OPTIMISATION OF STRAIN RATE SENSITIVITY DURING SUPERPLASTIC DEFORMATION OF Al-Li-ALLOY LITAL A

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OPTIMISATION OF STRAIN RATE SENSITIVITY DURING SUPERPLASTIC DEFORMATION OF

Al-Li-ALLOY LITAL A

N. Ridley, D. Livesey, J. Pilling

To cite this version:

N. Ridley, D. Livesey, J. Pilling. OPTIMISATION OF STRAIN RATE SENSITIVITY DURING

SUPERPLASTIC DEFORMATION OF Al-Li-ALLOY LITAL A. Journal de Physique Colloques,

1987, 48 (C3), pp.C3-251-C3-256. �10.1051/jphyscol:1987328�. �jpa-00226558�

J O U R N A L D E P H Y S I Q U E

Colloque C3, supplement au n09, Tome 48, septembre 1987

OPTIMISATION OF STRAIN RATE SENSITIVITY DURING SUPERPLASTIC DEFORMATION OF A1-Li-ALLOY LITAL A

N. R I D L E Y , D.W. L I V E S E Y and J. P I L L I N G ' ~ )

Department of Metallurgy and Materials Science, University of Manchester/UMIST, Grosvenor street, GB-Manchester M13 9 P L , Great-Britain

Abstract

A study has been made of the superplastic behaviour of A1-Li alloy (Lital A) sheet o f 3mm thickness deformed in uni-axial tension under constant strain rate and constant cross-head velocity conditions. Significantly higher strains t o failure were observed for constant velocity straining than for constant strain rate testing. Optical metallographic studies and measurements of the strain rate sensitivity of flow stress, m, as a function of strain showed that during superplastic deformation grain growth led to a displacement of the strain rate for maximum m, the optimum strain rate, to progressively lower levels. Hence, if the strain rate during a superplastic forming (SPF) process is correspondingly reduced, as it would be during a constant velocity uni-axial test, it should be possible to maintain a high m value during forming and to minimise non-uniform thinning and premature fracture (and maximise elongation to failure in a uni-axial test). A progressive fall in strain rate could also minimise the rise in flow stress due to grain growth and this could be beneficial if SPP is being carried out with a n imposed hydrostatic pressure.

1. Introduction

Superplastic bulge-forming of sheet products is becoming an increasingly important method of producing complex shapes in aluminium alloys. Interest in the superplastic forming (SPF) of alloys in the Supra1 range and in the 7000 series e.g. 7475, has recently been extended to the A1-Li based alloys.

Superplastic behaviour is associated with materials which have a small stable grain size, often in the range 5-lOum, but certainly less than 20um, when they are deformed at temperatures greater than 0.5Tm, at strain rates which include the range to 10-~s-l. Under these conditions a material will show a high strain rate sensitivity of flow stress, m. This is the most important feature of superplast%c materials, and that which confers a high resistance t o neck formation during tensile straining.

It is usually found that the higher the value of m the more uniform is the deformation, and the greater the strain at failure('). Microstructural changes which occur during deformation can have a 'marked effect o n strain rate sensitivity, and it has been suggested that the strain to failure depends more o n the m value at high strains than at low strainsc2). From a practical point o f view it would be desirable to optimise the strain rate path in order to maintain the m value as high as possible throughout a superplastic forming (SPF) operation so as to minimise non-uniform thinning of the formed part and the possibility of premature failure.

In the present work, studies have been made of the effect of strain o n m values o f a commercially produced superplastic A1-Li alloy (8090; Lital A) sheet material deformed under both constant strain rate and constant cross-head velocity conditions, in an attempt to identify the optimum strain rate path required to maintain a high m value throughout deformation. Changes in m value have been related to changes in microstructure.

("~epartment of Metallurgical Engineering. Michigan Technological University. Houghton. MI 49931, USA

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987328

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2. Experimental

The Al-Li alloy examined was Lital A of nominal wt% composition:

A1-2.5%Li-1.2%Cu-O.6%Mg-O.l%Zr, and was received in the form of cold rolled sheet of 3mm thickness. Tensile specimens of lOmm gauge length and 5mm gauge width were machined from the sheet with their longitudinal axes parallel to the rolling direction. All tests were carried out at 520°C, in a furnace attached to the cross-head of a n Instron tensile machine.

T o investigate the superplastic deformation potential of the material specimens were pulled to failure at a range of constant strain rates and at a range of constant cross-head velocities. T o determine the variation of m value during superplastic flow, specimens were deformed at various constant strain rates to predetermined strains. When the pre-strain had been attained, the m value of the material was determined as a function of strain rate by cross-head velocity cycling. Separate specimens were used for each pre-strain. The variation o f m with strain was also measured at the cross-head velocity which gave the maximum superplastic elongation. Each m value recorded was determined for the geometric mean strain rate between two cross-head velocity steps.

Grain size measurements were made using the mean linear intercept (m.1.i.) method o n specimens etched in Keller's reagent.

3. Results and discussion 3.1 Elongation to failure tests

Specimens were pulled to failure at constant strain rates extending from 1 . 6 ~ 1 0 - ~ s - ~ to 2.5x10-~s-l and at constant cross-head velocities which extended across the range of constant strain rates examined. The elongations measured have been plotted in Fig. 1. For the constant velocity tests the data is presented as a function of the mean strain rate. The ranges of strain rates involved in the constant velocity tests are shown for several experimental points in Fig.1.

Pig. 1. Elongations to failure for constant strain rate and constant cross-head velocity testing. Al-Li, 520°c.

The main features o f Pig. 1. are that the elongations to failure go through well defined maxima, and the elongations obtained in the constant velocity tests are significantly higher than those obtained for comparable constant strain rate deformation. The relatively high tensile elongations observed showed that the material had a considerable potential for S P F , and that t o optimise the elongation to failure a continual decrease in strain rate is required. Although the highest tensile elongation was recorded for a cross-head velocity equivalent to a n initial strain rate o f 2%min-l (3.3x10-~s-l) it is clear that appreciable superplastic behaviour (3570% elongation) can be obtained for initial strain rates up to 20% min-l (3.3~10'3s-~),

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3.2 Influence of strain and strain rate on m

To investigate the differences in material behaviour under constant strain rate and constant velocity conditions, the variation of m with increasing strain was measured at three constant strain rates of 8x10-~s-l, 2x10-~s-l and 5x10-~s-l, and at the constant velocity which gave the maximum tensile elongation, namely 0.2mm min-' (initial strain rate of 3 . 3 ~ 1 0 - ~ s - ~ or 2%min-l).

STRAIN RATE, 5'

c i s . 2. Variation of m with strain rate after strains of

0.5(65%), 1.0(172%) and 1.5(348%) at constant strain rates of (a) 8x10-~s-' and (b) 5 ~ 1 0 - ~ s - l .

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Measurements of m were made for predetermined strains ranging from 0.5(65%) to 1.5(348%), and in some cases up to 1.75(475%). Results for the highest and lowest constant strain rates are shown in Figs. 2a and b. For each of the curves in this figure, it can be seen that at low strain rates the m values are low and they increase with increasing strain rate, reach a maximum and decrease. This is consistent with the sigmoidal relationship which is observed in a logarithmic plot of flow stress versus strain rate for the alloys. It is evident that as the strain imposed prior to performing the step strain rate test is increased, the strain rate at which the maximum m is observed moves to lower strain rates.

If the values of m at each pre-strain rate (superimposed on Figs. 2a and b)

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C3-254 JOURNAL DE PHYSIQUE

are plotted as a function of strain it is seen that the strain rate sensitivity decreases with increasing superplastic deformation at strain rates greater (e.g.

8~10-~s-') than the optimum (Fig. 3). However, if the strain rate is less than the optimum (e-g. 2x10-~s-l) the m value first increases before passing through a maximum. For the pre-strain rate of 5x10-~s-l the value of m has reached the maximum value and would be expected to fall with further strain (Fig.3).

For the constant cross-head velocity tests the position of the instantaneous strain rate is shown for each strain (Fig. 4). It can be seen that the falling strain rate combined with the progressive displacement of the peak m value to lower strain rates, leads to the development and maintenance of a relatively high m value as strain increases (Fig. 3 ) . This behaviour is consistent with the high elongation to failure recorded (855%) for this test.

Fig. 3. Variation of m with strain for constant strain rate and constant velocity testing.

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0 0.5 1.0 1.5 2.0 TRUE STRAIN

The variations of m with strain shown in Fig. 3 for constant strain rate testing are also qualitatively in accord with the tensile elongations recorded in Fig. 1. The rapidly falling m values at higher strains (Fig. 3) for strain rates of 2 ~ 1 0 - ~ s - l and 8 ~ 1 0 - ~ s - ~ soon lead to failure at elongations of 564% and 490%, respectively, while a higher elongation of 660% was recorded for the commercially unattractive strain rate of 5x10-~s-~. However, as will be seen later, cavitation may also play some role in determining the strain to failure.

3.3 Microstructural and flow stress characteristics

The progressive displacement of the peak m values to lower strain rates seen in Figs. 2 and 4 is clearly associated with microstructural changes during deformation. Microstructural evolution during superplastic flow has been examined for a wide range of materials including titanium alloys(3), aluminium a l l ~ ~ s ( ~ - ~ ) , brasses(') and stainless steels(8).

Studies of the optical microstructure were made for specimens deformed to various strains at constant strain rates of 8x10-~s-l and 5x10-~s-l. The initial microstructure could not be discerned metallographically as there were probably few high angle boundaries in the cold worked alloy. After heat cycling to the deformation temperature of 520°C, the material appeared to have recrystallised to give a grain size of -6pm. Some microstructural banding was observed due to variations in grain size, but this was appreciably less marked than that previously reported by the authors in earlier work on an Al-Li based alloy(9). In the present work the banding was progressively removed during superplastic straining. The main microstructural change observed was grain growth and the variation of grain size (m.1.i) with strain is shown in Fig.5. Grain growth is more marked at the lower strain rate and reflects the longer deformation times involved. Although the material appeared to have statically recrystallised, there

is evidence that A1-Li alloys may undergo dynamic recrystallisation to develop a superplastic microstructure during the early stages of deformation(lO*ll), a s is observed for the Supra1 a l l ~ ~ s ( ~ * ~ ~ ) .

STRAIN RATE, s1

Fig. 4. Variation of m with strain rate after deformation at a constant velocity of 0.2- min-l (initial strain rate 3.3~10-~s-~).

Hence, it is grain growth during deformation which leads to a displacement of the optimum strain rate (for maximum m) t o progressively lower strain rates. T o compensate for this the strain rate during SPF should be correspondingly reduced and the strain rate path thereby optimised to maintain a s high an m value a s possible throughout forming, consistent with sensible forming times and pressures.

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TRUE STRAIN

0.5 1.0 1-5 TRUE STRAIN

Fig. 5. Effect of superplastic strain Fig. 6. True stress-true strain o n grain size (m.1.i) for two strain curves for constant velocity and

rates. constant strain rate tests.

Previous work has shown that A1-Li alloys may undergo cavitation dur:ng superplastic flow, and that cavities are often associated with localised regions of large grain sizec9). The present material did not show metallographically

JOURNAL DE PHYSIQUE

significant cavitation until strains of -i.5 were exceeded, but in specimens pulled to failure the volume fraction of cavities near the fracture surface was large, indicating that cavity interlinkage did play a role in the final failure process. However, the magnitude of m (Pig. 3) will influence the rate at which failure occurs through its effect on both external and internal neck development.

It has also been shown previously that the volume fraction of cavities in an A1-Li alloy can be reduced to negligible proportions by the application of hydrostatic pressure equal to approximately one half of the effective flow stress, during superplastic def~rmation(~). Grain growth during superplastic deformation under constant strain rate conditions leads to an increasing flow stress and hence to a progressive increase in the minimum level of hydrostatic pressure necessary to inhibit cavitation. Commercially it is difficult to maintain high hydrostatic pressures (>3MMPa; 500psi) during forming. However, the increase in flow stress due to grain coarsening could be offset by a fall in flow stress if the strain rate decreased during forming. This could lower both the maximum flow stress attained and the level of hydrostatic pressure req"ired to eliminate cavitation.

True stress-true strain data measured in the present work is shown in Fig.6.

If a strain of 1.3 is selected as the maximum likely to be encountered during SPF, then a forming time of -54 minutes would be required for both a constant strain rate of 4x10-~s-l and a constant cross-head velocity of 5%min-I (initial strain rate 8.3x10-~s-l). It can be seen that the maximum flow stress attained is clearly less for the constant cross-head velocity deformation.

Hence, not only could a progressively decreasing strain rate lead to an optimisation of m during forming but it could also be beneficial in reducing the maximum flow stress encountered during an SPF process.

4. Summary

The strains to failure observed for an A1-Li sheet material superplastically deformed in uni-axial tension under a range of constant velocity conditions were significantly higher than those obtained for comparable constant strain rate testing. From metallographic studies and measurements of m as a function of strain for constant strain rate and constant velocity tests it was seen that grain growth during superplastic flow led to a displacement of the optimum strain rate i.e. that for maximum m, to progressively lower strain rates. If the strain rate in an SPF process is correspondingly reduced, as it would be in uni-axial constant velocity testing, it should be possible to maintain m at a high value throughout forming and so minimise non-uniform thinning and premature fracture. A progressive reduction in strain rate could also minimise the rise in flow stress due to grain growth and this could be beneficial if SPF is being carried out with superimposed hydrostatic pressure.

5. References

1. D.A. Woodford, Trans. ASM. 62 (1969) 291.

2. A.K. Ghosh and A. Ayres, Metall. Trans. 7A (1976) 1589- -

C.H. Hamilton and A.K. Ghosh, Titanium '80, (ed.)H. Kimura and 0. Izumi, TMS-AIME, Warrendale, PA. 1980, 1001.

A.K. Ghosh and R. Raj, Superplasticity. (ed.)B. Baudelet and M. Suery, Editions de CNRS, Paris 1985. 11.1.

J. Pilling and N. Ridley, Aluminium Technology '86. (ed.) T. Sheppard, Institute of Metals. London. 1986, 206.

B.P. Kashyap and K. Tangri, Metall. Trans. (1987) 417.

M. Suery and B. Baudelet, Superplastic Forming of Structural Alloys, (ed.)N.E. Paton and C.H. Hamilton, TMS-AIME, Warrendale, PA. 1982, 105.

B.P. Kashyap and A.K. Mukherjee, Mater. Sci. and Tech. L(1985) 291.

J. Pilling and N. Ridley, Aluminium-Lithium Alloys 111, ed. C. Baker et al., Institute of Metals. London.

R. Grimes, and W.S. Miller, Aluminium-Lithium Alloys 11, (ed.) T.H. Sanders and E.A. Starke, TMS-AIME, Warrendale PA. 1983, 153.

J. Wadsworth, A.R. Pelton and R.E. Lewis, Metall. Trans.

16A

12. R. Grimes, C. Baker, M.J. Stowell and B.M. Watts, Aluminium, 51 ⁽¹⁹⁷⁵⁾

720.