Bending of amorphous alloys

Journal of Materials Processing Technology, 33 (1992) 215-227

215

Elsevier

Bending of amorphous alloys S. Suto a, K. Matsuno a, T. Sano ~ and K. Matsui b aMechanical Engineering Laboratory, Agency of Industrial Science and Technology, MITI, 1-2 Namiki, Tsukuba, 305 Japan bAmada Co., Ltd., Isehara, Japan (Received March 27, 1991; accepted in revised form August 26, 1991)

Industrial Summary Fe-based and Ni-based amorphous foils with thicknesses of 25-30 ttm have been used for bending experiments. First, the mechanical and physical properties of the foils were measured, after which bending experiments were carried out by four bending methods, i.e., conventional V-bending, L-bending with a lateral force, multi-stage V-bending and V-bending with a soft metal. SEM observations have been made to look for shear bands and crack initiation along the bend lines. The maximum strain caused by bending is greater than that of the uniaxial tension test, the temperature rise being effective in suppressing crack initiation. The relationship between the bending condition and the bend angle has been obtained for each bending method. As a result, the superiority of multi-stage V-bending and V-bending with a soft metal to the two other methods is established, crack initiation is prevented, and the minimum spring-back angles for these methods are 4 ° and 6 °, respectively, in the case of Ni-based amorphous bending (ez=90 ° ). As for Vbending with a soft wire, the bending mechanism is elucidated by computer simulation. The high accuracy bending of this method can be attributed to the application of hydrostatic pressure through the soft metal and the bending deformation occurring under the stress state of tension.

1. Introduction

Amorphous metals will be used widely in industry in the future because of their excellent mechanical, magnetic and chemical properties. The effort is concentrated on the practical applications of the material. However, only few actual applications have been reported so far. One of the most serious problems, preventing the material from being used, is the difficulty in processing. Plastic working is the most effective processing technology for the material, although plastic working using a punch and a die is very difficult due to the high hardness and strength, and the low toughness, of the material. Other factors, such as low dimensional accuracy and a low crystallization temperature, also limit its formability. Correspondence to: S. Suto, Mechanical Engineering Laboratory, Agency of Industrial Science and Technology, MITI, 1-2 Namiki, Tsukuba, 305 Japan.

0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

216

Presently, studies on plastic working methods such as shearing, bending, rolling, drawing, etc., are performed. As for shearing, in particular, high-speed and punchless blanking methods [ 1,2 ] have been developed. Water-jet cutting is also effective [3 ] from the viewpoint of suppressing crystallization. Bending, though a highly demanded plastic working method, has not yet been realized. The purposes of this paper are to clarify the bending mechanism and to attain accuracy in bending of amorphous foil. Magnetic and anti-corrosion amorphous foils have been chosen for the experiments. First, some characteristics such as the accuracy and the mechanical and physical properties of the foils are determined. The bending process is studied with regard to four bending methods: conventional V-bending, L-bending with a lateral force, multistage V-bending, and V-bending with a soft metal. The effect of bending conditions such as the shape of the tools, the bending force, and the temperature on the bend angle, the crack initiation, etc., are evaluated quantitatively. Furthermore, SEM observation and numerical analysis of the bending process are carried out. 2. Materials and experimental apparatus

2.1. Properties of amorphous foil Characteristics of the foil. In these experiments two types of amorphous foil were used: Ni-based (anti-corrosive material, MBF50) and Fe-based (soft magnetic material, 2605S-2 ) amorphous foil. The foil was made by a single roll quenching method, so that one side of the foil surface (chilled by air) is glossy and the other side (chilled by the roll) is dull. Defects are sometimes found on the glossy surface. Other characteristics such as roughness, friction coefficient, etc., are different on each side of the foil. Usually, the thickness of the amorphous foil varies widely, however, the experimental foil had only a small amount of scatter, less than 4% of the thickness. The physical and dimensional properties of the foil are shown in Table 1. The thermal conductivity was measured by the radiation heat transfer method [4]: the values are about 1/8th of those of crystalline metals with the same composition. Mechanicalproperties. The tensile strength, the maximum strain and Young's modulus of the foil were measured by means of a tensile test. As the mechanical properties are affected strongly by the temperature T and the strain rate, the properties were examined varying these parameters. Figure 1 (a) shows the tensile strength of the Ni-based, amorphous foil, with a maximum tensile strength of about 1 GPa. The strength decreases as the strain rate and the temperature increase ( T < 300 ° C ). On the other hand, the strain-rate dependency of the tensile strength shows the opposite tendency above 300 ° C. Figure 1 (b) shows the change in maximum strain, which value was less than 3% at room temperature. However, plastic strain was virtually unobserved at room temperature. As can be seen from Fig. 1 (b), when the strain rate is low and the temperature is high, the material has better ductility, and a maximum

217 TABLE 1 Characteristics of amorphous foils Properties

composition thickness (mm) width (ram) hardness Hv (0.1) surface roughness Ra (~tm) glossy surface dull surface friction coefficient glossy surface dull surface crystallization temp. ( ° C ) density (mg/mm 3) saturation induction B, (kG) maximum permeability (/~ax) dC annealed thermal conductivity (W/InK)

Materials MBF50

2605S-2

Ni72.sCrls.sSiT.sB1.5 0.030_+ 0.001 25.1 814

FeTsBl~Si9 0.025 _+0.001 25.0 907

0.85 1.35

0.65 1.10

0.091 0.562 0.111 0.592 466 7.9

8.1

541 7.2 15.6 60000 8.5

1Diamond 2Steel ball

plastic strain greater than 10% is obtained. Nevertheless, the foil deformed at elevated temperature showed remarkable brittleness, even if the temperature was below the crystallization temperature, Tc. Young's modulus decreases with an increase in temperature and strain rate (Fig. 1 (c)). The Fe-based amorphous foil had a higher tensile strength, a greater maxim u m strain and a stronger temperature-dependency than the Ni-based amorphous foil.

2.2. Experimental apparatus and procedure Conventional V-bending. The punch-and-die set used for V-bending is illustrated in Fig. 2. The punch and die had the same corner angle ~ (30 °, 60 °, 90 ° ), corner radius R (0.1 mm, 0.3 mm, 0.6 m m ) and width (10 ram). In this paper, the inner angle measured after the bending process was considered as the bend angle ft. L-bending with lateral force. L-bending was performed using the tool shown in Fig. 3. The experimental procedure was as follows: the foil was clamped onto the die by a hydraulic ram, then the punch was released and a lateral force was

218

= 1000 ~ - o ~

"o

Tension Speed

(a) loo

,

¢

x •~

50 40O

~

~,.

lO ~ lo-3 lO-2 m-' Tension Speed

-f .~ 5o

>

(b)

fl ~?@0~2

~1°

Te:ConSpeed [mm/s]

(c) Fig. 1. Mechanical properties of Ni-based amorphous foil: (a) strength; (b) maximum strain; (c) Young's modulus.

applied to the foil through the punch. The specific temperature of the foil was kept constant during bending. The experimental conditions are listed in Table 2. Multi-stage V-bending. This method comprised several V-bending stages. As shown in Fig. 4, the corner radius R of the punch and the die in each stage was different: the radius R decreased sequentially from start to finish. Three stages were used in the experiment, the radii being 0.6, 0.3, and 0.2 mm, respectively. Each punch and die had the same corner angle of a = 90 ° and a width of 5 mm. V-bending with a soft metal. Figure 5 shows a schematic diagram of this

219

i

160 -CN- SUS304(t = Fb , ,~[v, ¢ 1 4 0 J

l[6jJ

,oo

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i

~'~

I

(8}

I

30

60 90 a [deg]

0 (b)

0.2

I

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0.4 R[mm]

0.6

Fig. 2. Influence of the die shape on the bend angle in V-bending: (a) die angle; (b) die radius.

Cl~p

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F1 110

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Foil [

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Fo [kN] Fig. 3. Schematic diagram of L-bending with a lateral force.

Fig. 4. Relationship between the bending force and the bend angle in multi-stage bending for material MBF50 (Q - conventionalbending; O - multi-stage bending; V-width = 10 ram).

220 TABLE 2 Experimental conditions for L-bending

Parameters

Material MBF50

die angle c~ (deg) die radius Ra (mm) bending temperature T (°C) lateral force F1 (kN) clamp force (kN) duration ofFl t (s)

60,75,90 0.1,0.2,0.3,0.4 RT, 100,200,300,400 70-124 77 5-20

/

Punch

Soft Metal Wire

..-4--.

/

Fig. 5. Schematic diagram of V-bending with a soft wire.

method. Soft metal wire was set at the punch end in a round-shaped groove. As the punch went down, the wire was deformed and pressed the foil into the die. This method gives a better fit between the tool and the foil than conventional rigid tools. The experimental conditions of this method are shown in Table 3. 3. Results and discussion

3.1. Conventional V-bending This V-bending was employed to examine how difficult the bending of amorphous foil is by the conventional method. Therefore, stainless steel, SUS 304, was bent also for comparison. Figure 2 (a) shows the effect of c~ on the bend angle ft. The bending force Fb is 2 kN. The maximum strain caused by bending is greater than that of uniaxial tension. The springback ~( = fl-c~) of the MBF50 is 80-110% greater than that of SUS304 for 30 ° ~<~ ~<90 °, illustrating the difficulty of bending amorphous material. The value of fl increases as

221 TABLE 3 Experimental conditions for soft metal bending Parameters

Materials MBF50, 2605S-2

die angle c~ (deg) die radius R (ram) die width soft metal wire material diameter D (mm) bending force Fb (kN) duration OfFb t (s) bending temperature

90, 60, 45 0.2 5 lead 1.0, 1.2, 1.5 - 1.0 10 room temperature

decreases for both materials, although MBF50 shows a stronger tendency than SUS304. The relationship between fl and R is shown in Fig. 2 (b). The value offl rises with an increase in R, i.e., a greater bending accuracy is obtained for smaller values of R. On the other hand, cracks along the bent line appear for R < 0.3 ram. Amorphous bending takes place with the generation of shear bands, and the strain concentrates on these shear bands due to non-strain hardening, which leads to crack initiation. Therefore, the value of R should not be too small from the viewpoint of crack initiation. In addition, the generation of a tool with a small value of R is difficult. The results obtained above show that conventional V-bending is not applicable to the low spring-back bending of amorphous materials.

3.2. L-bending with lateral force The influences of lateral force FI and temperature T on the bend angle were examined mainly in L-bending. Figure 6 shows the change in fl as a function of F~ and T. The springback 7 falls to 15 ° and 20 ° , when F~ increases to 0.8 kN at room temperature and 100 ° C, respectively. The temperature rise up to 100 ° C does not improve the bending accuracy. However, SEM observation showed that the temperature rise had the tendency to suppress crack initiation. The optimum temperature, giving the minimum value of 7, depends on the value of R. As shown in Fig. 7, the optimum temperature is 100°C for R < 0.3 mm, and 200°C for 0.3 m m < R . The optimum temperature is not affected by c~ (Fig. 8). The temperature dependency offlbecomes stronger as ~ increases. As for crack initiation, the number of microcracks decreased as the temperature rose to 300 ° C, and larger cracks were observed above the temperature of 300 ° C. Sometimes fracture took place during the bending process when the temperature was beyond 400°C. This is caused by enhanced brittleness above

222

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R[mm] Fig. 6. Influence of the lateral force on the bend angle ( G - the glossy surface of the foil was set outside; D - the dull surface of the foil was set outside; R = 0 . 2 ram; t = 10 s).

6;75

120

Fig. 7. Influence of the die radius on the bend angle ( G - the glossy surface of the foil was set outside; D - the dull surface of the foil was set outside; FI = 0.5 kN; t = 10 s).

0 A •

7~ 100 O

80

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200

400

T [°C1 Fig. 8. Influence of the temperature and the bend angle for material MBF50. (R = 0.2 ram; Fi = 0.5 kN; t = 10 s)

T

223

300°C. Embrittlement occurs readily under stressed states even though the temperature is less than Tc [5]. More faults were observed on the glossy surface of the foil, and more cracks occurred when the glossy surface was set to the outside.

3.3. Multi-stage V-bending The occurrence of cracks and the large spring-back could not be prevented by the previous two bending methods. Therefore, multi-stage bending was proposed to solve these problems. The results are shown in Fig. 4, from which is noted that lesser spring-back takes place compared with conventional V-bending. The minimum spring-back of 4 ° is achieved by this method without cracking under the bending condition of F b = 5 kN and a -- 90 °. The distribution of the strain in a wider area makes it possible to give a greater total bending strain without cracking. This method is applicable to other types of bending such as U-bending and should be an effective production method if a progressive die is used. 3.4. V-bending with a soft metal Another bending method using a soft metal wire has been introduced. Figure 9 shows deformed shapes of the soft metal in steps. The shape of the wire

-R

Soft Metal Wir~ 0.4

~

B = 108.9

80.2

~

0.2

O

~

;8

O

Amorphous Foil

/X

Soft Metal Wire

5

10

[kN] Fig. 9. Change of t h e radii of t h e soft material a n d the a m o r p h o u s foil for material 2605S-2. ( O a m o r p h o u s foil; A - SOft metal wire; lead wire D = 1.2 ram; ~ = 6 0 ° )

224

changes and fits to the die radius as the bending force Fb increases. The radii of the amorphous and the soft wire decrease with an increase in Fb. The difference between the two radii, related to the amount of spring-back, shows the same tendency. Figure 10 gives the relationship between Fb and fl, the wire diameter, D, being kept constant in each curve. The value offl for conventional V-bending is also shown in this figure. Soft metal V-bending gives greater accuracy than the conventional method without the occurrence of cracking. A minimum springback of 6 ° is obtained for Fb = 10 kN and D = 1.5 mm. If a soft metal is used as the transmitter of the bending force, hydrostatic pressure is applied to the foil to prevent crack generation. Furthermore, this method can adapt flexibly to thickness change. The optimum value of D depends on both the die angle and the shape of the groove in the end of the punch. The results for material 2605S-2 are shown in Fig. 11. The bending force Fb has the same effect as in the case of material MBF50, i.e., fl decreases with an increase in Fb. However, the minimum spring-back is 33 °, which is greater than that for MBF50. Computer simulations were carried out to elucidate the bending mechanism of this method. Deformation of the foil and the wire were simulated by an FEM 180

150

120

90

a =90 °

6O ....

X ....

-----O--.m

45

m !

|

!

5

Fb

1.2 1.5

1-'~ m

I

10

[kN]

Fig. 10. Influence of the bending force and the wire diameter on the bend angle for V-bending with a soft metal for material MBF50 (C.B. - conventional bending with R=0.2 ram; D - the wire diameter).

225 180

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--X.\

.

.

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"~'¢"

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150 > o \

. . . . ×. . . .

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D=I.0 1.2 1.5

45 I 5

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Fb tkN] Fig. 11. Influence of the bending force and the wire diameter on the bend angle for V-bending with a soft metal for material 2605S-2 (C.B. - conventional bending with R=0.2 ram; D - the wire diameter). TABLE 4 Input data for simulation die angle a (deg) bending force Fb (kN) wire diameter D (mm) friction coefficient/~ material of wire Young's modulus E (GPa) yield strength ay (MPa)

90 - 10 1.0, 1.2, 1.5 0.2 lead 8 20

c o m p u t e r p r o g r a m J N I K E 2-D. T h e c o n d i t i o n s u s e d for t h e analysis are s h o w n in T a b l e 4; t h e m a t e r i a l c o n s t a n t s a n d t h e friction coefficient were o b t a i n e d e x p e r i m e n t a l l y . T h e tools, i.e., t h e p u n c h a n d t h e die, were t r e a t e d as rigid bodies. Figure 12 is t h e r e s u l t f r o m t h e c o m p u t e r simulation. T h e d e f o r m e d shapes g e n e r a t e d b y t h e s i m u l a t i o n agree w i t h t h e e x p e r i m e n t a l results, a n d t h e dist r i b u t i o n s o f stress a n d s t r a i n in t h e wire a n d t h e a m o r p h o u s m a t e r i a l are m a d e clear. T h e b e n d i n g m e c h a n i s m is as follows. T h e a m o r p h o u s foil is p r e s s e d

226

Fig. 12. Analysisof the bendingprocesswith the use of FEM. (~ = 90°; D = 1.2 mm) against the die wall by means of the deformed wire. The movement of the foil is constrained by the force exerted through the deformed wire while the wire is deformed. Thus, the wire fits into the die corner. Therefore, the foil is bent under the stress state of tension, and high-accuracy bending becomes possible. The computer simulation to find the optimum bending condition is still being studied: results will be obtained soon.

4. Conclusions

The bending of amorphous foil has been attempted using four different bending methods. The following conclusions are obtained: (1) Bending deformation gives a greater strain than uniaxial deformation. (2) A temperature rise up to 300 ° C is effective in suppressing crack initiation for the Ni-based amorphous metal. However, embrittlement takes place above 300°C even though the temperature is below the crystallization temperature. (3) The spring-back angle with conventional V-bending and lateral L-bending is greater than that with the other two methods, and more cracks are introduced by these methods. (4) Multi stage V-bending can prevent strain concentration and allow the generation of large strain. Thus, low spring-back bending without cracks can be achieved. (5) In the case of V-bending with a soft metal, bending deformation under tensile stress and the application of hydrostatic pressure are the main reasons for the securing of low spring-back bending without the occurrence of cracks.

227

Acknowledgements T h e a u t h o r s are m u c h i n d e b t e d to Mr. S. K u d o , Mr. G. Kagaya, Mr. Y. Suzuki a n d Mr. M. O n i z a w a for help w i t h t h e e x p e r i m e n t s a n d t h e c o m p u t e r simulation.

References 1 Y. Murakoshi, M. Takahashi, M. Terasaki, T. Sano, K. Matsuno and H. Takeishi, High speed blanking of amorphous alloy, Adv. Technol. Plast., 1 (1987) 323-328. 2 T. Sano, M. Takahashi, Y. Murakoshi, K. Matsuno and H. Takeishi, Punchless blanking of an amorphous alloy, Adv. Technol. Plast., 3 (1991) 1447-1452. 3 T. Sano, M. Takahashi, Y. Murakoshi and S. Suto, Abrasive water-jet cutting of amorphous alloys, J. Mater. Process. Technol., 32 (1992) 571-583. 4 S. Suto, M. Takahashi, T. Sano, A. 0no, A. Nishikawa, K. Takahashi and H. Takeishi, Measurement of mechanical and thermal properties of amorphous alloys, Proc. 38th Japanese Societyfor Technology o/Plasticity, Toyama, Japan, 1987, pp. 189-192. 5 C.A. Panpilo, Review, flow and fracture in amorphous alloys, J. Mater. Sci., 10 (1975) 11941227.