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condition is the phase distribution cal- culated in the solidification simulation. Te initial concentration profile

empty spot density, the faster the dif- fusion processes are performed. Te diffusion velocity is driven by the local dendrite arm spacing and the fraction of Mg2

Si or Al2 Cu phases. Starting

calculation considers the back diffu- sion of copper or magnesium during solidification. Te concentration at the outer rim of a cell uses values based on the thermodynamic equilibrium. Te results of a solidification simulation that started by measuring copper- distribution has shown good correla- tion with copper distribution at several solution treatment points for smaller secondary dendrite arm spacing. Quenching creates a state of satura-

tion of the magnesium/copper concen- tration, as well as the empty spot con- centration. Te material must be cooled as fast as possible to avoid unwanted precipitations or crystal structure changes. Cooling that is too slow creates precipitants that are neither optimally sized nor in sufficient numbers for the precipitation-hardening process to pro- vide the intended increase in strength. Previous experiments confirmed

peak yield strengths after aging depend on the cooling rate during quench- ing. Te simulation model used in the current research uses data derived from quenching experiments to determine the consumed amounts of magnesium and copper. According to the authors, additional experiments are needed to better determine the consumed amounts, particularly for low cooling rates.

Fig. 1. The measured and simulated yield strength profile for AlSiMg alloys with several magnesium concentrations proved to correlate well.

Aging provides a controlled process

to generate the desired number and size of precipitants. Te over-saturated empty spot concentration results in an acceleration of the process. Similar to the solution treatment, modeling in the current experiment was based on the calculation of magnesium/copper diffu- sion in solution in the crystal matrix to spherical precipitants, with the radius and the concentration and their subsequent growth. Te concentration at both sides of the boundary between the particle and the matrix equate to the thermodynamic equilibrium concentration. Te cell’s size is determined through the number of inoculation sites and the amount of magnesium or copper available to create precipitants.

When all the magnesium and cop-

per is consumed, precipitations only cluster together, so the total fraction of precipitants increase their individual size. In this experiment, modeling the mechanical properties was based on the size and volume fraction of the particles, the average distance between them and the magnesium/copper content of the crystal matrix. Te strength profile for AlSiMg

alloys with magnesium concentrations have shown a steep increase in yield strength through the precipitation hardening, a subsequent plateau with the final peak value, followed by a slow decrease in yield strength due to over aging, according to measured and simulated results (Fig. 1).

Fig. 2. The casting used in the experiment was poured inside a core package with a water-cooled steel mold on the firing deck and open risers. September 2012 MODERN CASTING | 35

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