In this paper, the influences of cold rolling and annealing time on the microstructure and mechanical behaviors of AA 5052 aluminum alloy were investigated. An aluminum sheet was cold-rolled at room temperature at 15, 30, and 45 % rolling reduction. The 45 % cold-rolled sample was subsequently annealed at 370 °C for 2, 4, and 6 h. The microstructure was observed using optical microscopy whereas the mechanical behaviors were evaluated through hardness and tensile tests. It was found that the equiaxed grains are severely elongated along the rolling direction with increased rolling reduction. The hardness and strength increase significantly with an increase of rolling reduction but the ductility decreases. The annealing treatment reduces hardness and strength but improves the ductility

C. Bora

Kusmono

U. A. Salim

English

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485METALURGIJA 59 (2020) 4, 485-488KUSMONO, C. BORA, U. A. SALIMEFFECTS OF COLD ROLLING (CR) AND ANNEALING TIME ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AA 5052 ALUMINUM ALLOY Received – Primljeno: 2020-05-30Accepted – Prihvaćeno: 2020-07-12Original Scientific Paper – Izvorni znanstveni radKusmono (kusmono@ugm.ac.id), C. Bora, U.A. Salim, Department of Mechanical and Industrial Engineering, Faculty of Engineering, Uni-versitas Gadjah Mada, Jl. Grafi ka No.2, Yogyakarta, 55281, IndonesiaIn this paper, the influences of cold rolling and annealing time on the microstructure and mechanical behaviors of AA 5052 aluminum alloy were investigated. An aluminum sheet was cold-rolled at room temperature at 15, 30, and 45 % rolling reduction. The 45 % cold-rolled sample was subsequently annealed at 370 °C for 2, 4, and 6 h. The mi-crostructure was observed using optical microscopy whereas the mechanical behaviors were evaluated through hardness and tensile tests. It was found that the equiaxed grains are severely elongated along the rolling direction with increased rolling reduction. The hardness and strength increase significantly with an increase of rolling reduc-tion but the ductility decreases. The annealing treatment reduces hardness and strength but improves the ductility. Keywords: AA 5052; cold rolling; annealing; microstructure; mechanical properties  INTRODUCTIONDue to its high strength-to-weight ratio, excellent corrosion resistance, toughness, and good weldability, the non-heat treatable AA 5052 aluminum alloy has been extensively used in various industries such as au-tomotive, marine and construction industries [1]. How-ever, this alloy is not strong enough for structural mate-rial applications [2]. It has been well- documented that there are two famous methods to enhance the strength of non-heat-treatable Al-Mg alloy, namely work hard-ening or strain hardening and micro-alloying [3]. Cold rolling is one of the most widely used strain hardening methods that improves the strength and hardness sig-nificantly but reduces the ductility of A-Mg alloy due to the increased dislocation density [4].It is well-known that the annealing heat treatment is one of the most used methods to enhance the ductility of cold-rolled aluminum alloys [5]. Several factors play a role in the success of annealing such as temperature, holding time, and heating rate of annealing. Numerous studies have been conducted on enhancing the mechan-ical properties of non-heat treatable Al-Mg alloy using strain hardening methods followed by annealing. It was reported that strain hardening methods such as accumu-lative roll-bonding (ARB), cryogenic rolling [6], and warm rolling [7] subjected to the aluminum alloy can drastically improve strength and hardness but decrease ductility. So far, investigations on the mechanical prop-erties of cold-rolled 5052 aluminum alloy followed by annealing were still limited.ISSN 0543-5846METABK 59(4) 485-488 (2020)UDC – UDK 669.715:621.771:621.785:620.18.620.17=111In this experiment, the AA 5052 aluminum alloy sheet was first cold-rolled under different rolling reduc-tions, namely 15, 30, and 45 %. The 45 % reduction cold-rolled AA 5052 sheet was then annealed at 370 °C at various holding times: 2, 4, and 6 h. The influences of cold rolling and annealing time on the microstructure and mechanical properties of AA 5052 alloy are dis-cussed in this article.  MATERIALS AND EXPERIMENTAL METHODSA commercial AA 5052 alloy sheet with a thickness of 3 mm was used in this experiment. Its nominal com-position was 2,37 % Mg, 0,31 % Fe, 0,22 % Cr, 0,08 % Mn, 0,11 % Si, and with the balance of Al (all in wt%). The sheet was first homogenized at 470 °C for 6 h with a heating rate of 5 °C/min and then cooled in air. The homogenized sheet was cold-rolled at room tempera-ture with 15, 30, and 45 % rolling reductions. The 45 % cold-rolled sample was then annealed at 370 °C at vari-ous holding times of 2, 4, and 6 with the heating rate of 5 °/min.The microstructures of as-received, cold-rolled, and annealed samples were examined using optical micros-copy on the short transverse and longitudinal (rolling) directions. The specimens were made using the stand-ard metallographic procedures such as grinding, polish-ing, and etching using Keller’s reagent. Mechanical behaviors of as-received, rolled, and an-nealed samples were determined through hardness and tensile tests. Vickers microhardness was measured us-ing a Boehler Micromet hardness tester under a load of 50 g and a loading duration of 15 s. The strength and elongation were assessed through a tensile test using a 486KUSMONO et al.: EFFECTS OF COLD ROLLING (CR) AND ANNEALING TIME ON MICROSTRUCTURE... METALURGIJA 59 (2020) 4, 485-488servo-hydraulic universal testing machine. All hardness and tensile specimens were taken along the rolling di-rection.RESULTS AND DISCUSSIONThree main directions in AA 5052 aluminum alloy with the appropriate microstructure are presented in Figure 1a. The microstructures of the α(Al) matrix and a large number of second phases dispersed within the grain are observed in all samples. Figure 1b shows the microstructure of the as-received sample in a longitudi-nal direction where large equiaxed grains with no pre-ferred orientation can be observed. The elongated grain is exhibited in the microstructures of as-received and rolled samples in a short transverse direction indicating the characteristic of rolled alloy structure (Figure 1c-f). The cold-rolling reduces the grain size of the AA 5052 alloy. The fine-grain alloy after cold rolling may be at-tributed to the increased number of nucleation sites when the higher rolling reduction is applied [8].   The optical microstructures in the short transverse di-rection of the rolled sample with a reduction of 45 % and rolled samples subjected to annealing for 2, 4, and 6 h are shown in Figure 2. The elongated grains can still be dom-inantly observed in the rolled sample with a reduction of 45 % and rolled samples subjected to annealing for 2 and 4 h (Figure 2a, b, c). Moreover, severe equiaxed grains as a result of recrystallization during annealing are also shown in the structure of rolled samples followed by an-nealing for 2 and 4 h. On the other hand, more equiaxed grains are exhibited in the microstructure of the rolled sample followed by annealing for 6 h (Figure 2d). This indicates that annealing at 370 °C for 6 h has changed the elongated grain structures to become the equiaxed grains. This may be associated with the recrystallization phe-nomenon that occurred in the rolled AA 5052 alloy fol-lowed by annealing at 370 °C. The hardness values of as-received and rolled AA 5052 alloy at various rolling reductions are presented in Figure 3. It is obvious that the hardness was significant-ly increased with rolling reduction. This may be attrib-uted to three reasons as follows [9,10]. Firstly, the cold rolling results in the decrease in grain size, and the grain boundary increases with an increase of rolling reduc-tion. The smaller grain size is more effective at inhibit-ing the dislocation movement and multiplication result-ing in enhanced hardness. Secondly, the density of dis-locations was remarkably increased during cold rolling. Furthermore, the existing dislocations hinder the nu-cleation and movement of new dislocations leading to the increased hardness of this material. Thirdly, the de-velopment of texture orientation due to cold rolling has Figure 1  (a) Three main directions in AA 5052 sheet with    corresponding microstructures: (b) As-received sample in longitudinal direction (c) As-received sample (d) 15 % reduction rolled sample (e) 30 % reduction rolled sample (f ) 45 % reduction rolled sampleFigure 2  Optical micrographs of (a) rolled sample, (b) rolled sample followed by annealing for 2 h, (c) rolled sample followed by annealing for 4 h, (d) rolled sample followed by annealing for 6 h020406080100As-recevied 15 30 45Hardness / VHNRolling reduction / %Figure 3 Variation of hardness with different rolling reduction 487KUSMONO et al.: EFFECTS OF COLD ROLLING (CR) AND ANNEALING TIME ON MICROSTRUCTURE...METALURGIJA 59 (2020) 4, 485-488an effective role in improving the hardness and mechan-ical properties of AA 5052 alloy. The influence of annealing time on the hardness of the 45 % rolled sample is presented in Figure 4. The hardness of all rolled and annealed samples is lower than both as-received and rolled samples. This means that the annealing treatment drastically reduces the hardness of rolled AA 5052 alloy. This is ascribed to recrystallization leading to coarsening and growth of grains [5,11]. The dislocation density decreased due to recrystallization during annealing resulting in the re-duced hardness of rolled AA 5052 aluminum alloy. From Figure 4, it can be also observed that hardness is unchanged with an increase in annealing holding time. This means that an increase in annealing time did not influence the hardness. Full recrystallization of grains that occurred after an annealing time of 2 h may be re-sponsible for the unchanged hardness with increasing annealing time.Figure 5 displays the yield and tensile strength of as-received and rolled AA 5052 aluminum alloy under dif-ferent rolling reductions. It was found that the yield and tensile strength of the rolled 5052 aluminum alloy were higher than as-received. This implies that cold rolling im-proved the strength of AA 5052 aluminum alloy. This is in agreement with the hardness results as demonstrated in Figure 3. Figure 6 presents the elongation of as-received and rolled AA 5052 aluminum alloy under different roll-ing reduction. The ductility was decreased by increasing annealing holding time. This is especially associated with the more obvious effect of work hardening and an in-crease in the dislocation density [12]. Besides, by in-creasing the rolling reduction, the grains become very elongated through the rolling direction, which forms the fiber texture and finally results in increased tensile strength and reduced ductility [5]. Figure 7 depicts the influence of annealing time on the yield and tensile strength of AA 5052 aluminum al-loy. It is clear that the yield and tensile strength were drastically reduced after annealing at 370 °C for 2 h. This reduction might be attributed to a larger decrease in dislocation densities during annealing at 370 °C for 2 h. However, the annealing holding time for 4 and 6 h does not affect the strength. On the other hand, it can be observed in Figure 8 that elongation significantly in-creases after annealing for 2 h but further annealing time has no effect. The rearrangement of dislocation and reduction of dislocation densities during recrystal-020406080100As-recevied CR-45 % 2 4 6Hardness / VHNAnnealing time / hFigure 4 Variation of hardness with different annealing timeFigure 5  Variation of yield and tensile strength with different rolling reductionFigure 6  Variation of elongation with different rolling reductionFigure 7  Variation of yield and tensile strength with different annealing time0510152025As-received 15 30 45Elongation / %Rolling Reduction / %488KUSMONO et al.: EFFECTS OF COLD ROLLING (CR) AND ANNEALING TIME ON MICROSTRUCTURE... METALURGIJA 59 (2020) 4, 485-488lization in annealing treatment might be responsible for the increase in elongation [13]. From Figures 7 and 8, it was found that the strength and ductility were un-changed with increasing annealing time. CONCLUSIONSThe inf luences of cold rolling and annealing time on the microstructure and mechanical properties of AA 5052 aluminum alloy was studied. Results displayed the equiaxed grains are severely elongated along the rolling direction by increasing rolling reduction. The elongated microstructures are changed to equiaxial structures due to recrystallization after annealing for 6 h. With an increase in rolling reduction, the hardness and strength enhance significantly but the ductility de-creases drastically. Annealing treatment leads to a re-duction in the hardness and strength but enhances the ductility. Increasing annealing time does not affect the hardness and strength of AA aluminum alloy.AcknowledgmentsThe authors would like to thank Universitas Gadjah Mada for the financial support of this work through the research grant of Rekognisi Tugas Akhir (RTA) Pro-gram 2019 (contract no. 2129/UN1/DITLIT/DIT-LIT/LT/2019). REFERENCES[1] S. L. Xia, M. Ma, J. X. Zhang, W. X. Wang, W. C. Liu. 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Won, Effect of annealing temperature on microstructures and mechani-cal properties of a 5083 Al alloy deformed at cryogenic temperature, Scripta Mater 51(2004), 355-359.[12] J. Zhang, M. Ma, F. Shen, D. Yi, B. Wang. Influence of deformation and annealing on electrical conductivity, me-chanical properties and texture of Al-Mg-Si alloy cables, Materials Science & Engineering A 710(2018), 27-37.[13] J. Huang, K. M. Zhang, Y. F. Jia, C. C. Zhang, X. C. Zhang, X. F. Ma, S. T. Tu. Effect of thermal annealing on the micro-structure, mechanical properties and residual stress relaxa-tion of pure titanium after deep rolling treatment, Journal of Materials Science and Technology 35(2019), 409-417.Note:  The responsible English translator is Emily Peterson - the Lan-guage Center, Syiah Kuala University, IndonesiaFigure 8 Variation of elongation with different annealing time051015202530As-received CR-45 % 2 4 6Elongation / %Annealing time / h

Effects of cold rolling (CR) and annealing time on microstructure and mechanical properties of AA 5052 aluminum alloy

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