subject: When You Are Coating Aluminum [print this page] The life of aluminum and magnesium die castings dies can be extended considerably with the process. This already has been proven using the coating on dies, especially those affected by heat checking. When the treatment is applied early in the life of the die, the number of castings made from the die can be increased and more profits generated.
Heat checking is a common cause of failure in die casting dies made of steel (I.e.: H-13) and is commonly attributed to the over stressing of thermal fatigue of the steel used. This thermal fatigue is caused by alternate heating and cooling of the surface of the steel die in conjunction with the continuous thermal shocks caused during the introduction of the molten die cast materials.
Pretreatment of dies with the process adds strength to the steel and helps overcome damage already done. The process does not remove the heat checks completely, but it does reduce the severity of existing damage, holding it in check so further heat checking continues at a much reduced rate.
Successful die casting processing requires that heat generated be removed from the molten metal to produce the solidified casting. The heat energy, therefore, must be transferred to the die steel. To get into the die, the heat must pass through the surface of the steel, heating it very rapidly. The surfaces then cool. When this sequence is repeated in rapid succession, the heat checks soon begin. These checks may not be visible under inspection by the naked eye for several hours or even days, but they have started on the die steel and usually can be found near the point where the molten metal enters the die cavity. As they grow longer and penetrate deeper into the steel surface, the checks will increase in number forming cross connecter cracks, many times perpendicular to the original flow lines. The die, and the castings produced, get progressively worse until the die must be reworked along with the castings. Both operations are expensive and time consuming.
A crack or from the die surface into the steel accompanies all heat checks. Another obvious characteristic is that these cavities usually are found first near the source of the metal flow following in its same direction that the metal flows. A fast moving stream of water will cut a groove in hard rock in the direction of the water flow. It is logical to assume that molten metal will have a similar effect on the die steel.
A most logical solution might be to fill in these crevices with a strong material that would not be easily washed away. This "crack filling" can and is being done with the coating.
There is an optimum time to process die casting dies. The most logical time is when the cracks are deep enough to need filling but not so deep that they cannot be filled completely.
The process is a rapidly applied process and it is applied most successfully on new cavities, after prototype samples are run. With older dies, however, the standard procedure for treatment is as follows. When the die is removed from the machine, the impressions are taken out and cleaned of all aluminum/magnesium particles, since they contaminate the die surface. This contamination is not good for the process materials. Excess lubricants must be removed and the die cavities thoroughly stoned and cleaned. With proper planning and coordination with, generally less than two days are lost in the treatment facility. Your experience and knowledge will tell you how many shots to run before retreating of the die.
