This page contains a Flash digital edition of a book.
in better mechanical properties. Solution heat treatment dissolves Cu phases and creates


a condition where empty spots are present within the crystal structure as much as possible. T e higher the emp- ty spot density, the faster the diff usion processes are performed. T e diff usion velocity is driven by the local dendrite arm spacing and the fraction of Mg2 or Al2Cu phases. Starting condition is


the Mg2Si or Al2 Si


the phase distribution calculated in the solidifi cation simulation. T e initial concentration profi le


calculation considers back diff usion of copper or magnesium during solidifi - cation. T e concentration at the outer rim of a cell uses values based on the thermodynamic equilibrium. T e results of a solidifi cation simulation started by measuring copper-distribu- tion has shown good correlation with copper distribution at several solution treatment points for smaller secondary dendrite arm spacing. Quenching creates a state of satura-


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


tion of the magnesium/copper con- centration, as well as the empty spot concentration. T e material must be cooled as fast as possible to avoid un- wanted precipitations or crystal structure changes. Cooling that is too slow creates precipitants that are neither optimally sized nor in suffi cient numbers for the precipitation-hardening process to pro- vide the intended increase in strength. Previous experiments confi rmed peak


yield strengths after aging depend on the cooling rate during quenching. T e simulation model used in the current re- search uses data derived from quenching experiments to determine the consumed amounts of magnesium and copper. Ac- cording to the authors, additional experi- ments are needed to better determine the consumed amounts. Aging provides a controlled process


to generate the desired number and size of precipitants. T e 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 diff u- sion in solution in the crystal matrix to


spherical precipitants, with the radius and the concentration and their subsequent growth. T e concentration at both sides of the boundary between the particle and the matrix equate to the thermodynamic equilibrium concentration. T e 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. T e strength profi le for AlSiMg


alloys with magnesium concentrations has shown a steep increase in yield strength through the precipitation hardening, a subsequent plateau with the fi nal 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 fi ring deck and open risers.


46 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2012


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60