Mold Materials for Permanent Molding of Aluminum Alloys
The specific goals of this project are to extend the life of permanent molds for aluminum permanent mold castings, reduce the cost of permanent molds, and improve surface quality and soundness of aluminum permanent mold castings.
At present, the metal molds used in permanent molding of aluminum alloys are selected from a fairly large field of materials ranging from gray iron to high strength tool steels. The selection of the mold is based primarily on the prior history of the foundry and limited experience of the operators. What is needed by the permanent mold foundries is a definite means of establishing how long or the number of castings that can be produced in a mold of a given mold material.
The usual mode of failure of permanent molds for aluminum casting is thermal fatigue cracking produced by the cyclic thermal stress on the mold face. Resistance to thermal fatigue can be increased by using mold materials that have a combination of high thermal conductivity, high strength at elevated temperature, low coefficient of thermal expansion and a low modulus of elasticity.
The primary goals of this project are to extend the life and improve the quality of permanent molds utilized in casting aluminum. The relative mold life under the thermal conditions that prevail in permanent molds that experience exposure to molten aluminum will be determined for a range of materials such as gray iron (Meehanite), ductile iron, and compacted graphite iron. These materials may be used plain or alloyed to provide microstructures with different life and stabilities at elevated temperatures. Other candidate materials are cast and wrought 4140 type steels, and cast and wrought H-13 steels.
An additional goal is to reduce the cost of the molds and improve the surface and soundness quality of the aluminum castings by selective application of coatings. Different coatings can control the rate at which heat is dissipated into the mold material. This heat transfer is a function of the diffusivity of the mold material, but the heat transfer in a metal mold is controlled to a large extent by the heat transfer at the interface of the mold with the casting. The coating, or mold washes employed in the industry, exert a direct effect on the surface heat transfer coefficient. In this project, state of the art coatings used in the permanent mold industry will be evaluated and compared. The heat transfer coefficients will be determined and correlated to the thickness of the coating.
In the study of alloy microstructure performance interaction, WPI developed an understanding of the interactions between alloy chemistry, casting microstructure, and properties of die cast parts for aluminum die casting. In addition, test specimens have been cast and property testing is underway. A comprehensive review of the subject has been completed and is available from NADCA.