Elsevier

Materials Characterization

Volume 61, Issue 11, November 2010, Pages 1074-1079
Materials Characterization

Effects of Cu content on microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy

https://doi.org/10.1016/j.matchar.2010.06.022Get rights and content

Abstract

This study elucidates how Cu content affects the microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy, by adding 4.65 wt.% and 0.52 wt.% Cu. Different Fe-bearing phases were found in the two alloys. The acicular β-Al5FeSi was found only in the high-Cu alloy. In the low-Cu alloy, Al8Mg3FeSi6 was the Fe-bearing phase. Tensile testing indicated that the low-Cu alloy containing Al8Mg3FeSi6 had higher UTS and elongation than the high-Cu alloy containing the acicular β-Al5FeSi. It is believed that the presence of the acicular β-Al5FeSi in the high-Cu alloy increased the number of crack initiators and brittleness of the alloy. Increasing Cu content in the Al–14.5Si–0.5Mg alloy also promoted solution hardening and precipitation hardening under as-quenched and aging conditions, respectively. The hardness of the high-Cu alloy therefore exceeded that of low-Cu alloy.

Research highlights

► Variable Copper content can change morphology of the Iron-rich phase in Al-14.5Si-0.5Mg alloy. ► The acicular β-Al5FeSi was found only in the high-Cu alloy. ► In the low-Cu alloy, Al8Mg3FeSi6 was the Fe-bearing phase. ► Tensile testing indicated that the low-Cu alloy containing Al8Mg3FeSi6 had higher UTS and elongation than the high-Cu alloy containing the acicular β-Al5FeSi.

Introduction

The excellent high specific strength, corrosion resistance, weldability and castability of Al–Si alloys make them the most commonly used aluminum foundry alloys [1]. Solution and aging treatment improve the mechanical properties of Cu and Mg-containing Al–Si alloys by the precipitation of strengthening phases. These Cu and Mg-containing Al–Si alloys with high strength are extensively used in the automotive and defense industries [2].

Copper and magnesium are usually added to increase the strength and other properties of the Al–Si foundry alloys. Iron, manganese, chromium and nickel that are not classified as alloying elements are considered as impurities in most of Al–Si foundry alloys. During the solidification process, the impurities and alloying elements partially go into Al solid solution and partially form various constituent particles including Al2Cu, Al5Cu2Mg8Si6, β-Al5FeSi, Al8Mg3FeSi6, Mg2Si, Al15(Mn,Fe)3Si2 and/or other particles under different conditions [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The most common impurity in the aluminum alloys is iron. In general, Fe forms various Fe-bearing phases during the solidification process due to its low solubility in the Al solid solution. Acicular β-Al5FeSi phase and other Fe-bearing phases cannot be dissolved in the Al matrix by solution heat treatment [4], [5], [6], [7], [8], [9], [10], [11], [12]. The tip of the acicular β-Al5FeSi in aluminum alloy under extension loading is likely to undergo stress concentration and become a crack initiator.

H.G. Kang found that θ′ (Al2Cu) and λ′ (Al5Cu2Mg8Si6) were strengthening precipitates in Mg and Cu-containing Al–7Si alloys following T6 treatment [13]. Y.J. Li found a threshold copper content in the range of 1.0–1.5 wt.% that the θ′ phase precipitates out in Al–7Si–0.45 Mg alloys during artificially aging [14]. The precipitation sequence of the θ and λ phases are supersaturated solid solution  cluster  G.P.(I)  G.P.(II)  θ′  θ phase and supersaturated solid solution  cluster  G.P.→λ′  λ phase, respectively. Both the θ′ and λ′ metastable phases that are semi-coherent with the Al matrix can enhance the strength and hardness of the Al alloys. G. Wang demonstrated that θ′ and λ′ phases were precipitated out in Al–8Si–0.4 Mg–xCu alloys with more than 1 wt.% copper [15]. Copper is an important element for precipitating θ′ and λ′ phases. The effect of precipitation strengthening on the mechanical properties therefore depends strongly on the amount of Cu solute atoms in the Al matrix.

Although Al–Si alloys that contain Cu and Mg are extensively used in the automotive and defense industries, this subject has not been thoroughly studied. The effects of Cu content on the microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy are investigated herein by microstructural examination, differential scanning calorimetric analysis, electrical conductivity measurement (%IACS), hardness testing, and tensile testing in the present study.

Section snippets

Experimental Procedures

Two alloys were prepared by melting pure aluminum in an electrical resistance furnace using a graphite crucible. Suitable amounts of silicon, copper and magnesium were sequentially added to the melt to generate high-Cu (4.65 wt.%) and low-Cu (0.52 wt.%) Al–14.5Si–0.5Mg alloys. Table 1 presents the chemical compositions of both alloys, determined by inductively coupled plasma-mass spectrometry (ICP-MS).

The experimental alloys were solution heat treated at 500 °C for 8 h in an air furnace, quenched

Microstructure

Under the as-cast conditions, the microstructure of high-Cu alloy contains primary silicon, eutectic silicon, eutectic Al2Cu, Al5Cu2Mg8Si6 and acicular β-Al5FeSi phases, as presented in Fig. 1(a) and (b). The microstructure of low-Cu alloy has primary silicon, eutectic silicon, Al5Cu2Mg8Si6, Mg2Si and Al8Mg3FeSi6 phases, as shown in Fig. 2(a) and (b). These intermetallic compounds are confirmed by EPMA and tabulated in Table 2. In the low-Cu alloy, all Cu atoms reacted with some Mg atoms to

Conclusions

This study elucidates how Cu content affects microstructure, precipitation kinetics and mechanical properties of Al–14.5Si–0.5Mg alloy. The experimental results support the following conclusions.

  • (i)

    In the low-Cu alloy, all of the Cu atoms reacted with some Mg atoms to form Al5Cu2Mg8Si6 phases during the solidification process. The remaining Mg atoms form Al8Mg3FeSi6 and Mg2Si phases.

  • (ii)

    The UTS and elongation of aging low-Cu alloy were superior to those of aging high-Cu alloy. The presence of the

Acknowledgments

The authors would like to thank the National Science Council of Taiwan under contract NSC-96-2622-E-008-009-CC3 for their financial support of this research.

References (21)

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