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Formability of superplastic PbSn60 alloy
Materials Letters 59 (2005) 2156 – 2158 www.elsevier.com/locate/matlet
Formability of superplastic PbSn60 alloy G. GiulianoT, L. Carrino, S. Franchitti Department of Industrial Engineering, University of Cassino, Cassino (FR), Italy Received 5 July 2004; accepted 22 February 2005 Available online 14 March 2005
Abstract The forming limit diagram of superplastic PbSn60 alloy was investigated in this paper. The superplastic sheet was bulged using dies with aspect ratios of 1:1, 15:11, 5:3 and 5:2, at the constant gas pressure and at the temperature of 293 K. Square sheets of dimensions 80 80 mm were machined and printed with grid circles of 3 mm diameter. In the test, the specimen is fixed on an elliptical shape die by the action of a blank holder; the sheet is stressed mechanically by a pressure gas, inducing on the pole a biaxial strain state. It was found that the forming limit diagram is not sensitive to die aspect ratio and it is different from a typical cold forming limit diagram. D 2005 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Deformation and fracture; Limit strains
1. Introduction The superplastic forming is a manufacturing process used to fabricate a number of sheet metal components in the aerospace and automotive industries. Superplasticity indicates an exceptional ductility exhibited by some finegrained alloys (grain size of the order of 10 Am) deformed under proper conditions of temperature and strain rate. The forming temperature should be greater than about half the material absolute melting point; moreover, the strain rate ranges generally between 10 3 and 10 5 s 1 [1]. The formability of sheet metal is a very important aspect of a manufacturing process design because it represents the ability of the material to induce permanent strains without fracturing itself. The forming limit diagram (FLD) shows the boundaries between the regions of failure and safe of sheet metal deformed during a bulge test process. Therefore, the industrial forming process will be executed without danger of fracture if the state of deformation remains constantly under the FLD of the sheet metal [2,3].
T Corresponding author. E-mail address: [email protected] (G. Giuliano). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.053
In this paper, the superplastic bulge test is investigated to predict the forming limit diagram of PbSn60 alloy and to analyze the path of the in-plane plastic strain ratio. In order to vary the strain path, elliptical dies, with various ratios
Fig. 1. Scheme of (a) the equipment used for the superplastic forming and (b) of superplastically bulged elliptical specimen.
G. Giuliano et al. / Materials Letters 59 (2005) 2156–2158
Fig. 2. Dies used for free bulging: aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively.
2. Experimental procedure A schematic drawing of the self-designed equipment is shown in Fig. 1a [4]. The equipment consists mainly of: (i) an air compressor with a proportional valve that controls the gaspressure (the gas pressure being measured by a high-accuracy pressure sensor, positioned in the proximity of the forming die); (ii) one set of plates (Fig. 2) having an hole of aperture of 60 mm length and 60, 44, 36 and 24 mm width, respectively (i.e. the aspect ratios of the dies are 1:1, 15:11, 5:3 and 5:2), and (iii) an upper plate that acts as a blank holder. PbSn60 alloy sheets with thickness of 0.3 mm were investigated in this paper. The material presents superplastic properties at the room temperature: a summary of its mechanical and physical properties is reported in [5]. The chemical composition of the alloy (wt.%) is: 60% Pb and 40% Sn. This material is supplied as bars. The sheets are drawn by the cold-rolling in order to obtain a very fine grain. In the first step, four sheets are made-up by the processing of four bars. By the cold-rolling, in the
Fig. 4. Failure point in bulge test.
second step, a sandwich of four sheets completely welded is fabricated. The resulting sheet is folded several times on itself and is cold-rolled until the thickness is equal to 0.3 mm. In this way, it is possible to obtain sheets composed of several layers of grain dimensions of about 0.1 Am: this condition is necessary for superplastic behaviour. Further details about sheets fabrication are reported in [6]. The constitutive equation of the material was obtained using the self-designed equipment (Fig. 1): the procedure for the biaxial tests of the PbSn60 alloy was outlined in [7]. The apparatus is composed of a conical die: the sheet evolution at a constant pressure was analyzed using a video camera. During each forming test, the polar heights of the
1,0 0,8
major strain
between the lengths of their major and minor axes, can be used.
2157
0,6
2B = 60 mm
0,4
2B = 44 mm
0,2
2B = 36 mm 2B = 24 mm
0,0 0,0
0,2
0,4
0,6
0,8
1,0
minor strain Fig. 5. Strain path in elliptical bulge test. 3,5 2B = 60 mm 2B = 36 mm
strain ratio
3,0
2B = 44 mm 2B = 24 mm
2,5 2,0 1,5 1,0 0,5 0,0 0,2
Fig. 3. Sheet bulged until to fracture: aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively.
0,6
1,0
1,4
equivalent strain Fig. 6. Strain ratio path versus equivalent strain.
1,8
G. Giuliano et al. / Materials Letters 59 (2005) 2156–2158
deformed specimens have been measured as a function of the processing time. The strain-rate sensitivity index was equal to 0.33. Square sheets of dimensions 80 80 mm were machined and printed with grid circles of 3 mm diameter. Then, the sheets were bulged at a temperature of 293 K with a constant gas pressure until fracture occurs. These tests have been considered for die aspect ratios of 1:1, 15:11, 5:3 and 5:2. In Fig. 1b, the axes of the specimen and the polar height are shown schematically. The in-plane plastic strain of the specimens was obtained measuring the grid circles along the major and the minor axes under the optical microscope.
polar height (mm)
2158
40 35 30 25 20 15 10 5 0 0,0
2B = 60 mm 2B = 36 mm 0,4
0,8
1,2
2B = 44 mm 2B = 24 mm 1,6
2,0
equivalent strain Fig. 7. Influence of the die aspect ratio on the polar height.
the die aspect ratio on the polar height: the higher the die aspect ratio, the smaller the polar height.
3. Experimental results and discussions 4. Conclusions Fig. 3 shows the specimens bulged until to fracture in dies with aspects ratio of 1:1, 15:11, 5:3 and 5:2; the failure point is localized at the bulge apex (Fig. 4). The strain paths of sheets are shown in Fig. 5. It is possible to note that the major strain is relative to the minor axis and in similar way, the minor strain is relative to the major axis. Moreover, the strain ratio increases with increasing die aspect ratio. The experimental limit strains have been measured at fracture; Fig. 5 shows that the shape of forming limit diagram to fracture is found to be different from that of a typical cold forming limit diagram. We believe that it depends by the hot forming and the high strain rate sensitivity index value. In particular the FLD is not sensitive to the die aspect ratio as it is also illustrated in [8] where the analytical prediction of a material characterized by a strain-rate sensitivity index equal to 0.3 is shown. The strain ratio may be assumed constant only when the die with the aspect ratio is equal to 1; if the die aspect ratio A/B is greater than 1, the strain path decreases increasing the polar height, H [3]: this effect is higher increasing the die aspect ratio, A/B (see Fig. 6). Fig. 7 shows the influence of
In this paper, an experimental activity was investigated to predict the limit strains of PbSn60 alloy under biaxial stress at the room temperature and at constant gas pressure. The sheet metal was bulged using self-designed equipment with dies of aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively. The experimental forming limit diagram to fracture was found not sensitive to die aspect ratio because strain-rate sensitivity index value was low. References [1] C.H. Hamilton, A.K. Ghosh, Metals Handbook 14 (1988) 852. [2] D. Banabic, H.J. Bunge, K. Pohlandt, A.E. Tekkaya, Formability of Metallic Materials, 2000. [3] G. Giuliano, S. Franchitti, International Conference Euro-SPF 2004, Albi, France, 2004. [4] L. Carrino, G. Giuliano, G. Napolitano, Finite Elements in Analysis and Design 39 (11) (2003) 1083. [5] W. Nicodemi, Metallurgia, Masson Italia, 1986. [6] L. Carrino, G. Giuliano, Advanced Performance Materials 6 (2) (1999) 159. [7] G. Giuliano, Materials and Design 26 (2005) 373. [8] T. Naka, G. Torikai, R. Hino, F. Yoshida, Journal of Materials Processing Technology 113 (2001) 648.
Formability of superplastic PbSn60 alloy G. GiulianoT, L. Carrino, S. Franchitti Department of Industrial Engineering, University of Cassino, Cassino (FR), Italy Received 5 July 2004; accepted 22 February 2005 Available online 14 March 2005
Abstract The forming limit diagram of superplastic PbSn60 alloy was investigated in this paper. The superplastic sheet was bulged using dies with aspect ratios of 1:1, 15:11, 5:3 and 5:2, at the constant gas pressure and at the temperature of 293 K. Square sheets of dimensions 80 80 mm were machined and printed with grid circles of 3 mm diameter. In the test, the specimen is fixed on an elliptical shape die by the action of a blank holder; the sheet is stressed mechanically by a pressure gas, inducing on the pole a biaxial strain state. It was found that the forming limit diagram is not sensitive to die aspect ratio and it is different from a typical cold forming limit diagram. D 2005 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Deformation and fracture; Limit strains
1. Introduction The superplastic forming is a manufacturing process used to fabricate a number of sheet metal components in the aerospace and automotive industries. Superplasticity indicates an exceptional ductility exhibited by some finegrained alloys (grain size of the order of 10 Am) deformed under proper conditions of temperature and strain rate. The forming temperature should be greater than about half the material absolute melting point; moreover, the strain rate ranges generally between 10 3 and 10 5 s 1 [1]. The formability of sheet metal is a very important aspect of a manufacturing process design because it represents the ability of the material to induce permanent strains without fracturing itself. The forming limit diagram (FLD) shows the boundaries between the regions of failure and safe of sheet metal deformed during a bulge test process. Therefore, the industrial forming process will be executed without danger of fracture if the state of deformation remains constantly under the FLD of the sheet metal [2,3].
T Corresponding author. E-mail address: [email protected] (G. Giuliano). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.053
In this paper, the superplastic bulge test is investigated to predict the forming limit diagram of PbSn60 alloy and to analyze the path of the in-plane plastic strain ratio. In order to vary the strain path, elliptical dies, with various ratios
Fig. 1. Scheme of (a) the equipment used for the superplastic forming and (b) of superplastically bulged elliptical specimen.
G. Giuliano et al. / Materials Letters 59 (2005) 2156–2158
Fig. 2. Dies used for free bulging: aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively.
2. Experimental procedure A schematic drawing of the self-designed equipment is shown in Fig. 1a [4]. The equipment consists mainly of: (i) an air compressor with a proportional valve that controls the gaspressure (the gas pressure being measured by a high-accuracy pressure sensor, positioned in the proximity of the forming die); (ii) one set of plates (Fig. 2) having an hole of aperture of 60 mm length and 60, 44, 36 and 24 mm width, respectively (i.e. the aspect ratios of the dies are 1:1, 15:11, 5:3 and 5:2), and (iii) an upper plate that acts as a blank holder. PbSn60 alloy sheets with thickness of 0.3 mm were investigated in this paper. The material presents superplastic properties at the room temperature: a summary of its mechanical and physical properties is reported in [5]. The chemical composition of the alloy (wt.%) is: 60% Pb and 40% Sn. This material is supplied as bars. The sheets are drawn by the cold-rolling in order to obtain a very fine grain. In the first step, four sheets are made-up by the processing of four bars. By the cold-rolling, in the
Fig. 4. Failure point in bulge test.
second step, a sandwich of four sheets completely welded is fabricated. The resulting sheet is folded several times on itself and is cold-rolled until the thickness is equal to 0.3 mm. In this way, it is possible to obtain sheets composed of several layers of grain dimensions of about 0.1 Am: this condition is necessary for superplastic behaviour. Further details about sheets fabrication are reported in [6]. The constitutive equation of the material was obtained using the self-designed equipment (Fig. 1): the procedure for the biaxial tests of the PbSn60 alloy was outlined in [7]. The apparatus is composed of a conical die: the sheet evolution at a constant pressure was analyzed using a video camera. During each forming test, the polar heights of the
1,0 0,8
major strain
between the lengths of their major and minor axes, can be used.
2157
0,6
2B = 60 mm
0,4
2B = 44 mm
0,2
2B = 36 mm 2B = 24 mm
0,0 0,0
0,2
0,4
0,6
0,8
1,0
minor strain Fig. 5. Strain path in elliptical bulge test. 3,5 2B = 60 mm 2B = 36 mm
strain ratio
3,0
2B = 44 mm 2B = 24 mm
2,5 2,0 1,5 1,0 0,5 0,0 0,2
Fig. 3. Sheet bulged until to fracture: aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively.
0,6
1,0
1,4
equivalent strain Fig. 6. Strain ratio path versus equivalent strain.
1,8
G. Giuliano et al. / Materials Letters 59 (2005) 2156–2158
deformed specimens have been measured as a function of the processing time. The strain-rate sensitivity index was equal to 0.33. Square sheets of dimensions 80 80 mm were machined and printed with grid circles of 3 mm diameter. Then, the sheets were bulged at a temperature of 293 K with a constant gas pressure until fracture occurs. These tests have been considered for die aspect ratios of 1:1, 15:11, 5:3 and 5:2. In Fig. 1b, the axes of the specimen and the polar height are shown schematically. The in-plane plastic strain of the specimens was obtained measuring the grid circles along the major and the minor axes under the optical microscope.
polar height (mm)
2158
40 35 30 25 20 15 10 5 0 0,0
2B = 60 mm 2B = 36 mm 0,4
0,8
1,2
2B = 44 mm 2B = 24 mm 1,6
2,0
equivalent strain Fig. 7. Influence of the die aspect ratio on the polar height.
the die aspect ratio on the polar height: the higher the die aspect ratio, the smaller the polar height.
3. Experimental results and discussions 4. Conclusions Fig. 3 shows the specimens bulged until to fracture in dies with aspects ratio of 1:1, 15:11, 5:3 and 5:2; the failure point is localized at the bulge apex (Fig. 4). The strain paths of sheets are shown in Fig. 5. It is possible to note that the major strain is relative to the minor axis and in similar way, the minor strain is relative to the major axis. Moreover, the strain ratio increases with increasing die aspect ratio. The experimental limit strains have been measured at fracture; Fig. 5 shows that the shape of forming limit diagram to fracture is found to be different from that of a typical cold forming limit diagram. We believe that it depends by the hot forming and the high strain rate sensitivity index value. In particular the FLD is not sensitive to the die aspect ratio as it is also illustrated in [8] where the analytical prediction of a material characterized by a strain-rate sensitivity index equal to 0.3 is shown. The strain ratio may be assumed constant only when the die with the aspect ratio is equal to 1; if the die aspect ratio A/B is greater than 1, the strain path decreases increasing the polar height, H [3]: this effect is higher increasing the die aspect ratio, A/B (see Fig. 6). Fig. 7 shows the influence of
In this paper, an experimental activity was investigated to predict the limit strains of PbSn60 alloy under biaxial stress at the room temperature and at constant gas pressure. The sheet metal was bulged using self-designed equipment with dies of aspect ratios of 1:1, 15:11, 5:3 and 5:2, respectively. The experimental forming limit diagram to fracture was found not sensitive to die aspect ratio because strain-rate sensitivity index value was low. References [1] C.H. Hamilton, A.K. Ghosh, Metals Handbook 14 (1988) 852. [2] D. Banabic, H.J. Bunge, K. Pohlandt, A.E. Tekkaya, Formability of Metallic Materials, 2000. [3] G. Giuliano, S. Franchitti, International Conference Euro-SPF 2004, Albi, France, 2004. [4] L. Carrino, G. Giuliano, G. Napolitano, Finite Elements in Analysis and Design 39 (11) (2003) 1083. [5] W. Nicodemi, Metallurgia, Masson Italia, 1986. [6] L. Carrino, G. Giuliano, Advanced Performance Materials 6 (2) (1999) 159. [7] G. Giuliano, Materials and Design 26 (2005) 373. [8] T. Naka, G. Torikai, R. Hino, F. Yoshida, Journal of Materials Processing Technology 113 (2001) 648.