Macrosegregation and Porosity during Directional Solidification of a Ternary Al-9wt%Si-3wt%Cu Alloy

Macrosegregation and porosity formation have been investigated by both a numerical model and by transient directional solidification experiments. The macrosegregation pattern, the theoretical and apparent densities are presented as a function of the casting length. X-ray fluorescence spectrometry was used to determine the macrosegregation profiles. The measurement of microporosity was performed using pyknometry analysis. The local composition along an Al-9wt%Si-3wt%Cu casting length was used as an input parameter for simulations of microporosity evolution. The results have demonstrated that the presence of Si in the alloy composition has inhibited the inverse copper segregation, which is a typical result of directionally solidified Al-Cu castings. The numerically simulated trend is in good conformity with the experimental scatter.

Numerical Study and Experimental Investigation of Solidification Phenomena in Semi-Continuous Casting of Bronze

In semi condinuous casting of technincal bronze alloys homogenuous microstructure is very important for the assurance of material properties. The improvement of the knowledge about both, thermodynamics of the ternary system Cu-Sn-P and the solidification process is of main interest for the involved industry. To describe solidification of these alloys, the ternary system Cu-Sn-P in the Cu-rich corner is experimentally investigated. DSC measurements, diffusion and annealing experiments have been performed and compared with computational thermodynamics based on the Calphad approach. The so defined thermodynamic information is coupled with solidification simulation. For this, CFD calculations are done with a two phase solidification model including mass, momentum, energy and concentration transfer and applied to semi-continuous casting of technical Bronze alloys. The predicted macrosegregation pattern is in good qualitative agreement with experimentally observed result.

Comparison between Simulation and Experimental Results of the Effect of RMF on Directional Solidification of Al-7wt.%Si Alloy

A numerical investigation of directional solidification of Al-7wt.%Si alloy stirred by a rotating magnetic field is compared with experimental results. Experimental study of such process has revealed periodical macrosegregation in axial direction of the samples in the shape of a “Christmas tree”. Similar macrosegregation pattern is obtained in simulations for the two values of magnetic field. Numerical simulations have shown also that formation of the periodical structure depends not only on the external conditions but on the permeability of the mushy zone.

Macrosegregation Mechanisms in Direct-Chill Casting of Aluminium Alloys

This review paper summarizes the results of recent studies on different mechanisms of macrosegregation upon direct-chill (DC) casting of aluminium alloys. In general, the main mechanisms of macrosegregation have been identified quite some time ago as thermo-solutal convection, free-moving crystals, shrinkage- and deformation-induced flow, and forced convection. Despite this general knowledge, the separation of the effects of these mechanisms on the overall macrosegregation pattern and the ratio of their contribution remained largely unexplored. With the advances in computer simulations and in experimental techniques it becomes possible to look at the impact of individual mechanisms in relation to the macroscopic parameters of the transition region of a DC cast billet and to the microscopic parameters of billet structure. Our systematic research helps in interpreting the apparently contradictory experimental macrosegregation profiles reported in literature. Paper is illustrated by own experimental and computer-simulation results.

Convective Transport Phenomena and Macrosegregation During Solidification of a Binary Metal Alloy: I—Numerical Predictions

A continuum model is used to simulate transient convective transport phenomena numerically during solidification of a Pb-19 percent Sn alloy in an experimental test cell. Solidification occurs in an axisymmetric, annular mold of stainless steel, cooled along its outer vertical wall. Results show that, during early stages of solidification, double-diffusive convection and liquid exchange between melted and mushy zones are responsible for the formation of channels in the outer periphery of the ingot, which ultimately lead to a form of macrosegregation known as A-segregates. During intermediate stages of solidification, solutally driven natural convection spawns a cone segregate in the interior region of the ingot. The final macrosegregation pattern is characterized, in general, by increasing Sn concentration with increasing height throughout the ingot and by increasing Sn concentration with decreasing radius in the upper portion of the ingot.