during the forming process. Fig. 1 represents a schematic of this pro-cess used for forming deep-drawn cups. The punch deforms the sheet to its final shape by moving against a controlled pressurized fluid. The aim is to form deep drawn cups wit-hout any kind of forming instability such as buckling, wrin-kling or bursting. In order to successfully obtain the final de-sired parts, it is necessary to study the influence of the for-ming parameters on the hydroformability.

The influence of process parameters and material proper-ties on the hydroforming process has been studied by means of analytical models and experiments. Amino et al [2] found that limiting drawing ratio (LDR) improved significantly in case of hydro mechanical deep drawing when compared with conventional deep drawing. Elsebaie and Mellor [3] determi-ned that high chamber pressure is required to achieve maxi-mum possible LDR, but when the chamber pressure is too high blank may be ruptured at the die profile. Zhang et al [4] using finite element simulation showed that prebulging pres-sure has greater influence on the thickness distribution along the cup wall. Kim et al [5] performed hydro mechanical for-ming and concluded that the hydro mechanical forming pro-cess could produce part with more uniform and sound thick-ness distribution com-pared to stamping operation. Conside-ration of the influence of the geometry, material and process parameters simultaneously in one process has not been found in the literature.

The Taguchi method adopts a set of orthogonal arrays to analyze the effect of parameters on specific quality chara-cteristics to determine the optimal combination of parame-ters [6]. Considering the wide application of super alloy in the fields of aviation and spaceflight, in this work, a method is developed to analyse the effects of the forming parame-ters on hydroformability of Inconel 625 by combining expe-rimental analysis and the Taguchi method. First, the quality characteristics and the forming parameters are selected, and the appropriate orthogonal array is constructed. Experimen-

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tal investigation is performed based on the arrangement of the orthogonal array and the results are then transformed into the Taguchi signal-to-noise (S/N) ratios. Statistical ana-lysis of variance (ANOVA) is performed to see which are significant.

2 Tooling and material for study

The experimental apparatus was built around a 60 tone hydraulic press, shown in Fig. 2. In hydro mechanical deep drawing process, when the fluid pressure in the chamber is too high, it results in premature rupturing of the sheet material. On the other hand if the fluid pressure is too low, it leads to wrinkling. Hence the process pressure has to be maintained in a safe working zone between the ruptures and wrinkling zones to ensure a defect free part. To maintain the fluid pressure punch travel path in the working zone, ma-terial specific wrinkle and rupture governing equations were formulated by Yossifon S. and Triosh J [7]. In this work, the safe working zone was established by interfacing the punch travel by means of Linear Variable Transducer (LVDT) with the Pressure Relief Valve (PRV) of the hydro mecha-nical deep drawing system (Fig. 3). This interface was controlled by a Personal Computer (PC) with the help of commercial software LabVIEW [8]. Experiments are car-ried using sheets of 1 mm thick sheet metal of nickel based super alloy (Inconel 625).

Fig. 2 Experimental setup

Obr. 2 Experimentlna zostava

Fig. 3 Arrangement of valves and electronic data acquisition

system Obr. 3 Usporiadanie valcov a elektronickho systmu

na zber dt

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Table 1 Parameters used Tab. 1 Pouit parametre

2.1 Design of Experiments Geometry parameters, material parameters and process

parameters are the three categories of parameters influen-cing hydroformability. Tab 2 shows the forming parameters selected for study and their levels. Based on Taguchi met-hod, the experimental layout of an L18 orthogonal array (Tab 3) is selected considering the eight parameters and three levels of each parameter. Although there are many different proposed criteria for predicting fracture in metal forming processes, there is no standard approach. Therefore, the commonly used thinning ratio criteria is used as a measure of forming quality. The successfully formed cups are sectioned at 90 and thickness is measured at several po-ints along the cup wall from the top of the cup to center of the cup bottom. Table 2 Forming parameters selected for study and their levels Tab. 2 Vybran parametre tvrnenia a ich rove

Table 3 Taguchis L18 orthogonal array Tab. 3 Taguchiho ortogonlne pole L18

Forming parameters Trial No K L M N X U V W 1 1 1 1 1 1 1 1 1 2 1 1 2 2 2 2 2 2 3 1 1 3 3 3 3 3 3 4 1 2 1 1 2 2 3 3 5 1 2 2 2 3 3 1 1 6 1 2 3 3 1 1 2 2 7 1 3 1 2 1 3 2 3 8 1 3 2 3 2 1 3 1 9 1 3 3 1 3 2 1 2 10 2 1 1 3 3 2 2 1 11 2 1 2 1 1 3 3 2 12 2 1 3 2 2 1 1 3 13 2 2 1 2 3 1 3 2 14 2 2 2 3 1 2 1 3 15 2 2 3 1 2 3 1 3 16 2 3 1 3 2 3 2 1 17 2 3 2 1 3 1 2 3 18 2 3 3 2 1 2 3 1

3 Results and Discussions Based on the experimental results, the forming quality

characteristic thinning ratio is studied using Taguchi method. The signal-to-noise (S/N) ratio and analysis of variance (ANOVA) are considered for analysis. For a successful deep drawing process, the maximum value of thinning is not beyond 20%. When drawing ratio reaches 2.6, the thickness distribution is shown in Fig. 4. Along the cup wall the thickness strain varies from negative values at the most wall height to positive values at the top of the cup.

Fig. 4 Distribution of thickness thinning ratio (to=1mm) Obr. 4 Rozloenie pomeru zmenenia hrbky(to=1mm)

3.1 S/N analysis In the Taguchi method, the S/N ratio is defined by

S/N = -10 log (Mean square deviation for the quality characteristic) ----- (1)

Generally there are three categories of quality characte-ristic in the analysis of the S/N ratio, i.e. the-lower-the-better, the higher-the-better, and the-nominal-the-better. Thinning ratio is the quality characteristic with the objective the-lower-the-better. The mean square deviation (MSD) for the lower-the-better quality characteristic is defined by

n MSD = 1/n yi2 ----- (2)

i=1 Where yi is the value of the-lower-the better quality cha-

racteristic and n is the number of tests for a trial condition. Regardless of the category of the quality characteristic, a greater S/N ratio corresponds to better quality characteri-stics. After conducting experiments and applying S/N analy-sis, the results of the thinning ratio and its S/N ratio in the 18 trial conditions are shown in Tab 4. The average S/N ratio of the thinning ratio for each parameter at levels 1 to 3 are shown in Tab 5 and plotted in Fig. 5. Table 4 Thinning ratio values and S/N ratio Tab. 4 Hodnoty pomerov ztenenia a pomery S/N Trial No

Thinning ratio

S/N ratio

Trial No

Thinning ratio

S/N ratio

1 0.254 10.722 10 0.365 8.345 2 0.496 6.127 11 0.439 7.786 3 0.435 6.978 12 0.355 8.742 4 0.399 7.015 13 0.321 9.567 5 0.565 5.678 14 0.436 7.893 6 0.312 10.543 15 0.543 5.934 7 0.484 6.987 16 0.477 6.784 8 0.338 10.643 17 0.362 8.416 9 0.456 7.556

18 0.451 7.324

-25-20-15-10

-505

1015202530

0 4 8 12 16 20 24 28 32 36 40

Curvelinear distance to the cup center (mm)

Thin

ing

ratio

(%)

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Table 5 Forming parameters and average S/N ratio Tab. 5 Parametre tvrnenia a priemern pomery S/N

Fig. 5 Effect of forming parameters on the thinning ratio

Obr. 5 Vplyv parametrov na pomer zmenenia hrbky

3.2 Analysis of variance (ANOVA) ANOVA is carried out to fine the effect of forming

parameters quantitatively. By comparing variances, ANOVA tests for significant differences between the parameters. The overall average S/N ratio is given by

___ m S/N = 1/m (S/N)i ----- (3)

i=1 Where m is the number of tests in the orthogonal array

and (S/N)i is the S/N ratio of the ith test. The sum of the squares due to the variation from the overall average S/N ratio is

m ___ SS = [(S/N)i S/N]2----- (4) i=1

The sum of the squares due to the variation from the average S/N ratio for the ith factor is

t ___ ____ SSi = Tt X [(S/N)it S/N]2----- (4) t=1

Where t is the number of the factor levels, Tt is the number of the tests of the ith factor at the tth level.

(S/N)it is the average S/N ratio of the quality characteristic for the ith factor at the tth level. The percentage contribution of the ith factor is defined by

Pi(%) = (SSi/SS ) X 100----- (5)

Table 6 ANOVA for thinning ratio Tab. 6 ANOVA pre pomery stenenia

The result of ANOVA for thinning ratio is shown in Tab

6. Tab 6 shows that the internal pressure is the most signi-ficant forming parameter affecting the thinning ratio follo-

wed by friction coefficient. The effect of thickness of the sheet and die profile radius are relatively small compared to that of internal pressure.

4 Conclusions The following conclusions are made:

In this work, a method to study the effect of forming parameters in sheet hydro mechanical deep drawing process has been developed.

Experiments designed based on the orthogonal array of the Taguchi method can used to identify the most sig-nificant forming parameter affecting the hydroformabi-lity

The greatest effects on thinning ratio in sheet hydro me-chanical deep drawing is contributed by internal pres-sure. The second greatest effect is from friction coef-ficient.

The results from this work open the avenue of determi-ning the optimal internal pressure range for better quality products.

The forming parameters except internal pressure and fri-ction coefficient do not contribute much to the hydrofor-mability. As a result other insignificant forming parame-ters like thickness of the sheet and die profile radius etc can be eliminated from the optimization study which will results in significant saving of computational time

Acknowledgements

This work was carried out in the Net shape Manufactuing Lab, Department of Mechanical Engineering, PSG College of Technology, Coimbatore, India. The authors wish to thank all the faculty and staff members for their contribution.

D. Rajenthirakumar, Lecturer, Tele Phone: 91 9486382138, 91 422 2572177

E-mail: d_rajentkumar@yahoo.co.in G. Chandramohan, Professor,

Tele Phone: 91 9443723728 Department of Mechanical Engineering, PSG College of Technology, Peelamedu

Coimbatore 641004, Tamil Nadu, India. References [1] LANGE K.: Handbook of metal forming: McGraw-Hill, New

York, 1985. [2] AMINO H, Nakamura K, Nakagawa T.: Counter-pressure deep

drawing and its application in the forming and automobile parts: Journal of Material Processing Technology, 1990:23, 243-265.

[3] EL-SEBAIE M G, Mellor M G.: Pressure assisted deep drawing: Ann. CIRP, 1973: 22(1), 71-72.

[4] ZHANG S H, Danckert J.: Development of hydro mechanical deep drawing: Journal of Material Processing Technology, 1998:83, 14-25.

[5] KIM J, Son B M, Kang B S, Hwang S M, Park H J.: Comparison stamping and hydro mechanical forming process for an automobile fuel tank using finite element method: Journal of Material Processing Technology, 2004:153/154, 550-557.

[6] KO D, Kim A, Kim B.: Application of artificial neural network and Taguchi method to perform design in metal forming considering workability: International Journal of Machine Tool Manufacturing, 1999:39, 771-785.

[7] YOSSIFON S, Tirosh J.: On the permissible fluid-pressure path in hydro forming deep drawing processes-analysis of failures and experiments: Journal of Engineering for Industry, 1988: 110, 146-152.

[8] Lab VIEW 7.1 Users Manual: National Instruments, 2002.