A study on the re-solution heat treatment of AA 2618 aluminum alloy

Materials Characterization 58 (2007) 312 317 A study on the re-solution heat treatment of AA 2618 aluminum alloy I brahim Özbek Department of Metallurgy and Materials, Sakarya Vocational High School, Sakarya University, Sakarya MYO, Adnan Menderes Cad., 54200 Sakarya, Turkey Received 3 May 2006; accepted 4 July 2006 Abstract In the present study, the effects of re-solution treatment of AA2618 aluminum alloy has been investigated. Solution heat treatments of 520 640 C for 14 24 h were applied followed by artificial aging. Characterization studies that were carried out by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques showed that recrystallisation was not observed by solution treatment at 530 C whereas it did occur as the solution treatment and the duration time were increased above 530 C. Increasing the solution treatment temperature further coarsened both the grains and the precipitates, resulting in significant reduction in hardness. Al 9 FeNi-type intermetallics are not completely dissolved by these solution treatments. 2006 Elsevier Inc. All rights reserved. Keywords: 2618 Aluminum alloy; Age hardening; Solution heat treatment; Recrystallisation 1. Introduction At the present time, technological demands for materials having high strength to weight ratio, high specific modulus, low coefficient of thermal expansion, good wear resistance, low density and good thermal conductivity are constantly increasing. Aluminum offers one answer for the combinations of such attractive properties. The aluminum alloy 2618, which contains copper and magnesium, is especially attractive due to its age hardenable properties. These alloys are used for applications involving high temperature exposures up to 300 C, such as engines for both automotive (engine cylinder heads, pistons Tel.: +90 264 2774001; fax: +90 264 2786518. E-mail address: iozbek@sakarya.edu.tr. etc.) and aircraft applications. This alloy was successfully used as the primary structure of the supersonic Concorde airplane [1,3]. The use of a solution treatment-plus-aging heat treatment for the 2618 aluminum alloy allows machining to a smoother finish than is the case for annealed or water quenched conditions. Solution heat treatment is done at 530 C for a period of time which may be extended up to 24 h for thick sections following by water quenching [4]. As a result of cold work, annealing may be required, and is usually limited to Table 1 The chemical composition of aluminum 2618 alloy, values are in wt.% Cu Mg Ni Fe Si Ti Mn Al 2.19 1.43 0.9 0.96 0.17 0.05 0.035 Balance 1044-5803/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.07.002

I. Özbek / Materials Characterization 58 (2007) 312 317 313 Table 2 Durations (h) and temperatures ( C) of the solution heat treatments Sample Solution heat treatment temperature, C Quenching no. 520 530 545 560 575 600 640 1 12 h Salt+ice 2 14 h 3 24 h 4 24 h +1/2 h 5 1h Water 6 1h +1h 7 1h +1h +1h 8 1 h +1 h +1 h +1/4 h approximately 385 C for 4 h followed by slow cooling. The aging heat treatment (T 61) is accomplished by first solution treating at 530 C for up to 24 h (depending on section size) followed by a quench in boiling water and subsequent aging at 200 C for up to 20 h (minimum of 5 h) dependent upon section size [2]. At the present study, selected properties of solution heat treated 2618 aluminum alloy were evaluated. The starting samples were extruded and thermally aged. Generally, some regions were not fully recrystallized, and all precipitates were not dissolved by the treatments used. Brinell hardness tests, optical and scanning electron microscopy, and energy dispersive X-ray spectroscopy were used to understand the reasons for the incomplete recrystallization and particle dissolution. Determining the optimum solution treatment conditions is a major consideration in the fabrication of 2618 alloy. Therefore, the present study was conducted to characterize the microstructures of 2618 aluminum alloy as a function of solution treatment conditions. 2. Experimental details Aluminum 2618 alloy used in this study was in the form of extruded angular bar with the chemical composition given in Table 1. The samples were cross-sectioned from the supplied bar having dimensions of 10 10 10 mm. The solution heat treatment program used for the samples is shown in Table 2. The heat treatments were performed in an electrical resistance furnace having temperature variation of ± 1 C under atmospheric Fig. 1. Optical microscopy cross-sectional view of non-treated 2618 aluminum alloy; (a) in the extrusion direction, (b) perpendicular to the extrusion direction. Fig. 2. Optical microscopy cross-sectional view of solution treated samples; (a) at 530 C for 24 h (sample 3), (b) at 530 C for 24 h +545 C for 1/2 h (sample 4).

314 I. Özbek / Materials Characterization 58 (2007) 312 317 4, respectively (see Table 2). It was noted that solution treatment at 530 C for 24 h. (sample 3) or an additional treatment of 545 C for 1/2 h. (sample 4) did not effect full dissolution of the precipitates in the microstructure. However, as the solution treatment temperature increased, the precipitates and the grains coarsened (Fig. 3). Increasing the solution treatment temperature to 560 C for 1 h (sample 5, Fig. 3a) or 560 C for 1 h +575 C for 1 h (sample 6, Fig. 3b) resulted in slight coarsening of the precipitates. Fig. 4a shows the microstructure after solution treatment of 560 C for 1 h + subsequent 1 h treatments at 575 and 600 C (sample 7). After this thermal exposure, most of the precipitates have further coarsened and are located at the grain boundaries. Fig. 4b shows the microstructure after a brief 1/4-h thermal excursion at 640 C; the occurrence of localized melting is apparent. The nature of the precipitates that formed in the matrix as fine and coarsened particles was examined by EDS analyses. In Fig. 5a, the precipitates are identified as particle 1 and particle 2 in the pre-solution treated alloy. Scanning electron microscopy (SEM) and energy Fig. 3. Optical microscopy cross-sectional view of solution treated samples; (a) at 560 C for 1 h (sample 5), (b) at 560 C for 1 h+575 C for 1 h (sample 6). pressure. The process temperature ranged from 520 C to 640 C while the cycle time ranged from 1 to 24.5 h followed by quenching in water or in water containing a mixture of salt and ice. All samples were then aged at 200 C for 2 h. The Brinell hardness values of the samples solution heat treated and artificially aged (aa) after the solution treatment were carried out on a universal hardness tester under 62.5 kg load with 2.5 mm diameter indenter. Optical microscopy examinations were carried out with an Olympus BHM-313 optical microscope. Energy dispersive X-ray spectroscopy (EDS) was also used to determine the distribution of elements in the particles that are present in the matrix. 3. Result and discussion Fig. 1 shows cross-sectional microstructures of asreceived 2618 aluminum alloy. In the as received samples, precipitates were evident both in the grains and along grain boundaries (Fig. 1). Fig. 2 depicts the microstructure after solution treatment for samples 3 and Fig. 4. Optical microscopy cross-sectional view of solution treated samples; (a) at 560 C for 1 h+575 C for 1 h+600 C for 1 h (sample 7), (b) as-melted sample after 1/4 h 640 C thermal treatment (sample 8).

I. Özbek / Materials Characterization 58 (2007) 312 317 315 Fig. 5. Original sample: (a) SEM back-scattered electron image (BEI), (b) matrix EDS spectrum, (c) particle 1 EDS spectrum, (d) particle 2 EDS spectrum. dispersive X-ray spectroscopy (EDS) analysis showed that the matrix consisted of a solid solution of Al, Cu and Mg, whereas both particles 1 and 2 consist of Al, Fe and Ni as given in Fig. 5b d. The results of similar analyses for sample 6 (solution treated 1 h at 560 C, plus an additional hour at 575 C, Table 2) are presented in Fig. 6a d. The results of these matrix and precipitate analyses are summarized in Table 3. The precipitates are believed to be Al 9 FeNi intermetallics. As the treatment temperature and duration time increased, a majority of the precipitates migrated to the grain boundaries and coarsened. However, as seen in Table 3, only rather modest changes in precipitate composition occurred. Other research [5 7] has reported that Cu and Mg contribute to the strengthening primarily through solid solution strengthening, whereas Ni and Fe form the intermetallic compound of Al 9 FeNi which contributes dispersion hardening in alloy 2618. The observations reported here are also consistent with research of [5 7] which report that Al 9 FeNi precipitates do not dissolve entirely in the matrix after solution treatment temperatures up to nearly 640 C for times up to 24 h. The Brinell hardness (HB) value of as-received 2618 aluminum alloy was measured to be 128 HB. The results of Brinell hardness measurements after solution treatment and solution treatment-plus-aging are shown in Fig. 7. Solution treatment alone decreased the hardness significantly, whereas after subsequent aging the hardness increased more or less to the original hardness of the as-received condition. Solution treatment of samples 1 and 2 decreased the hardness to 71 and 76 HB, respectively. This decrease can be attributed to the dissolution of residual S phase (a matrix hardening phase) that was present in the asreceived material. This result is in agreement with the work of Youdelis and Fang [8] who reported that maximum hardness is achieved by the presence of the S phase whereas the transformation of S phase to the S phase by solution treatment reduces the hardness

316 I. Özbek / Materials Characterization 58 (2007) 312 317 Fig. 6. Solution treated and aged sample 6: (a) SEM BEI, (b) matrix EDS spectrum, (c) particle 1 EDS spectrum, (d) particle 2 EDS spectrum. considerably. From Figs. 1 and 2, recrystallisation had not occurred during the lowest time-temperature heat treatments applied. As the solution treatment temperature and time was raised to 530 C for 24 h or over, recrystallisation occurs (see Fig. 2b, sample 4) resulting in an increase in hardness to 102 104 HB. Gilman and Sankaran [9] observed similar behavior in an Al 4Cu 1Mg 1.5Fe 0.75Ce alloy, where solution heat treating extrusions at 500 C for 1 h did not result in observable recrystallization. Merchant et al. [10] argued that in order to dissolve some intermetallics in Al Li Cu Mg alloys it is essential vital to apply temperatures of 530 C or higher, well above conventional aluminum homogenization treatments. However, in the present Table 3 SEM-EDS spectrum analysis of particles and matrix of sample 6 and original sample (at.%) Sample Al Ni Fe Cu Mg Si Formula of particles Original Matrix 97.46 1.40 1.14 Particle 1 77.50 10.71 9.07 1.64 0.08 Al 9 FeNi Particle 2 81.04 9.77 8.57 1.20 Al 9 FeNi Sample 6 Matrix 98.74 0.71 0.55 Particle 1 79.80 10.35 8.70 1.64 Al 9 FeNi Particle 2 80.73 9.28 9.32 0.67 Al 9 FeNi Fig. 7. Variation of the Brinell hardness values of the solution treated and aged samples.

I. Özbek / Materials Characterization 58 (2007) 312 317 317 work as the solution treatment temperature was increased to 560 C, grain coarsening was observed (Fig. 3) and as a result of this the hardness was considerably reduced. Further increase of the solution treatment temperature to 600 C, led to grain coarsening and a coarsening of the intermetallic particles at the grain boundaries (Fig. 4a). This coarsening reduced the hardness dramatically down to 48 HB. 4. Conclusions Recrystallisation was not observed in extruded and artificially aged 2618 aluminum alloy by solution treatment at 530 C for 14 h. Hardness was considerably reduced that was attributed to dissolution of S phase during artificial aging. Recrystallisation was observed at the 530 C solution treatment temperature when the duration time was increased to 24 h; this led to a substantial increase in hardness. Further increasing the solution temperature to 560 C coarsened both the grains and the precipitates, leading to a significant reduction in the hardness. The Al 9 FeNi-type intermetallic precipitates that form do not dissolve entirely in the matrix even when the solution temperature is increased to 600 C. Maximum hardness of the 2618 alloy was achieved by aging at 200 C after solution treatments at approximately 530 C for 24 h. Acknowledgements The author would like to thank Mr. Hüsnü ÖZTÜRK for helping with SEM study at ASSAN Aluminum A.Ş., and Prof. Dr. Fehim FINDIK for supplying of the 2618 aluminum alloy at Sakarya University. References [1] Metals Handbook, vol. 2. Properties and Selections: Nonferrous Alloys and Pure Metals, 10th Ed.; 1979. p. 81 2. [2] The European Aluminum Association (EAA) web pages (http:// www.eaa.net). [3] Williams James C, Starke Jr Edgar A. Progress in structural materials for aerospace systems. Acta Mater 25 November 2003;51(19):5775 99. [4] Cavaliere P. Hot and warm forming of 2618 aluminium alloy. J Light Metals Nov. 2002;2(4):247 52. [5] Polmear IJ. Light Alloys-Metallurgy of the Light Metals. 3rd ed. London: Arnold Publishers; 1981. p. 73 81. [6] Yu Kun, Li Wenxian, Li Songrui, Zhao Jun. Mechanical properties and microstructure of aluminum alloy 2618 with Al 3 (Sc, Zr) phases. Mater Sci Eng A Struct Mater Prop Microstruct Process 15 March 2004;368(1-2):88 93. [7] Youdelis WV, Fang W. Effect of beryllium on age hardening, defect structure, and S formation in Al 2.5Cu 1.2Mg alloy. Mater Sci Technol Dec. 1994;10:1031 41. [8] Testani C, Ielpo FM, Alunni E. AA2618 and AA7075 alloys superplastic transition in isothermal hot-deformation tests. Mater Des 1 Aug. 2000;21(4):305 10. [9] Gilman PS, Sankaran KK. Dispersion strengthening of precipitation hardened Al Cu Mg alloys prepared by rapid solidification and mechanical alloying. Proceedings of the Dispersion Strengthened Aluminum Alloy. Phoenix, Arizona, January 25 29, 1988. TMS Publication; 1988. p. 631 45. [10] Merchant HD, Kattamis TZ, Scharf G. Homogenization of aluminum alloys. Proceedings of the Homogenization and Annealing of Aluminum and Copper Alloys. Cincinnati, Ohio, October 12-13, 1987. The Metallurgical Society Publication; 1987. p. 1 53.