Applied Mechanics and Materials Vols. 465-466 (2014) pp 958-961 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.465-466.958
The Effect of Metallic Addition on Mechanical Property of Aluminum (LM6) Alloy R. Ahmad1,a, M.B.A. Asmael1,b , R. Sadeghi1,c, H. Mohamad2,d, Z. Harun1,e, S. Hasan1,f 1
Department of Manufacturing and Industrial Engineering, Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia 2
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang
a
[email protected],
[email protected],
[email protected],
[email protected], e
[email protected],
[email protected]
Keywords: Aluminum alloys, Chromium, mechanical property, hardness
Abstract. This article investigates the effects of addition of alloying element on the impact toughness of as-cast aluminum (LM6) alloy. Presence of 0.1 wt.% Cr increases the toughness up to 38%. In fact the sharp tips Fe intermetallics which have needle shape act as stress raisers with a general reduction of the ductility and ultimate tensile strength. On the other hand by increasing the Cr content in the base alloy the size distribution of the compounds becomes more spread. Therefore addition of Cr improves toughness impact by two mechanisms. First eliminating harmful βintermetallics and second providing microstructure with more spread particles. Introduction Aluminum–silicon based alloys are well-known casting alloys with high wear resistance, low thermal-expansion coefficient, good corrosion resistance, and improved mechanical properties at a wide range of temperatures. These properties lead to the application of Al–Si alloys in the automotive industry, especially for cylinder blocks, cylinder heads, pistons and valve lifters [1–5]. Most of the mechanical properties reported about cast Al–Si alloys are the outcome of tensile testing. Significant scatter is typically observed in the results since this type of testing is extremely sensitive to additions of alloying elements to the sample. Furthermore, the test results are not a strong function of silicon morphology. Charpy impact testing was thus chosen for use in this study while this particular test has always been found to be extremely susceptible to the addition of alloying elements and to silicon morphology [6], despite the fact that data on impact properties is comparatively rare for these alloys. According to prior studies [7], impact strength is the most sensitive of all the mechanical properties to silicon content for alloy compositions containing 3– 15% Si [6, 7]. Iron is a frequent impurity element in aluminum alloys. In commercial Al–Si foundry alloys Fe forms brittle intermetallic compounds that have long been identified to be harmful to mechanical properties [8-11]. Even with a very low amount of Fe, the monoclinic particles of βAl5FeSi shape up during solidification [9]. The spiky tips of these needles act as stress raisers with a general fall of the ductility and ultimate tensile strength [10]. The creation and amount of porosity is also dependent on iron content [8]. It has been publicized that the volume and quantity of iron-containing phases is strongly affected by solidification speed [7] and that alloying elements such as Mn and Cr can modify the morphology of the intermetallic phases or improve the precipitation of phases which are less destructive than β-Al5FeSi, i.e. a Chinese script or polyhedral morphology [8-11]. Manganese is the most general alloying addition, but it has been exposed that Cr has parallel effect [9-11]. Modification of Fe-bearing intermetallics with Mn and Cr addition has also some weaknesses. The complex intermetallic compounds formed with Fe, Mn and Cr have a high specific gravity and settle to the floor of the furnace as sludge [12]. Sludge formation not only changes the chemical composition of the molten metal, but also diminishes the castability of molten metal [13-15]. 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 TTP, www.ttp.net. (ID: 60.52.2.48-05/11/13,17:29:46)
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Further, entrapment of the sludge into the mould cavity has a harmful effect on the mechanical and physical properties of the casting part [16]. Sludge formation has been shown to be dependent on the alloy’s chemistry, melting and holding temperatures, and time. Gobrecht [15] and Jorstad [17] have defined a sludge factor (SF) for Al–Si–Cu alloys. It is useful to determine Fe, Mn, and Cr contents to prevent sludge formation. This factor is calculated from the formula shown as Eq.1. Sludge Factor (SF) = (1 × wt.%Fe) + (2 × wt.%Mn) + (3 × wt.%Cr)
(1)
It is obvious that Cr is the most deleterious element for the formation of sludge and also changing the morphology of intermetallic compounds. Experimental Procedures The LM6 alloy ingot with chemical composition illustrated in Table 1 was melted as a base material. Another alloy was prepared, corresponding to additions of 0.1 wt.% Cr and melted in a SiC crucible in an electrical resistance furnace. The melt was stirred by a ceramic coated steel rod for five minutes to ensure the dissolution and homogeneity of Cr in the melt. Neither of grain refiner, modifier and degasser was used during melting and casting. The melt temperature was quantified by a K-type thermocouple. For casting required specimens green sand molds were utilized. The pouring temperature was 670oC and the melt was stirred before pouring. Table 1. Chemical composition of LM6 alloy ingot used as the base material in this research Elements
Al
Si
Cu
Mg
Mn
Fe
Zn
Ni
Ti
wt %
Reminder
10.5-13.5
0.10
0.10
0.55
0.55
0.55
0.10
0.15
After casting the specimens were machined into desired shape and dimension according to ASTM standard for notched Charpy impact test. In this study, the Charpy impact test was conducted using an impact test machine-Impact Tester MT3016, Wolpert. The geometry and dimension of impact specimen is illustrated in Fig. 1.
Fig. 1 Schematic illustration of Charpy impact specimen (all dimensions in mm) Results and Discussion The results of impact toughness test are illustrated in Table 2. There is an obvious increase in impact energy after adding 0.10 wt.% Cr. The amount of this boost is up to 38%. This improvement in fracture energy is thanks to modification of intermetallic compounds. Fig. 2 demonstrates better comparison between average impact energy for both base materials before and after Cr addition.
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Table 2 The results of Charpy impact tests The
The
percentage
Fracture energy (J)
percentage
The value of
Cr in
impact
Aluminum
strength
LM6
(%) of increases in
(kJ/m²)
impact strength
1
2
3
Average
0%
1.04
0.96
1.02
1.01
12.5
--------
0.1%
1.68
1.60
1.61
1.62
20.25
38.27
Average fracture energy
According to Shabestari [18] addition of Cr up to 0.1 wt% Cr needed to convert all harmful iron platelet phases, which are usually in needle form, to star-like intermetallics. In fact it has been shown that the size and amount of iron-containing phases is strongly influenced by solidification rate [18, 19] and Cr in addition can change the morphology of the intermetallic phases or enhance the precipitation of phases which are less harmful than β-Al5FeSi, i.e. a Chinese script or polyhedral morphology. The sharp tips Fe intermetallics which have needle shape act as stress raisers with a general reduction of the ductility and ultimate tensile strength. On the other hand increasing the Cr content in the base alloy, the size distribution of the compounds becomes more spread. Therefore addition of Cr improves toughness impact by two mechanisms. First eliminating harmful β-intermetallics and second providing microstructure with more spread particles [19].
wt% Cr VS Average Fracture Energy 2
1 0
LM6 LM6+ 0.10 wt% Cr
Fig. 2 Chart of average fracture energy related to addition of Cr Conclusion The effect of Cr addition on the impact toughness of aluminum (LM6) alloy has been investigated. Based on results obtained in the present study, addition of Cr has increased the amount of impact toughness in the aluminum (LM6) alloys. The mechanisms of this increase in fracture energy are listed below; 1) In fact the sharp tips Fe intermetallics which have needle shape act as stress raisers with a general reduction of the ductility and ultimate tensile strength.
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2) On the other hand increasing the Cr content in the base alloy, the size distribution of the compounds becomes more spread. 3) Therefore addition of Cr improves toughness impact by two mechanisms. First eliminating harmful β-intermetallics and second providing microstructure with more spread particles. 4) The consequence of addition of Cr up to 0.10 wt.% is formation of α-Al(Fe,Mn,Cr)Si precipitates which lead to the improvement of the ductility of material, more than any other mechanisms like strengthening. Acknowledgment This research is funded by Research Acculturation Collaborative effort (RACE) grant scheme, Ministry of Education. References [1] H. Lio, Y. Sun, G. Sun, Mater. Sci. Eng. A 335 (2002) 62–65. [2] M. Zeren, J. Mater. Process. Technol. 169 (2005) 292–298. [3] L. Lasa, J.M. Rodrigues-Ibade, Mater. Sci. Eng. A 363 (2003) 193–2002. [4] L. Lasa, J.M. Rodrigues-Ibade, Mater. Charact. 48 (2002) 371–378. [5] W. Reif, J. Dutkiewicz, R. Ciach, S. Yu, J. Krol, Mater. Sci. Eng. A 234/236 (1997) 165–168. [6] Komatsu N, Nakamura M, Yamamoto Y. Relationship between Si crystallized form and impact strength of Al–Si alloys observation of impact strength of Al– Si alloys. Japanese J Light Metal 1997/1998;43:398–408. [7] Tsukuda M, Koike S. The heat treatment of Al–7%Si–0.3%Mg alloy. J Jpn Inst Light Metals 1978;28(3):109–15. [8] Z. Ma, A.M. Samuel, F.H. Samuel, H.W. Doty, S. Valtierra, Mater. Sci. Eng. A 490 (2008) 36– 51. [9] S. Seifeddine, I.L. Svensson, Proceedings of the International Conference High Tech Die Casting, Montichiari, Italy, April 9–10, 2008, paper no. 15. [10] S. Seifeddine, S. Johansson, I.L. Svensson, Mater. Sci. Eng. A 490 (2008) 385–390. [11] J.Y. Hwang, H.W. Doty, M.J. Kaufman, Mater. Sci. Eng. A 488 (2008) 496–504. [12] S.G. Shabestari, Mater. Sci. Eng. A 383 (2004) 289–298. [13] S.G. Shabestari, J.E. Gruzleski, Metall. Mater. Trans. A 26 (1995) 999–1006. [14] E. Taghaddos, M.M. Hejazi, R. Taghiabadi, S.G. Shabestari, J. Alloys Compd. 468 (2009) 539–545. [15] X. Cao, J. Campbell, Metall. Mater. Trans. A 35 (2004) 1425–1435. [16] X. Cao, J. Campbell, AFS Trans. 108 (2000) 391–400. [17] J.L. Jorstad, Die Cast. Eng. 11/12 (1986) 30–36. [18] S.G. Shabestari, Materials Science and Engineering A 383 (2004) 289–298 [19] Giulio Timelli, Franco Bonollo Materials Science and Engineering A 528 (2010) 273–282
