Dendrite structure control in directionally solidified bronze castings

Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between -0.338°C/mm and 0.167°C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys.

Section: Introductionmentioning

confidence: 93%

Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum Alloys

Ares

Schvezov

2007

“…Here, we focus on experiments where the heat flow was essentially unidirectional and the alloy was a simple binary system, namely, Al-Cu, [17,23,24] Al-Si, [21] Sn-Pb, [16,24] Cu-Sn, [25,26] Pb-Sn, [27] and Al-Mg. [28] In these experiments, the position of the CET in the solidified samples was studied as a function of process variables, including pouring superheat, addition of inoculants, alloy composition, heat extraction rate, and average cooling rate. In all cases, the experimental apparatus basically consisted of a thermally insulating sleeve resting on a metallic base, through which the heat was extracted.…”

Section: B Experiments On the Cet In Directional Solidification Of Bmentioning

confidence: 99%

A solutal interaction mechanism for the columnar-to-equiaxed transition in alloy solidification

Martorano

Beckermann

Gandin

2003

Self Cite

239

217

A multiphase/multiscale model is used to predict the columnar-to-equiaxed transition (CET) during solidification of binary alloys. The model consists of averaged energy and species conservation equations, coupled with nucleation and growth laws for dendritic structures. A new mechanism for the CET is proposed based on solutal interactions between the equiaxed grains and the advancing columnar frontas opposed to the commonly used mechanical blocking criterion. The resulting differences in the CET prediction are demonstrated for cases where a steady state can be assumed, and a revised isotherm velocity (V T ) vs temperature gradient (G) map for the CET is presented. The model is validated by predicting the CET in previously performed unsteady, unidirectional solidification experiments involving Al-Si alloys of three different compositions. Good agreement is obtained between measured and predicted cooling curves. A parametric study is performed to investigate the dependence of the CET position on the nucleation undercooling and the density of nuclei in the equiaxed zone. Nucleation undercoolings are determined that provide the best agreement between measured and calculated CET positions. It is found that for all three alloy compositions, the nucleation undercoolings are very close to the maximum columnar dendrite tip undercoolings, indicating that the origin of the equiaxed grains may not be heterogeneous nucleation, but rather a breakdown or fragmentation of the columnar dendrites.

“…As opposed to the strategy adopted here, in the literature [34][35][36][37][38] the identification of the CET position has always been uniquely based on the visual examination of the grain macrostructures, which, to a large extent, is a subjective process. In the present work, the previous ranges of aspect ratios were defined for the best agreement between the visually identified CET and those obtained from the proposed method.…”

Section: Aspect Ratio Criterion To Determine the Cetmentioning

confidence: 98%

Mechanical Blocking Mechanism for the Columnar to Equiaxed Transition

Biscuola

Martorano

2008

Mechanical blocking of the columnar front during the columnar to equiaxed transition (CET) is studied by quantitatively comparing the CET positions obtained with one stochastic model and two deterministic models for the unidirectional solidification of an Al-7 (wt pct) Si alloy. One of the deterministic models is based on the solutal blocking of the columnar front, whereas the other model is based on the mechanical blocking. The solutal-blocking model and the mechanical-blocking model with the traditional blocking fraction of 0.49 give columnar zones larger than those predicted with the stochastic model. When a blocking fraction of 0.2 is adopted, however, the agreement is very good for a range of nucleation undercoolings and number density of equiaxed grains. Therefore, changing the mechanical-blocking fraction in deterministic models from 0.49 to 0.2 seems to model more accurately the mechanical-blocking process that can lead to the CET.