Scholarly article on topic 'Superplasticity of the rolled and friction stir processed Al–4.5 Mg–0.35Sc–0.15Zr alloy'

Superplasticity of the rolled and friction stir processed Al–4.5 Mg–0.35Sc–0.15Zr alloy Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Anton Smolej, Damjan Klobčar, Brane Skaza, Aleš Nagode, Edvard Slaček, et al.

Abstract The article describes the effect of friction stir processing (FSP) on the superplastic behaviour of the Al–4.5Mg alloy containing Sc and Zr. The results yielded by the FSPed sheet were compared with the superplastic behaviour of the same alloy produced conventionally by cold rolling. The measurements of the superplasticty included the flow stresses and the maximum elongations of the alloy at initial strain rates ranging from 1×10−3 s−1 to 1s−1, and at testing temperatures from 350°C to 500°C. The inclusion of the FSP step considerably enhanced the superplastic behaviour of the alloy in comparison with its rolled counterparts, which was reflected in higher elongations at higher strain rates and lower forming temperatures.

Academic research paper on topic "Superplasticity of the rolled and friction stir processed Al–4.5 Mg–0.35Sc–0.15Zr alloy"

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F^llll^ Materials Science & Engineering A

ELSEVIER journal homepage: www.elsevier.com/locate/msea

Superplasticity of the rolled and friction stir processed Al-4.5 Mg-0.35Sc-0.15Zr alloy $

Anton Smoleja,n, Damjan Klobcarb, Brane Skazaa, Ales Nagodea, Edvard SlaCekc Vukasin Dragojevic c, Samo Smoleja

a Faculty of Natural Science and Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia b Faculty of Mechanical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia c Impol, Aluminium Industry, 2310 Slovenska Bistrica, Slovenia

ARTICLE INFO ABSTRACT

The article describes the effect of friction stir processing (FSP) on the superplastic behaviour of the Al-4.5 Mg alloy containing Sc and Zr. The results yielded by the FSPed sheet were compared with the superplastic behaviour of the same alloy produced conventionally by cold rolling. The measurements of the superplasticty included the flow stresses and the maximum elongations of the alloy at initial strain rates ranging from 1 x 10~3s_1 to 1 s_1, and at testing temperatures from 350 °C to 500 °C The inclusion of the FSP step considerably enhanced the superplastic behaviour of the alloy in comparison with its rolled counterparts, which was reflected in higher elongations at higher strain rates and lower forming temperatures.

© 2013 The Authors. Published by Elsevier B.V. All rights reserved.

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Article history: Received 10 June 2013 Received in revised form 9 October 2013 Accepted 14 October 2013 Available online 22 October 2013

Keywords: Aluminium alloy Friction stir processing Superplasticity

1. Introduction

The Al-Mg type alloys, such as AA5083 (Al-Mg-Mn), belong among the principal Al alloys for superplastic forming (SPF). The superplastic behaviour of these alloys has been investigated extensively [1-3], and the requirements for achieving the super-plasticity of the Al alloys are now well understood [4,5]. Recently, the SPF operations have required Al materials which are capable of exhibiting high strain-rate superplasticity (HSRS) at forming rates faster than 1 x 10_ 2 s_ \ and low-temperature superplasticity (LTSP) at temperatures lower than 400 °C [2,6,7-9]. The superplastic forming at higher strain rates increases the productivity of the practical industrial fabrication, whereas the lower forming temperatures prevent grain growth and reduce cavitation, which enhances the superplastic flow of the material [7]. Good superplastic formability under the working conditions mentioned above can be achieved by further reduction in the grain size of the material [7]. There are several methods for modifying the grain size: (1) an appropriate thermo-mechanical treatment involving high reductions during the cold rolling of the material, (2) new

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forming processes, such as equal channel angular pressing (ECAP), high-pressure torsion, accumulative roll-bonding [7,9], friction stir processing [10,11], and (3) minor additions of certain transition elements, such as scandium (Sc) and zirconium (Zr), to the base alloy [12,13].

It is now established that the addition of small quantities of Sc to Al alloys significantly increases their superplasticity [12-16]. Recent researches have shown that the combined addition of small quantities of Sc and Zr to Al-Mg alloys improves the super-plasticity more than the separate addition of the one or the other element does [17-19]. The presence of both elements stabilizes the crystal grains due to a fine and uniform distribution of Al3(Sc, Zr) precipitates [18]. The highest tensile elongation known so far -4100% - has been obtained with the ECAP-treated Al-5 Mg-0.18Mn-0.2Sc-0.08Zr alloy at a strain rate of 5.6 x 10~2 sand a forming temperature of 450 °C [17].

The fine-grained 1-10 ^m microstructure, which is the basic condition of good superplasticity, is likewise formed in metallic materials treated by friction stir processing (FSP) [10,11]. During FSP, the material in the processed zone undergoes intense plastic deformation with stirring and simultaneously receives heat input from the friction, which results in significant microstructure changes with extremely fine crystal grains [10,11]. FSP has been used to investigate the superplastic behaviour of various Al alloys, such as AA7075 [20-23], AA2024 [24], AA5083 [25,26], Al-Mg alloys with additions of Sc and Zr [27-31], and others. The treatment has also been applied to such casting alloys as A326 [32]. The investigations have focused on the impact of the FSP

0921-5093/$-see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.027

parameters and tool dimensions on the microstructure and super-plasticity of the alloys [26,27,30], on the direct use of FSP to achieve superplasticity in cast light alloys and continuous cast aluminium alloys [25,33-37], on the impact of FSP parameters on the thermal stability of microstructure [6], and on the mechanisms of superplastic forming [6,21,26].

The FSP-treated Al-Mg alloys containing Sc and/or Zr can exhibit high strain rate superplasticity (HSRS) as well as low temperature superplasticity (LTSP). An elongation of 240% was achieved for the FSPed Al-4 Mg-1Zr alloy at a low temperature of 175 °C for initial strain-rates between 1 x 10"4 and 3 x 10"4 s [27], and a maximum elongation of 2150% was achieved for the FSPed Al-Mg-Sc alloy at a high strain-rate of 1 x 10"1 s"1 and at 450 °C [30]. The addition of Sc and Zr stabilizes the crystal grains of the basic Al-Mg alloy and prevents abnormal grain growth [6]. FSP also enables the formation of relatively high elongations at higher strain rates and at lower temperatures in standard aluminium alloys without added transition elements. In the case of the AA7075 alloy, low-temperature superplasticity of 350% to 540% was achieved at 200-350 °C and at strain-rates up to 1 x 10"2 s" 1 [22]; in the case of AA5083, a maximum HSRS ductility of 315% was achieved at 1 x 10"2 s"1 and 450 °C [26]. In addition, FSP is applied to continuously cast aluminium alloys since it enables their superplastic forming without previous thermo-mechanical treatment [25,35]. The FSP modifies the casting microstructure by refining the crystal grains, breaking the coarse primary dendritic structure and intermetallic phases, dissolving most or part of the precipitates, and eliminating the casting porosity [34]. The superplastic-forming mechanism of FSPed aluminium alloys is grain boundary sliding, which occurs also at lower temperatures and at higher forming rates [6,22,36].

Another advantage of FSP is the possible local formation of fine-grained microstructure in the material, which causes a corresponding local alteration of the basic material properties. In this way, FSP can be applied selectively to a desired location with specific properties without altering the basic characteristics of the material outside the FSP region [34,36]. A still further advantage of FSP is the possibility of fine-grained microstructure forming in thicker plates (above 5 mm) and thus the possibility of superplasticity, which is difficult to achieve with conventional thermo-mechanical processes involving a combination of heat treatment and rolling. The refining of the crystal grains in the desired regions and in thicker products of aluminium alloys leads to new concepts of superplasticity, such as selective superplastic forming and thick-plate superplastic forming [20,34,36].

The article describes the effect of friction stir processing on the superplastic behaviour of the Al-4.5 Mg alloy with minor additions of Sc and Zr. The cold-rolled plates of the alloy were treated with FSP at various tool rotation rates (TRR). The measurements included the flow stresses as a function of strain and the maximum elongations at testing temperatures between 350 °C and 500 °C and at initial strain rates from 1 x 10 "3s" 1to1 s"1. The results of the FSPed alloy were compared with the superplastic behaviour of the alloy produced conventionally by cold rolling without the FSP step.

2. Experimental procedure

The investigated alloy Al-Mg-Sc-Zr, with the chemical composition shown in Table 1, was prepared by induction melting, using Al99.99, Mg99.8, and the master alloys AlSc2.1, AlZr7.5, and AlTi5B1. The melt was repeatedly cast into steel moulds with dimensions of 175x80 x 27 mm3.

The as-cast ingots were homogenised for 4 h at 440 °C and for 4 h at 460 °C. The scalped ingots were hot- and cold-rolled into

Table 1

Chemical composition of the alloy investigated (in wt%).

Si Fe Mg Ti Zr Sc Al

0.007 0.016 4.72 0.012 0.168 0.35 Bal.

Table 2

Tool rotation rate (m) and designation of samples.

Samples E95/2 E150/3 E235/4 E475/5 E750/6

m (rpm) 95 150 235 475 750

Fig. 1. Schematic presentation of the coincidence of the tensile sample with the stirring region.

plates with a thickness of 4 mm and 2 mm. The thicker plates (of 4 mm) were further treated by FSP. The FSP tool used had a concave shoulder of 16 mm in diameter, and a threaded pin of 2.37 mm in diameter and 3.9 mm in length. Different combinations of TRR were tested at a fixed tilt angle of 3°. The TRR varied from 95 rpm to 750 rpm at tool traverse speed of 73 mm/min (Table 2). To create a broader region of the stirring zone, two parallel passes of the rotating tool were carried out on the plates. The second pass was parallel-shifted 5 mm from the axis of the initial pass. The overlap of the stirring zones in the workpieces resulted in regions with fine-grained microstructure, sufficiently broad to produce suitable tensile samples. The surfaces of the FSPed plates were scalped on both sides to a thickness of 2 mm.

The samples for tensile testing with a gauge section of 10 x 5.4 mm2 were machined from cold-rolled plates and FSPed plates, with the tensile axis aligned with the direction of rolling and of the FSP rotating tool movement. The gauge section fully coincided with the stirring region (Fig. 1).

Tensile tests, intended to evaluate the superplastic behaviour of the rolled alloys as compared to their FSPed counterparts, were carried out using a Zwick Z250 machine with a 0.5 kN load cell. The machine was equipped with a three-zone split electrical resistant furnace that wrapped around the samples and pull rods. The controlled temperature testing chamber was 300 mm in length. The temperature was measured with three NiCr-Ni thermocouples arranged along the testing chamber. The test temperatures were maintained at a constant plus/minus°C by electronic temperature regulation. All the samples were heated to the desired temperatures within 30 min and then held at these temperatures for 30 min longer to ensure thermal equilibrium in the heating chamber prior to starting the test. After the conclusion of the test, they were cooled in the air. The testing procedure and the evaluation of the results were controlled with the software system TestXpert II. The measurements included the true stress-true strain dependences and the maximum elongations. The testing temperatures and strain rates ranged from 350 °C to

500 °C and from 1 x 10 _ 3s _ 1to1 s respectively. The tests were carried out at fixed cross-head speeds, where the initial strain rate decreased with increasing strain. The elongation values were calculated from direct measurements of the elongated gauge length. The total strain was not corrected because of the pin hole deformation undergone by some samples.

The microstructural investigations of the alloy, focusing on the effect of the FSP parameters on the size of the crystal grains and their thermal stability, were carried out by means of light microscopy. The grain structure in the stirring zone was revealed using Baker's reagent. The investigated sections were parallel or perpendicular to the FSP direction. The average crystal grain size as a function of TRR was examined with the linear intercept technique at 2000x magnification. To check the thermal stability of grains in the stirring zone, the samples were statically annealed for two hours at temperatures of 300, 400 and 500 °C, which roughly corresponds to the temperatures and duration of the superplastic forming applied.

3. Results

3.1. Microstructure of the FSPed alloy

The microstructure in the stirring zone was analysed at various TRR of the single pass run. Fig. 2 shows the microstructure of the E95/2 alloy, both in the parent material and after FSP. Following the FSP, the fibrous crystal grains are considerably refined into fine recrystallised grains. The average grain size increases with the increased TRR from 1.3 ^m at 95 rpm to 7.4 ^m at 750 rpm (Fig. 3). The crystal grains are equiaxed and up to 235 rpm not aligned with the rotation or traversing direction of the tool. At 750 rpm, the microstructure is recrystallised in the vicinity of the stirring zone as well, which is a consequence of the higher heat input into the material during the FSP. The stirring zone has a typical basin shape produced in a 4 mm thick plate, with the top and bottom surfaces measuring 16 mm and 5 mm respectively. The FSP region displays no defects, such as cracks or porosity. Under the testing conditions applied, a second pass run does not change the average grain size in the overlapping region.

The influence of the test parameters on the FSP process can be expressed with the 'revolution per feed' (RPF) factor, that is, with the quotient between tool rotation rate (m) and tool traverse speed (v). The RPF provides information about the heat input per FSP length. The dependence of the RPF factor on the tool rotation rate is shown in Fig. 3. The RPF factor changes almost linearly, which implies that the temperature rises during the FSP with the increasing quotient m/ v. This rise affects the size of the emerging recrystallised grains in the stirring zone [10,39].

The thermal stability of crystal grains was examined for the E95/2, E235/4 and E475/5 alloys. Fig. 4a and b shows the microstructures of the E95/2 and E475/5 alloys after annealing at 500 °C in comparison with the not-annealed FSPed alloy (Fig. 4c). The FSPed Al-4.5 Mg alloy with additions of Sc and Zr exhibits good thermal stability under the annealing conditions applied (Fig. 4b). Some grain growth occurs only at 500 °C in the E95/2 alloy at the lowest TRR. In this case there forms a duplex microstructure, containing a minor number of larger grains surrounded by arrays of fine original grains (Fig. 4a).

3.2. Superplastic behaviour of rolled and FSPed alloy

The true stress-true strain curves were established through fixed cross-head speed tensile tests at various initial strain rates and temperatures. Figs. 5 and 6 show the sets of stress-strain curves for rolled and FSPed samples (at m=95 rpm), as they were yielded in tests carried out at 500 °C and at initial strain rates from 1 10 3 s 1 to 1 x 10 s . The curve shapes and flow-stress values of the rolled samples differ from their FSPed counterparts. All rolled samples undergo immediate hardening, followed by softening at higher strains (Fig. 5).

The FSP stress-strain curves are similar at all tested strain rates (Fig. 6). The maximum flow stresses during the superplastic flow are 2-2.5 times lower than those of the rolled samples. There is no explicit stress transient. The samples exhibit curves with the flow stress remaining almost constant during the superplastic flow at lower strain rates. As expected, the flow stresses increase with the increasing strain rates

0 200 400 600 800

Tool rotation rate [rpm]

Fig. 3. The average grain size and RPF factor as a function of tool rotation rate at a single pass run.

Fig. 2. Microstructure outside the FSP region (a) and in the middle of the stirring zone (b and c) of the E95/2 alloy after a single pass run. The micrographs are parallel to the rolling and tool traverse directions.

Fig. 4. Microstructures of FSPed E95/2 (a) and E475/5 (b) alloy after static annealing at 500 °C for 2 h in comparison with the not-annealed FSPed E475/5 alloy (c).

Fig. 5. True stress-true strain curves of the rolled alloy at 500 °C and at various initial strain rates.

7" = S00°C

w = 95 rpm

¿ = 1X10-Is-1

£ = 5 X Kr2s-1 P-nJ NI

é = 2.5 xj.0 ~2s__

£ = 1 X 10" 2S

True strain

Fig. 6. True stress-true strain curves of the FSPed alloy (m=95 rpm) at 500 °C and at various initial strain rates.

Since the superplastic behaviour of material depends not only on the chemical composition of the material but also on the working conditions, it is often measured with the elongation to failure. This elongation is significantly affected by the strain rate, testing temperature, and processing parameters. Fig. 7 shows the variation of elongation with the initial strain rate at a temperature of 500 °C for rolled and FSPed samples, which have been treated by tool rotation rates of 95, 235 and 475 rpm. The elongations of the FSPed samples decrease at higher strain rates (1 x 10"2 s"1) with an increasing TRR. The limited length of the controlled-temperature testing chamber has precluded tensile tests with elongations over 1900%; in these cases, the unbroken samples

Fig. 7. Elongations of FSPed samples at various tool rotation rates as a function of the initial strain rate and at 500 °C, in comparison with the conventional rolled alloy.

are marked with arrows. The E95/2 and E235/4 alloys exhibit their maximum elongation without failure, c. 1900%, at strain rates from

(E95/2) and at 1 x 10-

2.5 x 10- 2 1880%, is

s-1, whereas the highest elongation of E475/4, that is achieved at a lower strain rate (1 x 10-3s-1). The elongation of the rolled alloy first increases to the maximum value at 1 x 10- 2 s-1, which is followed by a gradual decrease with the increasing strain rate. At higher strain rates, the E95/2 alloy exhibits elongations 1.5-3 times higher than the rolled alloy does under the same testing conditions. Fig. 8 shows the appearance of the rolled and FSPed (m=95 rpm) tensile samples at 500 °C and at different strain rates. The FSP samples at lower initial strain rates (1 x 10- 1s-1) show a uniform deformation within the gauge length without any necking. At lower TRR ( ~ 95 rpm), the FSPed Al-Mg-Sc-Zr type alloys clearly exhibit good superplasticity, with elongations ranging from 470% to over 1000% at strain rates from 1s-1 to 1 x 10-1 s-1 and at 500 °C. These results indicate the occurrence of HSRS.

Figs. 9 and 10 show the tensile ductility of the rolled and FSPed (m=95 rpm) alloy at various forming temperatures and at 1 x 10-2 s-1. The FSPed alloy achieves good superplasticity over a wide temperature range, from 350 °C to 500 °C, whereas the elongations of the rolled samples are 7 to 2.5 times lower at temperatures below 500 °C. The maximum ductility of the FSPed alloy at the lowest testing temperature (350 °C) is 1450%. The FSP alloy exhibits elongations of nearly 1900% without failure at temperatures between 380 °C and 500 °C. These FSP samples show the uniform straining which is characteristic of the superplastic flow (Fig. 10).

The microstructure of the samples was tested for the phenomena of crystal grains under conditions of static annealing in the grip section, as well as during superplastic forming in the gauge

1 x10-2s-1

5x10 - 2 s -1

£ = ls"1 e = 470%

Fig. 8. Samples of rolled and FSPed alloy after the tensile tests at various initial strain rates and at 500 °C. (a): rolled samples and (b): FSP samples at ш—95 rpm.

sr 1400

с о 1200

о 1000

с о 800

ä А А -о

-о-ш = £ = lxl

О-2 s1

Temperature [°C]

Fig. 9. Elongations of the rolled and FSPed alloy (m—95 rpm) at various testing temperatures and at 1 x 10_2 s_1.

section, for the E95/2 alloy samples pulled at an initial strain rate of 1 x 10 _ 2 s _ 1 and at 500 °C. The starting microstructure consists of fine recrystallised grains, with an average grain size of c. 1.3 ^m (Fig. 2a and c). Fig. 11 presents the optical micrographs of the grains in the grip (Fig. 11a) and gauge (Fig. 11b) sections for the sample which has been pulled to a 1880% elongation. The grip contains the duplex microstructure described earlier (Fig. 11a). The gauge-length microstructure, on the other hand, differs from that of the grip: the grain size in the deformed region increases from the starting 1.3 ^m to 11.4 ^m. This means that the dynamic grain growth during the superplastic forming is faster than the static growth (Fig. 11b). The sample yields no occurrence of abnormal grain growth, which suggests that the fine-grained structure of the FSPed alloy is relatively stable during high-temperature superplastic deformation

4. Discussion

The Al-4.5Mg-0.35Sc-0.15Zr alloy investigated here is a modification of the commercial AA5083 alloy. It has been established that the addition of Sc to Al alloys acts as a grain refiner during the casting and stabilizes the grain structure with Al3Sc dispersoids, which is an essential condition of good superplasticity. The effect of grain refinement is enhanced when Sc is added to the melt in combination with minor quantities of Zr [38,40,41].

The microstructure of the FSPed Al-4.5-0.35Sc-0.15Zr alloy consists of fine recrystallised grains in the stirred zone. The grain size of the alloy investigated chiefly depends on FSP parameters, among which the tool rotation rate (TRR) is the most important since the average grain size increases together with the TRR. At TRR ranging from 95 to 750 rpm, the grain size increases from 1.33 to 7.42 ^m but never exceeds 10 ^m. Its growth is a consequence of the higher temperatures produced by friction at faster tool rotation. The 'revolution per feed' (RPF) factor, which provides information about the heat input, is directly linked to the temperature during the FSP [10,39] and to the grain size, since the RPF and grain size change almost linearly with the TRR (Fig. 3). At an RPF of 1.33, the heat input is lowest, which accords with the lowest temperature and smallest crystal grains.

In a 4 mm thick plate, a single pass run of the tool produces a basin-shaped stirred zone nearly 20 mm in diameter on the top surface and 5 mm on the bottom (Fig. 1). This FSPed region can be expanded to 24 mm and 7 mm with an additional run, in which the tool is parallel-shifted 5 mm to the right side. In this way, the tensile samples to be tested for superplasticity achieve a fine FSP microstructure along the entire gauge length, with dimensions of 10 x 5.4 x 2 mm3. A single overlapping of the stirred zone does not essentially affect the orientation or size of the initial fine crystal grains.

The fibrous microstructure in the thermally affected zone remains unaltered. Static recrystallisation occurs only in the narrow thermo-mechanically affected zone at the highest tool rotation rate of 750 rpm and the highest RPF of 10.3. The FSPed alloy exhibits good thermal stability and retains the fine grains produced at tool rotation rates ranging from 150 to 750 rpm even during protracted annealing at 500 °C. Under identical annealing conditions, partial grain growth with a duplex microstructure occurs only in the 95 rpm-processed alloy. Clearly, thermal stability - in contrast to the formation of the fine-grained structure -increases with the higher tool rotation rate, which corresponds with the results found in literature [25,33].

The superplastic behaviour of the material is characterised by the dependence of true stress on true strain. Rolled alloy yields different s-e plots from its FSPed counterpart. The curves exhibit quick incipient hardening, followed by continuous softening (Fig. 5). The flow stresses remain almost constant at lower strain rates and at strains above e— 1. The optimum strain rates for achieving the maximum elongations in rolled alloys are nearly 1 x 10~2 s~1.

There is no explicit initial stress transient in the FSPed alloy, where a smooth, increasing hardening takes place during the initial superplastic flow (Fig. 6). The gradual increase of stress is more intense at

Fig. 10. Examples of superplasticity in the rolled (a) and FSPed alloy (b) after tensile testing at various temperatures and at 1 x 10 2 s

Fig. 11. Microstructures in the grip (a) and gauge sections (b) of the E95/2 alloy, strained to a 1880% elongation at 1 x 10 2 s 1 and at 500 °C.

higher strain rates. The hardening behaviour can be explained with the dynamic grain growth during the tensile test [25]. After reaching the maximum value, the stress continuously decreases. At increased strain rates, the unexpressive peak stress is shifted to lower strains. The softening at higher strains can be attributed to progressive cavitation. It should be noted that the shapes of the FSP stress-strain curves are similar to the curves yielded by the fine-grained Al-6Mg-3Sc alloy, which has been processed by ECAP and examined under similar testing conditions [14].

The elongation provide quantitative information about the superplasticity of the materials. At strain rates over 2.5 x 10-2 s-1, the FSPed E95/2 alloy exhibits nearly 1.5-3 times higher elongations than the rolled one (Fig. 7). The differences are more pronounced at higher strain rates. The elongations depend on the TRR as well, decreasing as the latter increases at equal initial strain rates. This decrease is due to the formation of coarser crystal grains at the higher heat input during the FSP, which agrees with the other results [6]. The highest elongations without failure of the E95/2 and E235/4 samples were achieved at 500 °C between 1 x 10-2 and 5 x 10-2 s-1 (Fig. 7). The elongations of the E95/2 alloy amounting to 1020% and 470%, attained at strain rates of 5 x 10-1 and 1s-1, respectively, correspond to the HSRS criteria. The size of the crystal grains along the gauge length of tensile samples increases during superplastic straining because of the dynamic grain growth. After FSP, the microstructure along the gauge length consists of equiaxed, somewhat elongated grains of equal size (Fig. 11b). The grain growth along the gauge length is more intense than the static growth in the grip, where a duplex

microstructure is formed (Fig. 11a). The samples pull out uniformly, without any necking, even at higher strain rates (Fig. 8b).

At forming temperatures of 380 °C and 450 °C, the FSPed E95/2 alloy exhibits 7 times and 2.5 times higher elongations, respectively, than its rolled counterpart (Fig. 9). Under these testing conditions, no flow localisation is developed along the gauge length. FSP enables such superplastic ductility even at lower temperatures. However, high elongations at temperatures over 350 °C can occur in FSP-treated Al-Mg-X alloys only in the presence of a suitable thermally stable microstructure, which is ensured by the additions of Sc and Zr.

5. Conclusions

1. The thrust of the present work is a comparison between the superplastic behaviour of the rolled and FSPed Al-4.5Mg-0.35Sc-0.15Zr alloy.

2. The grain size in the stirred zone increases during FSP with the increasing tool rotation rate (TRR) from 1.3 ^m at 95 rpm to 7.4 ^m at 750 rpm. The FSPed alloy exhibits good thermal stability at temperatures up to 500 °C.

3. During superplastic forming (SPF), the maximum flow stresses of FSPed alloys are 2-2.5 times lower than those of the rolled samples.

4. Elongations without failure in the vicinity of 1900% are obtained for the FSPed alloy at TRR r 235 rpm, at initial strain rates ranging from 1 x 10-2 s-1 to 5 x 10-2 s-1, and at 500 °C. The elongations

at strain rates between 2.5 x 10 2 s 1 and 1 x 10 1 s 1 are 1.5-3 times higher than those of the rolled alloy.

5. Between 350 °C and 500 °C and at 1 x 10~2s~\ the elongations of the FSPed alloy are 7 to 2.5 times higher than those of the rolled alloy.

6. The FSP treatment of the Al-4.5Mg-0.35Sc-0.15Zr alloy results in good superplasticity, which is reflected in elongations up to c. 1900% (without failure) within an interval of higher strain rates (1 x 10~2 s_1) and lower forming temperatures (500 °C) in comparison with a rolled alloy of identical composition.

Acknowledgement

This work was supported by Slovenian Research Agency (ARRS), Government of the Republic of Slovenia through project L2-4138.

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