Effect of Solution Treatment Temperature on Microstructure and Mechanical Properties of Al-5.1Zn-2Mg-0.1Ti (wt. %) Produced by

Squeeze Casting WIJANARKO Risly, ANGELA Irene, and SOFYAN Bondan Tiaraa

Department of Metallurgy and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok 16424, Indonesia

abondan@eng.ui.ac.id

Keywords: Al-Zn-Mg, Ti, solution treatment, second phase dissolution, precipitation hardening

Abstract. Al 7xxx alloy is a heat treatable Al alloy with superior strength. Solution treatment in precipitation hardening sequence of the alloy has an important role to dissolve second phases and bring vacancies out to form precipitates in the ageing process. Another strengthening can be done by Ti addition as grain refiner. As cast alloy by squeeze casting was homogenized at 400 °C for 4 h. Solution treatment was conducted at 220, 420, and 490 °C, followed by rapid quenching. Subsequent ageing was conducted at 130 °C for 48 h. Characterization was performed by optical microscope, SEM-EDS (Scanning Electron Microscopy – Energy Dispersive Spectroscopy), Rockwell hardness testing, XRD (X-Ray Diffraction), and STA (Simultaneous Thermal Analysis). Ti added alloy showed rounder grains, lower hardness, and more reduction in second phase volume fraction along with increasing solution treatment temperature than those in alloys without Ti addition. Otherwise, the alloy final hardness was increasing and higher after the ageing process due to higher second phase dissolution which leads to more precipitates formed.

Introduction

Al-Zn-Mg (Al 7xxx) is one of heat treatable Al alloys which has high strength. The special strengthening characteristic of Al 7xxx alloy is the formation of η (MgZn2) phase after the ageing process. Syakuura [1] performed precipitation hardening to the alloys with Zn:Mg ratio of 1:1 with resulting the formation of β (Mg3Zn3Al2) and η phases. Al-Zn-Mg alloys are commonly treated by T6 precipitation hardening with the sequences of solution treatment, quenching, and artificial ageing. Solution treatment is necessary to dissolve the second phases to the matrix (α-Al) and form vacancies, which subsequently beneficial to the formation of precipitates after ageing process [2]. Increasing solution treatment temperature lead to higher second phase dissolution and vacancies formation, but overly high temperature reduced the mechanical properties [3]. Fan, et al. [4] found that interdendritic volume of Al-Zn-Mg decreased 1.01 % and 2 % after solution treatment at 450 and 475 °C. This supported earlier work by Lu, et al [5] that second phases were decreasing as the solution treatment temperature increasing from 430 to 490 °C. However, the solution treatment done by Putra [6] to Al-5.1Zn-1.9Mg alloy at 220 °C resulted in a similar interdendritic volume with alloy before solution treatment. It was due to the dissolution process was started at 277 °C [7].

Strengthening of Al alloy also can be achieved by the addition of grain refining, such as Ti from Al-5Ti-1B master alloy [8]. Generally, the grain refinement mechanisms done by Ti are grain growth restriction by solute Ti, heterogeneous nucleation, and peritectic reaction [8]. Optical microscopy by Mostafapoor [9] showed finer and even grains after the addition of 0.05 wt. % Ti, while alloy without Ti addition had dendritic structure. Alipour, et al. [10] found that grain size reduction was performed up to 0.1 wt. % Ti addition and higher addition resulted to constant grain size. However, the study on the effect of Ti addition combined with treatment by various solution treatment temperature is still very limited. In this study, we aimed to get a deep understanding of the Ti added Al-Zn-Mg alloy and its behavior during solution treatment at different temperatures.

Materials Science Forum Submitted: 2018-08-01ISSN: 1662-9752, Vol. 939, pp 38-45 Accepted: 2018-09-06doi:10.4028/www.scientific.net/MSF.939.38 Online: 2018-11-20© 2018 Trans Tech Publications, Switzerland

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TransTech Publications, www.scientific.net. (#110928854-16/11/18,18:48:30)

Materials and Method Materials used to produce Al-5.1Zn-2Mg-0.1Ti (wt. %) were pure Al (99.76 %), Zn (99.99 %), Mg (99.9 %) ingots, and Al-5Ti-1B master alloy rods. The materials were melted in a graphite crucible at 800 °C and Ar degassed for 2 minutes during stirring. The melts were poured into a preheated metal mould and squeeze cast at 76 MPa for 10 min. The nominal composition of the alloy is listed in Table 1. Homogenization was then conducted at 400 °C for 4 h. Samples were cut, solution treated at 220, 420, and 490 °C, water quenched, and aged at 130 °C up to 48 h. Characterization was done through compositional analysis using ARL 3460 Optical Emission Spectroscopy (OES), Rockwell E and B hardness testing in accordance with ASTM E18 standard, X-Ray Diffraction testing using Philips PW3719, and quantitative image analysis by MagniSci. The images were examined on QUANTA 650 Scanning Electron Microscope – Energy Dispersive Spectroscopy (SEM-EDS) and Carl Zeiss Primotech optical microscope with metallographic preparation using Keller’s reagent (5 ml HNO3, 1.5 ml HCL, 1 ml HF, and 95 ml aquadest). To investigate the phase transformation, Simultaneous Thermal Analysis (STA) was conducted using 6000 Parkin Elmer.

Table 1. Chemical composition of the sample (wt. %). Al Zn Mg Ti Fe Si

Rem. 5.0747 1.9796 0.1007 0.0870 0.0302

Results and Discussion

Microstructure and Hardness before Solution Treatment. In as-cast condition, Al-5.1Zn-2Mg-0.1Ti was compared to a similar Al alloy without Ti addition, Al-5.1Zn-1.9Mg [6], to differentiate the characteristics. Both alloys consisted of second phases which were showed in dark contrast in Fig. 1. Fig. 1 (a) shows dendritic structures of alloy without Ti addition with the SDAS (Secondary Dendrite Arm Spacing) average of 37.5 µm and second phase volume fraction of 10.53 %. Meanwhile, the addition of 0.1 wt.% Ti led to fine and equiaxed grains with the size of 103.4 µm and 10.76 % of second phase volume fraction, as can be seen in Fig. 1 (b). Furthermore, the hardness value of Al-5.1Zn-2Mg-0.1Ti was 47.1 HRB which lower than Al-5.1Zn-1.9Mg [7] with hardness of 53.5 HRB. It can be clearly seen that the studied alloy structure underwent inconsiderable grain refinement, but grain rounding. It even possessed higher grain size when compared with higher Ti content Al alloy, Al-5.1Zn-1.8Mg-0.4Ti [11], which grain size was 67.3 µm. This phenomenon was due to the lack of solute Ti which subjected to segregate and restrict the growth of grain. It was predicted that the refinement of Al-5.1Zn-2Mg-0.1Ti mainly came from the heterogeneous nucleation by Ti which can induce constitutional supercooling. Meantime, Fig. 1 (c) presents the structure of as-homogenized Al-5.1Zn-2Mg-0.1Ti which second phase volume fraction was 7.07 %. It was lower than the second phase volume fraction in as-cast condition due to the dissolution process of second phases. The studied alloy underwent dissolution process at 282 °C, based on Mondolfo [7], while the homogenization temperature was 400 °C. Moreover, the hardness obtained after homogenization process was 42.46 HRB, which was lower than the as-cast condition.

Figure 1. Microstructures of (a) as-cast Al-5.1Zn-1.9Mg [6], (b) as-cast Al-5.1Zn-2Mg-0.1Ti, and (c) as homogenized

Al-5.1Zn-2Mg-0.1Ti alloy.

a b c

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The distribution of elements in as-homogenized structure can be seen in Fig. 2 (a-d). It can be identified that Zn and Mg elements, which were showed in yellow and purple contrast, were uniformly distributed in α-Al matrix. Further, Ti element was also distributed in the α-Al matrix for which role was as nucleants. Meanwhile, aggregation of TiAl and TiAl3 were predicted at grain boundaries which aimed to restrict the grain growth. The phases were then clarified with SEM micrograph shown in Fig. 2 (e) and the EDS results in Table 2. α-Al matrix was pointed out by point 1. Furthermore, point 2 indicated the second phases (T and β) and TiAl3. At point 3, higher Ti content which also proportional to the Al content can be detected. This proved that there were TiAl segregation, which in blocky shape [12], in the grain boundaries. However, most of Ti element was dissolved into the matrix to be α–Al nucleants.

Figure 2. As-homogenized Al-5.1Zn-2Mg-0.1Ti alloy: (a-d) X-ray mapping and (e) SEM micrograph.

Table 2. EDS data at positions as shown in Fig. 2 (e).

No. Element content (at. %) Phase Al Zn Mg Ti O 1. 90.89 2.40 2.13 - 4.57 α–Al 2. 13.83 3.15 2.73 7.25 - T, β, TiAl3 3. 33.04 3.67 1.87 36.65 16.66 T, β, TiAl

Microstructure and Hardness after Solution Treatment. Fig. 3 and Fig. 4 present the microstructures of Al-5.1Zn-2Mg-0.1Ti alloy after solution treatment with various temperatures and subsequent quenching. Alloy with solution treatment temperature of 220 °C showed similar structure with as-homogenized alloy. It was due to the dissolution of second phases did not occur at this temperature. Hence, the second phase volume fraction at this condition showed similar value, which was 6.74 %, with as-homogenized condition. Meantime, the EDS results in Table 3 showed α–Al matrix at point 4, second phases (T and β) at point 5, and blocky TiAl at point 6.

Meantime, second phase dissolution was occurred at solution treatment temperature of 420 and 490 °C, which can be seen in Fig. 3 (b) and (c), respectively. It can be observed that the second phases in dark contrast seemed to be faded in both figures. At temperature of 420 °C, the second phase volume fraction was 3.50 %, which was lower than the previous condition. Furthermore, second phase dissolution was much higher at 490 °C, which resulted in 2.75 % of second phase volume fraction. Meanwhile, the EDS results present the presence of TiAl3 and TiAl particles in the alloy with solution treatment temperatures of 420 and 490 °C, which pointed out by point 8, 11, and 12. It concluded that both particles stayed in the grain boundaries while the dissolution process occurred.

Figure 3. Microstructures of Al-5.1Zn-2Mg-0.1Ti after solution treatment at (a) 220, (b) 420, and (c) 490 °C.

Al Kα1 Zn Lα1_2 Mg Kα1_2 Ti Kα1 a b c d e

a b c

1

2 3

40 Advanced Manufacturing and Materials

Figure 4. SEM images of Al-5.1Zn-2Mg-0.1Ti after solution treatment at (a) 220, (b) 420, and (c) 490 °C.

Table 3. EDS data at positions shown in Fig. 4.

No. Element content (at. %) Phase Al Zn Mg Ti O Fe 4. 88.55 2.82 2.04 - 3.65 - α–Al 5. 68.35 1.54 1.08 - 7.69 16.24 T and β 6. 12.25 0.26 0.58 12.40 2.35 0.47 TiAl 7. 90.65 3.06 2.20 - 4.10 - α–Al 8. 89.54 3.20 2.77 0.28 4.20 - T, β, TiAl3 9. 79.79 2.67 0.88 0.09 - 16.57 T

10. 81.28 2.37 1.78 0.29 6.01 - α–Al 11. 53.80 0.56 0.21 0.34 3.59 13.04 TiAl3

12. 12.85 0.54 0.25 11.90 4.24 - TiAl

Second phase volume fraction reduction led to a decrease in mechanical properties, as can be seen in Fig. 5. The hardness value of Al-5.1Zn-2Mg-0.1Ti alloys with solution treatment temperature of 220, 420, and 490 °C were 41.68 HRB, 28 HRE, and 18.2 HRE, respectively. A great second phase dissolution developed at 420 and 490 °C was giving significance decrease on the hardness values. The trend was due to that the dissolution caused the α-Al matrix to be super-saturated solid solution (SSSS). At this point, the alloy was relatively soft and weak [2].

Figure 5. Hardness values of Al-5.1Zn-2Mg-0.1Ti before and after solution treatment with subsequent quenching.

XRD Characteristics. The XRD pattern of the studied alloy is shown in Fig. 6. As illustrated in Fig. 5, only α–Al matrix peaks that can be easily found. It was due to the fact that the alloy consisted of very low alloying elements. Hence, the phase transformation in the studied alloy presented unclear peaks data. According to data, β phase was detected at 36.4 – 36.6 ° and 40 – 41 °, while T phase at 37 – 38 °. After solution treatment at 420 and 490 °C, T and β phases dissolved into α–Al matrix, which made it hard to be detected by XRD analysis. TiAl3 was also detected in every condition, as can be seen in Fig. 5 (c). However, due to a slightest content of Ti in the studied alloy, the resulted peak was noticeably small.

a b c

4

5

6

7

8 9

10

11

12

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Figure 6. XRD pattern of Al-5.1Zn-2Mg-0.1Ti at (a) 20 – 90 °, (b) 35 – 44 °, and (c) 68 – 71 ° after homogenization and solution treatment with subsequent quenching.

Effect of Ti during Solution Treatment. As is well known, the studied alloy possessed heterogeneous nucleation with low grain growth restriction, thus whose grains were equiaxed and quite big. In fact, the structure affected the second phase dissolution process as can be seen in Fig. 7. The studied alloy was compared with Al-5.1Zn-1.9Mg [6] and showed analogous trend since as-cast condition until solution treatment with temperature of 220 °C. As the temperature of solution treatment increasing, it can be observed that second phase volume fraction of studied alloy was lower than alloy without Ti addition. The grain rounding by Ti caused to a low surface tension at α–Al matrix interface so that the dissolution kinetics were increasing [13]. Meanwhile, the second phase dissolution kinetics of Al-5.1Zn-1.9Mg [6] were delayed as a result of the dendritic structure which caused to high surface tension.

Figure 7. Second phase volume fraction of Al-5.1Zn-1.9Mg [6] and Al-5.1Zn-2Mg-0.1Ti at as-cast condition up

to after solution treatment with subsequent quenching.

Microstructure and Hardness after Ageing. Fig. 8 illustrates the microstructures of Al-5.1Zn-2Mg-0.1Ti alloy after ageing process up to 48 h at 130 °C. Dissolved second phases formed precipitates of η (MgZn2) phase as the Zn : Mg ratio was approximately 1. However, the precipitates were generally undiscovered in the microstructures because they were very small and finely dispersed. Over all condition, the microstructures were comparable to the alloys after solution treatment. The presence of precipitates can be clarified by the hardness values shown in Fig. 9.

c)

a) b)

[this work]

42 Advanced Manufacturing and Materials

Figure 8. Microstructures of Al-5.1Zn-2Mg-0.1Ti after solution treatment at (a) 220, (b) 420, and (c) 490 °C, followed by

ageing at 130 °C for 48 h.

The hardness testing of studied alloy with various solution treatment temperature after ageing process at 130 °C were examined every 12 h up to 48 h, which results can be seen in Fig. 8. As can be seen at 220 °C, the studied alloy showed similar value, which is 49.18 HRB, with that of the as-quenched one, 41.68 HRB. Meanwhile, the hardness of studied alloy was significantly increased to 52.46 HRB after ageing with solution treatment temperature of 420 °C. Furthermore, the values were higher than Al-5.1Zn-1.9Mg [6]. It was indicated that higher amount of dissolved second phases led to higher precipitates formation, which increased the age strengthening effect. At solution treatment temperature of 490 °C, the hardness value was much higher than studied alloys at lower solution treatment temperature. Aside from the higher amount of precipitates formed, solution treatment at 490 °C also produced vacancies at great amount of 67.18 x 1022. Meantime, the number of vacancies at 220 and 420 °C were 0.13 x 1022 and 21.23 x 1022, respectively [2]. Those vacancies were then being the site for the formation of precipitates. The hardness was 70.98 HRB, while Al-5.1Zn-1.9Mg [6] was 66.76 HRB.

Figure 9. Hardness values Al-5.1Zn-2Mg-0.1Ti compared with Al-5.1Zn-1.9Mg [6] after ageing at 130 °C for 48 h.

Phase Transformation Analysis with STA. Fig. 10 shows the STA heat flow curves and their

derivatives of Al-5.1Zn-2Mg-0.1Ti alloy at various solution treatment temperatures. As is well known, the precipitate which predicted to be formed is η phase. It can be easily found that reaction occurred in studied alloy with solution treatment temperature of 220 °C was only the dissolution of T and β phases. However, the dissolution temperature was higher than prediction by Mondolfo [7], which is at 282 °C. It was due to the low grain surface tension thus the second phases were much easier to occur [13]. The studied alloy encountered the precipitates formation at solution treatment temperature of 420 and 490 °C, as can be seen in Fig. 8 (b) and (c). There were 3 peaks of exothermic reaction and 2 peaks of endothermic reaction, which represented formation and dissolution reaction, respectively.

b c

[6]

[6]

[6]

a

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Figure 10. Heat flow curves of Al-5.1Zn-2Mg-0.1Ti wt. % alloy after solution treatment at (a) 220, (b) 420

and (c) 490 °C.

At solution treatment temperatures of 420 and 490 °C, the studied alloy sustained a good precipitation process for there were the reactions of GP zone, η’ phase, and η phase formation and dissolution at certain temperatures. Compared to Al-5.1Zn-1.9Mg [6], as can be seen in Table 4, phase transformation of the studied alloy was occurred at lower temperature. It was due to higher amount of dissolved second phase which caused the alloy became more saturated and led to a lower free energy for precipitation [14]. The trend also appeared at the comparison between the studied alloy with solution treatment temperature of 420 and 490 °C, where the amount of dissolved second phases was higher at 490 °C.

Table 4. Phase formation and dissolution temperature (°C) of Al-5.1Zn-1.9Mg [6] and Al-5.1Zn-2Mg-0.1Ti at solution treatment temperature of 420 and 490 °C.

Phase Al-5.1Zn-1.9Mg [6] Al-5.1Zn-2Mg-0.1Ti Formation Dissolution Formation Dissolution

Solution treatment temperature at 420 °C GP Zone 50.14 131.04 48.44 110.18 η' 153.98 230.11 150.08 226.63 η 273.90 340.10 269.67 350 Solution treatment temperature at 490 °C GP Zone 50.02 110.50 48.38 90.34 η' 120.02 230.46 120.91 153.95 η 253 341.11 230 295.47

Summary Investigation on Al-5.1Zn-2Mg-0.1Ti alloy at various solution treatment temperature resulted in the following conclusion:

1) The addition of 0.1 wt. % Ti promoted an alloy with 103.4 µm of equiaxed grains without considerable grain reduction and hardness value of 47.1 HRB. The hardness decreased to 42.46 HRB after homogenization.

(b) (a)

270.72 °C

48.44 °C

110.18 °C

150.08 °C

226.63 °C

269.67 °C

350 °C

48.38 °C 120.91 °C

153.95 °C

230 °C

295.47 °C

90.34 °C

(c)

44 Advanced Manufacturing and Materials

2) Solution treatment at the temperature of 420 and 490 °C significantly decreased the second phase volume fraction from 7.07 % at as-homogenized condition to 3.50 and 2.75 %, respectively. The resulted hardness at solution treatment temperature of 220, 420, and 490 °C were 41.68 HRB, 28 HRE, and 18.2 HRE, successively.

3) The grain rounding by 0.1 wt. % Ti decreased the surface tension thus dissolution of second phases could easily occur. Aggregated TiAl and TiAl3 phase were undissolved to the matrix during solution treatment.

4) Ageing process extremely increased the hardness values, especially in studied alloys with solution treatment temperature of 420 and 490 °C, to 52.46 and 70.98 HRB, respectively. The hardness of the studied alloy showed higher values than alloy without Ti addition.

5) Based on STA analysis, precipitation was occurred in studied alloys with solution treatment temperature of 420 and 490 °C. Higher amount of dissolved second phases decreased the phase formation and dissolution temperatures.

Acknowledgements This work was supported by Directorate of Research and Community Services, Universitas Indonesia through Hibah PITTA (Publikasi Internasional Terindeks untuk Tugas Akhir Mahasiswa) UI 2018.

References

[1] A. Syakuura, B. T. Sofyan, and S. P. Ringer: Majalah Ilmu dan Teknologi – Metalurgi Vol. 27(2) (2012), pp. 85 – 94

[2] W, Callister and D. G. Rethwisch: Materials Science and Engineering An Introduction – Eight Edition (John Wiley and Sons Inc, Hoboken, 2009)

[3] D. A. Porter, K. E. Easterling, and M. Y. Sherif: Phase transformations in metals and alloys (CRC Press, Boca Raton, 2009)

[4] Y. Fan, K. Wen, Z. Li, X. Li, Y. Zhang, B. Xiong, and J. Xie: Rare Metals Vol. 36 (2017), pp. 256-262

[5] J. T. Lu, H. Huang, H. Wu, S. P. Wen, K. Y. Gao, and Z. R. Nie: Mat. Sci. Forum Vol 887 (2016), pp. 303-309

[6] D. P. Putra and B. T. Sofyan: IOP Proceeding (2018) in-press. [7] L. F. Mondolfo: Aluminium Alloys: Structure and Properties (Butterworth & Co Ltd, Boston,

1976) [8] R. Guan and D. Tie: Acta Met. Sinica (English Letters) Vol. 30 (2017), pp. 409-432 [9] S. Mostafapoor, M. Malekan, and M. Emamy: J. Ther. Anal. Calorim. Vol. 127 (2016), pp.

1941-1952 [10] M. Alipour and M. Emamy: Materials & Design Vol. 32 (2011), pp. 4485-4492 [11] D. Nursyifaulkhair, R. Wijanarko, I. Angela, and B. T. Sofyan: Int. J. of Mech. Eng. and

Robotics Research (2018) in-press. [12] X. An, Y. Liu, J. Ye, L. Wang, ad P. Wang: Acta Met. Sinica (English Letters) Vol. 29 (2016),

pp 742-747 [13] F. Vermolen, K. Vuik, and S. V. Zwaag: Mat. Sci. and Eng. Vol. 246 (1998), pp. 93-103 [14] P. Li, B. Xiong, Y. Zhang, and Z. Li: Trans. of Nonferrous Metals Soc. of China Vol. 22 (2012),

pp. 546-554

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