UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

350 IMMC 2018 | 19th International Metallurgy & Materials Congress

Eff ect of Secondary Aging on Microstructure and Mechanical Properties of Al-Si Alloys

Rıdvan Özsoy¹,²,³, Mehmet Yıldırım¹,²

¹Selçuk University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Konya, Turkey²Konya Technical University, Faculty of Engineering and Natural Sciences, Department of Metallurgical and Materials Engineering, Konya, Turkey

³Şirvanlı Aluminum Casting and Machining Incorporation, Kocaeli, Turkey

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Abstract

Eutectic and near-eutectic Al-Si alloys are highly used in automotive industry due to their high strength to weight ratio, excellent abrasion and corrosion resistance. The mechanical properties of these materials are strongly affected by the size, volume fraction, morphology and distribution of the silicon particles in the matrix phase as well as the presence of Sr as a microstructure modifier and proper aging treatments. In this study, the effect of secondary aging on the microstructure and mechanical properties of AlSi10Mg and AlSi7Mg have been investigated in detail. The microstructural examination was carried out using optical microscopy, mechanical properties were investigated by hardness and, tension tests. Age-hardened samples exhibited very similar tension behavior. Among age-hardened samples, 4h aged samples had highest hardness value for both alloys. After secondary aging, further hardening was observed for AlSi7Mg alloy, while AlSi10Mg hardened slightly.

1. Introduction

Eutectic and near-eutectic Al-Si alloys have many potential applications in automotive industry such as cylnder blocks, cylinder heads, pistons, valve lifters and steering knuckles due to their low density, good corrosion and abrasion resistance, low coefficient of thermal expansion, good castability and high strength-to-weight ratio [1-7]. Mechanical properties of Al-Si cast alloys depend on several factors such as chemical composition (Si content), microstructural features such as the morphology of -Al dendrites, the size, distrubition and morphology of eutectic Si, the presence of modifiers and the presence of some other intermetallic compounds present in the microstructure [2].

The age hardening characteristics of Al-Si cast alloys have also strong effect on mechanical properties. Compared to standard T6 treatment, multiple-stage or secondary ageing treatments can further improve the hardening response. An important amount of study has been conducted in order to investigate the multistage aging treatments and effective

results have been obtained about the influence of this treatment on strength and toughness of Al-alloys [8-12]. Secondary ageing is a heat-treatment that involves an aging treatment at relatively lower temperatures (80-110°C) for an extended period of time after the T6 treatment. In this study, it is aimed to investigate the effect of secondary ageing treatment on microtructures and resultant mechanical properties of commercial AlSi10Mg and AlSi7Mg alloys.

2. Experimental Procedure

2.1. Production of the samples

AlSi10Mg (EN-AC 43000) and AlSi7Mg0.3 (EN-AC 42100) alloys were produced by a commercially available Ecotherm Sem 600/82 model induction furnace with a capacity of 600 kg. The induction furnace was filled with both primary AlSi10Mg and AlSi7Mg0.3 ingots obtained from ETI ALUMINYUM and scrap runners and risers obtained from previous castings. The relative amounts of primary ingots and scraps were 40% and 60%, respectively. The melting temperature was maintained as 720-750 °C. Both alloys were grain-refined by adding Ti was added as Al-5%Ti-1%B form and modified by adding Sr in an Al-10%Sr master alloy form (500 gr). All melts were degassed using pure, dry argon injected into the melt for ~15 min by means of a rotary degassing impeller in order to ensure homogenous mixing of the modifiers. The degassed melt was carefully poured into tension test specimen shaped 1045 steel metallic molds.

2.1. Heat-treatment

The samples were solution heat-treated at 540 °C for 5h, then rapidly quenched in warm water at 45 °C, followed by artificial aging at 180 °C for 4, 6, 8, 10 and 12 h. Finally, samples were cooled in air. Then, secondary aging treatment were applied to aged samples. In this treatment, samples were re-aged at relatively lower temperature of 100 °C for 135 hours. The solution and aging treatments were carried out in a Protherm PLF 130/9 electric furnace

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equipped with a programmable temperature controller (±2°C). The aging delay was less than 10 s.

2.3. Characterization

The chemical analysis of the as-cast samples were performed using a GNR Solaris-CCD NF model optical emission spectrometer. Phase analysis of the samples was carried out using X-ray diffraction (XRD) analyses. XRD analyses were conducted using a Bruker D8 Discover model diffractometer with Cu-K radiation ( = 1.54056 Å) and an X-ray source operating voltage of 40 kV. XRD scans were performed in the 2 range of 20°–100° using a scanning rate of 2°/min. For metallographic examinations, cylindrical specimens of 12 mm diameter and 4 mm length were sectioned off to represent each condition. Then, samples were mechanically grinded (with 120-1200 grit SiC papers), and polished with 1 μm Al2O3 suspension. Finally, samples were etched with Keller solution. Microstructures of the sample surfaces were examined using a Nikon Eclipse MA100 model optical microscope. Mechanical properties of the alloys were determined with hardness and tension tests. Brinell hardness was measured using a Digirock RBOV model hardness tester on samples polished to 1 μm, with an applied load of 62.5 kgf and a steel ball of 2.5 mm. The mean hardness values were determined by averaging at least 5 independent measurements made on for each condition. The tension test specimens were machined to the standard shape and dimensions as specified in the ASTM E8/E8M (Standard Test Methods for Tension Testing of Metallic Materials). The aged specimens were pulled to fracture at room temperature at a strain rate of 5 x 10-4 s-1 using a servo-hydraulic MTS Mechanical Testing equipment.

3. Results and Discussion

Table 1 lists the chemical compositions of the investigated as-cast samples. The measured compositions of the AlSi10Mg and AlSi7Mg alloys generally agreed well with the composition given in the standards. Si contents of the alloys (8.642% and 6.435%) were a bit lower than minimum value given in the EN-AC 43000 and EN-AC 42100 standards (9% and 7%, respectively), whereas Cu contents (~0.06%) were a little higher than the standard value of 0.03%. these deviations are mainly due to the commercial purity pre-alloys and scraps used in the production of the samples.

Figure 1 shows the XRD patterns of as-cast samples. According to the XRD patterns, secondary intermetallic -Al4FeSi2 phase was present besides Al and Si phases. The relative intensity of the diffraction lines corresponding to the -Al4FeSi2 phase were higher in the patterns of AlSi10Mg alloy. This result revealed that amount of -Al4FeSi2 phase was higher for AlSi10Mg alloy due to higher Fe content (0.352%) compared to that of AlSi7Mg (0.165%). The solid solubility of Fe in -Al phase is very low (< 0.04%), and ternary intermetallic -Al4FeSi2, -Al5FeSi and -AlFeSi secondary phases can easily be formed in Al-Si alloys even if Fe concentration is low.

Figure 1. XRD patterns of as-cast AlSi10Mg and AlSi7Mg alloys.

Microstructures for AlSi10Mg and AlSi7Mg alloys in the as-cast and age-hardened condition are shown in Figures 2 and 3. The microstructures of all samples were composed primary -Al dendrites (light phase) and eutectic Silicon particles. However, needle-like -Al4FeSi2 particles were also observed in the micrographs. In the microstructures of as-cast samples, fine eutectic morphology revealed the presence of grain refiners or modifiers added during the casting period. After aging fine Si eutectic coarsened and transformed into rods or particles. At high temperatures, eutectic structures may tend to coarsen to reduce the interfacial area and improve the interfacial bond strength [13]. However, after secondary-aging, morphology and size of the eutectic Si did not change too much but amount of Si increased especially for AlSi10Mg alloy.

Table 1. Chemical compositions of the as-cast AlSi10Mg and AlSi7Mg alloys.Element Al Si Fe Mg Cu Mn Ti Sr Cr Ni Zn Sn

AlSi10Mg 90.312 8.642 0.356 0.333 0.063 0.152 0.053 0.003 0.003 0.010 0.000 0.000

ChemicalComposition

(wt.%) AlSi7Mg 92.804 6.435 0.158 0.443 0.065 0.032 0.016 0.001 0.000 0.006 0.000 0.000

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

352 IMMC 2018 | 19th International Metallurgy & Materials Congress

Figure 2. Optical micrographs of AlSi10Mg alloys.

Figure 3. Optical micrographs of AlSi7Mg alloys.

Figure 4. Optical micrographs of AlSi10Mg and AlSi7Mg alloys after secondary aging.

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Results of the hardness measurements after aging and secondary-aging were given in Table 1 and Table 2, respectively. Both alloys were hardened after aging and the highest hardness values were observed for 4h aged samples. Increasing the aging time resulted in decreasing the hardness. Moreover, the samples of AlSi7Mg alloy had higher hardness values for all aging conditions compared to the samples of AlSi10Mg alloy

Table 1. Hardness measurements for AlSi10Mg and AlSi7Mg alloys.

AlloysAlSi10Mg AlSi7MgConditionHardness(Brinell)

Hardness(Brinell)

As-cast 50 46 Solutionized 61 65

4h 127 127 6h 114 124 8h 100 119

10h 114 116 12h 105 113

After secondary aging, the hardness of AlSi7Mg samples incraesed, while the hardness of AlSi10Mg samples did not change too much. This is due to the fact that microstructural features such as size, morphology and volume fraction of constituent phase did not change much after secondary aging for AlSi10Mg alloy. However, for AlSi7Mg alloy, secondary-aging led to an increase in volume fraction of silicon. Thus, AlSi7Mg samples hardened with secondary aging.

Table 2. Hardness measurements for AlSi10Mg and AlSi7Mg alloys.

AlloysAlSi10Mg AlSi7MgConditionHardness(Brinell)

Hardness(Brinell)

4h 128 135 6h 114 128 8h 100 126

10h 116 118 12h 106 118

Room temperature tensile properties of age-hardened AlSi10Mg and AlSi7Mg alloys are listed in Table 3. Results for as-cast and 8h age-hardened samples were not included since the experiments are still being conducted. According to the results, AlSi10Mg samples had higher yield and tensile strength with lower ductility compared to AlSi7Mg samples. In order to evaluate the room temperature mechanical properties in detail, further analysis and measurements should be performed.

Table 3. Tensile properties of AlSi10Mg and AlSi7Mg alloys.

Alloy Aging(hours)

Yield Strength

(MPa)

TensileStrength

(MPa)

FractureStrain

(%) 4h 245 282 2.3 6h 244 282 1,2

10h 244 274 1.0 AlSi10Mg

12h 245 285 1.66 4h 273 315 2.4 6h 270 311 1.93

10h 273 304 1.49 AlSi7Mg

12h 276 313 1.93

4. Conclusion

The effect of secondary-aging on microstructures and room temperature mechanical properties of AlSi10Mg and AlSi7Mg alloys were investigated in detail, and the following conclusions can be drawn:

• Fine eutectic silicon coarsened and transformed to particles with age-hardening treatment.

• 4h age-hardened samples exhibited the highest hardness.

• After secondary aging, new Si particles precipitated for AlSi7Mg alloys and these particles further hardened the alloy.

References[1] S.B. Hassan and V.S. Aigbodion, Journal of Alloys and Compounds, 486 (2009) 309-314. [2] A.M.A. Mohamed, A.M. Samuel, F.H. Samuel and H.W. Doty, Materials and Design, 30 (2009) 3943-3957. [3] O.M. Dupuich and J. Stolarz, Material Science Forum, 217-222 (1996) 1343-1348. [4] H. Liao, Y.Sun and G.Sun, Materials Science and Engineering A, A335 (2002) 62-66. [5] M. Zeren, Journal of Material Processing Technology, 169 (2005) 292-298. [6] L. Lasa and J.M.R. Rodrigues, Materials Characterization, 48 (2002) 371-378. [7] S. McDonald, K. Nogita and A. Dahle, Acta Materilia 52 (2004) 4273-4280.[8] R.N. Lumley, I.J. Polmear and A.J. Morton, Materials Science and Technology, 19 (2003) 1483–1490. [9] R.N. Lumley, I.J. Polmear and A.J. Morton, Material Science Forum, 426–432 (2003) 303. [10] R.N. Lumley, I.J. Polmear and A.J. Morton, Material Science Forum, 28 (2004) 85–90. [11] R.N. Lumley, I.J. Polmear and A.J. Morton, Materials Science and Technology, 21 (2005) 1025–1032. [12] D. D. Risanti, M. Yin, P.E.J. Rivera Diaz del Castillo and S. van der Zwaag, Materials Science and Engineering A, A523 (2009) 99-111. [13] H. Li, J. Guo, K. Huai and Y. He, Journal of Crystal Growth, 290 (2006) 258-265.