Microsegregation during Solidification of an Al-Cu Binary Alloy at Largely Different Cooling Rates (0.01 to 20,000 K/s): Modeling and Experimental Study

Abstract

Microsegregation in the Al 4 wt pct Cu alloy was investigated experimentally in a large range of cooling rates from 0.01 to 20,000 K/s using different solidification techniques. The microstructure was modeled using two-dimensional (2-D) pseudo-front tracking (PFT) developed by Jacot and co-workers. The experimentally determined amount of nonequilibrium eutectics increases with the cooling rate in the range 0.01 to 3 K/s and then decreases in the range 20 to 20,000 K/s. The fraction of eutectic calculated from the 2-D PFT model shows not only the same tendency, but also agrees quantitatively very well with the experiments over the range of cooling rates. It can also be explained qualitatively how the observed in terms of coarsening of the secondary dendrite arms and the back diffusion in the way both depend on the local solidification time.

Estimation of Cooling Rates During Close-Coupled Gas Atomization Using Secondary Dendrite Arm Spacing Measurement

“…We have therefore re-calculated the fitting parameters on the basis of the ARTEMIS and EM levitation data, obtaining λ 0 = 54.8 μm and n = 0.339. We note however that these are not very different to the combined value given by [18]. The data from all the sources used here is summarised in Table 2.…”

“…For this reason we have omitted the drop-tube data from the fitting. This however also highlights that there is a systematic difference in the cooling rate-SDAS data between the ARTEMIS and EM levitation data on the one hand and the mould data of [18] on the other hand. We have therefore re-calculated the fitting parameters on the basis of the ARTEMIS and EM levitation data, obtaining λ 0 = 54.8 μm and n = 0.339.…”

“…A comprehensive analysis of the secondary dendrite arms spacing in Al-4wt.% Cu has been given by Kasperovich et al [18] combined data set with a power law with λ 0 = 54.5 μm and n = 0.328. However, for their drop-tube samples [18] were unable to measure cooling rates directly, instead using a heat balance model similar to that defined by Equations (1-2) to estimate the cooling rate.…”

“…However, for their drop-tube samples [18] were unable to measure cooling rates directly, instead using a heat balance model similar to that defined by Equations (1-2) to estimate the cooling rate. In terms of the current study this would introduce an unacceptable circularity to the methodology if we were to use data reliant on Equations (1-2) as part of the calibration from which we will then measure cooling rates to assess the extent to which Equations (1-2) can be applied to gas atomization.…”

“…The correlation between SDAS and cooling rate in Al-Cu alloy at the Al-rich end of the phase diagram has been widely studied and consequently a number of conversions from SDAS are available [14,15,16,17,18]. The effect on the estimated cooling rate of applying these various scaling laws is considered fully in the Section 3.…”