Effect of annealing and aging treatment on mechanical properties of ...

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Effect of annealing and aging treatment on mechanical properties of ultrafine grained Al 6061 alloy P. N. Rao, S. K. Panigrahi and R. Jayaganthan* A comparative study of aging and a combined treatment of short annealing and aging on mechanical properties and microstructure of cryorolled (CR) Al 6061 alloy is investigated in the present work by using tensile tests, hardness tests, electron backscattered diffraction and transmission electron microscope. The pre-CR solid solution treatment combined with post-CR short annealing (200uC, 5 min) followed by aging treatment (100uC, 57 h) of the Al 6061 alloy showed an improved ductility and well defined ultrafine grain structure as compared to the samples subjected to pre-CR solid solution treatment followed by post-CR aging treatment (100uC, 60 h). Keywords: Cryorolling, Al 6061 alloy, Ultrafine grained structure

Introduction

Experimental

Al 6061 alloy is used extensively for fabricating the automotive body sheets due its light weight, high formability, and fairly good corrosion resistance as compared to 2xxx and 6xxx Al alloys.1 The substantial improvement in mechanical properties of the Al alloys could be realised by refining its grain structure to the ultrafine regime as reported in the literature.2 Cryorolling is a novel route to produce ultrafine grained sheets of pure metals and alloys3,4 from its bulk alloys. An apt short annealing (SA) and aging treatment of the cryorolled (CR) Al alloys is expected to improve both its strength and ductility. Zhao et al.5,6 have reported the enhancement of strength and ductility in Al 7075 and Al 2024 alloys due its ultrafine grain structure and precipitate morphology. The effect of plastic deformation conditions on mechanical properties and microstructure of Al 6063 alloy has been reported in the authors’ earlier work.7 It has been observed that the precryorolled (pre-CR) solid solution treatment combined with the post-cryorolled (post-CR) aging (100uC for 44 h) treatment is the optimum processing condition to achieve the improved tensile strength (277 MPa) and good tensile ductility (11%) in this alloy. It is essential to identify the optimum annealing and aging conditions for achieving the superior mechanical properties in the Al 6061 alloy. Therefore, the main objectives of the present work are to study the influence of aging treatment and the combined treatment of short annealing and aging on the mechanical properties of CR Al 6061 alloy.

The high purity 6061 Al alloy, in ingot form with 250 mm diameter, with Al–0?8Mg–0?6Si–0?04Fe– 0?2Cu–0?012Mn–0?03Zn–0?02Cr (wt-%) was procured from HAL (Bangalore, India). It was machined to a thickness of 10 mm and then solutionised at 530uC for 120 min and quenched in water. These plates were rolled at liquid nitrogen temperature to a strain of 2?2. The details of cryorolling are discussed elsewhere.4,7 The CR and the un-CR samples upon solutionising treatment (ST) were subjected to artificially aging at 100uC before and after SA at 200uC for 5 min to study its influence on their mechanical properties. The microstructural characteristics of the CR Al 6061 alloy samples, peak static aged CR samples, and short annealed and aged CR samples were obtained by using electron backscattered diffraction (EBSD, field emission gun SEM Quanta) and TEM (Technai). The samples for EBSD characterisation were prepared by mechanical polishing up to 1000 grit emery paper, fine polishing to mirror finish with diamond paste, and then finally electropolished at 215uC using an electrolyte of methanol/perchloric acid (80 : 20) at 11 V dc. The EBSD measurements of the CR materials were performed at the centre of the samples with a scan area of 1606160 mm. However, the peak static aged CR samples, short annealed CR samples, and CR samples, subjected to short annealing and aging, were scanned with a scan area of 15615 mm. The step size of 0?1 mm was chosen for all the samples during the measurements. TSL OIM analysis 4?6 software developed by TEXSEM Laboratories Inc. was used to analyse the EBSD maps. The average confidence index after scanning of CR Al 6061 alloy samples, peak static aged CR samples, short annealed, followed by aged CR samples was 0?31, 0?38, 0?41 and 0?42 respectively. Since it is difficult to identify orientation of the grains at grain

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India *Corresponding author, email [email protected]

ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 2 February 2009; accepted 9 April 2009 DOI 10.1179/174328409X443227

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Mechanical properties of ultrafine grained Al 6061 alloy

1 a EBSD map and b TEM image of CR Al 6061 Al alloy (1–5 denotes ﬁve subgrains; two sets of arrows point out dislocation tangles)

boundaries where the patterns are often made up of two superposed diffraction patterns from both crystal lattices separated by the grain boundary, dilation clean up method from TSL software has been used to clean up the data at the grain boundaries. Tensile and hardness tests were carried out to evaluate the strength and ductility of the ST and CR Al 6061 alloys subjected to aging before and after SA. Vickers hardness (HV) was measured on the plane parallel to longitudinal axis (rolling direction) by applying a load of 5 kg for 15 s. An average of at least six readings on the surface of the specimen was taken to obtain a hardness value. The tensile specimens were machined as per ASTM E-8 subsize specifications parallel to the rolling direction with gauge length of 25 mm.

Results and discussion The EBSD scan/orientation image microscopy map of the CR material at a true strain of 2?2 is shown in Fig. 1a. The black and grey lines indicate the location of high (>15u) and low (1?5–15u) angle grain boundaries (higher fraction) respectively. The cryorolled sample shows severely fragmented and elongated grains along

the rolling direction with low aspect ratio. The heavily deformed grains with high density of dislocations (two sets of arrow marks shows the tangle of dislocations) are observed for CR Al alloy (true strain 2?2) shown in Fig. 1b. Only few subgrains (the numbers 1–5 of Fig. 1a denote five subgrains) are observed with clear grain boundaries, within 500 nm size. To increase the strength and ductility, the CR materials were aged at low temperature (100uC) for a prolonged period of time before and after SA at 200uC for 5 min. It has been reported in the authors’ earlier investigation4 that aging at 175 and 150uC of the CR Al 6063 alloys resulted in the decrease of hardness. Hence, a low temperature (100uC) aging of the samples was adopted in the present work and a comparative study of hardness versus aging time plot of CR Al 6063 alloy and CR Al 6061 alloy at the aging temperature of 100uC was made to find out if the similar trend of mechanical properties of 6063 alloy could also be observed in the 6061 alloy, as shown in Fig. 2a. In both CR materials, 10–15% increase in hardness is observed. The peak hardness of CR Al 6063 alloy and CR Al 6061 alloy is obtained at 48 and 60 h respectively. The improved hardness observed for the post-aging of the CR samples

2 Hardness versus ageing time of (a) CR Al 6063 alloy and CR Al 6061 alloy at a ageing temperature of 100uC, (b) CR and un-CR ST Al 6061 alloy after SA and ageing treatment at 100uC

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3 Images (EBSD) of CR Al 6061 alloy a aging at 100uC for 60 h; b SA at 200uC for 5 min and c SA and then aging at 100uC for 57 h

is due to precipitation hardening. Figure 2b shows the combined effect of SA and aging on hardness of CR and un-CR ST Al 6061 alloy. The CR material subjected to aging before and after SA exihibits a similar trend in the hardness. The precipitation hardening effect is only the dominating factor in un-CR material but both recovery and precipitation hardening manifests simultaneously in the CR materials. The colour coded orientation (EBSD) maps along with the [001] inverse pole figure of the CR 6061 alloy subjected to peak aging treatment (100uC for 60 h), SA treatment (200uC for 5 min) and a combined treatment of SA and aging treatment (200uC for 5 minz100uC for 57 h) are shown in Fig. 3. The microstructure of the peak aged sample is inhomogeneous, where some region


shows relatively high density of low angle grain boundaries and the other region appears as elongated lamellar with ultrafine grain structure. The grey line shows the low angle grain boundaries, which corresponds to the dislocation substructures. The higher fractions of dislocation substructures are still present in the CR material after the peak aging treatment. In the SA samples, the dislocation substructures transform into the subgrain boundaries as shown in Fig. 3b. As compared to the peak aged material, the fraction of the subgrain structures is higher in the short annealed material. The fraction of subgrain structures are higher in the CR material subjected to combined treatment of SA and peak ageing (Fig. 3c), as compared to the aged cryorolled samples (Fig. 3a). The microstructure of the CR materials subjected to annealing treatment before and after ageing (Fig. 3b & Fig. 3c) looks similar. The frequency histogram of the CR material subjected to SA treatment and aging treatment before and after SA is plotted in Fig. 4. The misorientation angles between 0 and 2u were not considered in order to prevent noise in the EBSD maps. The distribution of higher fractions of low angle misorientation boundaries is observed in CR material (Fig. 4a). The low angle boundaries are reduced for the CR material subjected to peak aging treatment, but the difference is less. In the CR materials subjected to SA treatment, and the combined treatment of SA and aging, a marked decrease in low angle misorientation boundaries but a corresponding increase in high angle boundaries is observed. The frequency histogram data of misorientation angles shows that the aging treatment at 100uC for 57 h is not sufficient to recrystallise the subgrain structure in the CR materials. The tensile test results for the CR materials subjected to SA treatment and peak aging condition before and after SA treatment are shown in Fig. 5. A large enhancement of yield strength (YS, 270 MPa) and ultimate tensile strength (UTS, 296 MPa) is observed in the CR material as compared to the ST material. The elongation to failure (ductility) has improved from 4?9 to 7?4, without the loss of strength, for the short annealed CR samples. The tensile results and EBSD micrograph of SA samples show that the SA of CR samples before aging treatment relieves the internal stress and realign the dislocation structures. The YS and UTS of the CR materials after peak aging have increased from 270 to 304 MPa and 296 to 360 MPa respectively, as compared to the CR materials. The CR materials subjected to SA and aging treatment shows an improved ductility without compromising the strength as compared to static aged CR materials. An increase of 16% UTS and 9% YS is observed in CR material subjected to SA and aging treatment than that under T6 treated condition.8 The pre-equal channel angular pressing solid solution treatment combined with post-equal channel angular pressing aging of Al alloys exhibited a significant improvement in strength but without improvement of ductility.9,10 However, in both of the post-CR aging treated materials, before and after SA, a large enhancement of both strength and ductility was observed. It may be due to the combined effect of dislocation strengthening, grain refinement, precipitation hardening and substructure coarsening.


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4 Frequency histograms of misorientation angles of CR Al 6061 alloy a as CR state, b aging at 100uC for 60 h, c SA at 200uC for 5 min and d SA and aging at 100uC for 57 h

subjected to post-CR aging treatment (100uC, 60 h) without annealing (ductility510 and tensile strength5360 MPa). The SA of CR samples relieves the internal stress and realigns the dislocations structures. The improvement of both strength and ductility in ultrafine grained Al 6061 alloy is due to the influence of dislocation strengthening, grain refinement, precipitation hardening and substructure coarsening.

References 5 Tensile properties of CR Al 6061 alloy subjected to (A) as CR state, (B) aging at 100uC for 60 h, (C) SA at 200uC for 5 min, (D) SA and then aging at 100uC for 57 h and (E) T6 condition

Conclusions The influence of aging and the combined effect of SA/ aging on the mechanical properties of CR Al 6061 alloy were investigated. A substantial improvement in ductility (11?5) without compromising the tensile strength (359?2 MPa) was observed for the CR Al 6061 alloy subjected post-CR SA (200uC, 5 min) followed by aging treatment (100uC, 57 h) than that of the similar samples

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