Multicomponent Al-Si piston alloy was consisted of primary Si, eutectic Si, and various types of intemetallic compounds(IMC) such as δ-Al3CuNi, α-Al15(Mn,Fe)4Si2, ε-Al3Ni, θ-Al2Cu, Μ-Mg2Si, and Q-Al5Cu3Mg8Si6. This alloy has been widely used in high temperature and pressure application such as automobile engine piston or cylinder due to its good castability, high stength-to-weight ratio, and low thermal expansion. Recently, with the increase of the demand for high fuel efficiency, this alloy is required to be available at higher temperature and pressure.
In this study, Al-12Si piston alloy was added with the various transition metal elements and ultrasonic melt treated, and their effects on the microstructure and mechanical properties were systematically investigated. The mechanical properties of Al-Si piston alloy are strongly influenced by the type, size, and distribution of primary Si and IMC as well. It has been well known that UST refine primary Si and IMC which result to improve the mechanical properties of the alloy. A detailed analysis of microstructure was performed using by optical microscopy and scanning electron microscopy. Tensile tests were carried out at room temperature and 350 ℃ and their fractured surfaces were observed.
The addition of alloying elements of transition metals such as Cu, Ni, Fe, Mn increased the volume fraction of IMC. Regardless of the content of transition metal, the primary Si, IMC, and grain were significantly refined and uniformly distributed by UST which could be caused by the cavitation-enhanced heterogeneous nucleation under ultrasonic irradiation. The ultimate tensile strength(UTS), elongation, and qulity index(Q) of the samples with large content(> 6.66wt.%) of transition metals were much improved by UST, which mainly resulted from the refinement of microstructure. On the other hand, the yield strength could not be measured at room temperature owing to the low elongation(< 0.73%), and seemed not to be critically changed by UST at 350℃. With the increase of the area fraction of primary Si and IMC, tensile strength at 350 ℃ was increased by the refinement of primary Si and IMC by UST, while the strength at room temperature was not changed by UST.