The effects of heat treatment on microstructure and mechanical ...

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ScienceDirect Materials Today: Proceedings 5 (2018) 9476–9482

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The 10th Thailand International Metallurgy Conference (The 10th TIMETC)

The effects of heat treatment on microstructure and mechanical properties of rheocasting ADC12 aluminum alloy Rungsinee Canyooka*, Ruethairat Utakruta, Chanakarn Wongnichakorna, Kittichai Fakpana, Saowalak Kongiangb a

Department of Materials and Production Technology Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, 1518 Pracharat I Rd., Wongsawang, Bangsue, Bangkok, 10800, Thailand b Material and Process Engineering Technology, Faculty of Engineering and Technology, King Mongkut’s University of Technology North Bangkok (Rayong Campus), 19 Tumbon Nonglalok, Amphur Bankhai, Rayong, 21120, Thailand

Abstract The effects of heat treatment on microstructure and mechanical properties of rheocasting ADC12 aluminium alloy produced by gas induced semi-solid (GISS) technique were studied. The solution heat treatment performed at the temperature of 520°C under various solution treatment times for 2, 4, 6, 8, 10, and 12 h and followed by water quenching at room temperature. The results show that the optimum solution heat treatment condition for the non-dendritic structure of ADC12 aluminium alloy was 520°C for 8 h. The age hardening was also carried out at 170°C for different aging times of 6, 8, and 10 h, respectively. It was found that the artificial aging at 170°C for 6 h was sufficient to achieve the highest hardness of 73.2 HRB. This was due to the appropriate refinement of acicular eutectic structure. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: ADC12 aluminium alloy; T6 heat treatment; Gas induced semi-solid (GISS); Squeeze casting; Rheocasting

1. Introduction Aluminium alloys are favorable candidates as lightweight automotive materials for developing vehicles performance and fuel efficiency [1]. ADC12 aluminium alloy is a cast alloy widely used due to their excellent castability, high productivity and high strength. However, the aluminium alloy casting has not been suitable for

* Corresponding author. Tel.: 66025 874 335; fax: +66 25 874 335 E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.

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safety-critical parts in the automotive field because of cast defects. Therefore, there have been considerable efforts to minimize these problems then semi-solid metal technique introducing [2-3]. It deals with semi-solid slurry metal processes, which uses temperature lower and the viscosity is higher than conventional casting. Moreover, semi-solid process can be reduced cycle time gives improved productivity and reduction in production costs. Lower porosity is also expected due to the higher viscosity of the flow and reduces the shrinkage. The semi-solid metal process can be carried out by 2 routes are thixocasting and rheocasting. Rheocasting process has increasingly gained attentions from many researchers because of cost advantages over thixocasting, because the liquid alloy has processed into semi-solid metal at the production site and scrap metals can be recycled in-house [3]. Various techniques were later proposed such as cooling slope [4], electromagnetic stirrer [5] and gas induced semi-solid (GISS) technique [6]. In the GISS techniques, fine inert gas bubbles are injected through a graphite diffuser into a molten alloy. The solid particles produced at the cold surface of the graphite diffuser are vigorously agitated and dispersed in the bulk liquid by the flow of the gas bubbles. It is a simple, economical and efficient process. Additionally, this technique has succeeded to produce non-dendritic structure of ADC12 aluminium alloy [7]. Heat treatment of aluminium alloys used for improving the mechanical properties by the well-known precipitation hardening mechanism, which occur during appropriate heating and subsequent cooling. The T6 heat treatment consists of (i) solution heat treatment of as-cast samples for dissolution of certain intermetallic phases and for changing the morphology of the eutectic phase; (ii) quenching, to obtain a supersaturated solid solution; and (iii) age hardening, to controlled decomposition of a super-saturated solid solution to form a finely dispersed precipitate. The heat treatment for conventional cast aluminium alloys are well understood, but the different microstructure and solidification history of semi-solid metal components indicate that heat treatment conditions which optimized for conventionally cast materials are not applicable to SSM components. The present study investigates the effect of T6 heat treatment on the microstructure and mechanical properties of rheocast ADC12 aluminium alloy produced by GISS technique. 2. Experimental procedures A commercial ADC12 aluminium alloy is used in this study. The liquidus temperature of the alloy is 582°C. The chemical composition was measured using the optical emission spectrometer (OES) is given in Table 1. Table 1. Chemical compositions of the ADC12 aluminium alloy used in this study (wt.%)

Alloy

Si

Fe

Cu

Mn

Mg

Zn

Ti

Cr

Al

ADC12

11.88

0.89

1.62

0.22

0.21

0.98

0.06

0.03

Balance

In this experiment, the ADC12 aluminium alloy ingots were melted in the graphite crucible in an electric resistance furnace at the temperature of 680°C. Approximately 500 g of the molten alloy was taken from the crucible using a ladle cup. The graphite diffuser was immersed in the molten alloy to produce a semi-solid slurry at the rheocasting temperature of 590°C for 5 seconds. The schematic diagram of the GISS technique as shown in Fig. 1. The slurry poured into a die cavity, which preheated to 250 - 300°C. The squeeze cast applied pressure at 80 MPa. The as-cast plates dimension is 100 mm x 100 mm x 15 mm. To study the effect of solution heat treatment on microstructure of the alloy, the as-cast alloy was solution treated at temperature of 520°C for 2, 4, 6, 8, 10, and 12 h and followed by water quenching at a temperature of 25°C. Artificial aging was performed at the temperature of 170°C for various times of 6, 8, and 10 h, respectively. The microstructure characterized by using an optical microscope (OM) and a JEOL JEM 6340J scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry (EDX). For optical and scanning electron examinations, the specimens were ground, polished, and then etched with Keller’s reagent for up to 3 seconds before rinsing with distilled water. Hardness was measured using Rockwell hardness test under B scale. An average hardness value of at least five measurements calculated.

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.

Fig. 1. Schematic diagram of gas induced semi-solid (GISS) process.

3. Results and discussion 3.1. Microstructure Figure 2 shows the non-dendritic microstructure of the rhocasting ADC12 aluminium alloy was obtained using the GISS technique with squeeze casting process. According to the results of SEM-EDX, it is found that mainly consists of primary -Al phase (white phase) and -AlFeSi phase (grey particles) surrounded by eutectic phase of acicular-like silicon particles (grey phase), as clearly revealed in Fig. 2. It can observe that the microstructures of primary -Al particles with various morphology: dendritic, rosette, equiaxed, and globular. The non-dendritic structure of the particles occurs from the dendrite fragmentation by remelting mechanism created by GISS process [9]. From the EDX analysis carried out for Si, Cu, and Fe, it is clear that the morphology of -AlFeSi phase was compacted polyhedral, as shown in Fig. 3. The compacted polyhedral form of the intermetallic compound phase was coherent with Al matrix, is the advantage to the mechanical properties of casting parts due to strengthening mechanism, especially in form of fine particles.

Si -Al

-AlFeSi

Fig. 2. Optical micrographs of the rheocast specimen showing the microstructure of as-cast specimen.


(a)

(b)

(c)

(d)

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Si K

Cu K

Fe K

Fig. 3. EDX maps for Si, Cu, and Fe in (a) as-cast of rheocast specimen (b) Si-K image, (c) Cu-K image, and (d) Fe-K image.

After solution heat treatment at the temperature of 520ºC for 2, 4, 6, 8, 10, and 12 h, the acicular-like silicon particles initial disintegration and fragmented, as depicted in Fig. 4 (a) – (c), and exhibit perfectly rounded shape at the solution heat treatment for 8 h, is shown in Fig. 4 (d). After solution treatment at the temperature of 520ºC for 10 and 12 h, which diffusion is easier and cause whole the silicon particles become coarsening as shown in Fig. 4 (e) – (f). The size of the visible -AlFeSi phase quite decreased when increasing solution heat treatment times. The length of the -AlFeSi phase reduces due to the dissolution effect demonstrated by Narayanan et al. [10]. The hardness variations during solution heat treatment at the temperature of 520ºC are showing in Fig. 5. The results showed that the as-cast specimen has a hardness value of 45 HRB. The hardness value of the short solution heat treatment at 520ºC for 2 h specimen immediately increased to 63 HRB. The solution heat treatment times for 4 and 6 h show a slight decreased to 60 HRB and decreased again to about 57 HRB after solution heat treatment at 520ºC for 8 h. It is well known that the change size and morphology of the eutectic silicon phase have a significant influence on the mechanical properties of the alloy. The fragmentation of silicon particle initial occurs at the short solution heat treatment time, when a longer solution heat treatment times help to reduce the vacancies and distortions formed and become less and less active as nuclei, resulting in fewer precipitates and slowing the increase in strength. It is clear that the hardness values are decreasing for the solution heat treatment at the temperature of 520ºC for 8 h. Moreover, it can explain concerning the uniform distribution of spherical particles of eutectic silicon. The results can conclude that the solution heat treatment at the temperature of 520ºC for 8 h is sufficient to achieve the microstructure and hardness.

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4. Optical micrographs of the rheocast specimen showing the microstructure of solution heat treatment at 520ºC for (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h, (e) 10 h, and (f) 12 h, respectively.

3.2. Age hardening After solution heat treatment at the temperature of 520ºC for 8 h, the specimen is artificially aged at the temperature of 170ºC for 6, 8, and 10 h. The microstructures of aged samples at 170ºC for 6, 8, and 10 h, as shown in Fig. 6. The shapes of the microstructures are changed after the aging process, with fragmented and spherical


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eutectic particles, as shown in Fig 6 (a). Fig. 6 (b) – (c) show the large size and plate-like morphology of eutectic silicon. 70

Hardness, HRB

60 50 40 30 20 10 0 0

2 4 6 8 Solution Heat Treatment Times, hr

10

12

Fig. 5. Hardness variations during solution heat treatment at the temperature of 520ºC.

Fig. 7 shows the variation of hardness at the different aging time at 170ºC. The result attributed to the slow speed of coherent precipitate formation. It can note that hardness of the aged sample decreased with increasing aging time. The maximum hardness of the aged samples produce by GISS technique are aged at the temperature of 170ºC for 6 h which is 73.2 HRB. At the long aging times, overaging occurs and hardness drop due to extensive aging, where the precipitation turns to be the equilibrium phase.

(a)

(b)

(c)

Fig. 6. The microstructure of aged samples at 170ºC for (a) 6 h, (b) 8 h, and (c) 10 h, respectively.

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Hardness (HRB)

73

72

71

70

69 6

8 Aging times (h)

10

Fig. 7. Hardness values with different aging time at 170ºC.

4. Conclusions The effects of heat treatment on the microstructure and the mechanical properties of rheocasting ADC12 aluminium alloy were studied. It can be concluded that: 1. The non-dendritic microstructure of the rheocast ADC12 samples consists mainly of an -Al matrix and AlFeSi phase surrounded by eutectic phase of acicular-like silicon particles. 2. The solution heat treatment at the temperature of 520ºC for 8 h enough to dissolved and uniformly dispersion of spherical particles of the eutectic silicon for the alloy. 3. The artificial aging at the temperature of 170°C for 6 h is sufficient to achieve the average highest hardness of 73.2 HRB. Acknowledgements The authors gratefully acknowledge the financial support of the Faculty of Engineering, King Mongkut’s University of Technology North Bangkok (Contract no. 57-10-09-215). References [1] [2] [3] [4] [5] [6]

Aluminum promises greater fuel-efficiency: metallurgy, Mater. Today 5 (2002)12. D.H. Kirkwood, M. Suery, P. Kapranos, H.V. Atkinson, K.P. Young, Semi-solid processing of alloys.New York:Springer Verlag; 2009. M.C. Flemings, R.G. Riek, K.P. Young, Mater Sci Eng 25 (1976) 103-107. D. Liu, H.V. Atkinson, P. Kapranos, W. Jirattiticharoean, H. Jones, Mater. Sci. Eng. A 361 (2003) 213-224. Z. Xiao-Li, L. Ting-Jua, X. Shui-Sheng, T. Hai-Tao, J. Jun-Ze, J. Mater. Process. Technol. 209 (2009) 2092-2098. J. Wannasin, S. Janudom, T. Rattanochaikul, R. Canyook, R. Burapa, T. Chucheep, S. Thanabumrungkul, Trans. Nonferrous Met. Soc. China 20 (2010) 1010-1015. [7] S. Janudom, T. Rattanochaikul, R. Burapa, S. Wisutmethangoon, J. Wannasin, Trans. Nonferrous Met. Soc. China 20 (2010) 1756-1762. [8] J. Wannasin, R.A. Martinez, M.C. Flemings, in: Solid State Phenom.116-117 (2006) 366-369. [9] R. Canyook, S. Petsut, S. Wisutmethangoon, M.C. Flemings, J. Wannasin, Trans. Nonferrous Met. Soc. China 20 (2010) 1649-16551. [10] L.A. Narayanan, F.H. Samel, J.E. Gruzleski. Metall. Mater. Trans. A 25 (1994) 1761-1773.