Microstructure evolution and mechanical behaviour of pure aluminium and aluminium alloys processed through constrained groove pressing

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ScienceDirect Materials Today: Proceedings 18 (2019) 2335–2344

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ICMPC-2019

Microstructure evolution and mechanical behaviour of pure aluminium and aluminium alloys processed through constrained groove pressing Vindala Poojithaa,*, T.Raghub, V. Pandurangaduc a, c

Jawaharlal Nehru Technological University Ananthapur,Saradha nagar,Ananthapuramu,515002,India b Defence Metallurgical Research Laboratory,Kanchanbagh,Hyderabad,500058,India

Abstract Constrained groove pressing (CGP) is a severe plastic deformation technique for sheet materials resulting in significant microstructural refinement and enhanced mechanical behaviour. CGP involves alternate pressing of sheets between corrugated and flattened dies under the plane strain deformation condition. The applicability of CGP for obtaining fine grained structures has been well documented for various metals and alloys. Processing and characterization of ultra-fine grain structures in aluminium(Al)and its alloys has been a subject of research over a period of time due to their application in wide range of industries such as automobiles aerospace, medical and so on. This study is aimed at reviewing the effect of CGP processing on the microstructural evolution and mechanical behaviour of pure aluminium and few commercially important Al alloy sheets reported in literature. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords:Constrained groove pressing; Aluminium and Aluminium alloys; Mechanical properties.

1. Introduction Grain refinement is essential for attaining superior properties in metallic materials. It is evident from Hall-Petch equation that smaller grain size leads to increased strength. There are different types of grain refinement operations such as heat treatments and mechanical straining for achieving values in the range of 3-10μm [1]. Later with the research study published in 1988, nanostructured grains were possible through the application of severe plastic deformation (SPD) to coarse grained materials. Severe plastic deformation is one of the most effective methods for producing nanocrystalline (NC) (grain size<100nm) or ultrafine-grained (UFG) (100nm
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SPD process is currently defined as “any method of metal forming under an extensive hydrostatic pressure that may be used to impose a very high strain on a bulk solid without the introduction of any significant change in the overall dimensions of the sample and having the ability to produce exceptional grain refinement"[3].There are numerous methods to process ultra-fine grain materials such as equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB), repetitive corrugation and straightening (RCS), tubular channel angular pressing (TCAP) and constrained groove pressing process (CGP). From the above mentioned methods ARB, RCS and CGP are applicable for sheet materials. Among these, CGP process invented by Shin et. al., [4] is one of the most suitable methods for fabrication of UFG sheets. Depending on the limitation of a sheet test piece elongation during the process, many variants of CGP such as semi-constrained GP (SCGP) [5] and unconstrained groove pressing (UGP) [6] have emerged. This study briefly overviews the improved microstructure and mechanical properties of pure aluminium and aluminium alloys sheet materials processed by CGP. 2. CGP process In 2002, Shin et. al., [4] described the CGP method as schematically shown in Fig. 1. In this process, a set of asymmetrically grooved and flat dies are used to corrugate and flatten a sheet material as shown Fig. 1(a). During this process a gap of sheet thickness (t) is maintained between the upper and lower dies (Fig. 1(b)). During first pressing test piece is corrugated, resulting in alternate flat and inclined regions as shown in Fig. 1(c). Flat regions remained undeformed (unhatched area), whereas the inclined regions are subjected to shear deformation, inducing an effective strain of 0.58. During second pressing, the corrugated test piece is flattened (Fig. 1(d)) resulting in reverse shear deformation of earlier deformed region. The effective strain in the deformed region is increased to 1.16. Before the next pressing, test piece is rotated 180º (Fig. 1(e) around the axis perpendicular to its plane, which allows the undeformed regions to be deformed in the third and fourth pressing. Further after four pressings (one cycle) material with a uniform effective strain of 1.16 is attained (Fig. 1(f), Fig. 1(g)).

Fig. 1. Schematic view of CGP process. [7]

Fig. 2. Geometrical die details of CGP process. [8]

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Magnified view of the groove die with the geometrical details such as the groove width (T), groove angle (θ), groove height (H) and sample thickness (t) are presented in Fig. 2. In CGP die the interface distance and groove width are maintained equal, while a groove angle of θ=45º is seen commonly used. 3. Strain imposed Fig. 3 shows the evolution of shearing strain in the test piece during CGP process. An engineering shear strain of γxy=1 is induced in single pressing as shown in Eqn. (1) below: tan 45°

γ

1 (1)

Fig. 3. Shearing strain during CGP process. [9]

Where, T is the width of the shear zone, ∆ is the distance sheared by specimen in the shear region as shown in Fig.3. The effective strain after each pressing of CGP is given by Eqn. (2) as follows [9]: ɛ

ε

ɛ

ɛ

ɛ

ɛ

ɛ

ɛ

ɛ

ɛ

(2)

Eqn. (2) is simplified by assuming ɛ =ɛ =ɛ =0 as deformation takes place with no expansion in both longitudinal and transverse direction [10]. Due to the plane strain condition displacements are considered to be limited to x-y ɤ plane, therefore strains in the z direction are neglected i.e ɛ =ɛ =0 and considering shear strain ɛ = [9]. Therefore,ɛ 0.58 √ Hence, each cycle of CGP has four pressings with a shear strain of 1.0 and an effective strain of0.58 after each pressing in the deformed region. 4. Pure aluminium Aluminium is one of the most abundant and versatile metal which is soft, ductile and has high electrical conductivity. In order to widen the applicability of pure aluminium in sheet form, its properties can be enhanced using constrained groove pressing process. Some of the investigations reported on change in structural behaviour and mechanical behaviour of CGPed pure aluminium are summarised here. Shin et. al., [4] subjected high purity aluminium sheet of dimension 70mm × 70mm × 6mm to four cycles of CGP at room temperature. Prior to pressing, sheet was annealed at 500ºC for 4h. The annealed aluminium sheet was characterised by dislocation cells of diameter 3μm as shown in Fig. 4(a).With the TEM micrographs shown in Fig. 4(b-f), the grain refinement progression can be clearly observed. Though first pressing (Fig. 4(b)) did not show a notable grain refinement, second pressing (Fig. 4(c)) ensued homogenous microstructure with equiaxed grains of size 0.5μm. Microstructure after the first cycle was similar to the microstructure in Fig. 4(c). Almost identical microstructural refinement was reported by ECAP [11-12]. In the fifth pressing (Fig. 4(d)), elongated grains were evolved and higher density of dislocation cells were revealed indicating UFG materials. After sixth pressing (Fig. 4(e)) the sheet specimen exhibited microstructural characteristics such as high dislocation density, ill-defined boundaries etc. An increased grain size of 0.8μm was evolved after four cycles (Fig. 4(f)), which was coarser than that of the grain size after sixth pressing. Hardness of the sample after two cycles increased twice as the annealed sample whereas it decreased by 10% after fourth cycle. Yield strength (YS) and ultimate tensile strength (UTS) increased after first cycle and decreases in the later cycles. Unlike YS and UTS, elongation decreased after first cycle and increased in later cycles.

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Fig.4. TEM micrographs indicating (a) annealed Al sheet and microstructural evolution of deformed aluminium sheets (b)1 pressing (c) 2 pressings (d) 5 pressings (e) 6 pressings (f) 16 pressings[4]

However in a different study by Zrnik et. al., [13], commercial purity Aluminium sheets were CGP processed up to four cycles at room temperature. Prior to deformation 70mm × 50mm × 7mm sheets were annealed at 250ºC for 1.5h. For the understanding of micro structural refinement and tensile behaviour, samples were extracted from undeformed region and sheared region after every pressing as shown in Fig. 5(a). After four cycles (Fig. 5(b)), the microstructure was characterized by a combination of elongated sub grains and equiaxed polygonised grains of about ˜1μm size when compared to its initial grain size of ˜100μm.The microstructural characteristics obtained after four cycles of CGP were similar to that obtained during initial cycle. YS and hardness of the initial heat treated test piece was 50MPa and 25HV respectively. Due to severe straining by CGP, YS is increased to 109MPa (118%) and hardness to 37HV (48%), distinguishing values between annealed state and deformed sheets were observed. The increase in Ultimate tensile strength (UTS) from 59MPa to 131MPa showed the impact of severe plastic deformation on the mechanical properties.

Fig.5. (a) Representation of CGP plate for TEM and tensile test specimen (b) TEM micrograph after four cycles of CGP. [13]

In the previous studies, high purity aluminium sheets were constrained groove pressed upto four cycles [4, 13]. However Satheesh et. al., [14] reported the effect of CGP on microstructural and mechanical behaviour of high purity aluminium sheets when processed upto five cycles. Prior to deformation these hot rolled sheets of dimension 130mm × 70mm × 5mm were annealed to 550ºC for 4h. The optical micrograph in Fig. 6(a) of pure aluminium in annealed condition revealed coarse grains ranging 300-1000μm. Fig. 6(b) indicates the onset of fragmentation of the coarse grains into fine grains after first cycle. The complete transformation of coarse grained structure into fine

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grained structure after the five cycles was revealed in TEM microstructure (Fig. 6(c)). Final grain size obtained after five CGP cycles was about 0.9μm. Fig. 7 summarises room temperature tensile properties of CGP processed pure aluminium sheets. A significant increase in YS (17MPa to 90MPa) and UTS (41MPa to 95MPa) were observed only after the first cycle. The yield strength was increased by ˜5.3 times after first cycle when compared to earlier investigations [4] on CGP processed pure Al which revealed ˜4 times increase in YS during similar processing conditions. Marginal drop in YS and UTS were observed in later passes, attributed to increased dislocation recovery with higher strains. This tensile behaviour was similar to that reported in other investigations on CGP [15, 16]. Drastic decrease in ductility associated with lowered strain hardening ability was observed. Micro hardness increased significantly up to two cycles and maintained uniform trend upto five cycles. Strain homogeneity evaluated using Vickers hardness profile indicated improvement in strain homogeneity with higher accumulated strain. Table 1 summarises grain size and tensile property of pure aluminium as reported in various investigations discussed above.

Fig. 6.(a) Optical micrograph of annealed Aluminium (b) TEM micrograph after first cycle (c) TEM micrograph after fifth cycle. [14]

Fig. 7. Tensile properties of severe plastically deformed Al sheets with increasing no. of passes (cycles) at room temperature. [14] Table1. Summary of grain size and tensile property of Pure Al Purity(%) Dimension(mm) Strain

Grain size(μm) Initial Final

99.9

1200

70×70×6

4.64

0.8

Tensile Property(MPa) YS UTS Initial Final Initial Final 38 118 50 122

Ref

[4]

99.9

70×50×7

4.64

100

1

50

109

59

131

[13]

99.9

130×70×5

5.8

(300-1000)

0.9

18

86

40

95

[14]

5. Aluminium alloys Among all the aluminium alloys series, Al-Mg (5xxx series) and Al-Mg-Si (6xxx series) are most widely used. These have various functional and structural applications in automotive and aerospace industries because of its low cost, high strength to weight ratio, good corrosion resistance and so on [17]. CGP process improves strength and hardness but reduces ductility. Post deformation annealing improves ductility simultaneously maintaining strength and hardness. An attempt was made to summarize the effect of CGP and also post deformation annealing on the microstructural evolution and mechanical behaviour of few aluminium alloy sheet materials.

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Moradpour et. al., [17] studied the effect of constrained groove pressing on 5052 aluminium alloy only upto two cycles, as further cycles led to the formation of cracks. The 84mm × 84mm × 3mm H34 tempered alloy was annealed to 500ºC for 2h. Fig. 8 (a-i) shows optical micrographs of surface section of the Al-Mg alloy after one cycle of CGP at different magnifications. These images revealed the fragmentation of coarse grain structure and the formation of deformation flow pattern after CGP processing. XRD analysis showed that after first cycle of CGP process, the grain size was reduced from ~50μm to ~600nm. After second cycle, the grain size was reduced to ~500nm which indicated lower grain refinement. This was accordant with the general fact that the grain refinement was higher in the initial cycles of severe plastic deformation process than in the later cycles [18].TEM studies revealed that after first cycle (Fig. 9(a-d)), a combination of equiaxed and elongated grain structure was seen. A homogenous equiaxed subgrains structure was developed after second cycle (Fig. 9(e-h)). With the increasing no. of cycles, fragmentation of grains with well demarcated boundaries was evolved. Identical microstructural changes were reported by other researchers for different materials [18-20].The hardness of annealed sample was in the range of 55HV-57HV, which increased to 85HV after two cycles. YS and UTS of annealed Al-Mg alloy were 110MPa and 225MPa respectively which increased to 234MPa and 260MPa respectively after two cycles, while elongation showed a downfall from initial condition to deformed stages.

Fig. 8. Optical micrographs of CGPed Al-Mg alloy after one cycle at different magnification. [17]

Fig. 9. Al-Mg alloy TEM images after one cycle (a-d) and after two cycles (e-h). [17]

Nagaraju et. al., [21] investigated the mechanical behaviour of aluminium alloy 6061 when processed upto five cycles of CGP. Sheet specimen of dimension 100mm × 20mm × 5mm was solid solution heat treated at 525ºC for 2h. Fig. 10 shows the variation of UTS and hardness with the increasing number of CGP. An increase of 19% in tensile strength (~270MPa to 335MPa) was observed when deformed upto four cycles. This could be because of the

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grain refinement after CGP. The hardness of the undeformed samples increased almost twice (~23HV to 52HV) when deformed upto four cycles, because of the presence of ultra-fine grains. Overall, an increasing trend in mechanical properties of CGPed test pieces were seen only upto four cycles. Ultimate Tensile strength Hardness

55 50

330 320

45

310

40

300

35

290 30

Hardness (Hv)

Ultimate Tensile strength ( MPa)

340

280 25

270 260

0

1

2

3

4

5

20

No.of Cycles

Fig. 10. Variation of UTS and hardness with increasing number of CGP cycles. [21]

Further to study the thermal stability of these severely strained test pieces, they were exposed to 125 ºC for various time periods i.e 25min, 50min, 75min, 100min, 125min. Fig. 11(a) and Fig. 11(b) shows the variation of UTS and hardness of deformed samples and thermally exposed samples with increasing number of cycles respectively. Decrease in UTS and hardness was reflected by thermally exposed samples when compared to the CGPed samples. Similarly, thermally exposed deformed samples showed higher UTS and hardness than the thermally exposed unprocessed samples, which reveals the thermal stability of CGPed Al alloy. This can be attributed, firstly due to the existence of trident grain boundary which has a significant influence on the ultra-fine grained materials [22]. Hence, the movement of trident grain boundary might increase the overall length of grain boundary, thereby enhancing the grain boundary energy [23].Therefore the grain boundaries are unlikely to move during heating, further stabilizing UFG structured materials. Secondly due to the precipitate phases, which pin the grain boundary at high temperature thereby stabilizing the grain size.

Fig. 11. Effect of CGP passes (cycles) and thermal exposure duration at 125ºC on (a) UTS and (b) Hardness of Al 6061 alloy. [21]

Hesam et al [24] gave an insight on change in microstructure and mechanical properties of CGPed Al-Mn-Si alloy after annealing. Sheet specimens of dimension 84mm × 70mm × 3mm were used and heat treated to 450ºC for 3h attaining a grain size of 464.89μm. In this study individual sheet test pieces were cold strained to different effective strains of 1.16, 2.32, and 3.48. Fig. 12 shows the optical micrographs of Al-Mn-Si alloy after CGP cycles and post

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deformation annealing. The increasing number of cycles resulted in a wavy structure because of the groove die profile and grain refinement due to the increased dislocations (Fig. 12(a-c)). With increasing strain process, YS increased from 85MPa in AA (as annealed) test piece to 98MPa in C2 (second cycle) test piece. UTS increased from 113MPa in AA test piece to 129MPa in C2 test piece. Elongation depleted from 7% in AA specimen to 4.9% in C2 test piece. This depletion could be attributed to the accumulation of dislocation density during SPD. Hardness increased from 29HV in AA specimen to 47HV after two cycles.

Fig. 12.OM micrographs of different specimens of Al-Mn-Si alloy (a) C1 (b) C2 (c) C3 (d) C2-150ºC (e) C2-250ºC (f) C2-350ºC. [24] Table 2. Summary of grain sizes of different test pieces [24] Sample Condition

NomenclatureAvg. length(μm)

Avg. width of grains (μm) Grain size(μm)

Deformed

AA

502.07

427.71

464.89

Deformed

C2

13.08

6.34

9.81

Post deformation annealed

C2-150ºC24

9

16

Post deformation annealed

C2-250ºC29

8

19

Post deformation annealed

C2- 350ºC31

10

20

AA-As annealed, C2-two cycles, C2-(150ºC, 250ºC, 350ºC) - two cycled specimen at different annealing temperatures.

In addition to that, C2 sheets were heat treated at 150ºC, 250ºC, 350ºC. Upto 250ºC annealing temperature change in microstructure was insignificant (Fig. 12(d-e)), but at 350ºC (Fig. 12(f)) increased grain growth and depletion of groove depth in wavy region was observed. Table 2 demonstrates the grain sizes of various test pieces. With the increasing annealing temperature, grain growth reached to a maximum of 20μm at C2-350ºC. While C2-150ºC was considered optimum as it attained minimum grain size among all the post deformation annealing conditions. Further, an increasing trend in length and width of grains indicate the emergence of elongated grains during annealing. This inhomogeneous grain growth in the heat treated samples indicated the occurrence of strain induced grain boundary migration phenomenon. The driving force for which was provided by imposed shear strain and dislocation proliferation.

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Fig. 13. Different mechanical characteristics of Al-Mn-Si Alloy (a) YS (b) UTS (c) El% (d) Hardness after CGP and post annealing [24].

Fig. 13 illustrates the mechanical characteristics of all test pieces. After post deformation annealing, a remarkable increase of 157% upto 350ºC in ductility is observed. Meanwhile YS, UTS and hardness showed improvement only at 150ºC which further decreased at higher temperatures (upto 350ºC). Table 3 summarises grain size and tensile property of pure aluminium as reported in various investigations discussed above. Table3. Summary of grain size and tensile property of Aluminium Alloys (AA) Purity(%) Dimension(mm) Strain

Grain size(μm) Initial Final

Al-Mg

84×84×3

2.32

50

Al-Mg-Si

100×20×5

5.8

-

Al-Mn-Si

84×70× 3

3.48

465

0.4-0.5

Tensile Property(MPa) YS UTS Initial Final Initial Final 110 234 225 260

Ref

[17]

-

-

-

270

330

[21]

9.84

85

118

112

141

[24]

6. Summary This review aims to summarize the effect of constrained groove pressing on microstructural evolution and mechanical property of pure aluminium and aluminium alloys. • Pure aluminium could be successfully constrained groove pressed upto an effective strain of 5.8 resulting in significant grain refinement upto ~0.9 to 1μm irrespective of the cases studied. Meanwhile increase in yield strength as high as ~5.3 times were observed in pressed sheets. • From the literature reported on CGP processed commercially important aluminium alloys like Al-Mg, Al-MgSi, Al-Mn-Si, it was evident that an effective strain up to 5.8 is imposed, leading to significant grain refinement. The yield strength of CGPed Al alloys were almost doubled with concomitant decrease in ductility. Acknowledgment We thank Dr.SS.Satheesh, NNSG, DMRL for his valuable suggestions that greatly helped in improving the manuscript. Authors also acknowledge the support received from DRDO for this work. References [1] [2]

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