Penetration

 

This defect may occur with all molds made on the basis of sand molding processes (in particular green sand processes), irrespective of the material.

It is primarily observed in places of the mold parts with significant temperature increases or around the gate, in places of low compaction of the mold material as well as in thick-walled castings. It frequently affects entire casting sections. If the defect is restricted to a certain area and often occurs in the part of the mold cavity filled last, then it is generally a case of explosive penetration. The defect is clearly visible to the human eye on the casting.

Penetrations are adhesive sand crusts on the casting which lead to rough casting surfaces (s. Rough surface), increased cleaning effort or rejection (Figures 1 to 3). The roughness depths are larger than the average grain radius of the molding base material used (unlike the roughness where the roughness depths are smaller than the average grain radius of the molding base material).

 

Explosive penetration also manifests itself in an increased formation of burrs and gas bubbles close to the surface (water vapor). Penetration occurs if no chemical reaction takes place between the mold material and the metal that has penetrated it. It is also referred to as real penetration. If the a chemical reaction takes place between the metal and the mold material, this is referred to as burn-on (burnt-on sand), sintering or in most cases metal penetration.

 

General causes for real (mechanical/physical) penetration are the metallostatic pressure, the dynamic pressure during casting, the crystallization pressureduring solidification and, in case of explosive penetration, an additional gas pressure. Sand molds have a certain pore system according to their packing density. At the boundary surface between metal and mold material, there is a balance between the metallostatic pressure, the capillary forces of the mold material, wetting and the surface tension of the metal During casting, the molten metal hits the grains of the mold surface and can enter the pores of the mold surface under the effect of the metallostatic pressure until a balance is achieved between the interfacial tension on the mold surface and the penetration pressure (critical pressure value at which the molten metal penetrates the uppermost grain layer). The consequence is a rughness of the casting surface.

The interfacial tension counteracting the penetration pressure is influenced by the capillary forces of the mold material (primarily by the porosity), the wettability of the mold surface, and the surface tension. The interfacial tension of mold materials on the basis Fe-C reaches relatively high values. The extent of this tension is mainly influenced by the chemical composition of the main alloy elements and the presence of interfacially active elements such as bismuth, lead, phosphorous, silicon etc. A strong increase of the interfacial tension is also achieved through additions of cerium, sodium, and zirconium.

The melt reflects the sandstructure of the mold surface wetted by it with varying intensity (roughness, wetting depth). The wetting depth values increase with increasing penetration pressure, grain radius of the mold material, pore radius of the mold material, density of the grains and decreasing interfacial tension. The wetting angle and thus the wettability of a mold surface can be significantly influenced by the formation of a lustrous carbon layer.

For a solidified mold part, the size of the pore radius primarily depends on the grain structure (grain size, grain distribution), the additives (binder content, sediments), the intensity of compaction (packing density), and the sinter behavior of the mold material.

Penetration occurs if the roughness depth is greater tahn or equal to the grain radius of the mold material grains. This means that a differentiation between roughness and penetration is only possible when the grain size of the mold material is taken into account.

 

Thus, penetration can only occur depending on the following influence factors:

 

  • too large grain size and wide grain distribution of the mold material
  • too low binder and sediment contents
  • the number of lustrous carbon-forming substances is too small
  • too high compressibility of the mold material system (packing density)
  • too low gas permeability
  • unfavorablel chemical composition of the casting material in combination with too high casting temperatures and too high metallostatic pressure
  • insufficient and uneven compaction of the molds or cores
  • insufficient gate system and thus excessive superheating of moldsand core sections

A special form of real penetration is the explosive penetration. In addition to the static and dynamic loads during casting, an explosive evaporation of water occurs when the liquid metal strikes the wet mold wall. This is connected to a gas surge which results in metal entering the pores. As the entered metal solidfies very quickly,
the path of the vapor bubbles to the pores is blocked so that gas bubbles (water vapor) remain in the casting close to the surface.

This defect occurs more frequently with coarse recirculating system sands and excessively compacted molds. The high casting outputs of modern molding plants where increased heat transfer conditions lead to spontaneous (explosive) evaporation of the water can also contribute to the occurrence of defects.

Measures for prevention (acc. to S. Hasse, FT&E):

1. Use of finer sands or refine the sand grain of the recirculating system sand trough addition of fine-grained (core) sands. For core sands, add additives

2. Increase of the lustrous carbon-forming additives. Under the effect of the casting heat, the coal becomes soft and forms a protective film, the wettability is reduced

3. Increase binder and sediment contents, reduce compressibility of the mold material system, this will increase the flowability of the sand allowing for better compaction

4. Separate at least mold and core sections at risk, this will significantly reduce the pore volume directly at the mold/core surface

5. Largely avoid interfacially active elements in the chemical composition of the material such as phosphorous and lead

6. Reduce the metallostatic pressure and improve the gate system so that casting joints and excessive superheating of mold and core sections are avoided

7. Reduce the casting temperature

In order to avoid explosive penetration the following applies in particular:

8. Reducing the heat withdrawal speed in combination with avoiding high heat-up speeds by filling the mold more slowly and evenly

9. Reducing the thermal conductivity of the mold material, e.g. by reducing the water content or by means of surface drying. As the occurrence of evaporated water mainly depends on the amount of free water, optimum conditioning of the mold material is essential (s. Sand conditioning)

10 Reducing the mold density to ensure sufficient gas permeability

11. Improving the discharge of casting gases (containing water vapor) (improved ventilation)

 

  • Fig. 1: Massive penetration on an aluminum casting, the adhesive sand crusts could be removed after extensive blasting (left side)
  • Fig. 2: Detail image of the section highlighted in Figure 1, the individual penetrated sand grains and the clean surface of the aluminum casting alloy after intensive blasting are clearly visible, 12:1
  • Fig. 3: REM images of the penetrated sections (highlighted in Figure 1), no traces of sintering can be observed
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