Scholarly article on topic 'Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming'

Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming Academic research paper on "Materials engineering"

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{cavitation / superplasticity / "AZ31 alloy" / "hydrostatic stress ;"}

Abstract of research paper on Materials engineering, author of scientific article — Yong-Nam Kwon, Sang-Hyun Kim, Young Seon Lee

Abstract Cavitation behaviors of fine grained AZ31 alloy sheet during high temperature blow forming have been investigated in terms of deformation and microstructure evolution. Especially, effect of hydrostatic stress by applying backward pressure on blow formability has been investigated by examining cavitation behavior. A commercial grade AZ31 magnesium alloy sheet with the grain size of 15μm could have an elongation over 300% over 400°C. However, elongation did not show a strong strain rate and temperature dependencies which could be easily found in most superplastic aluminum alloys. It might result from that AZ31 would be vulnerable to cavitation even at the early stage of deformation. It was also found that hydrostatic stress was not effective to prevent evolution of cavities during blow forming, which was much different with the case in most superplastic alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature where non basal slip systems were activated.

Academic research paper on topic "Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming"

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Procedia Engineering 10 (2011) 3471-3476

Engineering

Procedia

Cavitation behavior of fine grained AZ31 alloy sheet during

hot blow forming

Yong-Nam Kwona*, Sang-Hyun Kimb, Young Seon Leea

a Korea Institute of Materials Science, Changwondaero 797, Changwon 641-832, Korea b Pusan National University, Pusandeahakro 63, Busan 609-735, Korea

Abstract

Cavitation behaviors of fine grained AZ31 alloy sheet during high temperature blow forming have been investigated in terms of deformation and microstructure evolution. Especially, effect of hydrostatic stress by applying backward pressure on blow formability has been investigated by examining cavitation behavior. A commercial grade AZ31 magnesium alloy sheet with the grain size of 15 ^m could have an elongation over 300% over 400°C. However, elongation did not show a strong strain rate and temperature dependencies which could be easily found in most superplastic aluminum alloys. It might result from that AZ31 would be vulnerable to cavitation even at the early stage of deformation. It was also found that hydrostatic stress was not effective to prevent evolution of cavities during blow forming, which was much different with the case in most superplastic alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature where non basal slip systems were activated.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICM11

Keywords: cavitation, superplasticity, AZ31 alloy, hydrostatic stress;

1. Introduction

Fine grained materials are ready to show superplastic elongation at elevated temperature. Fine grained superplastic aluminum alloys usually bring more than several hundred percent elongations at optimum conditions[1,2]. Cavitation along grain boundaries has been known to be the most important mechanism to lead a fracture in superplastic aluminum alloys when grain boundary sliding is main deformation

* Corresponding author. Tel.: +82-55-280-3375; fax: 82-55-280-3499. E-mail address: kyn1740@kims.re.kr.

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.04.572

mechanism. Recently, magnesium alloys have drawn a lot of attention due to their lightness[3,4]. However, most magnesium parts are made by casting since wrought magnesium alloys have such a poor formability. On the other hands, wrought magnesium alloys are quite easily recrystallized to have a relatively small grained microstructure during thermo-mechanical processes. Therefore, blow forming of fine grained magnesium sheets at the elevated temperature would be one of options to avoid poor formability. However, magnesium alloy such as AZ31 sheet just showed much lower elongation compared with aluminum alloys with similar grain size, which seemed to be originated from magnesium alloys were prone to cavitation quite easily. It has been well known that hydrostatic stress could prevent cavity growth in superplastic aluminum alloys by closing existing cavities[5,6]. It has been reported that a substantial cavitation damage could be inhibited when the confining pressure reached half of flow stress of superplastic aluminum alloys[5]. Cavitation mechanism usually comprised into two stages. In the nucleation stage, all the cavities have spherical morphology due to diffusion process. Nucleated cavities are going to grow into plastically stressed direction and to final fracture eventually.

In the present study, cavitation behavior was investigated to understand an effect of hydrostatic stress on blow formability in a commercial grade AZ31 alloy sheet. Firstly, a series of tensile and load relaxation tests were carried out to get a general deformation characteristics of AZ31 sheet. Blow forming at the elevated temperature was followed to investigate how to enhance a poor formability of magnesium sheets.

2. Experimental procedures

A commercial grade AZ31 sheet used in the present study has the average grain size of 15 ^m as shown in Fig. 1, which is still in the range that superplasticity could be expected. The gauge thickness of a-received sheet was 2.5mm. Firstly, a series of load relaxation tests at the temperature range from 375 to 450°C were followed to get strain rate sensitivity as well as stress-strain rate relation more precisely. Tensile tests were carried out to get general deformation characteristics of AZ31 sheet such as elongation. All test specimens were taken out along the rolling direction. Gage length of test coupon was 20mm in tensile test and 25mm in load relaxation test.

Blow forming at the elevated temperature was carried out by using a cylindrical die with a diameter of 55mm and a depth of 80mm. All the corner radii of forming die were machined to have 6mm. Constant pressure blowing method was used to compare formability under different test conditions relatively by measuring a dome height. Blow formed domes were cut into several pieces to measure strain, cavity volume fraction.

Fig. 1 Optical micrograph of AZ31 alloy sheet used in the Fig. 2 Blow forming test and schematic die geometry

present study showing the average grain size of 15^m.

3. Results and Discussion

3.1. Deformation behavior at elevated temperature

Fine grain size of smaller than 10|im has been known to be suitable for superplastic deformation at some specific temperature regime for each material. The most important deformation mechanism for superplasticity is grain boundary sliding. However, accommodation processes are also required to prevent premature failure, which could result from stress concentration at the triple points that is aftermath of grain boundary sliding[7,8].

Fig. 3 shows flow curves of AZ31 sheet with the grain size of 15|im tested at the temperature of from 350 t0 450°C. Flow stress shifts toward lower and right corner of the graph implying that forming could be faster and easier by increasing forming temperature. Strain rate sensitivity, m, was plotted with the variation of strain rate. Irrespective of test temperature, strain rate sensitivity was measured around 0.45 at the strain rate of between 10"3 to 10"5/s. It has been well known that deformation conditions where the strain rate is over 0.5 would be suitable for superplastic deformation. As strain rate gets higher above 10" 3/s, strain rate sensitivity becomes lower, which implies that a role of grain boundary sliding would get lower compared to other deformation mechanism such as slip or diffusion.

Based on the flow curves shown in Fig. 3, tensile tests were carried out at the same temperature range. Fig. 4 shows tensile curves at 400°C with an initial strain rate varying from 10"1 to 10"4/s. As easily expected from as-received grain size, tensile elongation is increased as strain rate is lowered. Maximum elongation of 270% was measured with the strain rate of 10"4/s. Current elongation of AZ31 seemed to be quite lower compared with other superplastic aluminum alloys even considering grain size of 15 |im. Also, elongation of AZ31 sheet shown in Fig. 4 did not show strong dependencies on temperature as well as strain rate, which is one of important features of superplastic deformation. However, tensile stress was found to increase with lowering test temperatures. These tensile results imply that some mechanism might lead to premature failure irrespective of both strain rate and temperature.

1e-6 1 e-5 1e-4 1e-3 û .01 1e-6 1 e-5 1e-4 1e-3 0.01

strain rate(/s) strain rate(/s)

(a) (b)

Fig. 3. (a) Flow curves of AZ31 alloy sheet with grain size of 15^m; (b) strain rate sensitivity with strain rate variation

Fig. 5 shows tensile specimen surface morphologies after the deformation of 50% with the strain rate of 10-1 and 3 x 10-4/s, respectively. Vertical direction corresponds to tensile axis. While the faster specimen of Fig. 5(a) has elongated grains into tensile direction, equiaxed grain morphology could be

examined with the slowly deformed specimen in Fig. 5(b). Also, it is worth to note that protuberant surfaces were found in both deformed surfaces, which implies a robust activity of grain boundary sliding during high temperature deformation of AZ31 sheet. As result of active grain boundary sliding, cracks begin to open along the grain boundaries, especially at the triple points. It has been reported in superplastic aluminum alloys that cavity nucleation and growth could be controlled effectively when hydrostatic stress is applied during any type of deformation. Since cavity nucleation is dependent on diffusion, hydrostatic stress could not influence much on the initial stage. During growth stage, however, plastic deformation is the most important mechanism enhance crack opening. Therefore, crack closure by applying hydrostatic stress seemed to be an efficient way to control cavitation during high temperature deformation of AZ31 alloy sheet.

en <s)

<L> 20

@400°C

-0.1/s

-0.01 Is

- 0.001/s

-0.0003/s

1 0.0001/s

I ' 1 ^

400 -r

f- 250 -

-t—1 m 200 -

u 150 -

—^375°C

—*— 400°C

—A— 425°C

—▼— 450°C

strain (mm/mm) Strain Rate(/s)

(a) (b)

Fig. 4. (a) Tensile curves of AZ31 alloy at 400°C; (b) elongation with the variation of temperature and strain rate.

Fig. 5. Surface morphology after 50% under the strain rate of (a)10-2/s and (b) 3 x 10-4/s.

3.2. Blow forming characteristics at elevated temperature

Forming pressure was selected as a difference between forward and backward pressures as shown in Fig 2. When forward and backward pressures are 10 and 5kgf/cm2 respectively, a forming pressure is 5kgf/cm2 in this case. Blow forming data in the present paper were carried out at 400 °C. Strain rate level

used in this study corresponds to the strain rate of between from 10-5/s(5kgf/cm2) to 10-3/s (20kgf/cm2) at 400°C. As shown in Fig. 6, blow forming results in terms of dome height and thickness did not change much irrespective of backward pressure. Backward pressure might correspond to the level that hydrostatic stress reached more half of flow stress as each forming condition. Micrographically, cavitation seemed to be successfully suppressed with backward press as shown in Fig. 7. Then, cavity volume fraction was measured using density measurement of blow formed dome pieces with the size of 6x6mm2. Cavity growth rate parameters (-) in Eq.(1) were fitted and listed in Table 1. It was reported that q tended to decrease with the hydrostatic stress implementation in the most Al superplastic alloys. In the present AZ31 case, however, q was not to be dependent on backward pressure level.

C=Coexp(/ne)

O) <D ■C

a> E o ■o

{^OO'C

AP = 5kg/crrf -■-ap= 10kg/cm" -A^AP= 15kg/cm: AP = 20kg/cm:

forward pressure(kg/cm )

Fig. 6. Blow forming results of AZ31 alloy varying lorwaru anu uacKwaru pressures ai wu x.

apex (ess 1.3-1.5) in between (e«0.4~0,6) Peripheral(e®0.1~0.2)

Mr H Hi

1 ' IB

Fig. 7. Microstructures of formed dome at three different regions after forming pressure of 20kgf/cm2 (a) without back pressure and (b) with back pressure.

Cavity growth rate parameter at forming pressure of 5kgf/cm2 was found to get lower with implementation of back pressure, that is, hydrostatic stress as well known in aluminum superplastic materials. However, cavity growth rate parameter did not show any physically meaningful feature when forming pressure went up to 20kgf/cm2. Cavity growth rate parameter depends on hardening exponent (n«0), strain rate sensitivity (m«0.45), stress state (k=2~2.5, biaxial stress), flow stress (e) and hydrostatic pressure (P). For the present AZ31 sheet, it is about 2 to 2.5, which corresponds to the values listed in Table 1. Even though beneficial effect of hydrostatic stress on every metal forming process has been well reported, it is still unclear why hydrostatic stress did not influence in current AZ31 sheet as expected. It needs to be understood thoroughly later on. One of plausible assumption is that AZ31 sheet still has an inefficient slip system even at high temperature due to anisotropic slip system. It is possible that anisotropic plasticity of AZ31 made back pressure not to work properly at cavity growth stage.

3/n + l\ r (2 - m\ k Pi

n1,-sinh 2k—Rö-— )

2\ m ) L V2 + rtiV aJi

Table 1. Cavity growth rate parameter at 400°C with variation of forming pressure

forward pressure (kg/cm2) backward Pressure (kgf/cm2) n Co

5 0 2.6172 0.2836

10 5 1.3775 1.1105

15 10 1.9131 0.2829

20 15 0.7768 1.8493

20 0 2.4414 0.5743

25 5 2.6724 0.4385

30 10 2.6887 0.2874

4. Conclusion

Blow formability of AZ31 alloy with the grain size of 15 |im did not improve much in terms of dome height and thickness irrespective of backward pressure level. Hydrostatic stress was found to be much less effective to prevent cavitation when it was compared with the case in most superplastic Al alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature.

References

[1] D. H. Shin, etc., Materials Sci. and Eng. A, vol. 201, p118.

[2] J. A. Wert, etc., Metall. Trans. Vol. 12A, p1267.

[3] Yuichi Miyahara, etc., Materials Sci. and Eng. A, vol. 420, p 240.

[4] R. Panicker, etc., Acta Mater., vol. 57, p3683.

[5] J. Pilling and N. Ridley, Acta Metall., vol. 34, p669.

[6] C. C. Bampton and R. Raj, Acta Metall., vol. 30, p2043.

[7] R. Z. Valiev and T. G. Langdon, Acta Metall., vol. 41, p949.

[8] T. L. Spingarn and W. D. Nix, Acta Metall., vol. 26, p1389.