flash gap optimization in precision blade forging · design the flash gap design in closed-die...

Flash Gap Optimization in Precision Blade

Forging

S. Javid Mirahmadi MAPNA Group, R&D Department, Tehran, Iran

Email: [email protected]

Mohsen Hamedi Faculty of Mechanical Engineering, University of Tehran, Tehran, Iran

Email: [email protected]

Abstract—The main process for manufacturing of

compressor blade made of titanium alloys is forging. In the

precision forging, airfoil dimensions are made to the net

shape and formation of a flash around the forging ensures

the complete die filling. Some factors such as flash gap

dimensions affect the accuracy of the forged blade. The

elastic die deflection during the forging results in airfoil

thickness error. In this paper, effects of flash gap

dimensions and forging parameters on elastic die deflection

are studied by an experimentally verified finite element

method (FEM) model. These effects are analyzed by the

response surface method (RSM). The results show that by

selection of appropriate flash gap dimensions as well as the

process temperature and die speed, acceptable forged blade

dimensions can be achieved.

Index Terms—blade forging, elastic die deflection, flash gap

dimensions, precision forging

I. INTRODUCTION

Closed-die forging with flash is a metal forming

process in which usually two halves of dies move toward

each other to deform a billet. At the end of a successful

process, the billet fills the die cavity to make the desired

shape. During the process, the excess material is

squeezed out of the die cavity through a restrictive

narrow gap-named flash gap-that forms a flash around the

forging. The most important function of the flash is to

force the flowing material to completely fill the die cavity.

Although inappropriate design of flash gap may result

in a complete die filling, it will lead to excessive die wear

and a considerable elastic die deflection. In the

conventional close die forging, the elastic die deflection

usually is not a problem; but in the precision forging it

should be considered.

Lee et al. analyzed the forging load, die cavity filling

and strain distribution in the precision forging process

with and without flash by using an upper bound elemental

technique. They reported a good agreement between their

model and experimental results [1]. Ranatunga et al.

proposed an upper bound elemental technique (UBET) to

Manuscript received January 10, 2017; revised April 25, 2017.

design the flash gap design in closed-die forging of

axisymmetric shapes. They reported a good agreement

between finite element method simulations and UBET

results [2]. Samolyk and Pater used the slip-line field

elemental technique (SLFET) method in the closed-die

forging flash design process [3]. Tomov et al. simulated

the closed-die forging process by considering the flash

gap dimensions. They investigated the various rules of

flash gap design and reported a considerable difference

between the results of various flash design rules [4], [5].

Samolyk and Pater used SLFET for flash gap design with

V-shaped notches in the flash land [6]. Fereshteh-Saniee

and Hosseini studied the flash dimensions and bar size

effects on the forging load and metal flow experimentally

[7]. Shahriari et al. optimized the flash gap dimensions in

the superalloy blade forging by finite element method [8].

Zhang et al. investigated a novel flash structure with

resistance wall and its effects on the forging load and die

wear. They concluded great influence of the proper

resistance wall on the forging load and die wear [9].

Recently Langer et al. studied a movable slash gap in hot

forging [10]. Up to now, based on the experimental observation,

several rules are developed to design the flash gap. Some

of them are presented as mathematical equations and the

others are as tables or graphs. Sleeckx and Kruth

reviewed all of the rules and their origins [11]. Most of

the rules are developed for the steel forgings and use the

weight of the forging in their equations. Since some

materials like titanium alloys are approximately half the

density of steel, application of available rules for flash

gap design is inappropriate.

This paper is focused on the flash gap design in

isothermal precision forging of a blade made of a

titanium alloy. As concluded by Tomov et al. [4], the

flash design rules result in a very different flash gap

dimensions that is difficult to choose one of them; so in

this paper the effects of flash gap dimensions, the percent

of the excessive material, process temperature and

forging speed on the forging load and elastic die

deflection are investigated by the finite element method

(FEM) and analyzed by the response surface method

(RSM).

International Journal of Mechanical Engineering and Robotics Research Vol. 6, No. 3, May 2017

© 2017 Int. J. Mech. Eng. Rob. Res. 200doi: 10.18178/ijmerr.6.3.200-205

II. BLADE FORGING

Due to the mechanical and metallurgical properties of

the material under deformation, forging of a compressor

or turbine blade is a complex metal forming process [8].

In the precision blade forging, the desired dimensional

and geometrical tolerances increase complexity of the

process. The blade forging process starts with locating a

preform with a circular or oval cross-section on the lower

die (Fig. 1). After finishing the process, the airfoil forms

inside the cavity with flash around it according to Fig. 2.

The flash has two main dimensions: thickness and width.

By appropriate determination of the flash gap dimensions,

as well as excessive material to fill the flash gap and

compensate the enlargement of die cavity due to the

elastic die deflection, complete cavity filling and die life

will be achieved.

Figure 1. A perform with a circular cross section located on the lower die.

Figure 2. A forged blade cross-section with flash.

III. MODELING AND SIMULATION

A. FEM Model Development

In order to simulate the isothermal forging process and

study the effect of flash gap dimensions as well as the

process temperature and deformation rate on the elastic

deflection, a coupled thermomechanical modeling based

on the rigid-viscoplastic finite element method was

developed. Deform-2D, a commercially available FEM

package was utilized to perform the simulations.

According to the airfoil geometry, the airfoil forging can

be considered as a plane-strain deformation. The flow

stress of the material was determined as a function of

strain rate, temperature and strain by isothermal hot

compression tests. The flow stress data were corrected for

the effect of the friction and adiabatic temperature rise

and were implemented in the simulations. The friction

factor in the billet-die interface was determined as a

function of temperature and die velocity [12] and used in

the modeling. The geometry of the billet and dies were

modeled in a CAD package and imported to the FEM

software via the standard format IGES. The dies and the

billet were considered to be elastic and rigid-viscoplastic,

respectively. The developed FEM model is shown in Fig.

3.

Figure 3. The developed FEM model.

B. Parameters Selection to Study Their Effects

In order to study the effects of gap dimensions as well

as the process parameters on the accuracy of isothermal

forged blades, the response surface method (RSM) was

utilized to model and analyze of elastic die deflection as a

function of flash gap dimensions. The selected

parameters and their level are presented in Table I.

TABLE I. THE STUDIED FACTORS WITH THEIR LEVELS

Factor Low level High level ‒α +α

A: Flash thickness (mm) 0.5 1.0 0.16 1.34

B: Flash width to thickness ratio 2.0 4.0 0.62 5.38

C: Excessive material (%) 8 15 3.18 19.82

D: Temperature (°C) 875 920 844 950

E: Die speed (mm/s) 1 1.8 0.45 2.35


© 2017 Int. J. Mech. Eng. Rob. Res. 201

IV. FEM MODEL VERIFICATION

In order to verify the developed FEM model, a set of

experiments was done. Two halves of dies were designed

and manufactured by a Ni-base superalloy to withstand

the stresses as well as the high temperature of the tests.

After isothermal forging and trimming the blades, they

were subjected to coordinate measuring according to Fig.

4. The blades were located according to 3-2-1 rule at the

root section. The increasing of the blade thickness

compared to its nominal thickness was considered as

elastic die deflection. The measured dimensions were in a

good agreement with the simulation results. The

isothermal forged blade is shown in Fig. 5.

Figure 4. The locating and measuring points of the blade.

Figure 5. The isothermal forged blade.

V. RESULTS AND DISCUSSION

As stated before in section 1, flash gap dimensions

have a considerable effect on the elastic die deflection

and as a result airfoil thickness error. Other process

parameters, process temperature and forging speed,

which have effects on the elastic die deflection can be

controlled accurately in the isothermal blade forging.

The lower die elastic deflection for flash thickness and

flash width to thickness ratio equal to 0.75 and 3

respectively is shown in Fig. 6. The excessive volume

fraction of material is 3.18% and the maximum elastic die

deflection is approximately equal to 0.068 mm at

temperature 897.5 °C and die speed 1.4 mm/s. The die

filling status in Fig. 7 shows that because of insufficient

excessive material, die cavity is not filled completely. By

decreasing the process temperature to 844 °C, although

the lower die elastic deflection increased to 0.146 mm,

but increasing the excessive material volume fraction to

11.5% resulted in completely die filling (Fig. 8). In both

Fig. 6 and Fig. 8, the lower die geometry before the

forging start is shown by a red profile and the deflected

die geometry is scaled by 50.

Figure 6. Lower die elastic deflection (mm) for flash thickness=0.75

mm, flash width to thickness ratio=3, excessive material=3.18%, temperature=897.5°C, die speed=1.4 mm/s.

In order to evaluate the effects of the factors presented

in Table I and their interaction effects on the elastic die

deflection, analysis of variance (ANOVA) was performed

and the results are presented in Table II. The results show

that the flash thickness, flash width to thickness ratio,

excessive material volume fraction, process temperature

and forging speed have a considerable effect on the

elastic die deflection and as a result airfoil thickness error.

According to P-values, the process temperature and flash

thickness have the most significant effect on the elastic

die deflection. The fitted model to the elastic die

deflection by the coded values is presented in equation

(1). In Fig. 9 and Fig. 10 the model is plotted for 8% and

15% excessive material volume fraction, respectively, for

temperatures 875°C and 920°C and speeds 10 and 63

mm/min.

3 3

3 3 3 2

Deflection 0.23 0.021 5.941 10 5.623 10

0.027 9.920 10 5.166 10 8.314 10

A B C

D E BC C

(1)

TABLE II. ANOVA TABLE FOR ELASTIC DIE DEFLECTION.

Source Squares×10-4 df Square×10-4 F-Value P-Value

Model 619 7 88.5 54.63 > 0.0001

A 192 1 192 118.47 > 0.0001

B 15.3 1 15.3 9.44 0.0041

C 13.7 1 13.7 8.46 0.0063

D 311 1 311 192.12 > 0.0001

E 42.6 1 42.6 26.32 > 0.0001

BC 8.54 1 8.54 5.27 0.0278

C2 36.2 1 36.2 22.36 > 0.0001

Residual 56.7 35 1.62

Cor Total 676 42



Figure 7. Not completely filled die cavity at the end of the die stroke.

Figure 8. Lower die elastic deflection (mm) for flash thickness=0.75 mm, flash width to thickness ratio=3, excessive material=11.5%,

temperature=844°C, die speed=1.4 mm/s.

In the first comparison of Fig. 9 and Fig. 10 it is

concluded that for 8% excessive material, the flash width

to thickness ratio has no considerable effect on the elastic

die deflection; however for 15% excessive material, the

flash width to thickness ratio effect is significant. For 8%

excessive material the flash land is not fully covered by

the flash; so, increasing the flash width to thickness ratio

has no any effect on the elastic die deflection.

The plots in Fig. 9 and Fig. 10 show that more flash

thickness and less flash width to thickness ratio results in

less forging force and elastic die deflection. So the extent

of elastic die deflection can be controlled by increasing

the flash thickness. However, the trimming process is a

limiting criterion. If the flash thickness exceeds the

appropriate dimension, the elastic die deflection may be

acceptable but the trimming process may results in some

other geometrical and dimensional errors. If the flash

width is not chosen wide enough, it will not have

adequate strength and its wear will results in increasing

the flash thickness. So, according to the desired

dimensional tolerances, selection of appropriate flash

thickness and flash width to thickness ratio, insures

complete die filling as well as minimizing the process

force.

Figure 9. Elastic die deflection for 8% excessive material, A) Temperature=875°C, speed=60 mm/min, B) Temperature=920°C, speed=60 mm/min, C) Temperature=875°C, speed=100 mm/min and D) Temperature=920°C, speed=100 mm/min.



Figure 10. Elastic die deflection for 15% excessive material, A) Temperature=875°C, speed=60 mm/min, B) Temperature=920°C, speed=60 mm/min, C) Temperature=875°C, speed=100 mm/min and D) Temperature=920°C, speed=100 mm/min

Assessment of the plots reveals that even though

increasing the temperature reduces the die material elastic

modulus, but the forging material flow stress is more

affected by the temperature; so, increasing the

temperature reduces the elastic die deflection

significantly.

More forging speed reduces the friction factor,

however it increases the elastic die deflection; but its

effect is not stronger than temperature. This effect is

because of increasing the material flow stress with raising

the deformation rate.

Assessment of the plots show that by selection of

appropriate flash gap dimensions as well as the process

temperature and forging speed, the desired dimensional

tolerance can be achieved.

VI. CONCLUSION

In this paper the effects of the flash gap dimensions as

well as the process parameters are studied by a verified

FEM model and analyzed by response surface method.

The results show that flash thickness and flash width to

thickness ratio have a considerable effect on the elastic

die deflection and consequently thickness error. The

effect of the flash width to thickness ratio depends on the

amount of the excessive material. Between the process

parameters, the process temperature has a great effect on

the elastic die deflection and thickness error. By

appropriate selection of the flash gap dimensions as well

as the process parameters, the desired dimensional

tolerances can be achieved.

ACKNOWLEDGMENT

This study was supported by MAPNA Group and

performed at Mavadkaran Engineering Company under

the supervision of Mr. Mohammad Cheraghzadeh which

is appreciated.

REFERENCES

[1] J. H. Lee, Y. H. Kim, and W. B. Bae, “A study on flash-and flashless-precision forging by the upper-bound elemental

technique,” Journal of Materials Processing Technology, vol. 72,

no. 3, pp. 371-379, 1997. [2] V. Ranatunga, J. S. Gunasekera, W. G. Frazier, and K. D. Hur,

“Use of UBET for design of flash gap in closed-die forging," Journal of Materials Processing Technology, vol. 111, no. 1, pp.

107-112, 2001.

[3] G. Samolyk and Z. Pater, “Application of the slip-line field method to the analysis of die cavity filling,” Journal of Materials

Processing Technology, vol. 153, pp. 729-735, 2004. [4] B. Tomov, V. Gagov, and R. Radev, “Numerical simulations of

hot die forging processes using finite element method,” Journal of

Materials Processing Technology, vol. 153, pp. 352-358, 2004. [5] B. Tomov, R. Radev, and V. Gagov, “Influence of flash design

upon process parameters of hot die forging,” Journal of Materials Processing Technology, vol. 157, pp. 620-623, 2004.

[6] G. Samołykn and Z. Pater, “Use of SLFET for design of flash gap

with V-notched lands in a closed-die forging,” Journal of



Materials Processing Technology, vol. 162–163, pp. 558-563, 2005.

[7] F. Fereshteh-Saniee and A. Hosseini, “The effects of flash

allowance and bar size on forming load and metal flow in closed die forging,” Journal of Materials Processing Technology, vol.

177, no. 1, pp. 261-265, 2006. [8] D. Shahriari, M. Sadeghi, M. cheraghzadeh, and M. Taghipour.

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[10] J. Langner, M. Stonis, and B. A. Behrens, “Investigation of a moveable flash gap in hot forging,” Journal of Materials

Processing Technology, vol. 231, pp. 199-208, 2016. [11] E. Sleeckx and J. P. Kruth, “Review of flash design rules for

closed-die forgings,” Journal of Materials Processing Technology,

vol. 31, no. 1, pp. 119-134, 1992. [12] S. J. Mirahmadi, M. Hamedi, and M. Cheraghzadeh,

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S. Javid Mirahmadi was born in Isfahan, a

historical city in Iran. He received his B.Sc. from University of Science and Technology

(2007), M.Sc. from KN Toosi University of

Technology (2009) and Ph.D. from University of Tehran (2015) all in manufacturing

engineering. Dr. Mirahmadi was with Mavadkaran

Engineering Company, a subsidiary of

MAPNA Group, for 5 years as a researcher in the field of metal forming processes. Then he

joined MAPNA Group R&D, active in developing advanced manufacturing technologies. He has published several papers in the field

of titanium and superalloy forming.

Mohsen Hamedi is professor of

manufacturing in School of Mechanical Engineering, University of Tehran, Iran where

he teaches graduate and undergraduate

courses. He obtained his Ph.D. (1995) from UNB, Canada and M.Sc. and B.Sc. from

University of Tehran respectively. Dr. Hamedi has supervised more than 60

Ph.D. and M.Sc. dissertations. He has

published more than 120 papers in the refereed international journals and conferences. His research area of

interests are micro-sensor and micro-actuator design and fabrication, mechanical energy harvesting and manufacturing process optimization.



flash gap optimization in precision blade forging · design the flash gap design in closed-die...

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