Microstructural Development in Al-Ni Alloys Directionally Solidified under Unsteady-State Conditions

Abstract

Three Al-Ni hypoeutectic alloys were directionally solidified under upward unsteady-state heatflow conditions. Primary (k 1 ) and secondary (k 2 ) dendrite arm spacings were measured along the castings for all alloys and correlated with transient solidification thermal variables. A combined theoretical and experimental approach was used to quantitatively determine such thermal variables, i.e., transient metal/mold heat-transfer coefficients, tip growth rates, thermal gradients, tip cooling rates, and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al-1.0, 2.5, and 4.7 wt pct Ni alloys are compared with the main predictive dendritic models from the literature.

Rapid solidification of an Al-5Ni alloy processed by spray forming

“…This is probably due to the higher cooling rates obtained as a consequence of the higher G/M ratio. Figure 4 shows the mean experimental λ 1 values of primary dendritic arm spacing as a function of the cooling rate (black squares) as determined by Canté et al 15 during unidirectional solidification of an Al-5Ni alloy. It can be seen that the proposed power function λ 1 = 135 T -0.5 fits adequately the experimental scatter.…”

“…It can be seen that the proposed power function λ 1 = 135 T -0.5 fits adequately the experimental scatter. The mentioned authors 15 have performed unsteady-state directional solidification experiments with a number of hypoeutectic Al-Ni alloys followed by characterization of the transversal and longitudinal as-cast structures and the determination of experimental dendritic growth evolution as a function of growth rate and tip cooling rate.…”

“…Summary of the thermophysical properties necessary to perform the calculations with the Al-5.0 wt. (%)Ni alloy 14,15 .…”

“…The four size ranges were >106 µm, 106 to 45 µm, 45 to 32 µm and <32 µm. The triangle method was employed to measure λ 1 15,16 . At least 20 measurements were performed for each selected powder size range.…”

“…However, for particles having sizes <8 µm, the Sn-rich phase appears finely dispersed within the α-Al matrix. A simple experimental linear law has been proposed by Tourret et al The dendritic and eutectic growth dependences on the cooling rate are already known for directionally solidified hypoeutectic Al-Ni alloys 15 . Canté et al 15 stated that the variation in the experimental primary dendritic arm spacing with the cooling rate for Al-1.0 to 5.0 wt.…”