Coating Aluminum Die Castings

Armoloy's Treatment to Retard Heat Checking

TThe life of aluminum and magnesium die castings dies can be extended considerably with the Armoloy Process. This already has been proven using the Armoloy coating on dies, especially those affected by heat checking. When the Armoloy 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 Armoly 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 cavitation 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. The 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 Armoloy 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 Armoloy 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 Armoloy 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 Armoloy Process materials. Excess lubricants must be removed and the die cavities thoroughly stoned and cleaned. With proper planning and coordination with Armoloy, generally less than two days are lost in the Armoloy treatment facility. Your experience and knowledge will tell you how many shots to run before Armoloy retreating of the die. A good guide to be used is provided by keeping marked castings from each day of production as a visual reference.

To help you in making Armoloy retreatment decisions, The following points should be considered:

  1. After Armoloy coating, the casting finish is better because the crevices have been filled by producing a smoother finish and appearance to the castings. Threads, cracks and tool marks are all smoothed and the die surface can now produce castings with a finish as fine as 8 R.M.S. when required.
  2. Heat Checks are greatly reduced following the Armoloy treatment but are not removed completely. However, it is noticeable that the usual rapid progress of the heat checking is virtually stopped for a period following the Armoloy coating. After the Armoloy treatment, heat checks still may be visible on the castings and may not be completely eliminated. Later, when checking does progress, it is at much reduced rate.
  3. Before additional Armoloy retreatments, the die impressions always should be stoned and/or polished, especially in the areas of severe checks. This action may seem foolish since the existing Armoloy chromium coating cannot be completely stoned off the die surface. However, the edges of heat check cracks will be smoothed to give a more even surface, or at least one that is smoother than it was before the die is to be Armoloy retreated. This practice will be helpful in ensuring a good Armoloy coating retreatment time after time.
  4. In many cases, material flow will be enhanced after the Armoloy coating has been applied. The 78Rc Armoloy surface resists sticking and material build up making for a cleaner die environment. Please keep this in mind when you consider Armoloy. In addition, this 78Rc hardness will enhance the parting line edges, helping to reduce or eliminate flashing. Since the Armoloy coating will not peel, crack, or chip, the surface integrity of the Armoloy coating on the die surface is absolute.
  5. When Armoloy recoating is called for it is absolutely important that you prepare, clean, weld and repair the die steel to its best condition to ensure that Armoloy coating will deliver its many advantages.
  6. Profits can be increased by reducing the cost of salvaging castings and increasing die life. It should be considered that when heat checked castings are produced, they require more and separate handling. The cost of the Armoloy treatment is very reasonable and is an excellent option for all die casters to consider in reducing these operations.
Please contact us at Armoloy to work with you to aid in extending your die life and to produce a better quality die casting product. For more information, call the leader in corrosion resistant coatings at: 412-823-1030.