Elsevier

Journal of Alloys and Compounds

Volume 693, 5 February 2017, Pages 201-210
Journal of Alloys and Compounds

Combined effects of ultrasonic melt treatment, Si addition and solution treatment on the microstructure and tensile properties of multicomponent Alsingle bondSi alloys

https://doi.org/10.1016/j.jallcom.2016.09.006Get rights and content

Highlights

  • Ultrasonic treatment (UST) significantly refines Si and aluminides.

  • Efficiency of primary Si refinement by UST is pronounced in highly alloyed system.

  • Si addition reduces alloy strength at 25 °C, but increases strength at 350 °C.

  • UST greatly improves both tensile strength and ductility at 25 °C and 350 °C.

  • Strength can deteriorate at 350 °C through spherodization of secondary phases.

Abstract

The effects of ultrasonic melt treatment (UST) on the microstructure and tensile properties of three multicomponent Alsingle bondSi alloys with different Si contents (12, 15, and 18 wt%Si) and solution treatments were systematically investigated at room and elevated (350 °C) temperatures. The microstructures of these alloys consisted of primary Si, eutectic Si, Mg2Si, and various types of aluminides, but the average size and area fraction of the primary Si increased with Si content. The application of UST caused a transformation from dendritic to equiaxed cells and a reduction in grain size, with a significant reduction in the size of all secondary phases (i.e., Si, Mg2Si and aluminides). Both the strength and ductility at room and elevated temperatures were greatly improved by UST, mainly due to microstructure refinement. Improvements in the high-temperature tensile strength as a result of UST increased with increasing Si contents, due to the enhanced refinement of primary Si by UST with increasing Si contents. The spherodization of rigid phases induced by solution treatment can improve the strength and ductility at room temperature, but can also cause a loss of strength at elevated temperatures due to the relaxation of the interconnected network structure of rigid phases caused by their spherodization.

Introduction

Multicomponent Alsingle bondSi alloys have been widely used in high-temperature applications such as automobile engine pistons due to their high strength and fatigue resistance at elevated temperatures and their excellent castability [1]. These alloys generally contain Si (11–23 wt%), Cu (0.5–5.5 wt%), Mg (0.6–1.3 wt%), Ni (0.5–3.0 wt%), Fe (<1.3 wt%), and Mn (<1.0 wt%), which are necessary to attain the required mechanical properties by forming thermally stable phases [2], [3]. Owing to increasing demand for greater fuel efficiency, many researchers are seeking to develop lighter piston alloys that are capable of withstanding even higher temperatures through optimization of their chemical composition (e.g., Si [4] and transition elements [5], [6], [7], [8], [9], [10]) or heat treatment [11], [12], [13], [14].

Ultrasonic melt treatment (UST) is one of the more promising means of improving the mechanical properties of Al alloys, as it effectively reduces the porosity while simultaneously refining the microstructure through cavitation-induced dendrite fragmentation and/or cavitation-induced heterogeneous nucleation [15], [16], [17], [18]. Indeed, the present authors [19] have shown that both the strength and ductility of near-eutectic Al-12.2Si-3.3Cu-2.4Ni-0.8Mg-0.1Fe (wt.%) alloy are greatly improved at ambient and elevated temperatures due to refinement of its grains, eutectic cells, and secondary phases. Khalifa et al. [20] also reported that the wear resistance of a hypereutectic Alsingle bond17Si-4.5Cu-0.6 Mg (wt.%) B390 alloy is improved by UST owing to refinement of the primary Si and Fe-intermetallic compounds.

The refining effect of UST implies that the mechanical properties of a multicomponent Alsingle bondSi alloy can be further improved by increasing the amount of Si or transition elements, assuming that the size of the secondary phase is sufficiently reduced by UST. Lin et al. [21] have examined the combined effects of UST and Mn addition on the microstructure and mechanical properties of hypereutectic Alsingle bond17Sisingle bond2Cusingle bond1Ni-0.4 Mgsingle bond2Fe-(0.4,0.8)Mn (wt.%) alloys, and found that high-temperature tensile properties are improved through an increase in the amount of secondary phases and a decrease in their size, as well as the transformation of needle-like β-Al5FeSi to a more compact α-Al15(Fe,Mn)3Si2 phase. Sha et al. [22] reported similar results with a combination of UST and Co addition, achieving improved high-temperature tensile strength of a hypereutectic Alsingle bond20Sisingle bond2Cusingle bond1Ni-0.6Mg-0.7Fe (wt.%) alloy.

Although Si also plays an important role in strengthening by forming a large amount (10–20 vol%) of rigid Si particles, there are few reports pertaining to the combined effect of UST and Si addition on the microstructure and mechanical properties of multicomponent Alsingle bondSi alloys. This study therefore examines the effects of UST on the microstructure and tensile properties of multicomponent Alsingle bondSi alloys containing different Si contents (12, 15, and 18 wt%) in order to understand the effect of Si addition on the beneficial effects of UST (i.e., the refining efficiency of primary Si and strength improvement). The combined effects of UST and solution treatment on the multicomponent Alsingle bondSi alloy were also examined to further improve its mechanical properties at room and elevated temperatures.

Section snippets

Experimental procedure

Ingots of three multicomponent Alsingle bondSi alloys containing different Si contents (12, 15, and 18 wt%) were provided by Dong Yang Piston Co. (Ansan, Republic of Korea). The ingots were remelted at approximately 800 °C in a clay-graphite crucible (Ø120 mm × 160 mm) located within an electric resistance furnace, and then degassed by Ar gas bubbling filtration (GBF). Once degassed, 1.5 kg melts were poured into a copper book mold (245 × 70 × 200 mm3) that had been preheated to 200 °C, and were then

Calculation of Scheil-Gulliver solidification

Fig. 1(a) shows the temperature vs. solid fraction (fs) curve of 12Si alloy without UST during Scheil-Gulliver solidification, which was calculated using Thermo-Calc software [30] and the TCAL 3 database. The alloying elements Si, Cu, Ni, Mg, Fe, and Mn were included in the calculation along with all of their formable phases. Note that in the pre-eutectic region, primary Si, ε-Al3Ni and β-Al9Fe2Si2 phases are formed at 576.5, 570.3 and 568.6 °C, respectively, with the same pre-eutectic phases

Conclusions

The combined effects of UST, Si addition, and solution treatment on the microstructure and tensile properties of multicomponent Alsingle bondSi alloys were systematically investigated at room and elevated temperatures. The main findings of this research can be summarized as follows:

  • (1)

    The microstructures of the multicomponent Alsingle bondSi alloys consisted of primary Si, eutectic Si, Mg2Si, and a variety of aluminides (e.g., ε-Al3Ni, δ-Al3CuNi, γ-Al7Cu4Ni, Q-Al5Cu2Mg8Si6, θ-Al2Cu, and Al3Ti). Both the formation

Acknowledgements

This work was supported by the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science and Technology) of Republic of Korea (Grant CCP-13-17-KIMS) and the Main Research Program of Korea Institute of Materials Science (PNK4711). The material support from Dong Yang Piston Co. (Ansan, Republic of Korea) is gratefully acknowledged.

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