Metallizations: Defect Pattern & Causes

Preferred locations where metallization occurs are edges in the mold or core where the metal remains liquid for a long time due to the geometry of the cast piece and where the mold parts are strongly heated. This defect can occur in all cast parts that are produced with the sand molding method (preferably with the green sand method), regardless of the material, as a result of physical/chemical interaction between the melt and the components of the mold material.

Close connection between metallization and penetration

Melting phases that lead to burnt sand (sintering) and metallization occur as a result of reactions between the metal and the mold material. This defect occurs frequently when casting alloyed cast steel in particular. It is visible to the naked eye on the cast part. Copper-tin and copper-zinc alloys are also at risk.

In contrast to real physical/chemical penetration, metallization is a chemically caused penetration. If a thin sand crust made of individual quartz grains that adheres firmly to the cast piece occurs, we use the term burnt sand (sintering). If this thin layer that now adheres to the cast piece very firmly consists of melted sand and if the entire surface is also pimpled, we use the term metallization.

It is not possible to clearly distinguish between the defects of penetration and metallization. Metallization is always preceded by penetration and sintering. In any case, the defect leads to significantly increased cleaning work, if not to rejections.

Causes

When metallization occurs, the wetting stress can be influenced in such a way that the penetration pressure (see penetration) decreases under the present metallostatic pressure, which significantly intensifies the penetration on the cast part. On the other hand, melting phases that lead to the formation of silicate layers (metallization) can occur as a result of reactions between the metal and the mold material, with the atmospheric oxygen in the mold cavity and the pores of the mold material making a significant contribution.

Influence factor

Both the silica sand and the disintegration products of the clay react with metal oxides (e.g. FeO) at higher temperatures and form low-melting orthosilicates according to the following reaction scheme:

2Fe + O2 _ 2FeO

Al2 O3 · SiO2 + 4FeO _ 2(2FeO · SiO2) + Al2 O3

fayalite

The iron oxides that develop in this process wet the mold material much better than the metallic iron and this increases the possibility of sintering and occurrences of burning. In general, the formation of intermediate layer phases can be expected during reactions between the metal and the mold material. These phases behave differently depending on the composition of the reaction partners with regard to wettability and reactivity. Preceding penetration of metal vapors increases the metallization tendency. The manganese oxide slags that develop during the production of manganese steel are very aggressive and increase the material's metallization tendency.