Scholarly article on topic 'Superplasticity of Spray Deposited 5083 Al-Mg Alloy'

Superplasticity of Spray Deposited 5083 Al-Mg Alloy Academic research paper on "Materials engineering"

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Chinese Journal of Aeronautics
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{superplasticity / "spray deposition" / "5083 Al-Mg alloy" / "thermomechanical process" / microstructure}

Abstract of research paper on Materials engineering, author of scientific article — Shou-jie YANG, Shu-suo LI, Sheng-long DAI, Ya-fang HAN

Abstract The superplasticity of spray deposited and thermomechanical processed 5083 Al-Mg alloy is investigated in this paper. The results show that spray deposited 5083 A1 exhibits an equiaxed grain morphology with an average size of 15μm and porosity in the range of 0.1 vol. % to 5 vol. %. Two distinct TMP procedures are employed to close porosity and refine grain size: extrusion plus rolling and direct rolling. The material processed using the former method exhibits a relatively high superplasticity with a maximum superplastic elongation of 465%, whereas that processed using the latter method exhibits a maximum superplastic elongation of 295%. Materials processed using extrusion plus rolling and direct rolling both exhibit similar stress-strain behavior and strain rate sensitivity factors. The strain rate factors are in the 0.3 to 0.5 range. The difference in their superplastic elongation is possibly the result of differences in grain size and available cavity nucleation sites provided by closed gas pores.

Academic research paper on topic "Superplasticity of Spray Deposited 5083 Al-Mg Alloy"

Superplasticity of Spray Deposited 5083 Al-Mg Alloy

YANG Shou-jie1, LI Shirsuo2, DAI Sheng-long1, HAN Ya-fang1,2 (1. Beijing Institute of Aeronautical Materials, Beijing 100095, China) (2. School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China)

Abstract: The superplasticity of spray deposited and thermomechanical process«! 5083 Al- M g alloy is investigated in this paper. The results show that spray deposited 5083 A1 exhibits an equiaxed grain morphology with an average size of 15Pm and porosity in the range of 0 1 vol. % to 5 vol. % . Two distinct TMP procedures are employai to close porosity and refine grain size: extrusion plus rolling and direct rolling. The material professed using the forme' method exhibits a relatively high superplasticity with a maximum superplastic elongation of 465%, whereas that processed using the latter method exhibits a maximum superplastic elongation of 295%. Materials processed using extrusion plus rolling and direct rolling both exhibit similar st ress-strain behavior and strain rate sensitivity factors. The strain rate factors are in t he 0. 3 to 0. 5 range. The difference in their superplastic elongation is possibly the result of differences in grain size and available cavity nucleation sites provided by closed gas pores. Key words: superplasticity; spray deposition; 5083 At Mg alloy; thermomechanical process; microstructure

PflliÏÏfR 5083 AJ Mg ^t^iltt. tH, *

2004, 17(1): 47- 52.

m W: W^TW^WB^&^^&aW 5083A1- Mg ^âWHtt. 5083A1- M g ^

^««Mi^É^R^ №m w^ttftfflrô,a 1%- 5%( » *

295% »

0.30.5»

gffitt; 5083 At M g ««Mi

1000- 9361(2004)01-0047- 06 ^ffi^^^: TG132.3;V252

Recently, spray deposition processing has received considerable interest as a method of producing bulk materials with uniform, refined mr crostructures and improved mechanical properties as compared to those of conventionally processing materials. M icrostructural development during spray deposition is governed primarily by dendrite arm fragmentation, enhanced nucleation, and constrained grain growth in the semi"solid and/or solid state. Consequently, the morphology of grains in most spray deposited materials is generally equiaxed with grain sizes in the range of less than 50^m[1"4].

Received date: 2003 02-21; Revision received date: 2003 06 12

The aforementioned microstructural characteristics render spray deposited materials potentially suitable for superplastic applications. In principle, refining grain size by spray deposition processing would inherently reduce the number of thermomechanical processing( T MP) steps and the total rolling reduction needed to achieve superplasticity. Another important characteristic of spray deposited materials is their reported extension of solid solubility well beyond the equilibrium values. Moreover, it is possible to introduce dispersoids or refine intermetallics phase using spray deposition processing. Such dis~

i^miiirlon ieu- Mrt^al N it,.^ Sc;ience ^d^m, °f Ch™ (598r0°5) fo

persoils help to restrain coarsening during elevated temperature deformation, presumably contributing to high superplasticity'5,6].

Commercial alloy 5083 Al exhibits superplas ticity following thermomechanical processing. Be cause of the limited Mg ( 4. 5wt%) and Mn (0. 7wt%) content, this alloy contains only a small amount of secondary phases. Discontinuous recrys tallization is considered to be a mechanism that may lead to further grain size refinement in 5083AL Grain refinement is typically accomplished by pre" cipitation of fine particles, thermomechanical treatment to produce a heavily deformed or partially re crystallized micro structure, and finally an annealing treatment to produce a recrystallized microstructure with fine equiaxed grains' T9]. In view of the above discussion, the objective of the present investigation is to study the superplas tic response of spray deposited and thermomechanical processed materials.

1 Experimental

A commercial 5083 ( AM. 5Mgr0. 7Mn, wt%)Al alloy is selected as the starting material. Fig. 1 illustrates schematically the spray deposition apparatus used for the present experiments, and Table 1 summarizes the relevant spray deposition parameters. The experiments are conducted according to the following procedure. First, the environmental chamber is evacuated down to a vacuum of 0. 1 torr( 13. 33224Pa) and is backfilled w ith N2 gas to a static pressure of 0. 12MPa The starting alloy is then inductively heated, melted, and further superheated to the pouring temperature in a graphite Crucible, under N2 protection ( see Table 1). Second, the superheated alloy melt is delivered to an atomizer by means of a ceramic delivery tube, where it is atomized. During atomization, the molten alloy is disintegrated into a fine dispersion of micrometer-sized droplets using N2 gas at a dynamic pressure of 1. 24MPa. Following atomiza t ion, t he rapidly quenched and partially solidified droplets are directed towards a substrate where they imping-

subsequently on each other, thereby forming a cor herent preform.

Fig. 1 Spray deposition apparatus Table 1 Spray deposition parameters

Atomization gas pressure/ MPa 1. 24

Metalto-gas flow rate ratio 1.3

Pouring temperature/ °C 900

Deposition distance/cm 46

Substrate C u ( w at er c oo led)

Substrate rotation/ (r* min- ') 75

Substrate off center distance/ cm 3. 2

Atomization gas N2

____ on the substrate ,

inà Äcaaemic J ourna

ace and inic Pul

Two approaches are used to close porosity and to further refine grain size in the 5083 alloy. In the first method, porosity is eliminated by hot extrusion, using a cylindrical extrusion die, to an area reduction of 16! 1. The extruded bars are further processed using warm rolling at a reduction per pass of 10% to an overall reduction of 72%. Samples are annealed between rolling passes. In the second method, the as"spray deposited materials are sectioned into flat sheets, the sheets are solution heat treated at 500 °C for 1 hour and are rolled directly at elevated temperatures.

The microstructure are characterized using optical microscopy and quantitative image analysis. Elevated temperature tensile tests are conducted using plate geometry samples with a gage section of 20mm( long) x 9mm (wide) x 3mm (thick) . Tensile tests are conducted using an Instron tensile testing machine in the temperature range of 500"

550°C and initial strain rate range of 3 x 10 3-3 x

-5 - 1

ishirig House. All rights reserved, http://www.cnki.net

2 Results and Discussions

2.1 Microstructure of as- spray deposited materials

Figs. 2 and 3 compare the micro structure of as-spray deposited 5083 A1 with that of its asringot cast counterpart. As shown in Fig. 2, the microstructure of ingot cast 5083 A1 is characterized by equiaxed dendrites, typical of stow solidification conditions. Also clear from Fig. 2 is the presence of secondary phases in interdendritic areas. The microstructure of spray deposited 5083 A1 is characterized by equiaxed grains with a uniform size distribution, the only exception being the region near the substrate (see Fig. 3). The dendritic structure and interdendritic phases are not present in the spray deposited condition. Porosity is noted both in grains and at grain boundaries. The material adja" cent to the substrate exhibits a finer grain size and a higher volume fraction of porosity as compared to that away from the substrate. Grain size is mea" sured in spray deposited preforms, excluding the area in the vicinity of the substrate (spray thick"

(a) dendrite structure

(b) second phase at dendrite boundaries

Fig. 2 Microstructure of ingot- fast 5083 Al © 1994-2010 China Àcaderïïic Journal Electronic

(a)in the center of the perform

(b) in the vicinity of the substrate

Fig. 3 M icrostructure of spray-deposited 5083 Al showing the presence of equiaxed grains

ness of < 1cm). The overall average grain size measured through the entire thickness is 15. 2Um.

One of the most important characteristics of spray deposited materials is the presence of porosi" ty. Porosity is calculated from density according to the following formulation

P= 1 - Ps/ Pt

where P is porosity, Ps is density of spray deposited material, and pt is theoretical density.

It is worth noting that the porosity is generally highest close to the substrate and the upper surface of the spray deposited preform, where in the present investigation a porosity of 5 vol. % is determined. However, porosity in the central portion of the spray deposited materials is generally less than 2 vol. % .

2.2 Microstructural development during TMP

In order to achieve superplasticity the porosity

must be eliminated. Fig. 4 illustrates the variation

of density as a function of rolling reduction. It is

evident that density increases rapidly with increas" Misrnng House. AlTrights reserved. http://www.cnKi.net

ing rolling to an overall reduction of approximately 50% . Beyond this value, density remains relatively unchanged with further rolling. The dashed line in Fig. 4 represents the theoretical density of 5083 Al. Accordingly, complete densification of spray deposited 5083 Al is achieved for a reduction of less than 50% during rolling at 300 °C.

9 2.64

- Theoretical density of5083 Al

........... .....о ...Ч...Я...

i 1 t i i i

10 20 30 40 50 60 70 Reduction in thickness/%

Fig. 4 Density of spray deposited 5083 alloy as a

function of rolling reduct ion

( a) extruded plus rolled

( b) directly rolled Fig. 5 Microstructure of thermomechanically processed 5083 Al following thermal exposure at 500 °C for 1 h

The average grain size in 5083 alloys is also

dition to less than 3r5^m following a 72% reduction. The microstructure of extruded and rolled samples is finer than that of directly rolled ones. Fig. 5 compares the microstructures of both extrud" ed plus rolled and directly rolled samples following thermal exposure at 500 °C for 1 hour. Apparently, thermal exposure at the testing temperature leds to recrystallization and formation of equiaxed grains from the deformed micro structures developed during thermomechanical processing. The recrystal" lized grain sizes are similar in both materials. 2.3 Superplastic behavior

Superplasticity is observed for spray deposited 5083 Al following thermomechanical processing by both extrusion plus rolling and direct rolling. The superplastic testing results summarized in Fig. 6 indicate that elongation is a strong function of testing temperature and strain rate. For testing temperatures below 500 °C and strain rates greater than 3 x

- 4 - i

elongation is generally less than 200%.

-о—500TC

\ -i—5301С

/ —D—5501C

1 lililí J-1 1 1 1 1 1 III 1 1 1 1 ЩД

lO4 10-3

Strain raters-1

reduced from i5- ?.Vm in

tedcorr ОП1С

Fig. 6 Tensile elongation of spray deposited 5083 Al Elongation values greater than 200% are obtained at testing temperatures higher than 500°C and strain rates below 3 x l0 4 s '. A maximum superplastic elongation of 465% is achieved at 550 °C and a strain rate of 3 x l0 5 s '. Samples thermome chanically processed by direct rolling are tested at 550°C at strain rates of 9. 5 x Ю- 5 and 3 x l0- 4 s '. The superplastic samples are prepared from spray deposited 5083 Al by thermomechanical rolling at 10% per pass to a total reduction in thickness of 57%. M aterials are annealed at 300 °C for 10 minutes between rolling passes. As summarized in Table2, directly rolled specimens exhibit

tensile elongations lower than those obtained for Pubhsning Ноше. All rights reserved, http://www.cnki.net

spray deposited 5083 A1 thermomechanically treated by extrusion plus rolling.

Table 2 Tensile elongation of spray deposited 5083 Al at 550 C

The rm om e ch anical Strain Elongation

processing condition rat e7 s- 1 /%

3 x 10-4 256

Directly rolled at 300"C to 57%

9. 5 x 10-5 295

Extruded at 400"C to 16 1 + 3 x 10-4 187

rolled at 300 "C to 69% 9. 5 x 10-5 380

To further understand the superplastic behavior of spray deposited 5083 Al, an effort is made in the present investigation to determine stress"strain rate relationships and strain rate sensitivity factors as a function of testing temperature and initial strain rate. The strain rate sensitivity factors are determined using strain-rate-change tests. In such tests, specimens are deformed in tension at an initial strain rate to a predetermined strain. The strain rate then changes to higher values in a stepwise manner. The strain rate sensitivity factors are then determined from logarithmic flow stress" strain rate curves according to the following equations'10] o = k[ £exp( Q c/RT )] m

m = ( Sln o/ Sln £)d, t where d is grain size, o is flow stress, K is a material constant, £ is steady-state strain rate, Qc is activation energy, R is gas constant and T is temperature in absolute scale. Figs. 7 and 8 summarize the true flow stress of thermomechanically processed 5083 as a function of true strain rate. The results shown in these figures illustrate the general trend of decreasing flow stress with increasing temperature and decreasing strain rate. M aterials thermomechanically processed by both extrusion plus rolling and direct rolling exhibit a similar flow stress at various strain rate levels. Moreover, the flow stress in the strain rate range of less than 3 x 10 3 s is generally less than 20MPa For the lowest strain rate of less than 3 x 10 5 s the flow stress is normally less than 5MPa. Such flow stress ranges agree with those obtained for 5083 A1 pro

True strain rate/s

(a) true flow stress with ture strain rate

0.6 0.5 0.4

0.3 0.2 0.1

6 * z 5

e 4 500TS

_ □ o 5301C

A c 550-C

.........

10"* IO3 IO"2

True strain rate/s"'

( b) strain- rate; sensitivity factor with true strain rate Fig. 7 The relationship between the parameters in spray deposited 5083 thermomechanically processed by extrusion and rolling

True strain rate/s

(a) true flow stress with true strain rate

cessed using

ng conventional methocjf0unia . Figsi7 Publishing House. Alfrights reserved, http://www.cnki.net

( b) strain- rate; sensitivity factor with true strain rate Fig. 8 The relationship between the parameters in spray deposited 5083 thermomechanically processed by direct rolling

and 8 also summarize the strain rate sensitivity factors. For spray deposited 5083, the strain rate sensitivity factors are in the range of 0. 3r0. 5 and increase with decreasing strain rate. The high strain rate sensitivity factors associated with the 5083 Al samples processed by extrusion plus rolling suggest that this material should exhibit a relatively high superplastic elongation, consistent with the tensile test results. At strain rates higher than 3 x l0 4 s, the strain rate sensitivity factors of all samples fall below 0. 5. Accordingly, the micro struct ures of the present spray deposited 5083 Al need to be optimized further in order to improve superplasticiyty.

It is worth noting that spray deposited 5083 Al thermomechanically processed using direct rolling exhibits a similar range of strain rate values. Its elongation is, however, relatively low at low strain rates. One of the major factors limiting the achievable superplastic elongation in the present irr vestigation is considered to be grain size, the grain size in extruded and rolled samples is smaller than that of directly rolled ones. Another major factor may be the elevated number of cavity nucleation sites. Extrusion may have eliminated porosity more effectively than direct rolling. T herefore, there may be fewer available nucleation sites for cavities in extruded plus rolled samples than in directly rolled samples.

3 Conclusions

(1) Spray deposited 5083 Al exhibites an e quiaxed grain morphology with an average size of 15 Um and porosity in the range of 0. 1 to 5 vol. % . Such spray deposited 5083 Al is densified by TMP rolling when the reduction exceedes 60%.

(2) Two distinct TMP procedures are employed to close porosity and refine grain size: extrusion plus rolling and direct rolling. The material processed using the former method exhibites a relatively high supeplasticity with a maximum superplastic elongation of 465% , whereas that processed using the latter method exhibites a maximum superplastic elongation of 295% .

(3) Spray deposited 5083 Al processed using extrusion plus rolling and direct rolling both exhibit similar stress"strain behavior and strain rate sensitivity factors. The strain rate factors are in the 0. 3 to 0. 5 range. The difference in their superplastic elongation is considered to be a result of differences in grain size and available cavity nucleation sites provided by closed gas pores.

References

| 1| Lavernia E J, Wu Y. Spray atomization and deposiion| M | .

LK: John Wiley & Sons Jnc, 1996. | 2| WuY, Castillo L D, Lavernia E J. Superplasticity of 5083 alloys produced by spray deposition| J| . Scipta Mater, 1996, 34: 1235- 1243.

| 3| Annavarapu S, Apelian D, Law ley A. Spray casting of steel strip: process analysis| J| . Metall Trans A, 1990, 21A: 3237 - 3256.

| 4| Perez J F, M orris D G. Copper- A^Og composites prepared by reactive spray deposition| J | . Scr Metall M at er, 1994, 31: 231- 235.

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| 6| Zeng X, Nutt S R, Lavernia E J. Microstructural character ization of Ni3Al processed by reactive atomization and deposi tion| J| . Metal Mater Trans A, 1995 , 26A: 817- 827. | 7| Li F, Roberts W T, Bate P S. Dislocation distributions in an Al 4. 5% Mg alloy during superplastic deformation| J | . Scri Metall Mater, 1993, 29: 875- 880. | 8| Hales S J, M cN elley T R. M icrostructural evolution by continuous re crystallization in a superplastic Al" Mg alloy| J | . Acta Metall, 1988, 36: 1229- 1239. | 9| Hales S J, M cN elley T R, Munro J G. Superplastic it y in an AlMg-LrZr alloy at intermediate temperatures | J|. Scri Metall, 1989, 23: 967- 972. | 101 Pilling J, Ridley N. Superptasticity in crystalline solids, institute of metals| M | . London, 1989. | 111 Kannan K, Hamilton C H. Jnhomogeneities in initial cavity distribution in a superplastic Al 5083 alloy| J| . SciptaMater, 1997, 38: 299- 305. | 121 Verma R, Ghosh A K, Kim S, et al. Grain refinement and superplast icity in 5083 Al | J | . Metall Sci Eng A, 1995, 191A: 143- 150.

Biographies:

DAI Sheng long Born in 1966, researcher, He received his doctoral degree in 1995 in Beijing Institute of Aeronautical materials, and then became a visitor of the U niversity of California. Now he mainly goes in for advanced Aluminum alloy research. Tel. (010)6245662 2 5 020, E- mail: sheng long. dai @biam. ac. cn

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