Abstract
In the present work, individual/combined additions of transition elements (V, Zr and Mo) were introduced into Al-7Si-0.6Cu-0.35Mg foundry alloy at different cooling rates to study their influence on the precipitation behaviour of dispersoids. Results showed that both individual and combined additions of V, Zr, Mo lead to the formation of dispersoids but with different composition, morphology and number density during solution treatment. The addition of V produces the precipitation of both (Al,Si)3M dispersoids and α-dispersoids, while the Zr addition promotes (Al,Si)3M type dispersoids but inhibits the formation of α-Al(Mn,Fe)Si dispersoids. The addition of Mo effectively promotes α-Al(Mn,Mo,Fe)Si dispersoids and significantly reduces the dispersoid size and increase the number density of dispersoids. The combined addition of V, Zr and Mo produces the largest number of finer dispersoids among all five alloys studied, but the most dispersoids are (Al,Si)3M. The (Al,Si)3M dispersoids and α-dispersoids have the rod-like and block-like morphologies, respectively. High cooling rate can generally refine the dispersoids and increase their number density, while it also increases the proportion of (Al,Si)3M dispersoids.
Highlights
Since commercially replacing cast iron with Al-Si foundry alloys in automotive engine applications, the weight of automobiles has been significantly reduced and with better fuel efficiency
3.1 As-cast and solutionized microstructures In the present work, the as-cast microstructures of experimental alloys are observed to be composed of dendritic α-Al, eutectic silicon and various intermetallic phases, which exhibit a variety of types and morphologies and highly depend on the addition of transition elements
The typical as-cast microstructures of #4 alloy under two cooling rates are exemplarily given in Fig. 1, which displays the various intermetallics formed due to the combined addition of transition elements as well as the finer microstructures in S-sample (Fig. 1b)
Summary
Since commercially replacing cast iron with Al-Si foundry alloys in automotive engine applications, the weight of automobiles has been significantly reduced and with better fuel efficiency. The mechanical properties of Al-Si alloys will significantly decrease with increasing temperature (above 200 °C) owing to the coarsening of conventional strengthening precipitates (mainly β’-Mg2Si and θ’Al2Cu) [1, 2]. How to improve the elevatedtemperature properties of Al-Si foundry alloys is of primary importance for their sustainable development. The dispersion strengthening of thermally stable dispersoids has been proved to be one of the best strengthening ways for Al-Si alloys at elevated temperature [1, 3, 4]. Since the transition elements like V, Zr and Mo have much lower diffusion rate (4.85 × 10-24 m2 s−1, 1.20 × 10-20 m2 s−1 and 5.52 × 10-23 m2 s−1 at 400°C respectively) in the α-Al matrix than the common used dispersoid initiator of Mn (6.24 × 10-19 m2 s−1 at 400 °C) [6], the dispersoids containing these elements are expected to have better coarsening resistance
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