Please refer as:

Bondan T. Sofyan, Budi W. Utomo and Merindra B. Setyawan, 2005, Characteristics of AC2B Aluminium Alloy Modified with 2.0 wt. % Sn, Proc. Int Conf. on Recent Advances in Mechanical & Materials Engineering, ISBN 983 41728 26, Kuala Lumpur, Malaysia, 30-31 May 2005, p. 555 - 560.

Characteristics of AC2B Aluminium Alloy Modified with 2.0 wt. % Sn

Bondan T. Sofyan1*), Budi W. Utomo2, and Merindra B. Setyawan1

1Department of Metallurgy and Materials, Faculty of Engineering, University of Indonesia, Kampus UI Depok 16424, Indonesia

2PT. Astra Honda Motor, Jl. Raya Pegangsaan Dua Km. 2, Kelapa Gading, Jakarta 14250, Indonesia

*)Corresponding author: bondan@metal.ui.ac.id Abstract The use of aluminium alloys for automotive purpose is increasing in the last two decades, due to their light weight and corrosion resistance. One type of aluminium alloys used in automotive is AC2B (Al-6Si-2.8Cu) as casting products. Enhancement of mechanical properties of this alloy may be achieved through addition of alloying elements and heat treatment processes. This paper discusses the characteristics of AC2B alloy added with 2.0 wt. % of Sn and its ageing response. Tensile and hardness tests were conducted to investigate the mechanical properties, while observation on the microstructure of the materials was carried out by using light microscopy and SEM (scanning electron microscope) equipped with EDS (energy dispersive spectroscopy). Research results show that the addition of Sn for 2.0 wt. % led to formation of elemental Sn particles in the interdendritic structure of the alloy. Some Sn was also found to be associated with Si crystals. Since Sn particles are relatively soft, their presence gave rise to detrimental effects on mechanical properties. However, the Sn-modified alloys remain posses good age hardening response. Modified alloys which were cast in metal mould possess finer dendritic structure than that cast in resin-coated sand mould. Keywords: AC2B, Al-Si-Cu, alloying, ageing, dendritic structure

Introduction The excellent castability and mechanical properties of AC2B (Al-6Si-2.8Cu, or known as 319) aluminium alloy makes it a popular foundry alloy for automotive purposes. Its low specific gravity is essential to reduce consumption of energy of a vehicle, and its excellent corrosion resistance and low costs of recycling are also important considerations from an environmental point of view [1]. It is well known that alloying elements possess profound impact on the properties of AC2B alloy. Addition of Cu to eutectic Al-Si alloys leads to a slight increase in the alloy fluidity, and a depression in the Si eutectic temperature of ∼1.8 oC for every 1 wt% Cu added. Also, some of the mechanical properties obviously benefit from the addition of Cu as an alloying element (such as yield dan tensile strength) [2]. Copper forms an intermetallic phase with Al that precipitates during solidification either as block-like Al2Cu or in eutectic form as (Al+Al2Cu). In 319 alloys, the copper intermetallic phase precipitates in these two forms, according to a multicomponent eutectic reaction reported by Mondolfo [3]:

L Al + Al2Cu + β-Al5FeSi + Si at 525 oC Iron (Fe) is also one critical alloying elements for 319 alloys. During solidification, it forms several intermetallic compounds. Among these, the formation of hard brittle plates of the β-Al5FeSi phase is particularly deleterious to the alloy mechanical properties [4]. This intermetallic phase also acts as nucleants for the Al2Cu phase [5]. Magnesium (Mg) is found to considerably enhances the alloy response to artificial ageing, which is believed to be due to the formation of coarse particles of Al5Mg8Si6Cu2 [6]. New advancement to significantly increase the strength of aluminium alloys is through enhancement of precipitates formation within the alloys by addition of minor amount of alloying element. This has been widely studied for Al-Cu system. For example, Al-Cu alloys microalloyed with Cd, Sn and In are known to have a finer dispersion of θ' (Al2Cu) and exhibit an increased hardening response [7-10]. Various mechanisms have been proposed to account for this effect, i.e., one-dimensional atom probe (1DAP) experiments on an Al-1.7Cu-0.01Sn (at. %) alloy have shown that θ' nucleation is preceded by clustering of Sn atoms and the precipitation of β-Sn. The fine and uniform dispersion of θ' which follows occurs such that the incoherent rim of the precipitates is associated with Sn atoms [11]. Furthermore, Kanno et al. [12] have observed In particles in Al-Cu-In alloys and Nie et al. [13] as well as Sofyan et al. [14-15] have recently discussed the enhanced precipitation of θ' in Al-Cu-Sn/Cd alloys in terms of cluster-assisted nucleation. However, no study has been conducted on the effects of Sn on more complex alloys, such as 319 alloys. This research investigates the characteristics of AC2B alloy modified with 2.0 wt. % of Sn, as well as their response to heat treatment processes. The mechanism by which the Sn influence formation of precipitates and hardness of the alloys during ageing was also a concern in this work. Effect of mould material on the characteristics of the cast product was also studied. Observation on the microstructure of the materials was conducted by using SEM (scanning electron microscope) and light microscopy.

Experimental Method The base AC2B alloys were cast in a 300 kg industrial induction furnace ~ 300 kg. Gas buble floatation (GBF) was conducted to remove trapped hydrogen in the molten metal, by purging argon for around 10 – 15 minutes. Before pouring at 720 ± 5°C, Sn is charged into the molten metal by using plunger and then stirred. Alloys were cast into two different moulds, metal and resin-coated sand (RCS) mould. Tensile test on the as-cast products is conducted in accordance with JIS Z 2201 standard, in a Shimadzu universal testing machine, with strain rate of 0.04 mm/s and loading capacity of 200 kg. Samples of alloy were also cut into 10 x 10 mm blocks for hardness testing and microanalysis. After solution treatment at 505 oC for 1 h and water quenching, ageing was conducted at 30, 150, 175 and 200 oC. The age hardening response was monitored by Hoytom Brinnel hardness measurements using a 31.25 - kg load and a 2.5 mm diameter- steel ball indentor. Five indentations were measured from each sample.

The evolution of microstructure was followed by means of a light microscope and LEO 420 Scanning Electron Microscope (SEM). Samples for microanalysis were prepared by etching with 0.5 % Hidrogen Fluoride.

Results and Discussion As-Cast Characteristics Table 1 shows the composition of 2.0 wt. % Sn – added alloy in comparison with the standard composition of the base AC2B alloy. From Table 1 it is clear that the studied alloy possesses 6.02 % Si, 0.44 % Fe, 2.8 % Cu, 0.15 % Mg, and other trace elements together with 2 % of Sn. Meanwhile, the standard composition for AC2B alloy [16] is: 5.5 – 6.5 % Si, 1 % max Fe, 3 – 4 % Cu, and maximum of 0.1 % Mg. Comparing the two alloys reveals that the composition of the studied alloy is within the range of standard of AC2B. There is a slightly higher Mg content, which probably came from the charged aluminium ingot. However, the presence of Mg seems to not lead to significant effect to the properties of the alloy. Tabel 1. Comparison between composition of 2 % Sn – added alloy and that of standard AC2B alloy.

Element Composition of 2.0 wt. % Sn-added alloy (wt. %)

Standard composition for AC2B alloy[16]

(wt. %) Si Fe Cu Mg Zn Mn Ti Ni Sn Al

6.02 0.44 2.80 0.15 0.41 0.14 0.01 0.01 2.00

balance

5.5 – 6.5 1.0 max 3.0 – 4.0 0.1 max 1.0 max 0.5 max

0.25 max 0.35 max 0.5 max balance

Example of stress strain curves of as-cast 2 % Sn – added alloy is presented in Figure 1. Mechanical properties derived from the curves then tabulated in Table 2, which are compared to those of AC2B [16]. It reveals that addition of Sn for 2 % decreases the tensile and yield strength by 32 % and 19 %, respectively. The hardness and elongation are also below those of AC2B. All of these show that addition of Sn up to 2 % deteroriates the properties of AC2B. This seems to be due to the immiscibility of 2 % of Sn in aluminium, so that Sn may form independent phase within the structure. And because Sn is softer and weaker that Al, the presence of independent Sn will decrease the hardness and strength of the alloy.

0

50

100

150

200

0 0.01 0.02 0.03 0.04 0.05Strain

Stre

ss (M

Pa)

Figure 1 Stress-strain curve of as-cast 2 % Sn-added AC2B alloy

Table 2. Mechanical properties of cast alloys and comparison with that in literature

2 % Sn added alloy AC2B [16] Mechanical Properties Metal mould Sand mould Metal mould Sand mould

Tensile strength 177 MPa - 235 MPa 185 MPa Yield strength 109 MPa - 130 MPa 125 MPa

Elongation 2.2 % - 2.5 % 2.0 % Hardness 58 BHN 56 BHN 85 BHN 70 BHN

Figure 2 shows comparison of as-cast microstructure of 2.0 wt. % Sn - added AC2B alloys, both in metal and sand moulds. Alloy which was cast in metal mould possess finer dendritic structure than that in sand mould, so that leads to higher hardness of metal mould alloy, as can be seen in Table 1. The finer structure is due to faster heat transfer in metal mould. The as-cast microstructure consists of Al-rich dendrites with interdendritic structures, which are thought to be Al2Cu, Al5Cu2Mg8Si6, Al5FeSi or Si-rich crystals [17].

Figure 2. As-cast microstructure of 2.0 wt. % Sn – added AC2B alloys cast in (a) sand mould, and (b) metal mould.

200 μm 200 μm

a b

Age Hardening Response Comparison of hardening curves of the base AC2B alloy and 2.0 wt. % Sn-containing alloys following ageing at room temperature are presented in Figure 3. In general, not much difference was encountered between the two alloys. The base AC2B alloy shows significant decrease in hardness upon quenching. This behaviour is common in Al-Si-Cu alloys which are sensitive to quenching stress [8]. However, with minor alloying of Sn, hardness reduction during initial period in ageing diminishes. It is thought that the presence of Sn in Al matrix cause compressive lattice strain, so that tensile stress produced during quenching is nullified and no further stress relieve occurs upon subsequent ageing process. It is noted that up to 200 h, the ageing process remain in the incubation period.

Metal Mould, Natural Ageing

40

50

60

70

80

90

100

110

0.01 0.1 1 10 100 1000Ageing time (h)

Har

dnes

s (B

HN

) 0 % Sn2 % Sn

Figure 3. Hardness response of AC2B and 2.0 wt. % Sn-added alloys cast in metal mould, upon ageing at room temperature.

50

60

70

80

90

100

0.01 0.1 1 10 100 1000Ageing time (h)

Har

dnes

s (B

HN

)

25 C

150 C

175 C

200 C

Sand mould

50

60

70

80

90

100

0.01 0.1 1 10 100 1000 Ageing time (h)

Har

dnes

s (B

HN

)

25 C

150 C

175 C

200 C

Metal mould

(a) (b) Figure 4. Hardness response of 2.0 wt. % Sn – added alloy cast in (a) sand mould, and (b) metal mould, upon ageing at various temperatures.

Figure 4 (a) and (b) provides age-hardening curves of 2 % Sn-modified alloy during ageing at 30 °C (natural aging), 150 °C, 175 °C dan 200 °C (artificial aging). An incubation period for 20 minute to 1 h was detected, which then followed by an increase to peak hardness It is clearly revealed that the time needed to reach peak hardness decreases with the increase in ageing temperature, but the peak hardness decreases with the increase in ageing temperatures.

Figure 5. Evolution of microstructure of 2 % Sn - modified AC2B alloys cast in (a – c) sand mould, and (d – f) metal mould, during ageing at 200 oC.

200 μm

sand mould metal mould a

b

c

d

unde

rage

d pe

ak a

ged

over

aged

e

f

2 % Sn

The shorter time needed to reach peak hardness at higher ageing temperature is due to faster diffusion, which leads to higher growth rate of strengthening precipitates. On the other hand, at lower ageing temperature, precipitate nucleation rate is faster and the precipitate growth rate is slower. Therefore, finer but denser distribution of precipitates are expected to occur. This gives rise to higher peak hardness due to effective impediment of dislocation movement by the precipitates. However, at low temperature, diffusion rate is slower so that need longer time to reach the peak hardness. Among all condition, the highest peak hardness for 2 % Sn added alloy was achieved during ageing at 150 oC which the peak hardness values for sand and metal moulds are 90 and 92 BHN, respectively. The difference between the metal and sand mould hardness is ± 3 BHN, which is the same with the difference in as-cast condition. This indicates that the difference in characteristics due to type of mould is preserved during ageing. Microstructural Analysis As-quenched microstructures (not shown here) revealed slightly finer dendritic structure, indicating that dissolution of the structure may occur during solution treatment. A series of microstructural evolution of AC2B alloys added with 2.0 wt. % Sn, for both sand and metal moulds, during ageing at 200 oC, is presented in Figure 5. All micrographs have the same magnification as shown by the scale bar. It is confirmed that alloys cast in metal moulds have relatively finer dendritic structure that that of the sand mould. With further ageing, growth of dendritic structure is apparent. Interdendritic space is filled with various intermetallics that may be in the form of Al2Cu, Al5FeSi and Si crystals [4-6]. Spherical particles (arrowed) were detected to form in all microstructure in various size, that is thought to be elemental Sn particles. These particles were not found in AC2B base alloy.The type of intermetallic and the spherical particles will be confirmed by SEM/EDS observation. However, fine and nanoscale precipitates were not detected here due to the limitation of light microscopy. To retrieve the presence of Sn, microanalysis by using EDS/SEM was conducted. Figure 6 shows SEM micrograph of a 2 % Sn - modified AC2B alloy cast in metal mould and peak-aged at 150 oC. Composition of each position in Figure 6 is presented in Table 3. White round particle (position 1) is confirmed to be Sn-rich with a small amount Si, while bright intermetallic structure (positions 3 and 4) seems to be α-Al-Fe-Mn-Si, similar to findings by Cayron [17]. The grayish structure (position 5) is rich in Si with a slight enrichment of Sn. The association of Sn with the Si-rich structure may indicate that Sn plays a role in the formation of these structures. As has been well known, Sn atoms have high binding energy with vacancy due to its large atomic size. The vacancy associated with Sn will facilitate formation of Si-rich structures through to accommodation of shear strain [13]. However, addition of Sn in this alloy seems to be more than the amount needed to promote formation of intermetallics, so that the excessive Sn tends to form elemental particles due to its miscibility in Al. These particles are soft, so that leads to detrimental effect to the properties of bulk AC2B alloys.

Figure 6. SEM micrograph of a 2 % - Sn added AC2B alloys peak-aged at 150 oC.

Table 3. Microanalysis result on positions shown in Figure 6. Content (wt. %) Position

Al Si Cu Fe Mn Sn Colour Possible phase

1 32.5 0.4 - - - 66,51 White Elemental Sn 3 67.1 8.3 6.0 16.3 2.2 - White α-Al-Fe-Mn-Si 4 65.6 9.2 6.0 16.8 2.4 - White α-Al-Fe-Mn-Si 5 15.2 82.1 - - - 2.6 Greyish Si-rich crystal 6 93.2 2.5 2.9 - - - Greyish Al matrix

Conclusions

1. Addition of 2.0 wt. % of Sn into AC2B aluminium alloys seems to lead to

detrimental effects, such as reduction in strength and hardness, because of the tendency of Sn to form elemental particles which possess low hardness.

2. Heat treatment process that was found to be effective in producing 2.0 wt. % Sn – added AC2B aluminium alloys with high hardness was artificial ageing at 150 oC for 45 hours.

3. Microstructure of Sn-modified AC2B aluminium alloys consists of aluminium-rich dendrites with intermetallic particles, such as: elemental Sn particles, α-Al-Fe-Mn-Si Si-rich crystals. Most of the Si-rich crystals are associated with Sn, which indicates that Sn plays a role in the formation of this structures.

4. Alloys aged in metal moulds seem to have higher mechanical properties due to faster heat transfer which then lead to finer dendritic structures.

Acknowledgement This work is partly funded by Indonesia Toray Science Foundation. Assistance from Mr. Dwi Marta Nurjaya for SEM work is highly appreciated.

References

1. J.E. Hatch (ed.), Aluminium: Properties and Physical Metallurgy (Ohio, OH: American Society for Metals, 1984), 143.

2. M.A. Moustafa, F.H. Samuel, H.W. Dotty and S. Valtierra, Int. J. Cast Metals Res., 14 (2002), 235.

3. L.F. Mondolfo, Aluminum Alloys: Structure and Properties (London: Butterworths, 1976).

4. A. Couture, AFS Int. Cast Metals J., (1981), 9. 5. P.N. Crepeau, AFS Trans. 103 (1995), 361. 6. P. Ouellet, F.H. Samuel, J. Mat. Sci., 34 (1999), 4671. 7. H.K. Hardy, J. Inst. Met., 80 (1951-52), 483. 8. I.J. Polmear and H.K. Hardy, J. Inst. Met., 81 (1952-53), 427. 9. J. M. Silcock, T. J. Heal and H. K. Hardy, J. Inst. Met., 82 (1953-54), 239 10. J.M. Silcock, T.J. Heal and H.K. Hardy, J. Inst. Met, 84 (1955-56), 23. 11. S.P. Ringer, K. Hono, and T. Sakurai, Metall. Mater. Trans. A, 26A (1995), 2207 12. M. Kanno, H. Suzuki, and O. KanohJ. Japan Inst. Light Metals, 44 (10) (1980),

1139. 13. J.F. Nie, B.C. Muddle, H.I. Aaronson, S.P. Ringer, and J.P. Hirth, Metall. Mater.

Trans. A, 33A (2002), 1649. 14. B.T. Sofyan, K. Raviprasad and S.P. Ringer, Micron, 32 (8) (2001), 851. 15. B.T. Sofyan, I.J. Polmear and S.P. Ringer, Mater. Sci. Forum, 396 – 402 (2002),

613. 16. ASM International, ASM Specialty Handbook: Aluminium and Aluminium Alloys,

(Ohio, OH: American Society for Metals, 1993). 17. C. Cayron, “TEM Study of Interfacial Reactions and Precipitation Mechanisms in

Al2O3 Short Fibers or SiC Particles Reinforced Al-4Cu-1Mg-0.5Ag Squeeze-Cast Composites” (PhD. Thesis, Ecole Polytechnique Federale de Laussane, France, 2000)